We designed a three-dimensional (3D) hierarchical pore structure to improve the current production efficiency and stability of direct electron transfer-type biocathodes. The 3D hierarchical electrode structure was fabricated using a MgO-templated porous carbon framework produced from two MgO templates with sizes of 40 and 150 nm. The results revealed that the optimal pore composition for a bilirubin oxidase-catalysed oxygen reduction cathode was a mixture of 33% macropores and 67% mesopores (MgOC33). The macropores improve mass transfer inside the carbon material, and the mesopores improve the electron transfer efficiency of the enzyme by surrounding the enzyme with carbon.

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MgO-templated porous carbon (MgOC) was developed for D-fructose dehydrogenase (FDH) electrodes. MgOCs with an average pore diameter ranging from 10 to 100 nm were used in this study. FDH adsorbed on a MgOC electrode exhibited significant catalytic currents for D-fructose-oxidation without a redox mediator. When the pore size of MgOC was much larger than the size of FDH, a sufficient amount of FDH was adsorbed in the mesopore on and even inside the MgOC structure. In contrast, when the pore size of MgOC was comparable to the size of FDH, the catalytic current depended only on the amount of enzyme adsorbed in mesopores formed at the surface of the carbon particles; however, an enhanced thermal stability of FDH was observed. Thus, FDH was stabilized through encaging in carbon mesopores with a size comparable to that of the enzyme.

Porous carbon materials, including carbon nanotubes and carbon blacks, and mesoporous carbons, carbon cryogel and carbon aerogel, have attracted attention for improving the performance of enzymatic electrode. Carbon nanotubes and carbon blacks aggregate and create mesospaces suitable for enzymatic reactions. However, much of the mesospaces is unused because the pore size is random and controlling the pore structure and pore size is difficult. In contrast, carbon cryogel or aerogel has a well-controlled mesopore size distribution with an average pore diameter ranging from 10 to 100 nm. However, the procedure of carbon gel production is time-consuming and not suitable for mass production. We focused on a new mesoporous carbon material, MgO-templated carbon (MgOC) as an alternative to carbon gels. The mesopore size can be controlled by alternating the MgO template size[1]. One of the striking advantages of MgOC over the previously reported mesoporous carbons is its simple and cost-effective production procedure. The effects of the pore structure of MgOCs on the direct electron transfer (DET) reaction of redox enzyme, D-fructose dehydrogenase, and bilirubin oxidase, were investigated[2]. We found two effects of optimizing pore structure. First, the macropores of MgOC can enhance current production. When the pore size of MgOC was much larger than the size of enzyme, the MgOC could adsorb a sufficient amount of enzyme even on the pores formed inside the MgOC particle structure, and the enzymes adsorbed inside the MgOC worked well as an electrocatalyst. In contrast, when the pore size of MgOC was comparable to the size of enzymes, the enzymes could not penetrate the MgOC particles through the inter-connecting pores, and only the enzyme adsorbed on the mesopores formed at the surface of the carbon particles produced catalytic current. Second, the mesopores which were close to the size of enzymes improve thermal stability of electrode. When enzymes molecules were embedded in the carbon mesopores with pore size comparable to that of enzymes, an enhanced contact area between enzymes and the MgOC surface led to strong adsorption of enzymes on the MgOC surface as well as to enzyme stabilization by preventing the denaturation, aggregation and collision of enzyme molecules. Furthermore, we developed the novel MgOC which have hierarchical structure composed by meso-macro pores. This material improved current density and thermal stability of enzyme modified electrodes. Reference [1] T. Morishita, T. Tsumura, M. Toyoda, J. Przepiórski, A. W. Morawski, H. Kanno, and M. Inagaki, Carbon, 48, 2690 (2010) [2] H. Funabashi, K. Murata, S. Tsujimura, Electrochemistry, 83(5), 372 (2015).