Effect of ε-Poly-L-lysine on a Glucose Sensor Based on Glucose Oxidase and Ferricyanide Ion

ε-Poly-L-lysine (εPL) is a homopolymer of L-lysine residues with linkages between the α-carboxyl and ε-amino groups. εPL exists as a polycationic species under acidic and neutral conditions, and can form a polyion complex with negatively charged glucose oxidase (GOx). We previously reported that εPL significantly promoted a GOx enzymatic reaction using ferricyanide ion ([Fe(CN)₆]^3-) as the oxidant. Here, we construct a GOx-immobilized electrode using εPL and glutaraldehyde, and show that [Fe(CN)₆]^3- can efficiently act as the mediator in this electrode. Ferricyanide ion failed to function adequately as the mediator when bovine serum albumin or other polyamines (e.g., polyallylamine, α-poly-L-lysine) were used instead of εPL. The GOx-immobilized electrode using εPL successfully responded to glucose even when the [Fe(CN)₆]^3- concentration was as low as 1 mM, and exhibited a wide dynamic range of up to several tens of mM. Thus, εPL is considered to be a useful additive for glucose sensors based on the [Fe(CN)₆]^4-/3--mediated GOx catalytic current.