(744f) Toward the Retention of Enzyme Activity In High-Surface-Area Electrode Made of Redox Polymer Grafted Carbon Black

Biofuel cells attracted attention as candidates for portable and implantable energy sources because a variety of fuels like glucose and ethanol can be used. One important issue to be addressed in enzyme-base biofuel cells is their low power density caused by their low current density. The rate-limiting step in mediated enzyme electrodes has been electron conduction via a redox polymer, which shuttles electrons from the enzyme to the electrode, due to the low apparent electron diffusion coefficient in the redox polymer (D_m).[1]

We have proposed a high-surface-area three-dimensional enzyme electrode made of redox-polymer-grafted carbon black with particle diameters of about 30 nm to obtain high current density.[2] In this electrode, the three-dimensional carbon electrode with high surface area and high electron conductivity plays the primary role in conducting electrons; thus, the electron conduction distance in the redox polymer was reduced, overcoming the rate-limiting step even when the redox polymer used has a low D_m. The electrode was used as a bioanode using glucose oxidase as the enzyme. The effectiveness of the high-surface-area electrode was verified by experiments and a mathematical model. The model calculation also suggests that an increase in the surface coverage of the enzyme allows an increase in the current density to the order of 10² mA/cm².[3] However, the low enzyme surface coverage, mainly due to the deactivation of enzymes upon adsorption on carbon black, limited the current density.[4]

In this work, we investigated two methods to retain the enzyme activity upon adsorption on carbon black. One was to utilize a hyperthermophilic laccase. In general, hyperthermophilc enzymes have rigid structures, and thus, have a potential to retain the activity during adsorption. The catalytic activity of laccases was measured after adsorption on carbon black. The activity of the hyperthermophilic enzyme did not change by adsorption, while the activity of the laccase with the optimal temperature around 60 C decreased to approximately 20% of that in solution after adsorption. In addition, for electrochemical measurements of laccases, a redox mediator for laccase was chemically immobilized to a polymer grafted on the carbon black. The other method for the retention of the enzyme activity was modification of carbon black to make its surface hydrophilic. Surface treatment of carbon black with a hydrophilic polymer reduced the deactivation of enzyme to some extent. Then, chemical modification of carbon surface was investigated to further retain the enzyme activity.

References [1] S. C. Barton, J. Gallaway et al., Chem. Rev., 2004, 104, 4867. [2] T. Tamaki and T. Yamaguchi, Ind. Eng. Chem. Res., 2006, 45, 3050. etc. [3] T. Tamaki, T. Ito, T. Yamaguchi, Fuel Cells, 2009, 1, 37., [4] T. Tamaki, A. Hiraide, T. Ito, H. Ohashi, T. Yamaguchi, Ind. Eng. Chem. Res., 2010, 49, 36394.