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Electrochimica Acta 52 (2007) 8118–8124

Electrocatalytic oxidation of ascorbic acid using a single layerof gold nanoparticles immobilized on 1,6-hexanedithiol

modified gold electrode

A. Sivanesan, P. Kannan, S. Abraham John ∗Department of Chemistry, Gandhigram Rural University, Gandhigram 624302, Dindigul, India

Received 30 April 2007; received in revised form 21 June 2007; accepted 10 July 2007Available online 18 July 2007

bstract

This paper describes the electrocatalytic oxidation of ascorbic acid (AA) in phosphate buffer solution by the immobilized citrate capped goldanoparticles (AuNPs) on 1,6-hexanedithiol (HDT) modified Au electrode. X-ray photoelectron spectrum (XPS) of HDT suggests that it forms aonolayer on Au surface through one of the two SH groups and the other SH group is pointing away from the electrode surface. The free SH

roups of HDT were used to covalently attach colloidal AuNPs. The covalent attachment of AuNPs on HDT monolayer was confirmed from thebserved characteristic carboxylate ion stretching modes of citrate attached with AuNPs in the infra-red reflection absorption spectrum (IRRAS)n addition to a higher reductive desorption charges obtained for AuNPs immobilized on HDT modified Au (Au/HDT/AuNPs) electrode in 0.1 MOH when compared to HDT modified Au (Au/HDT) electrode. The electron transfer reaction of [Fe(CN)6]4−/3− was markedly hindered at theDT modified Au (Au/HDT) electrode while it was restored with a peak separation of 74 mV after the immobilization of AuNPs on Au/HDT

Au/HDT/AuNPs) electrode indicating a good electronic communication between the immobilized AuNPs and the underlying bulk Au electrodehrough a HDT monolayer. The Cottrell slope obtained from the potential-step chronoamperometric measurements for the reduction of ferricyanidet Au/HDT/AuNPs was higher than that of bare Au electrode indicating the increased effective surface area of AuNPs modified electrode. Theu/HDT/AuNPs electrode exhibits excellent electrocatalytic activity towards the oxidation of ascorbic acid (AA) by enhancing the oxidationeak current to more than two times with a 210 mV negative shift in the oxidation potential when compared to a bare Au electrode. The standardeterogeneous electron transfer rate constant (ks) calculated for AA oxidation at Au/HDT/AuNPs electrode was 5.4 × 10−3 cm s−1. The oxidationeak of AA at Au/HDT/AuNPs electrode was highly stable upon repeated potential cycling. Linear calibration plot was obtained for AA over

he concentration range of 1–110 �M with a correlation coefficient of 0.9950. The detection limit of AA was found to be 1 �M. The commonhysiological interferents such as glucose, oxalate ions and urea do not show any interference within the detection limit of AA. The selectivity ofhe AuNPs modified electrode was illustrated by the determination of AA in the presence of uric acid.

2007 Elsevier Ltd. All rights reserved.

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[oatw

eywords: Gold nanoparticles; 1,6-Hexanedithiol; XPS; IRRAS; Cyclic voltam

. Introduction

Gold nanoparticles (AuNPs) are among the most extensivelytudied nano materials to date because of their unique sizeependent physical, optical and electronic properties when com-

ared to bulk gold [1–3]. Recently, modification of electrodeurfaces with AuNPs has received much attention mainly dueo their interesting electrocatalytic and biosensing applications

∗ Corresponding author. Tel.: +91 451 245 2371; fax: +91 451 245 3071.E-mail address: abrajohn@yahoo.co.in (S. Abraham John).

