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Electrochemistry Communications xxx (2010) xxx–xxx

ELECOM-03682; No of Pages 5

Contents lists available at ScienceDirect

Electrochemistry Communications

j ourna l homepage: www.e lsev ie r.com/ locate /e lecom

Flexible PDMS-based three-electrode sensor

Wen-Ya Wu a, Xiaoqin Zhong a, Wei Wang a,b, Qiang Miao a,⁎, Jun-Jie Zhu a,⁎a Key Laboratory of Analytical Chemistry for Life Science (MOE), School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210093, Chinab School of Chemical and Biological Engineering, Yancheng Institute of Technology, Yancheng, 224051, China

⁎ Corresponding authors. Tel./fax: +86 25 83594976E-mail address: [email protected] (J.-J. Zhu).

1388-2481/$ – see front matter © 2010 Published by Edoi:10.1016/j.elecom.2010.09.005

Please cite this article as: W.-Y. Wu, et aelecom.2010.09.005

a b s t r a c t

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Article history:Received 5 August 2010Received in revised form 5 September 2010Accepted 6 September 2010Available online xxxx

Keywords:FlexibleGoldPDMSElectrochemical sensingHydrogen peroxide

In this article, we report a flexible poly(dimethylsiloxane) (PDMS)-based three-electrode sensor (FPT-Sensor). In PDMS basis, gold was chemically deposited as working and counter electrodes, and silver as thereference one, the device was flexible without inducing irreversible deformation or fatigue afterelectrochemical testing with forced deformations (the device was twisted, rolled, and stretched). Thissensing system provides a route for producing in situ diagnosis sensing device which requires excellentflexibility to fit in various situations.

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1. Introduction

A great challenge for analytical chemist is to develop newmethodsthat match the growing need in rapid “in situ” analysis. Thesemethods should be sensitive and accurate, and able to determinevarious substances with different properties in “real-life” samples [1].In situ detection devices and on-body wearable sensors are ofconsiderable interest owing to their great promise for monitoringthe people's health and their surrounding environment recently [2,3].The sensing devices could be used for a wide range of healthcare,military of sport applications. Point-of-care testing devices are widelyapplied in health care and the literature on flexible devices has beenexpanding [4–9]. Poly(dimethylsiloxane) (PDMS) which is a siliconeelastomer with a low Young's modulus (b2 MPa) has recently beenused as the supporting substrate for some sensing devices [10–12].

One promising route that other groups are exploring for addres-sing the challenge of stretchable electrochemical sensors involves theuse of flexible thick-film (screen-printed) transducers. Since the wideuse of screen-printed technology for fabricating disposable biosensorin 1980s, amperometric and potentiometric single-use sensors havebeen largely developed for a growing number of biomedical andenvironmental applications [13–16].

This article describes the fabrication and performance of flexiblePDMS-based electrochemical sensing device for the first time, whichis a three-electrode system comprising two gold films as both theworking and counter electrodes and one silver film as the reference

electrode (Fig. 1). The flexible poly(dimethylsiloxane) (PDMS)-basedthree-electrode sensor (FPT-Sensor) was validated by hydrogenperoxide detection, and showed high sensitivity and accuracy. Thedetection limit of H2O2 with the FPT-Sensor is 2.496 μM, which isapproximate to the results of traditional gold electrode. FPT-Sensorprovided a simple, fast and sensitive platform for the quantitativedetection of analytes. This type of detection device has manyadvantages stemming from the properties of PDMS and its use aselectrode substrate. PDMS is routinely used in the fabrication ofminiaturized lab-on-a-chip and microfluidic devices because smalland flexible microstructures can be built reproducibly with it. Inaddition to being flexible, which allows for advanced implantablebiomedical devices, PDMS does not interfere with electrochemicalsignals, so that highly sensitive, accurate, and disposable light-weightsensors can be produced.

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2. Experimental

2.1. Chemical reagents

PDMS was obtained from Dow Corning (Midland, MI, USA).HAuCl4·4H2O was from Shanghai Chemical Reagent Company(Shanghai, China). Ag ink was purchased from Shanghai BaoyinElectronic materials Co., Ltd. Fe(CN)63−/4− aqueous solution contained2 mM Fe(CN)63−/4− and 0.1 M KCl.

The supporting electrolyte was 0.1 M PBS prepared by mixing thestock solutions of 0.1 M NaH2PO4 and 0.1 M Na2HPO4. All the otherreagents were of analytical grade, and deionized water was usedthroughout.

or, Electrochem. Commun. (2010), doi:10.1016/j.

