Towards nanostructured red-ox active bio-interfaces: Bioinspired antibacterial hybrid melanin-CeO₂ nanoparticles for...

Redox-active nano-biointerfaces are gaining weight in the field of regenerative medicine since they can act as enzymes in regulating physiological processes and enabling cell homeostasis, as well as the defense against pathogen aggression. In particular, cerium oxide nanoparticles (CeO₂ NPs) stand as intriguing enzyme-mimicking nanoplatforms, owing to the reversible Ce⁺³/Ce⁺⁴ surface oxidation state. Moreover, surface functionalization leads to higher catalytic activity and selectivity, as well as more tunable enzyme-mimicking performances. Conjugation with melanin is an adequate strategy to boost and enrich CeO₂ NPs biological features, because of melanin redox properties accounting for intrinsic antioxidant, antimicrobial and anti-inflammatory power. Herein, hybrid Melanin/CeO₂ nanostructures were designed by simply coating the metal-oxide nanoparticles with melanin chains, obtained in-situ through ligand-to-metal charge transfer mechanism, according to a bioinspired approach. Obtained hybrid nanostructures underwent detailed physico-chemical characterization. Morphological and textural features were investigated through TEM, XRD and N₂ physisorption. The nature of nanoparticle-melanin interaction was analyzed through FTIR, UV-vis and EPR spectroscopy. Melanin-coated hybrid nanostructures exhibited a relevant antioxidant activity, confirmed by a powerful quenching effect for DPPH radical, reaching 81 % inhibition at 33 μg/mL. A promising anti-inflammatory efficacy of the melanin-coated hybrid nanostructures was validated through a significant inhibition of BSA denaturation after 3 h. Meanwhile, the enzyme-mimicking activity was corroborated by a prolonged peroxidase activity after 8 h at 100 μg/mL and a relevant catalase-like action, by halving the H₂O₂ level in 30 min at 50 μg/mL. Antimicrobial assays attested that conjugation with melanin dramatically boosted CeO₂ biocide activity against both Gram (-) and Gram (+) strains. Cytocompatibility tests demonstrated that the melanin coating not only enhanced the CeO₂ nanostructures biomimicry, resulting in improved cell viability for human dermal fibroblast cells (HDFs), but mostly they proved that Melanin-CeO₂ NPs were able to control the oxidative stress, modulating the production of nitrite and reactive oxygen species (ROS) levels in HDFs, under physiological conditions. Such remarkable outcomes make hybrid melanin-CeO₂ nanozymes, promising redox-active interfaces for regenerative medicine.