Molecular imprinted polymer films on RFID tags: a first step towards disposable packaging sensors

Phys. Status Solidi A 210, No. 5, 938–944 (2013) / DOI 10.1002/pssa.201200743 p s sa

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Molecular imprinted polymer films onRFID tags: a first step towardsdisposable packaging sensors

Dieter Croux*,1, Tim Vangerven1, Jeroen Broeders1,2, Jan Boutsen2, Marloes Peeters1, Stijn Duchateau1,2,Thomas Cleij1, Wim Deferme1,2, Patrick Wagner1,3, Ronald Thoelen1,2, and Ward De Ceuninck1,3

1 Institute for Materials Research, Hasselt University, Wetenschapspark 1, 3590 Diepenbeek, Belgium2Xios University College, Agoralaan Gebouw H, 3590 Diepenbeek, Belgium3 IMEC, Division IMOMEC, Wetenschapspark 1, 3590 Diepenbeek, Belgium

Received 21 September 2012, revised 30 November 2012, accepted 3 December 2012

Published online 18 February 2013

Keywords biomimetic sensors, molecular imprinting, screen-printing, wireless sensors

*Corresponding author: e-mail [email protected], Phone: þ32 11 26 88 04, Fax: þ32 11 26 88 99Web: http://www.uhasselt.be/IMO

Molecular imprinted polymer (MIP) sensors offer a high

potential in the development of cheap small-scale disposable

biomimetic sensors. Molecular imprinting leads to the for-

mation of inert polymer particles with nanocavities, which can

exhibit similar selectivity and specificity to target molecules as

antibodies or enzymes. These sensors open up many possible

applications in the field of mass-market consumer products

such as food packaging sensors. One such application is the

detection of histamine in spoiled fish, which causes scombroid

poisoning, a common seafood poisoning.

This contribution provides one possible solution for easing the

use of these sensors in field applications.A screen-printed short-

range wireless MIP-based biosensor based upon passive radio

frequency identification (RFID) tags was developed as a proof

of principle. Histamine molecules binding to an MIP recog-

nition layer induce a dielectric change in the sensor capacitance,

resulting in a resonance frequency shift that is transmitted by

inductive coupling. This wireless sensor is capable of detecting

histamine concentrations as low as 50 nM at a range of a few

centimeters.

� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Suited for awide variety of interestingbiological targets [1] and compatible with many differentmeasurement techniques such as gravimetric sensing [2] andimpedance spectroscopy [3], Molecular imprinted polymers(MIPs) offer huge potential in the development of cheapsmall-scale disposable biomimetic sensors. These syntheticbiomimetic polymers with imprinted nanocavities formpreferential binding sites for a desired target molecule. Aftercareful optimisation their synthesis is usually comparativelystraightforward and based on conventional polymer chem-istry [4, 5]. During this synthesis a mixture of functionalmonomers and target molecules will form a pre-polymeris-ation complex due to the non-covalent interactions betweentarget and functional monomer. Once stabilised in a suitableporogen, crosslink monomers are added to create a stablematrix. After subsequent radical polymerisation a bulkpolymer is obtained containing the embedded targets.Subsequently the target is extracted from the polymer matrix

resulting in nanocavities, which act as the complementarybinding sites. MIPs have already been proven to beextremely suited for the detection of various low molecularweight targets such as histamine [6, 7], L-nicotine [8],serotonin [9] and adenine [10]. They have also been founduseful with antibody [11] and bacteria [12] templates. As anadded bonus they are robust and completely inert in manydifferent environments and within a wide temperature andpH range [13]. A complementary non-imprinted polymer(NIP) reference is often employed in MIP-based detection.Synthesised identically as the MIP but without the templatemolecules, this polymer lacks the imprinted nanocavitiesand thus allows actual target detection to be easilydistinguished from non-specific binding events in biologicalsamples and environmental noise.

One important application for such biomimetic sensingcould be found in the packaging industry: cheap disposablesensors incorporated into the commercial packaging of


Phys. Status Solidi A 210, No. 5 (2013) 939

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Figure 1 Equivalent circuit model of the sensor tag readout, withthe antenna inductor on the left and the sensor tag on the right.

foodstuff. Such sensors should be easy to use and cheap tomass-produce. Ideally, their size is as small as possible foreasy integration and the sensors are required to be easilydisposable with the rest of the packaging.

