Ceramic templated melanin nanostructures: a biomimetic synthesis approach to bio-functional hybrid materials

Organo-inorganic hybrids provide the opportunity to invent a huge set of new multifunctional materials with a large spectrum of known and as yet unknown properties. Melanins, hydrophobic natural pigments have been emerging as a powerful organic component for developing biologically active materials because of their numerous biological functions, such as photo-protection, photosensitization, free radical quenching, metal ion chelation and even intrinsic antimicrobial behavior. Furthermore, due to their semiconductor behavior and electrical properties they hold great promise for next-generation photovoltaics and bioelectronics. Melanins are produced in-vivo by oxidative polymerization of phenolic or indolic compounds within melanosomes that template melanin formation. Following a bioinspired approach, herein we propose a novel synthesis approach towards hybrid materials, that exploits inorganic ceramic systems as catalysts and structure directing agents in melanin biopolymers building up. In this route we disclosed TiO2 ability to drive 5,6-dihydroxyindole-2-carboxylic acid (DHICA) polymerization via complex mediated electron transfer (LMCTC) from DHICA to the TiO2 lattice, that enables photo-activation under visible light [1]. This strategy led to eco-friendly, non-cytotoxic, melanin-TiO2 hybrid nanostructures with unique antimicrobial activity even higher than bare melanin under visible light and peculiar antimicrobial mechanism (Fig.1A-B) [2,3]. This synthesis approach was successfully extended to the design of melanin-silica hybrid nanoparticles, integrating the potent antioxidant properties of DHICA melanin into a stable, bioactive and biocompatible silica scaffold with high antioxidant
and cytoprotective effects associated with a specific subcellular
localization (Fig.1C-D) [4]. Both systems prove that melanin’s biofunctional and physical-chemical properties can be markedly enhanced through its templated polymerization in the presence of a ceramic phase, disclosing the manifold potentialities of this approach, that can ultimately lead 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.