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Electrochimica Acta 56 (2011) 10159– 10165

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta

irect electrochemical detection of pyruvic acid by cobalt oxyhydroxideodified indium tin oxide electrodes

ingyi Wang, Peng Diao ∗

ey Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, PR China

r t i c l e i n f o

rticle history:eceived 11 July 2011eceived in revised form 30 August 2011ccepted 31 August 2011vailable online 9 September 2011

a b s t r a c t

An enzyme-free electrode was fabricated by anodic electrodeposition of cobalt oxyhydroxide film onan ITO electrode (CoOx(OH)y/ITO) for direct electrochemical detection of pyruvic acid (PA) in solution.Scanning electron microscopy (SEM) and atom force microscopy (AFM) were employed to characterizethe morphology of CoOx(OH)y film. Cyclic voltammetry (CV) was used to investigate the electrochemicalproperties of PA on CoOx(OH)y/ITO in order to select the optimal potential for the chronoamperometric

eywords:yruvic acidobalt oxyhydroxidenzyme-free electrodelectrochemical detection

detection of PA. It was found that the CoOx(OH)y/ITO electrode served as an excellent PA sensor witha linear detection range of 1.00 �M to 1.91 mM, a detection limit of 0.55 �M, and a high sensitivity of417.1 �A mM−1 cm−2. Moreover, the response time of CoOx(OH)y/ITO to PA is less than 10 s, which is theshortest for PA detection reported in literature using electrochemical method. These properties and thehigh stability of CoOx(OH)y/ITO made it a good candidate for developing electrochemical enzyme-freePA sensing device.

. Introduction

Pyruvic acid (PA), one of the smallest biomolecules as the sim-lest �-keto acid, is an important molecule due to its fundamentaloles in biological systems, such as acting as an intermediate inhe metabolism of carbohydrates, proteins and fats. Dietary withA supplementation can help individuals lose weight [1], decreasehe plasma cholesterol concentration [2], and enhance the exer-ise duration [3]. Supraphysiological amounts of PA may increaseardiac mechanical performance due to its inotropic effects [4].onsequently, PA can serve as a diagnostic aid in clinical analysis5,6] and an ingredient in medicine and food. Therefore, the mea-urement of PA concentration is required in clinical, biochemicalrocess and food analysis.

Several techniques have been developed to determine theoncentration of PA on the basis of different separation andetection mechanisms, including spectrometry [7,8], chromatog-aphy [9–11], flow injection analysis [12], capillary electrophoresis13,14] and enzyme biosensors [15–18] as well. Light and elec-ricity are the most common detective signals involved in theseechniques. However, PA cannot be directly detected by UV spec-

roscopy or fluorescence spectroscopy because it neither absorbsight nor fluoresces to an extent high enough to allow quanti-ative detection [13]. Electrochemical methods, which combine

∗ Corresponding author. Tel.: +86 01 82339562; fax: +86 01 82339562.E-mail address: pdiao@buaa.edu.cn (P. Diao).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.08.113

© 2011 Elsevier Ltd. All rights reserved.

advantages of fast response, high sensitivity, low cost and easyoperation, are proved to be useful in the direct detection of smallbiomolecules. For electrochemical detection of pyruvic acid, theelectrodes were usually modified with enzyme that was used asthe key sensing element to detect biological analytes [15–18]. How-ever, enzyme biosensors face the challenge of instability in practicalapplication, such as denaturation during immobilization and mea-surement [19], and deactivation by strong acids or bases [12]. Thusnon-enzymatic sensors are considered to overcome these draw-backs. However, few works are found to study the electrochemicaldetection of PA without enzymes.

Cobalt oxyhydroxide films contain Co(III)/Co(IV) redox couplehas received much attention for their high electrocatalytic activityin alkaline solution toward the oxidation of small organic moleculescontaining OH and NH2 groups, such as carbohydrates, aldehydes,alcohols and amino acids [20]. In this work, the cobalt oxyhydroxidefilm was prepared on ITO substrates by anodic electrodeposition.The obtained CoOx(OH)y/ITO electrode was used to investigate theelectrochemical behavior of PA in a basic solution. It was demon-strated that direct electrochemical quantitative detection of PA canbe realized by using cobalt oxyhydroxide modified ITO electrodes.

