Novel Polyaniline-based Ammonia Sensors on

Plastic Substrates

A thesis submitted to the University of Manchester for the degree of Doctor of

Philosophy in the Faculty of Engineering and Physical Sciences

2014

Ehsan Danesh

School of Chemical Engineering and Analytical Science

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CONTENTS

ABSTRACT ................................................................................................................ 15

DECLARATION .......................................................................................................... 17

COPYRIGHT STATEMENT ........................................................................................ 18

DEDICATION ............................................................................................................. 19

ACKNOWLEDGMENTS ............................................................................................. 20

CHAPTER ONE

1. Introduction: FlexSmell project ............................................................................. 21

CHAPTER TWO

2. Background ......................................................................................................... 25

2.1. Food spoilage ............................................................................................... 25

2.2. Food quality indicators .................................................................................. 26

2.2.1. Examples of FQIs products currently available ...................................... 27

2.3. Selection of the target analyte: Ammonia ..................................................... 31

2.4. Ammonia sensing principles ......................................................................... 35

2.4.1. Metal oxide sensors ............................................................................... 36

2.4.2. Chem-FET sensors ............................................................................... 37

2.4.3. Conducting polymer sensors ................................................................. 38

2.5. Intrinsically conducting polymers .................................................................. 40

2.5.1. Background ........................................................................................... 40

2.5.2. Electrical conductivity in ICPs ................................................................ 43

2.5.3. Polyaniline ............................................................................................. 48

2.5.4. Polymerisation of aniline ........................................................................ 49

2.5.5. Polyaniline as an ammonia sensing material ......................................... 53

2.5.6. Solution processable PANI .................................................................... 55

2.6. References ................................................................................................... 59

CHAPTER THREE

3. Hybrid polyaniline sensors ................................................................................... 65

3.1. Introduction................................................................................................... 65

3.2. Materials and methods ................................................................................. 67

3.2.1. Materials ................................................................................................ 67

3.2.2. Preparation of doped PANI .................................................................... 67

3.2.3. PANI/CB hybrid composites .................................................................. 68

3.2.4. Thin film characterisation ....................................................................... 68

3.2.5. Fabrication of sensors ........................................................................... 70

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3.2.6. Sensing chamber and circuit board ....................................................... 70

3.2.7. Ammonia sensing characterisation ........................................................ 72

3.3. Results ......................................................................................................... 75

3.3.1. UV-Vis spectroscopy ............................................................................. 75

3.3.2. Atomic force microscopy........................................................................ 79

3.3.3. X-ray photoelectron spectroscopy ......................................................... 81

3.3.4. Thermogravimetric analysis ................................................................... 85

3.3.5. Conductivity of polyaniline films ............................................................. 87

3.3.6. Ammonia sensing results....................................................................... 91

3.4. Discussion .................................................................................................... 99

3.5. References ................................................................................................. 102

CHAPTER FOUR

4. Vapour-phase deposition polymerisation ........................................................... 104

4.1. Introduction................................................................................................. 104

4.1.1. VDP process ....................................................................................... 105

4.1.2. Gas sensors made by VDP method ..................................................... 107

4.2. Materials and methods ............................................................................... 109

4.2.1. Materials .................................................................................................. 109

4.2.2. PANI sensing layer preparation ........................................................... 109

4.2.3. Characterisation .................................................................................. 110

4.3. Results ....................................................................................................... 111

4.3.1. PAA-doped PANI ..................................................................................... 111

4.3.2. PSSA-doped PANI .............................................................................. 114

4.3.3. Nafion-doped PANI.............................................................................. 119

4.3.4. Bio-immobilisation ............................................................................... 124

4.4. Discussion .................................................................................................. 126

4.5. References ................................................................................................. 128

CHAPTER FIVE

5. Polyaniline Ammonia Sensors on Printed Polymeric Hotplates .......................... 131

5.1. Introduction................................................................................................. 131

5.2. Experimental section .................................................................................. 134

5.2.1. Heaters ................................................................................................ 134

5.2.2. Dielectric layer and interdigitated electrodes........................................ 135

5.2.3. Preparation of the PANI sensing layer ................................................. 136

5.2.4. Characterisation .................................................................................. 137

5.3. RESULTS AND DISCUSSION ................................................................... 138

5.3.1. Thermal characteristics of the µ-hotplates ........................................... 138

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5.3.2. PSSA-doped PANI thin films ............................................................... 139

5.3.3. Ammonia sensing properties ............................................................... 140

5.4. Discussion .................................................................................................. 149

5.5. References ................................................................................................. 150

CHAPTER SIX

6. CONCLUSIONS & FUTURE WORK .................................................................. 160

6.1. Conclusions ................................................................................................ 160

6.2. Future work ................................................................................................ 161

APPENDICES

APPENDIX I: Food spoilage ..................................................................................... 166

Microbial deterioration........................................................................................... 166

(Bio)chemical changes .......................................................................................... 168

References ........................................................................................................... 173

APPENDIX II: Dissemination .................................................................................... 174

Publications .......................................................................................................... 174

Conference presentations ..................................................................................... 174

Awards ................................................................................................................. 175

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LIST OF TABLES

Table ‎2-1. Ammonia sensing requirements in different applications (Reproduced from ref. (Timmer and al., 2005)) ............................................................................... 32

Table ‎3-1. Atomic percentage in polyaniline samples extracted from XPS wide scan spectra. ............................................................................................................. 84

Table ‎3-2. TGA parameters extracted from the TGA graphs of three polyaniline samples. ............................................................................................................ 86

Table ‎3-3. Resistance measurements data for 4 samples of doped PANI and hybrid composites. ....................................................................................................... 88

Table ‎3-4. Resistivity and conductivity of polyaniline samples. ................................... 90

Table ‎3-5. Some important physical properties of the two substrates used in this study.

CTE and CHE are the coefficient of thermal expansion and coefficient of hydroscopic expansion, respectively (Adapted from (Thomas Kinkeldei et al., 2012)). ............................................................................................................... 96

Table ‎5-1. Effect of temperature on the 2nd generation sensor response to ammonia vapour in dry and humid air. ............................................................................ 145

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LIST OF FIGURES

Figure ‎1-1. Overall picture of the FlexSmell training and transfer of knowledge (Adapted from the FlexSmell’s description of work, part B). ............................... 23

Figure ‎1-2. 2D grids of thematic work packages and methodical approaches with the

relevant tasks and groups in charge of the coordination (Adapted from the FlexSmell’s description of work, part B). ............................................................ 23

Figure ‎2-1. Freshpoint’s OnVu™ indicators accurately and consistently record and

display the freshness of the products, based on their time and temperature histories. Once packaged, the OnVu™ TTI is activated using an ultra violet (UV) light source. The evolution of the label colour as a function of time and temperature is demonstrated. ............................................................................ 26

Figure ‎2-2. Examples of food quality indicators available in the market: a) Freshness Guard (Smolander, 2008), b) FreshQ FQI label, c) Litmus FQI label. ................ 28

Figure ‎2-3. It's Fresh Inc.’s smart label products (from http://www.itsfresh.com/) ....... 29 Figure ‎2-4. RipeSense® ripeness indicator. ............................................................... 30

Figure ‎2-5. Ammonia content in water vs. pH of the solution ...................................... 35 Figure ‎2-6. Energy band diagram showing the Schottky barrier height (eVd and eVʹd) at

the grain boundary of TiO2 without and with a chemically reducing gas, R (Adapted from (Timmer and al., 2005)). Ec = conduction band, Ev = valence band, and Ef = Fermi level. .......................................................................................... 37

Figure ‎2-7. The structure of the two types of field-effect transistor sensors. a) TFT; b) IGFET. (Reproduced from (Janata and Josowicz, 2003)) .................................. 38

Figure ‎2-8. Sensing mechanism in conductive polymer composite sensors. The

vapour-induced expansion of the polymer composite causes an increase in the electrical resistance of the composite because the polymer expansion reduces the number of conducting pathways for charge carriers (Reproduced from Lewis research group website, Caltech, USA). ............................................................ 39

Figure ‎2-9. Electrical conductivity of halogen-doped polyacetylene as a function of

bromine and iodine concentration. Inset shows the structure of undoped trans-polyacetylene (up) and doped trans-polyacetylene with delocalised electrons (down). The former is an insulator, whereas the later can act as a metal (Reproduced from (Chiang et al., 1978)). .......................................................... 41

Figure ‎2-10. Chemical structures and conductivity range of some of the most popular conducting polymers (Adapted from (Terje A. Skotheim, 2007)). ....................... 42

Figure ‎2-11. Application fields of ICPs. (Reproduced from (Heeger, 2001)) ............... 43 Figure ‎2-12. Electronic band configuration in (a) metals, (b) insulators, and (c)

semiconductors from closely spaced atoms having quantized energy levels. .... 45 Figure ‎2-13. n-type (left) and p-type (right) conduction in semiconductors. ................ 46 Figure ‎2-14. (a) π-π* transition and (b) soliton (left) vs. polaron lattice (right) band gap

states. ................................................................................................................ 47 Figure ‎2-15. Proposed crystalline-amorphous (heterogeneous) structure of an ICP,

showing different conduction pathways: A: along the backbone, B: interchain and C: between metallic grains (Reproduced from (Hobday, 2009; Bhadra et al., 2009). ................................................................................................................ 47

Figure ‎2-16. Different forms of polyaniline, depicting the oxidative and protonic acid (non-oxidative) doping (Freund and Deore, 2007) ............................................. 48

Figure ‎2-17. Structure of EB after 50% protonation and formation of bipolarons (a),

unstable dication radicals (b) and stable polaron lattice (d) (Adapted from (Epstein et al., 1987; Bhadra et al., 2009)). ........................................................ 49

Figure ‎2-18. Electrophilic substitution reaction. .......................................................... 51

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Figure ‎2-19. Polyaniline polymerisation mechanism. (Adapted from (Sapurina and Shishov, 2012)) ................................................................................................. 51

Figure ‎2-20. Effect of pH on the oxidation of aniline in the presence of a strong

oxidising agent and in a high ionic strength medium (Reproduced from (Sapurina and Shishov, 2012)). pKAn=3.5 and pKPANI=2.5 are marked by the blue dashed line. The black line and dashed line are representative of aniline and chain oxidation potentials, respectively. ...................................................................... 52

Figure ‎3-1. Samples used for conductivity measurements. Left: SSA-doped

PANI/coated-CB, and right: SPhA-doped PANI/coated-CB hybrid composites. Au electrodes were thermally evaporated on the thin films...................................... 69

Figure ‎3-2. Sensing chamber design. All dimensions and measures are in mm. ........ 71

Figure ‎3-3. Left: PTFE sensor chamber and its Viton® sealing; Right: Substrate holder, circuit board and the sub-miniature ceramic heater (shown by yellow arrow) ..... 72

Figure ‎3-4. Permeation tube structure and function.................................................... 73 Figure ‎3-5. Automatic ammonia-vapour generation system. ...................................... 73

Figure ‎3-6. Diluted solutions of a) SSA-doped PANI, b) SPhA-doped PANI and c) EB in NMP. ............................................................................................................. 75

Figure ‎3-7. Band structures of protonated polyaniline with (left) a coil-like conformation

and (right) an expanded coil-like conformation, respectively. Numbers are indicative of the major wavelengths where peaks appear in UV-Vis spectra. Reproduced from Ref. (Xia et al., 1995) ............................................................ 76

Figure ‎3-8. UV-Vis spectra of emeraldine base, SSA- and SPhA-doped PANI solutions in NMP. ............................................................................................................. 76

Figure ‎3-9. Evolution of UV-Vis spectra of protonated PANI solutions in NMP during a 1-year period: (top) SSA-doped and (bottom) SPhA-doped PANI. ..................... 78

Figure ‎3-10. AFM image of (a) 20 wt.% Black Pearls 2000 and (b-d) PANI-coated CB

with SSA-doped PANI composite, deposited from suspension in NMP (measurements were done in non-contact mode). Black arrow shows a dispersed particle with ~100 nm in diameter ...................................................................... 80

Figure ‎3-11. Polyaniline/carbon nanoparticle core-shell structure. (Reproduced from W. Xiaorong et al., US Patent application No. 20100004398 (2010)) ................. 81

Figure ‎3-12. XPS N 1s core-level spectra of (a) undoped and (b) SSA-doped PANI

layers on silicon wafer. The quinoid and benzoid nitrogen atoms in emeraldine base show peaks at 398.7 and 399.9 eV, respectively. Upon doping, the quinoid type peak is replaced by polaronic N with a corresponding peak at higher BE (around 402 eV). Comparison of doped (red) and undoped (green) N 1s spectra is shown in (c). There is a clear shift in binding energy to higher values when the multifunctional dopant is used to protonate polyaniline imine sites. ................... 83

Figure ‎3-13. The wide scan XPS of emeraldine base (top) and SSA-doped PANI (bottom). ............................................................................................................ 84

Figure ‎3-14. C 1s spectra of (a) undoped and (b) SSA-doped PANI after a curve fitting, demonstrating the spectral contribution of different functional groups ................ 85

Figure ‎3-15. TGA graphs of undoped and multifunctional doped PANI powders. ....... 86

Figure ‎3-16. I-V curves at different gap distances for a) SSA-doped PANI, b) SPhA-

doped PANI, c) SSA-doped PANI/coated CB composite, and d) SPhA-doped PANI/coated CB composite. G1=0.2 mm, G2=0.6 mm, G3=1.0 mm, G4=1.4 and G5=1.8 mm are gap sizes of consecutive parallel electrodes. Solid lines are the best linear fit to the data points for each measurement. ..................................... 87

Figure ‎3-17. Calculation of sheet resistance (R□) and contact resistance (Rc): a) SSA-

doped PANI, b) SPhA-doped PANI, c) SSA-doped PANI/coated CB composite, and d) SPhA-doped PANI/coated CB composite. The R□ is the slope of the RxW vs. G plot. The intercept is equal to 2xWxRc. ..................................................... 89

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Figure ‎3-18. Examples of thickness measurement using AFM: a) SSA-doped PANI,

and b) SPhA-doped PANI/coated CB composite. The thin films on the glass slides were scratched by a scalpel to facilitate the measurement. ..................... 90

Figure ‎3-19. Transient response (as defined in Equation ‎3-4) of a SSA-PANI/coated-

CB dip coated sensor on Kapton toward 340, 550 and 1150 ppb at 80 °C. In all

cases exposure time is 300 sec (5 min), and the sensor has been given enough time to recover to the Rb. An off-set is applied to curves for clarification. The green and red arrows indicate the start and the end point of each exposure, respectively. ...................................................................................................... 92

Figure ‎3-20. Ammonia sensing behaviour of hybrid SSA-doped PANI/coate-CB sensor

(dip coated on Kapton; blue line) and Synkera’s MOS ammonia sensor (red line). The hybrid sensor kept at 80 °C with the sub-miniature ceramic heater. Ammonia

concentrations are indicated as bar graphs (black line) for each measurement. Exposure and recovery durations are 3 and 5 min, respectively. ....................... 94

Figure ‎3-21. Comparison of the relative response (as defined in Equation ‎3-5) between

the hybrid SSA-doped PANI/coated-CB sensor (dip coated on Kapton; blue bars) and Synkera’s MOS ammonia sensor (green bars). The hybrid sensor kept at 80 °C with the sub-miniature ceramic heater. The values are the average of five measurements. .................................................................................................. 95

Figure ‎3-22. Response of the spin coated SSA-doped/coated CB hybrid sensor to ammonia in air. The sensor is heated up to 80 ºC. ............................................ 95

Figure ‎3-23. Sensitivity enhancement by using spin coating instead of dip coating as

the deposition method. The intercept of the fitted lines has been fixed to zero. The error bars correspond to 5 measurements for the dip- and spin-coated devices and 3 repeats for the MOS sensor. ....................................................... 96

Figure ‎3-24. Effect of temperature and substrate material on the transient response of

a dip-coated SSA-doped PANI/coated-CB hybrid sensor toward 1 ppm ammonia. The zoom in area (b) shows the very beginning of the response. The exposure time is 3 min. ..................................................................................................... 97

Figure ‎3-25. The comparison of ammonia concentration measurements between a

commercial ammonia detector (red curve), and a spin-coated PANI/coated-CB hybrid nanocomposite sensor on PEN substrate working at 80 ºC. The response of the hybrid sensors is scaled to match the first point of the on-site sensing data. .......................................................................................................................... 98

Figure ‎3-26. Successful printing of a conducting ink: SSA-doped PANi in NMP, using MD-K-130 microdrop dispenser. The polymer concentration was 0.5 wt. %. The printing parameters were adjusted to produce a perfect droplet; a 320 mV pulse was applied to the piezo actuator for 72 µs. The droplet diameter in this image is ~ 80 µm corresponding to droplet volume of ~ 268 picolitres. .......................... 100

Figure ‎4-1. Two-step vapour-phase deposition polymerisation using solid-state oxidant and dopant. ..................................................................................................... 105

Figure ‎4-2. Illustration of a patterning example of conducting polyaniline using VDP-

mediated inkjet printing. (1) and (2) are the patterned polyaniline on plasma-treated PET (Reproduced from (Cho et al., 2010b)). ....................................... 107

Figure ‎4-3. Three different polymeric acids used as PANI dopant in the VDP process. ........................................................................................................................ 109

Figure ‎4-4. The homemade vapour-phase deposition polymerisation system used in

this study. During polymerisation, the colour of the coated substrates gradually changes to green, indicative of emeraldine salt formation. ............................... 110

Figure ‎4-5. UV-Vis transmittance spectrum of PAA-doped PANI layer fabricated by VDP on glass substrate. .................................................................................. 112

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Figure ‎4-6. Left: the sensor substrate before and after vapour-phase deposition

polymerisation of PAA-doped PANI layer. Right: AFM images of the sensing layer (the scale bar in inset is 500 nm). .................................................................... 113

Figure ‎4-7. RT response of the PAA-doped PANI sensor to various concentrations of

ammonia vapour in synthetic dry air. The exposure and recovery times are 5 and 15 min, respectively. Ii is the initial value of the current passing through the sensor at time=0. ............................................................................................. 114

Figure ‎4-8. Calibration curve of the PAA-doped PANI sensor response under RT

conditions. Imax-Ib represents the maximum change in sensor current during the exposure (5 min) to each concentration. The reported values and error bars correspond to the average values over 3 repeats of each measurements. ...... 114

Figure ‎4-9. The sensor was dried under dry air flow (100 sccm). The red line shows the fitted line to data. I is current in µA and t is time in sec. .............................. 116

Figure ‎4-10. The thermal aging of the sensor at 80 ºC. The red line represents the fitted line to the data. I is current in µA and t is time in sec. .............................. 116

Figure ‎4-11. The transient variation in current of the PSSA-doped PANI sensor (raw

response) in exposure to ammonia at 80 ºC. The sensor is exposed to nine successive concentrations of ammonia, and this process is repeated 4 times.. The 2nd measurement cycle is marked by an orange square for clarification; the x-axis of this marked square has been magnified on the top x-axis; the corresponding concentration profile of each cycle is indicated on the right y-axis, showing ammonia concentrations from 175 ppb to 1.75 ppm in dry air. The applied potential was kept constant at 100 mV. The exposure and recovery durations were 5 and 15 min, respectively for each exposure. (The initial resistance value of the sensor is about 330 kΩ.) ............................................. 117

Figure ‎4-12. The raw response of the PSSA-doped PANI sensor in 5 cycles of

exposure to a series of ammonia concentrations from 175 ppb to 1.75 ppm in humid air (AH=3000 mg m-3). Similar to previous figure, the 3rd repeat cycle is marked by an orange square for clarification; the x-axis of this orange square is magnified and shown on the top x-axis. The sensor was heated to around 80 ºC to enhance the recovery. The applied potential was kept constant at 100 mV. The exposure and recovery durations were 5 and 15 min, respectively for each exposure. (The initial resistance value of the sensor is about 42 kΩ.) .............. 118

Figure ‎4-13. Effect of AH on the relative response of the PSSA-doped PANI after a 3-min exposure to ammonia vapour at ~80 ºC. ................................................... 119

Figure ‎4-14. Humidity sensing behaviour of the Nafion-doped PANI sensor at RT. The inset shows the response magnitude. .............................................................. 120

Figure ‎4-15. Ammonia sensing RT transient response of the Nafion-doped PANI

sensor at AH=5000 mg m-3. The sensor exposed to each concentration until it reaches equilibrium, and then left in clean air for complete recovery. .............. 121

Figure ‎4-16. Ammonia sensing RT transient response of the Nafion-doped PANI

sensor at AH=8000 mg m-3. The sensor exposed to each concentration until it reaches equilibrium, and then left in clean air for complete recovery. .............. 121

Figure ‎4-17. Effect of absolute humidity on the response magnitude and sensitivity of Nafion-doped PANI sensor. ............................................................................. 122

Figure ‎4-18. Repeatability test of the Nafion doped-PANI sensor to 1.0 ppm ammonia

is shown. The sensor at humid environment works better. The exposure and recovery duration were 1500 and 3000 sec, respectively. An offset in y-axis has been applied to the data for clarification. ......................................................... 123

Figure ‎4-19. Cross-sensitivity test for Nafion-doped PANI sensor toward ammonia and several VOCs in ppth concentration range. ...................................................... 123

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Figure ‎4-20. The proposed mechanism for immobilisation of a biomolecule (shown by

blue circles) on a PAA-doped PANI (shown by green colour) using EDC-NHS coupling agent. ................................................................................................ 124

Figure ‎4-21. The immobilisation of GFP on PAA-doped PANI layer. Top left: the optical

image of the control sample under fluorescence microscope, top right and bottom left: the fluorescence microscopy images of the immobilised GFP on the surface in two difference magnifications, bottom right: AFM image of immobilised GFP showing the protein aggregates. ...................................................................... 125

Figure ‎5-1. Process flow of the micro-hotplates fabrication: (a) Inkjet printing of the

heater on the PEN substrate, (b) Lamination of a thin dielectric film on the heater for electrical isolation, (c) Inkjet printing of the comb electrode onto the thin dielectric and (d) Deposition of the polyaniline sensing layer. A dry film photoresist, 50 or 14 µm thick, was used as the dielectric film laminated onto the heater for the first and the second generations, respectively. Lamination was performed at 85 °C at 2 bar, at a speed of 2 m min-1. The IDE consisted of two interdigitated combs. The pitch of the electrodes was 120 μm, corresponding to a width of 68 ± 8 μm and an inter-finger spacing of 52 ± 8 μm. The electrodes’ total thickness was 260 ± 50 nm. ............................................................................ 133

Figure ‎5-2. Layout of the different designed heaters: the first generation (top) was

designed as a large meander with a total length of ~41 mm. Adjacent tracks had a pitch (centre-to-centre distance) of 600 µm, and 300 µm width on the design layout. The resulting printed lines were wider at 397 ± 6 µm due to the lateral spreading of the ink. The heater thickness was 1.2 ± 0.3 µm. The resistance of the heater was 25 ± 3 Ω and the resistivity was 22 ± 3 μΩ cm. The total surface area of the device was ~24 mm2. The second generation (bottom) was of a much smaller size (~1 mm2) and was designed as a square double meander of total length of ~7.75 mm for improved thermal performance. The line width after printing was 68 ± 8 μm, corresponding to a single drop-wide line. To further decrease the thermal gradient over the surface of the double meander heater, adjacent lines were placed closer to each other at the outermost part of the spiral than at the innermost part (160 µm pitch against 240 µm pitch), compensating for the larger heat dissipation occurring at the edge of the heater, compared to at its centre. A square shape was chosen because round shapes are challenging to inkjet print. The line thickness was 530 ± 90 nm and the measured resistivity of the line was 18 ± 7 μΩ cm. The total resistance of the second generation heaters was 95 ± 7 Ω. The contact pads of the second generation devices were 1.4 cm, to facilitate their connection using a zero insertion force (ZIF) connector. To minimise the heat dissipation on the long contact pads, they were designed to be much wider and thicker than the heater lines. The pad widths and thickness was 700 μm and 1 μm respectively. The average resistance of the pads was 2.3 ± 0.3 Ω, (negligible when compared to the heater resistance). ................................. 135

Figure ‎5-3. Optical images of µ-hotplates depicting examples of both generations,

including flexible arrays, close views of a single device and a device with gold-plated combed electrode. ................................................................................ 136

Figure ‎5-4. Temperature measured at the center of the micro-hotplate as a function of the heating power: Comparison between first and second generation. ............ 138

Figure ‎5-5. Thermal gradient at the surface of the second generation of heaters: (a)

thermographic simulation obtained by FEM and (b) thermal profile along x and y axis. ................................................................................................................. 139

Figure ‎5-6. Response of the 1st generation sensor towards various concentrations of

ammonia vapour from 250 ppb to 3.65 ppm in dry air, at four temperatures: RT, 40, 60 and 80 °C with exposure and recovery time of 10 min and 30 min, respectively. Here, the Ri is the resistance value of the sensor at time=0. ....... 140

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Figure ‎5-7. Effect of µ-hotplate temperature on sensitivity (open square) and recovery

(closed circle) of the 1st generation sensors. The solid curve and dashed line show curve fits for sensitivity and recovery data, respectively. The error bars represent the standard deviation. .................................................................... 142

Figure ‎5-8. a) The 2nd generation sensor response magnitude (ΔRmax/Rb) to ammonia

during a 3 min exposure at RT (closed square) and 95 ºC (open circle; the heater power consumption was 35 mW). b) Enhancement of the recovery of the sensor at 95 ºC compared to that of RT. The sensor was purged with clean dry air for 15 min after each ammonia exposure. .................................................................. 143

Figure ‎5-9. Comparison of the humidity response of the 2nd generation sensors at RT

(closed square) and 95 ºC (open circle). The exposure time is 3 min in all cases. ........................................................................................................................ 144

Figure ‎5-10. Selectivity test of the 2nd generation sensors: resistance response in

exposure to: 1) acetic acid (3.19 ppth), 2) acetone (1.16 ppth), 3) chloroform (2.26 ppth), 4) n-butyl acetate (0.58 ppth), 5) ethanol (3.12 ppth), 6) methanol (4.51 ppth) and 7) ammonia (2.83 ppth). The ordinate on the inset was multiplied by a factor of 50 from the ordinate on the main plot of the figure. The value at the low end and high end of the ordinate of the inset are 3.12 and 4.12 MΩ, respectively. The baseline value in the inset is 3.87 MΩ. ................................. 146

Figure ‎5-11. Baseline drift of the sensors over an aging period of 21 days. The sensors

were heated to about 95 °C and dry air passed over them. R0 is the initial resistance on day 1. The inset shows the optical image of the electrodes’ surface on the 2nd generation sensors after 2 months: (a) an oxidised silver electrode and (b) an intact gold-electroplated electrode. ........................................................ 148

Figure ‎6-1. Conceptual design of the FlexSmell tag. ................................................ 163 Figure ‎6-2. Image of the RFID FlexSmell tag including the sensor chip for temperature

and humidity which connects to the tag by a standard ZIF connector. ............. 164 Figure ‎6-3. Schematic of the inkjet-printed multi-sensor chip platform, including two

capacitive sensors, two resistive sensors, a heater and a temperature sensor. 165

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LIST OF ABBREVIATIONS

AFM Atomic Force Microscopy

AH Absolute Humidity

APS Ammonium Peroxydisulphate

AVGS Automatic Vapour Generation System

BE Binding Energy

CB Carbon Black

CHE Coefficient of Hydroscopic Expansion

Chem-FET Chemically Sensitive Field-effect Transistor

CNT Carbon Nanotube

CPC Conductive Polymer Composite

CSA Camphor Sulphonic Acid

CTE Coefficient of Thermal Expansion

DBSA Dodecylbenzene Sulphonic Acid

DMA Dimethylamine

DMF Dimethylformamide

DMMP Dimethyl methylphosphonate

DMPU n,n'-dimethylpropylene urea

DMSO Dimethylsulphoxide

EB Polyaniline Emeraldine Base

EDC 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide

EDOT 3,4-ethylenedioxy thiophene

EKUT University of Tübingen

EPA USA Environmental Protection Agency

EPFL École Polytechnique Fédérale de Lausanne

ES Polyaniline Emeraldine Salt

FEM Finite Element Method

FP7 Seventh Framework Program

FQI Food Quality Indicator

GC-MS Gas Chromatography-Mass Spectroscopy

GFP Green Fluorescent Protein

ICP Intrinsically Conducting Polymer

IDE Interdigitated Gold Electrodes

IGFET Insulated Gate Field-effect Transistors

IPA 2-propanol

IPN Interpenetrating Polymer Network

ISA Instrument Association of America

ITN Initial Training Networks

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MC m-cresol

MFC Mass Flow Controller

MOS Metal Oxide Semiconductor

MW Molecular Weight

MWNT Multi-walled Carbon Nanotube

NCTR USA National Centre for Toxicological Research

NHS n-hydroxysuccinimide

NMP n-methylpyrrolidone

oCVD Oxidative Chemical Vapour Deposition

OEL Occupational Exposure Limit

P3HT Poly(3-hexylthiophene-2,5-diyl)

PAA Poly(acrylic acid)

PANI Polyaniline

PDMS Poly(dimethylsiloxane)

PEDOT Poly(3,4-ethylenedioxythiophene)

PEN Poly(ethylene naphthalate)

PET Poly(ethylene terephthalate)

PMMA Poly(methyl methacrylate)

ppb Parts per Billion

ppm Parts per Billion

ppt Parts per Trillion

ppth Parts per Thousand

PPy Polypyrrole

PS Polystyrene

PSSA Poly(styrenesulphonic acid)

PTFE Polytetrafluoroethylene

PVA Poly(vinyl alcohol)

PVP Poly(4-vinylphenol)

PVSA Poly(vinylsulphonic acid)

RFID Radio Frequency Identification

RT Room Temperature

sccm Standard Cubic Centimetres per Minute

SDS Sodium Lauryl Sulphate

SPhA 4-sulphophthalic Acid

SSA Sulphosuccinic Acid

STEL Short Term Exposure Limit

SWNT Single-walled Carbon Nanotubes

TAN Total Ammonia Nitrogen

14

TBA 2-thiobarbituric Acid

TFT Thin Film Transistors

TGA Thermogravimetric Analysis

TIPS-pentacene 6,13-bis(triisopropylsilylethinyl)pentacene

TMA Trimethylamine

TTI Time Temperature Indicator

TNO Netherland’s Organisation for Applied Scientific Research

TVB-N Total Volatile Basic Nitrogen

UMAN University of Manchester

UNIBA University of Bari

USFD University of Sheffield

UV Ultraviolet

VDP Vapour-phase Deposition Polymerisation

VOC Volatile Organic Compound

VTT Finland’s Technical Research Centre

WF Work Function

WHO World Health Organisation

WRAP UK Waste and Resource Action Program

XPS X-ray Photoelectron Spectroscopy

ZIF Zero Insertion Force

15

ABSTRACT

The University of Manchester

Ehsan Danesh Doctor of Philosophy

Novel Polyaniline-based Ammonia Sensors on Plastic Substrates

| February 2014 |

This thesis describes the development of high performing low-cost and low-power

ammonia sensors on plastic substrates using solution processing techniques. As a part

of the Marie Curie Initial Training Networks, FlexSmell project aimed at the realisation

of such sensors as elements of a sensing system on flexible tags for wireless

compatible applications. Ammonia was selected as the target analyte due to its

importance in many application fields including food industry, air and water quality

monitoring. Polyaniline, a conjugated polymer, was used as the sensing layer for

chemiresistive detection of ammonia because of its well-known gas sensing properties.

