Electrochemical characterization of poly(eriochrome black T) modified glassy carbon electrode and its application to simultaneous determination of dopamine, ascorbic acid and uric acid

Electrochimica Acta 52 (2007) 6165–6171

Electrochemical characterization of poly(eriochrome black T) modified glassy carbon electrode and its application to simultaneous determination of dopamine, ascorbic acid and uric acid Hong Yao a,∗ , Yuanyuan Sun a , Xinhua Lin a , Yuhai Tang b , Liying Huang a a

Department of Pharmaceutical Analysis, Faculty of Pharmacy, Fujian Medical University, Fuzhou 350004, PR China b Department of Chemistry, College of Science, Xi’an Jiaotong University, Xi’an 710061, PR China Received 2 March 2007; received in revised form 25 March 2007; accepted 1 April 2007 Available online 5 April 2007

Abstract A polymerized film of eriochrome black T (EBT) was prepared on the surface of a glassy carbon (GC) electrode in alkaline solution by cyclic voltammetry (CV). The redox response of the poly(EBT) film at the GC electrode appeared in a couple of redox peak in 0.1 M hydrochloride and the pH dependent peak potential was −55.1 mV/pH which was close to the Nernst behavior. The poly(EBT) film-coated GC electrode exhibited excellent electrocatalytic activity towards the oxidations of dopamine (DA), ascorbic acid (AA) and uric acid (UA) in 0.05 mM phosphate buffer solution (pH 4.0) and lowered the overpotential for oxidation of DA. The polymer film modified GC electrode conspicuously enhanced the redox currents of DA, AA and UA, and could sensitively and separately determine DA at its low concentration (0.1 ␮M) in the presence of 4000 and 700 times higher concentrations of AA and UA, respectively. The separations of anodic peak potentials of DA–AA and UA–DA reached 210 mV and 170 mV, respectively, by cyclic voltammetry. Using differential pulse voltammetry, the calibration curves for DA, AA and UA were obtained over the range of 0.1–200 ␮M, 0.15–1 mM and 10–130 ␮M, respectively. With good selectivity and sensitivity, the present method provides a simple method for selective detection of DA, AA and UA in biological samples. © 2007 Elsevier Ltd. All rights reserved. Keywords: Poly(eriochrome black T) modified electrode; Electrocatalysis; Dopamine; Ascorbic acid; Uric acid

1. Introduction Dopamine is one of the most important catecholamine neurotransmitters in mammalian central nervous system [1–4]. As a cholinergic drug, DA is widely applied to the treatment of circulatory collapse syndrome caused by myocardial infarction, trauma, renal failure, cardiac surgery, or congestive cardiac failure. Consequently, it has attracted much of interest of electrochemists to develop the detection methods of DA no matter in route or in vivo analysis. However, in assay of DA, the electrochemical methods suffer from inferior selectivity because of the presence of AA and UA that have higher concentrations than DA in physiological fluids and whose oxidation potentials always are close to that

∗

Corresponding author. E-mail address: [email protected] (H. Yao).

0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.04.013

of DA. Therefore, it is a significant attempt to separate the peak potentials of oxidation between DA and AA, or UA. Of many electrochemical approaches have been used to implement the above goal [5–11]. Among these methods, using electroactive polymer-coated electrode to determine DA in the presence of AA, or UA displayed excellent selectivity and sensitivity. For example, a poly(phenosafranine) film-modified GC electrode exhibited potent and persistent electron-mediating behavior followed by well-separated oxidation peaks towards AA, DA and serotonin with activation overpotential, which is 200 mV lower than that of the bare electrode for AA oxidation [12]. Roy et al. also described an electropolymerized film of N,N-dimethylaniline modified electrode, which could separate the DA and AA oxidation peak potentials and could detect DA at its low concentration (e.g., 0.2 ␮M) in the presence of 1000 times higher concentration of AA [13]. In addition, there were reports using 2,2-bis(3-amino4-hydroxyphenyl)hexafluoropropane [14], sulfosalicylic acid

