Oxidative multilayer films and eumelanin analogs for biocompatible supercapacitors

Abstract

The future applications of implantable electronic medical devices set heavy requirements for the materials powering the devices. They need to be biocompatible or even biodegradable and have sufficient electrochemical properties. The primary goal of this work was to provide knowledge on materials that can be potentially used as electrode materials in biocompatible and/or biodegradable supercapacitors.

The first material of interest was eumelanin, pigment found e.g. in the human skin. We studied synthetic eumelanin analogs polydopamine (from dopamine) and DHI-melanin (from 5,6-dihydroxyindole (DHI)). Dopamine has multiple oxidation routes and we focused on the critical initial steps in the Raper-Mason pathway, which leads to the DHI-melanin-like compound. We studied the pH dependence of the dopamine oxidation in two cases: (1) oxidation by O2 only, and (2) oxidation in the presence of Ce(IV), Fe(III) or Cu(II) ion. Secondly, we oxidized DHI to DHI-melanin directly on top of the oxidative film and analyzed its chemical composition in detail.

As a second topic, we studied the oxidative layer-by-layer (LbL) films composed of polyphosphate (PP), graphene oxide (GO) and Ce(IV)/Ce(III) ions: (PP/Ce/GO/Ce)n. Ce(IV) ions in these films can oxidize monomers from their surrounding solution onto the film structure. In this work, we prepared oxidative films with advanced spin-spray LbL method. Build-up time of the films was much faster compared with the traditional dip-LbL method, the films were smoother, they had a stratified structure, and their thickness could be controlled with high precision.

Oxidative films can be used to prepare thin electroactive GO/PEDOT films with ca. 80 % w/w biocompatible conducting polymer PEDOT. These films have promising charge transfer kinetics and ca. 100 F/cm3 capacitance. By using electrochemical impedance and capacitance spectroscopies, we demonstrated that the main contribution to the capacitance comes from redox processes. (PP/Ce/rGO/Ce)15/(DHI-melanin) film had even higher capacitance. However, the results emphasize that DHI-melanin must be mixed with more electronically conductive materials to improve its dynamic properties.