Significance of the Length of Carbon Nanotubes on the Bioelectrocatalytic Activity of Bilirubin Oxidase for Dioxygen Reduction

Bilirubin oxidase (BOD) catalyzes 4e⁻-reduction of O₂ into H₂O and is one of the enzymes allowing direct electron transfer (DET)-type bioelectrocatalysis. Mesoporous carbon materials are often used for constructing BOD-modified DET-type biocathodes. We have examined the performance of several multi-walled carbon nanotubes (MWCNTs) as cathode platforms for BOD-catalyzed bioelectrocatalytic O₂ reduction. It has been found that the length of MWCNTs is a very important factor to control the performance. The π-π conjugated system with the carboxy groups on the surface of MWCNTs plays an important role in enhancing the DET-type bioelectrocatalytic activity of BOD. Water-soluble MWCNT with the length of as short as 1 − 4 μm is the most suitable material among the MWCNTs examined, and they have been used to construct a mesoporous carbon electrode without any binder. The catalytic current density has reached a diffusion-controlled level for the first time for BOD at an electrode rotating rate of 4000 rpm and at 25 °C (8 mA cm⁻²).