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Sensors and Actuators B 122 (2007) 549–555

An electrochemical sensor for l-dopa based on oxovanadium-salenthin film electrode applied flow injection system

Marcos F.S. Teixeira a,∗, L.H. Marcolino-Junior b, O. Fatibello-Filho b,E.R. Dockal b, Marcio F. Bergamini c

a Universidade Estadual Paulista-Faculdade de Ciencias e Tecnologia, Departamento de Fısica, Quımica e Biologia,Rua Roberto Simonsen, 305, P.O. 467 CEP, 19060-900 Presidente Prudente, SP, Brazil

b Departamento de Quımica, Universidade Federal de Sao Carlos, Sao Carlos, SP, Brazilc Instituto de Quımica, Universidade Estadual Paulista, Araraquara, SP, Brazil

Received 18 April 2006; received in revised form 26 June 2006; accepted 26 June 2006Available online 17 August 2006

bstract

An oxovanadium-salen complex (N,N′-ethylene-bis(salicylideneiminato) oxovanadium) thin film deposited on a graphite–polyurethane electrodeas investigated with regard to its potential use for detection of l-dopa in flow injection system. The oxovanadium(IV)/oxovanadium(V) redox

ouple of the modified electrode was found to mediate the l-dopa oxidation before its use in the FIA system. Experimental parameters, such asH of the carrier solution, flow rate, sample volume injection and probable interferents were investigated. Under the optimized FIA conditions, the

mperometric signal was linearly dependent on the l-dopa concentration over the range 1.0 × 10−6 to 1.0 × 10−4 mol L−1 (Ianodic �A) = 0.01 + 0.25l-dopa �mol L−1]) with a detection limit (S/N = 3) of 8.0 × 10−7 mol L−1 and a sampling frequency of 90 h−1 was achieved. For a concentrationf 1.0 × 10−5 mol L−1 l-dopa, the R.S.D. of nine consecutive measurements was 3.7%.

2006 Elsevier B.V. All rights reserved.

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eywords: Modified electrode; Oxovanadium-salen; Amperometric detection;

. Introduction

l-Dopa is a chemical substance used in the treatment ofatients with Parkinson’s disease, acting efficiently in the ease ofymptom. The substance is converted in dopamine by enzymaticeaction (dopa-descarboxilase) compensating the deficiency ofopamine in organism [1].

Several methods for the l-dopa determination have beenescribed in literature including spectrophotometric [2,3], higherformance liquid chromatography [4–6] and capillary zonelectrophoresis with electrochemical detection [7,8]. Neverthe-ess, each technique has often suffered from diverse disadvan-ages with regard to cost and selectivity, the use of organic

olvents, complex sample preparation procedures and long anal-sis time.

∗ Corresponding author. Tel.: +55 18 32295355; fax: +55 18 32215682.E-mail address: funcao@fct.unesp.br (M.F.S. Teixeira).

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925-4005/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2006.06.032

a; Flow analysis system

However, amperometric based method is one of the mostrequently used in the catecholamines determination using elec-rochemical sensors [9–13] and most of them are related toiosensors [14–17]. Chemically modified electrodes in gen-ral based on the incorporation of a catalyst or a redoxediator, have extended the applicability on flow injection

echniques with amperometric detection to pharmaceutical,ood, forensic and clinical sciences [18,19]. Electrodes mod-fied with inorganic complex materials have been largelysed in the area of electroanalysis to improve electrocat-lytic properties, to increase the stability and reproducibil-ty of the electrode response and to improve the selectivity12,20–22].

This paper reports the application of a graphite–polyurethanelectrode coated with thin film of oxovanadium-salenomplex (N,N′-ethylene-bis(salicylideneiminato) oxovanadium

VO(salen)]) as an amperometric sensor for l-dopa determina-ion in flow injection system. The influence of several parameterspotential, pH and interference) besides the parameters of theow system was studied.

