A

(Tsiwrd©

K

1

mrplAawftsf

epc

0d

Electrochimica Acta 52 (2007) 4622–4629

Electrocatalytic oxidation of some amino acids on a nickel–curcumincomplex modified glassy carbon electrode

S. Majdi a, A. Jabbari a,∗, H. Heli b, A.A. Moosavi-Movahedi b

a Department of Chemistry, Faculty of Science, K.N. Toosi University of Technology, P.O. Box 16315-1618, Tehran, Iranb Institute of Biochemistry and Biophysics, University of Tehran, Tehran, Iran

Received 10 December 2006; received in revised form 10 December 2006; accepted 13 January 2007Available online 30 January 2007

bstract

This study investigated the electrocatalytic oxidation of alanine, l-arginine, l-phenylalanine, l-lysine and glycine on poly-Ni(II)–curcumin filmcurcumin: 1,7-bis [4-hydroxy-3-methoxy phenyl]-1,6-heptadiene-3,5-dione) electrodeposited on a glassy carbon electrode in alkaline solution.he process of oxidation and its kinetics were established by using cyclic voltammetry, chronoamperometry and electrochemical impedancepectroscopy techniques. Voltammetric studies indicated that in the presence of amino acids the anodic peak current of low valence nickel speciesncreased, followed by a decrease in the corresponding cathodic current. This indicates that amino acids were oxidized on the redox mediator which

as immobilized on the electrode surface via an electrocatalytic mechanism. Using Laviron’s equation, the values of α and ks for the immobilized

edox species were determined as 0.43 ± 0.03 and 2.47 ± 0.02 × 106 s−1, respectively. The rate constant, the electron transfer coefficient and theiffusion coefficients involved in the electrocatalytic oxidation of amino acids were determined.

2007 Elsevier Ltd. All rights reserved.

urcum

paapTamra

motrs

eywords: Alanine; l-Arginine; l-Phenylalanine; l-Lysine; Glycine; Nickel–c

. Introduction

Amino acids are essential building blocks of biologicalolecules [1] and play key roles in many neuro-chemical

esponse mechanisms, such as memory, appetite control andain transmission [2–4]. The disruption of amino acid regu-ation has been linked to many disorders such as Huntington,lzheimer and Parkinson diseases [2–4]. The development ofsimple and universal detection method for these compoundsould improve our understanding of their biological role and

acilitate the design of new drugs for these diseases. However,he determination of underiviatized amino acids is not simpleince these compounds lack natural chromophore or fluorephoreor photometric and flurometric detections.

The oxidation and adsorption behaviors of amino acids on

lectrode surfaces are relevant to the interfacial behaviors ofroteins and also to the medical and industrial problems asso-iated with the proteins adsorption on the surfaces [5–7]. The

∗ Corresponding author. Tel.: +98 21 44 219 952; fax: +98 21 22 853 650.E-mail address: jabbari@kntu.ac.ir (A. Jabbari).

te

cfec

013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2007.01.022

in; Modified electrode; Electrocatalysis; Impedance spectroscopy

roblem with the application of electrochemical methods formino acid and protein analysis is the lack of electrochemicallyctive groups in most of these compounds. Thus, a derivatizationrocedure must be used prior to determination of amino acids.wo approaches are adopted: the first approach is to derivate thenalyte with an electrochemically active group prior to deter-ination. The second approach is to generate in situ chemical

eactions on electrode surfaces to produce electrochemicallyctive products for detection [8].

Extensive research has been directed towards the develop-ent of electrocatalysts aimed at lowering the normally large

verpotential and raising the faradaic current encountered inhe electro-oxidation of materials. A great deal of interest hasecently focused on the materials immobilized on the electrodeurface and is capable of mediating fast electron transfer underhe effect of external electric fields, namely chemically modifiedlectrodes [9–12].

