PSA III modified glassy carbon electrode for simultaneous determination of ascorbic acid, dopamine and uric acid

Journal of Electroanalytical Chemistry 689 (2013) 135–141

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Nano-Cu/PSA III modified glassy carbon electrode for simultaneous determination of ascorbic acid, dopamine and uric acid Lei Zhang ⇑, Wen-Juan Yuan, Bao-Qin Hou Department of Chemistry, College of Life and Environment Sciences, Shanghai Normal University, Shanghai 200234, PR China

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Article history: Received 31 May 2012 Received in revised form 26 October 2012 Accepted 5 November 2012 Available online 24 November 2012 Keywords: Cu nanoparticles Poly(sulfonazo III) Ascorbic acid Dopamine Uric acid

a b s t r a c t Cu nanoparticles (nano-Cu)–poly(sulfonazo III) (PSA III) modified glassy carbon electrode (nano-Cu–PSA III/GCE) had been fabricated and successfully used for simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA). The performance of the nano-Cu–PSA III/GCE towards the electrooxidation of AA, DA and UA had been investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The results showed that the modified electrode not only exhibited excellent electrocatalytic activity toward the electrooxidation of AA, DA and UA in a phosphate buffer solution (PBS, pH 4.0), but also could separate the overlapped oxidation waves of these species at the bare GCE into three sharp and strong oxidation peaks at 0.23 V, 0.37 V and 0.54 V using CV and 0.16 V, 0.33 V and 0.49 V using DPV, corresponding to the electrooxidation of AA, DA and UA, respectively. The separations of anodic peak potentials for AA–DA, DA–UA and AA–UA at nano-Cu–PSA III/GCE are large enough for the simultaneous detection of AA, DA and UA. Under the optimum conditions, the linear calibration curves were obtained over the range of 0.30–730, 0.02–65 and 0.25–107 lmol L1 with detection limits of 0.15, 0.01 and 0.10 lmol L1 for AA, DA and UA (S/N = 3), respectively. Due to the good selectivity and high sensitivity, nano-Cu–PSA III/GCE had been used for the simultaneous determination of AA and UA in urine samples and DA in human serum samples with satisfactory results. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction DA is one of the many important catecholamine neurotransmitters in our body and plays important roles in various biological, pharmacological, and physical processes [1], and also, it is believed to be related to several diseases, such as schizophrenia and parkinsonism [2–5]. Therefore, a simple, fast, and sensitive method is necessary for its determination in both biological fluids and pharmaceutical preparations. UA is the other important compound in our body and its abnormal concentration levels will lead to several diseases such as hyperuricaemia and gout [6,7]. AA is a vital component in human diet and has been widely used for the prevention of scurvy and treatment of common cold, mental illness, infertility, cancer and AIDS [8]. Usually, AA, DA and UA are coexisting in human body fluids, therefore, the selective and simultaneous determination of these molecules is very important for the clinical point of view. However, at bare electrodes, selective and simultaneous determination of AA, DA and UA is nearly impossible because they undergo an overlapping oxidation potential and electrode fouling takes place due to the adsorption of the oxidation products [9]. ⇑ Corresponding author. Tel./fax: +86 21 64322511. E-mail address: [email protected] (L. Zhang). 1572-6657/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2012.11.006

Various techniques have been applied to determine these biomolecules including fluorimetry [10], chemiluminescence [11], ion-exchange column chromatography [12], ultraviolet–visible spectroscopy [13] and capillary electrophoresis [14], etc. However, these methods are often complicated, very expensive, and suffered from sensitivity, selectivity and reproducibility. Fortunately, electrochemical techniques can offer the opportunity for portable, cheap and rapid methodologies. It has been demonstrated that the electropolymerised films have attracted much more attention in the fields of chemical sensors and biosensors [15–23], because they exhibit many advantages in the detection of some analytes due to their sensitivity, selectivity and homogeneity in electrochemical deposition, strong adherence to electrode surface and the chemical stability of the electro-polymer films [24–26]. Metal nanoparticles (MNPs) modified electrodes have also drawn much attention for a long time and been applied for the simultaneous determination of AA, DA and UA [27,28], because of their unusual physical and chemical properties [29,30], high surface area, effective mass transport, high electrocatalytic activities and control over local microenvironment compared to the macro-electrodes [31,32]. However, the problem that the resulting MNPs layer on electrode surface is usually fragile and thus electrically unstable limits their effective application in sensors. Based on the good stability and the high electroactivity of the polymer film, the