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013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2007.07.020

y; Ascorbic acid; Uric acid

1–7]. Several strategies have been used to immobilize AuNPsn the electrode substrate which includes electrodeposition [5]nd immobilization through covalent or electrostatic interac-ions with the self-assembled monolayers (SAMs) terminatedith suitable functional groups [1,8–10]. Although AuNPs cane successfully immobilized on SAM modified electrodes butomplete immobilization was not achieved on these surfacesue to inter-particle repulsion between the attached AuNPs and

uNPs in solution [11]. For example, it has been shown thatnly 10% coverage of AuNPs was achieved at the SAM termi-ated with amino functional group [10]. While higher surfaceoverage of AuNPs was attained only by preparing multilayer of

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A. Sivanesan et al. / Electroch

uNPs on the electrode substrate using layer by layer assemblyethod [9,12,13]. One way of assessing the surface coverage

f AuNPs on the electrode substrate is the determination oflectron transfer reaction of [Fe(CN)6]4−/3− or [Ru(NH3)6]2+/3+

n solution by cyclic voltammetry [9–13]. The electron trans-er reaction of these redox species is very sensitive to theurface coverage of AuNPs on the electrode surface. As theurface coverage of AuNPs on the electrode substrate increaseshe electron transfer reaction of [Fe(CN)6]4−/3− also increases9–13].

Determination of ascorbic acid (AA) by voltammetric meth-ds has received much attention in recent years [14]. It has beenhown that the content of AA in biological fluids can be used toccess the amount of oxidation stress in human metabolism andxcessive oxidative stress has been linked to cancer, diabetesnd hepatic disease [15]. It is known that accurate determi-ation of AA using conventional electrodes is very difficultecause of its high overpotential, poor reproducibility due toouling effect caused by the oxidized products of AA, low selec-ivity and low sensitivity. Therefore, several approaches haveeen used to modify the electrode surfaces which includes ion-xchange polymers [16], various inorganic and organic materials17,18] and SAMs terminated with different functional groupsnd [19–21]. In most of the cases, preconcentration is ofteneeded before measurement and further these electrodes oftenhow poor reproducibility.

Very recently, determination of AA using AuNPs attachedn glassy carbon electrode (GC) modified with 4-aminobenzoiccid (ABA) followed by coupling with 4-aminothiophenolATP) [22] and multilayers of AuNPs/redox polymers have beeneported [23]. The detection limit of 0.1 mM AA was reportedt multilayers of AuNPs/redox polymers [23] while 2.8 �M waseported at AuNPs attached on GC modified electrode [22]. Inhe reported papers tedious procedures were adopted for thettachment of AuNPs on the electrode substrate [22,23]. Forxample, the multilayers of NPs/redox polymers were preparedn GC electrode by first electrodepositing osmium polymer onC electrode, and then immersed in AuNPs solution followedy immersion in redox polymer solution and AuNPs solution,espectively for one more time [23]. In the other paper, first ABAas attached on GC electrode by potential cycling and then theBA modified GC electrode was immersed in a mixture of ATP

nd coupling agent for 1 h and then immersed in AuNPs solution22]. Herein we report a stable determination of AA with a detec-ion limit of 1 �M using AuNPs modified electrode prepared by aery simple one step procedure. We have used 1,6-hexanedithiolHDT) adsorbed on Au electrode for the attachment of AuNPshrough covalent binding. In the present work, we have chosenSH terminated thiol over NH2 terminated thiol for the immo-ilization of AuNPs because it is known that complete coveragef AuNPs on the SAM surfaces could not be achieved [10] andherefore it is expected that a possible electrostatic interactionetween the positively charged amine and negatively charged

scorbate ions may enhance the oxidation of AA. However, suchlectrostatic interaction is absent when SH terminated SAMas used. We found that AuNPs attached on HDT modified Au

lectrode enhanced the oxidation current of AA more than two

vpa

Acta 52 (2007) 8118–8124 8119

imes with 210 mV negative shift in the oxidation potential whenompared to bare Au electrode. The application of the AuNPsodified electrode is further demonstrated by determining AA

n the presence of uric acid.

. Experimental

.1. Chemicals

1,6-Hexanedithiol (Aldrich) and HAuCl4 (Lancaster) weresed as received. Sodium citrate, ascorbic acid and uric acidere purchased from Merck (India). Phosphate buffer solu-

ion (pH 7.2) was prepared by using Na2HPO4 and NaH2PO4.he Au working electrode was polished with alumina powder

0.5 �m) and sonicated in double distilled water for 5–10 min.he polished Au electrode was then electrochemically cleanedy cycling the potential between −0.2 and 1.5 V in 0.05 M2SO4 at a scan rate of 10 V s−1 for 5 min or until the CV

haracteristics for a clean Au electrode were obtained.