Construction of electrode materials that are based on macro-

yclic complexes that behave as fast electron transfer mediatorsor solution species is of great interest [13–16]. Althoughlectrochemistry and electrocatalytic properties of macrocyclicomplexes of some transition metals have been investigated

ica Acta 52 (2007) 4622–4629 4623

[irtts

ciNIamtc

oNea

2

atvEtewN11abwaas

crpNt1cTt

3

Nts

Fsn

oFacfeoeamtais readily oxidized and generates free-radical II. The presenceof methoxy group in the monomer makes the reaction possible.The methanol molecule can be eliminated from radical IIby alkaline hydrolysis giving an anion radical III, which is

S. Majdi et al. / Electrochim

13,15–19], few studies about the behavior of electropolymer-zed films in aqueous alkaline solution exist. One such studyeported that nickel macrocyclic complexes can be easily elec-ropolymerized onto an electrode surface in alkaline solutiono form modified electrodes that catalyze oxidation of severalubstrates [20–22].

The resulting films from polymeric metal complexes whichontain Ni(II)/Ni(III) redox couple showed high catalytic activ-ty towards electro-oxidation of organics containing OH andH2 groups [20,21] such as carbohydrates and amino acids.

n these polymeric metal complexes, nickel oxyhydride speciesct as redox mediators between a substrate and an electrode inany electro-oxidation processes. Moreover, the procedure of

heir preparation is simple and the properties of the resultingoating can be controlled carefully [20,21,23].

Following our recent studies on the electrocatalytic oxidationf some organics and pharmaceuticals [9–12], we employed ai(II)–curcumin complex chemically modified glassy carbon

lectrode for the study of electrocatalytic oxidation of somemino acids in alkaline solutions.

. Materials and methods

All chemicals used in this work were purchased from Mercks analytical reagent grade chemicals and used without fur-her purification. The Ni(II)-ammonia complex was preparedia the dissolution of 4 mM NiCl2 in a 25% ammonia solution.lectrochemical measurements were carried out in a conven-

ional three – electrode cell (from Metrohm) powered by anlectrochemical system comprising of an AUTOLAB systemith PGSTAT30 and FRA2 boards (Eco Chemie, Utrecht, Theetherlands). In impedance measurements, a frequency range of00 kHz to 25 mHz was employed, the ac voltage amplitude was0 mV and the equilibrium time was 5 s. The system was run byPC through the FRA and GPES 4.9 softwares. A glassy car-on (GC) disk electrode (from Azar Electrode Co., Iran) whichas modified, a dual Ag/AgCl–saturated KCl (from Metrohm)

nd a platinum disk (from Azar Electrode Co. Iran) were useds working, reference and counter electrodes, respectively. Alltudies were carried out at room temperature.

The GC electrode was further polished on a polishing micro-loth with 0.5 �M alumina powder and rinsed thoroughly withedistilled water prior to the modification. The electrode waslaced in 100 mM NaOH containing 10 mM curcumin and 4 mMi(II)-ammonia complex, subsequently applying the poten-

ial between 200 and 800 mV with a potential sweep rate of00 mV s−1 in a cyclic voltammetry regime. An excess of cur-umin was used for completion of Ni–curcumin complexation.he modified electrode has been denoted as NCGC throughout

he text.

. Results and discussion

The electrochemical oxidation of curcumin andi(II)–curcumin complex was carried out at the GC elec-

rode in aqueous 100 mM NaOH solution by using multiplecan cyclic voltammetries. Consecutive cyclic voltammograms

ig. 1. Consecutive cyclic voltammograms 8 mM curcumin in 100 mM NaOHolution using a GC electrode. Potential sweep rate was 100 mV s−1. The cycleumber is indicated on each voltammogram.

f 8 mM curcumin in 100 mM NaOH solution are depicted inig. 1. Curcumin undergoes an irreversible oxidation processnd produces an anodic peak located at 333 mV in the firstycle, however in the following sweeps the peak disappearsrom the voltammogram. This is the fouling effect of thelectrode surface (blocking the surface via chemical attachmentf a layer which alters the kinetics of the heterogeneouslectron transfer process of curcumin electro – oxidation). Thenticipated chemical structure of curcumin molecule and theain possible reactions involved in the electro-oxidation of

he curcumin molecule were depicted in Scheme 1A. In stronglkaline solutions, phenol converts into phenolate ion I which

Scheme 1.