L. Zhang et al. / Journal of Electroanalytical Chemistry 689 (2013) 135–141

electrodes coated with polymer films can be used as a substrate for deposition of MNPs, the more active sites can lead to larger analytical signals [33–36]. Sulfonazo III (SA III), a derivative of arsenazo III in which the – AsO3H2 groups have been replaced by –SO3H, is one kind of sensitive metallochromic reagent and has been mainly applied as an indicator for sulfate precipitation titrations using barium chloride [37]. Ensafi’s research group have fabricated PSA III modified GCE and used it for the selective determination of AA, DA, and UA [22]. Considering the special characteristics of Cu nano-particles with high catalytic activity and the electroactive PSA III, which not only can facilitate the electron transfer of analytes, but also can be used as a binder for the stable and homogeneous distribution of Cu nano-particles on electrode surface, we reported, in this work, the fabrication of the nano-Cu–PSA III composite film modified GCE and its use for the selective and sensitive simultaneous determination of AA, DA and UA.

2. Experimental 2.1. Reagents Sulfonazo III (SA III), CuSO4 and H2SO4 were purchased from Shanghai Chemical Reagents Company (China). DA, AA, and UA were purchased from Sigma. Phosphate buffer solutions (PBS, 0.1 mol L1) of different pH values were prepared from stock solutions of 0.1 mol L1 H3PO4, NaH2PO4, Na2HPO4 and NaOH. DA and UA were prepared by appropriate dilution of the stock solution in 0.1 mol L1 PBS, pH 4.0. Doubly distilled water was used to prepare all the solutions. All chemicals were of analytical reagent grade and used without further purification.

2.2. Apparatus All electrochemical experiments were performed on a CHI 660C electrochemical analyzer (Shanghai, China), with three electrodes consisting of a platinum wire as the auxiliary electrode, the nano-Cu–PSA III/GCE ,GCE or PSA III/GCE as the working electrode, and a saturated calomel electrode (SCE) as reference electrode.

2.3. Preparation of nano-Cu–PSA III/GCE GCE was firstly polished with 0.30 and 0.05 lm alumina slurries on a polishing cloth, and sonicated in absolute ethanol and water for 3 min, successively. Then, the electropolymerization of SA III on GCE was performed by cyclic scanning in the potential range of 0.10 to +1.20 V at 0.1 V s1 for 20 cycles in 0.2 mol L1 NaOH solution containing 1.0 mmol L1 SA III. To improve the reproducibility and stability, the modified electrode was washed with water and cycled again between 0.3 and 0.80 V for eight times in 0.1 mol L1 PBS pH 4.0 to polymerize the untreated SA III adsorbed on electrode surface. Then, the poly(sulfonazo III) modified GCE (PSA III/GCE) was obtained. To further deposite the Cu nano-particles on the PSA III modified GCE, the obtained PSA III/GCE was immersed in 1 mmol L1 H2SO4 solution containing 1 mmol L1 CuSO4 and treated by cycling between 0 and 1.10 V at 0.05 V s1 for 10 times [38]. After rinsing with water carefully, the nano-Cu–PSA III composite film modified GCE (nano-Cu–PSA III/GCE) was obtained and stored in 0.1 mol L1 PBS pH 4.0 for use.

