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Electrochimica Acta 55 (2010) 5905–5910

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

onstruction of hybrid nanocomposites containing Pt nanoparticles andoly(3-methylthiophene) nanorods at a glassy carbon electrode:haracterization, electrochemistry, and electrocatalysis

uan Zhoua, Hongying Xiana, Feng Lib, Shengnan Wua, Qiufang Lua, Yongxin Lia,∗, Lun Wanga

College of Science and Materials Science, Anhui Normal University, 1# Beijing East Rd., Wuhu 241000, ChinaDepartment of Chemistry, Kansas State University, 111 Willard Hall, Manhattan, KS 66506, USA

r t i c l e i n f o

rticle history:eceived 3 March 2010eceived in revised form 8 May 2010ccepted 11 May 2010

a b s t r a c t

Hybrid nanocomposites containing Pt nanoparticles (nano-Pt) and poly(3-methylthiophene) (P3MT)nanorods at glassy carbon surfaces have been successfully prepared by use of an in situ cyclic voltam-metry (CV) method. Field emission scanning electron microscope (FE-SEM), electrochemical impedancespectroscopy (EIS) and cyclic voltagrams were used to characterize the properties of these nanocom-

vailable online 16 May 2010

eywords:t nanoparticlesoly(3-methylthiophene) nanorodsethanol

posites. SEM images showed that nano-Pt were located on the surface of P3MT nanorods and that theyformed a three-dimensional (3D) porous nanostructure. EIS and CV results demonstrated that thesehybrid nanocomposites had good conductivities, and could accelerate the electron-transfer rates ofredox ions. From the results of electrochemical oxidation of methanol and nitrite, we observed that thisnanocomposite-modified electrode exhibited excellent electrocatalytic activity, which might be useful

cells.

itritelectrocatalysis

in biosensors and/or fuel

. Introduction

In recent years, nanosized materials have been shown toave promising technological applications in many different areasuch as microelectronic devices [1], photocatalysis [2], electrocat-lytic [3], and biomedical devices [4]. Noble metal nanoparticlesave been studied extensively because of their many applica-ions [5–8]. For example, gold nanoparticles can be used as direct

ethanol fuel cells [9], for electrocatalytic reduction of O2 [10],nd in all kinds of sensors [11,12]. On the other hand, poly-er films with modified electrodes are widely used because

hey can be stably modified on an electrode surface and canhus provide more active sites [13]. One of the most impor-ant conducting polymers, 3-methylthiophene, is easily depositednto an electrode surface by electrodeposition techniques [14].ur recent results [11,12] showed that Au nanoparticles coulde easily deposited on a pre-coated polymer film surface byeducing a gold precursor (HAuCl4). With this method, it was

ossible to control the particle size and shape by controllinghe deposition conditions. Pt nanoparticles (nano-Pts) have alsoeen used extensively in all kind of fields [15–19]. For exam-le, Kim et al. developed a composite hybrid electrode containing

∗ Corresponding author. Tel.: +86 553 3869302; fax: +86 553 3869303.E-mail address: yongli@mail.ahnu.edu.cn (Y. Li).

013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2010.05.043

© 2010 Elsevier Ltd. All rights reserved.

Pt-based nanoparticles and nanowires, which they character-ized in terms of its electrocatalytic activities [17,18]. Tegou etal. reported the occurrence of oxygen reduction on the sur-face of platinum-coated and gold-coated glassy carbon surface[19]. Therefore, it is important to develop new methodologies toconstruct Pt nanostructures and investigate their potential appli-cations.

In this paper, we describe the synthesis of nano-Pt/poly(3-methylthiophene) (P3MT) nanorod hybrid nanocompositesthrough an in situ electrochemical method on a glassy carbonsurface. Field emission scanning electron microscope (FE-SEM)and electrochemical impedance spectroscopy (EIS) results showedthat the nano-Pts were absorbed on the surface of P3MT nanorodsuniformly and formed porous three-dimensional (3D) structures.This homogenous nanostructure film exhibited high stability andremarkable catalytic activity for the oxidation of methanol andnitrite, which may be used to construct fuel cells and biosensors inthe future.