4624 S. Majdi et al. / Electrochimica A

Fig. 2. Consecutive cyclic voltammograms of 100 mM NaOH solution contain-isI

cot

ecacposlcpcbcib

irpgtmtstsktf

s

est

E

E

l

waα

tFttbtterbtt(2

it(as

I

wΓ

tf4oprrfion

1

ng 10 mM curcumin and 4 mM Ni(II)–ammonia complex using a GC. Potentialweep rate was 100 mV s−1. The cycle number increases from inner to outer.nset: The first cycle in main panel.

onverted to a highly reactive O-quinone (IV) via anotherne-electron reaction. Then O-quinone species are adsorbed onhe electrode surface [20].

Consecutive cyclic voltammograms recorded using the GClectrode in 100 mM NaOH solution containing 10 mM cur-umin and 4 mM Ni(II)-ammonia complex are depicted in Fig. 2nd the anticipated chemical structure of the Ni(II)–curcuminomplex is also shown in Scheme 1B. In the early stages ofotential cycling, oxidation of curcumin causes the appearancef an irreversible peak (Fig. 2, inset). However, in the laterweeps, a pair of peaks appears due to a film growth and immobi-ization of Ni(II)–curcumin on the electrode surface. Althoughurcumin undergoes an irreversible oxidation process and theroduct of the reaction is adsorbed at the GC surface, eitherurcumin or its oxidation product can act as a substrate for immo-ilization of nickel ions on the electrode surface. Moreover, theharged nickel species, their corresponding redox transition, andnvolvement of ionic species penetration into the film from theulk of solution, makes the film an ionic conductor (vide infra).

Fig. 3A represents cyclic voltammogram of NCGC electroden 100 mM NaOH solution recorded at different potential sweepates in a wide range of 2–1000 mV s−1. A pair of well definedeaks with the mid peak potential of 395 mV in the voltammo-ram appears and the peak-to-peak potential separation (withhe potential sweep rate of 10 mV s−1) is 130 mV. The voltam-

ogram is similar to that previously reported [11,20,23] andhe redox transition involved is attributed to the Ni(II)/Ni(III)pecies. The peak-to-peak potential separation is deviated fromhe theoretical value of zero and increases at higher potentialweep rates. This indicates a limitation in the charge-transferinetics arising from chemical interactions between the elec-

rolyte ions and the modifier film, dominated electrostaticactors, and/or non-equivalent sites present in the film.

Laviron derived general expressions for the linear potentialweep voltammetric response for the case of surface-confined

i7to

cta 52 (2007) 4622–4629

lectro-reactive species at small concentrations [24]. The expres-ions for peak-to-peak separation (�Ep) > 200/n mV where n ishe number of exchanged electrons, are as follows:

pa = E0 + A ln

[1 − α

m

](1)

pc = E0 + B ln[ α

m

](2)

nks = α ln(1 − α) + (1 − α) lnα − ln

(RT

nFv

)

− α(1 − α)nF �Ep

RT(3)

here A = RT/(1 − α)nF, B = RT/�nF, m = (RT/F)(ks/nν), Epand Epc are anodic and cathodic peak potential respectively, and, ks and ν are electron transfer coefficient, apparent charge-

ransfer rate constant and potential sweep rate, respectively.rom these expressions, α can be determined by measuring

he variation of the peak potential with respect to the poten-ial sweep rate, and ks can be determined for electron transferetween the electrode and surface deposited layer by measuringhe Ep values. Fig. 3A, inset shows the plot of Ep with respect tohe logarithm ν from cyclic voltammograms recorded for NCGClectrode in 100 mM NaOH solution recorded at potential sweepates 2–1000 mV s−1 for anodic (a) and cathodic (b) peaks. It cane observed that for potential sweep rates of 200–1000 mV s−1

he values of Ep are proportional to the logarithm of the poten-ial sweep rate indicated by Laviron. Using the plot and Eq.3), the values of α and ks were determined as 0.43 ± 0.03 and.47 ± 0.02 × 106 s−1, respectively.

