Bioelectrocatalytic Oxidation of Glucose on MgO-templated ...

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Glucose-oxidizing anodes utilizing glucose oxidase (GOx) have been developed for biofuel cells.1 In GOx-based anodes, because of poor electrical contact ...
CL-140189

Received: March 5, 2014 | Accepted: March 18, 2014 | Web Released: March 26, 2014

Bioelectrocatalytic Oxidation of Glucose on MgO-templated Mesoporous Carbon-modified Electrode Kazuki Murata, Wataru Akatsuka, and Seiya Tsujimura* Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573 (E-mail: [email protected]) MgO-templated mesoporous carbon (MgOC) with an average pore diameter of 38 nm was used as electrode material for a glucose-oxidizing anode. Anodic catalysts composed of a redox polymer, glucose oxidase, and crosslinkers were applied to MgOC-modified electrodes. The glucose oxidation current density on MgOC was 11.3 mA cm¹2 at 0.5 V, which was 5.3fold higher than that on glassy carbon.

Glucose-oxidizing anodes utilizing glucose oxidase (GOx) have been developed for biofuel cells.1 In GOx-based anodes, because of poor electrical contact between GOx and the electrode, the use of an electron-transfer mediator is necessary. Heller et al.2 have developed a promising “wired” GOx approach in which the enzyme is incorporated into a redox hydrogel formed on an Os-containing water-soluble redox polymer and crosslinkers. The GOx-loaded redox hydrogel adheres well to the electrode, enabling the immobilization of GOx and the redox polymer on an electrode surface at high concentrations. The resulting “wired” GOx-modified glassy carbon (GC) electrodes achieve a high glucose oxidation current density of ca. 400 ¯A cm¹2 at 25 °C.3 “Wired” GOx anodes aimed at practical biofuel cell applications demand a much higher and stable current output. To improve the efficiency of the anode, mesoporous carbon materials are expected to act as suitable scaffolds for biocatalysts and increase their electroenzymatically active surface area.4,5 Recently, Mano et al. coimmobilized a redox polymer and GOx on a Ketjen Black (KB)-based electrode.6 A current density of up to 5.1 mA cm¹2 was achieved on the KB-modified electrode, which was 60% higher than that on the GC electrode. KB has a large BET specific surface area (800 m2 g¹1); however, the wide pore size distribution of the KB-modified electrode suggested that a portion of its surface was inefficiently used for enzyme loading and electrochemical reactions.7 In this study, we report the use of MgO-templated mesoporous carbon (MgOC) as a scaffold for a redox hydrogel. MgOC used in this study was obtained from Toyo Tanso (CNovelμ, Japan) and has an average pore diameter of 38 nm with a narrow pore size distribution, and its BET specific surface area is 570 m2 g¹1.8 The redox hydrogel consisted of GOx from Aspergillus niger (Wako Pure Chemical), the redox polymer poly(vinylimidazole) with [Os(bpy)2Cl] (PVI-Os, bpy: bipyridine),3 and poly(ethylene glycol) diglycidyl ether (PEGDGE, molecular weight 500) as a crosslinker. One of the striking advantages of MgOC is its simple production procedure, compared to previously reported mesoporous carbon.7,9,10 PVIOs was prepared according to a previously reported method:3 a complex was first formed between PVI and [Os(bpy)2Cl] in a 10:1 molar ratio, and a part of imidazole rings of the resulting redox polymer was quaternized using 2-bromoethylamine to

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Figure 1. Scanning electron microscopy of KB (a), MgOC (b), and CP (c) with magnification of ©500, and MgOC-modified electrodes with magnification of ©2000 (d).