2. Experimental

2.1. Chemicals

3-Methylthiophene (P3MT) was purchased from Alfa AesarCo. Ltd. (Tianjin, China). Sodium nitrite, CH3OH, HClO4 andH2PtCl6·6H2O were purchased from Shanghai Chemical Co. Ltd.

5906 Y. Zhou et al. / Electrochimica Acta 55 (2010) 5905–5910

MT c

(a

ip

2

v(6e4eewtbde

wm

2e

wraau

Fig. 1. FE-SEM images of the P3MT (a, c) and the nano-Pt/P3

Shanghai, China). These chemicals were analytical-reagent gradend were used without further purification.

All aqueous solutions were prepared in doubly distilled, deion-zed water. High purity nitrogen was used for deaeration of therepared aqueous solutions.

.2. Apparatus

Cyclic voltammetry (CV), amperometry, differential pulseoltammetry (DPV), and electrochemical impedance spectroscopyEIS) were performed on the electrochemical workstation CHI60C (CH Instruments Co., Shanghai, China). A conventional three-lectrode system was used. A glassy carbon disk electrode (GCE, Ø.0 mm) was used as the basal electrode for fabrication. An Ag/AgCllectrode and a platinum wire electrode were used as the referencelectrode and the counter electrode, respectively. All potentialsere reported versus Ag/AgCl unless stated otherwise. The solu-

ions in the electrochemical cells were thoroughly deaerated by N2ubbling prior to experimentation. A N2 atmosphere was imposeduring all experiments. All experiments were carried out at ambi-nt room temperatures (approximately 20 ◦C).

Field emission scanning electron microscope (FE-SEM) imagesere obtained using an S-4800 field emission scanning electronicroanalyzer (Hitachi, Japan).

.3. Preparation of nano-Pt/P3MT nanorods composites modifiedlectrode

Prior to modification, the basal GCE was polished repeatedly

ith 6, 1 and 0.05 �m alumina slurries. After each polishing, it was

insed with doubly distilled water and ultrasonicated in ethanolnd doubly distilled water for 5 min, successively to remove anydsorbed substances on the electrode surface. Finally, it was driednder nitrogen atmosphere prior to use.

omposite (b, d) on the bare GCE at different magnifications.

Electrochemical polymerization of P3MT was carried out on thebasal GCE by CV from 0.0 to 1.7 V with a scan rate 20 mV s−1 for threecycles. The polymer film was grown in the potentiostatic mode at apotential of 0.7 V for 10 s in an acetonitrile solution containing 0.1 MP3MT and 0.1 M NaClO4 [13]. The electrode was then transferredinto a 0.1 M LiClO4 solution for 12 h of aging. The obtained mod-ified electrode, denoted as P3MT/GCE, was processed in a buffersolution (pH 7.0, 1/15 M PBS, and 0.5 M KCl) by repetitive scan-ning in the potential range of 0.0 and +0.7 V for 10 cycles and thenbetween −0.2 and +0.5 V at a scan rate of 100 mV s−1 until a stablebackground was obtained.

The formation of nano-Pts on the P3MT/GCE was carried out bycyclic voltammetry in a 0.1 HClO4 solution containing 4 × 10−3 MH2PtCl6·6H2O with a potential range from −0.3 to 0.6 V at a scanrate of 50 mV s−1 for thirty cycles. Changing on the cycle number,the potential window, and the scan rate of this potential cycling, cansignificantly change the properties of the fabricated electrode [20].The obtained modified electrode is denoted as nano-Pt/P3MT/GCE.An individual modified GCE, denoted as nano-Pt/GCE, was similarlyprepared for comparison.

3. Results and discussion

3.1. Characterization of nano-Pt/P3MT composite

The FE-SEM images of the electropolymerized P3MT film andnano-Pt/P3MT composite film coated on the surface of the electrodeare shown in Fig. 1. From Fig. 1(a), it can be clearly seen that P3MTnanorods are formed on the surface of the GCE, and that the diam-eter is between 150 and 250 nm, while its length is greater than

500 nm. The nanorods are aggregated together and form a porousstructure. It is well-known that the P3MT can be easily depositedon the surface of a GCE [21,22], and that many isolated P3MT pointsform because of the defects in the GCE surface. Moreover, there isan overall potential drop at the polymer–solution interface [23].

