Ceramic templated melanin nanostructures: a novel 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 are 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 the oxidative polymerization of phenolic or indolic compounds within melanosomes that are believed to template melanin formation. Following a bioinspired approach, herein we propose a novel synthesis strategy 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 photoactivation under visible light [1]. This strategy led to eco-friendly melanin-TiO2 hybrid nanostructures with unique antimicrobial activity even under visible light (Fig.1) [2,3]. This approach was successfully extended to other semiconductor oxides, such as LaFeO3 perovskite, thus providing the key to optimize interactions between the organic and inorganic components and overcome limiting transport mechanisms. Finally, this synthesis strategy was carried out to the design of novel eumelanin-silica hybrid nanoparticles, integrating the potent antioxidant properties of DHICA melanin into a stable, bioactive and biocompatible silica scaffold (Fig.2). These systems prove that melanin’s biofunctional and physical-chemical properties can be markedly enhanced, if its formation occurs through 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 behavior.