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[15], dinickel(II) 2,2 -bis(1,3,5,8,12-pentaazacyclotetradec-3yl)-diethyl disulfide perchlorate [16], glutamic acid [17], pyrrole [18], aminobenzoic acid [19], cobalt hexacyanoferrate [20], melanin-type [21], and nickel hexacyanoferrate(II–III) [22] as monomer to modify electrode for the detection of DA in the presence of AA. In this paper, an electropolymerized film of eriochrome black T (EBT) was prepared on the surface of a glassy carbon electrode by cyclic voltammetry (CV). The poly-EBT on the surface of GC electrode had a high concentration of negative-charged function group –SO3 − and the electron-rich oxygen atom on its surface. The poly-EBT film at the electrode conspicuously enhanced the redox peak current of DA and could separately determine DA at its low concentration (0.1 ␮M) in the presence of 4000 and 700 times higher concentrations of AA and UA, respectively. The separations of oxidation peak potentials of DA–AA and UA–DA were over more than 210 mV and 170 mV, respectively, by cyclic voltammetry. Using differential pulse voltammetry technique, the calibration curves for DA, AA and UA were obtained over a wide range (0.1–200 ␮M for DA, 0.15–1 mM for UA and 10–130 ␮M for UA at the poly-EBT GC electrode). The detection limit of DA in the presence of AA (0.4 mM) and UA (70 ␮M) was found to be 20 nM, which was more sensitive and excellent than that reported in literatures [12,15,17,18,21,22]. Thus, the present study provides a novel method for selective and sensitive detection of DA, AA and UA in the presence of the other two species, which has a significant attraction in biological and chemical fields. 2. Experimental 2.1. Chemicals UA was purchased from Fluka (Switzerland). Dopamine hydrochloride and l-ascorbic acid were obtained from National Institute for the Control of Pharmaceutical and Biological Products (Beijing, China). EBT was purchased from Shanghai Chemical Reagents Company (China). All reagents were of analytical grade and used without any further purification. Phosphate buffer solutions (PBS) were prepared by mixing 0.02 M NaCl and 0.05 M NaH2 PO4 –Na2 HPO4 , and then adjusting the mixture solution’s pH with 0.05 M H3 PO4 or 0.1 M NaOH. Double-distilled water was used to prepare all solutions. All experiments were performed at room temperature.

2.3. Preparation of pretreated and poly(EBT) modified GC electrode Before modifying, the GC electrode was polished with 0.3␮m and 0.05 ␮m alumina slurries for 2 min each step, followed by thoroughly rinsing with water and sonicating in turn with distilled water, ethanol and distilled water for 2 min each. After polished and rinsed, the electrode was examined by cyclic voltammetry using 1 mM potassium ferricyanide solution (by evaluating the oxidation peak current and peak potential). The voltammograms were performed between −0.2 V and 0.8 V at 100 mV/s. The polished electrode was electrochemically pretreated by cycling the potential scan between −0.4 V and 1.5 V in 0.1 M H2 SO4 at the scan rate of 100 mV/s for 25 times and then was scanned in 0.1 M NaOH under the same conditions for 25 times to obtain the pretreated electrode. The poly(EBT)coated electrode was fabricated on the same conditions with the pretreated electrode but in the presence of 0.1 M NaOH containing 0.5 mM EBT. After polymerization, poly(EBT) film was washed with water and cycled scan in pH 4.0 PBS between −0.2 and 0.8 to eliminate untreated EBT at 100 mV/s for five cycles. 3. Results and discussion 3.1. Formation of poly(EBT) film and its electrochemical properties Fig. 1 displays the CV graph of EBT electro-polymerization over the range of −0.4 V to 1.5 V at 100 mV/s for 25 cycles. During the polymerized process, an anodic peak (p1 ) at 0.11 V corresponding to the oxidation of EBT descended gradually with cyclic time increasing. A cathodic peak (p2 ) formed at approximate −0.37 V with incessant scans and the peak currents also decreased continuously. Both p1 and p2 trended to be stable after 20 scans. These facts suggest that the initial formed poly(EBT) film had a leaching process with scan cycle increasing up to 20 times, which maybe implied a self-adjustment of the polymer film thickness at the GC electrode. The electro-deposited

2.2. Apparatus CHI 660B Electrochemical Workstation (Shanghai CH Instruments, China) equipped with a personal computer was used for electrochemical measurements and treating data. A conventional three-electrode system was used throughout. The working electrode was a bare or a poly(EBT) film modified GC electrode (3.0 mm in diameter), the auxiliary electrode was a Pt foil and a saturated calomel electrode (SCE) was used as reference electrode. All electrode potentials were reported with respected to SCE in this paper.