Scombroid fish poisoning is an allergic reaction thatoccurs within a few hours after eating fish contaminated withhistamine. It is a very common type of seafood poisoning,and most commonly reported with fish from the Scombridaefamily such asmackerel or tuna, but also with related speciesof fish that were inadequately preserved after being caught[14]. Symptoms consist of skin flushing, throbbing head-ache, oral burning, abdominal cramps, nausea, diarrhoea,palpitations, a sense of unease, and, rarely, collapse or loss ofvision. These symptoms can show within minutes and rarelyexceed 12 days. At temperatures above 16 8C in contact withair, naturally present histidine is converted to histamine viathe enzyme histidine decarboxylase produced by entericbacteria. The presence of 50 parts per million (ppm) or morehistamine in fish is indicative of decomposition whether ornot there is gross evidence of spoilage, such as bad odour,and is not destroyed by normal cooking temperatures. Whenillness results, histamine levels have commonly been at least200 ppm and often greater than 500 ppm [15]. The sensitivedetection of histamine in the small amounts of liquid presentin packaged fish could thus give early warning indication ofspoilage setting in and aid in the prevention of scombroidfood poisoning.

As proof of principle a disposable wireless MIP-basedpackaging sensor tag for histamine will be presented in thiswork. The sensor principle is based upon inductive couplingwith a resonating inductor–capacitor (LC) circuit, similarto the humidity sensor presented by Tan et al. [16]. Thisproposed biosensor would be the first step on the roadtowards a disposable packaging sensor for the prevention ofscombroid poisoning.

2 Experimental2.1 Resonant inductive coupling Combining the

basic principles of the resonator behind a gravimetric sensorwith the change in layer capacitance commonly used inelectrochemical impedance spectroscopy, a new sensortechnique can be created based upon the tuned LC circuit.Often used in filters, tuners and frequency mixers, a parallelcircuit between an inductor and a capacitor exhibits a self-resonance at a certain frequency given by

www

v ¼ffiffiffiffiffiffi1

LC

r) f ¼ 1

2pffiffiffiffiffiffiLC

p : (1)

L

Using a nearby second inductor, this resonancefrequency can be remotely detected through air or interven-ing materials due to the mutual inductive coupling Mbetween the two inductors, a technique commonly used in forexample passive RFID tags [17]. If this capacitor is coveredwith a biomimetic layer, any chemical binding process withthat layer will change the capacitor dielectric and thus theresonance frequency. Similar non-wireless capacitive MIP

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sensing has already been achieved with impedance spec-troscopy by BelBruno et al. [18].

In a two-port network the scattering parameter matrix Sis defined in terms of the incident and reflected waves ateach port as

b1b2

� �¼ S11 S12

S21 S22

� �a1a2

� �; (2)

where a1 and a2 are the incident waves at eachcorresponding port, b1 and b2 are the reflected waves, S11is the input port voltage reflection coefficient, S12 is thereverse voltage gain, S21 is the forward voltage gain andS22 is the output port voltage reflection coefficient.

In effect the proposed antenna/sensor tag inductor pairforms a two-port network with one port connected to themeasurement hardware and the other port terminated by anon-matched load in the form of the sensor capacitanceCs, ascan be seen in Fig. 1. In this two-port network the oscillatingcurrent provided by the measurement hardware generates anoscillatingmagnetic field through the antenna coil. This fieldis predominately a non-radiative near field. If a second coilis brought close by in the magnetic field, part of the energyin the first coil can be picked up by this second coil. Atfrequencies far from the resonance frequency of this secondcoil nearly no energy is transferred and thus all energy putinto the antenna tag is reflected back to the measurementhardware. When the operating frequency approaches theresonance frequency and the second coil starts ringing due tothe induced oscillation, more energy is transferred to thesecond coil and thus less energy is reflected back. By simplymeasuring the attenuation in the reflected signal the exactresonance frequency of the tag can be discerned.

2.2 Fabrication of the sensor tags The chosen tagdesigns can be seen in Fig. 2, and consist of two different coildesigns coupled to two different capacitor types. This resultsin four different sensor tags and two corresponding antennatag designs. For the round coils, the inductance can begiven by

¼ mN2ðdo þ diÞ4

ln2:46ðdo þ diÞ

do � diþ 0:2

do � dido þ di

� �2 !

;

(3)


940 D. Croux et al.: Molecular imprinted polymer films on RFID tagsp

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Figure 2 (online colour at:www.pss-a.com)Overviewofdifferenttag designs.

where m is the electromagnetic permeability of air, di and doare the innermost and outermost diameter of the spiral and Nis the number of windings [19].

