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The development of technologies integrating solar energy conversion and energy storage functions is critical for limiting the anthropogenic effects on climate change and preventing possible energy shortages related to the increase of the world population. In our work, we explored the possibility to integrate the conversion and storage functions within the same multifunctional biosourced material. We identified the redox-active, quinone-based, melanin pigment, featuring a broadband absorption in the UV-vis region, as the ideal candidate for such an exploration. Electrodes of melanin on carbon paper were investigated for their morphological, optical, and voltammetric characteristics prior to being assembled into symmetric supercapacitors operating in aqueous electrolytes. We observed that, under solar light, the capacity and capacitance of melanin electrodes significantly increase with respect to the dark conditions (by 22 and 39%, respectively). Once in a supercapacitor configuration, besides featuring a Coulombic efficiency close to 100% after 5000 cycles, the capacitance and capacity of the electrodes, rated by the initial values, improve after prolonged illumination, as it is the case for the energy and power density.

Nature has provided a valuable source of inspiration for developing high performance multifunctional materials. Particularly, catechol-containing amino acid l-3,4-dihydroxyphenylalanine (l-DOPA) has aroused the interest to design hybrid multifunctional materials with superior adhesive ability. DOPA oxidative polymerization mediated by either melanogenic enzymes or an alkaline environment involving catechol intermolecular cross-linking, ultimately leads to melanin oligomers. Recently, relevant studies disclosed the ability of Ti-based nanostructures to tune melanin&rsquo, s supramolecular structure during its formation, starting from melanogenic precursors, thus improving both antioxidant and antimicrobial properties. In this work, we propose a novel biomimetic approach to design hybrid DOPA melanin-like nanostructures through a hydrothermal synthesis opportunely modified by using citric acid to control hydrolysis and condensation reactions of titanium alkoxide precursors. UV-Vis and Electron paramagnetic resonance (EPR) spectroscopic evidences highlighted the key role of citrate&ndash, Ti(IV) and DOPA&ndash, Ti(IV) complexes in controlling DOPA polymerization, which specifically occurred during the hydrothermal step, mediating and tuning its conversion to melanin-like oligomers. Trasmission electron microscopy (TEM) images proved the efficacy of the proposed synthesis approach in tuning the formation of nanosized globular nanostructures, with high biocide performances. The obtained findings could provide strategic guidelines to set up biomimetic processes, exploiting the catechol-metal complex to obtain hybrid melanin-like nanosystems with optimized multifunctional behavior.

Eumelanin integration in silica aerogel (SA) was achieved via supercritical adsorption of 5,6-dyhydroxyindole (DHI) from CO2. Notably, after the supercritical treatment, DHI evolved towards spontaneous polymerization, which resulted in uniform pigment development over the SA. The new material was characterized for its morphological and physicochemical properties, disclosing the formation of a eumelanin-like coating, as confirmed by UV&ndash, vis and electron paramagnetic resonance (EPR) spectroscopy.