- Email: [email protected]
A novel metal immobilized self-assembled surface for electrochemical sensing
Sensors and Actuators B 96 (2003) 523–526
A novel metal immobilized self-assembled surface for electrochemical sensing Dhesingh Ravi Shankaran1 , Ken-Iichi Iimura, Teiji Kato∗ Satellite Venture Business Laboratory, Utsunomiya University, Yoto-7-1-2, Utsunomiya 321-8585, Japan Received 12 February 2003; received in revised form 10 June 2003; accepted 26 June 2003
Abstract A new iron immobilized (3-mercaptopropyl)trimethoxysilane (MPS) self-assembled layer has been developed over a gold electrode surface by a combined sol–gel and self-assembly techniques. The sensor exhibited excellent sensing characteristics for the catalytic reduction as well as oxidation of hydrogen peroxide (H2 O2 ) at the potentials of +0.1 and +0.4 V, respectively, with good sensitivity, selectivity and reproducibility. The possible coordination of iron with the functional thiol group of the MPS favored highly stable and leakage free sensing surface. The proposed system is a promising mean for the construction of a variety of new chemical and biosensors. © 2003 Elsevier B.V. All rights reserved. Keywords: Self-assembly; Sol–gel; (3-Mercaptopropyl)trimethoxysilane; Iron; Electroanalysis; Hydrogen peroxide
1. Introduction In recent years, the self-assembly technique has received wide spread attraction in the fabrication of effective sensors for biomedical and environmental analysis. The easy and reliable fabrication procedure, rapid response, enhanced sensitivity and selectivity are the special advantages of the self-assembled sensors [1–3]. Despite the variety of materials employed to fabricate self-assembled surfaces, functional silane molecules received great attraction due to the possible coordination with the immobilized catalysts, favoring highly stable sensing surfaces with a possible control over the distribution and orientation of the catalytic molecules [2–7]. In this context, an iron immobilized MPS self-assembled layer has been developed onto a gold electrode. MPS contains functional thiol group in its molecular structure, which favors the stable immobilization of iron in the self-assembled surface. The catalytic activity of the metal ions in the reduction or oxidation of a variety of compounds is well known [7–11]. The favorable combination of the efficient and preferential catalytic activity of iron, advantageous features of self-assembly and stable immobilization makes the system highly effective for electroanalysis of compounds of ∗ Corresponding author. Fax: +81-28-689-6179. E-mail addresses: [email protected] (D. Ravi Shankaran), [email protected] (T. Kato). 1 Co-corresponding author.
0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-4005(03)00632-4
interest. The sensing characteristic of the proposed system was evaluated for the determination of H2 O2 , a compound of great importance in biological, clinical and environmental analysis. The modified iron immobilized self-assembled electrode exhibited good catalytic activity for the reduction and oxidation of H2 O2 with good sensitivity, selectivity and reproducibility.
2. Experimental 2.1. Reagents and apparatus (3-Mercaptopropyl)trimethoxysilane was purchased from Sigma, USA. Hydrogen peroxide, ferrous sulfate, ascorbic acid and uric acid were received from Wako chemicals, Japan. All other chemicals and reagents were of analytical grade. Ultra pure water (ρ = 18 M) was used throughout the experiment. High purity nitrogen was used for deaeration. Electrochemical experiments were performed with potentiostat/galvanostat GPIB HA 503G and HAB 151 (Hokuto Denko Ltd.) interfaced with a personal computer and recorder, respectively. The modified self-assembled electrode was used as the working electrode, while Ag/AgCl (saturated KCl) and a platinum wire were used as reference and counter electrode, respectively. Scanning electron micrograph (SEM) was obtained with S-3200 N SEM, Hitachi.
524
D. Ravi Shankaran et al. / Sensors and Actuators B 96 (2003) 523–526
2.2. Fabrication of the modified electrode Prior to modification, a well-polished gold (3.0 mm diameter) electrode was cleaned with piranha solution followed by electrochemical cycling in 0.1 M H2 SO4 solution in the potential range between 0.0 and 1.5 V for 15 cycles. A stirring 0.85 ml of 0.1 M MPS solution (in ethanol) was mixed well with 0.2 ml ultra pure water followed by 1.2 ml of 0.1 M ferrous sulfate solution. The resulting mixture was mixed well and a cleaned gold electrode was dipped into this mixture for nearly 10–12 h. After modification, the iron immobilized self-assembled (Fe/MPS/Au) electrode was rinsed well with ethanol followed by ultra pure water. A MPS coated gold electrode (MPS/Au) was prepared in the similar way except the addition of ferrous sulfate.