For the square coils, this inductance is approximated inair as

� 20

L ¼ 1:39� 10�6ðdo þ diÞN5=3log 4do þ dido � di

; (4)

where di and do are the innermost and outermost diameter ofthe squares and N is the number of windings [20].

Calculating the capacitance of such thin in-planecapacitors is more complex. The first of the capacitors isan interdigitated capacitor (IDC) of 10 fingers, whosecapacitance can be approximated as

Figure 3 (online colour at: www.pss-a.com) Schematic overviewof the MIP sensor tag fabrication.

C ¼ lcðNc � 1Þe01þ erK ð1� ðds=dcÞ2Þ1=2

h i2Kðds=dcÞ

; (5)

where lc is the length of the IDC fingers, Nc the number offingers on each IDC terminal, e0 the permittivity of vacuum,er the dielectric constant of the substrate, dc the total spacingbetween two adjacent fingers, ds the spacing between twoadjacent fingers that is not covered by the conductor andK isthe complete elliptical integral of the first kind [20].

The second sensor capacitor design is a simple flattenedparallel plate (FPP) model. Its capacitance can be derivedfrom the IDC equation as

C ¼ We01þ erK ð1� ðds=dcÞ2Þ1=2

h i2Kðds=dcÞ

; (6)

where W is the width of the capacitor. Both these equationsfor the capacitance were derived for standard IC technology,and might not prove to be fully accurate for conductive inkstructures screenprinted on very thin substrates.

A schematic overview of the sensor tag fabrication canbe seen in Fig. 3. The tags were produced using an Isimat1000P, a semi-automatic screen-printing machine. Dupont5064 conductive silver ink was scraped across the screen

13 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

mask, and deposited through the mesh onto the flexibleMcDermid Autostat CT3 Heat Stabilised poly(ethyleneterephthalate) (PET) substrate. The resulting conductivepatterns are approximately 2–5mm thick and show anelectrical resistance of a few Ohms after curing. For thesensor tags this first printed layer is then covered by a newlayer of PET-foil with two holes for interconnects and acentral opening for the MIP deposition. This prevents anymedium the tag is used in from changing the permeabilitybetween the coil windings of the inductor. The circuit is thenshorted by printing a small bridge between the pads at bothends of the circuit. An example of the finished tags can beseen in Fig. 4.

The screen-printing process allows for extremely fastmass-production at trivial cost in a production line. Foractual large-scale implementation the tags can be printeddirectly onto the required part of the food packagingmaterial, eliminating the risk of loose artificial objectswhich could cause health risks being present amongst thefood products.

The MIP particles were produced according to themethods documented by Horemans et al. [6]. A mixture of

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Figure 4 (online colour at: www.pss-a.com) Printed tag and send/receive antenna.

Table 1 Properties of the circular and square sensor tags.

typecapacitor

circularFPP

circularIDC

squareFPP

squareIDC

N 6.5 6.5 5.325 5.325Lcalc (mH) 2.71 2.71 2.54 2.54Lmeas (mH) 2.65 2.65 2.48 2.48Ccalc (pF) 1.8 3.0 1.8 3.0Cmeas (pF) 2.41 3.85 2.34 3.96fcalc (MHz) 72.6 55.82 74.43 57.66fmeas (MHz) 62.98 49.83 63.57 51.51

methacrylic acid (17.8mmol), ethylene glycol dimethacryl-ate (36mmol) and azobisisobutyronitrile (0.66mmol)was dissolved in 7mL dimethylsulphoxide together withthe histamine template (8.99mmol). After degassing thesolution was placed in a sealed vial for polymerisation andkept in a thermostatic bath at 65 8C for 12 h. The resultingbulk polymer was ground with a mechanical mortar andsieved. Only particles with a size smaller than 25mm werekept. Subsequently the histaminewas removed from theMIPpowders by consecutive Soxhlet extractions, starting withmethanol (48 h), followed by a mixture of acetic acid/acetonitrile (1/1) (48 h) and finally again methanol (12 h).The extracted MIP powders were dried in vacuum for 12 h.An NIP was synthesised in the same manner, but withoutthe presence of the target molecule.