Two distinctive strategies were adapted to tackle doped polyaniline’s lack of solution

processablity. Firstly, dopant engineering was utilised to prepare doped polyaniline

formulations in aprotic solvents such as n-methyl-2-pyrrolidone. Hybrid composites

were then prepared by simply mixing the polyaniline solutions and carbon

nanoparticles. Sensors made by spin coating the polyaniline hybrid composites on

plastic substrates operating at ~80 °C showed sensitivities more than 6 times higher

than that of a commercial metal oxide sensor when exposed to sub-ppm

concentrations of ammonia in air. The incompatibility of the multifunctional dopants

used in this method with printed electronics, as well as the high boiling point and

toxicity of the solvent led to the second approach. A two-step vapour-phase deposition

polymerisation method was exploited to in-situ polymerise different polymeric acid-

doped polyaniline thin films on plastic substrates. Polyaniline sensors doped with

poly(4-styrenesulphonic acid), demonstrated sensitive response to sub-ppm

concentrations of ammonia vapour under both dry and humid conditions. These

sensors showed enhanced recovery and repeatability when operated at elevated

temperatures. Moreover, room temperature ammonia sensors were realised using

Nafion as the dopant.

Finally, ammonia sensors were made on small (~1 mm2) printed polymeric micro-

hotplates using a vapour-phase deposited polyaniline sensing layer in order to allow

16

reliable operation at ~95 °C with power consumptions as low as 35 mW. Such low-

cost, low-power, sensitive and selective ammonia chemiresistors may be incorporated

in smart RFID tags for food, air and water quality monitoring.

17

DECLARATION

No portion of the work referred to in the thesis has been submitted in support of an

application for another degree or qualification of this or any other university or other

institute of learning.

Ehsan Danesh

March 2014

18

COPYRIGHT STATEMENT

i. The author of this thesis (including any appendices and/or schedules to this

thesis) owns certain copyright or related rights in it (the “Copyright”) and he

has given The University of Manchester certain rights to use such

Copyright, including for administrative purposes.

ii. Copies of this thesis, either in full or in extracts and whether in hard or

electronic copy, may be made only in accordance with the Copyright,

Design and Patents Act 1988 (as amended) and regulations issued under it

or, where appropriate, in accordance with licensing agreements which the

University has from time to time. This page must form part of any such

copies made.

iii. The ownership of certain Copyright, patents, designs, trademarks and other

intellectual property (the “Intellectual Property”) and any reproductions of

copyright work in the thesis, for example graphs and tables

(“Reproductions”), which may be described in this thesis, may not be owned

by the author and may be owned by third parties. Such Intellectual Property

and Reproductions cannot and must not be made available for use without

the prior written permission of the owner(s) of the relevant Intellectual

Property and/or Reproductions.

iv. Further information on the conditions under which disclosure, publication

and commercialisation of this thesis, the Copyright and any Intellectual

Property and/or Reproductions described in it may take place is available in

the University IP Policy (see

http://documents.manchester.ac.uk/display.aspx?DocID=487), in any

relevant thesis restriction declaration deposited in the University Library,

The University Library’s regulations (see

http://www.library.manchester.ac.uk/aboutus/regulations), and in The

University’s policy on presentation of Theses.

19

DEDICATION

Dedicated to

my wife, Kajal for her unconditional love and support

20

ACKNOWLEDGMENTS

This dissertation would not have been possible without the continued support and

guidance of my supervisor, Prof. Krishna Persaud. I sincerely thank him for his

unrelenting patience and enthusiasm which helped me throughout my studies. I have

been fortunate over the course of these three years to meet and work with a great deal

of talented and bright individuals. I would like to extend my gratitude and utmost

appreciation to the members of our research group, Elena Tuccori, Dr. Mara Bernabei

and Dr. Maria Daniela Angione for their encouragement and assistance during my

work. Special thanks to Dr. Anna Maria Pisanelli for her continued scientific guidance

and moral support.

I would like to acknowledge the FP7 EU Marie Curie Initial Training Network,

FlexSmell (grant n°238454) for funding this research. I am also grateful to all members

of the FlexSmell, especially Francisco Molina Lopez, Mohammad Yusuf Shafi Mulla,

and Dr. Danick Briand for their valuable inputs. I have thoroughly enjoyed collaborating

with all of them during the course of this project.

I would like to thank my Mum and my brothers, Omid and Armin for encouraging me,

supporting me and believing in me all my life. Finally, I would like to show my deep

appreciation to my wife, Kajal, and her parents, Kamyar and Kianeh for always

supporting my decisions and providing me with encouragement and faith in my

abilities.

21

CHAPTER I

1. Introduction: FlexSmell project

Waste is a major issue facing retail food and packaging industries all around the world.

According to the UK Waste and Resource Action Program (WRAP1), food and retail

sector produces more than 30% of all industrial waste in the UK. Approximately one

third of the total purchased food items are thrown away regardless of their quality or

condition, mostly because they have passed their ‘use by’ or ‘best before’ period on

date stamps. Interestingly, at least half of this food is edible. These figures

demonstrate the huge economic loss that can be avoided by understanding food

deterioration process and changing the food freshness/spoilage determination criteria.

Smart food packaging technologies open up new possibilities to monitor quality of food

and detect spoilage in its early stages, reducing the waste quantity and disposal cost.

Funded under the Seventh Framework Program (FP7), the FlexSmell project aims at

the design, investigation and realisation of printed chemical sensing systems on

flexible plastic substrates for wireless compatible applications. The focus will be on

very low-cost and low-power smart chemical sensing tags based on radio frequency

identification (RFID) for food freshness and quality traceability control through

packaging. The specific scientific objective is the development of a general platform,

implemented on a flexible substrate, for the detection of volatile compounds. The whole

platform development will be driven by the requirements for applications in wireless

systems, and in particular for manufacturing small RFID tags, providing services to the

individuals and to the community in the areas of food control though smart packaging,

as well as health, environment, communication and security.

Being a part of the Marie Curie Initial Training Networks (ITN), the training and

transfer of knowledge programme is an integral part of the FlexSmell initiative. The

1. www.wrap.org.uk (2007)

22

breadth and the innovation of the scientific scope of this project require the

development of totally novel teaching/training strategies, the creation of

interdisciplinary and intersectoral courses and curricula, and the extension of graduate

and continuing education programs. The interdisciplinary nature of the FlexSmell

research and training activities is mirrored by the composition of the consortium

comprising competences in: chemical sensors, sensing systems, printed organic

electronics, bio-inspired sensors, micro-electronics and microsystems, molecular

biology, analytical chemistry, material science, nanotechnology, device physics and

wireless communication. The intersectoral character is assured by the collaboration of

several partners from the University of Manchester (UMAN, UK), the University of Bari

(UNIBA, Italy), École Polytechnique Fédérale de Lausanne (EPFL, Switzerland), the

University of Tübingen (EKUT, Germany), the University of Sheffield (USFD, UK), the

Finland’s Technical Research Centre (VTT, Finland) and the Netherland’s Organisation

for Applied Scientific Research (TNO, Netherlands). Moreover, Carton Pack (Italy), an

industrial company active in the field of food packaging is involved in the project. Apart

from the specific scientific and technical objectives, the nature of this Marie Curie ITN

offers early-stage researchers the opportunity to improve their research skills, join

established research teams and enhance their career prospects.

The FlexSmell technology platform will be in principle suitable for different

applications such as in the field of logistics for monitoring perishable goods along their

transport and storage. Industries require such a system, where high performance

wireless technologies are combined with printed low-cost and low-power chemical

sensors, which is restricted to the detection of gaseous analytes. The FlexSmell is

focused on applications of such smart labels for food packaging to control the key

parameters connected to the food quality. The use of RFID tags enables the

identification and traceability of objects and goods. RFID, originally used in retail as a

security measure, is now widely used for smart stock control to identify the content of

shipments and to track goods through transit points. To achieve enhanced sensing

performance, a number of well-established transducers will be exploited, namely:

chemiresistors, organic field effect devices, and capacitors.

The UMAN is involved in the work tasks relating to the selection, synthesis and

deposition of organic polymers and odorant binding proteins as single or bilayers on

chosen transduction devices. As can be seen from Figure ‎1-1 and Figure ‎1-2, most of

the UMAN work falls into work package one (WP 1) which is: “organic semiconductors

and odorant receptors selection and deposition.”

23

Figure ‎1-1. Overall picture of the FlexSmell training and transfer of knowledge (Adapted from the FlexSmell’s description of work, part B).

Figure ‎1-2. 2D grids of thematic work packages and methodical approaches with the relevant tasks and groups in charge of the coordination (Adapted from the FlexSmell’s

description of work, part B).

The aim of the present work was to develop chemiresistors based on solution

processable conducting polymers on flexible substrates such as polyimide Kapton®,

polyethylene naphthalate (PEN) and polyethylene terephthalate (PET). Such flexible

sensors may be modified in order to detect chemicals related to the food

freshness/spoilage in very low concentrations. Special attention has been devoted to

minimise the cost (materials cost as well as manufacturing costs) and power

consumption.

24

To achieve these goals, it was essential to gain insight into the food spoilage

mechanisms and parameters affecting them. The major causes of deterioration of

different food categories, from fresh meat to vegetables, have been reviewed briefly in

Chapter II and Appendix I. Since the focus of the FlexSmell is on “gas” sensing, only

volatile compounds released during food decay were studied and ammonia was

selected as the target analyte. Intrinsically conducting polymers were introduced and

the research was focused on the properties of polyaniline (PANI) as an excellent

ammonia sensing material. The challenges regarding the fabrication of PANI-based

chemiresistors on plastic substrates were discussed.

Two distinctive approaches were adopted to tackle polyaniline’s inherent lack of

processability. In Chapter III, dopant engineering has been used to make solution-

processable PANI formulations. Hybrid composites of PANI and carbon nanoparticles

were then prepared and their electrical and sensing properties were investigated.

Having assessed the advantages and shortcomings of this approach, an alternative

method was used to directly synthesise polyaniline conducting layers on sensing

platforms. Vapour-phase deposition polymerisation (VDP) technique has been

described in Chapter IV. Chemiresistors were made based on the VDP method on

selected plastic substrates, and characterised for their ammonia sensing properties in

dry air as well as under humid conditions. Common organic acid dopants were

replaced by polymeric dopants in order to enhance the properties of the sensor.

Testing of both the hybrid nanocomposite and the VDP-made PANI sensors revealed

that the operation at elevated temperatures significantly improves the ammonia

sensing performance. Therefore, plastic micro-hotplates were designed and fabricated

in collaboration with other FlexSmell partners. These hotplates incorporated in the

sensor platform can be used to increase the PANI layer’s temperature during ammonia

sensing, with very low-power consumption. The development of plastic micro-hotplates

for polyaniline-based ammonia sensors has been reported in Chapter V. Both

aforementioned approaches, i.e. the solution-processable hybrid nanocomposites and

the VDP technique, are believed to be compatible with large-area fabrication of reliable

and high performing sensors, hence suitable for the FlexSmell’s aimed applications.

25

CHAPTER II

2. Background

2.1. Food spoilage

Food spoilage may refer to any changes that render the product unacceptable (and

possibly unsafe) for human consumption. Spoilage may occur at any stage of the food

supply chain and it affects the chemistry, aroma, texture and appearance of food.

Spoilage of food can be evident as physical damage, visible growth of microorganisms,

slime formation or insect damage. In general, food deterioration can be the result of:

a. Microbial activity (It is estimated that one-fourth of the world’s food supply is lost

through microbial activity (Huis in't Veld, 1996))

b. (Bio)chemical changes (including enzymatic reactions)

c. External conditions (also referred to as physical factors such as heat, light and

humidity)

Usually the (bio)chemical reactions caused by microorganisms, enzymes, etc.

produce several volatile organic compounds (VOCs), among other chemicals. There is

an overlap between (bio)chemical spoilage and microbial spoilage that gives rise to the

difficultly in modelling food spoilage and growth of microorganisms. Changes occurring

in the food depend on the chemical composition of the food as well as the

microorganisms present in it. Therefore, knowledge of the nutrient content of a food is

of great importance when predicting which type of chemicals will be released upon the

microbial/ enzymatic activity during food decay. For instance, fresh meats are rich in

proteins and lipids, while carbohydrates are abundant in vegetables and fruits.

Since the main objective of the FlexSmell project is to detect spoilage based on

direct headspace measurement inside the packaging (as a possible non-destructive

approach), it is crucial to focus only on the chemicals that are evolved in the vapour

26

phase over the food during deterioration process. Appendix I gives an overview of the

food spoilage process with a special focus on volatile compounds released at the early

stages of food decay.

2.2. Food quality indicators

Time Temperature Indicators (TTI) and Food Quality Indicators (FQI) are two enabling

technologies which their incorporation into smart labels is believed to considerably

increase the shelf-life of fresh food products (Rokka et al., 2004; Smolander, 2008). A

TTI device shows the accumulated time-temperature history of a product. TTIs employ

established concepts and mature technologies, and are commercially available with

low cost. A commercial TTI1 is shown in Figure ‎2-1.

However, there is no actual relationship between TTI output and food spoilage! A TTI is

concerned only with the external environment (i.e., temperature), so it has no method

of detecting the presence of bacteria and the chemicals corresponding to the spoiled

food. Additionally, a TTI label functions primarily with food stuffs that have to be kept at

a low and constant temperature (chilled items). FQIs, on the other hand, rely upon the

change in the headspace of the food, and often indicate the spoilage by change in

colour. One of the potential drawbacks of this technology is its sensitivity to spoilage. If

the lower limit of detection of the indicator is too high, then users may already be able

1. www.freshpoint-tti.com

Figure ‎2-1. Freshpoint’s OnVu™ indicators accurately and consistently record and display the freshness of the products, based on their time and temperature histories. Once packaged, the OnVu™ TTI is activated using an ultra violet (UV) light source. The evolution of the label colour as a function of time and

temperature is demonstrated.

27

to determine if the food is spoiled by visually checking it. On the other hand if the limit

is too low, then food that is edible would be deemed by the sensor to be unfit for

consumption. This also depends on the type of the product being labelled. Selectivity is

another issue, where the volatile has to be of the correct type and concentration to

react effectively with the sensing element on the FQI. FQI technology is still under

development, and research in this area focuses solely on meat, fish and some other

short shelf-life foods.

2.2.1. Examples of FQIs products currently available

FQIs are used to directly measure the changes in food items’ headspace which links to

the quality of the product. Many of the products that exist in this area claim to register

gas changes and inform the user of actual microbial growth. This compares to TTIs

which their operation depends on the deviations from the specified temperature.

Obviously, the added benefits of having actual data on food spoilage means that the

user would have better information on the quality and safety of the product. As

previously discussed, the difficulty of producing an effective FQI label is finding a

metabolite or gas that directly correlates to the quality of a specific food. Many FQIs

are similar in their function to TTIs in that they rely on a colour change to report

bacterial/(bio)chemical spoilage. The following is some examples of FQI products

currently available or on the latest stage of development1:

1. UPM Raflatac2, Finland: Freshness Guard

UPM’s Raflatac division is specialised in labels and labelling systems. The Freshness

Guard has been developed to be used as a detector of variations in the headspace

composition. This smart label is displayed in action in Figure ‎2-2-a. The company also

produces RFID labels and aims to combine both TTI and FQI technologies to monitor

the shelf-Life. Both of these label systems conform to the EU regulations and UPM

have shown interest in producing a similar system to be used for fish and seafood.

1. All the descriptions/specifications mentioned for the following products are based on the manufacturing company’s website, documentations and/or patents 2. http://www.upmraflatac.com/

28

Figure ‎2-2. Examples of food quality indicators available in the market: a) Freshness

Guard (Smolander, 2008), b) FreshQ FQI label, c) Litmus FQI label.

The label is manufactured to react with hydrogen sulphide using nanoscale silver

particles dispersed over the label surface. The original colour of the thin silver layer is

light brown; once silver sulphide is formed the layer becomes opaque and a warning

cross becomes visible on the layer.

2. Carrier Corp., USA: Fresh-Tag

The Fresh-Tag is a FQI used for the determination of seafood quality. The label is

designed to react to increased levels of amines as seafood spoils. The active sensor is

in the form of a wick that progressively changes colour as the amines build up inside

the packaging headspace. The technology present in the Fresh-Tag was developed by

the National Centre for Toxicological Research (NCTR, USA) and then licensed out to

Cox Recorders. Unfortunately, data regarding the product and the progress of research

has not been available as Cox Recorders was purchased by Sensitech in 2004, which

was then more recently acquired by Carrier Corp. These companies do not have any

records of the device.

3. Food Quality Sensor International (FQSI)1 Inc., USA: FreshQ

The FreshQ is designed by FQSI and incorporates research into using pH sensitive

dyes for the detection of volatile amines produced from food spoilage. The Figure ‎2-2-b

shows the design of the label which could be applied to the outside of fresh wrapped

meat and poultry packages and detects food-borne bacteriological levels right through

the wrap. The literature and the patent describe a device that changes colour to

1. www.fqsi.com

29

indicate spoilage, starting with an orange colour and changing to a grey shade once

bacteria have reached a critical level. The sensitivity of the label can be tuned by

adjusting the original pH of the dye when it is deposited on the label. Thus, the device

can also be used for detecting acidic compounds from spoilage if required (Hobday,

2009). The company is also working on a handheld device called SensorfreshQ for

meat and poultry freshness detection.

4. Litmus1 LLC., USA: LITMUS FQI

Litmus holds a breakthrough technology offering simple and quick methods for

determining the direct quality of food products. LITMUS FQI’s technology is based on a

substrate and an indicator compound provided on it. The indicator compound is

colourimetrically responsive to ammonia generated by food decomposition. LITMUS

FQI’s colour changing sensors provide an indication of the quality of a food product

inside a sealed container. Yellow is the “BEST” condition and when the sensor shows

any other colour, the decomposition gases have exceeded guidelines and the sensor

affirms “DO NOT EAT” (Figure ‎2-2-c).

5. It‘s Fresh Inc.2, USA: It‘s Fresh®

It‘s Fresh have a portfolio of products aimed at the food spoilage detection market.

Included within this collection they have developed a sensor that is only to be used by

the end consumers, the It‘s Fresh® indicator. The portfolio also contains a TTI for cold

chain purposes (Figure ‎2-3).

Figure ‎2-3. It's Fresh Inc.’s smart label products (from http://www.itsfresh.com/)

The It‘s Fresh® indicator is designed to be incorporated into either specially designed

food containers or sealable bags using them as the substrate. The sensor detects

extracellular enzyme activity by a colour change of the label. The drawbacks of this

system are the requirement of a sterile environment to function properly as well as a

need for the substrate to be in direct contact with the food stuff (Hobday, 2009).

1. http://litmusgti.com/FQI.html 2. www.foodfreshnesstechnology.com/

30

6. Toxin Alert Inc., Canada: Toxin Guard

This product is under development for commercial and military purposes. According to

company’s patent (Bodenhamer, 1998), the sensor is comprised of a polyethylene film

with immobilised labelled antibodies used to detect certain pathogenic bacteria, such

as Salmonella spp.; further applications of this technology include the capture and

detection of pesticide residues or genetically modified proteins.

7. Ripesense Limited1, New Zealand: RipeSense®

RipeSense® is reported to be the world’s first intelligent sensor label that changes

colour to indicate the ripeness of fruits. The RipeSense® sensor works by reacting to

the aromas released by a fruit as it ripens. The sensor is initially red and gradually

changes to orange and finally yellow (Figure ‎2-4).

Figure ‎2-4. RipeSense® ripeness indicator.

In spite of the great interest in food quality indicators both in academia and industry

over the past two decades, none of the products (including the ones from the

aforementioned companies) has become commercially successful. The development of

successful FQIs requires more complex knowledge of food properties and its

deterioration process. Efforts have been made to tackle main challenges in this domain

including reliability, cost effectiveness and compliance with the strict food safety

regulations. The future of such systems mainly depends on whether or not they can

appeal to both retailers and consumers as the main members of the supply chain. With

the advances in the development of nanotechnology-based sensors, and with the

emergence of printed electronics and RFID tags, realisation of food quality sensors

capable of monitoring food freshness based on direct evidence is feasible in near

future.

1. www.ripesense.com/

31

2.3. Selection of the target analyte: Ammonia

As discussed in Appendix I, the headspace of every rotten food consists of a complex

mixture of analytes from different chemical groups: aldehydes (e.g., hexanal) are often

released during the oxidation of unsaturated fatty acids, and are mainly responsible for

the rancid-smelling flavour in lipid-rich foods (Guillén-Sans and Guzmán-Chozas,

1995). Long-chain alcohols, like octanol and decanol, are emitted from gram-negative

enteric bacteria such as E. coli. Geosmin, a bicyclic alcohol, is the primary odorant

component associated with the presence of Penicillium expansum fungus which acts to

decay fruits such as apples, pears, and cherries (Roberts, 1992). Oct-1-en-3-one, a

ketone derived from the oxidation of fatty acids (linolenic and arachidonic acid)

(Wilkinson and Stark, 1967), is the compound mainly responsible for the “metallic”

flavour in the milk (Stark and Forss, 1962). The putrid odour of spoiled meat is caused

by the decomposition of proteins and sulphur-containing amino acids by bacteria (Huis

in't Veld, 1996), which releases sulphur compounds such as methanethiol and

hydrogen sulphide (Belitz H. D., 2004). 2-isopropyl-3-methoxypyrazine has been

identified as metabolic by-product of Pseudomonas perland and taetrolens. This

pyrazine is responsible for a musty/earthy off-flavour in eggs, dairy product and fish

(Belitz H. D., 2004). Volatile acids, esters, lactones, furans and phenols are among the

other classes of chemicals related to deterioration of different types of food stuffs.

Several studies have investigated the potential of using amines and biogenic amines

as potential markers of spoilage of meat, poultry and fish products. A close correlation

between biological spoilage and increased production of nitrogen-containing

compounds is suggested. Biogenic amines (e.g., histamine, cadaverine and

putrescine) are non-volatile organic bases of low molecular weight, mainly produced by

bacterial enzyme activity, through decarboxylation of free amino acids (Bulushi et al.,

2009). Volatile amines such as ammonia, dimethylamine (DMA) and trimethylamine

(TMA) are the result of the amino acid breakdown due to microbiological activity (esp.

Pseudomonas spp.), and are collectively known as total volatile basic nitrogen (TVB-

N). Hence, TVB-N levels are a potential indicator of the protein-rich food spoilage

(Kuswandi et al., 2012; Pacquit et al., 2007). Specifically, the European Commission

has specified that TVB-N levels may be used as indicators of seafood spoilage (Byrne

et al., 2002). Amino acids in meat products undergo microbial degradation and

ammonia, as well as TMA and DMA, are produced as the result (Newton and Gill,

1978; Falk and McGuire, 1919; Pacquit et al., 2006). TMA is also reported to be

responsible for the fishy odour in milk and other dairy products (Ampuero and Bosset,

2003). Kim et al. (Kim et al., 2009) have measured a list of unpleasant odorants

32

including nitrogenous compounds during the decay processes of several food products

(egg, fish and squid) during one month. The results showed that ammonia has the

largest odour release strength, followed by TMA.

As discussed earlier, the diversity and strength of odorants depend not only on the

type and composition of the food, but also on the spoilage mechanism (bacterial,

chemical or both). Historically, Gas Chromatography-Mass Spectroscopy (GC-MS) has

been used to analyse the complex mixtures in the headspace of foodstuffs during

spoilage. Though powerful, these systems are generally large and costly, suited to the

laboratory rather than low cost sensors aimed for smart packaging applications. It is

not feasible to realise a single detector for all of the analytes responsible for food

spoilage. Two different concepts may be applied to address this issue: a) the detection

of specific compounds, which are responsible for decaying process of a specific food

category, with highly selective sensors, or b) the characterisation of different odours by

patterns generated with sensor arrays (i.e., electronic nose) using sensor elements

with partly overlapping selectivities.

Here, we have selected ammonia as the representative of volatile amines group,

which is encountered in protein-rich food species. Hence, we have aimed at the design

of a sensitive and selective ammonia sensor. Apart from food, ammonia is a chemical

encountered in many different environments over a wide range of concentrations.

While substances like refrigerants, household cleaners, and industrial fertilisers are

among the main artificial sources of ammonia, the majority of ammonia in earth’s

atmosphere is produced in nature by the ammonification process from the degradation

of amino acids in animal cells, food putrefaction, and decomposition of waste and

sewage. Although there is much research into ammonia sensors with variety of

mechanisms, the different requirements for each application drive the need for

versatility. Timmer et al (Timmer and al., 2005) have summarised these necessities.

Five major areas that are of interest for measuring ammonia concentrations are:

environmental, automotive, chemical industry, medical diagnostics and food industries

which is described in Table ‎2-1.

Table ‎2-1. Ammonia sensing requirements in different applications (Reproduced from ref. (Timmer and al., 2005))

33

It is clear that concentration levels of interest are mostly dependant on the actual

application. This also determines the time resolution of the required analysis

equipment. For instance, ammonia levels in the natural atmosphere can be very low,

down to sub-ppb concentration. Hence, very accurate ammonia detectors with a

detection limit of 1 ppb or lower are required for measuring such concentrations. Near

intensive farming areas, ammonia concentrations are much higher, above 10 ppm. The

detection ranges required are as low as 50–2000 ppb in biomedical applications, such

Application Detection limit Required

response

time

Temperature

range

Remarks

Environmental

Monitoring ambient

conditions

0.1 ppb† to >200

ppm‡

Minutes 0-40‎˚C Reduce environmental

pollutions

Measure in stables/firms

1 to >25 ppm ~1 min 10-40‎˚C Protect livestock animals

and farmers

Water Quality (both

clean and waste water)

0.1 mg l-1

to

several tens of

mg l-1

Minutes Water acts as a strong

interference

Automotive

Measure NH3 emission

from vehicles

4-2000 g min-1

(concentration

unknown)

Seconds Up‎to‎300‎˚C NH3 emission is not

regulated at this time

Passenger cabinet air

control

50 ppm ~1 s 0-40‎˚C Automotive air quality

sensors mainly aim on NOx

and CO levels

Detect ammonia slip 1-100 ppm Seconds Up‎to‎600‎˚C Control urea injection in

SCR NOx reduction

Chemical

Leakage alarm (in

petrochemical plants,

production of industrial

refrigerants, fertilisers,

…)

20-1000 ppm Minutes Up‎to‎500‎˚C Concentrations may be very

high at NH3 plants (can

even be explosive)

Medical

Breath analysis 50-2000 ppb ~1 min 20-40‎˚C Diagnosis of peptic ulcer

caused by bacteria, small

gas volume

Food packaging

10 ppb to

several tens of

ppm

Minutes

Below‎0‎˚C‎to‎

room

temperature

Very broad dynamic range

needed for general food

spoilage detection,

Compatibility of detector

with packaging should be

met

†‎ppb:‎parts‎per‎billion

‡‎ppm:‎parts‎per‎million

34

as for the detection of urea imbalance induced by renal problems or ulcers brought on

by infection by Helicobacter pylori (breath analysis), and as wide as 0.1 ppb to >200

ppm for environmental monitoring purposes. Natural levels of ammonia in groundwater

are usually below 0.2 mg l-1. Ammonia may be present in drinking water as a result of

disinfection with chloramines. According to World Health Organisation (WHO)1, the

threshold odour concentration of NH3 in water is ca. 1.5 mg l-1. USA Environmental

Protection Agency (EPA) (2013) recommends an acute criterion magnitude of 17 mg

Total Ammonia Nitrogen (TAN) per litre at pH 7 and 20 ºC for a 30-day average

duration, and a chronic criterion magnitude of 1.9 mg TAN at the same condition, for

aquatic life ambient water quality. From the varying samples encountered -clinical and

environmental, to those from the automotive industry and chemical industry- it is

obvious that each class of samples will have its own detection limits, potential

interferents, required operating temperature conditions, sample phase (aqueous or

gas) and response time. The Instrument Association of America (ISA) specifies that

ammonia detectors should reach a minimum of 50% of response within 90 seconds

(i.e. t50 < 90 s) on exposure to a fixed concentration of ammonia gas. Likewise for

recovery, a t50 of 90 s is specified when the detector is exposed to clean air.

The potential harm that ammonia can cause to human health is linked to its high

hydro-solubility. Exposure to high concentrations of ammonia is associated with

pulmonary oedema amongst other pulmonary related disorders, and temporary

blindness. In the UK, the time-weighted occupational exposure limit (OEL) for ammonia

is 25 ppm (18 mg m-3) over an 8 h period and the short term exposure limit (STEL) is

35 ppm (25 mg m-3) over a 15 min period. However the reported lower limit of human

perception of ammonia is highly variant within 1.5-50 ppm. The variance in these

figures highlights the requirement for sensors that can be used for continuous

monitoring of ammonia, in contrast to ‘one/off’ tests.