3. Results and discussion It is believed that MPS undergoes hydrolysis and condensation (sol–gel process) immediately after mixing with water and ferrous sulfate. When gold electrode was dipped into this organically modified silicate (ormosil) gel solution, MPS assembles onto the gold surface [5–7]. The presence of thiol (–SH) group in the MPS favors the stable attachment to gold surface due to the strong interaction of Au with –SH group. Meanwhile, the iron in the gel mixture gets immobilized into the self-assembled surface. Thus, a porous three-dimensional self-assembled surface with immobilized iron will be formed at the gold surface. The possible interaction of iron with the thiol group of the MPS molecule in the three-dimensional matrix makes the surface highly stable and leakage free [5–7,11]. A uniform distribution and good coverage of the iron immobilized MPS assembled layer over the Au surface can be seen in the scanning electron micrograph (Fig. 1). The modified self-assembled surface can also be prepared by initial coating of MPS layer onto a gold sur-
Fig. 1. Scanning electron micrograph of the Fe/MPS/Au electrode surface.
Fig. 2. HDVs for 4.6 × 10−4 M H2 O2 at (a) Fe/MPS/Au, (b) bare Au and (c) MPS/Au electrode in a stirring 0.1 M KNO3 solution; pH 7.0 (0.05 M phosphate buffer); stirring rate: 300 rpm.
face followed by the immobilization of iron by dipping or by electrochemical cycling in ferrous sulfate solution. However, the modified electrode prepared with these procedures showed higher background current and less stability compared to the proposed method. The possibility of iron immobilization throughout the three-dimensional ormosil matrix and the interaction with the thiol functionality are the probable reasons for the low background current and enhanced stability of the present system. The hydrodynamic voltammograms (HDVs) obtained with (a) Fe/MPS/Au, (b) bare Au and (c) MPS/Au electrode in presence of 4.6 × 10−4 M H2 O2 were shown in Fig. 2. The HDVs were registered in a stirring 0.1 M KNO3 solution in the potential range from −0.2 to +0.6 V. Both MPS/Au and bare Au electrode showed poor current response to H2 O2 . Small current response was observed at MPS/Au and at bare Au electrodes at extreme potentials. In contrast, the Fe/MPS/Au electrode showed enhanced current response with significant decrease in the overpotential for the reduction as well as oxidation of H2 O2 allowing sensitive determination of H2 O2 . For reduction, the current response starts increasing from +0.2 V, reaching a limiting value around +0.1 V, Similarly for oxidation, the current starts increasing from +0.3 V, reaching a limiting value around +0.4 V. From the HDVs, a potential of +0.1 and +0.4 V can be applied for electrocatalytic reduction and oxidation, respectively, during amperometric quantification of H2 O2 . The results indicate that the iron present in the modified self-assembled surface exerted good catalytic activity for the reduction as well as oxidation of hydrogen peroxide favoring its quantification possible both by reduction and oxidation. To compare the electrochemical behavior of the iron immobilized gold electrodes with and without use of the MPS, gold electrodes were modified with only iron by electrochemical cycling and by dipping in iron salt solution. It was observed that these electrodes showed less sensitivity and poor stability compared to Fe/MPS/Au electrode. Fig. 3 shows the cyclic voltammograms (CVs) observed for the determination of H2 O2 by electrocatalytic reduc-
D. Ravi Shankaran et al. / Sensors and Actuators B 96 (2003) 523–526
525
Fig. 3. CVs of Fe/MPS/Au electrode in 0.1 M KNO3 solution (a) without H2 O2 , (b) with 4.8 × 10−4 M H2 O2 and (c) 1.2 × 10−3 M H2 O2 ; (d) and (e) correspond to MPS/Au and bare Au electrode in presence of 1.2 × 10−3 M H2 O2 , respectively; scan rate: 50 mV/s; pH 7.0 (0.05 M phosphate buffer).