3. Results and discussion 3.1. Characterization of the nano-Cu–PSA III/GCE 3.1.1. Cyclic voltammetry Fig. 1 shows the electrochemical oxidation of SA III on GCE over the potential range of 0.10 to 1.20 V at a scan rate of 0.1 V s1 for 20 cycles in a solution containing 0.2 mol L1 NaOH and 1.0 mmol L1 SA III. As can be seen, there is a oxidation peak located at the potential of 0.60 V, which should be ascribed to the generated SA III cation radical due to the electrooxidation of SA III monomer. Subsequently, the SA III cation radicals react each other or with SA III monomers to form PSA III film onto the electrode surface. This can be confirmed by the gradually decreased oxidation current of SA III with increase of the potential scan. These results indicate that the initially formed PSA III film on GCE surface exhibits a leaching process with the increased potential scan. While, when the potential scan increases up to 20 cycles, the current response tends to be stable, which is also a self-adjustment process for the PSA III film on GCE surface [18]. So, 20 potential scans was used to fabricate the PSA III modified GCE. To investigate the different electrochemical behaviors of the bare GCE, PSA III/GCE and nano-Cu–PSA III/GCE, the cyclic voltammograms (CV) of the bare GCE (a), PSA III/GCE (b), and nanoCu–PSA III/GCE (c) in 0.1 mol L1 PBS pH 4.0 are shown in Fig. 2. As can be seen, apart from the pair of redox waves located at 0.12 V and 0.10 V, which is ascribed to the oxidation and reduction of the small quantity of carboxylic groups generated on GCE surface [39], there is no other redox wave (a). This indicates that the polished GCE is clean. While, PSA III/GCE (b) exhibits another pair of redox waves located at 0.45 V and 0.33 V, which should be due to the oxidation and reduction of the PSA III deposited on GCE surface. This indicates the successful modification of PSA III film on GCE surface. However, after depositing Cu nanoparticles on electrode surface, the obtained nano-Cu–PSA III/GCE (c) shows a couple of negatively shifted wide redox waves centered at about 0.34 V and 0.01 V, which is different from that of the unique PSA III/GCE (b). This not only indicates that Cu nanoparticles have been immobilized effectively onto the electrode surface, but also confirms the existence of the interaction between Cu nanoparticles and PSA III. 3.1.2. Electrochemical impedance spectroscopy (EIS) EIS is an effective method for probing the features of surfacemodified electrode using the redox probe FeðCNÞ4=3 [40]. Fig. 3 6 shows the results of employing EIS to determine the interface properties of the modified electrodes. It can be seen that, at the bare GCE (Fig. 3a), a very small semicircle of about 150 X diameter with an almost straight tail line is obtain. And, the diameter of the

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Fig. 2. CVs of the bare GCE (a), PSA III/GCE (b), and nano-Cu/PSA III/GCE (c) in 0.1 mol L1 PBS pH 4.0 at 0.1 V s1.

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Fig. 3. The Nyquist plots of the EIS measurement of the bare GCE (a), PSA III/GCE (b), and nano-Cu/PSA III/GCE (c) in the presence of 1.0 mmol L1 K3Fe(CN)6 and 1.0 mmol L1 K4Fe(CN)6 mixture with 0.1 mol L1 KCl as supporting electrolyte.

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frequency semicircle is obviously increased to about 4800 X by the surface modification of the PSA III film (Fig. 3b), indicating that the PSA III film hinders the charge transfer. While, the diameter of the high frequency semicircle is reduced to about 2500 X at the nanoCu–PSA III/GCE (Fig. 3c), which should be attributed to the better conductivity of the Cu nanoparticles. The impedance change of the modified process indicates the successful modification and the better conductivity of the Cu nanoparticles on PSA III/GCE.

3.2. Electrocatalytic oxidation of AA, DA and UA on nano-Cu–PSA III/ GCE Fig. 4A–C show the cyclic voltammograms (CVs) of AA, DA and UA at GCE (curve a), PSA III/GCE (curve b) and nano-Cu–PSA III/GCE (curve c) in 0.1 mol L1 PBS pH 4.0 at a scan rate of 0.1 V s1, respectively. It can be seen from Fig. 4A that, at the bare GCE, the oxidation peak of AA is board and sluggish at a potential of 0.31 V. However, a sharp and strong oxidation peak appeared at 0.21 and 0.19 V on the surface of PSA III/GCE and nano-Cu–PSA III/GCE, respectively. The peak current of AA at nano-Cu–PSA III/ GCE increases up to 4.34 folds compared with that at the bare GCE and 1.71 times in comparison with the anodic peak current on the PSA III/GCE. The negatively shifted oxidation over-potential and obviously increased peak current for AA indicate that the fabricated nano-Cu–PSA III/GCE can effectively catalyze the electrooxidation of AA. Fig. 4B shows that DA presents a weak cyclic voltammogram peak response with a pair of redox waves located at 0.42 V and 0.25 V (DEp = 0.17 V) at bare GCE (curve a). Furthermore, the reduction peak current is smaller than the oxidation peak current.

