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The electrochemical copolymerization of 3,4-dihydroxybenzoic acid and aniline at microdisk gold electrode and its amperometric determination for ascorbic acid
Talanta 45 (1998) 851 – 856
The electrochemical copolymerization of 3,4-dihydroxybenzoic acid and aniline at microdisk gold electrode and its amperometric determination for ascorbic acid Jian-Jun Sun, Dong-Mei Zhou, Hui-Qun Fang, Hong-Yuan Chen * Department of Chemistry, State Key Laboratory of Coordination Chemistry, Nanjing Uni6ersity, Nanjing, 210 093, People’s Republic of China Received 7 February 1997; received in revised form 2 June 1997; accepted 4 June 1997
Abstract The electrochemical copolymerization of 3,4-dihydroxybenzoic acid (3,4-DHBA) and aniline was carried out at microdisk gold electrodes by means of cyclic voltammetric sweep. The polymer obtained on the electrode shows good electrochemical activity and high stability even though in neutral and weakly basic media. It was found that the response current of ascorbic acid was greatly enhanced at this composite polymer electrode. Moreover, the anodic overpotential was significantly reduced for about 200 mV (vs. SCE) compared with that obtained at bare gold electrodes. The electrode exhibits a rapid current response (less than 2 s) and a high sensitivity (0.21 AM − 1 cm − 2). The dependence of response currents on the concentration of ascorbic acid was linear in the range of 1.0 ×10 − 4 – 1.0 × 10 − 2 M. In addition this composite polymer modified electrode exhibits a high electrode stability for a long-term use. © 1998 Elsevier Science B.V. Keywords: Microelectrode; Copolymerization; Aniline; 3,4-Dihydroxybenzoic acid; Ascorbic acid; Catalysis; Amperometric determination
1. Introduction Ascorbic acid has been extensively studied because of its significance in bioelectrochemistry, neurochemistry and clinical diagnostics’ applications [1]. The oxidation of ascorbic acid at conventional electrodes is well documented and is known to proceed via two consecutive one-electron transfer processes involving the participation * Corresponding author. Fax: +86 25 3317761.
of a radical anion intermediate to form dehydroL-ascorbic acid. This species subsequently undergoes a hydration reaction characteristic of carbonyl groups to form an electroinactive product. The catalytic oxidation of ascorbic acid has been obtained at many mediator-modified electrodes [2–6]. However, most of them still suffered from the pollution of ascorbic acid due to its adsorption on the electrode surface, especially at inorganic film modified ones.
0039-9140/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 9 - 9 1 4 0 ( 9 7 ) 0 0 1 8 3 - 5
852
J.-J. Sun et al. / Talanta 45 (1998) 851–856
Polymer modified electrodes have been widely used for the immobilization of mediators or enzymes by their matrix structure, electrocatalysis of biological molecules and the construction of molecular devices, etc. [7 – 11]. Usually, polymer modified electrodes exhibited a good stability and catalytic effects resulted from their three-dimensional mediator distribution. The conductive polymers such as polyaniline, polypyrrole, polythiophene as well as their derivatives have been used for constructing various biosensors. Polyaniline has been studied extensively because of its high stability and conductivity. It was well known that the polyaniline could be electrochemically polymerized easily in acid media. Protons would participate in the redox process of polyaniline. However, as pH of the media increased above 4 the polymer would lose its electrochemical activity [12]. This has greatly limited its application in biological sample’s analysis. Recently, Bartlett [13] obtained the polyanilinepoly(vinylsulfonate) composite film that exhibited an off/on switching response when it was exposed to NADH at pH 7.0. The reduction of the oxidized form of polyaniline(pernigraniline) in the composite film by reacting with NADH in pH 7.0 buffer solution brought about a switch in the conductivity of devices. The negative charged PVS, which was doped into polyaniline, could provide protons for the nitrogen atom of linear structure of polyaniline and led this composite film to work in neutral media. Karyakin et al. [14,15] also reported that the copolymerization of aniline with m-aminobenzoic acid or metrillic acid could exhibit a wide pH-range electroactivity even in basic solution. However, the application of this composite polymer in neutral media has not been reported. The application of microelectrode to the studies of neurotransmitters, which has been pioneered by Adams, to monitor the concentration of neurotransmitters in the central nerve system (CNS) has had a special impact [16]. The advantages of microelectrode include fast-response time, small size, high mass transportation flux and low IR drop [17–19]. They could be further developed for
constructing miniaturized biosensors and used for the in-vivo analysis. Here, the copolymerization of 3,4-DHBA with aniline in acid media was obtained. It was found that the resulted polymer was self-doped one that exhibited a stable electroactivity even in neutral and weakly basic media. It suggested that 3,4DHBA in oxidized state could react with the radicals of aniline and produce a composite polymer. Because of the presence of -COOH and -OH groups in the polymer structure, a self-doped composite polymer was obtained. Moreover, the catalysis of ascorbic acid could be obtained at this polymer modified electrode. As a consequence, such a miniaturized biosensor could be further developed for the clinical applications.