tion. The curve (a) corresponds to the Fe/MPS/Au electrode in 0.1 M KNO3 solution at a scan rate of 50 mV/s. A redox curve with anodic and cathodic peak potential at +0.3 and +0.18 V, respectively, was observed corresponding to Fe2+ /Fe3+ reaction. The redox reaction is highly stable, no significant change in the peak current and the peak potential values was observed on multiple cycling. The curves (b) and (c) correspond to the presence of 4.8 × 10−4 and 1.2 × 10−3 M H2 O2 at the Fe/MPS/Au electrode. The curves (d) and (e) correspond to presence of 1.2 × 10−3 M H2 O2 at MPS/Au and bare Au electrode, respectively. As can be seen, the modified electrode showed good catalytic activity for the reduction of H2 O2 at the reduced potential (0.1 V) with enhanced current sensitivity compared to bare Au and MPS/Au electrode. Studies on the determination of H2 O2 by electrocatalytic oxidation showed similar response, in which, the Fe/MPS/Au electrode showed enhanced current response to oxidation of H2 O2 at a reduced potential around +0.4 V (figure not shown). Studies on the effect of pH on the catalytic reduction and oxidation of H2 O2 revealed that the Fe/APS/Au electrode showed good response in a wide pH range from 4 to 9. Reduced responses were observed at highly acidic and basic conditions, which is possibly due to the hydration and hydroxylation of the iron in the self-assembled surface, respectively [7,9]. Fig. 4 shows the interference effect of ascorbic acid (AA) and uric acid (UA) on the determination of H2 O2 by electrocatalytic reduction at +0.1 V (A) and oxidation at +0.4 V (B). Negligible contribution of these species was observed during electrocatalytic reduction at +0.1 V. In contrast, the H2 O2 response was accompanied by a significant contribution of AA and UA at +0.4 V during
Fig. 4. Amperometric response of the Fe/MPS/Au electrode for the addition of 4.0 × 10−4 M H2 O2 followed by the additions of 8.0 × 10−4 M ascorbic acid and uric acid; pH 7.0 (0.05 M phosphate buffer); stirring rate: 300 rpm. (A) Reduction at +0.1 V and (B) oxidation at +0.4 V.
electrocatalytic oxidation. Thus, covering of the electrode surface with a suitable permselective membrane is advisable for the quantification of H2 O2 by electrocatalytic oxidation at +0.4 V. The current–time amperometric response recorded at +0.1 V during the successive addition of 85 M H2 O2 to the stirring 0.1 M KNO3 solution maintained at pH 7.0 was shown in Fig. 5. The Fe/MPS/Au electrode showed rapid response to concentration changes (<8 s) and the response was linear in the concentration range from 9.5 × 10−6 to 1.2 × 10−3 M H2 O2 with a correlation coefficient of 0.995. The detection limit was 7.6 × 10−6 M (S/N = 3). A linear response to H2 O2 in the concentration range from 8.2 × 10−6 to 1.0 × 10−3 M with a correlation coefficient of 0.994 was observed for the amperometric measurement by electrocatalytic oxidation at +0.4 V and the detection limit was 6.4 × 10−6 M. Relative standard deviations of 3.0 and 3.4% were observed for eight successive determination of 2.0 × 10−4 M H2 O2 by electrocatalytic reduction and oxidation, respectively, indicating the reliable reproducibility. The electrode retained its initial response up to nearly 10 days on continuous use and more than 88% of the response was retained for nearly 30 days (stored in a desiccator). As most of the enzyme sensor involved the determination of enzymatically liberated H2 O2 as the measuring principle [10–12], the proposed system can be effectively employed for the fabrication of a variety of biosensors for the determination of glucose, cholesterol, etc.
526
D. Ravi Shankaran et al. / Sensors and Actuators B 96 (2003) 523–526
Fig. 5. Current–time response of the Fe/MPS/Au electrode in stirring 0.1 M KNO3 solution for the successive addition of 85 M H2 O2 at +0.1 V; pH 7.0 (0.05 M phosphate buffer); stirring rate: 300 rpm. Inset: Calibration plot.