As the aquatic environments are subject to pollution by agricultural run-off and

natural decomposition of nitrogenous material, they require sensitive monitoring for the

protection of fish stocks. Furthermore, another potential source of aqueous ammonia

contamination comes from refrigerants and their potential leakage into the environment

(Crowley et al., 2008b). There is a trend to observe ammonia toxicity and water pH

relationship, where the higher the pH of the water, the more of the unionised form,

NH3(aq), is present (Figure ‎2-5). The pKa of ammonia is 9.2, and thus at pH values

below it, the ammonia will be present substantially as the ammonium ion, NH4+. NH3 is

considered the more toxic form as this is the form that can diffuse into biological

1. www.who.int/water_sanitation_health/dwq/ammonia.pdf

35

membranes. This is important when dealing with foodstuffs containing large amounts of

water, since pH affects the headspace during spoilage process.

Figure ‎2-5. Ammonia content in water vs. pH of the solution

2.4. Ammonia sensing principles

It is very difficult to find a good definition for sensors. Hársanyi (Harsányi, 1995) gives

the following simple definition for a sensor: “a sensor is a transducer that converts the

measurand into a signal”. This signal may be electrical or optical. However, the nature

of the output does not seem to be important. The important thing is that there is a

signal or a change in signal, which holds the information about the measurand and can

be directly transmitted by one of the information transmission channels. All chemical

gas sensors interact with the analyte to induce a physical and/or chemical change in

the sensing layer, which produces a signal. Usually, gas sensors are categorised

based on their transduction principle and sensing material.

Many different principles and technologies for measuring ammonia have been

described in literature. As discussed before, each application field has its own

requirements which thrust the development and exploitation of a specific technology. A

different sensor is used for measuring ultra-low concentrations of ambient ammonia for

environmental monitoring, than in the exhaust pipe of automobiles. This section aims to

give an overview of the three most common “solid-state” technologies being used for

ammonia sensing and a succinct list of their benefits and drawbacks. We focus only on

36

ammonia sensors with electrical signals; optical sensors are out of the scope of this

thesis and therefore they are not discussed here.

2.4.1. Metal oxide sensors

Metal oxide semiconductor (MOS) sensors, with no doubt, constitute the most

abundant ammonia sensors in the market. Tin oxide (SnO2) is the mostly used sensing

material, usually doped with a catalytic metal like platinum or palladium (Timmer and

al., 2005). Other oxides have also been utilised including zinc oxide (ZnO), titanium

dioxide (TiO2) and tungsten oxide (WO3), which are all n-type semiconductors. MOS

sensors commonly consist of a planar substrate with a heater coil and an interdigitated

electrode coated with the sensing material. It is well-established that these sensors

operate on the principle of conductance change due to chemisorption of gas molecules

into the sensing layer. A thick film of MOS is in the form of a large number of grains in

contact at their boundaries (Srivastava et al., 1994). The electrical conductivity is

governed by the formation of double Schottky potential barriers at the interface of

adjacent grains, caused by charge trapping at the interface. The height of this barrier

determines the resistance. When the MOS surface is placed in air, oxygen molecules

are adsorbed at the surface. The adsorbed oxygen molecules accept electrons from

the trap states present at the boundaries and form oxygen-derived adsorbates such as

(superoxide), (charged atomic oxygen),

(peroxide) ions. This results in

reduction of the charge carrier concentration near the surface and increases the

depletion region thickness. When exposed to a chemically reducing gas, like ammonia,

co-adsorption and mutual interaction between the gas and the oxygen result in

oxidation of the gas at the surface. The removal of oxygen from the grain surface

results in a decrease in barrier height (decrease in resistance). The energy band

diagrams at the grain boundaries in air and in the presence of a reducing gas (R) are

shown in Figure ‎2-6.

37

Figure ‎2-6. Energy band diagram showing the Schottky barrier height (eVd and eVʹd) at the grain boundary of TiO2 without and with a chemically reducing gas, R (Adapted from (Timmer and al., 2005)). Ec = conduction band, Ev = valence band, and Ef = Fermi level.

From this model, we can immediately infer that MOS sensors are not selective to a

particular gas, as long as it is a reducing agent. This is a serious drawback in practical

applications. One approach to increase the specificity of these sensors is using metals

or additives that enhance the chemisorption of specific gases. Known additives for

optimising the ammonia sensitivity of SnO2-based ammonia sensors are Pd, Bi and

AlSiO3 as well as Pt and SiO2 (Timmer and al., 2005). Ammonia detection limit of 1

ppm is reported using a WO3 sensor modified with Au and MoO3 additives,

accomplished by operation at above 400 °C (Xu et al., 2000). The limit of detection of

most MOS sensors is reported to range within 1-1000 ppm, and the operating

temperature is within the range of 200 to 500 °C. Below 200 °C, the chemisorption

kinetics is very slow, and as the temperature reaches ambient condition, water covers

the oxide surface, inhibiting gas adsorption. Furthermore, grain boundaries’ quality has

a crucial effect on MOS sensor sensitivity; the reproducibility of grain boundaries

requires accurately controlling the preparation parameters of the sensitive material

(Karunagaran et al., 2007). The high power consumption resulting from their high

operating temperature limits MOS sensor applications. In particular, MOS sensors

cannot be used in handheld devices where the power is supplied by a battery. Also,

common inexpensive plastic substrates (e.g., PET) cannot be used because of the risk

of thermal degradation at high temperatures.

2.4.2. Chem-FET sensors

A chemically sensitive field-effect transistor (Chem-FET) is an example of the sensors

that work based on the work function (WF) modulation (Janata and Josowicz, 2003;

Janata, 2004). In a field-effect transistor, the current is established between source and

drain electrodes, when a potential is applied to the metal gate. At the semiconductor-

dielectric interface, a channel allows the current of charge carriers (electrons or holes,

depending on the semiconductor type and work function). In these devices, interaction

of the sensing layer with an analyte modulates the work function of the layer. The

change in the WF shifts the transistor threshold voltage, which is a function of the gas

type and concentration (Domanský et al., 1998). Classically, Chem-FETs have been

made using a metal oxide as the semiconductor and a catalytic metal as the gate

material, in the so called MOSFET devices. Pt-gate Si-based FET sensors showed

sensitive response to ammonia concentrations as low as 10 ppm (Ross et al., 1987).

Much attention has been devoted to organic field-effect transistors, where the sensing

38

material is often an organic semiconductor (in thin film transistors (TFT)) or a

conducting polymer gate material (in insulated gate field-effect transistors (IGFET))

(Figure ‎2-7) (Janata and Josowicz, 2003).

Figure ‎2-7. The structure of the two types of field-effect transistor sensors. a) TFT; b) IGFET. (Reproduced from (Janata and Josowicz, 2003))

Yu et al. (Yu et al., 2012), have reported an ammonia sensitive TFT using pentacene

as an active layer and poly(methyl methacrylate) (PMMA) as an insulator. Recently,

sensitive gate dielectrics have also shown promising ammonia sensing properties

(Klug et al., 2013). Huang et al. (Huang et al., 2013), fabricated NH3 sensors based on

pentacene FETs on glass substrates with four different polymer dielectrics, including

polystyrene (PS), poly(vinyl alcohol) (PVA), poly(4-vinylphenol) (PVP) and PMMA.

They demonstrated that the dielectrics/pentacene interface diversities are mainly

responsible for the variations of the sensing properties. TIPS-pentacene-based organic

thin-film transistors fabricated on flexible substrates showed reliable response to

ammonia over the range of 10-100 ppm (Yu et al., 2013). A P3HT-based field effect

transistor has been reported showing sensitive response to ammonia in the range of

0.1–25 ppm at room temperature (Tiwari et al., 2012). Humidity interference is a big

concern in Chem-FETs. Also, the interpretation of the chemical response of these

sensors is difficult due to existence of various forms of modulation of conductivity and

work function in a simple interaction of a gas with the device. Moreover, device

complexity makes it difficult to fabricate Chem-FETs in large scale and with low cost, a

requirement for many sensing applications.

2.4.3. Conducting polymer sensors

Among different materials used in gas/vapour sensors, conducting polymers have been

increasingly employed over the past years, due to their great design flexibility and low

cost. Conducting polymers may refer to two distinctive categories:

39

a) Extrinsic conducting polymers or conductive polymer composites (CPCs), which

are comprised of an insulating matrix (polymer) and conductive fillers (e.g., metal

particles, carbon black (CB), carbon nanotubes (CNTs), graphene, etc.). As the

conductor content in the composite increases, a point is attained (called the percolation

threshold) at which the first connected conductive pathway extending through the

composite is created and consequently, the resistivity of the composite film drops

several orders of magnitude (Danesh et al., 2011; Molla-abbasi et al., 2011). Generally,

in a CPC vapour sensor, the response of the sensor to an analyte is measured as a

change in the resistance of the sensor, thus imparting the name “chemiresistor” to this

type of sensors (Figure ‎2-8). Swelling of the polymer upon exposure to various gases

and vapours causes the dispersed conductive particles to move farther apart from each

other. This disrupts the formerly formed conductive pathways in the film, which in turn

raises the resistance of the sensor providing a simple yet effective mean for monitoring

the presence of a vapour (Lonergan et al., 1996). Chemiresistors based on CPCs have

shown some advantages including great stability, ability to work at room temperature,

great processability, low power consumption, cost effectiveness, etc., that make them

attractive for use in commercial sensors (Albert et al., 2000). However, these sensors

suffer from low sensitivity and high detection limits, due to the nature of sensing

(swelling).

Figure ‎2-8. Sensing mechanism in conductive polymer composite sensors. The vapour-induced expansion of the polymer composite causes an increase in the electrical resistance of the composite because the polymer expansion reduces the number of conducting pathways for charge carriers (Reproduced from Lewis research group website

1, Caltech, USA).

1. http://nsl.caltech.edu/research:nose

40

b) Intrinsically conducting polymers (ICPs), including polyaniline, polypyrrole,

polythiophene, etc., which all have a conjugated π-electron system along their polymer

backbone. These polymers are semiconductors in their ground state, and upon doping,

they can conduct electricity with conductivities approaching those of metals. ICPs can

form selective layers in which the physical and/or chemical interaction between the

analyte gas and the conjugated polymer modulates the electronic properties.

Chemiresistors are the most common type of intrinsically conducting polymer sensors.

Due to the porous structure of ICP layers (compared to inorganic films), the

adsorption/desorption kinetics is high at room temperature (Janata and Josowicz,

2003; Yoon, 2013). Detection limits in sub-ppb range has been demonstrated using

ICP nanomaterials for detecting a chemical nerve agent (Kwon et al., 2012a). The

ammonia detection limit of pristine conducting polymers is usually over 1 ppm, with

response times over a minute. However, the sensing characteristics can be tailored by

altering the functional group chemistry of the polymer, dopant engineering,

microstructural modifications and incorporation of inorganic materials (Albert et al.,

2000; Kwon et al., 2012b). The poisoning of ICP sensors overtime is one of their major

drawbacks. Moreover, the sensing mechanism of this category of materials is not fully

understood. Understanding gas sensing behaviour of conducting polymers is

impossible without having an insight into their electrical conductance mechanism. We

will discuss the proposed mechanism of conductivity, their composition and fabrication

in more details in next section.

2.5. Intrinsically conducting polymers

2.5.1. Background

From the onset of their invention, synthetic polymers have been considered to be

excellent electrical insulators, and have been applied to applications such as anti-

corrosive coatings and cable insulations. However, the discovery of ICPs which

possess a variety of properties related to their electrochemical behaviour, opened new

perspectives in polymer science and technology. Although these materials are known

as new materials in terms of their properties, the first report on conducting polymer

synthesis goes back to the 19th century. At that time ‘aniline black’ was obtained as the

product of oxidation of aniline, however its electronic properties were not established

(Freund and Deore, 2007). In early 1970s, the evolution of conducting polymers began

with the discovery of poly(sulphur nitride) (polythiazyl) which becomes superconducting

41

at temperatures near zero Kelvin. Later, polyacetylene, a linear conjugated polymer

was accidentally1 found to have metallic properties. Over the last four decades,

research has shown that electrically conducting polymers can be produced with

conductivities approaching those of the metals such as copper, while having other

properties that are expected of an organic polymer. This work was recognised with the

2000 Nobel Prize in chemistry being awarded to Shirakawa, MacDiarmid and Heeger

for their discovery and subsequent research on high conductivity iodine-doped

polyacetylene (Figure ‎2-9) (Ngamna, 2006). The halogen dopant removes an electron

from the π-electron system, creating a hole (soliton) via p-type doping, allowing

charges to flow. Examples of ICPs include polyacetylene, polypyrrole (PPy),

polythiophene and polyaniline. ICPs often have spatially extended π-bonding systems

which allow the transfer of electrons or other charge carrying species, therefore

referred to as conjugated polymers.

Figure ‎2-9. Electrical conductivity of halogen-doped polyacetylene as a function of bromine and iodine concentration. Inset shows the structure of undoped trans-polyacetylene (up) and doped trans-polyacetylene with delocalised electrons (down). The former is an insulator, whereas the later can act as a metal (Reproduced from (Chiang et al., 1978)).

1. www.rsc.org/chemistryworld/2013/08/polyacetylene-organic-conducting-polymer-podcast

42

In the neutral (undoped) state, these polymers are only semiconducting. The

electronic conductivity appears when the material is doped, i.e., when electrons or

holes are injected into the conjugation system, via oxidation or reduction reactions.

Counter-ions called dopants are simultaneously inserted into the polymer matrix to

maintain electro-neutrality (Harsányi, 1995). Dopants may consist of a wide variety of

chemical groups ranging from simple anions such as Cl−, HSO4−, ClO4

−, NO3−, to

bulkier species such as p-toluenesulphonate or camphor-10-sulphonate, to large

polyanions such as poly(styrene sulphonate), as well as amino acids and biopolymers,

including proteins and DNA (Gordon G. Wallace, 2009). Doping can be either chemical

or electrochemical. The electrical conductivity of ICPs lies above that of insulators and

extends well into the region of common metals; therefore, they are often referred to as

‘synthetic Metals’1. Figure ‎2-10, shows the structures, along with electrical conductivity

in the doped state for a few conjugated polymers. Most of the examples utilise the

electron rich benzene rings in their mechanism of charge transfer. The aromatic ring

allows delocalisation of π electrons across the polymer chain2.

Figure ‎2-10. Chemical structures and conductivity range of some of the most popular conducting polymers (Adapted from (Terje A. Skotheim, 2007)).

1. So far, the highest reported value has been obtained for iodine-doped polyacetylene (≈10

5 S

cm-1

) and the predicted theoretical limit is about 2×107, more than an order of magnitude higher

than that of copper. 2. Polyaniline has a range of different properties due to the overlap of pz orbitals from the nitrogen atoms in the chain.

43

Due to the growing amount of potential applications of this relatively new scientific

field, the area of conjugated polymers has received a great deal of interest. Potential

applications include polymer/oxide batteries, redox super capacitors, molecular

recognisers, sensors and light emitting diodes. Furthermore, conducting polymers can

be used in a wide variety of other applications (Figure ‎2-11). These vary depending on

the polymer characteristics and methods used for doping the polymer. The uniqueness

of these materials stems from the ability to conduct electricity and bearing the same

inherent properties of normal polymeric materials. However, Lack of processability of

common ICPs has been a major impediment to the commercial realisation of these

polymers.

Figure ‎2-11. Application fields of ICPs. (Reproduced from (Heeger, 2001))

2.5.2. Electrical conductivity in ICPs

ICPs are fundamentally different from CPCs, redox polymers and ionically conducting

polymers such as polymer/salt electrolytes. These polymers can act as semiconductors

or conductors. These terms refer to the formation of electron bands or shells within

compounds and elements. Band theory is the most reasonable theory used to explain

the mechanisms of conduction in solids. This theory is based on the formation of

energy bands in solid materials from discrete orbital energy levels found in single atom

44

systems. Bands form because of the effect of neighbouring nuclei in a 3D lattice on the

electronic energy levels. As soon as these bands are formed, they can facilitate

electrons passing from one quantum state to the next if there is an overlap between the

bands or sufficient energy to overcome any band gap. The “Fermi level” in a given

system represents the hypothetical energy level of electrons in a solid at which half of

the quantum states are occupied. In an insulator the Fermi level sits within the valance

band, whereas in a conductor, the Fermi level rests within the conduction band. For

semiconductors, the Fermi level lies within the band gap, i.e. the space between the

conduction and valance bands (Figure ‎2-12).

In terms of electronic configuration, the valence band has the outer most electrons. If

this band is full, there is nowhere for electrons to move to which means that the

material is an insulator. For electrons to be able to move, the Fermi level requires to be

within a partly filled band, known as a conductance band, which allows electrons to

transfer through these different quantum states. Most metals are good conductors

because the conduction band is not completely filled or there is sufficient overlap with

the valence band, allowing the conduction band to provide different electronic states. In

an insulator, there is no overlap of the valence and conduction bands and there is an

energy gap between the two bands. Electrons cannot easily move between the two

bands and therefore the flow of electrons is prohibited by this gap. The energy gap can

be made up of bands of forbidden electronic states or represent the gap between the

next available energy level for electrons. In an intrinsic semiconductor, this band gap is

much smaller than an insulator. Therefore, by providing enough energy to the solid via

a thermal change or disturbance in electronic field, electrons can jump and move from

the valence band into the conductance band, leaving behind a hole in the valence

band. At absolute zero the semiconductor would act like an insulator but at a given

temperature there is enough energy to allow electrons to transfer to the higher band

(Figure ‎2-12) (Atkins, 2001).

45

Figure ‎2-12. Electronic band configuration in (a) metals, (b) insulators, and (c) semiconductors from closely spaced atoms having quantized energy levels.

There is also a way to make semiconductors out of a material that would normally act

as an insulator. There are two types of semiconductors which are formed by placing

impurities into the original material. These are known as extrinsic semiconductors and

the band gap in these materials have foreign orbitals for electrons to use. In an

extrinsic semiconductor there are two types of mechanisms for conduction that occur.

Silicon for example, has a valence of four; that if doped with something of a higher

valance (for example arsenic) this creates an extra electron per atom. This allows

partially filling of the conduction band with these unpaired electrons. The resulting

material is called n-type semiconductor and uses electrons as the main charge carrier.

The other mechanism occurs when a material is doped by a chemical with a lower

valence, for instance silicon with boron (valence of three). Here, the valence band is

depleted of an electron, creating a hole for each electron missing from a bonding pair.

The main charge carriers therefore are the holes that are formed when the electrons

move out of the valence shell. These are known as p-type semiconductors. In ICPs,

dopant molecules act similar to impurities in silicon semiconductors. For most

conjugated polymers, n-type doping refers to a reduction whereas p-type doping refers

to an oxidation (Figure ‎2-13) (Atkins, 2001).

46

Figure ‎2-13. n-type (left) and p-type (right) conduction in semiconductors.

To better explain the electronic phenomena in ICPs, new concepts including solitons,

polarons and bipolarons have been introduced by solid-state physicists (Brédas and

Street, 1985). These structures are mainly differentiated by the changes they make in

the polymer backbone; in solid-state physics a charge associated with a boundary or

domain wall is called a soliton, because it has the properties of a solitary wave that can

move without deformation and dissipation. Solitons are the boundary separating two

regions of the polymer with different π-bond phases, while polarons (and bipolarons)

are the boundary between quinoid-type and non-quinoid (benzenoid) regions of

conjugation (i.e., they are mainly present in the π-conjugated systems based on

aromatic rings). A radical cation that is partially delocalised over a segment of the

polymer is called a polaron which is stabilised through the polarisation of the

surrounding medium (Freund and Deore, 2007). The energy diagrams of these

structures are different as well, as solitons result in only one mid-gap state, while two

adjacent polarons exhibit two mid-gap states with a secondary energy gap in between

(Figure ‎2-14).

47

Figure ‎2-14. (a) π-π* transition and (b) soliton (left) vs. polaron lattice (right) band gap states.

Generally ICPs are semi-crystalline, heterogeneous systems with a crystalline

(ordered) region dispersed in an amorphous (disordered) region. The crystalline

domains are metallic in nature, where conduction occurs through electron

delocalisation or hopping of the charge carriers through the polaron structure.

However, metallic grains are surrounded by an amorphous non-conducting region

consisting of disordered or folded chains. The overall conductivity of an ICP depends

on bridging among conductive regions by means of tunnelling (Figure ‎2-15) (Bhadra et

al., 2009).

Figure ‎2-15. Proposed crystalline-amorphous (heterogeneous) structure of an ICP, showing different conduction pathways: A: along the backbone, B: interchain and C: between metallic grains (Reproduced from (Hobday, 2009; Bhadra et al., 2009).

48

2.5.3. Polyaniline

Due to its interesting properties, polyaniline has attracted great attention in recent

years. Compared to other common ICPs, PANI has good thermal and environmental

stability, and provides reasonable conductivity values enough for most applications. In

terms of cost, aniline is more stable and far cheaper than any of the other monomers

used to synthesise conjugated polymers, making the use of this monomer much more

economic. The synthesis of polyaniline from aniline is also much easier compared to

methods required to produce the other conjugated polymers. The properties of

polyaniline can be tuned more easily than other conducting polymers. As an example,

PANI can be reversibly doped & undoped by either electrochemical or chemical

oxidation & reduction. These are some of the main reasons which make polyaniline

popular among research communities in this field. Depending on the redox potential

and the pH of the environment, PANI can exist in 4 different molecular structures.

Figure ‎2-16 shows the interconversion of these different forms.

Figure ‎2-16. Different forms of polyaniline, depicting the oxidative and protonic acid (non-oxidative) doping (Freund and Deore, 2007)

The undoped, insulating forms of PANI consist of: 1) transparent (or pale yellow) fully

reduced form (leucoemeraldine); 2) partially oxidised form which has an amine/imine

ratio of ~1.0 (emeraldine base, EB) with blue colour; and 3) fully oxidised form

(pernigraniline) which is violet. Out of all forms of polyaniline, only the emeraldine salt

(ES) which is the protonated form of PANI EB is electrically conducting. During the

protonation process (treating EB with protonic acids), the number of electrons

associated with the polymer remains unchanged. Hence, the protonation does not

affect the oxidation state of emeraldine base. However, upon doping, a polaron is

formed through the successive formation of localised bipolarons, the intermediate

virtual state of two polarons centred on adjacent N's structure, and more stable polaron

lattice (Figure ‎2-17) (Epstein et al., 1987).

49

Figure ‎2-17. Structure of EB after 50% protonation and formation of bipolarons (a), unstable dication radicals (b) and stable polaron lattice (d) (Adapted from (Epstein et al., 1987; Bhadra et al., 2009)).

In the polaron structure, the cation radical on a nitrogen acts as the hole and these

holes are the charge carriers. The electron from the adjacent nitrogen (neutral) jumps

to that hole and that hole becomes neutral. As a result, on the second nitrogen another

hole is created. Thus in the polaron structure an electron starts moving along the

polymer chain towards one particular direction and the corresponding hole is set into

motion along the chain length in the opposite direction leading to an electrical

conduction along the chain (Bhadra et al., 2009). However, in the case of a bipolaron

structure, this type of movement of the electron and hole is not possible since the two

holes are adjacently located. Moreover, in leucoemeraldine or pernigraniline structures,

the electronic environments of all the nitrogen atoms along the polymer chain are

similar. Protons from the dopant can be attracted by any nitrogen atom and there may

be a few (more than two) protonated nitrogen atoms or free nitrogen atoms situated

side by side across the chain. Hence, there is lower chance for chain regularity, and

thus lower chance for the formation of delocalised polarons which are responsible for

the conduction. As a result, protonated leucoemeraldine and pernigraniline forms are

insulating in nature.

2.5.4. Polymerisation of aniline

The polymerisation of aniline is believed to follow the oxidative polymerisation route.

Understanding the molecular mechanism of PANI synthesis helps in controlling the

supramolecular structure and physical properties of the polymer, especially regarding

its polyconjugation and electrical conductivity. Oxidative polymerisation is used for the

synthesis of oligomeric and polymeric products from a variety of monomer classes

50

including phenols, thiophenols, aromatic hydrocarbons, sulphur- and nitrogen-

containing heterocycles, and aromatic amines. Pronounced electron donor properties

and high oxidation tendency are inherent to these monomers. PANI synthesis consists

of two interrelated processes: during polymerisation, monomer undergoes a chain

reaction resulting in the formation of regular macromolecules, and growing chains are

simultaneously assembled into complex supramolecular structures with various

morphologies (Sapurina and Shishov, 2012).

2.5.4.1. The molecular mechanism of aniline polymerisation

Oxidative polymerisation of aniline is found to proceed according to the electrophilic

substitution mechanism (Sapurina and Shishov, 2012): the oxidised nitrogen-

containing structure attacks phenyl ring of another aniline molecule and substitutes one

H+ of the ring, producing a radical cation. Then both the nitrogen-containing structure

and the ring lose one proton, and monomer units bind to each other. In fact, the

polymerisation follows a chain-growth mechanism rather than a polycondensation. The

oxidising agent activates the dormant polymer chain (PANIm), converting it to PANIm+,

then an aniline molecule adds to yield a new deactivated chain with higher molecular

weight (MW):

→

→ Equation ‎2-1

In the initial step of polymerisation, the monomer is oxidised by the oxidant; however,

upon formation of oligomers, the terminal amino groups undergo oxidation due to their

lower oxidation potential. The oxidant is spent in every step of monomer addition, so

the molar concentration of the oxidising agent should be equal to the monomer

concentration. This is in contrast to other types of chain-growth polymerisation (e.g.,

radical polymerisation) where oxidant is only involved in the initiation step.

The most probable linkage between the two aniline monomers in the electrophilic

substitution is “head-to-tail”, since the electron donor nitrogen atom will most likely be

oxidised by a strong oxidising agent such as ammonium peroxydisulphate (APS). The

exact oxidation path of aniline in reaction with double-electron oxidant APS is still open

to discussion (Ćirić-Marjanović, 2013).

51

Figure ‎2-18. Electrophilic substitution reaction.

The electrophilic agent may attack hydrogen atoms on different positions in the phenyl

ring to produce units with ortho-, para- and meta- structure. As a result, the final PANI

may be heterogeneous.

The most widely accepted mechanism for oxidative polymerisation of aniline is

depicted in Figure ‎2-19. The aniline radical cations are formed by the oxidant in a slow

process called “induction period”. Radical cations then recombine according to the

electrophilic substitution mechanism to form the dimer, N-phenylphenylene-1,4-diamine

(p-semidine). The induction period is considered to be the rate-limiting step of the

polymerisation. It is then followed by a rapid exothermic step of chain propagation,

where the oxidised terminal amino group of oligomer/polymer attacks the monomer.

The oxidative polymerisation of aniline in acidic medium yields polymers with regular

structure containing more than 95% para-substituted units linked in the head-to-tail

fashion (Sapurina and Stejskal, 2008). The polymer oxidation state during

polymerisation oscillates between doped pernigraniline (violet) and emeraldine salt

(green).

Figure ‎2-19. Polyaniline polymerisation mechanism. (Adapted from (Sapurina and Shishov, 2012))

52

2.5.4.2. Effect of pH on the polymerisation of aniline

The acidity of the polymerisation mixture has a strong influence on the final PANI

product. A conducting polymer is produced only in pH < 2.5, while the reaction in mildly

acidic, neutral and alkaline media results in non-conducting oligomers. The effect of pH

on the polymerisation process of aniline is summarised in Figure ‎2-20.

Figure ‎2-20. Effect of pH on the oxidation of aniline in the presence of a strong oxidising agent and in a high ionic strength medium (Reproduced from (Sapurina and Shishov, 2012)). pKAn=3.5 and pKPANI=2.5 are marked by the blue dashed line. The black line and dashed line are representative of aniline and chain oxidation potentials, respectively.

Depending on the protonation constant1 of monomer (pKAn) and imino groups on the

chain (pKPANI) this process can be divided into three ranges:

pH > 3.5; here, all types of nitrogen-containing groups are deprotonated, and

oxidation process is similar to typical dielectric polymers, such as phenols, with

the formation of irregular chains having MW of up to 5000.

3.5 < pH < 2.5; monomer is protonated in this range, while imino groups on the

chains are not. The imbalance of the redox processes results in slow formation

of cyclic dimers with phenazine structure.

pH < 2.5; where all the nitrogen-containing groups are protonated, and the

balance of electron exchange between chain and monomer is restored. Para-

substitution is the major growth pattern, which leads to the formation of high

MW regular chains with long polyconjugation lengths.

1. Also known as acid dissociation constant, is often reported in logarithmic scale: ; and it depends on so many factors such as the solvation effect and ionic strength of

the medium.