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E / V vs SCE Fig. 4. CVs of 500 lmol L1 AA (A), 50 lmol L1 DA (B), and 30 lmol L1 UA (C) at GCE (a), PSA III/GCE (b), and nano-Cu/PSA III/GCE (c) in 0.1 mol L1 PBS pH 4.0. Scan rate: 0.1 V s1.

The larger redox peak separation and the weaker cathodic peak current exhibit the irreversible electrochemical reaction of DA at the bare GCE. This indicates the sluggish electron transfer kinetics of DA at the bare GCE, which may be related to the electrode fouling due to the deposition of the oxidation products of DA on GCE surface [28]. And, when the PSA III/GCE was used, DA shows a pair of improved redox peaks appeared at 0.39 V and 0.37 V with a DEp of 0.02 V and 3.02 times increase in peak current compared with that of DA at the bare GCE (curve b). However, the electrochemical reaction kinetics of DA is greatly improved as the copper nanoparticles are inserted into the PSA III film (curve c). As can be seen, at the nano-Cu/PSA III/GCE, DA exhibits a pair of reversible and wellbehaved redox peaks with a negatively shifted oxidation over-potential of 0.39 V (DEp = 0.015 V) compared with that of 0.42 V at the bare GCE. Moreover, the oxidation peak current is increased up to 6.01 folds in comparison with the anodic peak current on the bare GCE and 1.99 times compared with that at the PSA III/ GCE. The greatly enhanced peak current and the near-irreversible redox peaks with smaller peak separation for DA suggest that the nano-Cu/PSA III/GCE exhibits excellent electrocatalytic ability for the electrochemical oxidation of DA.

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Fig. 4C shows the CVs of UA at GCE (curve a), PSA III/GCE (curve b), and nano-Cu/PSA III/GCE (curve c), respectively. It can be seen from curve a that, at the bare GCE, UA only shows a small oxidation peak appeared at 0.56 V. While, under the identical conditions, both PSA III/GCE (curve b) and nano-Cu/PSA III/GCE (curve c) can dramatically improve the electrooxidation of UA with negative shifted oxidation potential and increased peak current. As can be seen, at the nano-Cu/PSA III/GCE, the oxidation peak potential of UA shifts negatively to 0.53 V compared with that of 0.56 V at the bare GCE, and the anodic peak currents increases up to 3.4 times compared with that at the bare GCE and 1.8 folds in comparison with the peak current on the PSA III/GCE. The decrease of oxidation over-potential accompanied by the obviously increase in oxidation peak current of UA suggest the strong catalytic ability of nano-Cu/PSA III/GCE on the electrochemical oxidation of UA.

3.3. Voltammetric separation of AA, DA, and UA at nano-Cu/PSA III/ GCE Fig. 5A displays the CVs of a mixture containing 500 lmol L1 AA, 50 lmol L1 DA and 30 lmol L1 UA in 0.1 mol L1 PBS pH 4.0 at GCE (curve a), PSA III/GCE (curve b), and nano-Cu/PSA III/ GCE (curve c), respectively. It can be seen from curve a that the bare GCE cannot resolve clearly the oxidation waves of AA, DA and UA, and thus no useful information can be obtained from the overlapped oxidation peaks. In contrast, when PSA III/GCE or nano-Cu/PSA III/GCE is used as working electrode, three welldefined oxidation peaks located at the potentials of 0.24 V, 0.36 V and 0.52 V at PSA III/GCE and 0.23 V, 0.37 V and 0.54 V at nanoCu/PSA III/GCE corresponding to the oxidation of AA, DA and UA

