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Sensors and Actuators B 194 (2014) 173– 179

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

journa l h om epage: www.elsev ier .com/ locat e/snb

reparation of flexible VOC sensor based on carbon nanotubesnd gold nanoparticles

ihat Tasaltin ∗, Fevzihan Basariraterials Institute, TÜBITAK Marmara Research Center, 41470 Gebze, Kocaeli, Turkey

r t i c l e i n f o

rticle history:eceived 1 August 2013eceived in revised form2 December 2013ccepted 16 December 2013vailable online 24 December 2013

a b s t r a c t

Novel flexible volatile organic compound (VOC) sensor was prepared by deposition of gold nanoparticle(AuNP) and amine modified multi-walled carbon nanotubes (MWCNT-NH2) on polyimide substrate viaelectrospraying technique. First, interdigitated electrodes (IDE) were fabricated on polyimide (Kapton®)substrate by photolithography. Then, the substrates were subjected to oxygen plasma etching, followedby electrospraying of AuNP and MWCNT-NH2 solution on the substrate. The thin films were character-ized by FE-SEM and AFM as well as electrical conductivity measurement. Chemical sensing behaviors of

eywords:lexible sensorOC sensingulti-walled carbon nanotubeold nanoparticle

the sensors were analyzed against polar (water, propanol and ethanol) and nonpolar (hexane, toluene,trichloroethylene and chloroform) VOCs. In addition, effect of the AuNP/MWCNT-NH2 ratio on the con-ductivity and sensing was investigated. Bending test (100 times) resulted in negligible resistance change,demonstrating the successful preparation of flexible sensor.

© 2013 Elsevier B.V. All rights reserved.

olyimide

. Introduction

During the last decade, fabrication of sensors on flexible poly-er substrates has attracted great interest, owing to their low-cost,

ightness, easy processing, conformability and flexibility [1]. Forexible sensors, polymers including polyimide (PI) [2], polyethy-

ene terephthalate (PET) [3], polydimethylsiloxane (PDMS) [4] andolyethylene naphthalate (PEN) [5] have been evaluated, with poly-

mide considered the most promising, due to its high thermal,echanical and chemical stability.Besides, volatile organic compound (VOC) sensing is a current

hallenging issue because they have great influence on the indoorir quality [6]. Sensing materials have been known to have signifi-ant effect on the sensor performance; therefore, there has beenreat research effort on development of new materials for VOCensing. Most of the commercial sensors in the current marketre based on metal oxides. Although they have good performance,etal oxide based sensors have extreme power consumption

nd poor chemical selectivity. Mentioned problems have pushedesearchers to develop new sensing materials based on organic andomposite materials since they have lower power consumption and

etter chemical selectivity. Phthalocyanine derivatives have beenidely studied in the early works, however, water sensitivity and

low response time as well as the problems in film formation have

∗ Corresponding author. Tel.: +90 262 677 3042; fax: +90 262 677 2309.E-mail address: cihat.tasaltin@tubitak.gov.tr (C. Tasaltin).

925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.snb.2013.12.063

limited their use [7,8]. Consequently, various conducting polymershave been developed [9,10]; however, despite their better perfor-mance, they have stability problems which prevent their long-termuse.

Recent achievements in nanotechnology resulted in develop-ment of nano-scale material based sensors.

VOC sensing with ligand-stabilized gold nanoparticles (AuNP)was first introduced in 1998 by the work of Wohltjen and Snow [11].They have successfully demonstrated that the octanethiol coatedAuNPs were highly sensitive to organic vapors and the responsewas fast and reversible. Since then, several research groups haveshown VOC sensing by coating of AuNPs with various ligands[12–17]. When the VOC was exposed to the AuNP film, the VOCpasses through the pores, which led to swelling of the film [18].It has been demonstrated that the current between neighboringnanoparticles decreases exponentially with increasing width ofthe tunneling barrier, resulting in increased resistance of the films[13,14,18]. Conductivity of these materials has been discussed inthe context of an activated tunneling model as defined in Eq. (1).