2. Experimental

2.1. Chemicals and apparatus

Pyruvic acid was purchased from Alfa Aesar. Cobalt chloridehexahydrate (CoCl2·6H2O) was obtained from Jinke Institute of

1 ica A

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0160 J. Wang, P. Diao / Electrochim

ine Chemical Industry (Tianjin, China). Sodium acetate anhy-rous (CH3COONa) and all other chemicals were purchased frominopharm Chemical Reagent Beijing Co., Ltd. (Beijing, China). Allhe chemicals were of analytical grade and used without furtherurification. The aqueous solutions were prepared using deion-

zed water and deaerated with high purity nitrogen before use. Thendium tin oxide (ITO) coated glass slides obtained from CSG Hold-ng Co. have a square resistance of 15 �. The ITO substrates were cutnto 1 cm × 2.5 cm segments for use. The morphology of modifiedlectrodes was characterized using scanning electron microscopySEM, JEOL JSM-6700F) with an accelerating potential of 5 kV andsing tapping mode AFM (Multimode IIIa, Veeco Instrument Inc.).

.2. Electrochemical preparation of CoOx(OH)y/ITO electrodes

Electrochemical experiments were performed on CHI750corkstation (ChenHua Instruments Co., Shanghai, China) with

three-electrode system. The electrodeposition and the cyclicoltammetric (CV) experiments were carried out in a conventionalhree-electrode cell at room temperature. A saturated calomel elec-rode (SCE) and a Pt foil were employed as reference and counterlectrodes, respectively. All potentials are reported with respect to

CE.

The ITO substrates were cleaned by sonication in 0.5 M KOH andcetone for 10 min and 15 min, respectively. Cobalt oxyhydroxidelms were prepared on ITO substrates according to the method

ig. 1. Typical (a) SEM and (b) tapping mode AFM images of cobalt oxyhydroxide film onhree-dimension AFM image of the edge of cobalt oxyhydroxide film, left: cobalt oxyhydr

cta 56 (2011) 10159– 10165

by Casella, et al. [21]. Anodic electrodeposition was performed bycyclic voltammetry (CV) with a potential sweep rate of 50 mV s−1

between 0.0 and 1.1 V for 10 cycles in 0.1 M CH3COONa solutionwith 10 mM CoCl2 as the precursor. After electrodeposition, theelectrodes were rinsed with copious water and then dried withhigh purity nitrogen. The resulting electrodes that were coatedwith brown madder films of cobalt oxyhydroxide were denotedas CoOx(OH)y/ITO.

2.3. Detection of pyruvic acid at CoOx(OH)y/ITO electrodes

The CoOx(OH)y/ITO was first cured by a consecutive cyclicpotential sweep within the potential range from −0.1 to 0.6 V at0.05 V s−1 for 5 cycles in 0.5 M KOH. Then, the cured CoOx(OH)y/ITOwas used for the chronoamperometric detection of PA. In detail, theCoOx(OH)y/ITO was immersed in 0.5 M KOH and a detection poten-tial (0.45 V) was applied to the electrode. The chronoamperometriccurves were recorded during the multiple addition of PA to thedetection system.

3. Results and discussion

3.1. Characterization of CoOx(OH)y/ITO

For the preparation of cobalt oxyhydroxide, ITO substrates wereselected as electrodes due to their good conductivity, high stability

an ITO substrate. (c) three-dimension AFM image of cobalt oxyhydroxide film. (d)oxide film, right: ITO substrate.