5 nd Actuators B 122 (2007) 549–555

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r2c(

2

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mai

2

tT

Fig. 2. Schematic diagram of the flow system used for evaluation of the sensorfta

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50 M.F.S. Teixeira et al. / Sensors a

. Experimental

.1. Apparatus

Cyclic voltammetric and amperometric measurements werearried out with an AUTOLAB PGSTAT-30 (Ecochemie) con-rolled by a personal computer using the GPES 4.8 software.he measurements were performed in a three-electrode flow cellonfiguration (Fig. 1) using a graphite–polyurethane electrodeGPE) coated with thin film of oxovanadium-salen complex asorking (surface area of 12.6 mm2), an Ag/AgCl (3 mol/L KCl)

s reference and a platinum auxiliary electrode (φ = 3 mm disk).he body of the electrochemical flow cell (Fig. 1A) was fab-

icated with polyurethane resin from vegetable oil [23]. Theffective volume of the flow cell was 77 �L.

For cyclic voltammetric measurements the potential wasanged from 0.0 to +0.8 V versus Ag/AgCl at a scan rate of5 mV s−1. Stationary solutions were used in such case. Theurrent measurements were performed using the GPES softwareEcochemie) by chronoamperometry (constant potential).

.2. Reagents and solutions

All the solutions were prepared using water purified with aillipore Milli-Q system. All chemicals were of analytical grade

nd used without further purification. The supporting electrolytesed for all experiments was a 0.10 mol L−1 KCl solution (pH.0). l-Dopa (Aldrich) standard solution (1.0 × 10−3 mol L−1)as prepared by dissolution of the appropriate amount of l-dopa

n 100.0 mL of KCl solution.The oxovanadium-salen (N,N′-ethylene-bis(salicylidenei-

inato) oxovanadium(IV) [VO(salen)]) complex utilized asctive material in electrode preparation was synthesized accord-ng to a method described by Zamian and Dockal [24].

.3. Preparation of oxovanadium-salen thin film electrode

The graphite–polyurethane electrode used as a base elec-rode was prepared as previously described in literature [23].his base electrode was gently borrowed by Prof. E.T.G. Cav-

otrv

ig. 1. Schematic diagram of the electrochemical flow cell (EFC) used in the amperomeference electrode (Ag/AgCl); 3, platinum electrode; 4, oxovanadium-salen thin filmoated.

or l-dopa. P, Peristaltic pump; I, manual injector; S, sample or reference solu-ions; L, sample volume; C, carrier solution; EFC, electrochemical flow cell; R,mperometer (recorder); W, waste.

lheiro. Prior to coating, the graphite–polyurethane electrodeas conditioned following a reported polishing/cleaning pro-

edure [23]. Coating was obtained by the droplet evaporationethod placing on the electrode surface. The [VO(salen)] solu-

ion (10 mmol L−1 in CH3CN) was prepared previously to beeposited in the electrode surface. One hundred microliter of thisolution was transferred onto a freshly cleaned surface and theolvent was allowed to evaporate. This procedure was repeatedwo more times in order to increase the thickness of the modifierlm and thus to obtain more robust sensor. The electrode coatedith [VO(salen)] was activated by cyclic voltammetry in KCl

olution.

.4. Flow injection analysis system with amperometricensor for l-dopa

The electrochemical flow cell was inserted in an one-channelow injection system schematically represented in Fig. 2. Theystem was assembled with a peristaltic pump (Ismatec, model618-40, Switzerland) and a manual injector made of Perspex®

ith two fixed sidebars and a sliding central bar. The manifoldonnections were made with polyethylene tubing (0.76 mm i.d.).