The sensor capacitance was spincoated with a poly-vinylchloride (PVC) solution (0.7 wt% in tetrahydrofuran)to obtain a layer of 100 nm thickness. The MIP and NIPparticles were stamped on the surface using a poly(dimethyl-siloxane) (PDMS) stamp, which resulted in a thin layer ofMIPs/NIPs on the surface. The functionalised electrodeswere heated on a hotplate to 120 8C, which is above the glasstransition temperature of PVC (80 8C), to ensure partialembedding of the particles into the layer. Subsequently thesamples were cooled and the excessive powder was removedby washing with isopropanol. This results in an averagesurface coverage of 37–39% [6].

3 Results and discussion3.1 Tag characterisation The fabricated antennas

were connected to a SDR-Kits DG8-SAQ vector networkanalyzer, which swept a frequency range around theresonance frequency and measured the attenuation andphase shift of the reflected signal.

Table 1 compares various measured equivalent circuitparameters of the tags to their calculated values. Themeasured resonance capacitances do not correspond to thecalculated values, partly due to the presence of crosstalkbetween the printed lines and through the short bridge. Thiscrosstalk can be represented as numerous parasitic capaci-

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tances both serial and parallel in the equivalent circuit. Mostof these are negligibly small and, as shown in the equivalentcircuit of Fig. 1, the primary influence is parallel with thesensor capacitor. Thus the total capacitance is given by

Ctot ¼ Cs þ Cp; (7)

with Cs being the capacitance of the sensor capacitor and Cp

the parasitic capacitance. As this parasitic capacitance isparallel to the sensor capacitance, it does not influence thesensitivity of the sensor capacitance but merely providesan offset value. Optionally a dielectric layer of highpermittivity could be printed on top of the coils to tweak Cp

and thus the resonance frequency if needed.Figure 5 shows the effect of a change in the dielectric

constant of the material surrounding the sensor capacitance.Data are shown for a clean sensor tag in air, the same sensortag where the printed side is covered in a phosphate bufferedsaline (PBS) solution and the tag with the non-printed sidecovered in the PBS solution. It can clearly be seen that theprinted side of the capacitance is more sensitive to dielectricchanges than the non-printed side. This is easily explaineddue to the presence of the interlying PET substrate, whichacts as a loss capacitance CPET in series with the sensorcapacitance when detecting through the substrate. The totalseries capacitance in that case can be calculated as

Ctotal ¼CprintCPET

Cprint þ CPET

: (8)

From this data the approximate loss capacitance dueto the PET foil was calculated around 26.6 pF, whichis consistent with the dielectric constant of 3 for a PETsubstrate at frequencies in the lower MHz range.

In Fig. 6 Bode plots for the attenuation of the reflectedsignal and the corresponding phase shift can be seen for anantenna tag with no sensor tag in its near field, an antennacoupled with a blank tag in air and an antenna coupled with ablank tag covered in PBS. Not only can the resonancefrequency of the tag be clearly discerned in both theattenuation and phase plots, but the capacitance change fromthe changing dielectric constant between the electrodes isalso easily observable. The amplitude of the attenuation andphase plot is however significantly reduced as compared tothe direct non-wireless measurement of the sensor tag. The



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Figure 6 (online colour at: www.pss-a.com) Reflection measure-ments of the antenna tag.

Figure 5 (online colour at: www.pss-a.com)Comparison betweena change in dielectric constant on the printed and non-printed sideof the sensor tag.

resonance frequencies measured with the antenna tag areslightly shifted upwards in frequency due to the parasiticeffects in the antenna tag and the reactance caused byinductive coupling between the non-matched loads of theantenna and sensor tags.

3.2 Wireless histamine sensor For the histaminedetection measurements once again the resonance frequencywas monitored with the DG8-SAQ vector network analyzer.A 50mV peak-to-peak sinusoidal signal was continuouslyswept in a range of 5MHz around the resonance frequency asmeasurement signal. The tags were then submerged intoPBS solutions (pH 7.1) to which increasing concentrationsof histamine were added. Histamine concentrations of 1.8–4.5mM are considered a risk for scombroid poisoning.

The measured concentration curves can be seen inFig. 7a–c and show extremely sensitive histamine detectionat very low concentrations down to 50 nM. Saturationgenerally occurs around 700–800 nM. Response time isextremely swift compared to more traditional impedancespectroscopy sensors, with the resonance frequency shiftfirst becoming noticeable within 1min after concentrationchanges and reaching a new steady-state after less than5min. The NIP tag concentration curve in Fig. 7d demon-strates the effects of non-specific binding inherent to


molecular imprinting and the influence of the slight changesin permittivity of the surrounding liquid due to the increasedhistamine concentrations. It demonstrates the high speci-ficity of the sensor tag for histamine.

