Fully Oriented Bilirubin Oxidase on Porphyrin-Functionalized Carbon Nanotube Electrodes for Electrocatalytic Oxygen Reduction

The efficient immobilization and orientation of bilirubin oxidase from Myrothecium verrucaria on multi-walled carbon nanotube electrodes by using pi-stacked porphyrins as a direct electron-transfer promoter is reported. By comparing the use of different types of porphyrin, the rational effect of the porphyrin structure on both the immobilization and orientation of the enzyme is demonstrated. The best performances were obtained for protoporphyrin IX, which is the natural precursor of bilirubin. These electrodes exhibit full orientation of the enzyme, as confirmed by the observable non-catalytic redox system corresponding to the T1 copper center associated with pure Nernstian electrocatalytic behavior with high catalytic currents of almost 5 mA cm(-2) at neutral pH.