pH-Sensitive On-Off Switching Behavior of Layer-by-Layer Films Assembled by Concanavalin A and Dextran toward Electroactive Probes and its Application in Bioelectrocatalysis

Concanavalin A (Con A) and dextran (Dex) were assembled into {Con A/Dex}(n) layer-by-layer films on electrodes by the biospecific interaction between them. At {Con A/Dex}(n) film electrodes, the cyclic voltammetric (CV) response of different electroactive probes, such as Fe(CN)(6)(3-), Ru(NH3)(6)(3+), ferrocenecarboxylic acid (Fc(COOH)), and ferrocenedicarboxylic acid (Fc(COOH)(2)), was very sensitive to the solution pH. For example, in pH 4.0 buffers, the films showed good permeability toward Fe(CN)(6)(3-), leading to a well-defined CV peak pair of Fe(CN)(6)(3-) with large peak currents at about 0.17 V vs SCE. In pH 9.0 buffers, however, the CV response of Fe(CN)(6)(3-) was significantly depressed or even could hardly be observed. This pH-sensitive on-off switching property of the films toward the probe should be attributed to the different electrostatic interaction between Fe(CN)(6)(3-) and the Con A constituent in the films at different pH and could be further used to control the electrocatalytic reduction of H2O2 by horseradish peroxidase (HRP) with Fe(CN)(6)(3-) as the diffusional electron transfer mediator. Fc(COOH)(2) showed the similar pH-dependent on-off behavior at {Con A/Dex}(n) film electrodes, and the corresponding pH-sensitive electrocatalytic oxidation of glucose by glucose oxidase (GOD) with Fc(COOH)(2) as the mediator was also realized. This work provides a new interface system that has the pH-sensitive on-off CV property, and the better understanding of the interactions involved in this model system may guide us to develop the novel kind of controllable biosensors based on enzymatic electrocatalysis.