� ∝ exp(−ˇı) exp(−Ea

kbT

)(1)

where ˇ, ı, Ea and kb represent the tunneling decay constant, edge-to-edge separation of the metal cores, activation energy, Boltzmann

constant, respectively. The first term defines the tunneling currentbetween the neighboring AuNPs, which decreases with increas-ing particle distance [13]. The second term describes the thermalactivation of charge carriers in which Arrhenius term is inversely

1 nd Ac

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imittcditlw

toifcirf

wCoemrsantdgawcts

Vd(to

74 C. Tasaltin, F. Basarir / Sensors a

roportional to the permittivity of the organic matrix. According tohe equation, conductivity is also dependent of the dielectric con-tant of the analyte which was ascribed to a change in permittivityf the matrix as the dominating component of sensing mechanism14,15].

On the other hand, carbon nanotube (CNT), another interest-ng nanomaterial, was first observed by Iijima [19]. Particularly,

ulti-walled CNT (MWCNT) based gas sensors have attracted greatnterest, owing to their large surface area-volume ratio, high elec-ron mobility and high capability of gas adsorption. In addition,he electronic properties suggest that MWCNTs might transfer theharges efficiently upon exposing to analytes [20,21]. Thus, up toate, MWCNTs have been frequently used to detect gases includ-

ng NOx [22,23], NH3 [24,25] and SO2 [26]. These works exhibitedhat MWCNTs are highly sensitive to the mentioned gases with aow detection limit and short response time. However, VOC sensing

ith good performance was a real challenge with MWCNTs.CNT behaves electrically as a thin-film semiconductor with p-

ype conductivity with the advantage of the sensing propertiesf the individual nanotubes or their bundles/ropes incorporatedn the network [23,27,28]. Increased surface coverage with CNTsorms “spaghetti like” structure which results in increased electrononducting paths between the electrodes [29]. This effect of CNTsn the sensing films can be explained by percolation theory andecent research shows that percolation effect must be consideredor sensor applications [30,31].

From gas sensing point of view, the metallic conductive path-ays of the CNTs are not desired. The electrical conductivity of theNT network can be altered by modifying the surface and sidewallsf the nanotubes. Thus, a strategy of altering the electronic prop-rties of nanotubes is by functionalizing with catalytically activeaterials for enhancing gas sensitivity and imparting selectivity at

educed cross-sensitivity. Several approaches have been attempteduch as introducing functional groups on MWCNTs [32,33] as wells functionalization of MWCNTs with polymer [34,35] and metalanoparticles [36,37]. These approaches have greatly enhancedhe sensitivity of the MWCNT based sensors. Recently, Penza hasemonstrated the fabrication of CNT–AuNP hybrid structure basedas sensors [38]. Enhanced gas response (NO2, NH3, CO, N2O, H2Snd SO2) was exhibited upon deposition of AuNPs on the CNT net-ork. However, CNTs and AuNPs were fabricated by high-cost and

omplex radio-frequency plasma enhanced chemical vapor deposi-ion (RF-PECVD) and sputtering, respectively. Moreover, this hybridtructure has never been utilized for VOC sensing.

In this work, therefore, it was attempted to prepare flexibleOC sensor on polyimide substrate by low-cost solution based

eposition of amine modified multi-walled carbon nanotubesMWCNT-NH2) and AuNPs simultaneously via electrosprayingechnique. First, interdigitated electrodes (IDE) were fabricatedn polyimide (Kapton®) substrate by photolithography. Then, the

Fig. 1. (a) Image of the prepared IDE electrodes on the flexible subst

tuators B 194 (2014) 173– 179

substrates were subjected to oxygen plasma etching, followed byelectrospraying of AuNP and MWCNT-NH2 solution on the sub-strate. Finally, sensor behaviors were measured against polar andnonpolar VOCs.

2. Materials and methods

2.1. Materials

HAuCl4·3H2O, (CH3(CH2)7)4NBr, C12H26S and NaBH4 were pur-chased from Sigma-Aldrich (St. Louis, USA) and used as received.Amine modified multi-walled carbon nanotubes (MWCNT-NH2)was obtained from NANOSHEL (USA) while the flexible polyimidefilm (Kapton®, 100 HN) with 25.4 �m thickness was kindly donatedby DuPont (USA).

