Synthetic Neuromelanin Particles for Metal-Free Photocatalysis

Neuromelanin, a brain pigment comprising a pheomelanin core encased in a eumelanin shell, offers a unique bioinspired blueprint for functional nanomaterials. Here, synthetic neuromelanin particles (NMPs) with tunable shell thicknesses are reported, prepared via co-polymerization of dopamine and L-cysteine under alkaline oxidative conditions. Electron microscopy and spectroscopy reveal a core-shell nanostructure in which sulfur-rich, spin-active pheomelanin cores are progressively shielded by eumelanin shells of variable thickness. The thinnest-shell NMPs exhibit the highest sulfur-to-carbon ratio and EPR spin concentration (mass-normalized double-integrated signal); their spectra show a single line at g = 2.005, characteristic of semiquinone/carbon-centered radicals. The spin concentration closely correlates with the photocatalytic activity in the visible-light oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid. Transient photocurrent measurements confirm superior charge-separation efficiency, consistent with radical-mediated photophysics intrinsic to the core. This neuromelanin-inspired design establishes a new class of metal-free organic photocatalysts, in which nanoscale architecture dictates radical density and catalytic performance.