.2. Preparation of Au nanoparticles

Monodispersed colloidal Au nanoparticles were prepared byhe reported procedure [24]. 75 mg of HAuCl4 in 250 mL ofistilled water (0.88 mM) was brought to a vigorous boil withtirring in a round-bottom flask fitted with a reflux condensernd then 26.25 mL of 1% sodium citrate (w/v) solution wasapidly added to the flask. The solution was boiled for another5 min, during which the solution changed from pale yellow toeep red. The solution was allowed to cool to room temperatureith continuous stirring and stored at 4 ◦C in a dark bottle until

urther use. The gold nanoparticles (AuNPs) prepared by thisethod were characterized by transmission electron microscopy

nd UV-Vis spectrophotometer. The TEM image and surfacelasmon resonance band at 520 nm indicate that the averageize of the AuNPs is 10–12 nm.

.3. Modification of electrode surface

Immobilization of Au nanoparticles on polycrystalline Aulectrode was carried out, as typically shown in Scheme 1.he monolayer of HDT was prepared on Au (Au/HDT) elec-

rode by immersing a clean polycrystalline Au electrode into a0 mM ethanolic solution of HDT for 3 h under nitrogen atmo-phere. Then the Au/HDT electrode was subsequently washedith copious amount of ethanol and water to remove the looselyound HDT molecules from the electrode surface and thenipped into Au colloidal solution for 12 h. The resultant elec-rode was washed with water and used for electrochemical

easurements.

.4. Instrumentation

Electrochemical measurements were performed in a con-entional two-compartment three-electrode cell with a polishedolycrystalline Au electrode (0.02 cm2) as working electrode,Pt wire as counter electrode and KCl saturated Ag/AgCl

8120 A. Sivanesan et al. / Electrochimica Acta 52 (2007) 8118–8124

er on

aPAmoWJIwTtcM

3

3Xa

stmcancaflf

tcA1tdtaefp

cnbgbdac

tlga

rtiAiatpellet shows bands at 1394 and 1583 cm−1, corresponding tosymmetric and asymmetric stretching of the carboxylate groupof citrate anion, respectively [36]. The appearance of carboxy-late stretching frequencies in the IRRAS for Au/HDT/AuNPs

Scheme 1. Schematic representation of (a) formation of HDT monolay

s reference electrode. UV–vis spectra were recorded with aerkin-Elmer Lambda 35 Spectrophotometer. The size of theuNPs was determined with a JEM-100S transmission electronicroscope. All the electrochemical measurements were carried

ut with CHI Model 643B (Austin, TX, USA) Electrochemicalorkstation. SEM measurements were carried out by JEOL,

SM-5610LV. IRRAS measurements were carried out on an FT-R spectrophotometer, JASCO FT-IR 460 Plus model equippedith an ATR attachment with a horizontal ZnSe crystal (Pikeechnologies). The resolution of the spectra was 4 cm−1 and

he scans were repeated 100 times. XPS measurements werearried out by using Shimadzu ESCA-850M instrument with ang K� source.

. Results and discussion

.1. Characterization of Au/HDT and Au/HDT/AuNPs byPS, infra-red reflection absorption (IRRA) spectroscopynd cyclic voltammetry

The adsorption behavior of HDT on Au surfaces has beentudied extensively both from gas phase and solution deposi-ion [25–29]. Some groups have reported flatly adsorbed HDT

olecules on Au surface [27,28]. Others reported that HDThemisorbed through one of the two SH groups [25,26,29–32]nd the molecular backbone are tilted 30◦ away from the surfaceormal [29]. Based on XPS studies, it has been recently con-luded that the conditions of deposition have an effect on HDTdsorption geometry [29]. Gaseous HDT was found to adsorbat on Au surface via its two SH groups whereas adsorptionrom solution leads to the formation of monolayer through oneSH group [29]. Thus the free SH groups of HDT were used

o prepare electroluminescent films consisting of multilayers ofopper ions [30], CdS clusters [31] and gold nanoparticles onu substrate, respectively [32]. Since the adsorption behavior of,6-hexanedithiol (HDT) on Au surface is still a matter of con-roversy and further the concept of using HDT to anchor AuNPsepends on the availability of the second thiol functionality athe open end of the monolayer, in the present study we have char-

cterized the structure of HDT monolayer prepared under ourxperimental conditions by XPS. Fig. 1 shows the XPS obtainedor HDT adsorbed on Au plate in the S 2p region. It shows aeak maximum at a binding energy of 163.5 eV. This is a typi-

