Abstracts

Melanin-based materials offer unique chemistry and opto-electronic properties that can be potentially leveraged as functional materials in medical devices. In this talk, recent process on elucidating the structure of natural and synthetic melanins will be presented. The use of melanins and melanin-like materials for applications in energy storage, adhesives, coatings, and flexible electronics will also be discussed. Each vignette will highlight structure-property-processing relationships of these materials. Fundamental knowledge can both inform and advance the prospective use of melanins and melanin-like materials in medical devices and beyond.

Living photosynthetic microorganisms or their functional components can be integrated as active elements in bio-optoelectronic platforms using polydopamine and related polymers as active interfaces. We have demonstrated the possibility to functionalize living unicellular diatoms microalgae with polydopamine , using the polymer coating to build-in additional functions by integrating moieties such as enzymes and magnetic particles. Moreover, polydopamine and its derivatives have been proved to be ideal interfaces to couple living cells of purple photosynthetic bacteria or extracted photoenzymes (e.g. reaction center from Rhodobacter sphaeroides) with electrodes in bio-electrochemical cells. Tuning the adhesive and optoelectronic properties of polydopamines enables to optimize interfaces and represents a powerful tool towards integration of photosynthetic systems in bio-electrochemical cells and bio-optoelectronics devices.

Melanins are ubiquitously found in nature, and much research has focused on the photoprotective role of this absorber. Some of these studies have examined the generation of reactive oxygen species in association with light absorption by melanin. We have studied the presence of photochemical and photoelectrochemical reactions on melanin thin-films, and found a dominance of oxygen reduction reactions. The products of these reactions are superoxide or hydrogen peroxide. We have found that while photogenerated electrons reduce oxygen, the fate of the holes represents a complex picture. When the melanin is tested in a photocathode configuration and efficient p-type transport is available, photogenerated holes can be extracted to an external circuit. If this is not possible or inefficient, the hole will either oxidize electron-donors in solution or precipitate autooxidation reactions of the melanin itself. The factors affecting this photocorrosion effect will be discussed, as well as principles leading to stabilization of melanin as a true catalyst rather than photochemical substrate. The photochemical oxygen reduction reaction on melanin opens opportunities for on-demand generation of reactive oxygen species in physiological environment using light.

Heterogeneity and poorly defined structure in both natural and synthetic eumelanin pigments hinder establishment of design principles for electronic materials based on this biopigment. We are developing ultrafast transient absorption spectroscopy approaches capable of disentangling the structure and photochemical properties of subsets of eumelanin chromophores distinguished by their optical energy gaps. Transient spectral hole burning and vibrational fingerprinting of eumelanin reveal molecular level insights into the chromophores making up eumelanin pigments and their ultrafast photochemical dynamics.

Melanin’s hydration dependent conductivity with an associated proton conductivity is well known. However, little attention has been paid to the nature of the water morphology in the hydrated state with the concomitant protonic conductivity mechanism. Presented is an inelastic neutron scattering experiment on hydrated melanin powders in order to investigate the water morphology. The melanin dry spectra is modelled with a minimalist approach whereas for higher hydration levels a difference spectra is obtained to extract out the water scattering signal. A key result is that the physi-sorbed water structure within melanin is made purely of interfacial water with the number of water layers being 3-5. We also detect the potential signatures for proton cations, most likely the Zundel ion. The nature of the water morphology opens up new questions about the potential proton charge transport mechanism within hydrated melanin.

Eumelanin, black-brown insoluble biomacromolecule, is known to be composed of two building blocks, 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxylic acid, however, the exact structure of Eumelanin remains unsolved. Meredith and coworkers groundbreaking work established Eumelanin as an electronic-ionic conductor rather than an organic semiconductor. Presented here will be our approach to well defined, soluble Eumelanin-inspired materials and their application as organic semiconductors in chemosensors and organic light emitting diodes.

In this contribution, we will focus on a study case in organic sustainable electronics that pertains to the melanin family of biopigments. Polymerization of the biopigment building blocks, self-assembly, electronic and proton-assisted electronic transport will be discussed together with molecular disassembly issues functional to melanin degradation by composting.