The 0.10 mol L−1 KCl solution was used as the carrier solu-ion (C) at a flow rate of 3.4 mL min−1 and a potential (0.61 Versus Ag/AgCl) was applied to obtain a stable baseline. Since

xygen did not interfere with the analysis at the detection poten-ials, no de-aeration was performed in this study. The l-dopaeference in 0.10 mol L−1 KCl solution contained in the sampleolume loop (L, 612.8 �L) was injected and transported by the

etric measurements in flow injection system. (A) 1, polyurethane resin block; 2,electrode; 5, polyethylene tubing (flow). (B) Graphite–polyurethane electrode

nd Ac

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M.F.S. Teixeira et al. / Sensors a

arrier stream after the baseline had reached a steady-state value.he analytical path was 30 cm and the entire flow injection sys-

em was kept at room temperature. Peak height of the oxidationurrent was used as a signal and three injections were made forach l-dopa standard concentration.

. Results and discussion

.1. Electrochemical studies of the oxovanadium-salen thinlm electrode

Cyclic scans were conducted in unstirred solution at a poten-ial sweep rate of 25 mV s−1 from 0 to 0.8 V versus Ag/AgCl,ecording firstly the oxidation step and then the reduction step atxovanadium-salen thin film electrode. Fig. 3A presents a typi-al cyclic voltammogram with two peaks at +0.52 V (anodic) and

−1

0.45 V (cathodic) in 0.10 mol L KCl solution (pH 6.0), whichemained stable after the third cycle. This is typical behavior forn electrode surface immobilized with a redox couple whichan be usually assumed like a quasi-reversible single-electron

ig. 3. (A) Cyclic voltammogram of the oxovanadium-salen thin film electrodend (B) cyclic voltammograms obtained for oxovanadium-salen thin film elec-rode in the absence (dash line – curve 1) and in the presence (solid line – curve) of 1.8 × 10−5 mol L−1 l-dopa. Scan rate 25 mV s−1 in 0.10 mol L−1 KClolution at pH 6.0.

wFrec

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3fi

cc(lcrebg0neTi(

V

V

tuators B 122 (2007) 549–555 551

eduction/oxidation of the oxovanadium(IV)/oxovanadium(V)ouple [22,25,26]. The ratio of cathodic to anodic peak currentst various scan rates was almost unity. The linear correlationf the peak current with the square root of scan rate showedhat the system is similar to the process controlled by diffusion.his behavior suggests a mobility of the counterions of the sup-orting electrolyte necessary for charge transport or to keep thelectroneutrality at the electrode surface during the redox pro-ess [27]. The peak-to-peak potential separations (�Ep is about0.0 mV) for scan rate below 25 mV s−1. This suggests facileharge transfer kinetics over this sweep range.

The surface concentration of electroactive species (Γ VO(salen)ol cm−2) was estimated from the background-corrected elec-

ric charge, Q, under the anodic peaks in accordance with theheoretical relationship [28] (Eq. (1)).

VO(salen) = Q

nFA(1)

here n represents number of electrons transferred (assume ≈1),the Faraday constant (96,485 C mol−1) and A is the geomet-

ic surface area of the electrode (0.126 cm2). After cycling thelectrode in 0.10 mol L−1 KCl solution, the estimated surfaceoncentration was found to equal 5.95 × 10−11 mol cm−2.

The electrochemical behavior of GPE coated thin film ofxovanadium-salen complex in 0.10 mol L−1 KCl solution wastudied over a large pH range using cyclic voltammetry and scan-ing the potential from 0 to 0.8 V (versus Ag/AgCl) at 25 mV s−1

can rate. The peak potential for oxovanadium-salen complex athe surface of GPE was pH independent in the 4.0–9.0 range.his behavior suggests that the [VO(salen)] complex is stable in

his pH range.