Y. Zhou et al. / Electrochimica Acta 55 (2010) 5905–5910 5907

FmP

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fotwIatetdtaRmtabrFFb

aretnotPr

was selected for an electrochemical experiment. Fig. 5 shows typ-ical CV responses at bare GCE (curve a), nano-Pt/GCE (curve b),and nano-Pt/P3MT/GCE (curve c) in 0.1 M HClO4 aqueous solu-tions containing 0.1 M CH3OH. At the nano-Pt/P3MT/GCE (curvec), a bell-shape oxidation peak of methanol appeared at approx-

ig. 2. Complex plane impedance plots in a 10 mM K3[Fe(CN)6]:K4[Fe(CN)6] (1:1)ixture containing 0.1 M KCl at the bare GCE (a), the nano-Pt/GCE (b), and the nano-

t/P3MT/GCE (c).

his potential drop causes more P3MT monomers to polymerizet the P3MT points on the GCE surface, and a nanorods/nanowireshape can then form if there exist suitable electropolymerizationonditions [24]. On the other hand, in the image of nano-Pt/P3MTomposites at the GCE surface shown in Fig. 1(b), it can be observedhat the nanorod-shape P3MT is covered by nano-Pts, and that theD nanostructures are homogeneously distributed on the surface.his distribution is beneficial to the deposition of Pt nanoparticlesnd the poly(3-methylthiophene). Fig. 1(c) and (d) are large-scalemages of Fig. 1(a) and (b), respectively. From Fig. 1(c) and (d),t can be seen that the morphologies of the P3MT film and theano-Pt/P3MT film consist of uniform 3D nanostructures, which

s important for improvement of the catalytic activity of nanoma-erials.

It is well known that EIS is an efficient tool for studying the inter-ace properties of a surface-modified electrode. Fig. 2 shows the EISf the bare GCE, the nano-Pt/GCE, and the nano-Pt/P3MT/GCE inhe presence of equivalent 1.0 × 10−2 M Fe(CN)6

4−/3− + 0.1 M KCl,hich were measured at the formal potential of Fe(CN)6

4−/3−.t can be seen that at the bare GCE, a large semicircle with anpproximate diameter of 250 � (curve a) and an almost straightail line is present. This demonstrates that there is a very lowlectron-transfer rate to the redox-probe dissolved in the elec-rolyte solution. It can also be seen that a much smaller semicircleiameter of about 150 � (curve b) is present. This circle is relatedo the nano-Pt modified GCE and suggests that the resistance to thenion redox was decreased for the nano-Pt/GCE. Interestingly, thect from the nano-Pt/P3MT/GCE markedly decreased to approxi-ately 20 � (curve c), which indicated a decreased resistance to

he anion redox reaction at the nano-Pt/P3MT/GCE. This is not onlyttributed to the good conductivity of the P3MT film, but may alsoe caused by the high double-layer capacitance arising from theoughness of the structure of the nano-Pt/P3MT composite film.rom the impedance changes at the different electrodes listed inig. 2, it is confirmed that the nano-Pts and the P3MT were immo-ilized on the surface of the GCE.

The electrochemical properties of the modified electrodes werelso investigated by CV. As shown in Fig. 3, a pair of quasi-reversibleedox peaks of Fe(CN)6

3− with a peak separation(�Ep) of 70 mVxists at the bare GCE (curve a). The peak currents increased andhe �Ep decreased slightly with the P3MT/GCE (curve b) and theano-Pt/GCE (curve c) due to either the good conductivity of P3MT

r the nano-Pt layer at the surface of the GCE. In comparison withhe CV measurements obtained from the P3MT/GCE or the nano-t/GCE, a significant enhancement of the peak currents with aeduced �Ep of 65 mV was obtained for the nano-Pt/P3MT/GCE

Fig. 3. CV responses of 5.0 mM K3[Fe(CN)6] + 1.0 M KCl at the bare GCE (a), theP3MT/GCE (b), the nano-Pt/GCE (c), and the nano-Pt/P3MT/GCE(d). Scan rate:50 mV s−1

(curve d). This demonstrates an effective facilitation of the electron-transfer rate constant of the Fe(CN)6

3−/4− redox reaction for thenano-Pt/P3MT/GCE. These results are in agreement with the EISdata, indicating that there was a successful modification of P3MTand Pt nanoparticles.