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are observed, respectively. Meanwhile, the oxidation peak currents of AA, DA and UA are greatly increased in comparison with the current responses on the bare GCE. Especially, the enhancement of the anodic peak currents of AA, DA and UA is more notable at the nanoCu/PSA III/GCE (ipa,AA = 5.82 lA, ipa,DA = 3.48 lA, ipa,UA = 3.51 lA) than the current responses at PSA III/GCE (ipa,AA = 3.83 lA, ipa,DA = 1.73 lA, ipa,UA = 1.71 lA). The separation of the anodic peak potentials at nano-Cu/PSA III/GCE is 0.14 V for AA–DA, 0.17 V for DA–UA, and 0.31 V for AA–UA, respectively, which is large enough for the selective and simultaneous determination of AA, DA and UA in their mixture. It is known that DPV technique exhibits lower background, higher current sensitivity, lower detection limit and better resolution than cyclic voltammetry, it has been employed to investigate the simultaneous electrochemical responses of AA, DA and UA in their mixture. Fig. 5B shows the DPV curves for the electrooxidation of AA, DA and UA in their mixture at GCE (inset), PSA III/GCE (curve a) and nano-Cu/PSA III/GCE (curve b), respectively. As can be seen, the bare GCE shows two broad and weak overlapped oxidation waves centered at 0.25 V and 0.46 V for the oxidation of AA, DA and UA in their mixture. And, when the experiment was performed at PSA III/GCE and nano-Cu/PSA III/GCE, three strong and well-resolved oxidation peaks are observed with the peak potentials at 0.22 V, 0.34 V and 0.50 V at PSA III/GCE and 0.16 V, 0.33 V and 0.49 V at nano-Cu/PSA III/GCE for the oxidation of AA, DA and UA, respectively. The separation of peak potentials at the nano-Cu/PSA III/GCE is 0.17 V for AA–DA, 0.16 V for DA–UA, and 0.33 V for AA–UA, respectively, which are large enough for the simultaneous determination of AA, DA and UA in a mixture solution. Meanwhile, the peak currents of AA, DA and UA at PSA III/ GCE and nano-Cu/PSA III/GCE are all increased in comparison with the current responses on the bare GCE. However, one can see that the enhancement of the peak currents of AA, DA and UA is more notable at nano-Cu/PSA III/GCE (ipa,AA = 4.23 lA, ipa,DA = 8.41 lA, ipa,UA = 4.90 lA) than the peak current responses at PSA III/GCE (ipa,AA = 1.58 lA, ipa,DA = 2.77 lA, ipa,UA = 1.36 lA). Thus, considering the improved current response and the large peak separation for the electrooxidation of AA, DA and UA at nano-Cu/PSA III/GCE, the following experiments were carried out using nano-Cu/PSA III/GCE as working electrode.

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3.4. Effect of pH on the electrooxidation of AA, DA and UA

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E / V vs SCE Fig. 5. (A) CVs of the mixture containing 500 lmol L1 AA, 50 lmol L1 DA, and 30 lmol L1 UA at GCE (a), PSA III/GCE (b), and nano-Cu/PSA III/GCE (c) in 0.1 mol L1 PBS pH 4.0. (B) DPVs of the mixture containing 500 lmol L1 AA, 50 lmol L1 DA, and 30 lmol L1 UA at the bare GCE (inset), PSA III/GCE (a), and nano-Cu/PSA III/GCE (b) in 0.1 mol L1 PBS pH 4.0. Scan rate: 0.1 V s1.

