On the stability of the “wired” bilirubin oxidase oxygen cathode in serum

Oxygen is electroreduced to water on the “wired” bilirubin oxidase (w-BOD) catalyst at a considerably lesser potential than on pure platinum. The w-BOD catalyst could be of value in an implantable glucose–O2 biofuel cell, operating living tissue, if it were stable in serum. We found, however, that w-BOD loses its activity in a few hours in the combined presence of the urate and O2, both of which are normal serum constituents (Bioelectrochemistry, 2004, 65, 83–88). Here we report a second major instability: When the disconnected w-BOD cathode is allowed, in the absence of urate, to poise itself at the potential of the O2/H2O half cell at pH 7.2, it loses its activity rapidly. Unlike the urate/O2 caused loss, this loss can be avoided either by applying a potential that is reducing relative to the O2/H2O half-cell potential, or by excluding O2 and adding a mildly reducing reagent, such as urate. The w-BOD cathode can be stored, therefore, in deoxygenated serum, which contains urate.