Disentangling melanin self-assembly via nanoprecipitation: from protoparticles to supraparticles

The physicochemical properties of eumelanin, the dark melanin pigment found in most organisms, depend sensitively on the size and morphology of the particles that make up this naturally occurring nanomaterial. Past strategies for controlling nanostructure have focused on modulating the rate of the oxidation reactions that yield melanin from precursor compounds, making it difficult to decouple oxidative polymerization from self-assembly. Here, we introduce a post-synthetic approach that uses an organic antisolvent to isolate nanoscale assembly, aggregating ultrasmall, disc-like melanin particles that are 2-6 nm in diameter and contain only a few stacked layers of planar, sheet-like polycyclic molecules into size-controlled spherical supraparticles with diameters that can be tuned from 30 to 100 nm. Following dialysis to remove the organic solvent, the supraparticles are stable to heating and sonication and their size remains constant in water for at least two weeks. The excellent stability of the supraparticles is attributed to self-crosslinking reactions that form covalent bonds between the aggregated subunits. The bonds form slowly over hours by spontaneous reactions that can be accelerated by UV irradiation. The nanoprecipitation approach to creating melanin supraparticles offers a straightforward means to manipulate the hierarchical structure of melanin with the precision needed to enable investigations that can uncover melanin's emergent properties through structure-function studies.