Quaternary Ammonium Assisted Synthesis of Melanin-like Poly(l-DOPA) Nanoparticles with a Boosted Photothermal Effect

Poly(levodopa) nanoparticles (P( l -DOPA) NPs) are another kind of melanin mimetic besides well-established polydopamine nanoparticles (PDA NPs). Due to the presence of carboxyl groups, the oxidative polymerization of l -DOPA to obtain particles was not as efficient as that of dopamine. Several established methods toward P( l -DOPA) NP fabrication do not combine convenience, morphological regularity, size controllability, low cost, and adaptability to metal-free application scenarios. In this work, P( l -DOPA) NPs were successfully prepared in hot water with the assistant of organic quaternary ammonium, due to the extra physical cross-linking mediated by cations. The employed physical interactions could also be affected by quaternary ammonium structure ( i.e., number of cation heads, length of alkyl chain) to achieve different polymerization acceleration effects. The obtained P( l -DOPA) NPs retained superior photothermal properties and outperformed PDA-based melanin materials. Furthermore, P( l -DOPA) NPs were used in photothermal tumor therapy and showed better efficacy. This study offers new insights into the synthesis of melanin-like materials, as well as new understanding of the interaction between quaternary ammonium and bioinspired polyphenolic materials.