J. Wang, P. Diao / Electrochimica Acta 56 (2011) 10159– 10165 10161

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ig. 2. Cyclic voltammograms of CoOx(OH)y/ITO in 0.5 M KOH. Potential sweep rate:.05 V s−1.

n alkaline solutions, and ease to cut into desired shapes. Moreover,ompared to noble metal and glass carbon electrodes, the low-costf ITO substrates makes them more suitable to be used as dispos-ble electrodes for industrial and clinical applications. Therefore,he cobalt oxyhydroxide modified electrode was prepared by elec-rodeposition on an ITO substrate. Fig. 1a and b shows the typicalEM and tapping mode AFM images of the obtained CoOx(OH)y/ITO,espectively. It is seen from these images that the ITO substratesere coated with a uniform, smooth and continuous film of cobalt

xyhydroxide. As is well known, AFM is highly sensitive to theeight of surface features and it was used here to provide topolog-

cal profiles of CoOx(OH)y/ITO surface. A typical three-dimensionFM image shown in Fig. 1c demonstrates a plane surface uni-

ormly decorated with nanoscaled tubers. These nano tubers canreatly increase the specific area of the CoOx(OH)y/ITO, and thenre believed to be beneficial to the enhancement of detection sen-itivity.

The thickness of the CoOx(OH)y film increases with increasinghe deposition cycles within the initial 20 cycles, as can be seenrom the increase of deposition peaks. The increase of depositioneak indicates that the specific area of the CoOx(OH)y film increasesith deposition cycles. This means that, within certain range, the

ctive sites on CoOx(OH)y film increases with film thickness. How-ver, large number of deposition cycles will result in the decrease ofhe mechanical stability of the CoOx(OH)y film. Therefore, to obtainoOx(OH)y films with constant specific area and good mechanicaltability, all CoOx(OH)y/ITO electrodes were prepared by CV depo-ition for 10 cycles, which results in an approximate thickness of8 nm, as can be seen in Fig. 1d.

Fig. 2 shows the typical cyclic voltammetric curves ofoOx(OH)y/ITO in the blank 0.5 M KOH. The CV curves exhibit threenodic peaks at 0.02 V (Ia), 0.15 V (IIa) and 0.50 V (IIIa) and twoathodic peaks at 0.10 V (IIc) and 0.43 V (IIIc), respectively. Thebtained CV features are coincident with the results reported in theiterature [22]. Peak Ia is attributed to an oxidative adsorption pro-ess of hydroxyl ions on the cobalt oxyhydroxide film in 0.5 M KOH.o cathodic reduction peak can be observed at potentials lower

han 0 V during negative-going sweep, indicating that the adsorp-ion of hydroxyl ions on CoOx(OH)y/ITO is an irreversible process.his means that the adsorbed hydroxyl groups cannot be removeduring cathodic scan, and as a result, the active sites for hydroxyldsorption on CoOx(OH)y/ITO will greatly decrease from the sec-nd potential sweep cycle. The great decrease of peak Ia in theecond potential sweep cycle in Fig. 2 provides direct evidence for

he irreversible adsorption of hydroxyl groups on the cobalt oxyhy-roxide. Peak IIa and IIIa are assigned to redox conversion of variousobalt oxidation states, such as Co(OH)2, Co2O3, Co3O4, CoOOH and

Fig. 3. CV curves of CoOx(OH)y/ITO in 0.5 M KOH (dashed line) and in 0.5 M KOHwith 6 mM PA (solid line). The CV curve of CoOx(OH)y/ITO in 0.5 M KOH (dashedline) is obtained after a curing step. Potential sweep rate: 0.05 V s−1.

CoO2. The predominant specie formed at peak IIa is CoOOH, whilethat formed at peak IIIa is CoO2 [21]. During the negative-goingpotential sweep, peak IIc and IIIc reflect the reverse cathodic pro-cess corresponding to IIa and IIIa. The electrocatalytic activity ofCoOx(OH)y/ITO is attributed to the formation of high-valence cobaltcompounds (especially Co(IV) species) during the positive potentialsweep [22]. Moreover, as shown in Fig. 2, when potential is higherthan 0.5 V, the oxygen evolution takes place in an alkaline solution,which results in an abrupt increase in the oxidation current.