2.2. Synthesis of AuNPs

The AuNPs (∼2–3 nm) capped with dodecanethiol were syn-thesized by two-phase reduction method, as reported previously[39].

2.3. Sensor fabrication and measurement

The flexible polyimide substrates were equipped with litho-graphically defined interdigitated electrodes (IDE) in order toinvestigate the electronic and vapor-sensing properties. As demon-strated in Fig. 1a, the IDE consists of 16 finger pairs with thefollowing dimensions; 5 mm electrode width, 100 �m spacing and100 nm electrode thickness). Prior to film deposition, the polyimidesubstrates were rinsed with acetone and ethanol, followed by oxy-gen plasma treatment (30 W, 40 mTorr and 30 s) to make the surfacehydrophilic (contact angle less than 10◦) [40].

AuNP and MWCNT-NH2 were coated on the polyimide substratevia electrospraying technique. As demonstrated in Fig. 1b, the elec-trospraying equipment is basically a two-compartment setup witha sample holder that rotates at a velocity of 1.000 rpm, exposing theIDE to the positive electrospray mist and a negative discharge cloud.The coating voltages were adjusted to ∼+3.5 kV for the needle and∼−1.5 kV for the tungsten tip [7,41]. The polyimide substrates weresupported by glass back panels in order to prevent the curling ofpolyimide film during the coating process.

In order to understand the effect of AuNP and MWCNT-NH2concentration on sensing properties, four different coating compo-sitions have been prepared (Table 1). Typically, 1.25 mg/ml AuNPand MWCNT-NH2 were dissolved in chloroform in different vials

and subjected to sonication in order to increase the dispersion.Then, the sensing materials were mixed in different ratios and thetotal volume was adjusted to 2 ml by adding chloroform, followedby coating 400 �l solution on the substrate. The morphology of

rate and (b) schematic diagram of the electrospraying system.

C. Tasaltin, F. Basarir / Sensors and Actuators B 194 (2014) 173– 179 175

Table 1Sensors and related compositions of sensing materials.

Sensor number AuNP (ml) MWCNT-NH2 (ml) Solvent (ml) Conductivity ofthe sensors (S/cm)

I 1 – 1 1.2 × 10−7

II 1 0.5 0.5 4.2 × 10−3

III 1 1 – 3.1 × 10−2

IV 0.5 1 0.5 1.2 × 10−1

Table 2Properties of analytes: saturation vapor pressure at −10 ◦C, the tested concentration range, the saturation vapor pressure at the environmental temperature of 22 ◦C ascalculated using Antoine’s equation.

Analyte Dielectric constant (ε) Dipole moment (�) Concentration (ppm) P◦ (−10 ◦C) ppm P◦ (22 ◦C) ppm

Min. Max.

Hexane (HEX) 1.89 0.08 860 5,160 34,600 177,130Toluene (TOL) 2.38 0.26 120 1,200 4,920 33,126Trichloroethylene(TCE) 3.40 0.81 550 3,100 15,800 87,000Chloroform (CHL) 4.81 1.15 1,150 5,800 46,200 220,000

t6

2

(tiicwd(Tltiso(6Gv

Propanol (PROP) 17.90 1.66

Ethanol (ETH) 24.50 1.69

Water 80.10 1.85

he coated sensing materials was characterized by FE-SEM (JEOL3335F JSM) and AFM (Quesant Ambios, Q-Scope).