The modified electrodes were further characterized in H2SO4.Fig. 4 shows the CV measurements of the nano-Pt/GCE and thenano-Pt/P3MT/GCE in 2 M H2SO4. The typical peaks of Pt appearedin the two potential ranges of 0.5–1.0 V and −0.2 to 0 V. This is due tothe absorption/desorption of the hydrogen and oxygen containingspecies [25]. The characteristic value of the charge density asso-ciated with a monolayer of hydrogen adsorbed on polycrystallineplatinum (210 �C/cm2) is widely used to determine the active sur-face area of Pt electrodes [26]. The results from Fig. 4 indicatethat the nano-Pt/P3MT/GCE has large surface areas (2.88 cm2) com-pared to the nano-Pt/GCE (1.43 cm2). The values of active surfaceareas can be used to estimate the electrocatalytic abilities of someredox species [27].

3.2. Electrocatalytic activities of nano-Pt/P3MT/GCE

3.2.1. Electrocatalytic oxidation of methanol atnano-Pt/P3MT/GCE

To test the practical application of nano-Pt/P3MT/GCE, methanol

Fig. 4. CV responses of the nano-Pt/GCE (a) and the nano-Pt/P3MT/GCE (b) in 2.0 MH2SO4. Scan rate: 200 mV s−1.

5908 Y. Zhou et al. / Electrochimica Acta 55 (2010) 5905–5910

Fn

iabchmth(tPptao

ribtstgp

3

d

FrT

concentrations of nitrite on the nano-Pt/P3MT/GCE to determine

ig. 5. CV responses of 0.1 M CH3OH containing 0.1 M HClO4 at the bare GCE (a), theano-Pt/GCE (b), and the nano-Pt/P3MT/GCE (c). Scan rate: 50 mV s−1.

mately 0.65 V and another bell-shape oxidation peak appearedround 0.49 V in the reverse scan. The second oxidation peak maye attributed to the removal of incompletely oxidized carbona-eous species formed during the forward scan [28,29]. On the otherand, it can be seen that there is almost no catalytic activity towardethanol oxidation at the bare GCE (curve a). Moreover, the oxida-

ion peak current obtained from the nano-Pt/P3MT/GCE is 2.7 timesigher than that from nano-Pt/GCE, and the peak current densitypeak current value divided by active surface area) obtained fromhe nano-Pt/P3MT/GCE is 2.0 times higher than that from the nano-t/GCE. These observations can be attributed to the dispersion oflatinum particles in the poly(3-methylthiophene) layer matrix ofhe composite film, which results in a significantly greater surfacerea and an increasing electrocatalytic activity toward methanolxidation.

To investigate the methanol oxidation process, the effect of scanate versus the peak current of methanol at the Pt/P3MT/GCE wasnvestigated, and the results are shown in Fig. 6. From Fig. 6, it cane seen that the peak currents of methanol oxidation were propor-ional to the scan rates in the range of 10–125 mV s−1 in 0.1 M HClO4olutions, as shown in the inset. The linear relationship betweenhe peak current obtained from the first scan and the scan rates isiven by ip (�A) = 142.8 + 0.96v (mV s−1) (r = 0.9942), indicating theresence of an adsorption-controlled process.

.2.2. Electrocatalytic oxidation of nitrite at nano-Pt/P3MT/GCEFurther study shows that the nano-Pt/P3MT/GCE has extraor-

inary electrocatalytic activity toward the oxidation reaction of

ig. 6. CV responses of 0.1 M CH3OH in 0.1 M HClO4 at the nano-Pt/P3MT/GCE. Scanates: 10, 25, 50, 75, 100, and 125 mV s−1 from the inner figure to the outer figure.he inset is the plot of peak current vs. sweep rates.