3.2. Electrooxidation of pyruvic acid at CoOx(OH)y/ITO

The electrochemical behavior of pyruvic acid at CoOx(OH)y/ITOelectrode was investigated in 0.5 M KOH. Herein, the KOH solutionwas used as a reaction medium because the cobalt oxyhydroxideexhibits good stability and high catalytic activity toward electroox-idation of small organic compounds in an alkaline solution [20–23].Fig. 3 shows the CV response of PA at CoOx(OH)y/ITO in 0.5 M KOH.A large oxidation wave that starts at about 0.3 V in the positive-going sweep is observed. The oxidation wave originates from theelectrooxidation of PA. Similar to the oxidation of carbohydrates[21], amines [22] and amino acids [23] on cobalt oxyhydroxides,the electrooxidation of PA occurs in the potential range where lowvalence cobalt species are oxidized to Co(III/IV) oxides. We believethat the high-valence cobalt species, especially the Co(IV) specieson CoOx(OH)y film, act as media for electrooxidation of PA. As iswell known that, the Co(IV) species are strong oxidizing agents andcan oxidize PA. After reaction, the Co(IV) is reduced to Co(III). Thenet reaction is the electron transfer from PA to electrode, whichresults in the electrooxidation of PA. However, only the surfaceCo(III/IV) couple can act as a catalyst for PA oxidation, those insidethe CoOx(OH)y film have no contact with PA and therefore are cat-alytically inert. As a result, the oxidation current is composed oftwo parts: the oxidation current originated from catalytic oxida-tion of PA and the oxidation of low-valence cobalt species insidethe CoOx(OH)y film. The absence of a reduction wave in the reversepotential sweep in Fig. 3 indicates that the electrocatalytic oxi-dation of PA at CoOx(OH)y/ITO is an irreversible process. For anirreversible electrode process at 25 ◦C, the peak current Ip and thepeak potential Ep can be expressed as [24]:

Ip = 2.99 × 105 n˛1/2AD1/2C0v1/2 (1)

Ep = B −2˛F

ln v (2)

where n is the number of electron transferred in reaction, A the sur-face area of electrode, ̨ the transfer coefficient of the reaction, �

10162 J. Wang, P. Diao / Electrochimica Acta 56 (2011) 10159– 10165

F 2.5 ms s a funf

ttrD

malcao0ctPgEE

I

I

I

Abttibb

ig. 4. (a) linear sweep voltammograms (LSVs) of CoOx(OH)y/ITO in 0.5 M KOH withubtraction of the corresponding blank curves from LSVs in (a). (c) variation of Ep arom (b).

he potential sweep rate, D the diffusion coefficient, C0 the concen-ration of electrochemically active species in solution, B a constantelated with temperature, formal potential, standard rate constant,

and ˛.Eq. (1) can be used to determine the concentration of PA by

easuring the oxidation peak current. However, we cannot obtainn oxidation peak of PA in Fig. 3 due to the oxidation current of theow valence cobalt species. Therefore, to obtain the pure oxidationurrent of PA, the oxidation current contributed by cobalt speciesnd the charging current should be removed from total currentbtained at CoOx(OH)y/ITO in PA-containing solution. In the blank.5 M KOH solution, the total current (Iblank) consists of the chargingurrent (Ic) and the faradaic current generated by Co(III/IV) redoxransition (If,blank), as is given in Eq. (3). While in the solution withA, the total current (It) consists of Ic, If,blank and the faradaic currentenerated by the oxidation of PA (IPA) (see Eq. (4)). On the basis ofqs. (3) and (4), the oxidation current of PA can be expressed usingq. (5).

blank = Ic + If,blank (3)

t = Ic + If,blank + IPA (4)

PA = It − Iblank (5)

ccording to Eq. (5), the oxidation current of PA can be obtainedy subtracting the blank linear sweep voltammogram (LSV) fromhat with PA. Fig. 4a and b shows the LSVs in presence of PA and

he current-potential curves after subtraction of the correspond-ng blank LSVs, respectively. From Fig. 4b, an oxidation peak cane observed and the peak potential Ep and peak current Ip cane obtained. Fig. 4c shows the variation of Ep as a function of lnv