.4. Sensor measurement

Chemical sensing behaviors were analyzed against polarwater, propanol and ethanol) and nonpolar (hexane, toluene,richloroethylene and chloroform) VOCs. The gas stream contain-ng VOC vapor was generated from cooled bubblers that weremmersed in a thermally controlled bath with synthetic air as thearrier gas. The gas stream saturated with the analyte was dilutedith pure synthetic air to adjust the gas concentration to theesired amount by using computer driven mass flow controllersMKS Instruments Inc., USA) at a constant flow rate of 200 ml/min.ypical experiments consisted of repeated exposure to the ana-yte gas (10 min) and a subsequent purging with pure air (10 min)o reset the baseline. The concentrations of each VOC were variedn the range of 100–5000 ppm (Table 2). The temperature of theensor chip was kept at room temperature (22 ◦C) with the helpf temperature controller (Lake Shore, USA). Electrical resistances

DC) were measured with a programmable electrometer (Keithley17). Instruments were controlled and read by computer using aPIB interface. For a better comparison, the responses of differentapor pressures’ relative concentrations pi/p0i are used, where pi

Fig. 2. (a) Energy dispersive spectrum

132 660 2,650 22,300460 2,300 9,200 66,2004,730 25,260 – 31,578

is the actual analyte concentration and p0i is the saturation vaporpressure at the measurement temperature [41].

2.5. Bending test

To investigate the mechanical flexibility of the sensor, bendingexperiment was performed by directly bending the sensor to 90◦

manually by hand. The test was at least repeated 100 times andresistance change was investigated.

3. Results and discussion

3.1. Morphological and electrical characterizations

It is well known that the sensing layer plays a crucial rolein the sensor and its structural and chemical characteristics willaffect directly the sensing performance. Sensing layers based onAuNP and MWCNT-NH2 has been analyzed by using AFM and SEM.According to Energy Dispersive Spectrum (EDS) and SEM analysis,it has been seen that MWCNT-NH2 and AuNPs were successfullycoated on the substrate (Fig. 2a and b) and EDS analysis indi-

cates that the coated film consists of Au and C atoms. The SensorI consisting only AuNPs demonstrated very smooth surface andinterconnected network structure but due to their small size, it wasdifficult to distinguish AuNPs (Fig. 3a). The coating thickness was

, (b) SEM image for Sensor III.

176 C. Tasaltin, F. Basarir / Sensors and Actuators B 194 (2014) 173– 179

sors (I

mnwwnmtstm

otiiiowtai

Fig. 3. FE-SEM images of Sen

easured as ∼200 nm and we assume that AuNPs were homoge-eously coated with MWCNT-NH2 on the substrate and the AuNPsrapped the surface of CNTs. As the concentration of MWCNT-NH2as increased in the coating solution (Table 1), the density of theanotubes increased in the sensing layer, as evidenced by FE-SEMicrographs (Fig. 3b–d). This also correlates well the with conduc-

ivity increase with increased CNT concentration. In addition, theensor surface was compared with FE-SEM before and after VOCreatment. However, as shown in Fig. S1, no change in the film

orphology was observed.Current–voltage measurements were carried out in the range

f ±1 V and steps of 0.05 V under dry air at RT. Conductivity ofhe sensing layers strictly depend on ratios of the components asndicated in Table 1. According to I–V curve (Fig. 4a), conductiv-ty of layer increases with increasing MWCNT-NH2 concentrationn the film which can be attributed to the increment in numberf electron pathways. In addition, the film conductivity increase

ith CNT concentration is a clear proof of being under percolation

hreshold. Also, we believe that the MWCNT-NH2 behave as par-llel resistors, which led to decrease in total resistance as shownn Fig. 4b.

Fig. 4. (a) I–V Curve of the Sensors (II–IV) an

–IV), (a) I, (b) II, (c) III, (d) IV.

The conductivity of the Sensor I is consistent with the previ-ous reports [13,18]. However, the conductivity increase is not sohigh although there is significant increase in the concentration ofMWCNT-NH2. This can be explained by the formation of defects onthe CNTs during the surface modification as well as the insulatingproperty of the organic groups on CNTs. However, metallic bareAuNP and CNT as well as the insulating organic groups on themhave strong influence on the total conductivity of device. The elec-trical conductivity in the Sensors II–IV occurs via tunneling betweenMWCNT-NH2’s and AuNPs have negligible effect.

Unfortunately, the conductivity data of Sensor I was notincluded in Fig. 4a, due to the large difference between the oth-ers. The CNT added films are at least ∼100 times more conductivethan Sensor I, which illustrates the dominant role of CNTs in totalconductivity.

