Electro-Induced Covalent Cross-Linking of Chitosan and Formation of Chitosan Hydrogel Films: Its Application as an Enzyme Immobilization Matrix for Use in a Phenol Sensor

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We report a novel electrochemical method for making covalently cross-linked chitosan films on the surface of a screen printed carbon electrode. In the presence of a freely diffusing ruthenium complex, Ru(bpy)(2)Cl(2), soluble chitosan becomes irreversibly cross-linked in response to a reducing potential and subsequently deposits on the electrode surface to form a hydrogel film which shows robustness in harsh acidic conditions. The cross-linked chitosan film presents excellent ability to facilitate the diffusion and electron transfer process of the negatively charged redox couple [Fe(CN)(6)](3-)/[Fe(CN)(6)](4-) at the electrode surface, while its impact on the positively charged redox couple [Ru(NH(3))(6)](2+)/[Ru(NH(3))(6)](3+) is minimal. By covalently immobilizing the enzyme tyrosinase in a preformed cross-linked chitosan film via a similar mechanism, a sensitive phenol sensor is constructed with a broad dynamic range. The potential advantages of this method are also discussed.