Fig. 7. CV responses of 5 mM NaNO2 at the bare GCE (a), the nano-Pt/GCE (b), theP3MT/GCE (c), and the nano-Pt/P3MT/GCE (d) in 1/15 M PBS (pH 4.0). Scan rate:10 mV s−1.

nitrite. Fig. 7 depicts the CV responses of 5 mM nitrite with pH4.0 PBS at bare GCE (curve a), P3MT/GCE (curve b), nano-Pt/GCE(curve c), and nano-Pt/P3MT/GCE (curve d). As can be seen, anirreversible oxidation peak appears at approximately 0.9 V on thebare GCE. However, a remarkable increase of oxidation current(1.7-fold over the bare GCE) with a reduction of the overpoten-tial (around 0.77 V) can be seen at the nano-Pt/P3MT/GCE, whichcorresponds to the conversion of NO2

− to NO3− through a two-

electron oxidation process [30]. The potential shift around 130 mVand the 1.7-fold enhancement of peak current indicate that thenano-Pt/P3MT/GCE has excellent electrocatalytic activity towardthe oxidation of NO2

−. Although similar electrocatalytic responsesto nitrite can also be observed at the nano-Pt/GCE and P3MT/GCE(Fig. 7, curves b and c), the best electrocatalytic activity with thelowest overpotential and the highest peak current occurred at thenano-Pt/P3MT/GCE. This excellent electrocatalytic activity may beattributed to the high specific surface area and excellent electron-transfer ability of the nano-Pt/P3MT film, which leads to the largerelectroactive surface of the modified electrode for the detection ofnitrite.

Chronoamperometry experiments were carried out at different

the diffusion coefficient by setting the working electrode potentialat 0.78 V (Fig. 8). The diffusion coefficient was obtained according

Fig. 8. Chronoamperometric responses of nano-Pt/P3MT/GCE in a pH 4.0 PBS solu-tion containing different concentrations of nitrite using a potential step of 0.78 V.From the lower to the upper curves, nitrite concentrations range from 4.0 × 10−5

to 5.6 × 10−3 M. The inset shows the relationship between the current and nitriteconcentration at 120 s.

Y. Zhou et al. / Electrochimica Acta 55 (2010) 5905–5910 5909

Fig. 9. Plots of i vs. t−1/2 obtained from chronoamperometric measurements for then0Tc

t

i

wc4rsatt

3

tcshsome

4s

FP

Fig. 11. DPVs of NaNO2 at the nano-Pt/P3MT/GCE in pH 4.0 PBS. NaNO2 concentra-

ano-Pt/P3MT/GCE in a pH 4.0 PBS solution containing different concentrations of.04, 0.2, 1.0, 1.8, 2.6, 4.0, 4.8, and 5.6 mM nitrite (from the lower to the upper curves).he inset shows the relationship between the slopes of the linear segments and theorresponding concentrations.

o the Cottrell equation:

= nFAD1/2C

�1/2t1/2(1)

here D is the diffusion coefficient (cm2 s−1) and C is the bulk con-entration (mol cm−3). A calibration curve was recorded in a pH.0 PBS solution containing different concentrations of nitrite thatanged from 4.0 × 10−5 to 5.6 × 10−3 M (Fig. 8, inset). Plots of I ver-us t−1/2 with a best fit line for different concentrations of nitritere shown in Fig. 9. The slopes of the resulting straight lines werehen plotted versus the concentrations of nitrite (Fig. 9, inset). Fromhis we can calculate a diffusion coefficient of 1.16 × 10−5 cm2 s−1.

.2.3. Effect of supporting electrolyte and solution pHIt is well-known that the type of supporting electrolyte and

he solution pH are two important parameters in an electrochemi-al reaction. Three kinds of supporting electrolytes, acetum bufferolution, B–R buffer solution, and phosphate buffer solutions (PBS),ave been chosen for use in this study. The results of our testshowed that a more sensitive peak current with respect to nitritexidation was obtained in PBS (data not shown), which is in agree-ent with a previous result [31]. Thus, PBS was chosen for all

xperiments.The effect of solution pH on the electrochemical oxidation of

mM nitrite was also checked in the range of 1.0–7.0 in PBS, ashown in Fig. 10. It can be seen from Fig. 10 that the peak current of

ig. 10. The effect of pH on nitrite detection at the nano-Pt/P3MT/GCE in 1/15 MBS containing 4 mM NaNO2.

tions (from a to h) are 40, 44, 48, 52, 62, 72, 84, and 104 �M. DPV conditions consistof an amplitude = 0.05 V, a pulsewidth = 0.05 s, and a pulse period = 0.2 s. The insetshows the linear relationship between the peak current and the concentration ofNaNO2.

nitrite oxidation was obviously influenced by the pH value. Whenthe pH was below 4.0, the peak current decreased with the decreasein pH. It is known that nitrite anions are not stable in strong acidicmedium and can undergo the following transformation [32].