53

2.5.5. Polyaniline as an ammonia sensing material

Perhaps, one of the most attractive properties of polyaniline is its reversible doping

behaviour through acids and bases. This property can be employed for realising gas

sensors. In fact, nanostructured PANI fabricated as nanofibers (Huang et al., 2002;

Huang et al., 2004), nanowires (Liu et al., 2004), core–shell particles (Wojkiewicz et al.,

2011), and nanocomposites (Gong et al., 2010; Wu et al., 2013; Danesh and Persaud,

2012) has recently received a lot of attention due to its superior gas sensing

characteristics. In particular, PANI is an excellent sensing material for ammonia (Kukla

et al., 1996; Wojkiewicz et al., 2011; Rizzo et al., 2010; Subramanian et al., 2013;

Crowley et al., 2008a; Crowley et al., 2008b; Dhawan et al., 1997; Nicolas-Debarnot

and Poncin-Epaillard, 2003), since basic NH3 deprotonates the amine group in

emeraldine salt, hence converts it to emeraldine base which may cause a drastic drop

in its conductivity:

This reaction can be classified as a reversible chemisorption, given by the following

equation:

Equation ‎2-2

In high enough concentrations of ammonia, a colour change from green to blue may

also occur which indicates the conversion of ES to EB. This property may be used for

making colourimetric sensors (Gu and Huang, 2013; Kuswandi et al., 2012). Kukla et

al. (Kukla et al., 1996), investigated sensing layers based on HClO4-doped PANI

deposited from a solution in dimethylformamide (DMF). They showed experimentally

that the resistance at equilibrium, R, of the PANI layer varies with ammonia

concentration, N, in the following way:

[( ) ] Equation ‎2-3

54

Here is the baseline resistance value (at a given temperature) in air; is the

dimensional coefficient (in their work ppm-1, where is in ppm); and

parameter equals 0.5. The kinetic adsorption and desorption curves of ammonia in

polyaniline has been fitted to a wide range of concentrations (10-1000 ppm) using the

Langmuir isotherm but considering heterogeneous adsorption sites on the doped PANI

surface (Hu et al., 2002). Following equation has been proposed for the adsorption

kinetics, considering two different adsorption sites:

( ) { (

)} { (

)} Equation ‎2-4

where correlates to the adsorption isotherm and is the inverse of the association

constant, in Equation ‎2-2. The adsorption isotherm is obtained when the system has

come to equilibrium, i.e., the rates of adsorption and desorption are equal, or in

Equation ‎2-4. A is defined based on Freundlich model, where is the equilibrium

response (proportional to the equilibrium concentration of gas molecules adsorbed on

PANI surface), and is the maximum possible response (proportional to the

concentration of total available adsorption sites):

Equation ‎2-5

here a and b are constants and 0 < b < 1. A approaches to at low concentrations,

and to 1 when concentration tends to infinity. The desorption process, considering two

different desorption sites, can also be written as:

( ) ( ) (

) ( ) (

) Equation ‎2-6

similarly,

, where is the dissociation constant in Equation ‎2-2. The thermal

dependence of both and can be described in terms of the Boltzmann factor:

( ) (

) Equation ‎2-7

where and are the activation free energy of adsorption and desorption,

respectively. The heat of adsorption is then defined as . According to these

kinetic equations, Hu et al. (Hu et al., 2002), have suggested a monolayer reversible

chemisorption process, where there are two energetically different types of adsorption

sites for ammonia molecules on the PANI backbone. It was proposed that the positive

charges of the doped polymeric chain become unequally distributed over all the

nitrogen atoms as well as on the benzenoid rings, where they will be partially

delocalised. The fraction of positive charge delocalised onto the benzenoid rings

55

affects the neighbour nitrogen atoms. Hence, at least two kinds of positive charges are

present on the nitrogen atoms of the polymeric chain in emeraldine salt state.

indicates high affinity of PANI for ammonia, which does have its drawbacks,

mainly with regards to sensor recovery where long timescales of minutes to hours are

required to return the sensor to its baseline. Baseline drift and irreversible reactions in

polyaniline-based sensors are among the most important issues that negatively affect

their long-term stability and impede their practical use.

2.5.6. Solution processable PANI

One of the major issues regarding the fabrication of sensors composed of PANI is the

difficulty in processing the polymer. PANI has been generally categorised as an

insoluble and infusible material (Wessling, 2010; Wessling, 1997; Liu, 1997). Melt

processing of PANI is not possible, since the polymer decomposes at temperatures

below a softening or melting point (Mark, 1999). Emeraldine base is only slightly

soluble in strong aprotic solvents such as n-methylpyrrolidone (NMP), n,n'-

dimethylpropylene urea (DMPU), dimethylsulphoxide (DMSO) and DMF (Tzou and

Gregory, 1993; Tzou and Gregory, 1994). Conventional conducting PANI (in doped

state) does not dissolve in common organic solvents. The origin of the difficulty for

solubilising PANI, like other conducting polymers, is its delocalised π-electron

structure, which leads to (1) a large electronic polarisability and (2) a stiff polymer

backbone. High electronic polarisability leads to strong inter-chain attraction forces

which favour aggregation instead of solvation of the polymer. The stiffness of the

polymer backbone contributes to unfavourable entropy changes of dissolution. An

additional problem comes from the fact that the conducting polymer is composed of

both hydrophobic hydrocarbon segments and ionic segments in a single polymer chain,

which practically makes PANI a copolymer. This problem leads to difficulty in selecting

a suitable solvent that can simultaneously solvate both segments. PANI in its

conductive form has high crystallinity, which may play a role in preventing conductive

PANI from dissolving in most solvents.

A great effort has been made to enhance the solution processability of polyaniline in

its doped state for sensor fabrication. Methods to improve solution processability of

polyaniline may be essentially reduced into two main categories: synthetic methods

and blending methods.

a) Synthetic methods

Synthetic methods are based on aniline polymerisation to in-situ form a conducting

layer on the target substrate (e.g., interdigitated electrodes), or to form emeraldine salt

dispersions in a suitable carrier solvent for further processing (e.g., aqueous PANI

56

nanoemulsions). In-situ chemical polymerisation (Herath et al., 2012; Grennan et al.,

2005), electropolymerisation (Arsat et al., 2009; Crowley et al., 2013) and vapour-

phase deposition polymerisation (Oh et al., 2013) have been employed to coat sensor

substrates with the desired polyaniline layer. However, reproducibility is a major

concern regarding these approaches, where polymerisation parameters have to be

controlled precisely for each batch. Emulsion polymerisation with the use of steric

stabilisers (referred to as surfactants) such as sodium lauryl sulphate (SDS) and

dodecylbenzene sulphonic acid (DBSA), results in the formation of spherical ES

nanoparticles in water (Ngamna et al., 2007). This dispersion could then be used to

deposit polyaniline on the substrate using a spin coater or an inkjet printer. Huang and

co-workers pioneered new methods of synthesising PANI nanofibers, namely

interfacial polymerisation (Huang et al., 2003) and "rapid mixing" (Huang, 2006), with

the potential for large-scale production of sensitive PANI-based chemical sensors.

Template-guided synthesis has been developed to fabricate polyelectrolyte:PANI

complexes (Liu, 1997). The polyelectrolyte component provides soluble functional

groups to the complex so that the processability for PANI is improved. The

polyelectrolyte also acts as a polymeric dopant in the complex so that the dopant

stability, and consequently stability for PANI in the conducting form is greatly enhanced

owing to the strong electrostatic binding between PANI and the polyelectrolyte. The

polyelectrolytes used are polyanions such as poly(styrenesulphonic acid) (PSSA),

poly(acrylic acid) (PAA) and poly(vinylsulphonic acid) (PVSA) (Sun et al., 1997).

Unfortunately, all the synthetic methods require contact with aniline, which is a

carcinogenic monomer. Moreover, aniline, similar to other conjugated monomers, is

subject to oxidisation in air and must be distilled prior to use.

b) Blending methods

On the other hand, in blending methods, a previously prepared emeraldine base is

mixed with specific dopants to make solution processable emeraldine salt. A big

progress in improving processability of ES is made by doping PANI with functional

protonic acids to render the resulting PANI complex soluble in common organic

solvents. The concept has been named “counterion-induced processability” (Cao et al.,

1992), which indicates that polyaniline can be processed in the doped state, provided

that appropriate functionalised dopants are used in the doping process. Historically, the

first dopants used for the improvement of solution processability of the conducting form

of PANI were surfactant anions such as DBSA (Heeger, 1993). DBSA interferes with

the polymer-polymer interactions and improves polymer-solvent interactions

simultaneously, through the large hydrophobic hydrocarbon tail attached to the

hydrophilic head. However, since surfactants are non-conducting, an increase in their

57

content can be associated with an increase in polymer resistivity. Polyaniline is also

dispersible in 80% acetic acid, 60% formic acid, pyridine and concentrated sulphuric

acid (Reece, 2003). Polyaniline in these solvents undergoes protonation with

simultaneous dissolution. However, the films cast from these mixtures show low

conductivities (0.1-0.5 S cm-1) and very poor mechanical properties. In addition, they

are not resistant against deprotonation (Olinga et al., 2000). Much higher conductivity

is obtained for the films cast from a three-component mixture: PANI, camphor

sulphonic acid (CSA) as the dopant, and m-cresol (MC) as the solvent. Non-oriented

films of PANI(CSA)0.5 show a conductivity exceeding 300 S cm-1 (MacDiarmid and

Epstein, 1994). Unfortunately, films processed from m-cresol also exhibit very poor

mechanical properties and have the tendency to slowly release the solvent, whose

large quantities (up to 15 wt.%) remain in the polymer film after casting. For

PANI/CSA/MC complex, a concept of “secondary doping” has been introduced: due to

molecular matching and H-bonding interactions, a molecular complex is formed

involving all three components of the processing system. The formation of this complex

facilitates the ordering of the polymer upon the removal of the solvent, and the resulting

PANI(CSA)0.5 films show enhanced crystallinity.

However, ES made by blending methods usually is not stable enough for practical

applications because of the ease of dedoping which leads to loss of the electrical

conductivity and change of optical properties. One of the causes for dedoping comes

from the fact that dopants are usually small molecules that can easily evaporate or

sublime out of PANI. Although employing large-molecule dopants may reduce the

possibility of dopant evaporation & sublimation, over time the dopants may undergo

phase segregation. This phenomenon is especially prominent at elevated

temperatures. Exposure to moisture or direct contact with water may also accelerate

dedoping. Conventional PANI is completely deprotonated even in a weak acidic

solution (pH > 4). One approach to solve this problem is to use macromolecular

dopants (e.g., polyelectrolytes) to from an interpenetrating polymer network (IPN)

which resists dedoping by water even at pH values above 9 (Tarver et al., 2008). A

similar approach has been introduced by Pron and co-workers (Olinga et al., 2000;

Dufour et al., 2001; Rannou et al., 2002; Dufour et al., 2003; Zagorska et al., 2003).

They have synthesised new macromolecular protonating agents for PANI, which

combine doping and plasticising properties simultaneously. These dopants are diesters

of 4-sulphophthalic acid (SPhA) and sulphosuccinic acid (SSA), and their use has led

to solution- and melt-processable conducting polyaniline with high conductivity,

enhanced mechanical properties and high environmental and thermal stability.

58

Obviously, each of these methods has its own advantages and limitations. The

crucial thing is that in the case of doped PANI, the main properties of the complex,

including processability, environmental stability, electrical conductivity, mechanical

properties and so on, depend very strongly not only on the chemical constitution of the

polymer chain and the dopant anion, but also on the supramolecular aggregation

induced by the presence of the dopant. The role of dopant in ICP-based sensors

becomes even more important, since it is shown that the dopant itself can alter sensing

behaviour and modify selectivity and specificity of the sensor (Anitha and

Subramanian, 2003; Cabala et al., 1997). Therefore, one can understand the

importance of “dopant engineering” in fabrication of polyaniline-based sensors. In the

following chapter we shall use special dopants to make solution processable

polyaniline formulations, based on ‘counterion-induced processability’ concept.

In summary, a brief description of food spoilage and its mechanisms in different food

categories are presented (see Appendix I). Microbial deterioration and (bio)chemical

changes are believed to be the major decaying processes in fresh food. Depending on

the food type and deterioration mechanism, various volatile organic compounds may

be released at the early stages of spoilage. Headspace measurement has been

demonstrated to be a practical way to monitor the quality/freshness of fresh food.

Several food quality indicators, employing this concept, are available commercially and

some more are under research and development to fit in smart packages. In order to

fabricate an indicator that meets the FlexSmell necessities, we need to narrow down

the selection of analytes. Having reviewed different aspects of our project, we

proposed ammonia as the target analyte. Ammonia is not only an important vapour in

food industry, but its sensors can also be used in air and water quality monitoring

applications. Currently, metal oxide semiconductors are the most commonly used

sensing materials in ammonia detectors. However, electrically conducting polymers

such as polyaniline have shown promising features for low-cost detection of ammonia

in ppb levels. Polyaniline is a conjugated polymer, and its electrical and sensing

properties can be altered by doping with acids. Unfortunately, polyaniline in its doped

(conducting) form lacks solution processability. A lot of effort has been devoted to

make solution processable polyaniline, and concepts such as counter-ion induced

processability have been proposed. In the following chapter, we exploit dopant

engineering to prepare polyaniline ammonia sensors based on solution processing

techniques.

59

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CHAPTER III

3. Hybrid polyaniline sensors

3.1. Introduction

Although pristine polyaniline sensors have been developed for ammonia detection,

they are found to exhibit insufficient sensitivity (when sub-ppm concentrations in

vapour phase need to be detected), as well as poor reversibility (Chabukswar et al.,

2001). Different strategies have been adapted to enhance the sensing performance of

polyaniline gas sensors in general and ammonia and amine sensors in particular. One

of the most successful strategies is the employment of polyaniline-inorganic nano-

hybrid structures. Nanocomposites of polyaniline with inorganic materials, such as

carbon-based nanomaterials (Lu et al., 2010; Ting et al., 2009; Sotzing et al., 2000;

Jeon et al., 2010; Zhang et al., 2006; Lobotka et al., 2011; Wu et al., 2013; Ding et al.,

2011), metals (Sharma et al., 2002; Athawale et al., 2006) and metal oxides (Ram et

al., 2005; Gong et al., 2010), have been developed successfully for making highly

sensitive gas and vapour sensors.

Sotzing and co-workers (Sotzing et al., 2000) have reported highly sensitive and

selective sensors based on DBSA-doped PANI/CB composites for detection of

biogenic amines. The sensor response was reversible to exposures of n-butylamine at

vapour phase concentrations between 10 ppt (parts-per-trillion) and 700 ppb. The

response time was as low as 30 sec1. However, response at higher concentrations,

between 50 ppm and 1 ppth (parts-per-thousand) showed ‘pseudo-reversibility’. Feller

and co-workers (Lu et al., 2010) have shown that chemo-electrical properties of PANI

nanoparticles/multi-walled carbon nanotube (MWNT) hybrid nanocomposites could be

tuned by varying polyaniline concentration. Interestingly, only a small amount of MWNT

1. Here, the response time was defined as time sufficient for the sensor to produce 90% of the limiting value of the electrical resistivity response.

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added to polyaniline nanoparticles could not only enhance the transducer’s

conductivity, but also switches the response from negative to positive in exposure to a

specific analyte. These hybrids showed higher sensitivity and selectivity than

formulations comprising of only PANI or only MWNT, demonstrating a positive synergy.

This synergy has been described by the formation of double percolated network inside

the composite. Chemical gas sensors using single-walled carbon nanotubes (SWNT)

electrochemically functionalised with polyaniline are reported by Zhang and co-workers

(Zhang et al., 2006). The results showed reproducible response to NH3 with sensitivity

of 2.44% ∆R/R per ppm. This value was more than 60 times higher than the sensitivity

of intrinsic SWNT based sensors. Moreover, detection limits as low as 50 ppb were

observed for this hybrid SWNT/PANI composite sensors. However, the typical

response time of the sensors at room temperature was on the order of minutes and the

recovery time was a few hours. Graphene/PANI nanocomposites showed linear

response to ammonia over the range of 1-6400 ppm, with 1 ppm detection limit (Wu et

al., 2013).

In an attempt to make high performing ammonia sensors, we decided to prepare

hybrid nanocomposites of polyaniline and carbon nanoparticles. The first step though is

to solubilise (disperse) the conducting polyaniline in a suitable solvent. Solution

processable PANI formulations are needed since the final target of the FlexSmell is to

use techniques which are compatible with large scale sensor fabrication methods (e.g.,

printing). Furthermore, effective dispersion and distribution of carbon nanoparticles is

much easier in a liquid medium. Without a homogenous mixture, it is not possible to

fabricate reproducibly uniform composite layers on the sensor substrate. “Counter-ion

induced processability” concept (Section ‎2.5.6) was acquired to prepare polyaniline

solutions in common organic solvents such as NMP. “Multifunctional dopants” which

have at least one sulphonic acid group and one carboxylic acid group are used. The

idea is to simultaneously dope and solubilise emeraldine base in the solvent. Selected

carbon nanoparticles will then be mixed with the conducting polyaniline mixture. We

then use this homogeneous mixture to deposit a hybrid PANI-based sensor on the

substrates for ammonia sensing characterisation.

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3.2. Materials and methods

3.2.1. Materials

All chemicals used were reagent grade and purchased from Sigma-Aldrich (UK),

unless otherwise stated. Polyaniline used is in emeraldine base form with MW~10,000

g mol-1. Two multifunctional dopants, selected from the large family of sulphonic acids,

were:

a) Sulphosuccinic acid (70 wt.% solution in water; ρ=1.438 g cm-3):

b) 4-sulphophthalic acid (50 wt.% solution in water; ρ=1.292 g cm-3):

These dopants were selected because they have two types of functional groups in their

structure: one, which is a sulphonic acid group (SO3−), dopes the PANI’s imine sites

due to its strong acidity. The other one, which is a carboxylic acid group (COO−),

enables the interaction between dopant and polar solvents. Two conductive fillers were

used to make hybrid nanocomposites: (1) Carbon Black nanospheres (Black Pearls

2000, Cabot corp., USA) and (2) 20 wt.% polyaniline on carbon black, which is a

polyaniline-coated carbon nanoparticle supplied by Sigma-Aldrich. Apart from aniline

which was distilled prior to use, all reagents were used as received.

3.2.2. Preparation of doped PANI

Polyaniline protonated with multifunctional dopants were prepared as follows:

emeraldine base powder was dissolved in NMP to make 1.0 wt.% and 5.0 wt.% dark

blue mixtures at 50 °C, using a high shear mixer (RZR 2040, Heidolph overhead

stirrer). The mixtures were filtered using a 0.45 µm Nylon membrane (Millipore) to

remove unsolved particles and gels. These stock solutions were used to prepare

doped polyaniline compositions and hybrid mixtures. The reason for preparing two

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different concentrations is to have the possibility of fabricating PANI layers with

different thicknesses using the same deposition technique.

Since both dopants are readily miscible with aprotic solvents (e.g., NMP) one can

easily prepare a doped solution by adding the dopant in required concentration to the

stock solution of EB in NMP. As discussed earlier in Chapter II, polyaniline is a

tetrameric polymer, basically comprised of four benzene rings and four nitrogen atoms.

In a fully-doped PANI, only two of the nitrogen atoms become protonated by the dopant

molecules to form polaronic structures (see Figure ‎2-16 and Figure ‎2-17). Any excess

dopant will not cause any further doping of the polymer. In other words, for a complete

doping process, two dopant molecules (units) are required for each tetramer of

polyaniline. To simplify this, we define a ‘doping unit’ for emeraldine base. Each doping

unit of EB corresponds to a polyaniline repeat unit containing two benzene rings and

two nitrogen atoms (the MW of the unit is 182 g mol-1):

thus, here we need one molecule of acidic dopant to fully dope this doping unit of

PANI. For instance, to prepare a solution of SSA-doped polyaniline, we added 1.0

mmol SSA to a typical EB/NMP solution containing 182.0 mg of emeraldine base (1

mmol doping unit). Doped-mixtures were filtered using a 0.22 µm

polytetrafluoroethylene (PTFE) syringe filter immediately before use.

3.2.3. PANI/CB hybrid composites

Composites of PANI and CB were prepared by mixing doped PANI solutions in NMP

with adequate amount of conductive filler. The Carbon:PANI mass ratio was fixed at

20:80. Mixtures were stirred overnight and placed in an ultrasonication bath for 1 hr

prior to use. From this point, we will use “CB” as nomenclature for the Black Pearls

2000 and “coated-CB” for the surface-treated carbon black particles.

3.2.4. Thin film characterisation

UV-visible spectroscopy (PerkinElmer lambda 35) was used to study the doping

behaviour of different polyaniline solutions/dispersions. Atomic force microscopy (AFM)

(Thermo Microscopes) in non-contact mode was used to characterise the topography

of hybrid polyaniline composite films prepared. X-ray photoelectron spectroscopy

(XPS) measurements were performed with a Theta Probe Thermo-VG Scientific

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instrument (at the University of Bari, Italy) using an Al Kα monochromatic source (15

kV; X-ray spot size 300 mm; take-off angle 37°; base pressure 10-9 mbar). Wide-scan

survey (0–1200 eV binding energy, BE) and high-resolution spectra were acquired at

150 and 100 eV pass energy, respectively. Three distinct points were analysed per

sample; the resulting atomic percentage values were reported as the mean value with

associated standard deviation for each set of measurements. Data were analysed

using the Thermo Avantage software, version 4.75. Peak components were normalised

to the highest peak. Thermogravimetric analysis (TGA) was performed on a Mettler-

Toledo TGA DSC 1 under nitrogen to study thermal stability of doped samples. To

attain the doped PANI powders, NMP solutions of polyaniline were precipitated in

acetone, filtered and washed several times with acetone and dried at 65 ºC overnight.

The measurement was run from 30 to 600 °C, where the heating rate was 10 K min-1.

In order to investigate the electrical conductivity of the PANI and its hybrid composite

samples, the required amount of dopant (SSA or SPhA) was added to the 5 wt.% EB

stock solution to make a fully-doped PANI composition, and thin films of doped PANI

and PANI/coated-CB were deposited on glass slides. In the case of composites,

carbon nanoparticles were added to achieve final Carbon:PANI mass ratio of 20:80.

The clean glass slides were then spin coated with the mixture at 3000 rpm for 60 sec.

An IR heater at the distance of 15 cm from the substrate was used in the last 30 sec of

deposition, to ensure complete evaporation of the solvent. The samples were further

dried in oven at 65 ºC overnight. A series of gold contacts were thermally evaporated

on the films using a metallic mask. The micro-electrodes’ length, width and gap were

0.2 mm, 4 mm and 0.2 mm, respectively. Figure ‎3-1 shows two hybrid composite

samples used for conductivity measurements.

Figure ‎3-1. Samples used for conductivity measurements. Left: SSA-doped PANI/coated-CB, and right: SPhA-doped PANI/coated-CB hybrid composites. Au electrodes were thermally evaporated on the thin films.

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Considering contact resistances between the gold contacts and the measuring micro-

probes, as well as between the composite layer and Au electrodes are crucial in

evaluating the accurate conductivity of thin films. Here, we have used Torsi and co-

workers’ method (Torsi et al., 2009): electrical current (I) vs. applied voltage (V) was

measured for parallel electrodes with various gap distances (G). The electrode width

(W) was constant for all electrodes. The electrical resistance, which based on the

Ohm’s law is the slope of the V vs. I curve for each measurement, is a function of G.

For each sample, both sheet resistance (R□) and contact resistance (Rc) values can be

calculated based on the relationship between R and G1. Using the average film

thicknesses (measured by AFM), we can then estimate the conductivity of the samples

in an accurate way. The thickness values are the average of at least 3 points on the

film surface.

3.2.5. Fabrication of sensors

The plastic substrates used were polyimide Kapton and PEN substrates incorporating

interdigitated gold electrodes (IDE). These were provided by IMT centre at EPFL

(partner of the FlexSmell project). The electrode gap was 20 µm and the sensing area

was ca. 20 mm2. The substrates were washed in sequence with methanol, water,

isopropanol (IPA), rinsed with Milli-Q water and dried under N2 flow prior to coating.

Two methods have been used in order to make sensitive layers made by hybrid

polyaniline/coated-CB composites:

a) Dip coating: hybrid compositions starting from 1 wt.% EB/NMP stock solution were

made in a 15 ml glass vials. Dip coating were done by immersing the IDE area of the

substrates into the solution, keeping for 15 sec. Sensors then removed from the

solution, kept in vertical position in air for 30 sec and then dried for 1 day inside a fume

cupboard at room temperature (RT), and finally annealed at 110 °C for 15 min;

b) Spin coating: all the substrate area other than IDE part were covered by Kapton tape

and aforementioned compositions were deposited on the substrates at 2000 rpm for 60

sec using a Chemat spin coater (model: KW-4A, Chemat Scientific, USA), and dried at

RT for 6 hr. Finally, the sensors were annealed at 110 °C for 15 min.

3.2.6. Sensing chamber and circuit board

A custom designed sensing chamber was made from PTFE to host the flexible

sensors. The effective sensing volume before putting the sensor and its connector is

less than 1000 mm3 (1 ml) (Figure ‎3-2).

1.

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Figure ‎3-2. Sensing chamber design. All dimensions and measures are in mm.

A circuit board with zero insertion force (ZIF) connector (Molex 52207-0685) suitable

for holding the flexible substrates used in this study was designed. The sensor is then

connected to a proprietary interface board capable of measuring and recording real-

time resistance change of the sensor. A sub-miniature ceramic heater (model: DN505-

05, RS components, UK) was used for heating the sensor.

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3.2.7. Ammonia sensing characterisation

A homemade automatic ammonia vapour generation system (AVGS) was designed

and utilised for characterising the sensing performance of prepared sensors. The

carrier air was filtered to remove moisture and dust particles, and produce a clean air.

Ammonia vapour source was a refilled ammonia permeation tube (Fine Permeation

Tubes, Italy) with recalibrated permeation rate of 548 ± 6 ng min-1 (at 25 °C). To

estimate the permeation rate at a different temperature from that of the calibration, one

can use following equation:

( ) Equation ‎3-1

where is the rate at (°C), is the new rate at (°C), and α is the temperature

coefficient which is 0.034 for the tube used here. (Reference conditions are defined as

25 °C and 760 mm Hg). To calculate the concentration of a gas generated by a

permeation tube in a dynamic carrier flow (Figure ‎3-4), we can use the following

equation:

(

)

Equation ‎3-2

Here, the analyte concentration in ppm by volume, the permeation rate in ng

min-1, the molecular weight of the analyte, the total flow of the calibration

mixture in ml min-1. The constant 24.04 is the molar volume at the ambient condition

(20 ºC, 760 mmHg). The laboratory temperature was 20 ± 2 ºC during the course of

measurements.

Figure ‎3-3. Left: PTFE sensor chamber and its Viton® sealing; Right: Substrate holder, circuit board and the sub-miniature ceramic heater (shown by yellow arrow)

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Figure ‎3-4. Permeation tube structure and function.

Figure ‎3-5 depicts the AVGS system designed for preparing very low concentrations of

ammonia in a reliable, repeatable and precise manner. All the tubes are made of PTFE

and stainless steel. MKS mass flow controllers (MFCs) as well as the pressure control

system, and the 3-way solenoid valve were all controlled by computer. The output of

the air cylinder is filtered using an Agilent moisture trap filled with molecular sieves and

carbon active powder, in order to remove any residual water, oil or solvent from the

clean air stream. The carrier flow and the analyte flow from the permeation tube pass

through a microfilter prior to reach the sensor chamber, to ensure a uniform and

efficient mixing of analyte with carrier flow. The total flow through the sensor was

adjusted to 100 sccm.

Figure ‎3-5. Automatic ammonia-vapour generation system.

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In order to investigate the capability of the hybrid sensors for use in real-life

applications, the sensor was placed in an Ammonia Monitor instrument (Multisensor

Systems1), and the ammonia concentration of treated water samples in Heigham water

treatment site (Norwich, UK) monitored continuously over one week. A 1 M sodium

hydroxide (NaOH) solution was used to evolve ammonia from the water samples, using

a proprietary auto sampling system. Data of an on-site electrochemical ammonia

detector (Aztec 600 ISE ammonia analyser, ABB) was used as a reference for

ammonia concentrations. The sampling interval was 15 min.

1 . http://www.multisensor.co.uk/

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3.3. Results

Using dopant molecules containing polar groups in their structure increases the

interaction forces between dopant and polar solvents. If this interaction overcomes the

intermolecular forces between the chains, it may help the solubilisation of polymer

complex in the solvent. Here, by “solution” we actually mean a mixture that passes

through 0.22 µm PTFE membranes (which was the extent of our effort to filter out any

unsolved particles). However, we suspect that it is a ‘true solution’ on a molecular level

(Section ‎2.5.6). In this study, both SSA and SPhA contain two carboxylic substituents,

in addition to their strong acidic sulphonic group. These functional groups result in

strong forces between polymer/solvent rather than polymer/polymer interactions.

These interactions render doped salts more soluble in aprotic solvents such as NMP,

DMF and DMSO. This result is in agreement with Tzou and Gregory (Tzou and

Gregory, 1993), which have shown the positive effect of carboxyl groups on solubilising

PANI in NMP and DMSO. Figure ‎3-6 shows images of diluted solutions of EB, SSA-

and SPhA-doped PANI which were used for UV-Vis spectroscopy.

Figure ‎3-6. Diluted solutions of a) SSA-doped PANI, b) SPhA-doped PANI and c) EB in NMP.

3.3.1. UV-Vis spectroscopy

UV-Vis spectroscopy is an invaluable tool to examine the doping behaviour in

conjugated polymers both in solution and as thin films. The understanding of doping

state in polyaniline samples is essential in controlling their electrical (and also sensing)

properties. UV-Vis spectra of different forms of polyaniline are usually interpreted on

the basis of their ‘band structure’. The delocalised π bonds available in polyaniline are

responsible for the semi-conducting properties of the polymer. The π orbital produces

the valance band and the π* forms the conduction band (Figure ‎3-7). The value of the

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difference in energy between π and π* orbitals (band gap or band energy) determines

both the electrical and optical properties of PANI in solution or solid state. Detailed

description of band theory, polaron and bipolaron formation in polyaniline, as well as

electron transport mechanisms in PANI can be found in (Section ‎2.5.2).

Figure ‎3-7. Band structures of protonated polyaniline with (left) a coil-like conformation and (right) an expanded coil-like conformation, respectively. Numbers are indicative of the major wavelengths where peaks appear in UV-Vis spectra. Reproduced from Ref. (Xia et al., 1995)

Figure ‎3-8. UV-Vis spectra of emeraldine base, SSA- and SPhA-doped PANI solutions in

NMP.

Figure ‎3-8 compares typical UV-Vis absorption spectra of EB form of PANI with SSA-

and SPhA-doped PANI solutions in NMP, used in this study. For emeraldine base, the

absorption band at ca. 635 nm is generally believed to be relate to the n-π* transition

or quinoid form structure, and the absorption band at ca. 328 nm is related to the π-π*

77

transition. For doped polyaniline the absorption at ca. 425 nm is due to the presence of

localised semiquinone population or the polaron absorption (equivalent to: polaron

band to π* transition in Figure ‎3-7). Usually there is also another absorption at ca. 800

nm is due to the trapped excitons centred on quinoid (imine) (Tzou and Gregory,

1993), equivalent to: π- to polaron band transition. However, as can be seen from

Figure ‎3-8, the peak at ca. 800 nm is absent in our case and it is replaced by a long

free tail extended to near IR region. This may be attributed to the highly delocalised

free electron state of polyaniline doped with these dopants. According to Xia and co-

workers (Xia et al., 1995), this may also be due to the expanded-coil conformation of

doped-PANI chains in this specific solvent. We have observed different spectra for

different PANI solutions, based on different solvents and dilution ratios. Further

characterisation by UV or other means is needed in order to fully clarify the effect of

different parameters on the final spectra in UV-Vis-Near IR range of polyaniline

solutions.