[4] I. Willner, B. Willner, E. Katz, Functional biosensor system via surface nanoengineering of electronic elements, Rev. Mol. Biotech. 82 (2002) 325–355. [5] D. Ravi Shankaran, Y.B. Shim, An amperometric sensor for hydrogen peroxide based on a (3-mercaptopropyl)trimethoxysilane self-assembled layer containing hydrazine, Electroanalysis 14 (2002) 704–707. [6] A. Walcarius, Electroanalysis with pure, chemically modified and sol–gel derived silica-based materials, Electroanalysis 13 (2001) 701– 718. [7] D. Ravi Shankaran, N. Uehara, T. Kato, Determination of sulfur dioxide based on a silver dispersed functional self-assembled electrochemical sensor, Sens. Actuators B 87 (2002) 442–447. [8] J. Wang, P.V.A. Pamidi, C.L. Renschler, C. White, Metal dispersed porous carbon films as electrocatalytic sensors, J. Electroanal. Chem. 404 (1996) 137–142. [9] D. Ravi Shankaran, N. Uehara, T. Kato, Sol–gel derived metal dispersed ceramic-graphite composite electrode for amperometric determination of dopamine, Anal. Chim. Acta 478 (2003) 321–327. [10] S.A. Miscoria, G.D. Barrera, G.A. Rivas, Analytical performance of a glucose biosensor prepared by immobilization of glucose oxidase and different metals into a carbon paste electrode, Electroanalysis 14 (2002) 981–987. [11] D. Ravi Shankaran, N. Uehara, T. Kato, A metal dispersed sol–gel biocomposite amperometric glucose biosensor, Biosens. Bioelectron. 18 (2003) 721–728. [12] A. Chaubey, B.D. Malhotra, Mediated Biosensors, Biosens. Bioelectron. 17 (2002) 441–456.
4. Conclusion
Biographies
We demonstrated a new approach in the fabrication of catalyst immobilized self-assembled surface. The sensor exhibited improved and highly desired characteristics for the sensing of H2 O2 with good sensitivity, selectivity and stability. The sensitivity range can be altered depending on the specific demand by varying the loading of immobilized iron. Moreover, the availability of the number of controllable variables promises for the design of wide variety of new chemical and biosensors by varying the metal catalysts and employing different enzymes, which is currently under progress.
Dhesingh Ravi Shankaran was born in Katary village, the Nilgiris, India in 1972. He obtained his Ph.D. degree from the Department of Analytical Chemistry, University of Madras, in July 2000. He worked as a post-doctoral researcher with Prof. Yoon-Bo-Shim at the Department of Chemistry, Pusan National University, South Korea and Prof. Teiji Kato at the Satellite Venture Business Laboratory, Utsunomiya University, Japan. Presently, he is a visiting researcher in the group of Prof. Norio Miura at the Advanced Science and Technology Center for Cooperative Research, Kyushu University, Japan. Currently, he is engaged in the development of SPR immunosensors for landmine detection.
Acknowledgements The authors are grateful to Satellite Venture Business Laboratory of Utsunomiya University for financial support.
References [1] A. Ulman, Introduction to Thin Organic Films from LangmuirBlodgett to Self-assembly, Academic Press, New York, 1991. [2] S. Ferretti, S. Painter, D.A. Russell, K.E. Sapsford, D.J. Richardson, Self-assembled monolayers: a versatile tool for the formation of biosurface, Trends Anal. Chem. 19 (2000) 530–540. [3] V.M. Mirsky, New electroanalytical applications of self-assembled monolayers, Trends Anal. Chem. 21 (2002) 439–449.
Ken-Iichi Iimura was born in Tochigi prefecture, Japan in 1969. He obtained his Ph.D. degree from Tokyo Institute of Technology in 2000. He is a research associate in the Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University. He worked with Prof. H. Moehwald and Dr. G. Brezesinski at the Max-Planck Institute of Colloids and Interfaces, Germany, in 2001–2002, as a post-doctoral researcher under a scholarship of the Alexander von Humboldt Foundation. His current interest is the controlled fabrication of functionalized monomolecular systems with self-organization processes. Teiji Kato was born in Saitama prefecture, Japan in 1941. In 1973, he received his Ph.D. degree from the Graduate School of Science, University of Tokyo. In September 1973, he joined as a lecturer in the Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University. He promoted to associate professor in 1975 and to professor in 1991. Presently, he is the director of the Satellite Venture Business Laboratory of Utsunomiya University and also the council member of the University. His main area of research include the studies on the properties of monolayers and mixed monolayer structures at the interface and has published nearly 100 papers from his research work at various international journals. His current interest is on the fabrication and characterization of ultrathin functional molecular films and their application in nano-technology.
