Optical Properties of Different Chemistries of Natural and Synthetic Melanin

Melanin, a ubiquitous black pigment, consists of at least five chemistries identified by the starting monomers involved in its polymerization. Natural melanin may also contain other components, such as lipids and proteins, that can affect its optical properties. Melanin is known for its high refractive index and broadband absorption. However, most optical-property observations are based on measurements of eumelanin-based chemistry. In this study, we used spectroscopic ellipsometry to measure the complex refractive index over a broad wavelength range (380–1700 nm) for the three main melanin chemistries: eumelanin, allomelanin, and pheomelanin. We compared the complex refractive indices of these materials with values obtained from UV absorption measurements, using an integrating sphere to minimize scattering effects. Our results show that melanin chemistry strongly influences optical properties. Pheomelanin from human red hair has the lowest absorption and refractive index, while poly(L-3,4-dihydroxyphenylalanine) (PDOPA; a polydopamine-based eumelanin chemistry) has the highest refractive index and absorption. We also observed variation in the optical properties of pheomelanin samples extracted from human red hair and rooster feathers. Thus, variations in melanin composition affect both the optical and thermal properties of melanin, highlighting the need to identify its precise chemistry in natural systems to understand melanin's impact on function and performance.