Amperometric Flow‐Biosensor for Cyanide Based on an Inhibitory Effect upon Bioelectrocatalytic Reduction of Oxygen by Peroxidase‐Modified Carbon‐Felt

A novel, simple and relative highly sensitive amperometric flow biosensor for cyanide was developed by using horseradish peroxidase (HRP)‐adsorbed carbon‐felt (CF), based on an inhibitory effect on the HRP‐catalyzed O2 reduction. The HRP‐CF showed a sufficient bioelecrocatalytic activity for O2 reduction in the potential region from 0 to −0.5 V at pH 5.0, due to a direct electron transfer‐based O2 reduction process via ferrous‐HRP and compound III. This HRP‐catalyzed O2 reduction was reversibly inhibited by cyanide, which enabled to fabricate a novel and simple reagentless (i.e., no requirement of the ordinary substrate, H2O2, and the electron transfer mediators) flow‐biosensor for cyanide. When air‐saturated 0.1 M phosphate buffer (pH 5.0) was used as a carrier under the applied potential of −0.2 V vs. Ag/AgCl, the steady‐state base‐current due to the HRP‐catalyzed O2 reduction was reversibly inhibited by the cyanide injection (200 µL), resulting in peak‐shape current responses. The magnitude of the inhibition peak currents linearly increased with increasing concentrations of cyanide up to 1 µM, and the detection limit was found to be 0.04 µM (S/N=2). The apparent inhibition constant Ki′ was estimated to be 0.87 µM.