Fig. 1. Electropolymerization graph of EBT by CV. Inset is the locally amplified electropolymerization graph of EBT at 100 mV/s.

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Scheme 1. Mechanism of the poly-EBT electrode reaction.

behavior of EBT at the GC electrode was similar to some reports [23,24] referring to the electrochemical responses of a few azo compounds at solid electrode. The reaction mechanism could be explained as follows (Scheme 1): (1) EBT (A) was first deposited at the surface of GC electrode and oxidized to form a benzoquinone diimine structure (B) (peak 1); (2) and then the benzoquinone diimine structure (B) was reduced to EBT (A) (peak 2) at the surface of GC electrode. Fig. 2 exhibits the typical voltammogram of the poly(EBT) electrode in the range of −0.2 V to 0.8 V at various sweep rates in the 0.1 M HCl. A pair of redox peaks (Ipa and Ipc ) was obtained in each of the cyclic voltammogram. And the anodic peak currents (Ipa ) were linearly dependent of scan rate with the linear equation: Ip (␮A) = 0.019 C (␮M) + 0.36 (r = 0.999), and the ratio of anodic peak current to cathodic peak current (Ipa /Ipc ) was almost equal to unity. The separation of peak potentials (Ep = Epa − Epc ) was 32 mV at low scan rate (5–10 mV/s). With increasing scan rate, Ep would not be changed. The above results suggest that the electrode reaction was a quasi-reversible electron transfer process [25]. Therefore, the peak current could be correlative with scan rate by the equation: Ip =

n2 F 2 AΓv 4RT

where Γ represents the surface coverage concentration (mol/cm2 ), ν the scan rate, A (0.0706 cm2 ) the electrode surface area and Ip is the current peak [26]. The slope of anodic peak current against scan rate is 0.019. Ep (=32 mV) is close to 2.3RT/nF (or 59/n mV at 25 ◦ C), which identifies that the

Fig. 2. Cyclic voltammograms of a poly(EBT) modified GC electrode in 0.1 M HCl at a scan rate of (a) 20 mV/s, (b) 50 mV/s, (c) 70 mV/s, (d) 100 mV/s, and (e) 120 mV/s.

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number of electrons involved in the reaction was 2 (n ≈ 1.8). Thus, the calculated surface concentration of poly(EBT) is 1.0 × 10−10 mol/cm2 , which further confirms the immobilized state of the poly(EBT). The effect of pH value of the supporting solution on the electrochemical behavior of poly(EBT) film modified electrode was also investigated. With increasing pH from 2 to 7, a pair of redox peaks was observed in each of the cyclic voltammograms, and both the anodic and the cathodic peaks potential shifted negatively. The anodic peak potential (Epa ) linearly depended on pH value varying from 2 to 7. The result showed the slopes of −55.1 mV/pH, implying that the ratio of the participated protons to the transferred electrons through the poly-EBT film was 1:1. That is to say that the numbers of electron and proton involved in the reaction were both equal to two, which further certified the poly(EBT) film reaction mechanism by illustrated in Scheme 1. 3.2. Electrocatalytic oxidation of DA at poly-EBT modified GC electrode Fig. 3 shows the cyclic voltammograms of DA in pH 4.0 PBS at a bare GC electrode and a poly(EBT) film modified GC electrode. At a bare GC electrode, a pair of redox peak was observed with the oxidation peak potential at 0.56 V and the reduction peak potential at 0.15 V. Under the same conditions, the poly(EBT) modified GC electrode gave birth to significantly enhanced peak current and a more reversible electron transfer process to DA. A well-defined redox wave of DA was observed with the anodic and cathodic peak potentials at 0.356 V and 0.323 V, respectively. The separation of peak potentials at the poly(EBT) film modified GC electrode, Ep (=Epa − Epc ), was 33 mV, which was on accordance with a Nernst reversible behavior and identified that the number of electrons involved in the reaction was about equal to two. Intensive increase in peak current was also observed owing to the improvement in

Fig. 3. Cyclic voltammograms of 0.05 M PBS (pH 4.0) containing (a) 20 ␮M DA at a bare GC electrode, (b) 20 ␮M DA and (c) 70 ␮M DA at a poly(EBT) modified GC electrode at a scan rate of 100 mV/s.