M pyruvic acid at different potential sweep rates. (b) current-potential curves afterction of ln v, and (d) dependence of Ip on v1/2. The values of Ep and Ip are obtained

with a slope of 0.0256 V. The linear dependence on lnv agrees wellwith Eq. (2), providing solid evidence that the electrooxidation ofPA at CoOx(OH)y/ITO is a totally irreversible process. Introducing0.0256 V into Eq. (2), the transfer coefficient of the reaction ̨ iscalculated to be 0.501. Thus, an irreversible one-electron transferreaction is the rate-determining step for pyruvic acid oxidation. Webelieve that the formation of Co(IV) species from Co(III) is the one-electron rate-determining step in the electrocatalytic oxidation ofPA. It should be pointed out that, at large overpotentials, the rate ofcharge transfer step is greatly increased, and the rate-determiningstep changes from a charge transfer step to a diffusion process.This is why one obtains a diffusion current peak for an irreversiblecharge transfer reaction in CV experiments [24]. This phenomenonwas also observed in our work. As shown in Fig. 4d, the linear corre-lation between Ip and �1/2 indicates a diffusion-controlled processin voltammetric measurement.

3.3. Amperometric measurement of pyruvic acid atCoOx(OH)y/ITO

A diffusion-controlled process benefits both the voltammet-ric and the amperometric detection of active species in solution,because the current is proportional to the concentration of activespecies in solution. In the case of PA detection at CoOx(OH)y/ITO,we believe that the CV and linear sweep voltammetry are notmuch of a precise method because the peak current Ip cannot be

directly obtained from CV or LSV curves, and the Ip value obtainedby subtraction of blank curves (Fig. 4b) may cause errors. Whilefor amperometry, the background current contributed from thecharging process and the oxidation of cobalt species can be eas-

J. Wang, P. Diao / Electrochimica Acta 56 (2011) 10159– 10165 10163

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Fig. 6. (a) Typical amperometric response of CoOx(OH)y/ITO to pyruvic acid in 0.5 MKOH. The applied potential is 0.45 V. Insert is the magnification of 0–600 s. (b)Calibration curve for (a) and the detection results of samples (red points). (For inter-pretation of the references to color in this figure legend, the reader is referred to theweb version of the article.)

Table 1The detection results obtained from the samples with different pyruvic acidconcentration.

Idetected (�A) Ccalculated

(mmol dm−3)Cadded

(mmol dm−3)Relativederivation (%)

0.49 0.00523 0.00548 4.565.05 0.0540 0.0552 2.30

−1

ig. 5. Amperometric curves of CoOx(OH)y/ITO in 0.5 M KOH under different appliedotentials.

ly removed by measuring the current increment after the additionf certain amount of PA. Therefore, in this work, an amperomet-ic technique was used to determine the concentration of PA inolution.

The selection of PA detection potential is very important inmperometric measurements. In order to keep the potential inhe diffusion-controlled zone, a relatively high detection poten-ial is preferred to reach the diffusion-limited current. However,xygen evolution, which can interfere with the PA detection, willccur if the potential is too high. To obtain the optimal value ofetection potential, chronoamperometric curves were recorded onoOx(OH)y/ITO at different potentials in 0.5 M KOH blank solutionnd the results are shown in Fig. 5. It is seen from Fig. 5 that aearly zero steady-state current is obtained at potentials 0.40 and.45 V, indicating that no sustained electrochemical reaction takeslace on CoOx(OH)y/ITO in the blank 0.5 M KOH. However, whenhe applied potential exceeds 0.5 V, a clear steady-state current cane seen and the current increases significantly as the potential isurther increased. This current is due to the start of oxygen evolu-ion, as can be seen from Fig. 2. According to the above discussion,.45 V is selected as the detection potential not only to obtain a highetection sensitivity but also to rule out the interference of oxygenvolution.

