Engineering proton conductivity in melanin using metal doping.

Long range electrical conduction in biomaterials is an increasingly active area of research, which includes systems such as the conductive pili, proteins, biomacromolecules, biocompatible conductive polymers and their derivatives. One material of particular interest, the human skin pigment melanin, is a long range proton conductor and recently demonstrated as capable of proton-to-electron transduction in a solid-state electrochemical transistor platform. In this work, a novel "doping strategy" is proposed to enhance and control melanin's proton conductivity, potentially enhancing its utility as a transducing material. By chelating the transition metal ion Cu(ii) into the bio-macromolecular matrix, free proton concentration and hence conductivity can be modulated. We confirm these observations by demonstrating enhanced performance in a next generation electrochemical transistor. Finally, the underlying mechanism is investigated via the use of a novel in situ hydration-controlled electron paramagnetic resonance study, deducing that the enhanced proton concentration is due to controlling the internal solid-state redox chemistry of the intrinsic polyindolequinone structure. This doping strategy should be open to any transition metal ions that bind to hydroquinone systems (e.g. polydopamine). As such, the tailoring strategy could make other soft solid-state ionic systems more accessible to applications in bioelectronics, leading to the creation of higher performance ion-electron coupled devices.

The structural impact of water sorption on device-quality melanin thin films.

The melanins are a class of pigmentary bio-macromolecules ubiquitous in the biosphere. They possess an intriguing set of physico-chemical properties and have been shown to exhibit hybrid protonic-electronic electrical conductivity, a feature derived from a process termed chemical self-doping driven by the sorption of water. Although the mechanism underlying the electrical conduction has been established, how the sorbed water interacts with the melanin structure at the physical level has not. Herein we use neutron reflectometry to study changes in the structure of synthetic melanin thin films as a function of H2O and D2O vapour pressure. Water is found to be taken up evenly throughout the films, and by employing the contrast effect, the existence of labile protons through reversible deuterium exchange is demonstrated. Finally, we determine a sorption isotherm to enable quantification of the melanin-water interactions.