The effect of solution acidity on the electrochemical response of nano-Cu/PSA III/GCE towards the individual determination of AA, DA and UA has been studied. The variations of oxidation peak current (Fig. 6A) and anodic peak potential (Fig. 6B) with respect to the pH values from 2 to 8 are shown in Fig. 6. It can be seen from Fig. 6A that the peak current of AA (pKa = 4.17) increases with increasing pH values until pH 4, and then it decreases gradually when solution pH value increases further. In case of DA, the oxidation peak current also increases with increasing pH until pH 8, and then it decreases in solutions with higher pH values. And also, the peak current response of UA (pKa = 5.75) initially increases in the solutions with pH 6 5, and then decreased from a pH value of 5.0 to higher pH values. As is known, the pKa value of R–SO3H (R = aryl group) is usually less than 4; therefore, the –SO3Na of poly(sulfonazo III) film could dissociate favorably into a negative charged group (ASO 3 ) when solution pH value is more than 4; while, under this acidic condition, AA (pKa = 4.17), DA (pKa = 8.9) and UA (pKa = 5.75) could obtain protons to form the positive charged AA, DA and UA ions, which has great affinity toward the negatively charged poly(sulfonazo III) due to the electrostatic interaction. This is the reason why the electrochemical responses of AA, DA and UA can be improved on the modified electrodes in comparison with that on the bare GCE. Fig. 6B shows that all the anodic peak

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potentials for the oxidation of AA, DA and UA shift negatively with the increasing pH values, indicating that the protons took part in the electrode reactions. To obtain high sensitivity and good selectivity, 0.1 mol L1 PBS with pH 4.0 was used for simultaneous determination of these compounds.

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To investigate the practicality of the nano-Cu/PSA III/GCE for the selective determination of AA, DA or UA in their mixture, the DPVs recorded at nano-Cu/PSA III/GCE in solutions containing various concentrations of AA in the presence of 30 lmol L1 DA and 30 lmol L1 UA are shown in Fig. 7A. It can be seen that the peak current response for the oxidation of AA increases linearly with the increase of AA concentration. While, the current responses for DA and UA oxidation keep nearly unchanged. This indicates that the presence of DA and UA do not interfere with the electrooxidation of AA. Similarly, as shown in Fig. 7B and C, when keeping the concentrations of the other two compounds constant, the oxidation peak current of DA or UA also increases linearly with the increase of their concentrations, while, the electrochemical responses of the other two coexisted species keep unchanged. This indicates that the proposed method can be used for selective determination of AA, DA and UA in the presence of the other two compounds.

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Fig. 6. Effect of pH on the peak current (A) and peak potentials for the oxidation of 25 lmol L1 DA, 500 lmol L1 AA, and 40 lmol L1 UA.

3.5. Selective determination of AA, DA and UA

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Fig. 7. (A) DPVs of AA at nano-Cu/PSA III/GCE in the presence of 30.0 lmol L1 DA and 30.0 lmol L1 UA in 0.1 mol L1 PBS pH 4.0. AA concentrations (a ? o): 50.0, 92.0, 125.0, 215.0, 310.0, 420.0, 600.0, 710.0, 930.0, 1080.0, 1247.0, 1354.0, 1422.0 and 1595.0 lmol L1; (B) DPVs of DA at nano-Cu/PSA III/GCE in the presence of 200.0 lmol L1 AA and 20.0 lmol L1 UA in 0.1 mol L1 PBS pH 4.0. DA concentrations (a ? h): 5.0, 25.0, 53.0, 100.0, 145.0, 210.0, 262.0 and 290.0 lmol L1; and (C) DPVs of UA at nano-Cu/PSA III/GCE in the presence of 5.0 lmol L1 DA and 40.0 lmol L1 AA in 0.1 mol L1 PBS pH 4.0. UA concentrations (a ? j): 0.1, 7.0, 15.0, 28.0, 38.0, 45.0, 60.0, 82.0, 94.0 and 100.0 lmol L1.

and 0.10 lmol L1 for AA, DA and UA, respectively. Compared with the other related studies [16,17,22,41–46] on the simultaneous determinations of AA, DA and UA, as shown in Table 1, the present procedure shows high selectivity, low detection limit, wide linear range and satisfactory application in real samples.

