Structural Color Production in Melanin-based Disordered Colloidal Nanoparticle Assemblies in Spherical Confinement

TL;DR

This study combines computational modeling and experiments to understand how melanin's broadband absorption influences structural color in nanoparticle assemblies, revealing effects on color wavelength, saturation, and the impact of particle size dispersity.

Contribution

It introduces a combined computational and experimental approach to analyze how melanin's absorption affects structural color in nanoparticle supraballs, highlighting the roles of dispersity and chemistry stratification.

Findings

Melanin absorption shifts reflectance peaks to longer wavelengths.

Size dispersity impacts color diversity more than packing fraction.

Binary mixtures enable more diverse and tunable colors.

Abstract

Melanin is a ubiquitous natural pigment that exhibits broadband absorption and high refractive index. Despite its widespread use in structural color production, how the absorbing material, melanin, affects the generated color is unknown. Using a combined molecular dynamics and finite-difference time-domain computational approach, this paper investigates structural color generation in one-component melanin nanoparticle-based supra-assemblies (called supraballs) as well as binary mixtures of melanin and silica (non-absorbing) nanoparticle-based supraballs. Experimentally produced one-component melanin and one-component silica supraballs, with thoroughly characterized primary particle characteristics using neutron scattering, produce reflectance profiles similar to the computational analogues, confirming that the computational approach correctly simulates both absorption and multiple scattering from the self-assembled nanoparticles. These combined approaches demonstrate that melanin's broadband absorption increases the primary reflectance peak wavelength, increases saturation, and decreases lightness factor. In addition, the dispersity of nanoparticle size more strongly influences the optical properties of supraballs than packing fraction, as evidenced by production of a larger range of colors when size dispersity is varied versus packing fraction. For binary melanin and silica supraballs, the chemistry-based stratification allows for more diverse color generation and finer saturation tuning than does the degree of mixing/demixing between the two chemistries.