3.2. Sensor responses

Hereby, sensor responses were indicated by resistance changebetween the response and baseline (�R = Rresponse − Rbaseline) inorder to show the resistance change numerically. For Sensor I

d (b) equivalent circuit model (I–IV).

C. Tasaltin, F. Basarir / Sensors and Actuators B 194 (2014) 173– 179 177

oncen

(NlavddtAtri

cnuiif

owgsMatI

Fig. 5. Sensor responses at various c

Fig. 5a) case, the film resistance increased upon analyte exposure.evertheless, the AuNP film response is more significant for nonpo-

ar analytes (HEX, TOL, TCE, CHL) than polar counterparts (PROP). Inddition, the films do not have any response to water and ethanolapor. Consistent with the previous work, the analytes with lowielectric constant (nonpolar VOCs) seems to have a higher ten-ency to increase the resistance [18]. This can be attributed tohe good interaction of nonpolar VOC vapors with alkyl chains onuNPs. As mentioned in the Introduction part, the distance between

he AuNPs increases during vapor sorption, which led to increase inesistance. Also it is known that length of the alkyl chain has strongnfluence on the electronic properties of the AuNP film [12,18].

The characteristics of the sensors changed significantly by thehanging the ratio of the AuNP and MWCNT-NH2. It is worthy toote that base-resistance (resistance values of without gases) val-es decreased and the magnitude of sensor response increased by

ncreasing the concentration of MWCNT-NH2. In addition, as shownn Fig. 5b–d, the direction of the polarity of the sensors has changedor Sensors II–IV by addition of MWCNT-NH2.

Hereby, we assume that sensing mechanism for the Sensors II–IVccurs in two stages. In the first stage; AuNPs that are believed torap the MWCNT-NH2 behaves as the filters and determine the

ases that will reach to the surface of MWCNT-NH2. In the secondtage; interactions of the VOC’s and organic functional group on

WCNT-NH2 determine the sensing behavior. For instance, AuNPs

re assumed to adsorb HEX vapor and do not allow it to reacho MWCNT-NH2, thus very small resistance change was observed.ncreasing the polarity of the VOCs (Table 2) has led to an increase in

trations (a) I, (b) II, (c) III and (d) IV.

resistance change, which is applicable for TOL, TCE and CHL. How-ever, for the polar analytes with higher dielectric constant (Water,ETH, PROP) almost no sensor response was detected because AuNPsdo not allow the passage of the said analytes. The assumption isalso consistent with the sensor response, as shown in Fig. 5a. Thehumidity influences on the sensors are illustrated in Figs. S2 andS3, respectively.

The VOCs that were allowed to reach MWCNT-NH2 surface arebelieved to inject electrons to the conductive path, thus, decreasedthe resistance of the devices since amine groups are known withtheir good electron-withdrawing property. For Sensors II–IV, thesensor responses are influenced by two facts; (1) swelling effect ofthe alkyl chains on AuNP and (2) electron injection to the conduc-tive path (interaction between VOC and sensing layer). However,the magnitude of the �CNT was much higher than the �AuNP whichhas resulted in decreased resistance (see Fig. 5b to d).

Previous report by Valentini demonstrated that CNTs containingdefects exhibited a greater sensitivity towards analytes comparedto defect free sensors [42]. This was explained by the interaction ofthe defect sites with the gases via weak charge, hydrogen bound-ing and �–� interaction and large charge transfer area. This wasmainly attributed to the strong sp2 carbon–carbon molecular bond-ing within CNTs which renders defect free CNTs less sensitive to gasmolecule bonding. In our report, we believe that the attached amine

groups on the MWCNT produced some defects sites along the side-wall of the CNTs. Thus, NH2 groups and defected CNTs might havean action during the sensing of VOCs. For Sensor II, MWCNT-NH2surface was supposed to be wrapped by the AuNPs and hence, VOC’s

178 C. Tasaltin, F. Basarir / Sensors and Actuators B 194 (2014) 173– 179

Fo

cewtdap

cntwdwA

3

canastsaccLbfit

Fig. 7. A biplot of loadings and scores in the PCA.

ig. 6. (a) Schematic drawing of the proposed sensing mechanism and (b) responsef Sensor II for TCE.

ould not reach to the CNT surface. Therefore, there is no influ-nce of defected surface on sensor signal (see Fig. 6a). However,hen the concentration of MWCNT-NH2 was increased (concen-

ration of AuNPs decreased), the possibility of VOCs to reach theefects was increased, which led to decreased response magnitudes proposed by Watts [43]. The sensor response decreases inverselyroportional to the ratio of MWCNT-NH2 (see Fig. 6b).

