Effects of Mesoporous Structures on Direct Electron Transfer-Type Bioelectrocatalysis: Facts and Simulation on a Three-Dimensional Model of Random Orientation of Enzymes

Direct electron transfer (DET)-type bioelectrocatalytic waves of bilirubin oxidase (BOD)-catalyzed O2 reduction and [NiFe] hydrogenase (H2ase)-catalyzed H2 oxidation are very small and un-detectable using glassy carbon (GC) electrodes, respectively; however, clear catalytic waves are observed when the enzymes are adsorbed on Ketjen black-modified GC (KB-GC) electrodes, in which KB provides mesopores for DET-type bioelectocatalysis. To explain the phenomena, we focus on the curvature effect of mesoporous structures on long range electron transfer kinetics and simulate steady-state voltammograms catalyzed by model redox enzymes adsorbed with a random orientation on planar and mesoporous electrodes based on a three-dimensional model. In the simulation, we assume a spherical enzyme with a radius of r, an active site located at a certain distance from the center of the enzyme, and a spherical pore with a radius of Rp in mesoporous electrodes in which the enzyme is trapped and adsorbed. The simulation reveals that mesoporous electrodes provide platforms suitable for DET-type bioelectrocatalysis of enzymes when Rp becomes close to r. Such curvature effects of mesoporous electrodes become especially notable for larger sized enzymes. Furthermore, the simulation reproduces the experimental data of BOD- and H2ase-catalyzed DET-type waves by considering the crystal structures of the enzymes. This work will open a route to improve the kinetic performance of the DET-type bioelectrocatalysis that has become very important in its practical application to a variety of bioelectrochemical devices.