Another point in the voltammograms represented in Fig. 3As that the anodic and cathodic peak currents are proportionalo the potential sweep rate at low values from 1 to 30 mV s−1

Fig. 3B and C). This can be attributed to an electrochemicalctivity of an immobilized redox couple at the surface. From thelope of this line and using [25]:

p =(

n2F2

4RT

)vAΓ ∗ (4)

here Ip is the peak current, A the electrode surface area and* is the surface coverage of the redox species and taking

he average of both cathodic and anodic currents, the total sur-ace coverage of the electrode with the modifier film of about.02 ± 0.03 × 10−8 mol cm−2 was derived. In the higher rangef potential sweep rates (60–1000 mV s−1, Fig. 3D and E), theeak currents depend on square root of the potential sweepate, signifying the dominance of a diffusion process as theate limiting step in the total redox transition of the modifierlm. This limiting diffusion process which was also reported forther Ni-modified electrodes [13,14] may occur for the chargeeutralization of the film during the oxidation/reduction process.

Fig. 4 shows cyclic voltammograms of NCGC electrode in00 mM NaOH solution in the absence (a) and presence of var-

ous concentrations of alanine (b: 10, c: 25, d: 30, e: 50 and f:0 mM) in the potential range of 200–800 mV by using a poten-ial sweep rate of 100 mV s−1. At NCGC electrode, oxidationf alanine resulted in a typical electrocatalytic response. The

S. Majdi et al. / Electrochimica Acta 52 (2007) 4622–4629 4625

F M Na5 Plotp odic (t oots o

ambtwaonsa

ptawct

ig. 3. (A) Main panel: cyclic voltammograms of NCGC electrode in 100 mM0, 75, 100, 200, 300, 400, 500, 600, 700, 800, 900 and 1000 mV s−1. (A) Inset:eaks (a) and cathodic peaks (b). (B–E) The dependency of anodic (B) and cathhe proportionality of anodic (D) and cathodic (E) peak currents on the square r

nodic charge greatly increased with respect to observed for theodified surface in the absence of alanine and it was followed

y decreasing the cathodic current upon increasing the concen-ration of alanine in solution. In the presence of 25 mM alanineith the potential sweep rate of 100 mV s−1, the anodic charge

ssociated with the anodic peak was quantitatively 98.9% of that

f the corresponding cathodic peak, while in the absence of ala-ine, this ratio was 48.0%. The anodic charge in the positiveweep was proportional to the bulk concentration of alanine andny increase in the concentration of alanine caused an almost

ttho

OH solution. Potential sweep rates from inner to outer are: 2, 5, 10, 20, 30, 40,of Ep vs. log ν for cyclic voltammograms depicted in the main panel for anodicC) peak currents on the potential sweep rate at lower values (1–30 mV s−1) andf sweep rate at higher values (60–1000 m Vs−1).

roportional linear enhancement of the anodic charge. In addi-ion, an anodic peak in the beginning of the cathodic half cycleppeared. The appearance of an anodic peak in the forward asell as in the reverse sweep was the distinct feature of electro-

atalytic oxidation of amino acids on noble metals [1]. Also,he cathodic current that ensued from the oxidation process in

he reverse cycle indicated that the rate determining step cer-ainly involves alanine and was incapable of reducing the entireigh valence nickel species formed in the oxidation cycle. More-ver, the regeneration of the anodic peak in the cathodic half

4626 S. Majdi et al. / Electrochimica Acta 52 (2007) 4622–4629

Fig. 4. Main panel: cyclic voltammograms of the NCGE electrode in 100 mMN3wc

cscraswwteps

N

ar

fpo

ttFfi2

Fig. 5. (A) Double steps chronoamperograms of NCGC electrode in 100 mMNaOH solution with different concentrations of alanine of: (a) 0 mM, (b) 10 mM,(c) 15 mM, (d) 20 mM, (e) 25 mM and (f) 30 mM. Potential steps were 500 mVaD(

1rsTuc

I

wtf

t

wocγ

c

aOH solution in the absence (a) and the presence (b) 10 mM; (c) 25 mM; (d)0 mM; (e) 50 mM; (f) 70 mM of alanine in the solution. Potential sweep rateas 100 mV s−1. Inset: Dependency of the charge under the anodic peak on the

oncentration of alanine in solution.

