Enhanced performance of the wired-bilirubin oxidase oxygen cathode with incorporation of carboxylated single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) were oxidatively treated to introduce carboxyl groups on the carbon nanotube (CNT) surfaces. The carboxylated CNTs (CNT-COOHs) were incorporated into a redox hydrogel film made from a copolymer of polyacrylamide (PAA) and poly (N-vinylimidazole) (PVI) complexed with [Os(4,4′-dichloro-2,2′-bipyridine) ₂Cl]^+/2+ (PAA-PVI-[Os(dCl-bpy)₂Cl] ^+/2+) wiring bilirubin oxidase (BOD) enzyme molecules. The constructed BOD cathode was employed for the electrocatalytic O₂ reduction. The performance was compared with that of the BOD cathode without the incorporation of CNT-COOHs. At an electrode rotating with a 500 rpm speed in an air-saturated 0.15 M NaCl buffer at pH 7.4, an enhancement of the O₂ reduction current density from 0.24 mA/cm² to 0.51 mA/cm² after the CNT incorporation was observed. The BOD cathode containing CNT-COOHs showed less decay against halide ion inhibitions. For the BOD cathode with no CNT-COOHs incorporated, the current decreased by 22% with 0.9 M NaCl added, and 27% with 0.9 M KBr added, but, with CNT-COOHs incorporated, only 6% and 5% current decays were measured, respectively. Finally, the constructed BOD cathode with CNT-COOHs incorporated showed better stability. When a potential of 0.1 V vs. Ag/AgCl was applied to the cathode being rotated with a speed of 200 rpm for 8000 s, almost no current decay was measured, while 7% current decay was measured for the BOD cathode with no CNT-COOHs incorporated.