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Scheme 2. Mechanism of DA reaction at the poly-EBT GC electrode.

the reversibility of electron transfer process and the larger real surface area of the poly(EBT) film. This suggests an efficient oxidation reaction toward DA at the poly(EBT) modified GC electrode. The effect of scan rate on the anodic peak current of DA was studied by cyclic voltammetry. With the scan rate increasing, the anodic peak current (Ipa ) increased. A good linearity between the scan rate and Ipa was obtained within the range of 20–200 mV/s, suggesting a surface-controlled process on the modified electrode surface. The linear equation was Ipa (␮A) = 0.1801 ν (mV/s) + 5.9635 (r = 0.998). This result also hinted that DA possessed good affinity to the poly(EBT) film structure.  Effect of pH on the formal potential (E0 ) and peak currents were examined by cyclic voltammetry in the presence of 20 ␮M DA in 0.05 M PBS. With the increasing pH from 2.5 to 7, both the Epa and Epc shifted negatively and were dependent linearly on pH with the slopes of −59.2 mV/pH and −58.5 mV/pH, respectively, indicating that the proportion of the electrons and protons involved in the redox of DA was 1:1. As the oxidation of DA at the poly(EBT) modified GC electrode had been inferred to be a double-electron course in the previous discussion, the number of proton involved could be calculated to be two. It could also be observed that both the oxidation and reduction peak current obtained a maximum at pH 4, which could be partly explained on the basis of the dissociation ability of –SO3 Na(H) group of poly(EBT) film in different pH environments. As it known, the pKa value of R–SO3 H (R: alkyl or aryl groups) is usually between 3 and 4. When the solution pH was equal to 4, the –SO3 Na group of poly(EBT) film could dissociate favorably into a negative charge group –SO3 − . Under this condition, the alkaline –NH2 group of DA molecular (pKa : 8.9) could obtain a proton and form the positive ion of DA. Therefore, the negative charge group –SO3 − on the surface of poly(EBT) modified electrode had a well affinity to the DA positive ions and could catalyze and promote the oxidation of DA in the weak acidic mediator (pH 4). While pH was below 4, the –SO3 Na group of poly(CCA) film could form –SO3 H and exclude the DA positive ions. However, when pH value was beyond 5, decreased DA peak currents seemed to be contradicted to the above-mentioned dissociation presumption. Actually, to interpret the effect of pH

on the electrocatalytic oxidation of DA, it should not be overlooked that poly(EBT) film at electrode acted as a mediator of electron transfer for DAs oxidation, whose electron transfer process could be affected by pH. Indeed, the anodic peak current of poly(EBT) film reaction at GC electrode was observed to decrease gradually with increasing pH from 2 to 7. It identified that with increasing pH the rate of electron transfer of the poly(EBT) film could also decrease gradually, which was disadvantageous to the electrocatalytic reaction of DA at the poly(EBT)-coated electrode. Thus, pH was a dual-conditioner to the oxidation of DA at the poly(EBT)-coated GC electrode. On one hand, increasing pH of electrolyte solution from 2 to 7 could promote the formation of the –SO3 − radical on poly(EBT) film resulting in that the poly(EBT) modified electrode had a well affinity to the DA positive ions and enhanced the oxidation of DA. On the other hand, it could lower the rate of electron transfer of the poly(EBT) leading to the decreased oxidation peak current of DA. The results told us that pH 4 was an optimum protocol for the electrocatalytic oxidation of DA at the poly(EBT) modified GC electrode. According to the above discussions, the course of DAs oxidation at the poly(EBT) membrane modified GC electrode could be described as in Scheme 2 [27,28]. 3.3. Electrocatalytic oxidation of AA at poly(EBT) modified GC electrode Fig. 4 shows the cyclic voltammograms of AA (0.5 mM) in a pH 4.0 PBS at a bare GC electrode and a poly(EBT) film modified GC electrode at 100 mV/s. At the bare GC electrode, a wide and dwarf oxidation peak at a potential of about 0.45 V was observed. However, at the poly(EBT) film modified GC electrode, strongly enhanced oxidation peaks were recorded with a potential at about 0.15 V, which was an evidence for the electrocatalytic oxidation of AA. In addition, with scan rate increasing, the oxidation peak potential shifted to more positive points, which was in line with the characteristic of irreversible electrochemical process. A good linear relationship was observed between Ipa and v1/2 within the range of 20–200 mV/s, implicating that the electrocatalytic oxidation of AA at the poly(EBT)-coated electrode was controlled by diffusion.