2H+ + 3NO2− → 2NO + NO3

− + H2O (2)

The decrease in peak current at lower pH (<4.0) may be attributedto the conversion of NO2

− to NO and NO3−. On the other hand,

because the pKa of HNO2 was 3.3, most nitrite anions were proto-nated in this acidic solution [33]. When the pH was above 4.0, theelectrocatalytic oxidation of nitrite became more difficult becauseof the shortage of protons [34], and therefore, the catalytic peakcurrent decreased with the increase in solution pH. The maximalcatalytic peak current was obtained at pH 4.0, which was adoptedas the optimum pH value in our experiments.

3.2.4. Performance of the nano-Pt/P3MT/GCE for thedetermination of nitrite

Under the optimal conditions, the relationship between theoxidation peak current and the concentration of nitrite at thenano-Pt/P3MT/GCE was recorded by the DPV method in staticsolutions. As shown in Fig. 11, the peak current was propor-tional to the concentration of nitrite in the range of 8.0 × 10−6

to1.7 × 10−3 M. The linear regression equation was given by Ipa(�A) = 0.028C (�M) + 0.078 (n = 8, R = 0.9996), with a low detectionlimit of 1.5 × 10−6 M (S/N = 3). In comparison with nitrite detectionperformances reported previously with various chemically modi-fied electrodes [31,35–37], the nano-Pt/P3MT/GCE had a relativelylow detection limit, high sensitivity, and a wide linear range. Thisresult may be attributed to the fact that the three-dimensionalnanostructured Pt/P3MT film greatly enhanced the active areas ofthe GCE, and thereby, strongly exhibited electrocatalytic activitiestoward the oxidation of nitrite.

3.3. The reproducibility and stability of the nano-Pt/P3MT/GCE

To investigate the reproducibility of the performance of thenano-Pt/P3MT/GCE, electrochemical experiments were repeatedlyperformed 10 times with the modified electrode in the solution

containing 4.0 × 10−3 M nitrite. The relative standard deviation(R.S.D.) was 3.4%, which revealed that the modified electrode wasexcellent. The stability of nano-Pt/P3MT/GCE was also investi-gated. After the electrode was used approximately 50 times overthe course of 20 days, only a small decreased in current sensi-

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[[[[[[[32] Q. Cai, W. Zhang, Z. Yang, Anal. Sci. 17 (2001) i917.[33] M.L. Guo, J.H. Chen, J. Li, B. Tao, S.Z. Yao, Anal. Chim. Acta 532 (2005) 71.

910 Y. Zhou et al. / Electrochim

ivity (about 10%) for methanol and nitrite was observed. Thisigh stability may be due to the excellent stability of the film. Foromparison, the stability of the nano-Pt/GCE was also checked. Aurrent decrease around 20% was observed when the nano-Pt/GCEas used approximately 50 times over 8 days.

. Conclusions

In this work, a nano-Pt/P3MT/GCE has been fabricated suc-essfully. SEM and electrochemical results show that this hybridanocomposite structure has a 3D structure, and can acceleratehe electron-transfer rates of redox ions. Methanol and nitrite oxi-ation experiments prove that this nanocomposite has excellentlectrocatalytic activities. We hope that these improved electro-atalytic activities can be used for new biosensors and/or fuel cellsn the future.

cknowledgements

This work was supported financially by the Natural Scienceoundation of China (No. 20975002) and Anhui Normal University.

eferences

[1] V.L. Colvin, M.C. Schlamp, A.P. Alivisatos, Nature 370 (1994) 354.[2] A.J. Hoffman, G. Mills, H. Yee, M.R. Hoffmann, J. Phys. Chem. 96 (1992) 5546.[3] S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 412 (2001)

169.[4] S. Santra, K. Wang, R. Tapec, J. Biomed. Opt. 6 (2001) 160.[5] G.L. Che, B.B. Lakshmi, E.R. Fisher, C.R. Martin, Nature 393 (1998) 346.[6] J.H. Pei, X.Y. Li, J. Electroanal. Chem. 441 (1998) 245.[7] K. Shimazu, K. Uosaki, H. Kita, Y. Nodasaka, J. Electroanal. Chem. 256 (1988)

481.