ycle indicated that an intermediate(s) generated in the anodicweep did not have enough time to undergo full oxidation and itsonsumption continued even after the potential sweep had beeneversed. These results indicated that alanine was oxidized byctive nickel moiety via a cyclic mediation redox process. Nickelpecies were immobilized on the electrode surface and the oneith a higher valence oxidized alanine via a chemical reactionhich resulted in the generation of low valence nickel. Along

his line, the high valence oxide was regenerated through thexternal electrical circuit. The redox transition of nickel speciesresent in the film is: and alanine is oxidized on the modifiedurface via the following reaction present in the film is:

i(II) � Ni(III) + e (5)

nd alanine is oxidized on the modified surface via the followingeaction:

Ni(III)–curcumin + Alanine

→ Intermediate + Ni(II)–curcumin (6)

Ni(III)–curcumin + Intermediate

→ Product + Ni(II)–curcumin (7)

NCGC electrode exhibited similar electrocatalytic responsesor other amino acids, l-arginine, glycine, l-lysine and l-henylalanine, thereby exhibiting its capability for selectivexidation of amino acids.

Double steps chronoamperograms were recorded by settinghe working electrode potentials to desired values and were used

o measure the catalytic rate constant on the modified surface.ig. 5A shows double steps chronoamperograms for the modi-ed electrode in the absence (a) and presence (b: 10, c: 15, d:0, e: 25 and f: 30 mM) of alanine over a concentration range of

FF

nd 290 mV, respectively. (B) Dependency of transient current on t−0.5. (C)ependence of Icatal/Id on t0.5 derived from the data of chronoamperograms of

a and d) in panel (A).

0–50 mM. The applied potential steps were 500 and 290 mV,espectively. Plotting of net current with respect to the minusquare roots of time, presented a linear dependency (Fig. 5B).herefore, a diffusion-controlled process was dominated. Bysing the slope of this line, the diffusion coefficient of alaninean be obtained according to Cottrell’s equation [25]:

= nFAD1/2Cπ−1/2t−1/2 (8)

here D is the diffusion coefficient and C is the bulk concentra-ion. The mean value of the diffusion coefficient of alanine wasound to be 1.00 ± 0.05 × l0−6 cm2 s−1.

Chronoamperometry was also be used for the evaluation ofhe catalytic rate constant according to [25]:

Icatal

Id= γ1/2

[π1/2erf(γ1/2) + exp (−γ)

γ1/2

](9)

here Icatal and Id are the currents in the presence and absencef alanine, γ = kCt is the argument of the error function, k is theatalytic rate constant and t is elapsed time. In the cases where> 1.5, erf(γ1/2) is almost equal to unity and the above equation

an be reduced to:

Icatal

Id= γ1/2π1/2 = π1/2(kCt)1/2 (10)

rom the slope of the Icatal/Id versus t1/2 plot, presented inig. 5C, the mean value of k for alanine was obtained as 3.65 ±

S. Majdi et al. / Electrochimica Acta 52 (2007) 4622–4629 4627

Table 1The electrocatalytic reaction rate constants (k), the diffusion coefficients (D) andthe electron transfer coefficient (α) for the amino acids

k (cm3 mol−1 s−1) D (cm2 s−1) α

Alanine 3.65 ± 0.02 × 105 1.00 ± 0.05 × 10−6 0.41 ± 0.03l-Phenylalanine 9.53 ± 0.05 × 103 1.48 ± 0.02 × 10−6 0.53 ± 0.04Glycine 2.03 ± 0.02 × 106 1.40 ± 0.02 × 10−6 0.52 ± 0.03ll

0cTdT

uri(Ac

E

wc

Fs59asa

Fig. 7. Main panel: Nyquist diagrams of NCGC electrodes recorded at oxidationpeak potential as dc-offset for 20 mM, 20 mM, 20 mM, 50 mM, 50 mM of ala-nine, l-arginine, glycine, l-lysine and l-phenylalanine, respectively, in NaOH11P

noS

-Lysine 1.45 ± 0.02 × 105 1.35 ± 0.04 × 10−6 0.54 ± 0.04-Arginine 9.73 ± 0.03 × 105 1.86 ± 0.03 × 10−6 0.43 ± 0.03

.02 × l05 cm3 mol−1 s−1. Similar chronoamperograms wereollected for l-arginine, glycine, l-lysine and l-phenylalanine.he values of D and k obtained according to the methodescribed in the above for these amino acids were reported inable 1.

