Electrical Communication Between Graphite Electrodes and Glucose Oxidase/Redox Polymer Complexes

Abstract

Abstract Redox polymers can fold along the glycoproteins of glucose oxidase (MW 160,000) at low electrolyte concentrations and thereby penetrate the enzyme. Upon penetration, the distance between the redox centers of the polymer and the FADH2 centers of the reduced enzyme is reduced sufficiently for electrons to be transferred and, therefore, for the mediated electro-oxidation of glucose on conventional electrodes. At high (1M) electrolyte (NaCl) concentrations the redox polymers coil. Such coiling prevents the penetration of the enzyme by the redox polymers. Consequently, electron transfer does not take place and glucose is not electro-oxidized. When an appropriate polycationic redox polymer is covalently bound to the enzyme, the electro-oxidation of glucose occurs even at high electrolyte concentrations. Electron transfer from the enzyme's FADH2 centers to copolymers of poly(N-methyl-4-vinylpyridinium) chloride with either poly(vinylferrocene), E° = 0.25V (SCE), or with poly(4-vinylpyridine) complexes of Os(bpy)2Cl, E° = 0.25V (SCE), or of Os(4,4′-dimethylbpy)2Cl, E° = 0.15V (SCE) is rapid. The polycationic poly(4-vinylpyridine) complex of Os(bpy)2Cl can be covalently bound to glucose oxidase by preparing the terpolymer with 4-aminostyrene, diazotization and reaction of the diazonium cations with tyrosine or tryptophane residues of the enzyme. Glucose electrodes made with the redox polymer modified enzyme are relatively stable and sensitive. Furthermore, simple and fast amperometric glucose electrodes can be made by adsorbing either non-quaternized or quaternized poly(vinylpyridine) complexes of [Os(bipyridine)2Cl]2+ (MW 300,000) on graphite, and adsorbing on these polymer films glucose oxidase. These electrodes do not require diffusing redox mediators or membranes to contain the enzyme and the redox polymer. The redox polymer is shown to electrically “wire” the enzyme's redox centers to the electrode. The glucose response of the electrodes is faster than 1 sec; their current increases linearly with glucose concentration through the 0–10 mM range. At 60 mM glucose, their current density is about 20 μA/cm2.