2. Experimental
2.1. Reagents and materials Dopamine was purchased from Fluka AG (Switzerland). Ascorbic acid was obtained from the Shanghai Biochemistry Research Institute. Aniline was distilled before use. All the other reagents were analytical grade. Water used in experiments was twice-quartz-distilled. The phosphate buffer was made of 1.0× 10 − 2 M K2HPO4 + KH2PO4 and 0.2 M KCl.
2.2. Apparatus Electrochemical experiments were carried out with a BAS-100B Electrochemical Analyzer equipped with a PA-1 Preamplifier (BAS, USA) which was used to amplify the current and to filter out noise. A microdisk gold electrode (®50 mm) was used as working electrode. The auxiliary electrode and the reference one were a platinum wire and a saturated calomel electrode, respectively. The test solution was deaerated by high pure nitrogen before experiments and experiments were all performed under nitrogen atmosphere. The experimental temperature was controlled at 2090.5°C.
J.-J. Sun et al. / Talanta 45 (1998) 851–856
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2.3. Procedure The copolymerization of 3,4-DHBA and aniline was carried out in 0.5 M sulfuric acid by cyclic voltammetric sweeping from − 200 to 850 mV at a scan rate of 50 mV s − 1. Amperometric determination of ascorbic acid was performed in phosphate buffer at a constant potential.
3. Results and discussion
3.1. The electrochemical copolymerization of 3,4 -DHBA and aniline The electrochemical copolymerization of aniline with 3,4-DHBA by cyclic voltammetry was depicted in Fig. 1. It can be seen that there were two couples of peaks, denoted as electrocouple I and electrocouple II, situated at about 0.12 and 0.47 V, respectively, and also a sharp peak, peak III, at about 0.7 V. It is shown that at the very
Fig. 1. The cyclic voltammograms at the microdisk gold electrode with 50 mV s − 1 in 0.5 M H2SO4 containing 0.1 M aniline and 5 ×10 − 3 M 3,4-dihydroxybenzoic acid.
Fig. 2. The cyclic voltammograms at the microdisk gold electrode with 50 mV s − 1 in 0.5 M H2SO4 containing 5×10−3 M 3,4-dihydroxybenzoic acid.
beginning of the polymerization there was no peaks appeared in the voltammograms but nuclearization process of aniline on the electrode surface (point IV in Fig. 1). Several cycles later, the peak values of I, II and III began to increase. The peak potential of peak III shifted negative with the increase of the cycling number, and it might correspond to the oxidation process of 3,4-DHBA by polyaniline, and relate to an electrocatalytic mechanism for the peak potential of III was less than that of the oxidation of 3,4DHBA at a bare gold electrode (ca. 0.83 V, see also Fig. 2). The structure of electrocouple II in Fig. 1 was very different from that in the voltammograms of the polymerization of aniline itself, the former had higher peak strength and larger peak area. In the process of aniline polymerization; the peak appeared at ca. 0.5 V might be ascribed to the formation of the oligomer in the film [20,21]. Whereas in the case of the copolymerization of aniline with 3,4-DHBA, the electrocouple II might ascribe not only to the formation of the oligomer of aniline itself but also to that of aniline with 3,4-DHBA. As a mater of fact, the oxidized form of 3,4-DHBA was electrophilic (see also scheme 1 in [22]), and it could react with the radicals of nucleophilic N-atoms in the chain of polyaniline. The products of this reaction might incorporate into the copolymerized film.
