Tailoring Synthetic Melanin Nanoparticles for Enhanced Photothermal Therapy

Melanin and its synthetic analogs (i.e., polydopamine nanomaterials) are able to transform a near-infrared (NIR) light energy source to heat for the selective killing of cancer cells. Although many of the effects on these nontoxic photothermal agents have been well documented, a concern has arisen that the extended usage of these natural and synthetic melanins might be hindered by their limited photothermal effects under low-density light irradiation. To address this issue, herein, we propose a rational and green fabrication strategy toward a new class of synthetic melanin nanoparticles (SMNPs) with superior photothermal effects via the one-pot copolymerization of two kinds of naturally occurring monomers (arginine and dopamine). The total photothermal efficiencies of these arginine-doped SMNPs could be significantly improved (i.e., ∼60% increase) by enhancing 808 nm NIR light absorption via the construction of donor-acceptor microstructures within SMNPs and decreasing nonthermal radiative transition processes via the increase of free radical concentrations within SMNPs. The resulting SMNPs demonstrated higher photothermal therapy efficiencies in both killing 4T1 cancer cells in vitro and suppressing tumor growth and recurrence compared with conventional agents. This work offers new opportunities in the structural and functional tailoring of melanin-inspired nanomaterials for cancer treatment via green fabrication strategies.