Sources of instability of ‘wired’ enzyme anodes in serum: urate and transition metal ions

The glucose electrooxidation currents of anodes with films of glucose oxidase and redox polymers electrically connecting (‘wiring’) its redox centers to electrodes decay much more rapidly in serum than in pH 7.2 saline phosphate buffer. Rapid decay is observed also in buffer in the presence of urate and transition metal ions, which are constituents of normal serum. Urate is electrooxidized to dimeric or trimeric products, which precipitate in the electrocatalytic film. Transition metal ions, like Zn2+ and Fe2+, coordinatively crosslink pyridine or imidazole functions of the redox polymers and also inhibit glucose oxidase.