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J.-J. Sun et al. / Talanta 45 (1998) 851–856
3.2. Characteristics of the composite polymer modified electrode Fig. 3 showed the voltammograms of the modified electrode in neutral media. There was an electrocouple appeared in the range of − 500–550 mV and its electroactivity did not decrease under the continuously sweeping. It was well known that polyaniline would lose its electroactivity easily at pH values above 4 because there were not enough protons in the media to be doped in the pores of the film to hold the charge equilibrium during the oxidation process. In the present case, the function groups of 3,4-DHBA incorporated in the film, such as -COOH and -OH, could donor protons to the nitrogen atoms of the linear structure of the polymer and cause the polymer hold electroactivity in neutral media. When this composite polymer modified electrode was cyclically swept in neutral media with different scan rates, it exhibited that the current response was linear with the square root of scan rates. Meanwhile, the anodic and cathodic potential would shift toward positive and negative direction with the increasing scan rates, respectively. In lower scan rates, the couple was more re-
Fig. 4. The cyclic voltammograms at the composite polymer modified electrode in pH 7.0 buffer solution at a scan rate of 10 mV s − 1 in (a) absence and (b) presence of 2.0 × 10 − 3 M ascorbic acid.
versible compared with that obtained at higher scan rates. Moreover, the cathodic potential had a larger change. This suggested that the reduction of this composite film was rather difficult. During the process of reduction the film would store the negative charged ions in its pores for keeping its charge equilibrium. With the increase of the pHs, this phenomenon could be further observed.
3.3. Electrocatalytic oxidation of ascorbic acid at the modified electrode
Fig. 3. The cyclic voltammograms at the composite polymer modified electrode in pH 7.0 buffer solution at the scan rate: (1) 10; (2) 30; (3) 50; (4) 70 and (5) 9O mV s − 1.
Fig. 4 showed that the cyclic voltammograms which was obtained at the composite polymer modified microelectrode in the absence and presence of 2.0×10 − 3 M ascorbic acid. With the addition of ascorbic acid, the anodic peak currents increased significantly. The anodic overpotential was reduced for 200 mV compared with that obtained at bare gold electrodes. This large increase of oxidation currents might not be due to the doping of ascorbic acid into the copolymer because its concentration was very smaller than that of the other anions in the support media. It might be ascribed to the fact that ascorbic acid diffused to the electrode surface, reacted with the electroactive couple of the copolymer and resulted in a current increase.
J.-J. Sun et al. / Talanta 45 (1998) 851–856
855
The dependence of the catalytic peak currents on the square root of the scan rates was linear. It meant that the electrode process was a semi-infinite diffusion one rather than a surface reaction one. When this modified electrode was swept in ascorbic acid solution for a long time, no pollution phenomenon was observed. Thus, it is promising for further developing biosensors due to its easy-fabrication and stability. The positive-charged dopamine was used as a probe for the feature examination of the copolymer films. The results showed that dopamine could not be catalyzed at the film. It suggested that this film was positive-charged and repulsive to dopamine because of the identical charge.
3.4. The effects of pH on the catalytic oxidation of ascorbic acid Fig. 5 depicts the dependence of catalytic currents on the pHs of the media. It is shown that the current responses decreased with the pHs increased from 5.0 – 9.0. The ratios of the current responses at pH 7.0 and pH 8.0 to that at pH 5.0 were about 0.3 and 0.14, respectively, and at pH 9.0, it became zero. Thus, this electrode could not be used in the media of the pHs over 8.5 and could be used in neutral media for the detection
Fig. 5. The dependence of catalytic currents on pH from CVs in 2.0 × 10 − 3 M ascorbic acid phosphate buffer solution at scan rate of 10 mV s − 1.