A novel metal immobilized self-assembled surface for electrochemical sensing Dhesingh Ravi Shankaran1 , Ken-Iichi Iimura, Teiji Kato∗ Satellite Venture Business Laboratory, Utsunomiya University, Yoto-7-1-2, Utsunomiya 321-8585, Japan Received 12 February 2003; received in revised form 10 June 2003; accepted 26 June 2003
Abstract A new iron immobilized (3-mercaptopropyl)trimethoxysilane (MPS) self-assembled layer has been developed over a gold electrode surface by a combined sol–gel and self-assembly techniques. The sensor exhibited excellent sensing characteristics for the catalytic reduction as well as oxidation of hydrogen peroxide (H2 O2 ) at the potentials of +0.1 and +0.4 V, respectively, with good sensitivity, selectivity and reproducibility. The possible coordination of iron with the functional thiol group of the MPS favored highly stable and leakage free sensing surface. The proposed system is a promising mean for the construction of a variety of new chemical and biosensors. © 2003 Elsevier B.V. All rights reserved. Keywords: Self-assembly; Sol–gel; (3-Mercaptopropyl)trimethoxysilane; Iron; Electroanalysis; Hydrogen peroxide
1. Introduction In recent years, the self-assembly technique has received wide spread attraction in the fabrication of effective sensors for biomedical and environmental analysis. The easy and reliable fabrication procedure, rapid response, enhanced sensitivity and selectivity are the special advantages of the self-assembled sensors [1–3]. Despite the variety of materials employed to fabricate self-assembled surfaces, functional silane molecules received great attraction due to the possible coordination with the immobilized catalysts, favoring highly stable sensing surfaces with a possible control over the distribution and orientation of the catalytic molecules [2–7]. In this context, an iron immobilized MPS self-assembled layer has been developed onto a gold electrode. MPS contains functional thiol group in its molecular structure, which favors the stable immobilization of iron in the self-assembled surface. The catalytic activity of the metal ions in the reduction or oxidation of a variety of compounds is well known [7–11]. The favorable combination of the efficient and preferential catalytic activity of iron, advantageous features of self-assembly and stable immobilization makes the system highly effective for electroanalysis of compounds of ∗ Corresponding author. Fax: +81-28-689-6179. E-mail addresses: [email protected] (D. Ravi Shankaran), [email protected] (T. Kato). 1 Co-corresponding author.
0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-4005(03)00632-4
interest. The sensing characteristic of the proposed system was evaluated for the determination of H2 O2 , a compound of great importance in biological, clinical and environmental analysis. The modified iron immobilized self-assembled electrode exhibited good catalytic activity for the reduction and oxidation of H2 O2 with good sensitivity, selectivity and reproducibility.
2. Experimental 2.1. Reagents and apparatus (3-Mercaptopropyl)trimethoxysilane was purchased from Sigma, USA. Hydrogen peroxide, ferrous sulfate, ascorbic acid and uric acid were received from Wako chemicals, Japan. All other chemicals and reagents were of analytical grade. Ultra pure water (ρ = 18 M) was used throughout the experiment. High purity nitrogen was used for deaeration. Electrochemical experiments were performed with potentiostat/galvanostat GPIB HA 503G and HAB 151 (Hokuto Denko Ltd.) interfaced with a personal computer and recorder, respectively. The modified self-assembled electrode was used as the working electrode, while Ag/AgCl (saturated KCl) and a platinum wire were used as reference and counter electrode, respectively. Scanning electron micrograph (SEM) was obtained with S-3200 N SEM, Hitachi.
524
D. Ravi Shankaran et al. / Sensors and Actuators B 96 (2003) 523–526
2.2. Fabrication of the modified electrode Prior to modification, a well-polished gold (3.0 mm diameter) electrode was cleaned with piranha solution followed by electrochemical cycling in 0.1 M H2 SO4 solution in the potential range between 0.0 and 1.5 V for 15 cycles. A stirring 0.85 ml of 0.1 M MPS solution (in ethanol) was mixed well with 0.2 ml ultra pure water followed by 1.2 ml of 0.1 M ferrous sulfate solution. The resulting mixture was mixed well and a cleaned gold electrode was dipped into this mixture for nearly 10–12 h. After modification, the iron immobilized self-assembled (Fe/MPS/Au) electrode was rinsed well with ethanol followed by ultra pure water. A MPS coated gold electrode (MPS/Au) was prepared in the similar way except the addition of ferrous sulfate.
