Significance of nano-structures of carbon materials for direct-electron-transfer-type bioelectrocatalysis of bilirubin oxidase

Abstract

Three types of widely used carbon materials were examined as scaffolds for the direct-electron-transfer (DET)-type bioelectrocatalysis of bilirubin oxidase (BOD) as an electrocatalyst for a 4-electron reduction of oxygen (O2). The carbon materials used were: Ketjen Black EC300J (KB) with a primary particle size (φp) of ca. 40nm and a hollow structure, Vulcan XC-72R (Vulcan) with φp of 37nm and a filled structure, and high purity graphite SP series (JSP) with φp of 10μm and well-developed micropore structures. For the three carbon materials, the rotating disk steady-state limiting catalytic current density of the O2-reduction (|jc,lim|) increased with the non-Faradaic current (|jb|) at small |jb| values and was saturated at large |jb| values. The |jc,lim/jb| ratio in low |jb| range was in the following order: JSP C Vulcan > KB. Electrochemical and microscopic data suggested that microporous structures of JSP are highly effective for the DET-type reaction of BOD. Gaps between several primary particles in the KB and Vulcan aggregates play important roles as scaffolds for BOD. The inner surface of partially broken KB particles is electrochemically active to give large |jb| but not effective as BOD scaffolds.