Fig. 6. Dynamic response of the composite polymer modified electrode to successive addition of ascorbic acid in 0.1 mM steps at a constant potential of 0.2 V vs. SCE in pH 7.0 buffer solution. Insertion: the corresponding calibration curve.
of ascorbic acid. It should be noted that the catalytic currents were seriously influenced by the pHs of the media. Hence the pHs of the media have to be controlled carefully in the measurements.
3.5. The determination of ascorbic acid Steady-state amperometric determination of ascorbic acid had been carried out at a constant potential to demonstrate the analytical usefulness of the electrode as an ascorbic acid sensor. Fig. 6 depicts a typical trace of the steady-state current response of the electrode. After the addition of ascorbic acid stock solution, the oxidation current rose steeply to reach a stable value within ca. 2 s. Hence the electrode exhibited a short current response time and could be used in the case of rapid determination of ascorbic acid. The calibration curve of the electrode response to ascorbic acid is depicted in the insertion of Fig. 6. The current responses were linear with the concentration of ascorbic acid in the range of 1.0× 10 − 4 –1.0×10 − 2 M. The detection limit was 5.0×10 − 5 M at a signal-to-noise ratio of 3. The sensitivity of the electrode was estimated from the calibration curve to be 4 nA mM − 1 or 0.21 AM − 1 cm − 2.
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J.-J. Sun et al. / Talanta 45 (1998) 851–856
3.6. The stability of this self-doped polymer modified electrode This modified electrode exhibited a high stability whenever it was placed in dry state or in phosphate buffer solutions at 4°C. No loss of electroactivity of the electrode was found for the continuously cyclically sweep for 500 cycles. Over ten successive assays of 0.1 mM ascorbic acid, the relative standard deviation was ca. 1.4%. The electrode was also not deteriorated even for long a month.
4. Conclusions The copolymerization of 3,4-DHBA and aniline may be easily carried out using cyclic voltammetry. This composite polymer modified electrode exhibits a good electrochemical activity even in neutral and weakly basic solutions. It was found that this electroactive polymer could act as mediators for the catalytic oxidation of ascorbic acid. The amperometric determination of ascorbic acid had been carried out at a constant potential. The electrode exhibits a rapid current response (less than 2 s) and a high sensitivity (0.21 AM − 1 cm − 2). Moreover, the linear response current was achieved in a concentration range from 1.0× 10 − 4–1.0×10 − 2 M. This composite polymer modified electrode could be developed for the construction of miniaturized biosensors to the amperometric determination of ascorbic acid.
Acknowledgements This project was supported by the National Natural Science Foundation of China and the
.
Found of the National Education Committee of China.
References [1] L.R. Faulkner, Chem. Eng. News 62 (1984) 28. [2] A.P. Doherty, M.A. Stanley, J.G. Vos, Analyst 120 (1995) 2371. [3] A.-M. Yu, D.-M. Sun, H.-Y. Gu, H.-Y. Chen, Anal. Lett. 29 (1996) 2633. [4] K.N. Kuo, R.W. Murray, J. Electroanal. Chem. 132 (1982) 37. [5] J. Facci, R.W. Murray, Anal. Chem. 54 (1982) 772. [6] C. Ueda, D.C.-S. Tse, T. Kuwana, Anal. Chem. 54 (1982) 850. [7] J. Ye, R.P. Baldwin, J.W. Schlager, Electroanalysis 1 (1989) 133. [8] S. Dong, Z. Sun, Z. Lu, J. Chem. Soc. Chem. Commun. (1988) 993. [9] B.L. Wheeler, G. Caple, A. Henderson, J. Francis, K. Cantrell, S. Vogel, S. Grey, D. Russell, J. Electrochem. Soc. 136 (1989) 2769. [10] Y. Kang, M.