.2. Electrocatalytic activity of the oxovanadium-salen thinlm electrode for l-dopa

Fig. 3B shows the cyclic voltammograms obtained forarbon paste electrode modified with oxovanadium-salenomplex in 0.10 mol L−1 KCl solution (pH 6.0) in the absencecurve 1 – dash line) and presence (curve 2 – solid line) of-dopa. With the addition of l-dopa in solution, the anodic peakurrent of the modified electrode increased significantly. Thisesult shows that the oxovanadium-salen complex at surfacelectrode promotes oxidation of l-dopa. The electrochemicalehavior of l-dopa (1.8 × 10−5 mol L−1) at the unmodifiedraphite–polyurethane electrode presents an oxidation peak at.75 V (versus Ag/AgCl) at 25 mV s−1 scan rate. A 280 mVegative shift of anodic peak potential indicates a stronglectrocatalytic effect toward l-dopa oxidation in this medium.he voltammetric response of the modified electrode for l-dopa

s based in two cycles, initiating with a electrochemical stepEq. (2)) and followed by chemical step (Eq. (3)):

(IV)O complex(surface) → V(V)O complex(surface) + e− (2)

(V)O complex(surface)+1

2l-dopa(aq) → V(IV)O complex(surface)

+ 1

2dopaquinone(aq) + H+

(aq) (3)

5 nd Actuators B 122 (2007) 549–555

Tocpseo

cwvtlMedwv

3ei

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mpoaToehfAo++diape

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Fig. 4. Transient current signals obtained in triplicate for studying the samplevolume effect on the magnitude of amperometric response of the sensor. Samplevolume: (A) 81.7, (B) 163.7, (C) 245.1, (D) 326.8, (E) 408.6, (F) 612.8 and (G)8Ai

maucr3dipthan 3.4 mL min is caused by the residence time in the detectorand probably due to the rate-limiting step for electron transferat the electrode. A plot of the amperometric response of modi-fied electrode for l-dopa ions versus cube root of the flow rate

52 M.F.S. Teixeira et al. / Sensors a

he reaction can be brought about electrochemically wherebyxovanadium(IV) complex is first oxidized to oxovanadium(V)omplex at the electrode surface. The oxovanadium(V) com-lex then undergoes a catalytic reduction by the l-dopa inolution back to oxovanadium(IV) complex, which can then belectrochemically reoxidized to produce an enhancement in thexidation current.

The effect of pH solution on anodic peak current of the GPEoated thin film of oxovanadium-salen in the presence of l-dopaas studied. The anodic peak current gradually increased withariation of pH range of 3.0–8.0. This behavior is probably dueo the influence of H+ on the kinetic chemical reaction between-dopa and oxovanadium complex on the surface (see Eq. (3)).oreover the chemical oxidation of dopa to dopaquinone is gen-

rally easier in alkaline medium than in acidic medium. The stan-ard heterogeneous rate constant values are found to increaseith increase of pH, indicating easier oxidation at higher pHalues [11,29].

.3. Application of the oxovanadium-salen thin filmlectrode as amperometric sensor for l-dopa in flownjection analysis system

It will be demonstrated that significant improvement in theensor sensitivity can be achieved by using a flow-through thin-ayer cell. The oxovanadium-salen thin film electrode was testedor the electrocatalytic oxidation of l-dopa in flow injectionnalysis (FIA).

A hydrodynamic voltammogram (HV) for 1.0 × 10−5

ol L−1 l-dopa at a flow rate of 1.7 mL min−1 under severalotentials (+0.20 to +0.80 V versus Ag/AgCl) was studied. Thexidation current for the l-dopa reached a maximum valuet about +0.60 V and the half-wave potential was of 0.45 V.his corresponded approximately to the peak potential for thexidation of oxovanadium(IV) in the electrode surface. Thenhancement of the response is attributed to the formation ofigh oxidation state species of oxovanadium which is highlyavorable to electrocatalysis of the l-dopa (see Eqs. (2) and (3)).

stable, approximately constant response for l-dopa can bebtained when the electrode potential was anywhere between0.60 and +0.80 V versus Ag/AgCl. An operating potential of0.61 V (versus Ag/AgCl) was selected for all further studies,ue to the good reproducibility, both to minimize the potentialnterference at the higher positive potentials and to maintain

relative low background signal. Also relative low operatingotential is beneficial to the long-term stability of the modifiedlectrode.