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Fig. 1. Pictures of FPT-Sensor. Working electrode: gold, reference electrode: silver and counter electrode: gold. Top view (a) and side view (b) of the three-electrode sensor, straight(c), bend (d) and twist (e) of the working electrode.

2 W.-Y. Wu et al. / Electrochemistry Communications xxx (2010) xxx–xxx

2.2. Apparatus

All the electrochemical experiments were performed with aCHI760c electrochemical workstation (Shanghai Chenhua Company,Shanghai, China). SEM images were taken on a scanning electronmicroscopy (SEM, LEO1530VP), and digital photos were taken by aNikon Digital Camera (D60).

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2.3. Design and fabrication of the PDMS-based flexible electrochemicalsensing device

PDMS film was prepared similar to the reference [17]. Briefly,PDMS monomer and the curing agent were firstly mixed in aproportion of 10:1 and cured at 90 °C for 20 min. Chemical goldplating solution containing 0.01 g mL−1 HAuCl4, 200 g L−1 KHCO3,and 2% (w/v) glucose (v/v, 2:1:1) was prepared just before use.Another PDMS frame with a certain inner area was sandwichedbetween two native PDMS thin films named “cover chip” and “basechip”, and then the solution for chemical gold plating was injectedinto this sandwich architecture. It was then incubated at 37 °C for 3 hto ensure that the elemental gold is deposited completely. Themechanism of gold deposition was explained in literature [17]. Goldelectrode area was determined by the size of PDMS cavity withpreventing gold solution leakage due to the viscosity of PDMS chips.Two independent PDMS-gold films were fabricated for the use asworking electrode and counter electrode separately. The referenceelectrode was also prepared by painting silver ink smoothly onto thePDMS film with a controlled area. The three electrodes could beassembled on the one PDMS film simultaneously to form a compactFPT-sensor as shown in Fig. 1.

Please cite this article as: W.-Y. Wu, et al., Flexible PDMS-based threelecom.2010.09.005

3. Results and discussion

3.1. Characterization of the flexible electrochemical sensing device

SEM images were employed to obtain gold film information. TheSEM images shown in Fig. 2 revealed that the gold film was composedof 100 nm nanoparticles coherently and its thickness is about 300 nm.Such surface microstructure can provide a better electrochemicalbehavior and a relatively stable signal.

The electrochemical behavior of FPT-Sensor was investigated bycyclic voltammetry in 0.5 M H2SO4 solution. As shown in Fig. 2c, theCV profile exhibited three anodic peaks and a cathodic peak, whichcorresponds to the surface gold oxidation and the consequentreduction of gold oxides, respectively.

Fe(CN)63−/4− aqueous solution was employed as a model redox-active compound to characterize electrochemical behavior of oursensing device (Fig. 2d). The peak shape of CVs shows a typicalreversible (Nernstian) electrochemical reaction in which the reactionrate is controlled by the diffusion of the electroactive species to thesurface of a planar electrode [18]. The dependence of the peak currenton the scan rate was investigated. The difference in potential betweenthe peaks of the reduction (Epc) and oxidation (Epa) curves becomeswide with the increasing scan rate, and the peak current ratio (Ipa/Ipc)is equal to 1.0 [19]. This reversible behavior indicates that no sidereactions take place, and that, as expected, the kinetics of electron-transfer is sufficiently rapid to maintain the surface concentrations ofredox-active species at the values required by the Nernst equation.The insert in Fig. 2d shows anodic and cathodic peak currents aredirectly proportional to the square root of the scan rate between 10and 500 mV s−1. The solid lines represent a linear fit to both oxidationcurrent and reduction current with the regression equation: up data,

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Fig. 2. (a) SEM image of the PDMS-gold film surface morphology, (b) cross section of the gold layer, (c) CV with FPT-Sensor in 0.5 M H2SO4, (d) CV in Fe(CN)63−/4− aqueous solutionwith FPT-Sensor at various scan rates (ascending along y-axis). The insert shows the plot of anodic peak current vs. the square root of the scan rate (v1/2) for CV experimentsconducted in a bulk solution.

3W.-Y. Wu et al. / Electrochemistry Communications xxx (2010) xxx–xxx

y=−0.9477+0.98829x (R2=0.99822); down data, y=1.10408–1.07029x (R2=0.99866). The linearity indicates that the current iscontrolled by diffusion.