-H. Lee, S.B. Rhee, Synth. Met. 52 (1992) 319. [11] T. Tatsuma, M. Gondaria, T. Watanabe, Anal. Chem. 64 (1992) 1183. [12] T. Hirai, S. Kuwabata, H. Yoneyama, J. Chem. Soc. Faraday Trans. I 85 (1989) 969. [13] P.N. Bartlett, J.H. Wang, E.N.K. Wallace, J. Chem. Soc. Chem. Commun. (1996) 359. [14] A.A. Karyakin, A.K. Strakhova, A.K. Yatsimirsky, J. Electroanal. Chem. 371 (1994) 259. [15] A.A. Karyakin, I.A. Maltsev, L.V. Lukachova, J. Electroanal. Chem. 402 (1996) 217. [16] R.N. Adams, Anal. Chem. 48 (1976) 1126A. [17] R.J. Forster, Chem. Soc. Rev. 23 (1994) 289. [18] E. Csoregi, L. Gorton, G. Marko-Varga, Anal. Chim. Acta. 273 (1993) 59. [19] M. Fleischman, S. Pons, D.R. Robinson, Schmidt, Ultramicroelectrodes, Datatech Systems, Morganton, NC, 1987. [20] D.E. Stilwell, S.-M. Park, J. Electrochem. Soc. 135 (1988) 2254. [21] S. Dong, D. Zhang, Z. Li, Chin. Chem. Lett. 3 (1992) 29. [22] F. Pariente, F. Tobalina, M. Darder, E. Lorenzo, H.D. Abruna, Anal. Chem. 68 (1996) 3135.
The electrochemical copolymerization of 3,4-dihydroxybenzoic acid and aniline at microdisk gold electrode and its amperometric determination for ascorbic acid Jian-Jun Sun, Dong-Mei Zhou, Hui-Qun Fang, Hong-Yuan Chen * Department of Chemistry, State Key Laboratory of Coordination Chemistry, Nanjing Uni6ersity, Nanjing, 210 093, People’s Republic of China Received 7 February 1997; received in revised form 2 June 1997; accepted 4 June 1997
Abstract The electrochemical copolymerization of 3,4-dihydroxybenzoic acid (3,4-DHBA) and aniline was carried out at microdisk gold electrodes by means of cyclic voltammetric sweep. The polymer obtained on the electrode shows good electrochemical activity and high stability even though in neutral and weakly basic media. It was found that the response current of ascorbic acid was greatly enhanced at this composite polymer electrode. Moreover, the anodic overpotential was significantly reduced for about 200 mV (vs. SCE) compared with that obtained at bare gold electrodes. The electrode exhibits a rapid current response (less than 2 s) and a high sensitivity (0.21 AM − 1 cm − 2). The dependence of response currents on the concentration of ascorbic acid was linear in the range of 1.0 ×10 − 4 – 1.0 × 10 − 2 M. In addition this composite polymer modified electrode exhibits a high electrode stability for a long-term use. © 1998 Elsevier Science B.V. Keywords: Microelectrode; Copolymerization; Aniline; 3,4-Dihydroxybenzoic acid; Ascorbic acid; Catalysis; Amperometric determination
1. Introduction Ascorbic acid has been extensively studied because of its significance in bioelectrochemistry, neurochemistry and clinical diagnostics’ applications [1]. The oxidation of ascorbic acid at conventional electrodes is well documented and is known to proceed via two consecutive one-electron transfer processes involving the participation * Corresponding author. Fax: +86 25 3317761.
of a radical anion intermediate to form dehydroL-ascorbic acid. This species subsequently undergoes a hydration reaction characteristic of carbonyl groups to form an electroinactive product. The catalytic oxidation of ascorbic acid has been obtained at many mediator-modified electrodes [2–6]. However, most of them still suffered from the pollution of ascorbic acid due to its adsorption on the electrode surface, especially at inorganic film modified ones.