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Fig. 4. Cyclic voltammograms of 0.05 M PBS (pH 4.0) containing (a) 0.5 mM AA at a bare GC electrode, (b) 0.5 mM AA and (c) 1 mM AA at the poly(EBT) modified GC electrode at 100 mV/s.

The effect of pH on the response of poly(EBT) modified electrode towards AA was examined by cyclic voltammetry in solution. With pH increasing from 2.5 to 7, the redox peak shifted to a more negative potential and the breakover point could be seen between pH 4 and 5, which was accordance with AAs pK1 (=4.17). It could be observed that the anodic peak current reach a maximum when pH was 3. 3.4. Electrocatalytic oxidation of UA at poly(EBT) modified GC electrode In a pH 4.0 PBS, the poly(EBT) modified GC electrode also possessed a strongly electrocatalytic action for UA. Regardless of at a bare GC electrode or at the poly(EBT)-coated electrode, only could the oxidation peak be observed in pH 4.0 PBS, which confirmed that the electrochemical reaction of UA was an

Fig. 6. (A) DPV graphs of (a) 0.1 ␮M, (b) 1 ␮M, (c) 2 ␮M, (d) 8 ␮M, (e) 20 ␮M, (f) 50 ␮M, (g) 100 ␮M, (h) 150 ␮M and (i) 200 ␮M DA in pH 4.0 0.05 M PBS containing 0.4 mM AA and 70 ␮M UA at the poly-EBT modified GC electrode. (B) DPV graphs of (a) 0.15 mM, (b) 0.45 mM, (c) 0.65 mM, (d) 0.85 mM and (e) 1 mM AA in pH 4.0 0.05 M PBS containing 20 ␮M DA and 140 ␮M UA at the poly-EBT modified GC electrode. (C) DPV graphs of (a) 10 ␮M, (b) 50 ␮M, (c) 70 ␮M, (d) 90 ␮M, (e) 110 ␮M and (f) 130 ␮M UA in pH 4.0 0.05 M PBS containing 20 ␮M DA and 0.4 mM AA at the poly-EBT modified GC electrode. Fig. 5. Cyclic voltammograms of a 50 mM PBS (pH 4.0) containing 20 ␮M DA, 0.5 mM AA and 50 ␮M UA at a (a) bare, (b) pretreated and (c) poly(EBT) film modified GC electrode at 100 mV/s.

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Table 1 Calibration curve parameters for the determination of DA, AA and UA Catecholamines

Concentration ranges (␮M)

Regression equations (Ip a vs. Cb )

r

Dopamine

0.1–20 20–200

Ip = 1.0728 C + 4.7038 Ip = 0.2286 C + 21.46

0.9998 0.9991

Ascorbic acid

150–1000

Ip = 0.01687 C + 2.8608

0.9913

10–130

Ip = 0.0910 C + 2.8976

0.9933

Uric acid a b

Detection limits (␮M) 0.02 10 1.0

Ip (␮A) is the peak current after background subtraction CL intensity. C (␮M) is the concentration of the analyte.

irreversible process. At the bare GC electrode, the oxidation peak observed for 50 ␮M UA was low and patulous with a potential of about 0.56 V. Meanwhile, at the poly(EBT) modified GC electrode, the anodic peak potential produced little negatively shifted and the shape of oxidation peak became high, sharp and symmetrical. It certified that the poly(EBT) film at the electrode could intensively catalyze the electrochemical oxidation of UA in pH 4.0 PBS. The effect of pH on electrochemical reaction of UA at the poly(EBT) film-coated electrode was also examined. With pH increasing from 2.5 to 7, the Epa also moved towards a more negative potential. When pH was 4, a maximum Ipa was obtained, suggesting that pH 4 should be selected as the optimum protocol for determination of UA. 3.5. Resolution of DA, AA and UA in the same sample by cyclic voltammetry Based on the electrocatalytic action of the poly(EBT) film to DA, AA and UA, it was supposed that the poly(EBT)-modified electrode could conspicuously improve the voltammetric resolution of DA, AA and UA. To ascertain the presumption, the cyclic voltammograms of a mixture solution (pH 4.0 50 mM PBS containing 20 ␮M DA, 0.5 mM AA and 50 ␮M UA) were recorded with the rate of 100 mV/s at a bare GC electrode, a pretreated GC electrode and a poly(EBT) film modified GC electrode. As shown in Fig. 5, the graph at the bare GC electrode appear the seriously overlapped peaks while three separated oxidation peaks can be found either at the poly(EBT) film modified electrode or at the pretreated electrode. For the separations of electrochemical responses of DA, AA and UA at the pretreated electrode, it might be ascribed to the increasing active site (the surface of oxide species) on the surface of GC electrode that resulted from the CV pretreatment of the electrode in 0.1 M NaOH solution [29]. But only at the poly-EBT coated electrode were the anodic peaks of DA, AA and UA separated perfectly. The differences of the oxidation peak potentials for DA–AA and UA–DA were 210 mV and 170 mV, respectively, which were enough large separations to allow the simultaneous determination of DA, AA and UA in a mixture. Meanwhile, it could be noticed that the peak currents of DA, AA and UA were enhanced strongly at the poly-EBT modified electrode and the peak potentials of DA and UA were approximately identical to that at the pretreated electrode. It further identified that the polyEBT modified electrode possessed the higher active surface area