improve the solubility. Furthermore, spherical glassy carbon particles (CP particle sizes: 2­12 ¯m, Sigma-Aldrich) and KB (300J, Lion Co.) were utilized as control materials for the anode to discuss the significance of electrode morphology on current production. Each carbon material was modified on the GC electrodes by drop-casting. The MgOC dispersion was obtained by adding MgOC (25 mg) and poly(vinylidene difluoride) (50 ¯L, PVDF #9305, Kureha) to N-methyl-2-pyrrolidone (200 ¯L, NMP). On the other hand, KB and CP dispersions were produced by adding KB (25 mg) or CP (50 mg) to PVDF (100 ¯L) and NMP (200 ¯L). After ultrasonic homogenization for 1 min, aliquots (2 ¯L) of these dispersions were drop-casted onto GC electrode surfaces (3 mm in diameter, BAS), which were sequentially polished with 3- and 0.3-¯m-particle-size alumina slurry, sonicated for 5 min, and rinsed with deionized water. The resulting electrodes were dried for 12 h at 45 °C, and the electrode surfaces were examined by scanning electron microscopy (SEM, JSM-5510SEM, JEOL) at 20-kV beam voltage. Figure 1 shows the SEM images of KB-, MgOC-, and CP-modified electrode surfaces at a magnification of ©500. The KB electrode surface exhibited cracks on the carbon layer (Figure 1a), consistent with previous reports.6 In contrast, the MgOC electrode surface showed a uniform surface with a grain size of less than 5 ¯m and fewer cracks compared with KB (Figures 1a, 1b, and 1d). The surface of the CP-modified electrode displayed carbon particle aggregates with large interparticle voids (Figures 1c and 1d). Next, the hydrogel was coated on the porous carbon electrode surfaces to assess the effect of morphology on the bioelectrocatalytic glucose oxidation reaction. Each porous

© 2014 The Chemical Society of Japan

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Figure 2. (a) Cyclic voltammograms for glucose oxidation on CP (gray solid line), GC (gray dotted line), MgOC (black solid line), and KB (black dotted line); a total hydrogel loading of 0.2 mg cm¹2, 200 mM glucose, pH 7, 25 °C, 5000 rpm, and 10 mV s¹1. (b) Dependence of the peak current on the scan rate for MgOC, KB, and CP electrode in the absence of glucose; pH 7, 25 °C.

carbon-modified electrode was first subjected to plasma treatment for 20 min to generate a hydrophilic surface. Subsequently, an aliquot (1.5 ¯L) of a mixture containing 10 mg mL¹1 PVI-Os in water (12 ¯L), 20 mg mL¹1 GOx in water (4 ¯L), and 3 mg mL¹1 PEGDGE in water (8 ¯L) was applied to the electrode surface for a total loading of 0.2 mg cm¹2. The electrodes were cured at 25 °C in an evacuated desiccator for 12 h. Electrochemical measurements of the modified electrodes were performed using a potentiostat (BAS 50W). A Ag«AgCl electrode and a platinum wire were used as the reference and counter electrodes, respectively. All the electrochemical measurements were performed in a waterjacketed cell in a 100 mM phosphate buffer (pH 7) maintained at 25 °C. The electrodes with different hydrogel loadings were simply prepared by changing the volume of the deposition solution while maintaining a constant composition. Figure 2a shows the cyclic voltammograms for glucose oxidation with each electrode rotating at 5000 rpm in the presence of 200 mM glucose. The GC electrode exhibited a glucose oxidation current density of 2.1 « 0.4 mA cm¹2 at 0.5 V (Figure 2a, gray solid line). The CP electrode (Figure 2a, gray dotted line) showed a slightly higher glucose oxidation current density (2.2 « 0.4 mA cm¹2) than GC electrodes at 0.5 V. In contrast to these mesoporous carbon electrodes, both KB (Figure 2a, black dotted line) and MgOC electrodes (Figure 2a, black solid line) displayed higher glucose oxidation current densities of 3.3 « 0.3 and 2.8 « 0.2 mA cm¹2, respectively. The 60% increase in the catalytic current observed for the KB electrode is in good agreement with a previous report by Suraniti.6 The effect of the scan rates on the peak current in the absence of glucose is shown in Figure 2b. The peak current density of the GC electrode is proportional to the square root of the scan rates (data not shown), as expected for a semi-infinite diffusion limited by the electron transport through the redox hydrogel film. On the other hand, the peak current densities are proportional to the scan rates for MgOC-, KB-, and CP-modified electrodes (Figure 2b). These results indicate that the adsorbed redox hydrogel forms a thin film on each electrode and that the hydrogel film layer thickness is less than the diffusion layer thickness. The slopes of the peak current density plots (Figure 2b) are approximately four times larger for KB and MgOC electrodes than for the CP electrode, which may result Chem. Lett. 2014, 43, 928–930 | doi:10.1246/cl.140189