3.6. Simultaneous determination of AA, DA and UA 3.7. Interference study Fig. 8 shows the DPVs for the simultaneous determination of AA, DA and UA with different contents at nano-Cu/PSA III/GCE. As can be seen, the peak current for the electrooxidation of AA, DA and UA increases linearly with the increase of their concentrations in solution. The linear concentration ranges are 0.30–730 lmol L1 for AA, 0.02–65 lmol L1 for DA, and 0.25– 107 lmol L1 for UA, respectively. The correlation coefficients for detecting AA, DA and UA are 0.9984 , 0.9973 and 0.9992, respectively. The detection limits (S/N = 3) are found to be 0.15, 0.01

The possible interferences from other related compounds are also investigated. The tolerance limits are shown as the maximum concentrations of the interfering species, which leads to the approximately ± 5 relative error, for Mg2+ (500 lmol L1), Ca2+ (500 lmol L1), citric acid (450 lmol L1), glucose (350 lmol L1), lysine (300 lmol L1), cysteine (300 lmol L1), ammonium (350 lmol L1), urea (200 lmol L1) and creatinine (300 lmol L1), which do not affect the simultaneous determination of

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0.8

E / V vs SCE Fig. 8. DPVs of AA, DA and UA mixture at nano-Cu/PSA III/GCE in 0.1 mol L1 PBS pH 4.0. AA concentrations (a ? k): 0.3, 110.0, 195.0, 245.0, 320.0, 375.0, 455.0, 530.0, 600.0, 640.0 and 730.0 lmol L1; DA concentrations (a ? k): 0.02, 0.6, 12.0, 18.0, 23.0, 32.0, 37.0, 46.0, 51.0, 55.0 and 65.0 lmol L1; UA concentrations (a ? k): 0.25, 19.0, 34.0, 43.0, 50.0, 57.0, 65.0, 74.0, 83.0, 92.0 and 107.0 lmol L1.

50 lmol L1 AA, 10 lmol L1 DA, and 10 lmol L1 UA. However, the presence of P10 lmol L1 epinephrine will interfere with the detection of AA, DA and UA. That is because that the oxidation potential of epinephrine is near the oxidation potentials of AA, DA and UA. 3.8. Reproducibility and stability To investigate the reproducibility of nano-Cu/PSA III/GCE, repetitive measurements have been carried out for simultaneous detection of 300 lmol L1 AA, 30 lmol L1 DA and 50 lmol L1 UA in

their mixture. The results of 15 successive measurements show the relative standard deviations of 2.6%, 2.1% and 2.4% for the electrooxidation of AA, DA and UA, respectively. This indicates that the modified electrode does not undergo surface fouling during the measurements. And, the precision at renewed nano-Cu/PSA III/ GCE has also been studied, which shows an acceptable extent with relative standard deviation of 2.9%, 2.4% and 2.6% for the determination of 300 lmol L1 AA, 30 lmol L1 DA and 50 lmol L1 UA. It indicates that the modified electrode exhibits good reproducibility. Also, the modified electrode shows high stability. When nano-Cu/ PSA III/GCE is stored in 0.1 mol L1 PBS pH 4.0 at room temperature after voltammetric measurements, the current signals decreased about 4.1% in 1 week, 7.8% in 2 weeks, and 8.8% of its initial responses in 1 month, indicating the good stability of the modified electrode. 3.9. Sample analysis The practical application of the fabricated nano-Cu/PSA III/GCE has been examined by measuring the concentrations of AA and UA in human urine samples and DA in human serum samples. The standard addition technique is used for the determination of AA, DA and UA. The human urine samples are diluted to 50 times with 0.1 mol L1 PBS pH 4.0 without any treatment and all serum samples are diluted to 100 times with 0.1 mol L1 PBS pH 4.0. The results are listed in Table 2. The recovery rates are in the ranges of 99.43–102.42% for AA, 99.70–100.91% for UA, and 93.33–103.53% for DA, respectively. The present method shows better recoveries of spiked AA and UA in urine samples and DA in human serum samples, indicating that the proposed method

Table 1 Characteristics of some electrochemical methods for the simultaneous determination of AA, DA and UA. Working electrode

Method

pH

Analyte

Peak potential (V)

Detection limit (lmol L1)

Dynamic range (lmol L1)

