Factors affecting the interaction between carbon nanotubes and redox enzymes in direct electron transfer-type bioelectrocatalysis.

The effects of three types of water-soluble carbon nanotubes (CNTs) of different lengths on the direct electron transfer (DET)-type bioelectrocatalysis of redox enzymes were investigated. Bilirubin oxidase (BOD), copper efflux oxidase (CueO), and a membrane-bound NiFe hydrogenase (H2ase) were used as model redox enzymes for four-electron dioxygen (O2) reduction (in the case of BOD and CueO) and two-electron dihydrogen (H2) oxidation (in the case of H2ase). As a result, diffusion-controlled O2 reduction in an O2-saturated neutral buffer was realized by BOD on CNTs of a length of 1μm, but the catalytic current densities decreased as the length of CNTs increased. However, almost opposite trends were obtained when CueO and H2ase were utilized as the biocatalysts. Factors of the CNTs and the enzymes affecting the characteristics of the DET-type bioelectrocatalysis of the three enzymes were discussed. Finally, the electrostatic interaction between an enzyme (especially the portion near the redox active center) and CNTs is proposed as one of the most important factors governing the performance of DET-type bioelectrocatalysis.