New frontiers in bio-sustainable multifunctional materials: ceramic templated eumelanin-like nanostructures

Organo-inorganic hybrids hold great promise for the design of new multifunctional materials with a large spectrum of known and as yet unknown properties. The need to address bio-sustainability issues has raised huge interest toward biocompatible and bioinspired materials boosting the investigation and the engineering of biomedical devices based on natural products. Among those, eumelanins, ubiquitous biological pigments, as well as eumelanin-like compounds, hold huge potential in developing biologically active materials, thanks to their intrinsic biocompatibility, biodegradability and multiple biological functions, including photoprotection, photosensitization, free radicals scavenging, metal ion chelation, protein binding and even intrinsic antimicrobial behavior. Furthermore, due to their semiconductor behavior and electrical properties they hold great promise for next-generation photovoltaics and bioelectronics. Eumelanins are produced in-vivo by oxidative polymerization of phenolic or indolic compounds within melanosomes that template melanin formation. We recently proved that eumelanins biological functions can be markedly enhanced if their formation occurs in the presence of a nanostructured ceramic phase, acting as catalyst and structure directing agent in biopolymers building up, thus mimicking melanosomes functions according to a bioinspired approach [1-4]. In this way, TiO2 high photocatalytic activity was exploited to drive 5,6-dihydroxyindole-2-carboxylic acid (DHICA) polymerization to eumelanin, via complex mediated electron transfer (LMCTC) involving DHICA and Ti4+ ions, that enabled TiO2 photo-activation under visible light [1]. Obtained eumelanin-TiO2 hybrid nanostructures showed striking antimicrobial activity even higher than bare melanin under visible light and peculiar antimicrobial mechanism [2,3]. This synthesis strategy was successfully extended to the design of stable, bioactive and biocompatible melanin-silica hybrid nanoparticles with potent antioxidant 
and cytoprotective activity associated with a specific subcellular
localization [4]. Both systems disclose the great potential of this approach, that can be extended to eumelanin like compound in agri-food wastes, ultimately leading to cutting-edge functional hybrid materials featuring relevant biological properties, such as antimicrobial activity, selective cell interaction and signaling, as well as ionic- and electronic-based charge transport.