Application

Reference

Nanostructured poly(2-amino-1,3,4-thiadiazole)/ GCE

5.0

AA DA UA

0.06 0.27 0.41

2.01 0.33 0.19

30–300 5.0–50 10–100

Human urine – Human urine

[16]

DPV

Poly(3-(5-chloro-2-hydroxyphenylazo)–4,5dihydroxynaphthalene-2,7-disulfonic acid/GCE

0.28 0.44 0.61

1.43 0.29 0.016

5.0–240 5.0–280 0.1–18.0

[17]

4.0

AA DA UA

Model sample

DPV

3.0

AA DA UA

0.2 0.37 0.52

0.17 0.03 0.11

0.5–1300 0.05–470 0.2–100

Vc tablet DA injection Human urine

[22]

DPV

7.0

AA DA UA

0.05 0.15 0.3

0.95 0.12 0.2

4.0–4500 0.5–2000 0.8–2500

Vc injection DA injection Human urine

[41]

DPV

6.0

AA DA UA

0.052, 0.172 0.296.

2 0.04 0.4

10–2000 1.0–150 1.0–180

– – Human urine

[42]

DPV

6.5

AA DA UA

0.006 0.201 0.342

1.3 0.11 1.4

25–500 1.0–20 2.5–30

– – Human urine

[43]

DPV Multi-wall carbon nanotube–poly(3,4dioxyetylenethiophene)/GCE

0.1 0.3 0.4

100 10 10

100–2000 10–330 10–250

[44]

7.0

AA DA UA

–

DPV

Polystyrene sulfonate wrapped multiwalled carbon nanotubes/graphite

0.023 0.187 0.306

0.5 0.15 0.8

500–2500 1.0–150 1.0–120

[45]

7.0

AA DA UA

–

DPV

2.0

AA DA UA

0.222 0.426 0.546

10 0.15 0.21

30–2000 0.5–150 0.5–60

– – Human urine

[46]

SWV

4.0

AA DA UA

0.16 0.33 0.49

0.15 0.01 0.1

0.30–730 0.02–65 0.25–107

Human urine DA injection Human urine

This work

DPV

Poly(sulfonazo III)/GCE

Screen-printed graphene electrode

Poly(2-aminothiazole– 2-amino-4-thiazoleacetic acid)/GCE Pyrolytic graphite electrode

Gold nanoparticles–b-cyclodextrin–graphene/GCE

Cu nanoparticles–poly(sulfonazo III)/GCE

141

L. Zhang et al. / Journal of Electroanalytical Chemistry 689 (2013) 135–141 Table 2 Determination of AA and UA in urine samples and DA in serum samples. Samples

Urine 1 Urine 2

Original (lmol L1)

Founda (lmol L1)

Added (lmol L1)

AA

UA

AA

UA

AA

UA

AA

UA

2.35 1.52

16.23 15.15

10 20

10 20

12.28 22.04

26.15 35.47

99.43 102.42

99.7 100.91

DA Serum 1

93.33 0.1

0.2

0.28

1

2

2.98

5

10

15.53

Serum 2

99.33

Serum 3 a

Recoveries (%)

103.53

Average value of six measurements.

can be applied to the determination of AA, DA and UA in real sample with satisfactory results. 4. Conclusions A novel sensor had been fabricated based on the PSA III polymer film modified with Cu nanoparticles by electropolymerization and electrodeposition methods. It was shown that the modified electrode cannot only exhibit good electrocatalytic activities towards the oxidation of AA, DA and UA, but also resolve the overlapped oxidation peaks of AA, DA and UA into three sharp and well-defined peaks at potentials of 0.16 V, 0.33 V and 0.49 V in DPV, respectively. The large peak separation and strong peak current response allow the selective and simultaneous determination of AA, DA and UA using nano-Cu/PSA III/GCE with good selectivity and sensitivity. The proposed method had been applied to the simultaneous or selective determination of AA, DA and UA concentrations in real samples with satisfactory results. Acknowledgment This work was supported by the Natural Science Foundation of Shanghai (09ZR1423500). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

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