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.