Evaluation of osmium(II) complexes as electron transfer mediators accessible for amperometric glucose sensors

In order to lower the redox potentials of Os(III/II) complexes, the mixed ligand complexes of Os(II) were synthesized. The redox potentials of Os(III/II) complexes could be lowered by the use of 4,4'-dimethyl-2,2'-bipyridine (dmbpy), imidazole (Him) or its derivatives, and chloride ion as ligands, e.g., values of the redox (formal) potentials of 628 mV vs. Ag/AgCl for [Os(bpy)3]3+/2+ (bpy: 2,2'-bipyridine) and -6 mV for [OsCl(Him)(dmbpy)2]2+/+ were given in deaerated 0.1 mol dm-3 phosphate buffer (pH 7.0). The evaluation of Os(II) complexes as electron transfer mediators accessible for amperometric glucose sensors was examined according to the determination of the redox potentials of Os(III/II) complexes and the second-order rate constants for electron transfer between glucose oxidase (GOx) in reduced form and the Os(III) complex. Although the Os(II) complexes with lower redox potentials tended to decrease the second-order rate constants ks, the ks values for the majority of Os(II) complexes synthesized in this study were greater than that for ferrocenecarboxylic acid. Acceleration of the electron-transfer reaction is attributable to the hydrogen bonding and/or the electrostatic interaction between the Os(II) complexes and GOx. It may be consequently concluded that the mixed ligand complexes of Os(II) with bpy (dmbpy), Him (its derivatives), and Cl- can act as more efficient electron transfer mediators for the fabrication of amperometric glucose sensors.