0039-9140/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 9 - 9 1 4 0 ( 9 7 ) 0 0 1 8 3 - 5
852
J.-J. Sun et al. / Talanta 45 (1998) 851–856
Polymer modified electrodes have been widely used for the immobilization of mediators or enzymes by their matrix structure, electrocatalysis of biological molecules and the construction of molecular devices, etc. [7 – 11]. Usually, polymer modified electrodes exhibited a good stability and catalytic effects resulted from their three-dimensional mediator distribution. The conductive polymers such as polyaniline, polypyrrole, polythiophene as well as their derivatives have been used for constructing various biosensors. Polyaniline has been studied extensively because of its high stability and conductivity. It was well known that the polyaniline could be electrochemically polymerized easily in acid media. Protons would participate in the redox process of polyaniline. However, as pH of the media increased above 4 the polymer would lose its electrochemical activity [12]. This has greatly limited its application in biological sample’s analysis. Recently, Bartlett [13] obtained the polyanilinepoly(vinylsulfonate) composite film that exhibited an off/on switching response when it was exposed to NADH at pH 7.0. The reduction of the oxidized form of polyaniline(pernigraniline) in the composite film by reacting with NADH in pH 7.0 buffer solution brought about a switch in the conductivity of devices. The negative charged PVS, which was doped into polyaniline, could provide protons for the nitrogen atom of linear structure of polyaniline and led this composite film to work in neutral media. Karyakin et al. [14,15] also reported that the copolymerization of aniline with m-aminobenzoic acid or metrillic acid could exhibit a wide pH-range electroactivity even in basic solution. However, the application of this composite polymer in neutral media has not been reported. The application of microelectrode to the studies of neurotransmitters, which has been pioneered by Adams, to monitor the concentration of neurotransmitters in the central nerve system (CNS) has had a special impact [16]. The advantages of microelectrode include fast-response time, small size, high mass transportation flux and low IR drop [17–19]. They could be further developed for
constructing miniaturized biosensors and used for the in-vivo analysis. Here, the copolymerization of 3,4-DHBA with aniline in acid media was obtained. It was found that the resulted polymer was self-doped one that exhibited a stable electroactivity even in neutral and weakly basic media. It suggested that 3,4DHBA in oxidized state could react with the radicals of aniline and produce a composite polymer. Because of the presence of -COOH and -OH groups in the polymer structure, a self-doped composite polymer was obtained. Moreover, the catalysis of ascorbic acid could be obtained at this polymer modified electrode. As a consequence, such a miniaturized biosensor could be further developed for the clinical applications.
2. Experimental
2.1. Reagents and materials Dopamine was purchased from Fluka AG (Switzerland). Ascorbic acid was obtained from the Shanghai Biochemistry Research Institute. Aniline was distilled before use. All the other reagents were analytical grade. Water used in experiments was twice-quartz-distilled. The phosphate buffer was made of 1.0× 10 − 2 M K2HPO4 + KH2PO4 and 0.2 M KCl.
2.2. Apparatus Electrochemical experiments were carried out with a BAS-100B Electrochemical Analyzer equipped with a PA-1 Preamplifier (BAS, USA) which was used to amplify the current and to filter out noise. A microdisk gold electrode (®50 mm) was used as working electrode. The auxiliary electrode and the reference one were a platinum wire and a saturated calomel electrode, respectively. The test solution was deaerated by high pure nitrogen before experiments and experiments were all performed under nitrogen atmosphere. The experimental temperature was controlled at 2090.5°C.
J.-J. Sun et al. / Talanta 45 (1998) 851–856
853
2.3. Procedure The copolymerization of 3,4-DHBA and aniline was carried out in 0.5 M sulfuric acid by cyclic voltammetric sweeping from − 200 to 850 mV at a scan rate of 50 mV s − 1. Amperometric determination of ascorbic acid was performed in phosphate buffer at a constant potential.
3. Results and discussion
3.1. The electrochemical copolymerization of 3,4 -DHBA and aniline The electrochemical copolymerization of aniline with 3,4-DHBA by cyclic voltammetry was depicted in Fig. 1. It can be seen that there were two couples of peaks, denoted as electrocouple I and electrocouple II, situated at about 0.12 and 0.47 V, respectively, and also a sharp peak, peak III, at about 0.7 V. It is shown that at the very
Fig. 1. The cyclic voltammograms at the microdisk gold electrode with 50 mV s − 1 in 0.5 M H2SO4 containing 0.1 M aniline and 5 ×10 − 3 M 3,4-dihydroxybenzoic acid.