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3.2. Flexible testing

PDMS is one of the most widely used polymer for fabricatingmicrofluidic chips because of its excellent transparency, outstandingelasticity, good thermal and oxidative stability, and ease of fabricationand sealed with various materials. In our fabrication process, PDMSsurface was considered to follow a two-step change as reportedelsewhere [17]. In brief, the residual Si–H in PDMS can start to reduceHAuCl4 directly as soon as they contact to each other, so that goldnanoparticles were imbedded into PDMS matrix or adhere to thePDMS surface firstly. Then these gold nanoparticles acted asnucleation centres for subsequent gold metal deposition orderly,which led to the formation of a compact and continuous gold layer onPDMS films. Such microstructure can prevent fracture of gold filmwhen the PDMS-gold electrode has undergone extreme deformations,such as bending and twisting. Besides, the PDMS substrate could notbe thicker than 1.0 mm, because the elastomer film would be moreflexible with a thinner one.

Please cite this article as: W.-Y. Wu, et al., Flexible PDMS-based threelecom.2010.09.005

The bending of flexible sensors, associated with daily activity ofwearer, requires a detailed evaluation of the influence of theirmechanical stress upon the sensor performance. To understand theelectrochemical behavior of flexible sensor under strain, the redoxbehavior of potassium ferrocyanide was used for testing. Theelectrochemical response of FPT-Sensor was measured upon bendinginward to angles of 90°, 180°, and twist for 720°. As shown in Fig. 3,the flexible sensing device on PDMS can be bent and folded, causingextremely deformation. After scanning for 30 CV cycles, well-definedredox peaks are observed without apparent change in the peakcurrents or peak separations, after inward longitudinal bending atdifferent angles or twisted for two turns. H2O2 in PBS buffer were alsotested, the results were similar to potassium ferrocyanide. Overall, allthese results indicated negligible changes in the CV responsefollowing bending to extremely small radii of curvature.

3.3. Amperometric response of H2O2

The hydrogen peroxide is an important compound in life systemand can be detected by electrochemical methods, One of the mostcommon electrochemical methods is the anodic oxidation of H2O2 at aplatinum or gold electrode [20,21]. Here we introduce gold film to aflexible substrate to form a noble metal electrode system based on

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Fig. 3. Cyclic voltammograms of Fe(CN)63−/4− aqueous solution using flexible electrochemical sensing device(a) before and after bending at different angles with 90°, 180° andtwisted for 720° (b). Bending time was 10 min; scan rate was 0.1 V/s. (c) Current–time response observed for H2O2 at the sensing device after subsequent spiking of 50 μΜ H2O2. (d)Calibration plot observed for H2O2 at the sensing device at an applied potential of −0.6 V (Ag) in 0.1 M PBS solution.Q1

4 W.-Y. Wu et al. / Electrochemistry Communications xxx (2010) xxx–xxx

flexible PDMS, which is more sensitive than common screen-printedcarbon electrodes. Because naked gold itself can catalyze thedecomposition reaction of H2O2, the FPT-Sensor is an enzyme freesensor for the determination of H2O2.

Home-made FPT-sensing device was used to construct anamperometric sensor for H2O2 at pH 7.0. Current–time responsecurves were recorded at −0.6 V during successive injection of 50 μΜH2O2. A typical H2O2 calibration curve based on the steady-state H2O2

reduction current is shown in Fig. 3d, and the catalytic current islinearly proportional to H2O2 concentration from 50 to 500 μΜwith acorrelation coefficient of 0.9990 (n=10), and from 600 to 1250 μΜwith a correlation coefficient of 0.9974 (n=14) independently.The detection limit was 2.496 μΜ. The response time was about10 s, which indicated a generally fast electron-transfer process atthis FPT-Sensor. Reproducibility of the electrodes was investigated inN2-saturated 0.1 M PBS (pH 7.0) solution containing 1.5 mM H2O2 bymeasuring CVs 5 times every 30 min after holding the electrode in thesolution, Relative standard deviation (RSD) was 1.3% (n=8). For 3FPT-Sensors independently constructed, RSD is 5.0% for the catalyticcurrent at −0.6 V in 1.5 mM H2O2 solution. The electrodes did notshow obvious decrease in current after 30 successive scans in 1.5 mMH2O2 (N2-saturated PBS solution), and retained 95% of its initialresponse under the same condition after 40 days shelf life, it indicatedan excellent stability.

Please cite this article as: W.-Y. Wu, et al., Flexible PDMS-based threelecom.2010.09.005

4. Conclusions

We demonstrated a simple method to produce highly deformableelectrochemical sensing device. It can withstand mechanical defor-mation without damage and show stable electrochemical properties.The flexibility of the sensing device integrated with bulk micro-machining may allow it to be incorporated into specialized system,such as implantable sensing devices for in situ diagnosis, or to berolled up for remote deployment.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (20635020, 20875080) and National 863program of China (2007AA022001).

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