Control of Structural Coloration by Natural Sunlight Irradiation on a Melanin Precursor Polymer Inspired by Skin Tanning.

Natural melanin affects the reflection and absorption of light, and it is known as an important element in producing bright structural colors in nature. In this study, we prepared core-shell particles using a melanin precursor polymer, that is, polytyrosine (PTy), as a shell layer by the oxidative polymerization of tyrosine ethyl ester (Ty) in the presence of cerium oxide (CeO2) core particles. Inspired by skin tanning, irradiating the CeO2@PTy core-shell particles with UV or natural sunlight caused melanization by extending the π-conjugated length of PTy, producing colloidal particles with the ability to absorb light. The pellet samples consisting of CeO2@PTy particles appeared whitish because of multiple scattered light. In contrast, the light absorption capacity of CeO2@PTy UV or CeO2@PTy Sun particles after light irradiation suppressed scattered light, dramatically improving the visibility of the structural color of the pellet samples made from these particles. Thus, a new method has been developed to control the visualization of structural colors to the human eye by irradiating the melanin precursor polymer with light.