and excellent electrocatalytic activity for the oxidation of DA, AA and UA. 3.6. Simultaneous determination of DA, AA and UA by differential pulse voltammetry (DPV) DPV was used for the determination of DA, AA and UA at the poly(EBT) film-coated GC electrode because of its higher current sensitivity and better resolution than cyclic voltammetry. It was observed that the oxidation peak currents and potentials of DA, AA and UA by DPV were closely correlative to the pH of the supporting solution. When pH was 4.0, the oxidation peak currents of DA and UA by DPV over the range of −0.2 V to 0.8 V had a maximum and the differences of peak potentials for DA–AA and UA–DA also appeared a maximum. Therefore, pH 4.0 was elected as the optimum pH condition for the simultaneous determination of DA, AA and UA. The determination of DA, AA and UA in their mixtures was performed at the poly(EBT) modified GC electrode when the concentration of one species changed, whereas the other two species remained constant. The results are shown in Fig. 6. From Fig. 6A, it can be seen that the peak current of DA is positively proportional to its concentration increasing from 0.1 ␮M to 200 ␮M when keeping the concentrations of UA and DA at 0.4 mM and 70 ␮M, respectively. No changes in the peak currents and potentials of both AA and UA can be found and the higher concentrations of AA (4000 times relative to DA) and UA (700 times relative to DA) can produce little interference for the detection of DA. Similarly, as shown in Fig. 6B and C, keeping the concentrations of the other two compounds constant, the oxidation peak current of AA or UA increases with increasing their concentrations. No interference can be observed for the determination of UA or AA by the coexisting other two species. All the results identifies that it is possible to simultaneously determine DA, AA and UA in real samples at the poly(EBT) modified GC electrode. The calibration parameters for the simultaneous determination of DA, AA and UA are listed in Table 1. 4. Conclusions The poly(EBT) film modified GC electrode exhibits highly electrocatalytic activity to the oxidations of DA, AA and UA. The modified electrode displays higher selectivity in voltammetric measurements of DA, AA and UA in their mixture solution. The separations of the oxidation peak potentials for AA–DA and

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DA–UA are about 210 mV and 170 mV, respectively, by cyclic voltammetry. With the good sensitivity, excellent detection limit and wide linear range, the proposed method provides a possibility for simultaneous detection of DA, AA and UA in biological samples. Acknowledgements This work was financially supported from the National Natural Science Foundation of China (20675015) and the Education Department Foundation of Fujian Province of China (2005k049). References [1] P. Damier, E.C. Hirsch, Y. Agid, A.M. Graybiel, Brain 122 (1999) 1437. [2] C. Martin, Chem. Br. 34 (1998) 40. [3] A. Heinz, H. Przuntek, G. Winterer, A. Pietzcker, Nervenarzt 66 (1995) 662. [4] R.M. Wightman, L.J. May, A.C. Michael, Anal. Chem. 60 (1988) 769A. [5] K.H. Xue, F.F. Tao, W. Xu, S.Y. Yin, J.M. Liu, J. Electroanal. Chem. 578 (2005) 323. [6] K.H. Xue, F.F. Tao, S.Y. Yin, W. Shen, W. Xue, Chem. Phys. Lett. 391 (2004) 243. [7] J.X. Wang, M.X. Li, Z.J. Shi, N.Q. Li, Z.N. Gu, Microchem. J. 73 (2002) 325.

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