3. Results and discussion It is believed that MPS undergoes hydrolysis and condensation (sol–gel process) immediately after mixing with water and ferrous sulfate. When gold electrode was dipped into this organically modified silicate (ormosil) gel solution, MPS assembles onto the gold surface [5–7]. The presence of thiol (–SH) group in the MPS favors the stable attachment to gold surface due to the strong interaction of Au with –SH group. Meanwhile, the iron in the gel mixture gets immobilized into the self-assembled surface. Thus, a porous three-dimensional self-assembled surface with immobilized iron will be formed at the gold surface. The possible interaction of iron with the thiol group of the MPS molecule in the three-dimensional matrix makes the surface highly stable and leakage free [5–7,11]. A uniform distribution and good coverage of the iron immobilized MPS assembled layer over the Au surface can be seen in the scanning electron micrograph (Fig. 1). The modified self-assembled surface can also be prepared by initial coating of MPS layer onto a gold sur-
Fig. 1. Scanning electron micrograph of the Fe/MPS/Au electrode surface.
Fig. 2. HDVs for 4.6 × 10−4 M H2 O2 at (a) Fe/MPS/Au, (b) bare Au and (c) MPS/Au electrode in a stirring 0.1 M KNO3 solution; pH 7.0 (0.05 M phosphate buffer); stirring rate: 300 rpm.
face followed by the immobilization of iron by dipping or by electrochemical cycling in ferrous sulfate solution. However, the modified electrode prepared with these procedures showed higher background current and less stability compared to the proposed method. The possibility of iron immobilization throughout the three-dimensional ormosil matrix and the interaction with the thiol functionality are the probable reasons for the low background current and enhanced stability of the present system. The hydrodynamic voltammograms (HDVs) obtained with (a) Fe/MPS/Au, (b) bare Au and (c) MPS/Au electrode in presence of 4.6 × 10−4 M H2 O2 were shown in Fig. 2. The HDVs were registered in a stirring 0.1 M KNO3 solution in the potential range from −0.2 to +0.6 V. Both MPS/Au and bare Au electrode showed poor current response to H2 O2 . Small current response was observed at MPS/Au and at bare Au electrodes at extreme potentials. In contrast, the Fe/MPS/Au electrode showed enhanced current response with significant decrease in the overpotential for the reduction as well as oxidation of H2 O2 allowing sensitive determination of H2 O2 . For reduction, the current response starts increasing from +0.2 V, reaching a limiting value around +0.1 V, Similarly for oxidation, the current starts increasing from +0.3 V, reaching a limiting value around +0.4 V. From the HDVs, a potential of +0.1 and +0.4 V can be applied for electrocatalytic reduction and oxidation, respectively, during amperometric quantification of H2 O2 . The results indicate that the iron present in the modified self-assembled surface exerted good catalytic activity for the reduction as well as oxidation of hydrogen peroxide favoring its quantification possible both by reduction and oxidation. To compare the electrochemical behavior of the iron immobilized gold electrodes with and without use of the MPS, gold electrodes were modified with only iron by electrochemical cycling and by dipping in iron salt solution. It was observed that these electrodes showed less sensitivity and poor stability compared to Fe/MPS/Au electrode. Fig. 3 shows the cyclic voltammograms (CVs) observed for the determination of H2 O2 by electrocatalytic reduc-
D. Ravi Shankaran et al. / Sensors and Actuators B 96 (2003) 523–526
525
Fig. 3. CVs of Fe/MPS/Au electrode in 0.1 M KNO3 solution (a) without H2 O2 , (b) with 4.8 × 10−4 M H2 O2 and (c) 1.2 × 10−3 M H2 O2 ; (d) and (e) correspond to MPS/Au and bare Au electrode in presence of 1.2 × 10−3 M H2 O2 , respectively; scan rate: 50 mV/s; pH 7.0 (0.05 M phosphate buffer).