Au and (b) immobilization of AuNPs on HDT modified Au electrodes.

al value for the S (2p3/2) peak of free thiol moieties which areot bonded to Au surface [29,33]. The shoulder peak at lowerinding energy around 162 eV reveals the presence of thiolate-old bond [29,33,34]. The absence of peaks with S (2p3/2)inding energies above 166 eV corresponding to oxygen coor-inated sulfur species indicates that adsorbed HDT monolayersre highly stable on Au substrate [26]. The lower energy peakorresponding to gold–thiolate appears less intense than the freeSH due to the increased attenuation of photoelectrons escaping

he thiolate–Au interface [34,35]. Thus our immersion procedureeads to the formation of HDT SAM through one of the two SHroups and therefore the free SH groups are utilized for thettachment of AuNPs. The attachment of AuNPs with the freeSH groups of the HDT monolayer was characterized by infra-

ed reflection absorption spectroscopy (IRRAS). Fig. 2 showshe IRRA spectra obtained for Au/HDT and Au/HDT/AuNPsn the region of 900–1850 cm−1 region. In contrast to Au/HDT,u/HDT/AuNPs shows a characteristic carboxylate ion stretch-

ng mode at 1394 and 1580 cm−1 corresponding to symmetricnd asymmetric C( O)2

− stretching modes of the citrate func-ionality [36,37]. The FT-IR spectrum of citrate ion in KBr

Fig. 1. XPS of HDT monolayer on Au surface.

A. Sivanesan et al. / Electrochimica Acta 52 (2007) 8118–8124 8121

Fi

nlaoSabmiawfisFf

FA

aeo6T[rcwroccurs.

Fig. 5 shows the CVs of 1 mM [Fe(CN)6]3−/4− obtained for

ig. 2. IRRA spectrum of (a) HDT and (b) AuNPs immobilized on HDT mod-fied Au surfaces.

ot only confirms the attachment of AuNPs on HDT mono-ayer but also suggests that citrate ion remains on AuNPs evenfter attached with HDT monolayer. The attachment of AuNPsn HDT SAM was further verified from SEM measurements.ince we have not observed a clear image for AuNPs afterttached with HDT we have seeded the immobilized AuNPsy the reported procedure [37] and then used for SEM measure-ents. AuNPs attached on HDT modified Au glass plate was

mmersed in a seeding solution containing 1 mM each HAuCl4nd hydroxylamine hydrochloride for 20 min and then washedith water and dried for SEM measurements. SEM image con-rms the attachment of AuNPs on HDT monolayer and the

eeded AuNPs have an average diameter of 50 ± 10 nm (Fig. 3).ig. 4 shows the typical linear sweep voltammograms obtainedor the reductive desorption of HDT modified Au (Au/HDT)

Fig. 3. SEM image of seeded AuNPs on HDT modified Au surface.

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FA1

ig. 4. LSVs obtained for reductive desorption of (a) Au/HDT, (b)u/HDT/AuNPs electrodes in 0.1 M KOH at a scan rate of 100 mV s−1.

nd AuNPs immobilized on HDT modified (Au/HDT/AuNPs)lectrodes in 0.1 M KOH. The Au/HDT shows a reductive des-rption at −0.90 V and the amount of charge was calculated as5 �C cm−2 without subtracting the roughness of the electrode.he observed value is consistent with a monolayer coverage

38]. For Au/HDT/AuNPs, the reductive desorption peak cur-ent increases at 0.90 V (curve b) and the amount of charge wasalculated as 95 �C cm−2 suggesting that the colloidal AuNPsere covalently attached with the free SH groups of HDT and

eductively desorbs at the same potential where Au–S reduction

are Au, Au/HDT and Au/HDT/AuNPs electrodes in 0.1 M

ig. 5. Cyclic voltammograms obtained for (a) bare Au, (b) Au/HDT, (c)u/HDT/AuNPs electrodes at a scan rate of 50 mV s−1 in 0.1 M KCl containingmM K3[Fe(CN)6].

