Several years ago, we observed that a catechol-modified chitosan hydrogel film was redox-active but non-conducting.  Specifically, electrons could not flow in response to an applied electrical potential (i.e., voltage), and the film could not directly exchange electrons with an electrode. To test whether these catechol-chitosan films possessed redox activities, we developed a mediated electrochemical probing (MEP) method in which an electrode coated with the catechol-chitosan film was immersed in a solution containing multiple mediators (i.e., diffusible electron shuttles) and an oscillating voltage was imposed. These experiments revealed that the catechol-chitosan film could repeatedly accept, store and donate electrons provided these electrons were shuttled by diffusible mediators. This MEP method is revealing important consequences of such redox activities and is catalyzing the development of new technological materials.  For instance, the catechol moieties can be viewed as two-state molecular memories (oxidized and reduced) which is providing opportunities in molecular electronics.  Further, the ability of the catechol moieties to exchange electrons with biological reductants and oxidants is being explored for the creation of abiotic materials that can engage in interactive redox-based communication with biology. Finally, the ability of mediators to control the redox-based flow of electrons through the catechol-chitosan film is providing opportunities for redox-based bioelectronics.  When we extended these MEP measurements to various natural materials, we observed that melanins, humic acids and insoluble dietary antioxidants all possess reversible redox activities.  These observations suggest to us that redox-active materials may not be inert “bystanders”, but rather active participants in redox biology.

Unique properties of redox-active but non-conducting catechol-based materials

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