The quantitative measurement of the PA concentration was car-ied out using amperometry at a working potential of 0.45 V in 0.5 MOH supporting solution. The results are shown in Fig. 6a, fromhich it is seen that the oxidation current of PA increases with

ncreasing PA concentration. A linear correlation between the oxi-ation current (baseline subtracted) and the PA concentration cane obtained, as shown in Fig. 6b. The linear range of the calibrationurve is from 1.00 �M to 1.91 mM and the sensitivity is calculatedo be 417.1 �A mM−1 cm−2 with a correlation coefficient of 0.999.he detection limit is 0.55 �M with a signal to noise ratio of 3 asecommended in the literature [25].

Consecutive measurements of the samples with different PAoncentration were executed on CoOx(OH)y/ITO and the results arehown in Table 1 and Fig. 6b (red points). The results indicate a gooderformance of CoOx(OH)y/ITO for PA detection with the relativeerivations lower than 10% (see Table 1). Various electrochemicalensors for the detection of PA reported in literatures and in thisork are summarized in Table 2. In terms of detection limit, results

n Refs. [14,15] is superior to this work, however, the upper limitf concentration reported in these literatures are not as high as the

alue we obtained, which means the detection range of this works nearly several times wider than that reported in Refs. [14,15].ince most work reported in literatures provided the informationf the geometry area, it is applicable to compare the sensitivities of

20.2 0.216 0.221 2.3664.2 0.686 0.716 4.17

132.0 1.41 1.54 8.36

different work by using the geometry area of the electrode. We findthat the sensitivity obtained at CoOx(OH)y/ITO in our work is a lit-tle smaller than that reported in Ref. [14], but they are of the sameorder of magnitude. Furthermore, the response time is another keyparameter to a sensor, and fast response is highly desirable for thepractical application of an electrochemical sensor. The PA responsetime of CoOx(OH)y/ITO in this work is less than 10 s, which is theshortest compared to the values reported in other work.

3.4. Stability of CoOx(OH)y/ITO for pyruvic acid detection

During 100 consecutive CV measurements of PA atCoOx(OH)y/ITO electrode within −0.1 to 0.6 V at a scan rate

of 0.05 V s in 2 mM PA solution, a current decrease can bedetected, as shown in Fig. 7a. For the current at 0.45 V in thepositive-going sweep, a relatively rapid decrease can be foundwithin 5 cycles (ca. 19% loss, see the inset in Fig. 7a). The rapid

10164 J. Wang, P. Diao / Electrochimica Acta 56 (2011) 10159– 10165

Tab

le

2C

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on

of

vari

ous

elec

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hem

ical

sen

sors

for

pyr

uvi

c

acid

det

ecti

on.

Elec

trod

e

Det

ecti

on

ran

ge

Lim

it

of

det

ecti

on

Sen

siti

vity

Elec

trod

e

geom

etry

area

Res

pon

se

tim

e/re

acti

on

tim

e R

efer

ence

Car

bon

fibe

r

bun

dle

0.01

–5

mol

dm

−38

�m

ol

dm

−32.

39

pA

(�m

ol

dm

−3)−1

–

–

[13]

Car

bon

fibe

r

4–10

0

�m

ol

dm

−30.

388

�m

ol

dm

−354

0a/1

300b

�A

(mm

ol

dm

−3)−1

cm−2

8

�m

in

dia

met

er

100–

200

�m

in

len

gth

–

[14]

Poly

ion

com

ple

x

mem

bran

e/Py

Ox

Up

to

0.5

mm

ol

dm

−350

nm

ol

dm

−351

.7

�A

(mm

ol

dm

−3)−1

cm−2

0.08

cm2

20

s

[15]

GC

/Pt/

Pty/

PyO

x0.

1–3

mm

ol

dm

−350

�m

ol

dm

−36

�A

(mm

ol

dm

−3)−1

cm−2

0.07

cm2

5

min

to

90%

stea

dy

stat

e

curr

ent

[16]

PNR

/PyO

x-C

0.09

–0.6

mm

ol

dm

−334

�m

ol

dm

−30.

13

�A

(mm

ol

dm

−3)−1

cm−2

0.2

cm2

–

[17]

PyO

x0.

