Biomimetic Approach to Confer Redox Activity to Thin Chitosan Films

Electron transfer in biology occurs with individual or pairs of electrons, and is often mediated by catechol/o-quinone redox couples. Here, a biomimetic polysaccharide-catecholic film is fabricated in two steps. First, the stimuli-responsive polysaccharide chitosan is electrodeposited as a permeable film. Next, the chitosan-coated electrode is immersed in a solution containing catechol and the electrode is biased to anodically-oxidize the catechol. The oxidation products covalently graft to the chitosan films as evidenced by electrochemical quartz crystal microbalance (EQCM) studies. Cyclic voltammetry (CV) measurements demonstrate that the catechol-modified chitosan films are redox-active although they are non-conducting and cannot directly transfer electrons to the underlying electrode. The catechol-modified chitosan films serve as a localized source or sink of electrons that can be transferred to soluble mediators (e.g., ferrocene dimethanol and Ru(NH 3 ) 6 Cl 3 ). This electron source/sink is finite, can be depleted, but can be repeatedly regenerated by brief (30 s) electrochemical treatments. Further, the catechol-modified chitosan films can i) amplify currents associated with the soluble mediators, ii) partially-rectify these currents in either oxidative or reductive directions (depending on the mediator), and iii) switch between regenerated-ON and depleted-OFF states. Physical models are proposed to explain these novel redox properties and possible precedents from nature are discussed.