Fig. 2. The cyclic voltammograms at the microdisk gold electrode with 50 mV s − 1 in 0.5 M H2SO4 containing 5×10−3 M 3,4-dihydroxybenzoic acid.
beginning of the polymerization there was no peaks appeared in the voltammograms but nuclearization process of aniline on the electrode surface (point IV in Fig. 1). Several cycles later, the peak values of I, II and III began to increase. The peak potential of peak III shifted negative with the increase of the cycling number, and it might correspond to the oxidation process of 3,4-DHBA by polyaniline, and relate to an electrocatalytic mechanism for the peak potential of III was less than that of the oxidation of 3,4DHBA at a bare gold electrode (ca. 0.83 V, see also Fig. 2). The structure of electrocouple II in Fig. 1 was very different from that in the voltammograms of the polymerization of aniline itself, the former had higher peak strength and larger peak area. In the process of aniline polymerization; the peak appeared at ca. 0.5 V might be ascribed to the formation of the oligomer in the film [20,21]. Whereas in the case of the copolymerization of aniline with 3,4-DHBA, the electrocouple II might ascribe not only to the formation of the oligomer of aniline itself but also to that of aniline with 3,4-DHBA. As a mater of fact, the oxidized form of 3,4-DHBA was electrophilic (see also scheme 1 in [22]), and it could react with the radicals of nucleophilic N-atoms in the chain of polyaniline. The products of this reaction might incorporate into the copolymerized film.
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J.-J. Sun et al. / Talanta 45 (1998) 851–856
3.2. Characteristics of the composite polymer modified electrode Fig. 3 showed the voltammograms of the modified electrode in neutral media. There was an electrocouple appeared in the range of − 500–550 mV and its electroactivity did not decrease under the continuously sweeping. It was well known that polyaniline would lose its electroactivity easily at pH values above 4 because there were not enough protons in the media to be doped in the pores of the film to hold the charge equilibrium during the oxidation process. In the present case, the function groups of 3,4-DHBA incorporated in the film, such as -COOH and -OH, could donor protons to the nitrogen atoms of the linear structure of the polymer and cause the polymer hold electroactivity in neutral media. When this composite polymer modified electrode was cyclically swept in neutral media with different scan rates, it exhibited that the current response was linear with the square root of scan rates. Meanwhile, the anodic and cathodic potential would shift toward positive and negative direction with the increasing scan rates, respectively. In lower scan rates, the couple was more re-
Fig. 4. The cyclic voltammograms at the composite polymer modified electrode in pH 7.0 buffer solution at a scan rate of 10 mV s − 1 in (a) absence and (b) presence of 2.0 × 10 − 3 M ascorbic acid.
versible compared with that obtained at higher scan rates. Moreover, the cathodic potential had a larger change. This suggested that the reduction of this composite film was rather difficult. During the process of reduction the film would store the negative charged ions in its pores for keeping its charge equilibrium. With the increase of the pHs, this phenomenon could be further observed.
3.3. Electrocatalytic oxidation of ascorbic acid at the modified electrode
Fig. 3. The cyclic voltammograms at the composite polymer modified electrode in pH 7.0 buffer solution at the scan rate: (1) 10; (2) 30; (3) 50; (4) 70 and (5) 9O mV s − 1.
Fig. 4 showed that the cyclic voltammograms which was obtained at the composite polymer modified microelectrode in the absence and presence of 2.0×10 − 3 M ascorbic acid. With the addition of ascorbic acid, the anodic peak currents increased significantly. The anodic overpotential was reduced for 200 mV compared with that obtained at bare gold electrodes. This large increase of oxidation currents might not be due to the doping of ascorbic acid into the copolymer because its concentration was very smaller than that of the other anions in the support media. It might be ascribed to the fact that ascorbic acid diffused to the electrode surface, reacted with the electroactive couple of the copolymer and resulted in a current increase.
