Eumelanin is a brown heterogeneous pigment that is composed of aggregates of indolic subunits. It behaves as a sunscreen, dissipating light as heat in melanosomes that protect human skin cells. Light absorption by eumelanin is known to produce radicals that can sensitize harmful reactive oxygen species, but the mechanisms behind these processes are poorly understood in part due to uncertainty in how the subunits responsible for melanin’s hierarchical structure work together. The goal of this project is to establish how subunit aggregation affects the excited state pathways responsible for relaxation and radical generation in eumelanin. Polydopamine, a synthetic melanin, was chosen for this study because it replicates the optical and electrical properties of naturally occurring eumelanin, and it is easily synthesized and purified. Adapting a previous approach, we synthesized polydopamine nanoparticles with diameters of 100 – 1000 nm and developed a method for controlling their disassembly into subunits, which are presumed to be oligomers of dihydroxyindole. The polydopamine dispersions were studied as a function of the degree of disassembly using dynamic light scattering and UV/Vis absorption spectroscopy. Polydopamine nanoparticle disassembly resulted in absorption spectra that resemble previously reported action spectra for radical photoproduction, suggesting that disassembly is correlated with increased photochemical reactivity. Transient absorption measurements of polydopamine samples at different levels of disassembly reveal that excited state relaxation dynamics are affected by aggregation. Ultimately, understanding ways to manipulate the structure and photophysical characteristics of eumelanin will benefit applications in both biology and materials science.

Discerning aggregation effects on polydopamine nanoparticle photochemical reactivity

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.