8 himica Acta 52 (2007) 8118–8124

KtswrcSabeocibmtueteeesaiapKbeAewlAci

3A

csAa(tAAAoftttd

Fig. 6. (A) LSVs obtained for 0.70 mM AA at (a) bare Au, (b) Au/HDT,(c) Au/HDT/AuNPs electrodes in 0.2 M phosphate buffer at a scan rate of5ts

AopfruaooAu

122 A. Sivanesan et al. / Electroc

Cl solution. The bare Au electrode exhibits a characteris-ic voltammetric response for [Fe(CN)6]3−/4− with a peakeparation of 72 mV (curve a) at a scan rate of 50 mV s−1

hereas Au/HDT electrode significantly blocked the redoxesponse of [Fe(CN)6]3−/4− (curve b). The absence of appre-iable faradaic current at Au/HDT electrode suggests that HDTAM is densely packed on Au electrode surface with no defectsnd thus it effectively blocked the electronic communicationetween [Fe(CN)6]3−/4− in solution and the underlying goldlectrode surface. However, after the immobilization of AuNPsn Au/HDT electrode the electron transfer reaction of ferri-yanide was restored with a peak separation of 74 mV (curve c),ndicating that a good electronic communication was achievedetween AuNPs and the underlying Au electrode through HDTonolayer [9,12,13]. We have calculated the standard electron

ransfer rate constant (ks) for the [Fe(CN)6]3−/4− redox couplesing Nicholson theory [39]. The ks value for Au/HDT/AuNPslectrode is calculated as ks = 1.4 × 10−2 cm s−1, which is higherhan that of bare Au electrode (ks = 1.1 × 10−2 cm s−1). Since thelectron transfer of [Fe(CN)6]3−/4− was hindered at Au/HDTlectrode and ks value was not calculated. It is known that thelectron transfer reaction of [Fe(CN)6]3−/4− depends upon theurface coverage of AuNPs and the peak separation decreasess the surface coverage of AuNPs on the electrode substratencreases [9,12,13]. We have determined the effective surfacerea of the bare Au, Au/HDT and Au/HDT/AuNPs electrodes byotential-step chronoamperometric measurements using 1 mM3[Fe(CN)6] in 0.1 M KCl solution [13]. The Cottrell slope forare Au electrode was 0.59 �A s1/2 and that of Au/HDT/AuNPslectrode was 0.74 �A s1/2 indicating that the surface area ofuNPs immobilized electrode was higher than that of bare Au

lectrode. But in the case of Au/HDT electrode the Cottrell slopeas 0.19 �A s1/2 which reveals that HDT forms compact mono-

ayer on Au electrode. A higher Cottrell slope value obtained atu/HDT/AuNPs in contrast to bare Au and Au/HDT electrodes

learly reveals that effective surface area was increased after themmobilization of AuNPs on HDT modified electrode.

.2. Electrocatalytic oxidation of ascorbic acid atu/HDT/AuNPs electrode

Since it has been already demonstrated that the electro-hemical properties of immobilized AuNPs on the electrodeubstrates are different from the bulk gold [1–3] we have utilizedu/HDT/AuNPs electrode for the determination of ascorbic

cid (AA). Fig. 6A shows the linear sweep voltammogramsLSVs) obtained for bare Au, Au-HDT, Au/HDT/AuNPs elec-rodes in 0.2 M phosphate buffer solution containing 0.70 mMA. The irreversible oxidation of AA occurs at 0.48 V at bareu electrode (curve a) whereas no response was observed atu-HDT electrode within the potential limit (curve b). On thether hand, the irreversible oxidation of AA occurs at 0.27 Vor Au/HDT/AuNPs electrode, which is 210 mV less nega-

ive than the oxidation of AA at bare Au electrode. Furtherhe oxidation current of AA was enhanced to more than twoimes at Au/HDT/AuNPs electrode. The observed higher oxi-ation peak current with 210 mV negative potential of AA at

stAi

0 mV s−1. (B) LSVs obtained for 0.70 mM AA in 0.2 M phosphate buffer solu-ion after 3 (solid), 6 (dotted) and 9 (dashed) continuous potential cycles at acan rate of 50 mV s−1.