Several research groups have systematically evaluated CNTomposites as sensing materials [35,36,44–46]. Sensing mecha-ism of those sensors was explained by the interaction betweenhe gas and composite structures and influence of composite ratioas generally ignored. This is the fundamental difference thatistinguishes this paper from the previous works. In addition,e have carried out PCA analysis to understate the influence ofuNP/MWCNT-NH2 ratio on the sensor characteristics.

.3. Data analysis

The similarity of the sensors was explored using the prin-ipal component analysis (PCA). PCA is a powerful method fornalyzing, classifying and reduction the dimensionality of theumerical datasets in multi-vitiate problem [47,48]. This methodllows the similarities or dissimilarities among the individual sen-ors to be explored according to their responses. The goal ofhe evaluation is to identify the possible variations in the sen-or characteristics caused by the changes of ratios of the AuNPnd MWCNT-NH2 on the IDE transducer. To obtain the best PCAlassification, each sensor must have a different sensing profileompared to others. The PCA was performed with the help of MAT-

AB and the sensor response after normalization and centering. Aiplot of the loadings and scores in the PCA is shown in Fig. 7. Therst two principal components contain approximately 97% of theotal variance.

Fig. 8. Influence of bending on resistance of Sensor II.

All the VOCs except Hexane are localized in the PCA map asshown in Fig. 7. The sensors with metamorphosed structures ofsensing coating formed by the same material generate differentsensor responses. These results could help to create a sensor arrayprepared using the same sensing material but having different sen-sor profiles. On the other hand, the Hexane response is disposed bythe negative effect of the MWCNT-NH2 (Fig. 5b–d).

3.4. Bending test

Bending test was manually performed by hand at 90◦ up to100 times and the I–V curve was obtained in order to understandthe influence of bending on sensor resistance. Minor resistanceincrease was obtained for Sensors I–IV and only data for Sensor II isillustrated as representative (Fig. 8). The small difference betweenthe resulting characteristics clearly indicates that bending of thesensor did not significantly affect its sensing performance.

4. Conclusions

Preparation of flexible VOC sensor based on MWCNT-NH2and AuNPs via solution based process was successfully achievedin this work. Highest gas sensitivity to CHL and TCE has beenobserved by MWCNT-NH2 and AuNP composite films at RT. Sensingmechanisms was modeled for electrical charge transfer betweenMWCNT-NH2 networks and adsorbed VOC molecules. We have sys-tematically investigated the effect of sensing layer composition onthe sensing mechanism. Minor resistance increase was obtained forSensors I–IV upon bending 100 times manually.

Acknowledgement

The authors would like to acknowledge the financial sup-port from FP7 (Grant No: PEOPLE-2011-CIG-303779), TÜBITAK

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ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.snb.2013.12.063.

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Biographies

Cihat Tasaltin received his Ph.D. degree from the Department of Physics of theGebze Institute Technology in Turkey. He is a researcher at the TUBITAK MarmaraResearch Center, Materials Research Institute, Gebze, Turkey. His research activitiesare in the field of chemical gas sensors using acoustic transducers for health, safetyand environmental applications as well as the investigation of sensing properties oforganic and inorganic materials.

Fevzihan Basarir is working as senior researcher in Materials Institute of

TUBITAK Marmara Research Center. He has received his M.Sc. and Ph.D.degree in materials science from Gwangju Institute of Science and Tech-nology (GIST) in 2004 and 2011, respectively. His research interest includesdevelopment and assembly of nanomaterials for electronics and optoelectronicapplications.