The effect of the sample volume from 81.7 to 817.1 �L waslso investigated by changing the length of the sample loop10–100 cm, 1.0 mm i.d.) for a 1.0 × 10−5 mol L−1 l-dopa in.10 mol L−1 KCl solution (pH 6.0) on the oxovanadium-salenhin film electrode. As can be observed in Fig. 4, the ampero-

etric response gradually increased with the increase of sample

olume from 81.7 to 817.1 �L for flow rate of 1.7 mL min−1.herefore, a sample volume of 612.8 �L was selected as theost appropriate since high flow rates produced short analysis

ime and improved the cleaning of the electrode surface.

Fi(

17.1 �L of 1.0 × 10−5 mol L−1 l-dopa. Applied working potential = 0.61 V vs.g/AgCl; flow rate = 1.7 mL min−1. The anodic current vs. sample volume is

llustrated in detail.

The effect of the flow rate on the magnitude of ampero-etric response of the sensor for l-dopa was investigated by

pplying the working potential of +0.61 V and sample vol-me of 612.8 �L. The results showed that the flow injectionurrent response is flow rate dependent (see Fig. 5). The cur-ent increases with flow rate reaching a maximum value at.4 mL min−1. Increasing in flow rates causes a change in theiffusion profile at the electrode surface and consequently anncrease of the diffusion current or a more efficient mass trans-ort [18,30]. The limited electrode response at flow rates higher

−1

ig. 5. Dependence of anodic current response with flow rate. Applied work-ng potential = 0.61 V vs. Ag/AgCl; sample volume = 612.8 �L of l-dopa1.0 × 10−5 mol L−1).

M.F.S. Teixeira et al. / Sensors and Actuators B 122 (2007) 549–555 553

Fig. 6. Analytical curve for l-dopa in carrier of 0.10 mol L−1 KCl solution (pH6.0) using the oxovanadium-salen thin film electrode. For each l-dopa concen-tAl

siFl

I

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twcswrlde

I

(

Asm(

tT

tovoaaosctfTetRdsvcmctdtp

4

fii

ration was realized in triplicate the anodic peak current measurements (n = 3).pplied working potential = 0.61 V vs. Ag/AgCl; sample volume = 612.8 �L of-dopa; flow rate = 3.4 mL min−1.

hows a linear behavior with a correlation coefficient of 0.996n the range flow rate of 0.9–3.4 mL min−1. Following Bard andaulkner [31] the mass-transport-limited current Ilim in a thin-

ayer channel flow cell is given by:

lim = knFC

(DA

b

)2/3

f 1/3 (4)

here k is the cell constant (k = 1.467 for a channel flow thin-ayer cell), n the number of electrons, F the Faraday constant, Che concentration of the species involved in the electrochemicaleaction, f the flow rate of the carrier, D the diffusion coefficientf the reacting species, A the electrode surface area and b thehannel height, respectively. This result is in agreement with pre-ious findings, since the catalytic reaction between the modifiedlectrode and l-dopa is relatively a laminar flow regime, whichs attained in the thin-layer of electrolyte at the electrode surface.owever, future studies about the thin-layer flow cell geometryill be necessary for explaining the relationship between the

urrent electrode and flow rate. A flow rate of 3.4 mL min−1

as used in the further experiments, which maintained a goodensitivity and stability in the amperometric response.

After optimizing the operating conditions for the elec-rochemical sensor for l-dopa, amperometric measurementsere carried out in KCl solution containing different l-dopa

oncentrations in order to obtain the analytical curve. Fig. 6hows that the anodic peak current of the modified electrodeas linearly dependent on the l-dopa concentration in the

ange from 1.0 × 10−6 to 1.0 × 10−4 mol L−1 with a detectionimit of 8.0 × 10−7 mol L−1 of l-dopa (three times the standardeviation of the base line/slope) [32]. The linear regressionquation is:

pa(�A) = 0.01 + 0.25(±0.01)[l-dopa](�mol L−1)

r = 0.9988, n = 9) (5)

l-dopa concentration level of 1.0 × 10−5 mol L−1 waselected to examine the repeatability of the amperometriceasurements of the sensor. The relative standard deviation

R.S.D.) of nine measurements was of 3.7%, indicating that

wwfit

Fig. 7. Molecular structures of catecholamines.

he modified electrode presents good stability and repeatability.he sampling frequency was 90 determinations per hour.

