ELECTROCATALYTIC OXIDATION OF ASCORBIC-ACID AT [OSMIUM(2,2'-BIPYRIDYL)(2)-(POLY-4-VINYLPYRIDINE)(10)CL]CL MODIFIED ELECTRODES - IMPLICATIONS FOR THE DEVELOPMENT OF BIOSENSORS BASED ON OSMIUM-CONTAINING REDOX RELAYS

The oxidation of ascorbic acid at electrodes modified with the redox polymer [Os(bipy)(2)(PVP)(10)Cl]Cl (where bipy = 2,2'-bipyridyl and PVP = poly-4-vinylpyridine) has been studied. The reaction proceeds electrocatalytically at 0.250 V versus SCE by mediation via- the Os-II/Os-III redox couple immobilized in the polymer film. The reaction is negative first order with respect to proton concentration and positive first order with respect to ascorbic acid concentration. At pH greater than or equal to 3.0, limiting currents are diffusionally controlled and the kinetic regime may be classified as an S-k mechanism. The second order rate constant for the surface electrocatalytic reaction k'' at pH 4.5 is 7.8 +/- 0.4 X 10(-4) dm(3) mol(-1) cm s(-1), while at pH 7.0 k'' is 9.1 +/- 0.6 X 10(-4) dm(3) mol(-1) cm s(-1). At pH less than or equal to 2.0 ascorbic acid permeates the polymer film and reacts at the mediator Sites within the polymer film giving an Lk mechanism, while at potentials >0.4 V versus SCE the reaction proceeds both at the underlying electrode and with the Os-III mediator sit es wit hin the polymer film. The modified electrode has been characterized as an electrochemical sensor for ascorbic acid in flow injection systems.