[[[[

cta 55 (2010) 5905–5910

[8] W.B. Song, M. Okamura, T. Kondo, K. Uosaki, J. Electroanal. Chem. 554–555(2003) 385.

[9] M. Sheffer, D. Mandler, Electrochim. Acta 54 (2009) 2951.10] G. Goncalves, P.A.A.P. Marques, C.M. Granadeiro, H.I.S. Nogueira, M.K. Singh, J.

Gracio, Chem. Mater. 21 (2009) 4796.11] X. Huang, Y. Li, P. Wang, L. Wang, Anal. Sci. 24 (2008) 1563.12] P. Wang, Y. Li, X. Huang, L. Wang, Talanta 73 (2007) 431.13] H.S. Wang, T.H. Li, W.L. Jia, H.Y. Xu, Biosens. Bioelectron. 22 (2006) 664.14] N.F. Atta, A. Galal, A.E. Karagozler, G.C. Russell, H. Zimmer, H.B. Mark, Biosens.

Bioelectron. 6 (1991) 333.15] B.D. McNicol, D.A.J. Rand, K.R. Williams, J. Power Sources 83 (1999) 15.16] L.Q. Rong, C. Yang, Q.Y. Qian, X.H. Xia, Talanta 72 (2007) 819.17] H.J. Kim, Y.S. Kim, M.H. Seo, S.M. Choi, J. Cho, G.W. Huber, W.B. Kim, Elec-

trochem. Commun. 12 (2010) 32.18] Y.S. Kim, H.J. Kim, W.B. Kim, Electrochem. Commun. 11 (2009) 1026.19] A. Tegou, S. Papadimitriou, E. Pavlidou, G. Kokkinidis, S. Sotiro-poulos, J. Elec-

troanal. Chem. 608 (2007) 67.20] P. Wang, F. Li, X. Huang, Y. Li, L. Wang, Electrocchem. Commun. 10 (2008) 195.21] J. Wang, R. Li, Anal. Chem. 61 (1989) 2809.22] M.D. Levi, N.M. Alpatova, E.V. Ovsyannikova, M.A. Vorotyntsev, J. Electroanal.

Chem. 351 (1993) 271.23] M.A. Vorotyntsev, L.I. Daikhin, M.D. Levi, J. Electroanal. Chem. 332 (1992) 213.24] H. Zhang, S.K. Lunsford, I. Marawi, J.F. Rubinson, H.B. Mark Jr., J. Electroanal.

Chem. 424 (1997) 101.25] G.S. Attard, P.N. Bartlett, N.R.B. Coleman, J.M. Elliott, J.R. Owen, J.H. Wang, Sci-

ence 278 (1997) 838.26] S. Trasatti, O.A. Petrii, Pure Appl. Chem. 63 (1991) 711.27] Y. Xu, X. Lin, J. Power Sources 170 (2007) 13.28] R. Manohara, J.B. Goodenough, J. Mater. Chem. 2 (1992) 875.29] Z. Liu, X.Y. Ling, X. Su, J.Y. Lee, J. Phys. Chem. B 108 (2004) 8234.30] S.M. Chen, J. Electroanal. Chem. 457 (1998) 23.31] P. Wang, Z. Mai, Z. Dai, Y. Li, X. Zou, Biosens. Bioelectron. 24 (2009) 3242.

34] X. Huang, Y. Li, Y. Chen, L. Wang, Sens. Actuators B 134 (2008) 780.35] R.R. Zhuang, F.F. Jian, J. Solid State Electrochem. 14 (2010) 747.36] E. Kazimierska, M.R. Smyth, A.J. Killard, Electrochim. Acta 54 (2009) 7260.37] D.Y. Zheng, C.G. Hu, Y.F. Peng, S.S. Hu, Electrochim. Acta 54 (2009) 4910.