Stimuli-Responsive Structurally Colored Films from Bioinspired Synthetic Melanin Nanoparticles

Nature has evolved a fantastic gallery of structural colors, offering a source of inspiration for the development of artificial, multifunctional photonic devices. Inspired by the widespread use of melanin particles to produce structural colors in bird feathers, we recently demonstrated that synthetic melanin nanoparticles (SMNPs) offer the same rare, but critical combination of high refractive index and high absorption as natural melanin. Here, we show for the first time fast, significant, and reversible changes of structural coloration in self-assembled SMNP films in response to changes in humidity. This process is driven by the hygroscopic nature of the particles, leading to changes in the thickness of the SMNP layer that alter the interference color. This mechanistic explanation is supported by water absorption measurements, an optical model, and environmental scanning electron microscopy. Humidity-induced dynamic colors arising from SMNP films offer possible routes for synthetic melanin as an important material in sensors and coatings.