u/HDT/AuNPs electrode is attributed to greater surface areaf the AuNPs immobilized on Au/HDT electrode when com-ared to bare Au electrode [28]. One of the major problemsor the determination of AA using bare Au electrode is the pooreproducibility due to fouling effect caused by the oxidized prod-ct of AA. Therefore the reproducibility of AA oxidation peakt Au/HDT/AuNPs was studied by recording LSVs at continu-us potential cycling. Fig. 6B shows the LSVs recorded for thexidation of AA after every three continuous potential cycles.s you can see the oxidation potential of AA remains stablepon nine repeated potential cycling (Fig. 6B). The observed

table oxidation of AA shows that the fouling effect caused byhe adsorption of the oxidized AA products is eliminated at theu/HDT/AuNPs electrode. The oxidation peak current of AA

ncreases linearly with the potential scan rate and a linear rela-

imica Acta 52 (2007) 8118–8124 8123

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ofe

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Fig. 7. Differential pulse voltammograms obtained for 1 �M (a) of AA atAu/HDT/AuNPs electrode in 0.2 M phosphate buffer. Each addition of AAic

3a

e(pand Au/HDT/AuNPs electrodes in 0.2 M phosphate buffer solu-tion containing a mixture of 0.70 mM AA and UA. Both bareAu and Au/HDT electrodes fail to separate the oxidation peaksof AA and UA (curves a and b). However, Au/HDT/AuNPs

A. Sivanesan et al. / Electroch

ion between the oxidation current and the square root of the scanate was obtained indicating a diffusion controlled oxidation ofA at Au/HDT/AuNPs electrode. Further, the oxidation peakotential of AA shifted towards more positive potential whene increased the scan rate, suggesting a kinetic limitation in the

eaction between Au/HDT/AuNPs and ascorbic acid.By determining the Tafel slope ‘b’ we can get the informa-

ion about the rate determining step. For a totally irreversibleiffusion controlled reaction, b was obtained by the Eq. (1)

p = b

2log ν + constant (1)

rom the Eq. (1), the plot of Ep vs. log ν gives a slope of b/2,here b is Tafel slope. We have determined the Tafel slope of.242 V for the oxidation of AA at Au/HDT/AuNPs electrodendicating an electron transfer process is the rate limiting step.

The charge transfer coefficient αnα, where nα is the numberf electrons involved in the rate determining step, was calculatedor the oxidation of AA at both bare and AuNPs immobilizedlectrode by the Eq. (2) [40].

nα = 0.048

(Ep − Ep/2)(2)

here Ep and Ep/2 are the anodic oxidation potential and anodicalf-wave oxidation potential, respectively. Since AA undergoesne electron oxidation at pH 7.2 [41] the charge transfer coef-cient α for bare Au and Au/HDT/AuNPs electrodes are 0.58nd 0.67, respectively.

Potential-step chronoamperometry was used to determine theiffusion coefficient (D0) of AA at Au/HDT/AuNPs electrode.ased on Cottrell equation the diffusion coefficient of AA was

ound to 3.39 × 10−5 cm2 s−1.The standard heterogeneous rate constant (ks) for totally

rreversible oxidation of AA at Au/HDT/AuNPs electrodeas calculated based on Eq. (3) [42] and found to be.4 × 10−3 cm s−1.

s = 1.11D1/20 (Ep − Ep/2)−1/2v1/2 (3)

Fig. 7 shows the differential pulse voltammogramsbtained for AA in the concentration range of 1–110 �Mt Au/HDT/AuNPs electrode. The oxidation current of AAncreases with a slight potential shift in the oxidation peakotential upon each increment of 1–110 �M and the oxida-ion currents have a linear relationship with the concentrationf AA with a correlation coefficient of 0.9950 (Fig. 7 (inset)).he detection limit of Au/HDT/AuNPs was found to be 1 �M.he present method of AA determination has several advantagesver the recently reported papers for the determination of AAsing AuNPs attached on GC modified with ATP electrode [22]nd multilayer of AuNPs/redox polymer modified GC electrode24]. In the present work, a single step is used to attach AuNPs

n the electrode substrate when compared to the tedious pro-edures adopted in those papers for the attachment of AuNPs22,23]. Further, the detection limit of 1 �M was achieved in theresent work when compared to the reported detection limits of.1 mM [23] and 2.8 �M [22].