We believe that UV-Vis spectra can also be used to assess the stability of these

solutions. Figure ‎3-9 shows the evolution of the UV-Vis spectra of doped PANI over

time (i.e., the experiment has been done on the same sample in the whole period).

Both solutions are stable for at least two months with a slight decrease in intensity of

the peaks characteristic to doped state. In the case of SSA-doped PANI, the peak at

ca. 425 gradually disappears and the peak intensity at ca. 635 increases. Eventually

the solution turns into EB solution (undoped form) with a clear change in the colour

from dark green to purple. However, for SPhA-doped PANI, even after one year the

solutions remain doped (pale green in colour), in spite of the decrease in the polaron

absorption intensity. These changes may be correlated to the slightly basic nature of

the solvent (NMP), as well as its strong hydrogen bonding capability which causes

interaction with polyaniline and possibly deprotonation.

78

Nevertheless, both of the solutions are stable for a couple of months from the day of

preparation, without any significant change in properties which make them suitable for

use in our target application. As mentioned before, two different formulations were

prepared: 1 wt.% and 5 wt.% solutions of EB in NMP and their doped counterparts. All

the formulations are “solution-processable”. The former is useful when very thin layers

of PANI are needed or when the spin coating is not possible; so, dip coating can be

used instead. The latter on the other hand is more suitable for spin coating and in

Figure 3-9. Evolution of UV-Vis spectra of protonated PANI solutions in NMP during a 1-year period: (top) SSA-doped and (bottom) SPhA-doped PANI.

0.0

0.0

2.5

2.5

79

situations where films with larger thicknesses are required. This variety gives us

flexibility, especially in terms of deposition method selection.

Solution processability of PANI also enables us to make hybrid composites by simply

adding inorganic (nano)particles (e.g., carbon black) into PANI solutions and mixing the

suspension. As a starting point, Black Pearls 2000 was chosen as the conductive filler

due to its well-known physical/electrical properties and its compatibility with polymeric

matrices (Lonergan et al., 1996).

3.3.2. Atomic force microscopy

AFM is used to characterize the surface topography of hybrid composite thin films. It

can give an estimation of the uniformity of dispersion and distribution of nanoparticles

in the layer. The primary particle size of Black Pearls 2000 is reported to be as low as

~12 nm (Zhu et al., 2004). However, when used as an additive within a polymeric

matrix, nanoparticles tend to aggregate and form rather large agglomerations and

clusters from several hundred nanometers to few microns. As can be seen from the

AFM image (Figure ‎3-10-a), the composite of PANI/Black Pearls 2000 has a non-

uniform distribution of carbon black particles. Moreover, clusters as large as several

microns are present in the layer, in spite of extensive mixing used (1 day stirring plus 1

hr in sonication bath).

80

Figure ‎3-10. AFM image of (a) 20 wt.% Black Pearls 2000 and (b-d) PANI-coated CB with SSA-doped PANI composite, deposited from suspension in NMP (measurements were done in non-contact mode). Black arrow shows a dispersed particle with ~100 nm in diameter

Several methods have been suggested in the literature in order to enhance the

dispersability of carbon-based nanomaterials in polymeric matrices (Grossiord et al.,

2006); for instance: (a) employing ultra-high energy mixers (such as sonication probes,

Ultra-Turrax mixers, etc.); (b) surface functionalization and (c) using surfactants (and

also polymers (Lin et al., 2003)) to enhance compatibility between nanoparticle and

polymer matrix. The first two methods often significantly deteriorate electrical and

mechanical properties of the nanoparticle, mainly because of the reduction in its aspect

ratio. For example, in nanotubes or nanofibers, the high shear and strong acids can

break and shorten the long tubes/fibres, distorting the π-electron system. Although

using surfactants as a shell around the nanoparticle greatly improves the dispersion, it

adversely affects the electrical conductivity of nanocomposite because of the insulating

nature of common surfactants. Hence, we decided to exploit a new approach: using

carbon nanoparticles covered by a conducting polymer layer. In this manner, the

polymeric layer enhances the wettability of particles without sacrificing the electrical

properties at the interface layer. Such materials are commercially available (from

Sigma-Aldrich). Here, we have used a polyaniline-coated carbon nanoparticle (the

polymer to carbon ratio is 20:80 wt.:wt.). From the material description provided by the

81

manufacturer, we believe that these particles possess a “core-shell” structure; with

carbon nanoparticles as the core and polyaniline chains as the surrounding shell

(Figure ‎3-11). Polyaniline in the shell will provide maximum compatibility by means of

strong interactions with the polyaniline chains in the solution.

Figure ‎3-11. Polyaniline/carbon nanoparticle core-shell structure. Reproduced from W. Xiaorong et al., US Patent application No. 20100004398 (2010)

These nanoparticles were used to make hybrid polyaniline nanocomposites with

exactly the same mixing and deposition methods employed in the case of Black Pearls

2000. Figure ‎3-10-b to d show the AFM images of the layer, with successful dispersion

of nanoparticles. The distribution of nanoparticles is comparatively uniform across the

whole area of the deposited film prepared by spin coating on glass slides.

Improvement of dispersability and distributability of carbon nanoparticles in polyaniline

composites may improve the repeatability of device fabrication when using these

hybrid materials to make gas/vapour sensors. Hence, these additives were selected to

make the hybrid nanocomposite sensors.

3.3.3. X-ray photoelectron spectroscopy

XPS is another method that gives further information about the surface composition of

the samples. Figure ‎3-12 shows the N 1s core-level spectrum for the emeraldine base

and SSA-doped PANI obtained in the present work. The spectrum of undoped PANI is

deconvoluted into two major component peaks centred at 399.9 ± 0.1 and 398.7 ± 0.1

eV (Figure ‎3-12-a). The two peaks are assigned to the amine (-N-) and imine (-N=)

structure, respectively. The two major peak components in Figure ‎3-12-a are of about

the same area, suggesting that equal amounts of quinoid imine and benzoid amine

structure are present in the emeraldine base. This is consistent with the suggested

82

structure for EB (Yue and Epstein, 1991). The slightly lower amount of imine structure

observed is probably associated with the presence of a trace amount of positively

charged nitrogen, as indicated by the residual N 1s high BE tail. This is consistent with

the reports in literature that protonation occurs mainly at the imine units (Tan et al.,

1989). In the protonated polyaniline, the N 1s peak component due to the imine at

398.7 eV should have been shifted to a substantially higher BE (ca. 402 eV).

Figure ‎3-12-b shows the curve fitted N 1s spectrum of SSA-doped PANI. The imine

peak component has disappeared almost completely and a high-BE shoulder,

attributed to the positively charged nitrogen becomes prominent. There is another

shoulder visible at ca. 405 eV in Figure ‎3-12-b (shown by red arrow); however the

origin of this rather small peak is not known at this moment. For better discrimination,

the merged XPS N 1s peaks of both samples are shown in Figure ‎3-12-c. The wide

XPS spectra and the relevant average atomic percentages for the two samples are

shown in Figure ‎3-13 and Table ‎3-1, respectively.

The effect of doping can be further investigated by comparing C 1s core-level spectra

of emeraldine base (Figure ‎3-14-a) and SSA-doped PANI (Figure ‎3-14-b). The EB film

surface was smoothly fitted into two carbon peaks: aromatic carbon (C-Ar) and carbon

atom with a single bond to nitrogen (C-N) binding energies are very close together and

they are seen as a quasi-single peak at 284.8 ± 0.1 eV, and carbon atom with double

bonds to nitrogen (C=N) at 286 ± 0.1 eV (Figure ‎3-14-a). The presence of dopant can

be confirmed by the presence of the peak at 289.1 ± 0.2 eV in SSA-doped PANI

corresponding to carboxylic acid functionalities of the dopant, which is absent in the

undoped PANI spectrum. This high tail in the C 1s core level spectrum of doped PANI

(red arrow in Figure ‎3-14-b) is poorly defined; it has been attributed to the residual

NMP in the film, surface oxidation products, weakly charge-transfer complexed oxygen

on the film, or a shake-up satellite (Lee et al., 2000). The presence of dopant is also

supported by appearance of S 2s and S 2p in wide scan spectra of doped PANI layers

(Figure ‎3-13).

83

Figure ‎3-12. XPS N 1s core-level spectra of (a) undoped and (b) SSA-doped PANI layers on silicon wafer. The quinoid and benzoid nitrogen atoms in emeraldine base show peaks at 398.7 and 399.9 eV, respectively. Upon doping, the quinoid type peak is replaced by polaronic N with a corresponding peak at higher BE (around 402 eV). Comparison of doped (red) and undoped (green) N 1s spectra is shown in (c). There is a clear shift in binding energy to higher values when the multifunctional dopant is used to protonate polyaniline imine sites.

84

Figure ‎3-13. The wide scan XPS of emeraldine base (top) and SSA-doped PANI (bottom).

Table ‎3-1. Atomic percentage in polyaniline samples extracted from XPS wide scan spectra.

Average atomic percentage (%)

N O C S

Undoped PANI 12.05 7.21 80.74 0

SSA-doped PANI 6.45 37.33 49.69 6.53

85

3.3.4. Thermogravimetric analysis

A series of thermogravimetric analyses were done on polyaniline in its emeraldine base

and doped states, to evaluate the thermal properties. TGA also gives insight into the

effect of dopant on the thermal stability and thermal aging of the conducting polymer.

The thermograms of the three samples: EB, SSA- and SPhA-doped PANI are

compared in Figure ‎3-15. For clarification, the values of the major mass losses of all

three samples are summarised in Table ‎3-2. Clearly, EB has superior thermal stability

compared to emeraldine salt. The emeraldine base is stable up to around 300 ºC, while

doped samples undergo major weight losses above 100 ºC. EB shows only one major

weight loss starting from ca. 285 ºC with an inflection point at ca. 450 ºC (Table ‎3-2),

which can be attributed to structural decomposition of the polymer (Wei and Hsueh,

1989). Doping has a strong influence on the thermal stability of polyaniline (Kulkarni et

al., 1989). The thermogram of the SSA-doped sample demonstrates three stages of

weight loss with peaks at around 60, 260 and 490 ºC. The 2% weight loss at low

temperatures is related to the evaporation of volatiles (such as residual acetone) and

moisture from the sample. The mass reduction at 260 ºC can be assigned to the

decomposition of the dopant or its release from doped polyaniline. Finally, at ca. 490

ºC the polymer structural degradation happens. Similarly, a 1% change in mass is

observed for the SPhA-doped sample indicative of the release of volatiles and

Figure ‎3-14. C 1s spectra of (a) undoped and (b) SSA-doped PANI after a curve fitting, demonstrating the spectral contribution of different functional groups

86

hydrogen-bonded water molecules. The major loss at 233 ºC is attributed to the

thermal dedoping and dopant removal. However, the total mass loss of the SPhA-

doped PANI at 600 ºC, as well as its weight reduction rate at high temperatures is

lower than other two samples. We suggest that some cross-linking reactions could take

place between the dopant and PANI (and possibly between dopant molecules itself)

during the thermal aging. The absence of low temperature mass loss step for EB

shows that the moisture content in ES form is higher due to the presence of the

hydrophilic dopants (Chen, 2003).

Figure ‎3-15. TGA graphs of undoped and multifunctional doped PANI powders.

Table ‎3-2. TGA parameters extracted from the TGA graphs of three polyaniline samples.

Emeraldine

base SSA-doped

PANI SPhA-doped

PANI

1st major stage

Left limit (ºC) - 32.57 33.72

Right limit (ºC) - 98.45 104.56

Inflection point (ºC) - 60.64 68.06

Mass loss (%) - 2.07 0.98

2nd major stage

Left limit (ºC) - 136.71 126.33

Right limit (ºC) - 342.62 244.37

Inflection point (ºC) - 261.24 233.07

Mass loss (%) - 28.90 5.13

87

3rd major stage

Left limit (ºC) 285.33 398.76 324.04

Right limit (ºC) 588.46 588.66 590.06

Inflection point (ºC) 453.48 493.20 500.57

Mass loss (%) 54.83 29.90 23.48

3.3.5. Conductivity of polyaniline films

Figure ‎3-16 shows current versus applied voltage (I-V) curves for 4 polyaniline

samples. The results are summarised in Table ‎3-3. The R values are calculated based

on the slope of the fitted lines in Figure ‎3-16-a to d. The RxW values were then plotted

against the gap size to give the sheet resistance (R□) of the films. The R□ values,

derived from the slope of the fitted lines, together with the corresponding curves are

plotted in Figure ‎3-17. Moreover, the contact resistance (Rc) can be calculated from the

intercept of the fitted line, for each graph.

Figure ‎3-16. I-V curves at different gap distances for a) SSA-doped PANI, b) SPhA-doped PANI, c) SSA-doped PANI/coated CB composite, and d) SPhA-doped PANI/coated CB composite. G1=0.2 mm, G2=0.6 mm, G3=1.0 mm, G4=1.4 and G5=1.8 mm are gap sizes of consecutive parallel electrodes. Solid lines are the best linear fit to the data points for each measurement.

88

Table ‎3-3. Resistance measurements data for 4 samples of doped PANI and hybrid composites.

Samples Gap size (cm) 1/R (Ω-1

)† R (Ω) RxW

‡ (Ω cm)

SSA-doped PANI

0.02 5.79E-7 1.73E+6 6.90E+5

0.06 3.36E-7 2.98E+6 1.19E+6

0.1 2.36E-7 4.23E+6 1.69E+6

0.14 1.80E-7 5.55E+6 2.22E+6

0.18 1.48E-7 6.73E+6 2.69E+6

SPhA-doped PANI

0.02 7.05E-6 1.42E+5 5.67E+4

0.06 3.81E-6 2.62E+5 1.05E+5

0.1 2.69E-6 3.71E+5 1.48E+5

0.14 2.10E-6 4.75E+5 1.90E+5

0.18 1.73E-6 5.79E+5 2.31E+5

SSA-doped PANI/coated CB composite

0.02 7.63E-7 1.31E+6 5.24E+5

0.06 9.14E-7 1.09E+6 4.38E+5

0.1 5.54E-7 1.81E+6 7.22E+5

0.14 4.60E-7 2.17E+6 8.69E+5

0.18 3.59E-7 2.78E+6 1.11E+6

SPhA-doped PANI/coated CB composite

0.02 2.85E-5 3.51E+4 1.40E+4

0.06 1.75E-5 5.71E+4 2.29E+4

0.1 1.28E-5 7.78E+4 3.11E+4

0.14 1.03E-5 9.68E+4 3.87E+4

0.18 8.63E-6 1.16E+5 4.63E+4

†: The inverse of the resistance values are the slope of the fitted line in Figure 3-16.

‡: W, the width of the gold electrodes, is 4 mm in all cases.

89

Figure ‎3-17. Calculation of sheet resistance (R□) and contact resistance (Rc): a) SSA-doped PANI, b) SPhA-doped PANI, c) SSA-doped PANI/coated CB composite, and d) SPhA-doped PANI/coated CB composite. The R□ is the slope of the RxW vs. G plot. The intercept is equal to 2xWxRc.

The resistivity (ρ) of a thin film is the product of its sheet resistance and thickness (t):

Equation ‎3-3

AFM was used to measure the average thickness of the layers as shown in

Figure ‎3-18. The sheet and contact resistance values, as well as resistivity and

conductivity (σ) of the polyaniline layers are reported in Table ‎3-4. The conductivity of

polyaniline doped with SPhA is higher than that for SSA-doped PANI. This can be

explained by the higher acidity of the sulphonic group in SPhA compared to SSA due

to the resonance stabilisation effect of the benzene ring. This causes stronger

protonation effect on PANI, and hence higher conductivity. Both hybrid composites

show approximately an order of magnitude lower resistivity compared to their pristine

forms. This is evident since carbon particles at this concentration form continuous

pathways throughout the film, facilitating the flow of electrons.

90

Figure ‎3-18. Examples of thickness measurement using AFM: a) SSA-doped PANI, and b) SPhA-doped PANI/coated CB composite. The thin films on the glass slides were scratched by a scalpel to facilitate the measurement.

Table ‎3-4. Resistivity and conductivity of polyaniline samples.

Samples Rc (Ω) R□ (Ω) t (cm) ρ (Ω.cm) σ (S cm-1

)

SSA-doped PANI 5.48E+5 1.26E+7 1.66E-5 209.09 4.78E-3

SPhA-doped PANI 4.70E+4 1.09E+6 4.05E-5 43.99 2.27E-2

SSA-doped PANI/coated CB composite

4.14E+5 4.02E+6 4.91E-6 19.73 5.07E-2

SPhA-doped PANI/coated CB composite

1.31E+4 2.01E+5 1.86E-5 3.74 2.67E-1

91

3.3.6. Ammonia sensing results

Sensors made from hybrid composites of doped polyaniline and carbon black were

made on plastic substrates with interdigitated electrodes. Figure ‎3-19 shows the

response of a dip-coated sensor from the 1 wt.% SSA-doped PANI/coated-CB mixture

on flexible Kapton substrate toward 3 different concentrations of ammonia vapour. The

heater keeps the sensor temperature at 80 °C. The response is recorded as the

transient change in the resistance value of the sensor relative to an initial value (Ri):

Equation ‎3-4

where R is the resistance of the sensor and Ri is the initial resistance value usually

taken at the beginning of each experiment. The ‘response magnitude’ of each

exposure was calculated based on the relative change in the resistance during the

exposure period:

Equation ‎3-5

where Rmax is the maximum resistance of the sensor during the exposure period (e.g.,

if the sensor is exposed to ammonia for 5 min, the Rmax will be the resistance value of

the sensor after 5 min, irrespective of the equilibrium being reached or not during that

time), and and Rb is the baseline resistance of the sensor, immediately before starting

the exposure to ammonia. As can be seen, the response is fast, recoverable and

repeatable for all three concentrations. The exposure time is equal in all three cases;

however the recovery time (time needed for the sensor to recover back to its baseline

resistance value) varies with the concentration; as the concentration increases from

340 ppb to 1150 ppb, the recovery time increases from about 10 min to 15 min. The

SPhA-doped polyaniline hybrid sensor did not show a measurable response in the

concentration range studied here even at RT condition, so it was not investigated

further. More work is needed to find the optimum concentration of SPhA dopant in

order to achieve suitable sensing performance.

92

Figure ‎3-19. Transient response (as defined in Equation ‎3-4) of a SSA-PANI/coated-CB

dip coated sensor on Kapton toward 340, 550 and 1150 ppb at 80 °C. In all cases

exposure time is 300 sec (5 min), and the sensor has been given enough time to recover to the Rb. An off-set is applied to curves for clarification. The green and red arrows indicate the start and the end point of each exposure, respectively.

To better investigate the sensing performance of our sensor, we compared the

sensing results from the sensor with a commercial MOS ammonia sensor from

Synkera Technologies, Inc. (P/N 705, USA). Sensors were placed in series and

exposed to successive concentrations of ammonia vapour from 425 ppb to 2.25 ppm.

The exposure and recovery times were fixed to 180 and 300 sec, respectively. The

results are shown in Figure ‎3-20 and Figure ‎3-21. Obviously, the hybrid sensor

outperforms the commercial MOS sensor in terms of response intensity and sensitivity.

However, the MOS sensor is faster both in terms of response and recovery. Since the

recovery of the hybrid sensor is not complete in 300 sec (recovery period), a significant

baseline drift is observed. This issue can be solved easily by dedicating more time for

recovery to the polymeric sensor, as can be seen in Figure ‎3-19. Nevertheless, the

power consumption of the hybrid sensor is much lower than the MOS sensor, due to

their significantly lower operating temperature1.

The sensitivity of the hybrid sensors may be increased even more, using spin coating

instead of dip coating as the deposition methods. The response of the sensor operated

1 . Commercial MOS sensors’ minimum operating temperature is 200-300 ºC.

93

at 80 ºC to a series of ammonia concentrations is depicted in Figure ‎3-22. Figure ‎3-23

compares the sensitivity of a spin-coated and a dip-coated sensor based on SSA-

doped PANI/coated-CB on PEN substrate, with that of the commercial MOS sensor.

The sensitivity of the spin-coated device is about 2 times that of the dip-coated and

more than 6 times the response of MOS sensor. The higher response magnitude of the

spin coated sensor compared to the dip coated sensor may be explained by the

difference in the kinetics of ammonia sensing. The ammonia vapour is absorbed more

quickly into the thinner layer of polyaniline (spin coated), hence during a specific

exposure time, the relative response will be higher compared to a thicker layer (dip

coated). Moreover, spin coating is known to be a robust method for the fabrication of

repeatable and reproducible thin film sensors.

These figures demonstrate the great potential of these novel materials for use in real-

time ammonia sensing applications. The high sensitivity of these hybrid sensors is

attributed to two sensing mechanisms: (a) dedoping of the emeraldine salt by the basic

ammonia, and (b) disruption of carbon black conductive pathways throughout the

nanocomposite layer, upon absorption of ammonia molecules by the polymer. These

two simultaneous phenomena are believed to act in a synergistic way, decreasing the

electrical conductance of the composite layer (Sotzing et al., 2000; Lonergan et al.,

1996).

It is important to explore the effect of operating temperature and the substrate

material on the response behaviour of the hybrid sensors in exposure to ammonia. A

SSA-doped PANI/coated-CB mixture was dip coated on Kapton and PEN substrates

and the response to 1 ppm ammonia measured vs. time. These effects are depicted in

Figure ‎3-24. Although the response intensity is higher at RT, it takes hours for the

sensor to recover to its baseline value. Increasing the temperature to 80 °C decreases

the recovery time to minutes, in spite of reduction in the sensitivity. It seems that the

effect of substrate material is not significant in this case. However, when we zoom in

the very beginning of the response curve, a two-step process in response is observed:

an unexpected initial increase in conductivity followed by the expected increase in

resistance. The exact origin of this phenomenon is not known, however it may be due

to the sudden variation in pressure of the sensing chamber or condensation of trace

moisture present in the analyte vapour/carrier flow. Water vapour can be condensed on

the substrate, adsorbed by the polymeric layer, and increase the electrical

conductance either by (a) an increase in charge carrier density of polyaniline or by (b)

mobility enhancement due to the electrolytic properties of water. These two effects

both result in increase of total conductivity according to following equation:

94

Here, σ is the electrical conductivity, which in any system is proportional to the product

of the density of charge carriers (n), the charge carried by each carrier (e) and the

mobility of each carrier (μ); e is the unit electronic charge (1.6 × 10−19 C), ‘n’ is in m−3

and μ is in m2 (V.s)-1 (Bhadra et al., 2009). The extent of resistance reduction in the

initial stage is directly related to the amount of water condensation and hence the

physical properties of the substrate, mainly its hydrophobicity. Important physical

properties of the substrates used in this study have been summarized in Table ‎3-5.

PEN is known to be less hydrophilic than Kapton so the water condensation on it is

less than Kapton. This may rationalise the higher resistance reduction in the sensor

with Kapton substrate.

Figure ‎3-20. Ammonia sensing behaviour of hybrid SSA-doped PANI/coated-CB sensor (dip coated on Kapton; blue line) and Synkera’s MOS ammonia sensor (red line). The

hybrid sensor kept at 80 °C with the sub-miniature ceramic heater. Ammonia

concentrations are indicated as bar graphs (black line) for each measurement. Exposure and recovery durations are 3 and 5 min, respectively.

95

Figure ‎3-21. Comparison of the relative response (as defined in Equation ‎3-5) between the hybrid SSA-doped PANI/coated-CB sensor (dip coated on Kapton; blue bars) and Synkera’s MOS ammonia sensor (green bars). The hybrid sensor kept at 80 °C with the sub-miniature ceramic heater. The values are the average of five measurements.

Figure ‎3-22. Response of the spin coated SSA-doped/coated CB hybrid sensor to ammonia in air. The sensor is heated up to 80 ºC.

96

Figure ‎3-23. Sensitivity enhancement by using spin coating instead of dip coating as the deposition method. The intercept of the fitted lines has been fixed to zero. The error bars correspond to 5 measurements for the dip- and spin-coated devices and 3 repeats for the MOS sensor.

Table ‎3-5. Some important physical properties of the two substrates used in this study. CTE and CHE are the coefficient of thermal expansion and coefficient of hydroscopic expansion, respectively (Adapted from (Thomas Kinkeldei et al., 2012)).

Physical properties of the plastic substrates used in this study

Kapton PEN

Tg 354 ºC 121 ºC

Tm - 269 ºC

CTE 16 ppm/ ºC 18 ppm/ ºC

Solvent stability ++ ++

Transparency - transparent

CHE 8 ppm/%RH n.a.

Water absorption 1.8 % 0.3 %

97

The sensor performance was tested on-field to monitor water quality in terms of the ammonia concentration in treated water as a function of time. The response was compared to the data from a commercial electrochemical ammonia detector installed in

parallel, and the results are shown in Figure ‎3-25.

Response, Δ

R/R

i (%

) R

esp

on

se

, Δ

R/R

i (%

)

Figure ‎3-24. Effect of temperature and substrate material on the transient response of a dip-coated SSA-doped PANI/coated-CB hybrid sensor toward 1 ppm ammonia. The zoom in area (b) shows the very beginning of the response. The exposure time is 3 min.

98

Figure ‎3-25. The comparison of ammonia concentration measurements between a commercial ammonia detector (red curve), and a spin-coated PANI/coated-CB hybrid nanocomposite sensor on PEN substrate working at 80 ºC. The response of the hybrid sensor is scaled to match the first point of the on-site sensing data.

The response of the hybrid sensor was scaled to fit the initial data value of the

electrochemical sensor, using the Equation ‎3-6:

Equation ‎3-6

As can be seen, the hybrid sensor response trend correlates with the on-site data

accurately. The two large peaks of the electrochemical sensor are due to the

calibration of the instrument, and not because of a change in ammonia environment.

These data show the potential of the hybrid polyaniline/coated-CB sensor as a real-

time ammonia sensor, even in high humidity condition.

99

3.4. Discussion

Solution-processable conductive polyaniline formulations have been successfully

prepared using ‘counter-ion induced processability’ concept. Two multifunctional

protonating agents, namely sulphosuccinic acid and 4-sulphophthalic acid, were

employed which have two functional group types: a sulphonic acid group which dopes

polyaniline (converting non-conducting emeraldine base to electrically conducting

emeraldine salt) and carboxylic acid groups which interact with the solvent molecules

and make the complex soluble in aprotic solvents such as NMP. Using the same

concept, other multifunctional materials may be synthesised and used to expand the

range of dopants for PANI. For example we expect the same advantages using

sulphosalicylic acid as the doping agent, since it contains both –COOH and –SO3H

substituents. We have demonstrated NMP as a suitable solvent for such doped PANI

structures. However, solvents like DMSO and DMF showed acceptable results, too. In

fact DMSO is more benign than NMP, so it can replace NMP when

environmental/health hazards are of concern. The only problem is that the solubility of

emeraldine base in DMSO is not as good as in NMP. Therefore, the long-term stability

of the solutions will be lower.

According to TGA, emeraldine salt made by these multifunctional dopants can be

heated to about 150 ºC without the risk of thermal dedoping. This is very important

since ammonia sensors based on polyaniline need to be operated at elevated

temperatures for optimum performance. Hence, using multifunctional dopant may

extend the life-time of the sensors.

Hybrid nanocomposites of polyaniline and surface-modified carbon black particles

were made using the aforementioned doped-PANI solutions in NMP. The proposed

core-shell structure of the surface-treated carbon black nanoparticles are believed to

be responsible for their fine dispersion and uniform distribution throughout the

composite. The same method can be adapted for other carbon additives, as well as

metallic and metal oxide nanoparticles. There are several publications demonstrating

improved properties of composites containing nanoparticles wrapped (physically or

chemically) with polyaniline in polymeric matrices (Xu et al., 2012; Zucolotto et al.,

2004).

Hybrid sensors were operated at an elevated temperature to enhance the recovery.

The responses were fast, recoverable and repeatable in the concentration range

studied between about 200 ppb to 2 ppm. The sensing behaviour at higher

concentrations is yet to be investigated in order to understand the real dynamic range

of the sensors. For the FlexSmell application though, the operation in exposure to sub-

100

ppm to several ppm concentration of ammonia vapour is enough. The substrate type

seems to be less effective on response behaviour, compared to temperature. At room

temperature the recovery is very slow and insufficient for real-time sensing. This is of

great concern since the sensor to be used in the FlexSmell needs to work with low

power consumption. One way to solve this problem is to fabricate low-power heaters.

This issue will be addressed in Chapter V.

The response intensity of the sensors was optimised by changing deposition method

from dip coating to spin coating. Spin-coated devices show sensitivity values 6 times

higher than that of a commercial MOS ammonia sensor, while consuming significantly

less power. Inkjet printing can be used to fabricate layers of the hybrid composite with

nanometric thicknesses on the sensing platform. We can then take advantage of the

enhanced performance of the sensor due to decrease in its thickness. We have

demonstrated the printability of SSA-doped PANI in NMP ink using a micro dispenser

(MD-K-130 dispenser head, Microdrop technologies GmbH) (Figure ‎3-26). However, to

be able to print the hybrid PANI-carbon nanomaterials inks, we need to assess the

long-term stability of the dispersions. Large agglomerations can clog the printer nozzle;

hence filtration of the ink is suggested immediately before use. The pore size of the

filter depends on the ink viscosity and size of the print-head orifice. This has to be

further investigated.

Figure ‎3-26. Successful printing of a conducting ink: SSA-doped PANi in NMP, using MD-K-130 microdrop dispenser. The polymer concentration was 0.5 wt. %. The printing parameters were adjusted to produce a perfect droplet; a 320 mV pulse was applied to the piezo actuator for 72 µs. The droplet diameter in this image is ~ 80 µm corresponding to droplet volume of ~ 268 picolitres.

A major drawback of using multifunctional dopants such as SSA is the strong acidity

of these materials, as well as the presence of the sulphonyl hydroxide moiety, which

make them highly corrosive and incompatible with some electrode metals and plastic

101

substrates. We have observed that the SSA-doped PANI layer attacks silver

interdigitated electrodes on plastic substrates. The reaction occurs immediately after

deposition, and quickly degrades the electrical conduction at the interface of sensing

layer and IDE. This problem becomes more prominent taking into account the

objectives of the FlexSmell project. In the FlexSmell, the primary goal is to reduce

manufacturing cost and complexity. One approach to achieve these goals is to use low

cost materials (such as silver instead of gold as the electrode, and PET instead of

Kapton as the substrate). Using commercially available silver inks and cheap plastic

foils allows us to exploit high throughput sensor fabrication technologies such as

printing. We will try to address this issue in the next chapters by using alternative

methods of synthesis and deposition of polyaniline sensing layers.

