Effect of Ionic Strength on the Behavior of Amperometric Enzyme Electrodes Mediated by Redox Hydrogels

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Abstract
The electron-transfer behavior of electroactive hydrogels formed by cross-linking ferrocene poly(allylamine) (Fc−PAA) and glucose oxidase is investigated as a function of electrolyte ionic strength using several techniques. Cyclic voltammetry and electrochemical impedance spectroscopy show that the quantity cDe1/2 increases with electrolyte concentration. Enhancement of enzyme catalysis for the oxidation of glucose mediated by Fc−PAA is also apparent at higher KNO3 concentration. The electroactive redox center concentration, c, and the diffusion coefficient due to electron hopping in the gel, De, are independently measured by chronoamperometry at ultramicroelectrodes. Larger electrolyte ionic strength induces an increase in electroactive redox center concentration while De slightly decreases. These results are rationalized in terms of the electrostatic interactions within the redox gel backbone due to water and ion exchange with the external electrolyte, producing swelling and shrinking of the hydrogel.