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Figure 3. (a) Cyclic voltammograms for glucose oxidation on CP (gray solid line), GC (gray dotted line), MgOC (black solid line), and KB (black dotted line); a total hydrogel loading of 1.0 mg cm¹2, 200 mM glucose, pH 7, 25 °C, 5000 rpm, and 10 mV s¹1. (b) Dependence of the peak current on the scan rate for MgOC, KB, and CP electrode in the absence of glucose; pH 7, 25 °C.

from the desorption of the hydrogel from the CP electrode surface. Considering the charging current, KB and MgOC electrodes exhibit larger specific surface areas and much thinner hydrogel films on their surface than the CP electrode. The effects of electrode morphology on glucose oxidation current density were investigated for a high hydrogel loading (1.0 mg cm¹2). Figure 3 shows the glucose oxidation current density on hydrogel-modified KB, MgOC, CP, and GC electrodes (Figure 3a) and peak current as functions of the scan rate on hydrogel-modified KB, MgOC, and CP electrodes (Figure 3b) at a constant loading of 1.0 mg cm¹2. The glucose oxidation current density on MgOC electrodes reached up to 11.3 « 1.3 mA cm¹2 at 0.5 V (black line in Figure 3a). The peak current was linear with the scan rate, and the peak current was five times larger than the electrode modified with 0.2 mg cm¹2 of hydrogel (Figure 2b). This result indicated that the hydrogel film was still very thin; the immobilized GOx and redox polymer efficiently mediated the glucose oxidation on the MgOC electrode. On the KB electrode, the current density was 40% lower than that on the MgOC electrode. The cyclic voltammogram obtained for the KB electrode in the phosphate buffer in the absence of glucose showed that the peak current was not linear with the scan rate but with its square root. This result agrees with a previous report by Suraniti and suggests that the hydrogel might fill up the micropore structure formed by the KB granules.6 The decrease in the current may stem from the mediator polarization within the hydrogel layer. A decrease in the current density was observed for the GC electrode when the hydrogel amount increased from 0.2 to 1.0 mg cm¹2. This indicates that glucose diffusion within the hydrogel from bulk solution to the enzyme close to the electrode surface restricted current production. On the CP electrode, the glucose oxidation current density was only 10% higher than that for a hydrogel loading of 0.2 mg cm¹2, which suggests that the surface was fully covered with the hydrogel and that the film was thicker than the reaction and diffusion layers. MgO-templated mesoporous carbon containing large specific surface area and controlled mesopore size enabled the immobilization of significant amounts of enzyme at the vicinity of the electrode surface. This material may find applications as scaffolds for the fabrication of stable hydrogel electrodes with high current density.

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The authors gratefully acknowledge the financial support from the Shimadzu Science Foundation and the Iwatani Naoji Foundation. References and Notes 1 S. C. Barton, J. Gallaway, P. Atanassov, Chem. Rev. 2004, 104, 4867. 2 Y. Degani, A. Heller, J. Am. Chem. Soc. 1989, 111, 2357. 3 T. J. Ohara, R. Rajagopalan, A. Heller, Anal. Chem. 1993, 65, 3512. 4 S. C. Barton, Y. Sun, B. Chandra, S. White, J. Hone, Electrochem. Solid-State Lett. 2007, 10, B96.

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S. Yoshino, T. Miyake, T. Yamada, K. Hata, M. Nishizawa, Adv. Energy Mater. 2013, 3, 60. 6 E. Suraniti, S. Vivès, S. Tsujimura, N. Mano, J. Electrochem. Soc. 2013, 160, G79. 7 N. Setoyama, T. Kajino, H. Takagi, T. Asaoka, Y. Fukushima, Jpn. Kokai Tokkyo Koho 2006-96653, 2006. 8 T. Morishita, T. Tsumura, M. Toyoda, J. Przepiórski, A. W. Morawski, H. Konno, M. Inagaki, Carbon 2010, 48, 2690. 9 J. Lee, J. Kim, T. Hyeon, Adv. Mater. 2006, 18, 2073. 10 S. Tsujimura, A. Nishina, Y. Hamano, K. Kano, S. Shiraishi, Electrochem. Commun. 2010, 12, 446.

© 2014 The Chemical Society of Japan