In spite of the aforementioned issues, the good sensing performance of the hybrid

sensors under humid condition shows the great potential of these novel materials for

use in real-time ammonia sensing applications such as air and water quality

monitoring.

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Chen, C.-H. (2003) Thermal and morphological studies of chemically prepared emeraldine-base-form polyaniline powder. Journal of Applied Polymer Science, 89(8), 2142-2148.

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Gong, J., Li, Y., Hu, Z., Zhou, Z. & Deng, Y. (2010) Ultrasensitive NH3 Gas Sensor from Polyaniline Nanograin Enchased TiO2 Fibers. The Journal of Physical Chemistry C, 114(21), 9970-9974.

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Jeon, I.-Y., Tan, L.-S. & Baek, J.-B. (2010) Synthesis and electrical properties of polyaniline/polyaniline grafted multiwalled carbon nanotube mixture via in situ static interfacial polymerization. Journal of Polymer Science Part A: Polymer Chemistry, 48(9), 1962-1972.

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Lee, Y. M., Kim, J. H., Kang, J. S. & Ha, S. Y. (2000) Annealing Effects of Dilute Polyaniline/NMP Solution. Macromolecules, 33(20), 7431-7439.

Lin, Y., Zhou, B., Shiral Fernando, K. A., Liu, P., Allard, L. F. & Sun, Y.-P. (2003) Polymeric Carbon Nanocomposites from Carbon Nanotubes Functionalized with Matrix Polymer. Macromolecules, 36(19), 7199-7204.

Lobotka, P., Kunzo, P., Kovacova, E., Vavra, I., Krizanova, Z., Smatko, V., Stejskal, J., Konyushenko, E. N., Omastova, M., Spitalsky, Z., Micusik, M. & Krupa, I. (2011) Thin polyaniline and polyaniline/carbon nanocomposite films for gas sensing. Thin Solid Films, 519(12), 4123-4127.

Lonergan, M. C., Severin, E. J., Doleman, B. J., Beaber, S. A., Grubb, R. H. & Lewis, N. S. (1996) Array-based vapor sensing using chemically sensitive, carbon black-polymer resistors. Chemistry of Materials, 8(9), 2298-2312.

Lu, J., Park, B. J., Kumar, B., Castro, M., Choi, H. J. & Feller, J.-F. (2010) Polyaniline nanoparticle -carbon nanotube hybrid network vapour sensors with switchable chemo-electrical polarity. Nanotechnology, 21(25), 255501.

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Sotzing, G. A., Phend, J. N., Grubbs, R. H. & Lewis, N. S. (2000) Highly sensitive detection and discrimination of biogenic amines utilizing arrays of polyaniline/carbon black composite vapor detectors. Chemistry of Materials, 12(3), 3.

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Xu, J., Yao, P., Jiang, Z., Liu, H., Li, X., Liu, L., Li, M. & Zheng, Y. (2012) Preparation, morphology, and properties of conducting polyaniline-grafted multiwalled carbon nanotubes/epoxy composites. Journal of Applied Polymer Science, 125(S1), E334-E341.

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Zhu, W., Miser, D. E., Geoffrey Chan, W. & Hajaligol, M. R. (2004) HRTEM investigation of some commercially available furnace carbon blacks. Carbon, 42(8–9), 1841-1845.

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CHAPTER IV

4. Vapour-phase deposition polymerisation

4.1. Introduction

As discussed in section ‎2.5.4, one approach to produce conducting polyaniline layers

is in-situ oxidative polymerisation directly on the substrate. There are several ways to

in-situ polymerise aniline. Electropolymerisation can be used to deposit polyaniline on

conducting surfaces such as metallic electrodes. This method often gives uniform

coatings with high quality. However, it is not suitable for large-scale production and

cannot be used when dealing with non-conducting substrates, which is the case for

chemiresistor platforms. Furthermore, the electrochemical polymerisation of aniline is

often carried out in aqueous electrolytes, and less frequently in organic solvents like

acetonitrile (Gvo denović et al., 2011). The direct contact of the substrate with the

solvents during the polymerisation increases the potential of swelling or degradation of

the substrate by the solvents (Bhattacharyya et al., 2012). Conducting polyaniline

layers on insulating substrates can be obtained by direct chemical polymerisation from

the solvent, dopant and oxidant mixture, but the solvent-substrate incompatibility issue

remains. Moreover, the corrosive nature of the oxidising agents in the solution restricts

the fabrication process to a limited number of chemically-stable substrates. These

difficulties may be overcome by applying the oxidant and monomer separately. For

instance, the oxidant can be applied on the substrate by solvent coating and the coated

surface can be subsequently exposed to monomer vapour in the so-called vapour-

phase deposition polymerisation method (Winther-Jensen et al., 2004).

Chemical vapour deposition of ICPs was first reported in 1986 when PPy thin films

were prepared by exposing pyrrole monomer vapour to ferric chloride vapour under

vacuum condition (Mohammadi et al., 1986). It was later adapted for the formation of

105

high quality thin films of other conjugated polymers such as poly(3,4-

ethylenedioxythiophene) (PEDOT) and PANI (Fabretto et al., 2008; Winther-Jensen

and West, 2004; Lock et al., 2006; Bhattacharyya et al., 2012; Cho et al., 2010b; Kim

et al., 2003; Kim et al., 2007; Gozde et al., 2012). One of the major advantages of this

process is elimination of aggressive solvents associated with solution processing.

Since most of the oxidising agents like APS are water-soluble, one can easily employ

solution-based deposition techniques to coat/pattern the substrate with the precursor

layer. This makes a VDP technique compatible with printing technologies. Importantly,

there is no restriction on the type of substrate that can be used in a VDP process as

opposed to electrochemical polymerisation. Also, in contrast to direct oxidative

polymerisation from solution, the substrate is exposed for a much shorter time to

corrosive chemicals. Therefore, inexpensive plastic substrates and high throughput

fabrication technologies can be used. Another major advantage of this method is

elimination of toxic solvents that are usually used in solution-processable PANI

formulations.

4.1.1. VDP process

Vapour-phase polymerisation closely follows the oxidative polymerisation mechanism

in solution (Bhattacharyya et al., 2012) (Section ‎2.5.4). As mentioned, VDP is a two-

step process in which an oxidant/dopant mixture is first deposited onto a substrate, and

the coated substrate is exposed to monomer/dopant vapour in an enclosed chamber

(Figure ‎4-1). The polymerisation can proceed at ambient pressure or under vacuum

condition. The resulting polymer layer is then rinsed with a solvent to remove

unreacted monomers, oxidant and other by-products. The properties of the final film

are strongly dependent on the polymerisation time, temperature, humidity, oxidant

type, precursor layer thickness, monomer flow, presence of additives and the washing

step.

Figure ‎4-1. Two-step vapour-phase deposition polymerisation using solid-state oxidant and dopant.

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Various oxidising agents have been employed in VDP of conducting polymers:

Iron(III) chloride (FeCl3) and Iron(III) p-toluenesulphonate (iron(III) tosylate) and APS

are among the major oxidants used for polymerising 3,4-ethylenedioxy thiophene

(EDOT), pyrrole and aniline. The ferric-based oxidants act as both the protonating

agent and the dopant, eliminating the need for external dopant. However, VDP using

APS results in a conducting polymer only when a dopant is present during the

synthesis. For instance, emeraldine salt has been formed using the VDP with APS as

the oxidant and HCl vapour as the dopant (Cho et al., 2010b). The washing step in

VDP method is crucial in a sense that any remained oxidant or monomer in the

polymeric layer may negatively affect the long-term stability of the conducting layer, as

well as its sensing properties. Bromine (Br2) in vapour phase has been demonstrated

as a novel oxidant/dopant for the formation of stable conducting PEDOT. Due to its

volatility, there is no need for post-processing rinsing step to remove excess oxidant

(Chelawat et al., 2010).

In addition to the synthesis of homopolymers, chemical vapour polymerisation

process has been successfully developed to produce copolymers of conjugated

polymers (Jang et al., 2010; Bhattacharyya and Gleason, 2011; Vaddiraju et al., 2008).

This further extends the possibilities of VDP method for making functional layers. Many

applications of ICP thin films often require patterning of layers. Patterned oxidant layers

have been successfully made on flexible PET substrates by an inkjet printer.

Conducting PPy (Cho et al., 2010a; Shin et al., 2012), PEDOT (Choi et al., 2010) and

PANI (Cho et al., 2010b) patterns are then formed when oxidant-coated substrates

were exposed to the monomer/dopant vapour (Figure ‎4-2).

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Figure ‎4-2. Illustration of a patterning example of conducting polyaniline using VDP-mediated inkjet printing. (1) and (2) are the patterned polyaniline on plasma-treated PET (Reproduced from (Cho et al., 2010b)).

4.1.2. Gas sensors made by VDP method

There are a limited number of publications on ICP-based gas sensors made by VDP

method. Stussi and co-workers (Stussi et al., 1996) reported the first VDP conducting

polymer sensor for detecting C2-C4 alcohols in vapour phase by using patterned

poly(3,3ʹ-diphenoxy-2,2ʹ-bithiophene) on glass substrate. The same group performed

another experiment where VDP-made PPy films with different thicknesses, deposited

on alumina substrate were tested against 20,000 ppm toluene vapour. They observed

an 8-fold increase in relative resistive response upon decreasing the film thickness

from 3 µm to 500 nm (Stussi et al., 1997). Conducting polymer-metal nanoparticle

hybrid films were successfully developed using oxidative chemical vapour deposition

(oCVD)1 for selective sensing of acetone and toluene vapours (Vaddiraju and Gleason,

2010). Functionalised conducting copolymer films of EDOT and thiophene-3-acetic

acid (TAA), poly(EDOT-co-TAA), were deposited using oCVD. Carboxylic groups on

the films were then used to covalently attach palladium (Pd) and nickel (Ni)

nanoparticles by means of 4-aminothiophenol conjugated linker molecules. Used as

chemiresistors, detection limits of 10–20 ppm for toluene using Ni particles and 40–50

ppm for acetone using Pd particles were achieved by the hybrid sensors. Composites

of multi-walled carbon nanotubes and polypyrrole were prepared by vapour phase

polymerisation using FeCl3 as the oxidant. The composite sensor showed reversible

room temperature response to a broad range of ammonia vapour concentrations from

1. oCVD, developed by Gleason and co-workers in MIT is fundamentally similar to the VDP technique, the only difference being that the oxidant is also introduced from vapour phase.

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250-12,500 ppm in N2 (Chen et al., 2007). Kwon et al (Kwon et al., 2012a) have

recently demonstrated the application of the VDP technique to realise ultrasensitive

sensors, based on hydroxylated PEDOT nanotubes on PET substrate. A detection limit

as low as 10 ppt was achieved for dimethyl methylphosphonate (DMMP) as a simulant

for sarin nerve agent. Rapid response times (less than 1 s), fast recovery (in 3−25 s)

and excellent flexibility made the sensor suitable for wearable applications. The same

concept was employed to produce chemiresistors based on conducting PPy nanotubes

on a custom polydimethylsiloxane (PDMS) substrate, capable of detecting ammonia

down to 10 ppb (Kwon et al., 2012b). However the device fabrication process was

rather complicated, requiring dry transfer of the VDP-deposited sensing layer to a

photolithographically patterned PDMS using dry-transfer method, and then depositing

microelectrodes by thermal evaporation on top of the polymeric film.

Here, we have used a two-step VDP technique to synthesise PANI-based sensing

layers in-situ on selected plastic substrates incorporating IDE. Water-soluble polymeric

acids including poly(4-styrenesulphonic acid), poly(acrylic acid) and Nafion® are used

as the dopant with the aim to enhance the thermal stability and film forming properties

(as well as mechanical stability) of the conducting polymer. In this manner, we

eliminate the need for solubilising polyaniline, reducing the environmental risks

associated with hazardous solvents such as NMP. Moreover, highly corrosive dopants

like SSA and SPhA can be replaced by benign polymeric dopants. The VDP process

employed is fully compatible with large scale fabrication methods including printing

technologies.

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4.2. Materials and methods

4.2.1. Materials

Aniline was distilled using a Kugelrohr ball tube distiller (Büchi) and kept in fridge for

further use. The monomer was used for polymerisation up to 2 months following the

distillation. The oxidising agent used was APS which used as received. Three different

polymeric acids were used as protonating agents in the VDP process: poly(acrylic acid)

(MW ~100,000, 35 wt.% in H2O), poly(4-styrenesulphonic acid) (MW ~75,000, 18 wt.%

in H2O), and Nafion (5 wt.% perfluorinated resin solution in lower aliphatic alcohols and

water). All reagents were purchased from Sigma-Aldrich. The aqueous solutions were

made using ultrahigh purity water purified using a Milli-Q 50 system (Millipore Co.).

Figure ‎4-3. Three different polymeric acids used as PANI dopant in the VDP process.

4.2.2. PANI sensing layer preparation

The VDP method was developed to make thin polymeric acid-doped PANI sensing

layers on the target substrates. In the case of PAA and PSSA, a 5 wt.% aqueous

solution of each polymer was prepared by dilution from the commercial solution using

Milli-Q water. For Nafion, a 2.5 wt.% solution was prepared by mixing with Milli-Q

water. 60 mg of the oxidising agent, APS, was then added into 1 ml of each solution to

make homogenous mixture of dopant and oxidant (precursor). The substrates were the

same as the ones used in the Chapter III, unless otherwise stated. Substrates were

cleaned by sequential rinsing with acetone, methanol, isopropanol and water, and dried

with N2 flow. The IDE areas of the substrates were spin coated with the precursor

solution at 3000 rpm for 30 sec. Then, the coated substrates were transferred to a

custom-built VDP chamber (Figure ‎4-4) immediately, where they were exposed to

monomer vapour for 15 min. The reactants container (a 50 ml round-bottom flask) was

heated to 75 ºC during the VDP process using a heating mantel in order to facilitate

aniline evaporation. The optimised reactant contents were found to be 100 and 200 µl

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of distilled water and aniline, respectively. Nitrogen flow (50 sccm) was used as the

carrier gas to transfer reactant vapours uniformly to the reaction chamber. The VDP

glass chamber was kept under ambient conditions (T~20 ºC). No other adjustment was

done for the VDP process. The resulting doped PANI film was removed from the VDP

chamber after the specified duration, rinsed with distilled water to remove unreacted

monomer, oxidant and excess dopant, and finally dried at 65 °C for 1 hr.

4.2.3. Characterisation

UV-visible spectroscopy and AFM were used to study the doping behaviour and

microstructure of PANI films deposited on cleaned glass substrates, respectively. The

same automatic ammonia vapour generation system in the previous chapter was used

to characterise the sensing behaviour of the sensors in dry air. A custom-built

humidifier was utilised to provide moist air in the system whenever needed. A high

precision potentiostat (EG&G 273A, Princeton Applied Research) was used to record

the current of sensor versus time at a constant applied potential.

Figure ‎4-4. The homemade vapour-phase deposition polymerisation system used in this study. During polymerisation, the colour of the coated substrates gradually changes to green, indicative of emeraldine salt formation.

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4.3. Results

We observed that the presence of water vapour is essential in the VDP process.

Without H2O, the synthesis did not result in any conducting PANI. The role of water has

been already highlighted in several papers (Fabretto et al., 2008; Fabretto et al., 2009;

Mueller et al., 2012; Ha et al., 2004; Mohammadi et al., 1986) as an effective proton

scavenger in the polymerisation process of PEDOT and PPy. We believe the same

mechanism is operative for polyaniline, where water deprotonates dimers formed in the

early stages of polymerisation (Section ‎2.5.4). The stabilised dimers then participate in

polymer chain growth.

Usually, electrically conducting polyaniline is made by protonation of emeraldine

base with low molecular weight acids such as hydrochloric acid. However, the electrical

properties of these materials change abruptly with increasing temperature, due to

evaporation or segregation of the dopant (Nicolas-Debarnot and Poncin-Epaillard,

2003). Using macromolecular polymeric acids to protonate polyaniline can enhance the

heat stability and life-time of the layer (Lu et al., 2003). Here, PAA, PSSA and Nafion

are used in the VDP precursor solution to produce polymeric-doped conducting

polymer thin films with good uniformity and high stability.

4.3.1. PAA-doped PANI

UV-Vis transmittance spectrum (Figure ‎4-5) of a vapour-phase polymerised PAA-

doped PANI layer on glass clearly shows characteristic peaks of doped PANI structure:

the peak at ca. 435 nm is due to the presence of localised semiquinone population or

the polaron absorption (equivalent to: polaron band to π* transition). The peak at ca.

810 nm is due to the trapped excitons centred on quinoid (imine) (equivalent to: π- to

polaron band transition) (Tzou and Gregory, 1993; Xia et al., 1995).

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Sensing area of the Kapton substrate with gold IDE (20 µm gap) was coated with a

PAA-doped PANI layer. The conducting layer’s surface was characterised by AFM

(Figure ‎4-6). The layer is highly porous with pore diameter around 500 nm. The pores

are believed to form during polymerisation where the APS molecules react with the

aniline monomer. Assuming there is no dopant involved, the reaction between aniline

and ammonium peroxydisulphate may proceed as follows (Sapurina and Shishov,

2012):

The by-products (sulphates, sulphites and acids) as well as unreacted compounds are

then washed away during the rinsing step, leaving behind semi-spherical pores. The

homogeneity of the pore size and distribution is therefore highly dependent on the

uniformity of the oxidant in the precursor layer. The same concept applies to other

sensing layers made by this method of film fabrication.

Figure ‎4-5. UV-Vis transmittance spectrum of PAA-doped PANI layer fabricated by VDP on glass substrate.

140

250

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Figure ‎4-6. Left: the sensor substrate before and after vapour-phase deposition polymerisation of PAA-doped PANI layer. Right: AFM images of the sensing layer (the scale bar in inset is 500 nm).

The room temperature response of the sensor towards ammonia vapour in dry air is

shown in Figure ‎4-7. Here, the response is presented as the ‘absolute change’ in

electrical current passing through the sensor at a constant potential over time. The

response definition is the same as in Equation ‎3-4, only the resistance values are

replaced by current values (i.e., the response is shown as |(ΔI/Ii)| x 100% = |((I-Ii)/Ii)|x

100%, where I is the current and Ii is the initial value of current. Likewise, the relative

response of the sensor is defined as |(ΔImax/Ib)| x 100%, where Imax and Ib are the

current value at the end of the exposure period and current value immediately before

exposure, respectively. The sensor shows a sensitive and fast response, however the

recovery at room temperature is not complete in the course of this experiment (900 sec

for recovery) especially at the start of the exposure. However, after continuous test for

several hours, the response recovers back to baseline completely, giving enough time

for sensor regeneration. The relative response is linear in the range of 350 ppb to 3.0

ppm with the RT sensitivity equal to 0.734 % ppm-1 (Figure ‎4-8).

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Figure ‎4-7. RT response of the PAA-doped PANI sensor to various concentrations of ammonia vapour in synthetic dry air. The exposure and recovery times are 5 and 15 min, respectively. Ii is the initial value of the current passing through the sensor at time=0.

Figure ‎4-8. Calibration curve of the PAA-doped PANI sensor response under RT conditions. Imax-Ib represents the maximum change in sensor current during the exposure (5 min) to each concentration. The reported values and error bars correspond to the average values over 3 repeats of each measurement.

4.3.2. PSSA-doped PANI

PSSA has been previously used in conducting polymer formulations, due to its high

thermal stability, good proton conduction and water-solubility. PEDOT:PSS is a well-

known example, which is commercialised under the name of CleviosTM (Heraeus,

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Germany), and it is available in the form of aqueous dispersions. Here, we have

investigated the use of PSSA as the counter-ion in the PANI sensing layer deposited

by the VDP on Upilex® substrates incorporating gold IDE (20 µm gap). The sensor

was stabilised in dry air for 1800 sec (Figure ‎4-9), and then heated to ~80 ºC using the

sub-miniature ceramic heater (Figure ‎4-10). The air flow dries the sensor, decreasing

its current following a simple power law equation. The enhancement of the conductivity

by the sorption of water molecules at room temperature is a well-known phenomenon

(Angelopoulos et al., 1987; Matsuguchi et al., 2003; Javadi et al., 1988), which can be

explained based on the presence of “metallic islands” surrounded by an insulating

matrix in the emeraldine salt structure (Angelopoulos et al., 1987). The absorption of

water into the sensing layer greatly decreases the interparticle resistance between the

metallic grains where H2O molecules act as carriers to transfer charge from one chain

to another through a proton exchange mechanism. The hygroscopic nature of the

polymeric dopant plays an important role in this behaviour, since water molecules can

form hydrogen bonds with the dopant chains and PANI backbone itself (Matsuguchi et

al., 2003). Therefore, desorption of water molecules forced by dry air flow causes a

significant drop in conductivity.

Increasing temperature facilitates the charge mobility in the conducting polymer,

enhancing its conductivity. However, the variation in current by heating follows a much

more complex equation (Sakkopoulos et al., 1998) mainly due to the combination of

several phenomena involved in the process, including change in ICP’s grain si e,

proton conductivity of the dopant, water evaporation, structural aging of both polymers,

etc. The exact physical description of the fitted equations is yet to be determined.

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Figure ‎4-9. The sensor was dried under dry air flow (100 sccm). The red line shows the fitted line to data. I is current in µA and t is time in sec.

Figure ‎4-10. The thermal aging of the sensor at 80 ºC. The red line represents the fitted line to the data. I is current in µA and t is time in sec.

In order to assess ammonia sensing performance of the sensors, a sequential

ammonia exposure test was designed and run by computer. Figure ‎4-11 shows the raw

response (transient current change) of the sensor to four cycles of successive

ammonia concentrations. Each cycle comprises of nine concentrations from 175 ppb to

1.75 ppm in dry air , and takes 12000 seconds to be completed. The sensor was kept

117

at ~80 ºC with an external heater as described before. The response is sensitive and

fast, however the recovery is not complete in 15 min and a baseline drift is observed.

Figure ‎4-11. The transient variation in current of the PSSA-doped PANI sensor (raw response) in exposure to ammonia at 80 ºC. The sensor is exposed to nine successive concentrations of ammonia, and this process is repeated 4 times.. The 2

nd measurement

cycle is marked by an orange square for clarification; the x-axis of this marked square has been magnified on the top x-axis; the corresponding concentration profile of each cycle is indicated on the right y-axis, showing ammonia concentrations from 175 ppb to 1.75 ppm in dry air. The applied potential was kept constant at 100 mV. The exposure and recovery durations were 5 and 15 min, respectively for each exposure. (The initial resistance value of the sensor is about 330 kΩ.)

The effect of humidity as a major interfering compound on electrical conductivity and

ammonia sensing properties of polymeric acid-doped PANI has to be investigated in

order to develop a reliable sensor. The sensor was exposed to humid air with different

absolute humidity (AH) levels of 1000 to 3000 mg m-3. The sequential response at

AH=3000 mg m-3 is demonstrated in Figure ‎4-12. Similar to the Figure ‎4-11, the

concentration range in each measurement cycle is from 175 ppb to 1.75 ppm; here, the

measurement is repeated 5 times. The sensor is responsive even under humid

conditiona. Interestingly, although a baseline drift is observed after repeating

exposures, the sensor can be fully regenerated by exposure to clean air for longer

periods.

118

Figure ‎4-12. The raw response of the PSSA-doped PANI sensor in 5 cycles of exposure to a series of ammonia concentrations from 175 ppb to 1.75 ppm in humid air (AH=3000 mg m

-3). Similar to previous figure, the 3

rd repeat cycle is marked by an orange square for

clarification; the x-axis of this orange square is magnified and shown on the top x-axis. The sensor was heated to around 80 ºC to enhance the recovery. The applied potential was kept constant at 100 mV. The exposure and recovery durations were 5 and 15 min, respectively for each exposure. (The initial resistance value of the sensor is about 42 kΩ.)

The initial electrical resistance of the sensor decreased with increase in the humidity

level, from ~330 kΩ in dry air to 42 kΩ in 3000 mg m-3 absolute humidity. Although

humidity enhances the conductivity of the layer, it negatively affects the sensing

properties. The response magnitude, (Imax-Ib)/Ib %, and sensitivity decrease with the

humidity rise (Figure ‎4-13). The response behaviour at this condition can be described

based on competitive sorption of NH3 and H2O molecules. During exposure to

ammonia, the NH3 molecules are able to displace weakly-sorbed H2O molecules,

directly interact with the polyaniline and deprotonate it. However, due to the opposite

effects of water vapour and ammonia on the conductivity, the overall sensitivity

remains lower in humid air compared to dry air (Matsuguchi et al., 2003). The change

in humidity from 1000 to 3000 mg m-3 did not alter the response significantly. The error

119

bars, representing the standard deviation of data, demonstrate the acceptable

repeatability of the sensor operating under dry and humid conditions.

Figure ‎4-13. Effect of AH on the relative response of the PSSA-doped PANI after a 3-min exposure to ammonia vapour at ~80 ºC.

4.3.3. Nafion-doped PANI

Nafion, a perfluorosulphonic acid polymer with PTFE backbone was selected as PANI

dopant due to its well-known thermal and chemical stability (Mauritz and Moore, 2004;

Barthet and Guglielmi, 1996). However, the Nafion-doped PANI composites have been

merely used in fuel cell applications (Tan and Belanger, 2005). Here, we take

advantage of the unique properties of Nafion to prepare room temperature polyaniline-

based ammonia sensor. Due to improved properties of the polyacid in the presence of

water molecules, we expect enhanced sensing properties in humid condition. The

response of the sensor to air at different absolute humidity is compared in Figure ‎4-14.

As described earlier, water vapour increases the current of doped PANI. The highly

hygroscopic nature of Nafion facilitates the absorption of H2O molecules inside the

sensing layer. The response is non-linear, and starts to level off at low humidity.

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Figure ‎4-14. Humidity sensing behaviour of the Nafion-doped PANI sensor at RT. The inset shows the response magnitude.

It is crucial to investigate the effect of humidity on the ammonia sensing properties of

the sensor. The Figure ‎4-15 and Figure ‎4-16 demonstrate the good sensing

performance of the sensor toward low concentrations of ammonia in 5000 and 8000

mg m-3 absolute humidity, respectively. Both the response and recovery are fast at

room temperature. In contrast to PSSA-doped sensors, the presence of water

increases both the response magnitude and sensitivity Figure ‎4-17. The difference may

be attributed to the different microstructure of the two composite films. Cluster

networks are suggested to form in the hydrated Nafion (Mauritz and Moore, 2004).

These nano-channels can absorb and hold water molecules. The water molecules, pre-

sorbed in the sensing layer, result in an increase in conductivity. After introducing

ammonia, NH3 molecules not only interact with polyaniline, but also react with water in

the cluster networks and produce ammonium hydroxide:

Equation ‎4-1

consequently, produced hydroxide ions further deprotonate PANI and increase its

resistance. The higher Rb in dry air may attribute to its lower relative response.

However, there is a limit to this enhancement in response: increasing the humidity from

8000 to 15000 mg m-3 did not change the relative response significantly.

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Figure ‎4-15. Ammonia sensing RT transient response of the Nafion-doped PANI sensor at AH=5000 mg m

-3. The sensor exposed to each concentration until it reaches equilibrium,

and then left in clean air for complete recovery.

Figure ‎4-16. Ammonia sensing RT transient response of the Nafion-doped PANI sensor at AH=8000 mg m

-3. The sensor exposed to each concentration until it reaches equilibrium,

and then left in clean air for complete recovery.

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Figure ‎4-17. Effect of absolute humidity on the response magnitude and sensitivity of

Nafion-doped PANI sensor.

Repeatability of the sensor was examined by exposing it repeatedly to 1.0 ppm

ammonia vapour in dry and humid air over a long period. As can be seen from

Figure ‎4-18, both sensors show recoverable and repeatable behaviour. In fact, the

sensor working in humid condition outperforms the one in dry air: the average

response magnitude in humid air is about two times that in dry air. The standard

deviations were calculated to be 0.65 and 1.56 in dry and humid air, respectively. The

improved performance of the Nafion-doped PANI in high humid environment, together

with room temperature response of this material, makes it very suitable for the

FlexSmell application where the sensor has to operate inside food packaging with very

high humidity. Another important feature of the sensor is cross-sensitivity to other

VOCs. To assess the cross-sensitivity, the change in current of the sensor in exposure

to variety of VOCs in dry air was measured (Figure ‎4-19). The concentrations of all

analytes were kept in the range of 1 to 5 ppth and the relative changes in the current

were compared to that for 2.83 ppth ammonia vapour. The sensor showed only a slight

increase in current in exposure to high concentrations of chloroform, acetone, n-

butylacetate, ethanol and methanol, while the response to ammonia in comparable

concentrations was significantly higher and in the opposite direction. This results show

the high selectivity of the sensor to ammonia.

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Figure ‎4-18. Repeatability test of the Nafion doped-PANI sensor to 1.0 ppm ammonia is shown. The sensor at humid environment works better. The exposure and recovery duration were 1500 and 3000 sec, respectively. An offset in y-axis has been applied to the data for clarification.

Figure ‎4-19. Cross-sensitivity test for Nafion-doped PANI sensor toward ammonia and

several VOCs in ppth concentration range.

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4.3.4. Bio-immobilisation

Using polymeric dopants in the polyaniline can add functionality to the sensing layer.

For instance, the carboxylic acid group on the backbone of PAA can be easily activated

using standard carbodiimide chemistry (Fischer, 2010). We have demonstrated the use

of EDC-NHS1 coupling agent to immobilise biomolecules on the surface of a PAA-

doped polyaniline layer, as proof of concept (Figure ‎4-20).

Figure ‎4-20. The proposed mechanism for immobilisation of a biomolecule (shown by blue circles) on a PAA-doped PANI (shown by green colour) using EDC-NHS coupling

agent.