Fig. 6A illustrates cyclic voltammograms of 30 mM alaninesing NCGC electrode recorded at different potential sweepates. The oxidation current of alanine on the modified surfacencreased linearly with the square root of the potential sweep rateFig. 6B), which indicated the mass transfer controlled process.lso, the value of electron transfer coefficient for the reaction

an be obtained from the following equation [26]:

=(

RT)

lnv + constant (11)

p2αF

hich is valid for a totally irreversible-diffusion controlled pro-ess. Using the dependency of anodic peak potential on the

ig. 6. (A) Cyclic voltammograms of the NCGE electrode in 100 mM NaOHolution in the presence of 30 mM alanine at various potential sweep rates of 2,, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 150, 200, 300, 400, 500, 600, 700, 800,00, 1000, 1100, 1200, 1300, 1400 mV s−1, 1500 mV s−1. (A) Dependence ofnodic peak current during the forward sweep on the square roots of potentialweep rate. (C) Dependence of the peak potential on log ν for the oxidation oflanine at NCGC electrode obtained from the data of panel (A).

gai

eidcptsgstotberbcides

00 mM solution. Inset: Typical cyclic voltammogram of the NCGE electrode in00 mM NaOH solution in the presence of 40 mM of l-arginine in the solution.otential sweep rate was 100 mV s−1.

atural logarithm of the potential sweep rate (Fig. 6C), the valuef electron transfer coefficient was obtained as 0.41 ± 0.03.imilar cyclic voltammograms were recorded for l-arginine,lycine, l-lysine and l-phenylalanine. Values of � were obtainedccording to this method for these amino acids and were reportedn Table 1.

The electrochemical impedance spectroscopy was alsomployed to shed light on the oxidation mechanism. The resultsndicated that although in the regime of cyclic voltammetry,ifferent amino acids represented similar patterns for electro-atalytic oxidation, Nyquist diagrams represented very differentatterns. Fig. 7 shows the Nyquist diagrams of NCGC elec-rode recorded at oxidation peak potential as dc-offset for someelected concentrations of the amino acids. The Nyquist dia-ram of NCGC electrode in l-arginine solution represented onelightly depressed capacitive semicircle which can be relatedo the combination of charge transfer resistance of transitionf Ni(II)/Ni(III) redox couple in the presence of l-arginine andhe double layer capacitance. The equivalent circuit compati-le with this Nyquist diagram was depicted in Scheme 2. In thislectrical equivalent circuit, Rs, CPEdl and Rct represent solutionesistance, a constant phase element corresponding to the dou-le layer capacitance and the charge transfer resistance. In thisircuit the charge transfer resistance of the electrode reaction

s the only circuit element that has a simple physical meaningescribing how fast the rate of charge transfer during l-argininelectro-oxidation changes with the electrode potential while theurface covered by reaction intermediate(s) is kept constant.

Scheme 2.

4628 S. Majdi et al. / Electrochimica Acta 52 (2007) 4622–4629

IiccosiwcaaerFtcreptdRoro[ia[wfescfa[

tfttltcoemd

Scheme 4.

dttstv

prbwropceaclatTe

4

mptwtwoot

Scheme 3.