0025

–0.0

5

�m

ol

dm

−3–

�D

O

1.66

38

mg

L−1(�

mol

dm

−3)−1

–

3

min

[18]

CoO

x(O

H) y

/ITO

1.00

�M

–1.9

1

mm

ol

dm

−30.

55

�m

ol

dm

−341

7.1

�A

(mm

ol

dm

−3)−1

cm−2

0.22

4

cm2

Less

than

10

s

This

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um

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Fig. 7. (a) Continuous 100 cycle CV curves of CoOx(OH)y/ITO in 0.5 M KOH with 2 mMPA at potential sweep rate of 0.05 V s−1. Insert is the decline of current at 0.45 V withcycle number, I0 is the current at 0.45 V in the first potential sweep cycle. (b) The

detection sensitivity decrease of CoOx(OH)y/ITO under dry air storage condition. S0

is the sensitivity of the first detection.

decrease stage may correspond to the surface reconstruction ofCoOx(OH)y film, during which the high active but unstable siteswere lost. When the sweep cycle is over 5, a gentle current decreaseappears with a current loss less than 8%. Thus, to minimize theerrors generated by the difference in the numbers of unstableactive sites, each CoOx(OH)y/ITO electrode was cured by 5 cyclesof CV sweep from −0.1 to 0.6 V at 0.05 V s−1 in 0.5 M KOH beforeuse. The curing step results in a good stability of CoOx(OH)y/ITOfor continuous detection of PA in alkaline solution.

The long-term stability of CoOx(OH)y/ITO for electrochemicaldetection of PA was examined by measuring the detection sensi-tivity of the freshly synthesized electrode and the same electrodepreserved in dry air under environment temperature after 10 days,20 days and 90 days. The results are shown in Fig. 7b. After 10 days,the sensitivity decays to ca. 71% of its original value, and remainsstable for the next 80 days. This result means that CoOx(OH)y/ITOdeserved in dry air under environment temperature is very stablefor PA detection after the first use.

As is discussed in Section 3.1, larger thickness of CoOx(OH)y filmresults in larger specific area, and then more active sites. It shouldbe pointed out here that the thickness of the CoOx(OH)y film doesnot significantly influence the detection stability of CoOx(OH)y/ITOelectrodes. Though a higher initial PA detection current and a higherfinal stable current are obtained at CoOx(OH)y/ITO with a thicker

CoOx(OH)y film, the decreasing trend of the detection current withpotential sweep cycles is almost the same for all CoOx(OH)y/ITOregardless of the film thickness. We believe this is due to the con-

ica A

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[[[[23] M. Tabeshnia, M. Rashvandavei, R. Amini, F. Pashaee, J. Electroanal. Chem. 647

J. Wang, P. Diao / Electrochim

tant percentage of the active sites per unit of the real surface areaf CoOx(OH)y film.

. Conclusions

Cobalt oxyhydroxide film has been electrodeposited on ITOubstrate and used as an enzyme-free sensing electrode to deter-ine the concentration of pyruvic acid in alkaline solution. We

emonstrate theoretically and experimentally that the electroox-dation of PA at CoOx(OH)y/ITO is an irreversible process andhe oxidation current is proportional to the concentration of PA.hronoamperometry was used to quantitatively detect pyruviccid and the optimal detection potential is 0.45 V. We show thathe CoOx(OH)y/ITO behaves as an excellent sensing electrode forA detection with a high sensitivity, a wide detection range, aow detection limit, and a good detection stability. Moreover, theesponse time of the electrode is less than 10 s, which is the shortestmong the methods for the electrochemical detection of PA. Thisork provides a cobalt-oxyhydroxide-based new sensing elementith excellent performance in the enzyme-free electrochemicaletection of pyruvic acid.

cknowledgements

We gratefully acknowledge the financial support of this worky National Natural Science Foundation of China (NSFC 20973020,

0773007 and 21173016), Doctoral Fund of Ministry of Educationf China (20101102110002), Program for New Century Excellentalents in University (NCET-08-0034), IRT0805, and Innovationoundation of BUAA for PhD students.

[

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cta 56 (2011) 10159– 10165 10165

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