tion. The curve (a) corresponds to the Fe/MPS/Au electrode in 0.1 M KNO3 solution at a scan rate of 50 mV/s. A redox curve with anodic and cathodic peak potential at +0.3 and +0.18 V, respectively, was observed corresponding to Fe2+ /Fe3+ reaction. The redox reaction is highly stable, no significant change in the peak current and the peak potential values was observed on multiple cycling. The curves (b) and (c) correspond to the presence of 4.8 × 10−4 and 1.2 × 10−3 M H2 O2 at the Fe/MPS/Au electrode. The curves (d) and (e) correspond to presence of 1.2 × 10−3 M H2 O2 at MPS/Au and bare Au electrode, respectively. As can be seen, the modified electrode showed good catalytic activity for the reduction of H2 O2 at the reduced potential (0.1 V) with enhanced current sensitivity compared to bare Au and MPS/Au electrode. Studies on the determination of H2 O2 by electrocatalytic oxidation showed similar response, in which, the Fe/MPS/Au electrode showed enhanced current response to oxidation of H2 O2 at a reduced potential around +0.4 V (figure not shown). Studies on the effect of pH on the catalytic reduction and oxidation of H2 O2 revealed that the Fe/APS/Au electrode showed good response in a wide pH range from 4 to 9. Reduced responses were observed at highly acidic and basic conditions, which is possibly due to the hydration and hydroxylation of the iron in the self-assembled surface, respectively [7,9]. Fig. 4 shows the interference effect of ascorbic acid (AA) and uric acid (UA) on the determination of H2 O2 by electrocatalytic reduction at +0.1 V (A) and oxidation at +0.4 V (B). Negligible contribution of these species was observed during electrocatalytic reduction at +0.1 V. In contrast, the H2 O2 response was accompanied by a significant contribution of AA and UA at +0.4 V during
Fig. 4. Amperometric response of the Fe/MPS/Au electrode for the addition of 4.0 × 10−4 M H2 O2 followed by the additions of 8.0 × 10−4 M ascorbic acid and uric acid; pH 7.0 (0.05 M phosphate buffer); stirring rate: 300 rpm. (A) Reduction at +0.1 V and (B) oxidation at +0.4 V.
electrocatalytic oxidation. Thus, covering of the electrode surface with a suitable permselective membrane is advisable for the quantification of H2 O2 by electrocatalytic oxidation at +0.4 V. The current–time amperometric response recorded at +0.1 V during the successive addition of 85 M H2 O2 to the stirring 0.1 M KNO3 solution maintained at pH 7.0 was shown in Fig. 5. The Fe/MPS/Au electrode showed rapid response to concentration changes (<8 s) and the response was linear in the concentration range from 9.5 × 10−6 to 1.2 × 10−3 M H2 O2 with a correlation coefficient of 0.995. The detection limit was 7.6 × 10−6 M (S/N = 3). A linear response to H2 O2 in the concentration range from 8.2 × 10−6 to 1.0 × 10−3 M with a correlation coefficient of 0.994 was observed for the amperometric measurement by electrocatalytic oxidation at +0.4 V and the detection limit was 6.4 × 10−6 M. Relative standard deviations of 3.0 and 3.4% were observed for eight successive determination of 2.0 × 10−4 M H2 O2 by electrocatalytic reduction and oxidation, respectively, indicating the reliable reproducibility. The electrode retained its initial response up to nearly 10 days on continuous use and more than 88% of the response was retained for nearly 30 days (stored in a desiccator). As most of the enzyme sensor involved the determination of enzymatically liberated H2 O2 as the measuring principle [10–12], the proposed system can be effectively employed for the fabrication of a variety of biosensors for the determination of glucose, cholesterol, etc.
526
D. Ravi Shankaran et al. / Sensors and Actuators B 96 (2003) 523–526
Fig. 5. Current–time response of the Fe/MPS/Au electrode in stirring 0.1 M KNO3 solution for the successive addition of 85 M H2 O2 at +0.1 V; pH 7.0 (0.05 M phosphate buffer); stirring rate: 300 rpm. Inset: Calibration plot.