n order to obtain a satisfactory fitting of Nyquist diagrams,t was necessary to replace the double layer capacitance with aonstant phase element, CPEdl, in the corresponding equivalentircuit. The most widely accepted explanation for the presencef this distributed element and the appearance of depressedemicircles in Nyquist plots, is microscopic roughness, caus-ng an inhomogeneous distribution in the solution resistance asell as in the double layer capacitance [27]. The result indi-

ated that l-arginine was oxidized on NCGC electrodes withoutny significant adsorption of reaction intermediate(s). This waslso supported by cyclic voltammetry studies which imply thatlectrocatalytic oxidation of l-arginine on NCGC electrodesepresented no significant anodic peak in the reverse sweep (seeig. 7, inset). The Nyquist diagram recorded for NCGC elec-

rode in the presence of alanine also showed a slightly depressedapacitive semicircle due to the combination of charge transferesistance of transition of Ni(II)/Ni(III) redox transition. How-ver, the Nyquist diagram rolled over at low frequencies andresented an inductive behavior. The equivalent circuit relatedo the Nyquist diagram recorded in the presence of alanine wasepicted in Scheme 3. In this electrical equivalent circuit, L andads are the electrical elements compatible with the adsorptionf reaction intermediate(s). It was reported that adsorption ofeaction intermediates during the electrocatalytic oxidation ofrganics can cause the inductive behavior in Nyquist diagrams9,28]. The inductive feature that appeared in Nyquist diagramss often found in electrochemical measurements of electrocat-lytic reactions controlled by intermediate absorbed species29]. In general, the inductive behavior appears in Nyquist plots,hen the variation of the electrode potential causes a variation of

aradaic current density via affection of both the strength of thelectric field in the double layer and another variable, usuallyurface coverage of adsorbed intermediate(s) [30]. In electro-atalytic reactions, the inductive behavior takes place when thearadaic current is governed by the occupation of an intermedi-te state, which decreases when the applied potential increases29].

The Nyquist diagram of NCGC electrodes in l-lysine solu-ion represented two depressed capacitive semicircles. The highrequency semicircle was related to the combination of chargeransfer resistance of transition of Ni(II)/Ni(III) redox couple inhe presence of l-lysine and the double layer capacitance. Theow frequency semicircle was related to the adsorption of reac-ion intermediate on the electrode surface. The equivalent circuitompatible with the Nyquist diagram recorded in the presence

f l-lysine was depicted in Scheme 4. CPEads and Rads are thelectrical elements related to the adsorption of reaction inter-ediate(s). Because the electrocatalytic oxidation of l-lysine

epicted capacitive semicircle in the Nyquist diagram, it can be

oior

Scheme 5.

educed that reaction intermediate(s) of electrocatalytic oxida-ion of l-lysine on NCGC electrode must be quite different inheir adsorption nature (mode of adsorption, charge of adsorbedpecies, type of adsorbed atom, etc.) from that for alanine and/orhe electrocatalytic oxidation of the amino acids that take placeia different rate-limiting steps.

Nyquist diagrams of NCGC electrode in glycine and l-henylalanine solutions represented quite different patterns withespect to the other amino acids. The equivalent circuit compati-le with the Nyquist diagram recorded in the presence of l-lysineas depicted in Scheme 5. Nyquist diagrams at high frequencies

epresented a capacitive semicircle related to the combinationf charge transfer resistance of transition of nickel species in theresence of glycine and l-phenylalanine and the double layerapacitance. However, the diagrams represented negative differ-ntial resistance and rolled over the real axis at low frequenciesnd went to the second quadrant. In this electrical equivalent cir-uit CPE1 and R1 < 0 are the electrical elements describing theow frequency response. The diagram crossed the negative realxis at some finite frequencies (hidden negative differential resis-ance) indicating a dynamic instability of the stationary state.his negative impedance is characteristic of systems capable ofxhibiting galvanostatic potential oscillation [31].

. Conclusion

This work presented the preparation and usefulness of aodified electrode consisting of nickel ions loaded into a

oly-Ni–curcumin matrix and electrodeposited in a solution con-aining Ni–curcumin complex on GC electrode. The electrodeas electro-catalytically active around 400 mV/Ag, AgCl where

he GC electrode possessed no activity. Chronoamperometricorks showed a large anodic current at the oxidation potentialf low valence nickel in further support of the mediated electro-xidation. The values for the catalytic rate constant and electronransfer coefficient for oxidation and diffusion coefficient were

btained for some amino acids. The different impedance behav-ors for different amino acids revealed that the amino acidsxidized on the modified surface via different mechanisms andate-limiting steps.