This corresponds to approximate capacitance changes ofbetween 0.04 and 0.09 pF for the MIPs and less than 0.01 pFfor the NIP at the saturation point around 800 nm histamine.No fundamental difference in sensitivity could be discernedbetween the different tag designs when taking the variabilityof the deposited MIP layers into account. Figure 8 showsSEM imaging of the variability in MIP particle coveragebetween two distinct samples and within a single MIPlayer. The vastly decreased sensitivity in the measurementsdisplayed in Fig. 7a and b can be explained by a belowaverage surface coverage of MIP particles on the sensorcapacitor, at around 25–30%. Despite this lower amount ofbiomimetic receptors sensor response still showed veryclear and easily distinguishable detection at relatively lowtarget concentrations. Noise levels and the correspondinglimit of detection are highly dependent on the quality of thespincoated PVC layer and the immobilised MIP layer, withmeasured noise levels varying between 10 and 100 kHz.However, as most of this noise is Gaussian white noise it canbe easily filtered. The detection limit has been found to varybetween 50 and 300 nm.

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Figure 7 Histamine MIP (a–c) and NIP (d) concentration curves detected through 3mm glass.

Additionally, a sensor tag was created where the MIPlayer was immobilised on the non-printed underside of thetag.As can be seen in Fig. 9 the presence of the thin PET layerbetween the sensor capacitor and the MIP layer does notsignificantly influence the sensitivity of the sensor, especi-ally not compared to the influence of the MIP depositionvariability. The relative permittivity er of both PET andPVC is around 3 (depending on actual composition) in theoperating frequency range, while for PBS it is around 76(again depending on actual composition). Thus the electric

Figure 8 (online colour at:www.pss-a.com) SEM imaging of the variside view of the variability in a single MIP layer (c).

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field lines experience little difference between the 1mmPETsubstrate and the 100 nm PVC immobilisation layer. Thisalso suggests that the thickness of the immobilisation layer isof little concern during tag readout, though this has yet to beexperimentally confirmed.

4 Conclusions The proposed tags have turned out tobe cheap and easy tomass-produce due to the screen-printingfabrication process. Based upon the principle of resonantinductive coupling they can be accurately read at very short

ability inMIPparticle coverage between different samples (a, b) and



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Figure 9 Concentration curve for a histamine MIP immobilisedon the non-printed side of the sensor tag.

range, dependent upon the power of the applied oscillationsignal, with a simple frequency sweep from an impedanceanalyzer or vector network analyzer. Coated with abiomimetic MIP layer, the tags have proven to be accurateand sensitive biosensors. All these advantages make themexcellently suited as disposable packaging sensors for allsorts of commercial food products, such as seafood freshnessindicators. With the ability to detect histamine even throughan interlaying layer of PET, the potential health risk of theconductive ink contaminating the food has been eliminated.

For actual commercial production, roll-to-roll printing orinkjet printing might also provide the same required low-costhigh-throughput [21]. Also, the resonance frequency of thetags should fall within one of the assigned inductive couplingRFID frequency bands at 13.553–13.567MHz or 26.957–27.283MHz. The currently produced test tags did not yet fallwithin these frequency spectra, as the full influence of theMIP layer on the resonance frequency was not yet knownwhen the tagswerefirst designed. Solving this problemwouldmerely require tweaking the printed dimensions and thus LCparameters. One additional concern in this case is that thefrequency shift of histamine detection might easily becomelarger than the assigned bandwidths. This perhaps requires alower concentration ofMIP particles in the biomimetic layer,which would also shift the detection limit towards the lowmillimolar range that is of interest in an actual scombroidpoisoning sensor.

Acknowledgements We would like to thank the SpecialResearch Fund (BOF) of Hasselt University to provide us with thenecessary financial resources. This work is supported by the LifeScience Initiative of the Province Limburg. Special thanks goes toAns Weustenraed and Frederik Horemans for the MIP synthesis.Furthermore,weowemany thanks to theexpertise andknowledgeofJanMertens, JanD’Haen, JohanSoogen, JohnnyBaccus andLievenDe Winter for their technical support and to Benny Dehingh forthe measurement work performed during his master’s thesis.


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Molecular imprinted polymer films on RFID tags: a first step towards disposable packaging sensors

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