Here, we used recombinant Green Fluorescent Protein (GFP)2 as a model for

biomolecules in the immobilisation process. The GFP is visible under fluorescence

microscope, so the successful immobilisation can be easily tracked and confirmed. A

glass microscope slide was coated with a PAA-doped PANI by VDP method as

described previously. The carboxylic groups on the surface were activated by

immersion the layer in EDC-NHS solution for 5 hr at RT. The substrate was then

removed from the solution and immediately immersed in a buffer (acetic acid, pH=4.4)

containing GFP, and kept overnight. Finally, the substrate was rinsed with pure water

and dried under nitrogen. An unactivated PAA-doped PANI sample was used as the

control. Figure ‎4-21 shows the result of the immobilisation as observed by fluorescence

microscopy3 and AFM.

1. EDC: 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide, and NHS: n-hydroxysuccinimide. 2. GFPs, isolated from the jellyfish Aequorea victoria, were expressed and purified in house. 3. The fluorescence images were obtained using an Olympus BH2-UMA vertical illuminator.

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Figure ‎4-21. The immobilisation of GFP on PAA-doped PANI layer. Top left: the optical image of the control sample under fluorescence microscope, top right and bottom left: the fluorescence microscopy images of the immobilised GFP on the surface in two difference magnifications, bottom right: AFM image of immobilised GFP showing the protein aggregates.

Clearly, the GFPs are present on the surface as aggregates with several microns in

diameter, instead of single particles. The exact explanation for this phenomenon is yet

to be determined. However, the immobilisation parameters such as the buffer type and

pH, concentration of GFP, submersion duration and temperature, and the EDC-NHS

composition can be optimised in order to achieve better results. GFP can be simply

replaced by other biomolecules such as enzymes, antibodies, receptor proteins, etc. to

fabricate biosensors based on polyaniline with a diverse range of specificity toward

target analytes. The research is ongoing.

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4.4. Discussion

We proposed vapour phase deposition polymerisation for making ammonia sensors on

plastic substrates, because this method offers several advantages over other methods

of polyaniline thin film fabrication. As opposed to conventional in-situ polymerisation

methods (e.g., electropolymerisation), low-cost plastic foils can be used as the

substrate in the VDP. Moreover, polymeric acids can be exploited as the doping agent.

Polymeric acids are less aggressive than strong multifunctional acids often used to

solubilise PANI in aprotic solvents. This expands the choice of electrode material for

the sensor (e.g., less expensive metals like Ag can be used). The possibility of using

silver as electrode, greatly simplifies the IDE fabrication process since silver inks are

commercially available and their printing process is well-established. As shown before,

the VDP process can be performed in ambient condition and there is no need for

complex, expensive high vacuum equipment. Although the sensors here are all made

in a batch process, there are publications that demonstrate the feasibility of a

continuous VDP process as proof of concept (Najar et al., 2007; Xu et al., 1995).

These characteristics make the VDP method fully compatible with low-cost, large scale

sensor fabrication on plastic foils.

The VDP has already been used to make gas/vapour sensors, however to our

knowledge; this is the first report on ammonia sensors based on vapour-polymerised

polyaniline incorporating polymeric dopants. We have shown that polymerisation can

be carried out in the presence of various polyanions including PAA, PSSA and Nafion

as the doping agents. In spite of difference in their chemistry, all sensors detected

ammonia vapour in sub-ppm range. In fact, we observed different sensing properties

by changing the dopant material. The most sensitive sensing layer was the one doped

with PSSA, with response magnitude (ΔImax/Ib) of ~20 % in exposure to 1.0 ppm

ammonia in dry air. However, the sensor had to be operated at elevated temperatures

(~80 ºC) for complete recovery. Using Nafion led to a sensor with faster response and

recovery at room temperature, although the response magnitude was much lower than

PSSA-doped PANI sensors. The presence of water vapour diminished the sensing

performance of the PSSA-based sensor, while it had a positive effect on the Nafion-

based sensor. The enhancement in the response of Nafion-doped PANI sensor to

ammonia under humid condition may be attributed to the microstructure of the

perfluorinated polymer. When hydrated, the sulphonic acid functional groups on the

backbone of Nafion are believed to self-organise into nanometric hydrophilic channels

(cylinders with diameter of ~2.4 nm) (Schmidt-Rohr and Chen, 2008). The interaction of

ammonia and the pre-sorbed water molecules inside these channels produce

127

hydroxide moieties which accelerate deprotonation of polyaniline and increase the

electrical resistance of the sensing layer. This property is of great significance in the

FlexSmell applications, where the sensor may be exposed to high humid

environments. In spite of its interesting ammonia sensing behaviour, working with

Nafion was not easy! The Nafion solution is very expensive compared to PAA and

PSSA solutions. The solution mainly consists of alcohols which are non-solvent for

APS. Thus, we faced some difficulties making homogeneous solutions of APS in

Nafion precursor. Although, these difficulties can be suppressed by adding water as

diluent to the solution or using alcohol-soluble oxidants, the final microstructure of dried

deposited layer is difficult to be controlled due to crystallisation of Nafion during spin

coating. This adversely affects the reproducibility of sensing layers based on Nafion-

doped polyaniline. PAA-based sensors showed the lowest sensitivity in exposure to

ammonia compared to that of PSSA- and Nafion-doped layers. However, the use of

poly(acrylic acid) is of great interest for functionalisation of the sensing layer especially

with biomolecules, in order to realise biosensors with improved specificity toward target

analytes.

Finally, the VDP method described here can be applied to other conjugated

monomers such as pyrrole and thiophene, in order to produce ICP-based sensors with

specificity toward various analytes. The choice of dopant material and oxidising agent

can significantly affect the final properties of the sensor.

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CHAPTER V

5. Polyaniline Ammonia Sensors on Printed Polymeric Hotplates

5.1. Introduction

Low cost integrated-on-plastic chemiresistor devices for chemical sensing require the

development of novel materials suitable for low cost deposition technologies such as

inkjet printing (Briand et al., 2011; Claramunt et al., 2013). Here, we report the

development of printed µ-hotplates on inexpensive PEN polymeric substrate for

resistive detection of very low concentrations of ammonia in sub-ppm range. This work

has been done in collaboration with one of our FlexSmell partners, IMT (EPFL,

Switzerland), and a research group from FEMTO-ST institute (France). The exact

contribution of each partner to this research is given in the experimental section in

details.

For fabrication of inexpensive chemiresistors, intrinsically conducting polymers such

as PPy and PANI as well as their derivatives are attractive due to their high sensitivity,

ease of synthesis and ability to act as chemical vapour sensors under ambient

conditions (Janata and Josowicz, 2003; Persaud, 2005). Poor processability of ICPs

including PANI (Wessling, 1999) restricts the mass production of ICP-based sensors

exploiting common solution-based thin film fabrication methods such as spin coating

and inkjet printing. Hence, we investigated the deposition of thin films of conjugated

polymers by in-situ oxidative polymerisation directly onto a substrate using a VDP

method (Chapter IV). Since the precursors in the VDP process are water soluble, and

there is no restriction on the type of substrate that can be used in a VDP process as

opposed to electrochemical polymerisation, inexpensive plastic substrates and high

throughput fabrication technologies can be used to manufacture chemical sensors.

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As described earlier in Chapter II, our target analyte was ammonia, since a great

need exists for robust and cheap systems for real-time measurement of ammonia at

ppb-ppm levels in environmental monitoring, food processing and medical applications

(Abel et al., 2012; Timmer et al., 2005). PANI has previously been well-characterised

for its sensitivity to ammonia gas (Hu et al., 2002; Wojkiewicz et al., 2011; Liu et al.,

2004). However, the high affinity of PANI for ammonia has drawbacks due to slow

desorption, so that recovery after exposure to ammonia may take a significant time.

This factor, together with baseline drift due to environmental influences, and the

possibility of irreversible reactions in PANI-based sensors, has hitherto impeded their

practical use for continuous monitoring applications. A strategy adapted by some

researchers is to maintain the sensing layer at a higher temperature using external

heaters (Crowley et al., 2008a; Danesh and Persaud, 2012), in order to enhance

recovery and reversibility of sensors based on PANI. This increase in operating

temperature affects the kinetics of binding between ammonia and the sensing layer

and facilitates desorption of ammonia from PANI. It is desirable for manufacture that

heaters be incorporated into the substrate. However fabrication of inexpensive

printable heaters that are reproducible is challenging (Briand et al., 2008; Courbat et

al., 2012).

A conducting polymer chemiresistor on printed polymeric micro-hotplates for

ammonia detection consisting of fully inkjet-printed silver heater and comb electrodes,

separated by a thin patternable dielectric film, and a polyaniline sensing layer was

developed (Figure ‎5-1).

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Figure ‎5-1. Process flow of the micro-hotplates fabrication: (a) Inkjet printing of the heater on the PEN substrate, (b) Lamination of a thin dielectric film on the heater for electrical isolation, (c) Inkjet printing of the comb electrode onto the thin dielectric and (d) Deposition of the polyaniline sensing layer. A 50 or 14 µm thick dry film photoresist was used as the dielectric film laminated onto the heater for the first and the second generations, respectively. Lamination was performed at 85 °C at 2 bar, at a speed of 2 m min

-1. The IDE consisted of two interdigitated combs. The pitch of the electrodes was 120

μm, corresponding to a width of 68 ± 8 μm and an inter-finger spacing of 52 ± 8 μm. The electrodes’ total thickness was 260 ± 50 nm.

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5.2. Experimental section

The sensor consisted of a PEN substrate, a meander-shaped metallic heater, a dry foil

dielectric, interdigitated metallic electrodes and a conducting polymer sensing layer.

Two different generations of µ-hotplates were designed and fabricated by inkjet printing

silver nanoparticle-based ink on PEN foil aiming to minimise the power consumption as

well as the thermal gradient over the sensing area. PEN was selected as a substrate

material because it presented the best trade-off between price and good heat stability,

with a glass transition of 155 °C and a maximum process temperature higher than 180

°C (Camara et al., 2013).

An initial development of a sensor that was 24 mm2 for test purposes, allowed

characterisation of both of the heater designs, the manufacturing process and the

sensor materials. This led to a 2nd generation sensor that was 24 times smaller (1

mm2), used thinner PEN substrate (50 μm instead of 125 μm) and also had a thinner

dielectric dry foil film between heater and combed electrodes (14 μm instead of 50 μm).

This reduction in device size and dielectric layer thickness reduced the power

consumption and the thermal inertia, and minimised heat dissipation.

5.2.1. Heaters

Figure ‎5-2 depicts the two generations of the heater designs that were fabricated in the

IMT, EPFL. The heaters from the 1st generation were designed as large meanders to

simplify printing, whereas the heater belonging to the improved second generation was

smaller in area and presented a symmetrical square double meander shape to

ameliorate thermal homogeneity. The thermal design of the heaters was supported by

finite element method (FEM) modelling done in the FEMTO-ST labs.

These were fabricated by inkjet printing (Dimatix DMP 2800 printer) of a silver

nanoparticles-based ink (SuntTronic et EMD506 from SunChemical) on 50 or 125 μm-

thick PEN (Teonex®Q65FA from Dupont Teijim Films). Figure ‎5-1 documents the

printed electrodes produced. Two layers were printed selecting a drop-to-drop distance

of 25 µm for the first generation of heaters, and three layers at a drop-to-drop distance

of 40 µm were printed to fabricate the second generation. The patterns were sintered in

a convection oven for 3 hr at 180 °C. Electrodeposition of a thin layer of gold on the

silver heater and electrodes for improved robustness was also carried out (Molina-

Lopez et al., 2013; De Koninck et al., 2012).

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Figure ‎5-2. Layout of the different designed heaters: the first generation (top) was designed as a large meander with a total length of ~41 mm. Adjacent tracks had a pitch (centre-to-centre distance) of 600 µm, and 300 µm width on the design layout. The resulting printed lines were wider at 397 ± 6 µm due to the lateral spreading of the ink. The heater thickness was 1.2 ± 0.3 µm. The resistance of the heater was 25 ± 3 Ω and the resistivity was 22 ± 3 μΩ cm. The total surface area of the device was ~24 mm

2. The

second generation (bottom) was of a much smaller size (~1 mm2) and was designed as a

square double meander of total length of ~7.75 mm for improved thermal performance. The line width after printing was 68 ± 8 μm, corresponding to a single drop-wide line. To further decrease the thermal gradient over the surface of the double meander heater, adjacent lines were placed closer to each other at the outermost part of the spiral than at the innermost part (160 µm pitch against 240 µm pitch), compensating for the larger heat dissipation occurring at the edge of the heater, compared to at its centre. A square shape was chosen because round shapes are challenging to inkjet print. The line thickness was 530 ± 90 nm and the measured resistivity of the line was 18 ± 7 μΩ cm. The total resistance of the second generation heaters was 95 ± 7 Ω. The contact pads of the second generation devices were 1.4 cm, to facilitate their connection using a ZIF connector. To minimise the heat dissipation on the long contact pads, they were designed to be much wider and thicker than the heater lines. The pad widths and thickness was 700 μm and 1 μm respectively. The average resistance of the pads was 2.3 ± 0.3 Ω, (negligible when compared to the heater resistance).

5.2.2. Dielectric layer and interdigitated electrodes

Once the heater was fabricated, a dielectric film was laminated and patterned on top of

it by the EPFL partner. The film must be as thin as possible to optimise the heat

transfer from the heater to the sensing layer onto the film. Combed electrodes were

then printed on the dielectric film to complete the fabrication of the transducer

(Figure ‎5-1). Silver is known to oxidise which limits chemical compatibility with many

sensing layers. Therefore, electrodeposition of 400 ± 270 nm of gold on top of the

printed electrodes was adopted to enhance compatibility and stability.

A photo-patternable dry film photoresist (PerMX™ 3000 from DuPont™) (Figure ‎5-1-

b), was laminated onto the heater as a dielectric film. The dry photoresist was

patterned via photolithography to open contact windows on the pads of the device. The

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electrodes were fabricated by inkjet printing over the dielectric with the same silver ink

and they were thermally annealed for 2 hr at 180 °C (Figure ‎5-1-c). Two layers were

printed to form the electrodes, using a drop-to-drop space of 40 µm.

Figure ‎5-3 shows optical images of µ-hotplate examples from both generations,

before deposition of the conductive PANI sensing layer. A device with combed

electrode plated with gold is also depicted.

Figure ‎5-3. Optical images of µ-hotplates depicting examples of both generations, including flexible arrays, close views of a single device and a device with gold-plated

combed electrode.

5.2.3. Preparation of the PANI sensing layer

Once the completed micro-hotplates were received, we deposited the sensing layer

onto the IDE area of the substrates in our lab (see Figure ‎5-1-d). The VDP method

adapted to make a thin polymeric acid-doped PANI sensing layer on the polymeric

hotplates. A 3 wt.% aqueous solution of PSSA was prepared by dilution from the

commercial solution (PSSA, MW ~75,000, 18 wt.% solution in water). 300 mg of the

oxidising agent, APS, was then added into 5 ml of the solution to make homogenous

mixture of dopant and oxidant. The substrate was spin coated with the precursor

solution and then exposed to monomer (freshly distilled aniline) vapour in a custom-

built VDP chamber for 1 hr. The monomer container was heated to 75 ºC during the

VDP process in order to facilitate its evaporation. The resulting PSSA-doped PANI film

on the µ-hotplate was removed from the VDP chamber, rinsed with distilled water and

dried at 65 °C. The process was the same for both generations of the µ-hotplates. All

137

the chemicals were purchased from Sigma-Aldrich. The aqueous solutions were made

using ultrahigh purity water purified using a Milli-Q 50 system (Millipore Co.).

5.2.4. Characterisation

The power consumption and the thermal distribution over the active area of the µ-

hotplates were characterised by the FEMTO-ST partner. The characterisation was

done using type S micro-thermocouples (1.3 µm diameter) mapping surface

temperature over the device area (Thiery et al., 2008; Bontempi et al., 2013), while

different DC currents were supplied to the heater. The results were compared to FEM

simulations carried out using COMSOL (version 4.2) software. For these simulations,

the thermal conductivity of both polymeric films (PEN substrate and dielectric layer

covering the heater) was fixed at 0.78 W m-1 K-1. The convection coefficient was

determined experimentally and fixed at 2.6 x 107 W m-3 K-1. This value fitted well the

experimental results obtained for both generations of substrates.

UV-visible spectroscopy was used to study the doping behaviour of PANI films

deposited on cleaned glass substrates using the VDP method. The transmittance

spectrum was recorded using a Perkin-Elmer lambda 35 spectrometer with a scanning

velocity of 120 nm min−1. A homemade automatic ammonia vapour generation system

was designed and utilised for characterising the sensing performance of the sensors.

The electrical response of the fabricated sensors towards different NH3 vapour

concentrations in dry air was measured as the change in the resistance. Ammonia

vapour over the range of 250 ppb to 3.65 ppm was generated using a permeation tube

with calibrated permeation rate of 1042 ± 14 ng min-1 at 25 ºC. A custom-built

humidifier provided moist air whenever needed.

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5.3. RESULTS AND DISCUSSION

5.3.1. Thermal characteristics of the µ-hotplates

Figure ‎5-4 shows the relationship between temperature and dissipated power for the

two generations of the fabricated heaters. The power ranges from 0 to 350 mW for the

first generation and up to 80 mW for the second generation, enough in both cases to

achieve operational temperatures without damaging the sensing layer. The reported

temperature is that at the centre of the device, on the surface of the dielectric

laminated above the heater. The data for both generations of heaters showed linear

relationship with a gradient of 0.248 ± 0.008 mW °C-1 and of 1.93 ± 0.02 mW °C-1,

respectively, for the 1st and the 2nd generations. The second generation of heaters

exhibited an efficiency of 7.8 times greater than the first one. The generated

temperature for a given power value matched the FEM simulation (Figure ‎5-5-a).

Figure ‎5-4. Temperature measured at the center of the micro-hotplate as a function of the heating power: Comparison between first and second generation.

139

Figure ‎5-5. Thermal gradient at the surface of the second generation of heaters: (a)

thermographic simulation obtained by FEM and (b) thermal profile along x and y axis.

Analysis of thermal distribution over the active area of the devices for the 2nd

generation device is reported in Figure ‎5-5-b. The surface temperature of the µ-

hotplates was registered following two perpendicular lines, along the x and y axes, that

cross the device and pass by its centre. For measurements at 40 mW, a good fit was

obtained between the experimental and simulated thermal profiles. The range of

temperatures along the x and the y axes was 90 °C to 105 °C and 83 °C to 105 °C,

respectively. The thermal gradient over the sensing area presents an almost flat profile

in the middle of the device along both x and y axes with a maximum temperature

difference of 2 °C (between 103 °C and 105 °C) in the x direction and of 4 °C (between

102 °C and 106 °C) in the y direction.

5.3.2. PSSA-doped PANI thin films

Polymeric acids such as PSSA are known to enhance the properties of doped PANI

(see Chapter IV). Here, the same concept has been employed to fabricate PANI

sensing layers by VDP on top of the micro-hotplates. UV-Vis transmittance spectrum of

the deposited PANI layer on glass showed characteristic peaks at 435 and 810 nm

which are entirely consistent with reports on the doped PANI structure: the peak at 435

nm is due to the presence of localised semiquinone population or the polaron

absorption (equivalent to: polaron band to π* transition). The peak at 810 nm is due to

the trapped excitons centred on quinoid (imine) (equivalent to: π- to polaron band

transition).

140

5.3.3. Ammonia sensing properties

5.3.3.1. The first generation sensors

As described before in section ‎2.5.3, emeraldine salt exhibits p-type semiconductor

characteristics; consequently, electron-supplying gases such as NH3 reduce the

charge-carrier (polaron) concentration and decrease the conductivity, in a reversible

chemisorption process (Equation ‎2-2). The 1st generation sensors were fabricated by

depositing thin films of PSSA-doped PANI on the sensing area of the first generation µ-

hotplates using VDP process. Figure ‎5-6 compares the response of the sensor

obtained for consecutive ammonia vapour concentrations over the range of 250 ppb to

3.65 ppm in dry air at different operating temperatures. The sensor was then purged

with air for recovery.

Figure ‎5-6. Response of the 1st

generation sensor towards various concentrations of ammonia vapour from 250 ppb to 3.65 ppm in dry air, at four temperatures: RT, 40, 60 and 80 °C with exposure and recovery time of 10 min and 30 min, respectively. Here, the Ri is the resistance value of the sensor at time=0.

The transient response is defined by the change in the resistance of the sensor

(Equation ‎3-5). Increase in the operating temperature from ~20 to ~80 ºC significantly

enhanced recovery of the response to initial value after exposure to ammonia. This can

be explained by the equilibrium interaction between doped PANI and ammonia

molecules as stated in Equation ‎2-2: heating increases the dissociation rate of

ammonium ion into ammonia and proton, shifting the equilibrium to the left. Hence, the

141

sensor resistance returns rapidly to its Rb due to reprotonation of PANI. The enhanced

recovery also diminishes the baseline drift over time due to faster regeneration of the

sensor at elevated temperatures. Nevertheless, less ammonia is absorbed by PANI

layer at elevated temperatures, so the response magnitude is reduced compared to

room temperature (~20 °C).

To ascertain the effect of temperature on the response performance of the 1st

generation sensors, sensitivity and recovery parameters at each temperature were

calculated, and the results are compared in Figure ‎5-7. Sensitivity, S, is defined as the

slope of the linear curve of the relative response vs. concentration for each

temperature (% ppb-1). Recovery, Rec, for each temperature was quantified based on

the average ratio of recovered response to maximum response ( ) for the

concentration range and time intervals studied here. As the temperature increases, S

decreases exponentially according to the equation obtained from curve fitting

(Figure ‎5-7, solid line): ( ), where T is the

temperature in degrees Celsius. Such behaviour is in agreement with Kukla and co-

workers’ report (Kukla et al., 1996) in which ammonia adsorption onto poly(methyl

methacrylate)-PANI composite was considered as a reversible chemisorption process.

On the other hand, Rec increases linearly from ~45 % at ambient temperature to ~90

% at 80 °C ( ( )) (Figure ‎5-7, dashed line). Thus, heating to 80

ºC results in desorption of the most of the tightly bound ions, followed by sensor

regeneration. Sensors operated at temperatures higher than 80 ºC showed no

measurable response up to 750 ppb.

142

Figure ‎5-7. Effect of µ-hotplate temperature on sensitivity (open square) and recovery (closed circle) of the 1

st generation sensors. The solid curve and dashed line show curve

fits for sensitivity and recovery data, respectively. The error bars represent the standard deviation.

5.3.3.2. The second generation sensors

The same procedure was used to deposit a thin PSSA-doped PANI on the 2nd

generation µ-hotplates’ sensing area. The sensor was exposed sequentially to

ammonia vapour and dry air for 5 min and 15 min, respectively, and the relative

change in resistance was compared at RT and 95 ºC (Figure ‎5-8-a). Although the

heated sensor showed a small drop in sensitivity compared to that of RT, it remained

operative in detection of ammonia at very low concentrations down to 300 ppb. The

average recovery of the heated sensor was improved by a factor greater than 2 when

compared to recovery of the sensor operated at RT (Figure ‎5-8-b). The 2nd generation

sensor showed a significant improvement in sensor performance at low concentrations

of ammonia in comparison to the 1st generation at comparable temperatures while

using much less power. In comparison to 1st generation, the response speed is much

higher in the 2nd generation sensors operated at 95 ºC, reaching equilibrium response

in approximately 5 min (the response of the 1st generation sensor at 80 ºC started to

level off after 10 min). The gold plating of the IDE did not result in any significant

change in response magnitude and response speed of the 2nd generation sensors.

143

Figure ‎5-8. a) The 2nd

generation sensor response magnitude (ΔRmax/Rb) to ammonia during a 3 min exposure at RT (closed square) and 95 ºC (open circle; the heater power consumption was 35 mW). b) Enhancement of the recovery of the sensor at 95 ºC compared to that of RT. The sensor was purged with clean dry air for 15 min after each ammonia exposure.

5.3.3.3. Effect of humidity

Humidity often affects many conducting polymer sensors, so the effect on electrical

conductivity and the ammonia sensing properties of the PSSA-doped PANI was

investigated. The 2nd generation sensor exposed to humid air with different absolute

humidity levels from 1000 to 8000 mg m-3 at RT and when heated (Figure ‎5-9). The

electrical resistance of the sensor decreased with increase in the humidity level, in both

conditions. The enhancement of the conductivity by the sorption of water molecules at

RT is already discussed in section ‎4.3.2. The hygroscopic nature of the polymeric

dopant plays an important role in this behaviour, since water molecules can form

144

hydrogen bonds with the dopant chains and PANI backbone itself (Matsuguchi et al.,

2003).

Figure ‎5-9. Comparison of the humidity response of the 2nd

generation sensors at RT (closed square) and 95 ºC (open circle). The exposure time is 3 min in all cases.

The response of the sensor during exposure to water vapour is much faster than

ammonia, reaching 90 % of the equilibrium value in less than 3 min, following by a

rapid recovery when purged with dry air. In general, the heated sensor shows higher

response magnitudes, while the sensitivity (slope) is lower than that observed at RT.

An explanation for this observation remains to be determined. However, it seems that

the moisture’s influence on the barrier potential between granular metallic emeraldine

salt particles is more pronounced at elevated temperatures, so the relative change in

the resistance is higher at a constant humidity level. This is consistent with Javadi and

co-workers’ report (Javadi et al., 1988) for HCl-doped PANI free standing films. Also,

sensor operation at higher temperatures decreases Rb which may further contribute to

the higher relative response, based on Equation ‎3-5. The reduced sensitivity may be

attributed to the significantly lower partition coefficient of water in the sensing layer at

95 ºC compared to that of RT.

145

Table ‎5-1. Effect of temperature on the 2nd generation sensor response to ammonia vapour in dry and humid air.

The effect of water vapour on the ammonia sensing of the sensor is summarised in

Table ‎5-1. The 2nd generation sensor was exposed to four different concentrations of

ammonia vapour in dry air and absolute humidity of 5000 mg m-3. At RT, the response

magnitude is greater for the sensor working in humid conditions. When operating in

humid air, water molecules sorbed in the sensing layer result in an increase in

conductivity. Following exposure to NH3 molecules, these not only interact with

polyaniline, but also react with water in the layer and produce ammonium hydroxide

(Equation ‎4-1). Hence, hydroxide ions produced further deprotonate PANI and

increase its resistance. Moreover, the higher Rb in dry air may attribute to its lower

relative response. Due to the opposing effects of water vapour and ammonia on the

conductivity, the overall sensitivity remains lower in humid air compared to dry air, in

spite of higher response magnitudes. When heater is operated at 95 ºC, the moisture

content is reduced in the sensing layer. The response behaviour at this condition can

be described based on competitive sorption of NH3 and H2O molecules. During

exposure to ammonia, the NH3 molecules are able to displace weakly sorbed H2O

molecules, directly interact with the PANI and deprotonate it. However, the

Equation ‎2-2 is shifted to the left at elevated temperatures and both the response

magnitude and sensitivity are lower compared to RT condition.

NH3 concentration

(ppb)

Relative Response† (ΔRmax/Rb, %)

Room Temperature 95 ºC (Heater Power=35 mW)

AH=0 AH=5000 (mg m-3

) AH=0 AH=5000 (mg m-3

)

350 1.87 (±0.42) 3.76 (±0.59) 1.59 (±0.56) 1.61 (±0.47)

500 2.89 (±0.56) 4.33 (±0.72) 2.26 (±1.20) 2.05 (±0.61)

1000 5.97 (±0.88) 7.51 (±0.45) 4.86 (±1.40) 3.14 (±0.64)

2000 10.94 (±2.65) 11.49 (±1.31) 8.89 (±1.65) 6.20 (±1.14)

Sensitivity‡

(%/ppb) 0.0053 (R

2=0.9963) 0.0047 (R

2=0.9885)

0.0044 (R

2=0.9964) 0.0028 (R

2=0.9952)

†: Average maximum response of the sensor over a 5-min exposure. The figures in parenthesis are the mean deviations.

‡: Sensitivity is calculated based on the slope of the linear regression fit to the relative response over the concentration range between 350 and 2000 ppb.

AH = absolute humidity

146

5.3.3.4. Cross-sensitivity

To assess the cross-sensitivity to other chemical vapours, the 2nd generation sensor

was characterized (Figure ‎5-10). The sensor showed only a slight increase in

conductivity in exposure to high concentrations of methanol, ethanol, chloroform,

acetone and n-butylacetate (Figure ‎5-10- inset), while the exposure to ammonia in

comparable concentrations (2.83 ppth) resulted in a drastic increase in resistance. This

results show the high selectivity of the sensor to ammonia. Although the sensor

response to these chemicals were fast and reversible, the regeneration of the sensor

after exposure to ammonia at such a high concentration was slow and it took 18 hr for

the sensor to recover to ~90 % of its original baseline resistance, even when heated.

The partial recovery indicates irreversible chemical reactions may have occurred in the

sensing layer at these high concentrations. Sensor poisoning was not observed for the

sensors exposed to low concentrations of ammonia (up to 5 ppm) even over extended

periods.

Figure ‎5-10. Selectivity test of the 2nd

generation sensors: resistance response in exposure to: 1) acetic acid (3.19 ppth), 2) acetone (1.16 ppth), 3) chloroform (2.26 ppth), 4) n-butyl acetate (0.58 ppth), 5) ethanol (3.12 ppth), 6) methanol (4.51 ppth) and 7) ammonia (2.83 ppth). The ordinate on the inset was multiplied by a factor of 50 from the ordinate on the main plot of the figure. The value at the low end and high end of the ordinate of the inset are 3.12 and 4.12 MΩ, respectively. The baseline value in the inset is 3.87 MΩ.

147

5.3.3.5. Sensor stability

The long term stability of the sensors was examined by monitoring their baseline drift

over a three-week period. The 1st generation sensors as well as the 2nd generation

sensors (with and without gold electroplating) were heated to around 95 °C and a flow

of dry air passed over the sensors at a constant rate. The data were recorded after an

initial 24 hr induction period and the results are compared in Figure ‎5-11. All the

sensors showed a continuous decrease in conductance over time. This is a common

phenomenon for conducting polymer-based sensors (Yoon, 2013; Crowley et al.,

2008a) and can be attributed to the gradual oxidation of PANI in the presence of

oxygen molecules in air which results in degradation of electrical conductivity. Besides,

dry air flow in combination with the elevated temperature accelerate the evaporation of

water molecules trapped in the bulk of sensing layer due to the hygroscopic nature of

the polymeric dopant, and cause further drop in conductivity. The baseline resistance

values remained comparatively stable up to 7 days for all three samples; however the

baseline resistance of the devices with silver IDE started to increase monotonically

after one week. Surface oxidation of the silver electrodes may cause this issue.