ica A

A

baGc

R

[

[

[

[[

[[

[[[

[

[[[[[

[[[

S. Majdi et al. / Electrochim

cknowledgements

The authors gratefully acknowledge the support of this worky Research Council of K.N. Toosi University of Technologynd University of Tehran. The authors are also grateful to Miss. Ghafourifar, Miss Z. Jabbari and Dr. F. Ricks for their fruitful

ollaboration.

eferences

[1] S. Mho, D.C. Johnson, J. Electroanal. Chem. 495 (2001) 152.[2] H. Shen, S.R. Witowski, B.W. Boyd, R.T. Kennedy, Anal. Chem. 71 (1999)

987.[3] K.L. Kostel, S.M. Lunte, J. Chromatogr. B 695 (1997) 27.[4] T. Hokfelt, Neuron 7 (1991) 867.[5] K. Ogura, M. Kobayashi, M. Nakayama, Y. Miho, J. Electroanal. Chem.

449 (1998) 101.[6] S.M. MacDonald, S.G. Roscoe, Electrochim. Acta 42 (1997) 1189.[7] D.G. Marangoni, I.G.N. Wylie, S.G. Roscoe, Bioelectrochem. Bioenerg.

25 (1991) 269.[8] Y.S. Fung, S.Y. Mo, Anal. Chem. 67 (1995) 1121.[9] M. Jafarian, M.G. Mahjani, H. Heli, F. Gobal, H. Khajehsharifi, M.H.

Hamedi, Electrochim. Acta 48 (2003) 3423.10] M. Jafarian, M.G. Mahjani, H. Heli, F. Gobal, M. Heydarpoor, Electrochem.

Commun. 5 (2003) 184.11] M. Yousef Elahi, H. Heli, S.Z. Bathaie, M.F. Mousavi, J. Solid State Elec-

trochem. 11 (2007) 273.

[

[[

cta 52 (2007) 4622–4629 4629

12] S. Majdi, A. Jabbari, H. Heli, J. Solid State Electrochem. 11 (2007)601–607.

13] G. Roslonek, J. Taraszewska, Electrochim. Acta 39 (1994) 1887.14] M. Vilas-Boas, C. Freire, B. de Castro, A.R. Hillman, J. Phys. Chem. B

102 (1998) 8533.15] J. Obirai, F. Bedioui, T. Nyokong, J. Electroanal. Chem. 576 (2005) 323.16] M. Beley, J.P. Collin, R. Ruppert, J.P. Sauvage, J. Am. Chem. Soc. 108

(1986) 7461.17] F. Xu, H. Li, S.J. Cross, T.F. Guarr, J. Electroanal. Chem. 368 (1994) 221.18] J. Obirai, T. Nyokong, Electrochim. Acta 50 (2005) 5427.19] K.I. Ozoemena, Z. Zhao, T. Nyokong, Inorg. Chem. Commun. 9 (2006)

223.20] A. Ciszewski, G. Milczarek, B. Lewandowska, K. Krutowski, Electroanal-

ysis 15 (2003) 518.21] T.R.I. Cataldi, D. Centonze, G. Ricciardi, Electroanalysis 7 (1995) 312.22] A. Ciszewski, G. Milczarek, J. Electroanal. Chem. 413 (1996) 137.23] A. Ciszewski, Electroanalysis 7 (1995) 1132.24] E. Laviron, J. Electroanal. Chem. 101 (1979) 19.25] A.J. Bard, L.R. Faulkner, Electrochemical Methods, John Wiley and Sons,

New York, 2001.26] J.A. Harrison, Z.A. Khan, J. Electroanal. Chem. 28 (1970) 131.27] A. Maritan, F. Toigo, Electrochim. Acta 35 (1990) 141.28] M.V. ten Kortenaar, C. Tessont, Z.I. Kolar, H. van der Weijde, J.

Elechtrochem. Soc. 146 (1999) 2146.

29] J. Bisquert, H. Randriamahazaka, G. Garcia-Belmonte, Electrochim. Acta

51 (2005) 627.30] C.N. Cao, Electrochim. Acta 35 (1990) 831.31] M.T.M. Koper, M. Hachkar, B. Beden, J. Chem. Soc. Faraday Trans. 92

(1996) 3975.