[4] I. Willner, B. Willner, E. Katz, Functional biosensor system via surface nanoengineering of electronic elements, Rev. Mol. Biotech. 82 (2002) 325–355. [5] D. Ravi Shankaran, Y.B. Shim, An amperometric sensor for hydrogen peroxide based on a (3-mercaptopropyl)trimethoxysilane self-assembled layer containing hydrazine, Electroanalysis 14 (2002) 704–707. [6] A. Walcarius, Electroanalysis with pure, chemically modified and sol–gel derived silica-based materials, Electroanalysis 13 (2001) 701– 718. [7] D. Ravi Shankaran, N. Uehara, T. Kato, Determination of sulfur dioxide based on a silver dispersed functional self-assembled electrochemical sensor, Sens. Actuators B 87 (2002) 442–447. [8] J. Wang, P.V.A. Pamidi, C.L. Renschler, C. White, Metal dispersed porous carbon films as electrocatalytic sensors, J. Electroanal. Chem. 404 (1996) 137–142. [9] D. Ravi Shankaran, N. Uehara, T. Kato, Sol–gel derived metal dispersed ceramic-graphite composite electrode for amperometric determination of dopamine, Anal. Chim. Acta 478 (2003) 321–327. [10] S.A. Miscoria, G.D. Barrera, G.A. Rivas, Analytical performance of a glucose biosensor prepared by immobilization of glucose oxidase and different metals into a carbon paste electrode, Electroanalysis 14 (2002) 981–987. [11] D. Ravi Shankaran, N. Uehara, T. Kato, A metal dispersed sol–gel biocomposite amperometric glucose biosensor, Biosens. Bioelectron. 18 (2003) 721–728. [12] A. Chaubey, B.D. Malhotra, Mediated Biosensors, Biosens. Bioelectron. 17 (2002) 441–456.
4. Conclusion
Biographies
We demonstrated a new approach in the fabrication of catalyst immobilized self-assembled surface. The sensor exhibited improved and highly desired characteristics for the sensing of H2 O2 with good sensitivity, selectivity and stability. The sensitivity range can be altered depending on the specific demand by varying the loading of immobilized iron. Moreover, the availability of the number of controllable variables promises for the design of wide variety of new chemical and biosensors by varying the metal catalysts and employing different enzymes, which is currently under progress.
Dhesingh Ravi Shankaran was born in Katary village, the Nilgiris, India in 1972. He obtained his Ph.D. degree from the Department of Analytical Chemistry, University of Madras, in July 2000. He worked as a post-doctoral researcher with Prof. Yoon-Bo-Shim at the Department of Chemistry, Pusan National University, South Korea and Prof. Teiji Kato at the Satellite Venture Business Laboratory, Utsunomiya University, Japan. Presently, he is a visiting researcher in the group of Prof. Norio Miura at the Advanced Science and Technology Center for Cooperative Research, Kyushu University, Japan. Currently, he is engaged in the development of SPR immunosensors for landmine detection.
Acknowledgements The authors are grateful to Satellite Venture Business Laboratory of Utsunomiya University for financial support.
References [1] A. Ulman, Introduction to Thin Organic Films from LangmuirBlodgett to Self-assembly, Academic Press, New York, 1991. [2] S. Ferretti, S. Painter, D.A. Russell, K.E. Sapsford, D.J. Richardson, Self-assembled monolayers: a versatile tool for the formation of biosurface, Trends Anal. Chem. 19 (2000) 530–540. [3] V.M. Mirsky, New electroanalytical applications of self-assembled monolayers, Trends Anal. Chem. 21 (2002) 439–449.
Ken-Iichi Iimura was born in Tochigi prefecture, Japan in 1969. He obtained his Ph.D. degree from Tokyo Institute of Technology in 2000. He is a research associate in the Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University. He worked with Prof. H. Moehwald and Dr. G. Brezesinski at the Max-Planck Institute of Colloids and Interfaces, Germany, in 2001–2002, as a post-doctoral researcher under a scholarship of the Alexander von Humboldt Foundation. His current interest is the controlled fabrication of functionalized monomolecular systems with self-organization processes. Teiji Kato was born in Saitama prefecture, Japan in 1941. In 1973, he received his Ph.D. degree from the Graduate School of Science, University of Tokyo. In September 1973, he joined as a lecturer in the Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University. He promoted to associate professor in 1975 and to professor in 1991. Presently, he is the director of the Satellite Venture Business Laboratory of Utsunomiya University and also the council member of the University. His main area of research include the studies on the properties of monolayers and mixed monolayer structures at the interface and has published nearly 100 papers from his research work at various international journals. His current interest is on the fabrication and characterization of ultrathin functional molecular films and their application in nano-technology.