J.-J. Sun et al. / Talanta 45 (1998) 851–856
855
The dependence of the catalytic peak currents on the square root of the scan rates was linear. It meant that the electrode process was a semi-infinite diffusion one rather than a surface reaction one. When this modified electrode was swept in ascorbic acid solution for a long time, no pollution phenomenon was observed. Thus, it is promising for further developing biosensors due to its easy-fabrication and stability. The positive-charged dopamine was used as a probe for the feature examination of the copolymer films. The results showed that dopamine could not be catalyzed at the film. It suggested that this film was positive-charged and repulsive to dopamine because of the identical charge.
3.4. The effects of pH on the catalytic oxidation of ascorbic acid Fig. 5 depicts the dependence of catalytic currents on the pHs of the media. It is shown that the current responses decreased with the pHs increased from 5.0 – 9.0. The ratios of the current responses at pH 7.0 and pH 8.0 to that at pH 5.0 were about 0.3 and 0.14, respectively, and at pH 9.0, it became zero. Thus, this electrode could not be used in the media of the pHs over 8.5 and could be used in neutral media for the detection
Fig. 5. The dependence of catalytic currents on pH from CVs in 2.0 × 10 − 3 M ascorbic acid phosphate buffer solution at scan rate of 10 mV s − 1.
Fig. 6. Dynamic response of the composite polymer modified electrode to successive addition of ascorbic acid in 0.1 mM steps at a constant potential of 0.2 V vs. SCE in pH 7.0 buffer solution. Insertion: the corresponding calibration curve.
of ascorbic acid. It should be noted that the catalytic currents were seriously influenced by the pHs of the media. Hence the pHs of the media have to be controlled carefully in the measurements.
3.5. The determination of ascorbic acid Steady-state amperometric determination of ascorbic acid had been carried out at a constant potential to demonstrate the analytical usefulness of the electrode as an ascorbic acid sensor. Fig. 6 depicts a typical trace of the steady-state current response of the electrode. After the addition of ascorbic acid stock solution, the oxidation current rose steeply to reach a stable value within ca. 2 s. Hence the electrode exhibited a short current response time and could be used in the case of rapid determination of ascorbic acid. The calibration curve of the electrode response to ascorbic acid is depicted in the insertion of Fig. 6. The current responses were linear with the concentration of ascorbic acid in the range of 1.0× 10 − 4 –1.0×10 − 2 M. The detection limit was 5.0×10 − 5 M at a signal-to-noise ratio of 3. The sensitivity of the electrode was estimated from the calibration curve to be 4 nA mM − 1 or 0.21 AM − 1 cm − 2.
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J.-J. Sun et al. / Talanta 45 (1998) 851–856
3.6. The stability of this self-doped polymer modified electrode This modified electrode exhibited a high stability whenever it was placed in dry state or in phosphate buffer solutions at 4°C. No loss of electroactivity of the electrode was found for the continuously cyclically sweep for 500 cycles. Over ten successive assays of 0.1 mM ascorbic acid, the relative standard deviation was ca. 1.4%. The electrode was also not deteriorated even for long a month.
4. Conclusions The copolymerization of 3,4-DHBA and aniline may be easily carried out using cyclic voltammetry. This composite polymer modified electrode exhibits a good electrochemical activity even in neutral and weakly basic solutions. It was found that this electroactive polymer could act as mediators for the catalytic oxidation of ascorbic acid. The amperometric determination of ascorbic acid had been carried out at a constant potential. The electrode exhibits a rapid current response (less than 2 s) and a high sensitivity (0.21 AM − 1 cm − 2). Moreover, the linear response current was achieved in a concentration range from 1.0× 10 − 4–1.0×10 − 2 M. This composite polymer modified electrode could be developed for the construction of miniaturized biosensors to the amperometric determination of ascorbic acid.
Acknowledgements This project was supported by the National Natural Science Foundation of China and the
.
Found of the National Education Committee of China.
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