Stabilization of a bilirubin oxidase-wiring redox polymer by quaternization and characteristics of the resulting O₂ cathode

Quaternization of poly-N-vinylimidazole partly complexed with [Os (4, 4′ -dichloro-2, 2′ -bipyridine)2 Cl]+²⁺ produced, after cross-linking with polyethylene glycol diglycidyl ether, a redox hydrogel with a redox potential of 0.38 V vs AgAgCl and an apparent electron diffusion coefficient of 1.5× 10^-7 cm2 s^-1 at 37.5°C. Electrostatic binding and cross-linking with bilirubin oxidase from Trachyderma tsunodae provided a mechanically robust O₂ -electroreduction catalyst withstanding a shear stress of 2× 10-2 N m-2 associated with the rotation of a 3.6 mm radius electrode at 500 rpm. Unlike other catalysts made with osmium complexes, comprising in their inner coordination sphere imidazole-exchangeable chloride, it did not appreciably deteriorate by cross-linking through ligand exchange, leading to dehydration. The threshold potential of O₂ -electroreduction at pH 7.4, 0.15 M NaCl, and 37.5°C was near that calculated for the reversible O₂ H2 O half-cell. The O₂ -electroreduction current reached half of its maximum at +0.44 V vs AgAgCl, 60 mV more positive than the potential for bilirubin oxidase wired with PAA-PVI- [Os (diCl-bpy)2 Cl]+2+ (PAA=polyacrylamide, PVI=poly -N-vinylimidazole).