FAr

ncreased the concentration by 10 �M (b–l). Inset shows the correspondingalibration plot.

.3. Determination of ascorbic acid in the presence of uriccid

We have tested the practical application of Au/HDT/AuNPslectrode by determining AA in the presence of uric acidUA), a major interference coexists with AA in many sam-les. Fig. 8 shows the LSVs obtained for bare Au, Au/HDT

ig. 8. LSVs obtained for 0.70 mM mixture of AA and UA at (a) bare Au, (b)u/HDT and (c) Au/HDT/AuNPs electrodes in 0.2 M phosphate buffer at a scan

ate of 50 mV s−1.

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124 A. Sivanesan et al. / Electroc

lectrode resolves the oxidation peaks of AA and UA, wherexidation of AA occurs at 0.29 V while the oxidation of UAccurs at 0.47 V at Au/HDT/AuNPs electrode (curve c). Thebserved 180 mV peak separation is more than enough to deter-ine the concentration of both AA and UA simultaneously

n a mixture. Even though there is a 20 mV positive shift inoth AA and UA oxidation peaks in the mixture when com-ared to individual oxidation potential (AA at 0.27 V and UAt 0.45 V), the oxidation peak potentials of them were highlytable upon repeated potential cycling. This reveals that theu/HDT/AuNPs electrode does not undergo fouling caused by

he oxidized products of AA in phosphate buffer solution. Thetability of Au/HDT/AuNPs electrode was tested by a seriesf 30 repetitive voltammetric measurements of sample solutionontaining 50 �M of AA. The results show a 3% relative stan-ard deviation, indicating that the Au/HDT/AuNPs electrodeoes not undergo fouling effect caused by the oxidized prod-cts of AA. The reproducibility of the results was tested byhree different AuNPs modified electrodes for the determina-ion of AA. All the three electrodes show the oxidation of AAt 0.27 V with same sensitivity, confirming that the results areeproducible.

.4. Interferent studies

The application of Au/HDT/AuNPs electrode for the deter-ination of AA at micro molar concentration was also checked

n the presence of 0.5–1 mM concentration of glucose, oxalateon and urea as interferents in phosphate buffer solution. In theresence of interferents, oxidation peak potential of AA was sta-le and the current response of AA was also not affected. Thusu/HDT/AuNPs electrode could be successfully used to deter-ine the concentration of AA in the presence of physiologically

ommon interferents.

. Conclusion

The experimental results reported here clearly demon-trated that the oxidation of AA was facilitated by themmobilized AuNPs on HDT modified Au (Au/HDT/AuNPs)lectrode in contrast to bulk Au electrode. Attachment ofuNPs on HDT monolayer was characterized by IRRAS

pectral and SEM studies. The present method of attach-ng AuNPs on the electrode surface through covalent bindingith the SH groups of the HDT SAM was very simple.he Au/HDT/AuNPs electrode exhibits a reversible electron

ransfer reaction for [Fe(CN)6]4−/3− with a peak separationf 74 mV confirms the electronic communication between themmobilized AuNPs and the underlying Au electrode. The deter-

ination of AA using Au/HDT/AuNPs electrode has severaldvantages over bulk gold electrode: (i) it does not undergo

ouling effect, (ii) 210 mV negative shift in the oxidationotential, (iii) detection limit of 1 �M and (iv) selectivelyetermine AA in the presence of uric acid. Thus the presentuNPs modified electrode could be used for real sample

nalyses.

[[[[

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Acta 52 (2007) 8118–8124

cknowledgements

Financial support by the University Grants Commission (No.-31-131/2005(SR)), New Delhi is gratefully acknowledged.e thank one of the reviewers for his critical comments and

aluable suggestions.

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