Finally, the generic nature of the modified electrode reac-ivity towards the oxidation of other catecholamine can bexidized using the amperometric procedure in FIA (0.61 Versus Ag/AgCl). The corresponding amperometric responsesf solutions containing 1.0 × 10−4 mol L−1 l-dopa, carbidopa,drenaline and dopamine (pH 6.0, 0.1 mol L−1 KCl solution)t oxovanadium-salen thin film electrode are studied. Analysisf the amperometric response obtained for each catecholaminepecies reveals that the anodic process for the oxidation of theatechol moiety but having different sensitivity. The results showhat the sensor sensititivity to four catecholamines increase in theollowing order: dopamine, l-dopa, adrenaline and carbidopa.his behavior is probably due the steric effect of the periph-ral group at the catechol group (see Fig. 7) that can influencehe reaction of oxovanadium complex with reacting species.ecently, Shim and co-workers [33] studied the simultaneousetermination of neurotransmitters using capillary electrophore-is with a modified electrode. The plot of applied potentialersus anodic peak current exhibited distinct waves for theatecholamine species, showing that properties of the surfaceodifier influence in the detection of analogous electroactive

ompounds. The responses, therefore, emphasize the fact thathe use of modified electrode produces an increase in the oxi-ation current for the catechol moieties, thereby demonstratinghe possible use of such electrodes in post-separation detectionrotocols [34].

. Conclusions

This work demonstrates the use of an oxovanadium-salen thinlm electrode in FIA as amperometric detector for l-dopa. Flow

njection analysis coupled with an electrochemical flow cell

as employed to increase the sensitivity. Cyclic voltammetryas applied to study the electrochemical behavior of the modi-ed electrode and the electrocatalytic activity of the complex in

he l-dopa oxidation. The redox behavior of the modified elec-

554 M.F.S. Teixeira et al. / Sensors and Actuators B 122 (2007) 549–555

Table 1Analytical features of different electrochemical sensors for l-dopa determination

Analytical parameters Oxovanadium-salen thin film electrode Disposable electrochemical sensor [13] Polycarbazole electrode [9]

Linear range (mol L−1) 1.0 × 10−6 to 1.0 × 10−4 9.9 × 10−5 to 1.2 × 10−3 1.0 × 10−3 to 1.0 × 10−2

Detection limit (mol L−1) 8.0 × 10−7 6.8 × 10−5 –S

totald

smtattrgorar

A

f

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

ensitivity (�A L mmol−1) 250

rode could be attributed to the reaction of oxovanadium(IV) toxovanadium(V) assumed to be a quasi-reversible one-electronransfer. The oxovanadium-salen thin film electrode exhibitedgood activity for the oxidation and detection of l-dopa. Ana-

ytical features of several electrochemical sensors for l-dopaetermination are presented in Table 1.

In addition, the mechanism response of the oxovanadium-alen thin film electrode showed to be more effective than theodified carbon paste electrode [35]. Overall, the trend in elec-

rochemical reactivity reflects the effect of the pasting liquidnd type of carbon upon the electron-transfer rate. The addi-ion of any pasting liquid is known to decrease the electron-ransfer rates compared to the ‘dry’ carbon limit [36]. Thisesult indicates that oxovanadium-salen in the surface of theraphite–polyurethane presents major interaction with the aque-us medium and consequently easy the reaction of mediator witheacting species. Thus the reaction thickness layer for electrocat-lytic process should have a pseudo first-order couple chemicaleaction.

cknowledgment

The financial support by the CNPq (474367/2004-5) is grate-ully acknowledged.

eferences

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