Moreover, the PSSA may slowly react with the silver at the interface of the sensing

layer and the electrodes, and convert it to silver sulphonate moieties (Figure ‎5-11-inset

a). Heating speeds up these processes which eventually increase the contact

resistance, and the measured conductance of the sensor drops. In contrast, the

chemically inert gold-plated IDE device showed no change in its surface (Figure ‎5-11-

inset b), and baseline resistance was significantly more stable with R/R0=1.928 after 21

days. However, in spite of the baseline drift, even the silver IDE sensors were

operational for at least 2 months.

148

Figure ‎5-11. Baseline drift of the sensors over an aging period of 21 days. The sensors were heated to about 95 °C and dry air passed over them. R0 is the initial resistance on day 1. The inset shows the optical image of the electrodes’ surface on the 2

nd generation

sensors after 2 months: (a) an oxidised silver electrode and (b) an intact gold-electroplated electrode.

149

5.4. Discussion

PSSA-doped polyaniline chemiresistors have been produced on inkjet-printed

polymeric µ-hotplates using vapour-phase deposition polymerisation method for robust

detection of ammonia vapour in sub-ppm concentration range in air. This work was

accomplished through the collaboration between three research groups from IMT in

EPFL, FEMTO-ST and the University of Manchester. The vapour-phase deposition

polymerisation employed here is a two-step method compatible with large scale

manufacturing. For instance, instead of spin coating, one can use printing methods to

deposit the precursor layer (oxidant/dopant) since both the oxidising and doping agents

are water-soluble. In fact there are several examples of such printed devices in

literature.

Two generations of printed micro-hotplates with different dimensions were developed

on PEN substrate. The hotplates were processed using a combination of silver ink-jet

printing and lamination processes compatible with large area manufacturing. The

second generation of devices outperformed the first due to their smaller size, thinner

substrate and thinner dielectric film used, demonstrating the crucial importance of good

printing resolution and structure design. Further reduction in hotplate dimension and

substrate/dielectric thickness can be achieved, which may result in enhanced thermal

and sensing performance of the device.

Both generations of micro-hotplates displayed improved sensing characteristics at

elevated temperatures. However, the devices based on the second generation of

heaters showed the best performance, achieving sensitive, fast and selective ammonia

detection when operated at about 95 °C. The sensors were operational even at a very

high humidity. The electrodeposition of gold on the silver seed patterns has been

demonstrated for improved long-term stability. Nickel is another possibility as modifying

coating for the electrodes.

Other sensing materials can also be made by the same method on the micro-

hotplates. Polypyrrole, polythiophene and other conjugated polymers doped with

various polymeric acids such as Nafion and poly(acrylic acid) can be developed to alter

the specificity of the sensor. Finally, an array of different sensors can be made on a

single plastic substrate with several micro-hotplate platforms in order to realize an

electronic nose for headspace analysis of complex mixtures.

150

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CHAPTER VI

6. CONCLUSIONS & FUTURE WORK

6.1. Conclusions

The primary goal of the FlexSmell project was to realise printable chemical sensing

tags capable of remotely monitoring food quality by means of wireless technologies.

My role as a PhD candidate was to develop chemiresistors based on conducting

polymers on plastic substrates using solution processing methods which are

compatible with large-area manufacturing. The spoilage mechanisms in different food

categories were studied and the major chemical vapours produced during food decay

process were reviewed. Having done that, ammonia was selected as the target analyte

for this study.

Polyaniline-based chemiresistors were successfully made on plastic substrates for

detecting sub-ppm concentrations of ammonia vapour in air. As discussed in Chapter

II, the electrical resistance of emeraldine salt, the conducting form of PANI, changes

upon exposure to basic NH3 molecules due to their influence on charge carriers’

transport inside the polymer. This is the underlying principle which enables polyaniline

to be used as an ammonia sensing layer. However, ammonia detection using

conventional polyaniline sensors is limited to ppm level concentrations. This problem,

together with slow recovery of the sensors after exposure to ammonia, drives the need

for more sensitive and reliable polyaniline-based ammonia sensors. Moreover, like

other conjugated polymers, conducting PANI suffers from poor solution- and melt-

processability which hinders the manufacturing of PANI-based devices.

In this thesis, we have adopted two approaches to produce solution processable

polyaniline formulations that can be used to produce PANI sensing layers. The first

strategy, discussed in Chapter III, was to exploit multifunctional dopants such as

sulphosuccinic acid to simultaneously dope and solubilise polyaniline in NMP. In order

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to enhance the conductivity and sensing performance of these formulations, carbon

nanoparticles were added to the system. Hybrid composites with uniform nanoparticle

dispersion were prepared by mixing a polyaniline solution and surface-modified carbon

nanoparticles. Spin coated PANI hybrid composites on Kapton and PEN substrates,

exposed to sub-ppm concentrations of ammonia in air, showed high sensitivity, rapid

response and recovery, and good repeatability when operated at elevated

temperatures. The sensor outperforms a commercial ammonia MOS sensor (Synkera

P/N 705) in terms of sensitivity, while consuming much less power.

In attempt to enhance the sensitivity even further, and to make the fabrication

process compatible with low-cost organic electronics manufacturing, a two-step

vapour-phase deposition polymerisation method was developed to produce polyaniline

sensing layers directly on a plastic substrate (Chapter IV). Poly(acrylic acid), poly(4-

styrenesulphonic acid) and Nafion were used as polymeric acid dopants in VDP-made

polyaniline sensing layers. All sensors showed sensitive response to NH3 vapour at

sub-ppm level. The PSSA-doped sensor operated at ~80 ºC demonstrated enhanced

recovery and repeatability under both dry and humid conditions. Using Nafion as the

dopant resulted in a sensor operational at room temperature.

Finally, the VDP method was utilised to make stable PSSA-doped PANI-based

ammonia sensors on printed polymeric micro-hotplates with ~1 mm2 sensing area. The

design and development of these devices which allowed reliable operation at ~95 °C

with power consumptions as low as 35 mW were described in Chapter V. The heated

sensor showed sensitive and rapid response in exposure to ammonia vapour over the

concentration range of 250 ppb-3.65 ppm. The effect of water vapour on ammonia

sensing was described based on the competitive sorption of NH3 and H2O molecules

into the sensing layer. The sensor was found to be highly specific to ammonia with

respect to several VOCs including methanol and ethanol.

The methods described in this thesis can be employed to fabricate versatile and high

performing ammonia detectors not only for food, but also for air and water quality

monitoring applications. Such devices may be incorporated in smart RFID tags where

low-cost, low-power consumption, flexibility and compatibility with printing processes

are of crucial importance.

6.2. Future work

Further work is needed to establish the exact dynamic range of both hybrid

composite and VDP-made ammonia sensors. The maximum studied exposure

concentrations were 2.25 and 3.65 ppm for SSA-doped PANI/CB hybrid

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nanocomposite and PSSA-doped vapour-phase deposited PANI, respectively. Apart

from food quality, another potential application of special interest is the sensors’

response behaviour over the range of 10-100 ppm for water quality monitoring

applications. To achieve this, anhydrous ammonia cylinders with standard

concentration (100 or 1000 ppm) can replace somehow-limited permeation tubes in the

automatic vapour generation system. Moreover, the influence of higher humidity values

on the sensing behaviour has to be investigated. The maximum studied absolute

humidity was 15000 mg m-3 which corresponds to 80% relative humidity at 20 ºC. For

many applications where the headspace of a water sample needs to be tested, the

relative humidity values may reach 90-100%.

The optimum operating temperature for the sensors was found to be around 80 ºC.

This value gave a compromise between response magnitude and recovery rate.

Depending on the application, it is possible to establish different operating

temperatures. For instance, if recovery is not critical (e.g., when the sensor is meant to

be used in a disposable alarm system, like in the FlexSmell’s food spoilage detector

tag) the sensor can be operated at room temperature in order to achieve higher

response magnitude/sensitivity. Otherwise, the sensor can be heated to accelerate

sensor regeneration. The maximum operating temperature for the sensors was not

assessed. However, the PSSA-doped PANI made by VDP was operational even at 95

ºC for short term measurements. The good thermal stability of these materials in

comparison to conventional doped polyaniline was attributed to the presence of the

polymeric dopant in the complex. For hybrid composite sensors, the TGA curves of

multifunctional-doped PANI suggest that these materials are stable at temperatures

exceeding 100 ºC. Designed experiments are needed to determine the thermal stability

of the sensors.

The range of multifunctional dopants for hybrid PANI nanocomposites, as well as

polymeric dopants in the VDP process can be extended further. This may result in

enhanced sensing performance and/or specificity of the sensors. The hybrid composite

PANI sensors doped with 4-sulphophthalic acid did not show measureable response to

sub-ppm concentrations of ammonia vapour, while SSA-doped PANI hybrid composite

sensing layers showed sensitivity values of up to ~1.5 % ppm-1. Therefore, the dopant

nature can significantly influence the final sensing properties. A VDP-made PANI

sensor doped with PAA was used as a platform for biomolecules immobilisation as a

proof of concept to fabricate biosensor with high specificity. Although only GFP

immobilisation was realised, there is a huge potential for functionalising the sensing

layer with chemically-sensitive biomolecules, such as enzymes and receptor proteins.

Finally, the same fabrication concepts, i.e. dopant engineering and vapour-phase

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deposition polymerisation can be used to produce sensing layers based on other

conjugated polymers including polypyrrole and polythiophene. Research is ongoing in

this area.

Our preliminary results of the headspace measurement on the raw salmon fish and

boiled egg samples clearly indicate that it is not practical to monitor food quality using a

‘single’ sensor. This is mainly due to the very complex nature of the volatile

composition of the headspace during food spoilage. Although ammonia is reported to

evolve as a result of the deterioration of aforementioned food samples, it is not yet

elucidated at what exact step(s) this volatile is being released; neither is clear the

influence of environmental parameters (like temperature and humidity), as well as the

pH of the medium, on the decay process and ammonia concentration in the

headspace. One approach to overcome this problem would be to utilise a sensing

array consisting of several sensing elements with different sensitive layers and various

transduction mechanisms. Such functionality is envisaged in the final FlexSmell

sensing tag.

In order to realise the final FlexSmell sensing tag, several partners including UMAN,

EPFL, TNO and EKUT have collaborated, and the outcome of the research at each

node has been integrated into the final device. Depicted in Figure ‎6-1 is the conceptual

layout of the FlexSmell platform tag which includes: one or more sensor chips (different

technologies and transducer principles), printed circuitry on flexible polymeric substrate

(PET and PEN), electronic read-out (circuitry, discrete component and silicon chips),

power supply (thin battery) and wireless communication (high frequency HF antenna).

Figure ‎6-1. Conceptual design of the FlexSmell tag.

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Figure ‎6-2. Image of the RFID FlexSmell tag including the sensor chip for temperature

and humidity which connects to the tag by a standard ZIF connector.

The actual fabricated RFID FlexSmell tag (the 1st version), indicating its main

components, is shown in Figure ‎6-2. The main circuitry and antenna of the tag were

fabricated using screen printing of silver-based ink, while the discrete components and

chips were integrated using anisotropic conductive adhesives. The inkjet-printed chip

platform that includes temperature and humidity sensors has already been fabricated

and validated by EPFL and TNO nodes. At the moment, the next version of the multi-

sensing platform including the heater and two resistive sensors, two capacitive sensors

and a resistive temperature sensor is under development (Figure ‎6-3). On a 250 µm-

thick PET substrate the first metal layer (silver-based nanoparticles) including the

capacitors, heater, temperature sensor and resistive sensors (optional – they could be

printed in the second layer on top of heater) was inkjet-printed. Electrodeposition of

nickel was performed on the temperature sensor in order to increase the temperature

coefficient of resistance and passivate the silver. Next, a dielectric layer (dry film

photoresist PerMX 3014) of 14 µm of thickness was laminated at low temperatures (<

85 °C) in order to separate the heater from the sensing devices. The second metal

layer was inkjet-printed, which includes the resistive sensors on top of the dry film and

heater. The capacitive structures are envisaged to be used as differential humidity

sensors (one of them used as reference to remove the effect of the substrate). The

resistive structures are envisaged to be used to detect volatile compounds (e.g., for

detecting ammonia using a conducting PANI layer casted on top of the device). The

Reader

AntennaSensor chip

Read-out

ZIF

Batterycontacts

RFID

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heater is foreseen to be used as a chemical reset, in order to bring the signal back to

the baseline after a specific test. The heater in combination with the resistive sensor for

ammonia detection has already been validated (Chapter V). Validation of more

analytes and analyte mixtures (using different conducting polymers/sensing layers) is

undergoing at EKUT and UMAN.

Figure ‎6-3. Schematic of the inkjet-printed multi-sensor chip platform, including two capacitive sensors, two resistive sensors, a heater and a temperature sensor.

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APPENDICES

APPENDIX I: Food spoilage

As mentioned in Chapter I and II, it is crucial to understand the food deterioration

mechanisms as well as the volatile compounds evolving during this process in order to

develop a gas sensing system for detecting spoilage in its early stages. Here, we

intend to give a brief overview of the two main kinds of food spoilage: microbial

deterioration and (bio)chemical changes.

Microbial deterioration

Food spoilage due to contamination by microorganisms results in production of a wide

range of chemicals including volatile compounds such as alcohols, ketones, aldehydes,

esters, carboxylic acids, sulphur- and nitrogen- containing compounds (Pasanen et al.,

1996). These compounds represent both primary and secondary metabolites. The

microbial spoilage which can lead to putrid, ammoniacal, musty/mouldy odours in a

range of products is mainly due to sulphur-containing volatiles and volatile amines

(Hodgins and Sirnmonds, 1995). Meat and poultry serve as the growth media for many

microorganisms responsible for the food off-flavour. Many factors, such as substrate,

temperature, oxygen concentration, age of the culture and microbial species have been

observed to affect the composition of volatiles.

Volatile compounds can be characterised on the basis of the involved

microorganisms, which are mainly bacteria and fungi:

a) Bacterial volatiles

Volatiles produced by bacteria generally come from the proteolytic degradation of

proteins as well as carbohydrates. Released compounds are generally methylamine,

dimethylamine, trimethylamine and ammonia. They can also be sulphur-based like

hydrogen sulphide, methyl mercaptan, dimethyl disulphide, derived from sulphur

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containing amino acids such as cysteine and methionine (Gibson et al., 1997). Several

types of volatile compounds, including ammonia, amines, indole, and sulphur

compounds are produced by microorganisms such as Pseudomonas spp. growing in

meats. Pseudomonas spp. are one the most common spoilage organisms for milk,

cheese, meat, and fish, due to their metabolic diversity and ability to grow at low

temperatures. Food spoilage due to Pseudomonas may occur in different ways: in

foods of animal origin the non-protein nitrogen fraction will be first metabolized.

Subsequently, the production of lipases or proteases will release fatty and amino acids

which can also result in off-odours, off-flavours and rancidity (Huis in't Veld, 1996).

Some anaerobic organisms such as Clostridium laramie can produce hydrogen

sulphide in beef even at low temperatures. Other bacteria, including Brochothrix

thermosphacta, Carnobacterium spp., Enterobacteriaceae and Shewanella

putrefaciens, produce a cheesy odour in refrigerated beef and pork, which is

associated with the formation of acetoin diacetyl and 3-methylbutanol (Borch et al.,

1996). Lactobacillus spp. and Leuconostoc spp. are lactic acid bacteria and they have

been identified as the major spoiling microorganisms of vacuum-packed meat and

poultry (Belitz H. D., 2004). Off-odours in ice-packed fish and seafood are produced

during spoilage by different microorganisms, including Pseudomonas spp.,

Acinetobacter spp., Moraxella spp. and Shewanella putrefaciens (Chinivasagam et al.,

1998).

Milk and other dairy products are excellent growth media for many types of off-flavour

microbes, such as Pseudomonas fluorescens, P. fragi, P. putida and P. aeruginosa

(Vichi et al., 2008). Pseudomonas fragi produces ethyl butyrate and ethyl hexanoate in

refrigerated milk with a strawberry-like odour. Psychrotrophic strains of Bacillus cereus,

which frequently survive pasteurization and grow in milk at 7 °C, produce a sweet

curdling and then a bitter off-flavour (Wilkes et al., 2000). Hydrogen sulphide,

methanethiol and esters are off-odours attributed to the vegetable spoilage. Not only

off-odours can indicate the presence of bacteria, but also they can be used to specify

the degree of spoilage.

b) Fungal volatiles

Contamination of foods and beverages by yeasts and moulds has been extensively

reported in fresh seafood, packaged meats, milk, cereals and fresh vegetable. The

deterioration of food properties is often due to the production of exoenzymes during

growth. Moulds can produce a vast number of enzymes: lipases, proteases, and

carbohydrases. Once inside the food, these enzymes may continue their activities

independent of destruction or removal of the mycelium. The enzymatic activities may

give rise to flavours like musty odours in wine and dried fruits. This is caused by the

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fungal transformation of 2,4,6-trichlorophenol to trichloroanisole by Penicillium

brevicompactum, P. crustosum, Aspergillus flavus and other species.

Moulds can produce volatiles such as dimethyl disulphide, geosmin and 2-

methylisoborneol, which can affect the quality of foods and beverages even when

present in very small amounts (Filtenborg et al., 1996). The fungi tend to attack

foodstuffs with a high carbohydrate content producing alcohols and esters, such as

ethanol and ethyl acetate, which are considered to be pleasant smelling by humans.

Saccharomyces cerevisiae is one of the most commonly isolated food contaminating

yeasts. During branched chain amino acid catabolism, it forms alcohols like 3-methyl-1-

butanol from leucine and 2-methyl-1-propanol from valine catabolism. Pichia anomala

which is associated with the spoilage of cakes and pastry is further known to produce

large quantities of ethyl acetate which gives an off-odour.

Fungi can degrade triacylglycerols to free fatty acids which provide a carbon and

energy source. The important food spoilage fungi Aspergillus, Penicillium,

cladosporium, and Fusarium have all been shown to produce volatile methyl ketones

and secondary alcohols from medium chain fatty acids (Schnürer et al., 1999). Marked

differences in the production of volatile metabolites have been reported between

closely related species and even between strains of the same fungus (Boerjesson et

al., 1993; Gibson et al., 1997).

Moulds can also spoil food through the formation of toxic secondary metabolites,

mycotoxins. One mycotoxin may be produced by several species, while many different

mycotoxins may be produced by the same mould species. Some volatile compounds

are correlated to the presence of mycotoxins in food. For example, volatile terpenes

are released by Fusarium sporotrichoides during the formation of mycotoxins on cereal

grains and straw (Schnürer et al., 1999).

(Bio)chemical changes

Although microbial contamination is an important cause for food spoilage, natural food

decay occurs even in the absence of any microorganism. Foods contain a number of

constituents which may undergo (bio)chemical changes during preparation and

processing. These constituents may either react with other constituents or simply

degrade into off-flavour compounds. External harsh conditions such as extreme heat,

light and humidity accelerate these processes. The major reactions leading to off-

flavours include: lipid oxidation, photocatalysis reactions, non-enzymatic browning

(Maillard reaction) and enzymatic changes (Intarapichet, 1996).

a) Lipid oxidation

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Edible oils and fats may be of vegetable or meat origin. The deterioration of fats is

usually described as “rancidity” which arises from partial decomposition due to the

action of heat, light, and contamination by certain metals or by the presence of

microorganisms. There are basically two kinds of rancidity known as “oxidative” and

“hydrolytic” (soapy rancidity). Oxidative rancidity arises from the action of oxygen (in

air) on the fats. The rate of non-enzymatic or chemical oxidation of fatty acids is related

to the number of double bounds in the carbon chain of the molecule. Thus fish and

vegetable oils with the greatest proportions of polyunsaturated fatty acids tend to be

less stable to rancidity. Chemical oxidation of fatty acids in food can be catalysed by

light and certain metals, especially iron and copper and the metallo-proteins (e.g.,

hemoglobin, myoglobin, etc.) and even metallo-enzymes (e.g., catalase, peroxidase,

etc.).

Phospholipids have also been shown to easily oxidise or discolour during heating or

prolonged storage. The highly unsaturated fatty acids in the phospholipid molecules

are susceptible to oxidation and lead to the formation of strongly flavoured

decomposition products at early stages of the oxidation process.

Lipid oxidation typically involves the reaction of the molecular oxygen with

unsaturated fatty acids via a free radical mechanism. Lipid hydroperoxides are

normally flavourless and very unstable and break down to produce short chain VOCs

like saturated and unsaturated aldehydes, ketones and alcohols (Intarapichet, 1996).

These primary reaction products can undergo further oxidation if unsaturated, or

secondary reactions to yield a host of off-flavour volatiles. The final products are

generally aldehydes, ketones, acids, alcohols, hydrocarbons, furans, lactones or

esters. The unsaturated aldehydes and ketones have the lowest sensory thresholds

and are therefore, most often cited with being responsible for oxidised flavours.

Methyl ketones, lactones and esters are also formed primarily by hydrolytic reactions.

Methyl ketones contribute a piercing sweet fruitiness. Some aliphatic acids contribute

to the flavour by being sour, fruity, cheesy or animal-like. Their contribution depends on

the number of carbons and their flavour threshold.

The off-flavour due to oxidative changes in meats following the cooking is generally

called “warmed-over flavour” and is basically the result of oxidation of the intramuscular

phospholipid. The compounds responsible for warmed-over flavour are n-hexanal and

2-pentyl furan. These chemicals are the source of the undesirable flavour of many lipid-

containing foods. The 2-thiobarbituric acid (TBA) test is often used to measure

oxidative deterioration of meat fats. An increase in TBA number is found to correlate

with a decrease in phospholipids and phospholipid polyunsaturated fatty acids.

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The most commonly used terms to describe the oxidised flavours in milk are “cabby”,

“cardboard”, “metallic”, “oily” and “tallow”. The polyunsaturated fatty acids in the

phosphatides at the interface of the milk fat globules are considered as the precursors

of the flavours. Most of off-flavours in oxidised milk are from aldehydes group.

b) Photo-induced off-flavours

Off-flavours may be produced via photocatalysis reactions in food. “Skunky” or “sun-

struck” flavour in beer and “burnt feathers” in dairy products are two of the more unique

examples of off-flavours in food due to light exposure. Methional is found to be the

major compound responsible for the activated flavour in milk. Strecker degradation

plays a role in converting methionine into methional, ammonia and carbon dioxide

(Intarapichet, 1996). Free radicals from methionine, methional and cysteine are found

to combine to form methanethiol, dimethyl disulfide and dimethylsulfide.

Like dairy products, the photo-induced change in beer flavour is complex and

involves photo-oxidation of lipids, photocatalysed decarboxylation and deamination of

amino acids and hydrolysis of the isohumulones. Unlike dairy products, however, beer

will take on an initial off-flavour often characterised as being skunky. This skunky off-

odour is attributed to the photocatalysed degradation of the isohumulones in hops,

resulting in 3-methyl-2-butene-1-thiol.

c) Thermally-induced off-flavours

Many flavour compounds found in cooked food or processed foods occur as the result

of reactions common to all types of foods regardless of whether they are of animal,

plant or microbial origin. These reactions take place when suitable reactants are

present and appropriate conditions (heat, pH, and light) exist. The flavours derived

from thermally induced reactions may or may not be desirable. Thermal degradation is

often studied in association with non-enzymatic browning.

d) Non-enzymatic browning

While non-enzymatic browning is very important to the production of desirable flavour,

it is also a primary source of undesirable flavours in food. When food is heat processed

or stored for long periods of time, a number of chemical reactions occur. Among these

is the Maillard reaction. The Maillard reaction is the reaction of an aldehyde (usually a

reducing sugar) with an amino acid in its initial stages, which is promoted by heat. As a

result of the initial interaction of sugars and amines, volatiles are produced, and dark-

coloured polymeric materials arise. This reaction is responsible for many flavours and

colours in foods: the browning of steak, toasted bread and biscuits, chips and roasted

coffee. Generally, volatiles are formed in browning reactions from the interaction of α-

dicarbonyl compounds (intermediate compounds in the Maillard reaction) with amino

acids through the Strecker degradation reaction. Although some flavour compounds

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are acyclic, many are heterocyclic, with nitrogen, sulphur, or oxygen substituent in

common. Some heterocyclic compounds associated with heating or browning of foods

are pyrazine, pyridine, trithiane, γ-pyrone, thiophene, furan, thiazole, trithiolane,

pyrrole, oxazole, 3- thiazoline and tetrahydrothiophene.

e) Enzymatic flavour changes

Direct enzymatic degradation is termed as autolysis and can cause undesirable

changes in colour, texture and flavour in uncooked foods rich lipids and proteins. Two

types of enzymes responsible for lipid degradation are lipoxygenases and lipases

(Wilkes et al., 2000). Lipoxygenase catalyses the hydro-peroxidation of

polyunsaturated fatty acids leading to the formation of hydro-peroxides, which are

subsequently degraded to form a variety of secondary products responsible for the off-

flavour. This enzyme is widely distributed in great variety of plants and animal tissues.

Soybean contains substantial amount of lipoxygenase enzymes. The grassy flavour of

soybeans is due to the linoleic acid hydro-peroxide produced by lipoxygenases

released in the damaged tissues of the bean. Lipoxygenase are present in fish and

meat and they can oxidise polyunsaturated fatty acids and produce short chain

carbonyls responsible for the oxidative off-flavour. Off-flavour can arise from lipase

activity in other types of foods as well. Lipolised flavour is due to the hydrolysis of fatty

acids from triglycerides. Lipases produce a significant off-flavour in foods containing

short chain fatty acid, such as dairy and oil products. The milk lipases catalyse the

hydrolysis of triglycerides and they cause the common flavour decrypted as rancid,

butyric or bitter.

Degradation of proteins by proteases has important implications for the quality of

many foods. These include beneficial effects in the development of desirable features,

such as texture and flavour, e.g. in cheese manufacturing, meat tenderisation, beer

brewing and fish manufacturing. However, protease can also produce unpleasant

flavours. Bitterness in foods is an off-flavour that may occur due the action of

proteinases. This is caused frequently by proteolysis of some peptide and amino acids,

eliciting bitter flavour in soy products, casein and cheddar cheese (Intarapichet, 1996).

In summary, food spoilage is indeed a complex phenomenon, in which a combination

of microbial and (bio)chemical activities may interact. The microbiology of food

spoilage has received considerable attention over the past years; however the major

problem is to find the relationship between the microbial composition and the presence

of microbial metabolites related to the microbial spoilage. Although the detection levels

for the compounds formed during (bio)chemical spoilage are generally low and thus

more accessible, a major disadvantage is that the (bio)chemical processes related to

172

food spoilage appear to be poorly understood. Even less is known about interactions

between microbial and chemical spoilage reactions.

173

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APPENDIX II: Dissemination

Publications

“Development of a New Generation of Ammonia Sensors on Printed Polymeric

Hotplates”, Danesh, E., Molina-Lopez, F., Camara, M., Bontempi, A., Vásquez

Quintero, A., Teyssieux, D., Thiery, L., Briand, D., de Rooij, N. F., Persaud, K. C.;

Submitted to the Journal of Analytical Chemistry; (2014).

“Flexible Ammonia Sensors Based on Polyaniline/Carbon Black Composites

Operating at Elevated Temperatures”, Danesh, E. and Persaud, K. C.; AMA

Association Proceedings - The 14th International Meeting on Chemical Sensors

(IMCS 2012), Nuremberg, Germany; P1.8 (2012) 1134-1136.

[DOI:10.5162/IMCS2012/P1.8.9]

“Printed Micro-hotplates on Flexible Substrates for Gas Sensing”, Camara, M.,

Molina-Lopez, F., Danesh, E., Mattana, G., Bontempi, A., Teyssieux, D., Thiery, L.,

Breuil, P., Pijolat, C., Persaud, K., Briand, D., de Rooij, N.F.; Proceedings of the

17th International Conference on Solid-State Sensors, Actuators and Microsystems

(TRANSDUCERS & EUROSENSORS 2013), Barcelona, Spain; (2013) 1059 - 1062.

[DOI:10.1109/Transducers.2013.6626953]

Conference presentations

“Facile Fabrication of Highly Sensitive Nanoporous Polyaniline-based Ammonia

Sensors on Flexible Substrates by Vapour-phase Deposition Polymerisation”,

Danesh, E. and Persaud, K. C.; the 15th International Symposium on olfaction and

Electronic Nose (ISOEN 2013), EXCO, Daegu, South Korea; July 2013. [Talk]

“Hybrid Composites of Polyaniline/Modified Carbon Nanoparticles Employed as

Highly Sensitive Ammonia Sensors on Flexible Substrates”, Danesh, E; Angione, M.

D., Persaud, K. C.; the Nanosensor Technologies for Monitoring- materials and

methods, the Royal Society of Chemistry, London, UK; Nov. 2012. [Talk]

“Sensors for Volatile Chemicals: Room Temperature Ammonia and Humidity

Sensing”, Danesh, E. and Persaud, K. C., European Chemoreception Research

Organization conference (ECRO 2013), Leuven, Belgium; Aug. 2013 [Poster]

“Fabrication of Polyaniline-based Ammonia Sensors on Plastic µ-Hotplates”,

Danesh, E., Molina-Lopez, F., Camara, M., Bontempi, A., Vásquez Quintero, A.,

Teyssieux, D., Thiery, L., Briand, D., de Rooij, N. F., Persaud, K. C.; Air Quality

175

Monitoring: New Technologies, New Possibilities, the Royal Society of Chemistry,

London, UK; Dec. 2013. [Poster]

“Highly Sensitive Detection of Ammonia Using Solution Processable

Polyaniline/Carbon Black Composite Sensors”, Danesh, E. and Persaud, K. C.; The

International Symposium on Olfaction and Taste (ISOT 2012), Stockholm, Sweden;

June 2012 [Poster]

Awards

Winner of the Wolfgang Göpel Award; the International Society for Olfaction and

Chemical Sensing (ISOCS), July 2013.

Winner of the first prize in the oral presentations at the PGR 2013 conference; the

University of Manchester- School of Chemical Engineering & Analytical Science,

June 2013.