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.

Effect of the Polydopamine Composite Method on Structural Coloration: Comparison of Binary and Unary Assembly of Colloidal Particles.

Melanin influences light reflection and absorption and is known to be one of the elements producing structural color, such as that in the feathers of birds. In this study, we used polydopamine (PDA), an artificial melanin, as a light-absorbing material and examined in detail the effect of its composite method on the structural color. The following two composite methods were investigated using cerium(IV) oxide (CeO2) particles as a core particle: binary coassembly of CeO2 and PDA particles and unary assembly of CeO2@PDA core-shell particles. Although both methods dramatically improved the visibility of the structural color by suppressing the scattered light owing to the light absorption capability of the PDA, there was a difference in the particle arrangement, angle dependence of the structural color, and color tone change. By selecting the PDA composite method, the guidelines for providing high visibility and the desired structural color were presented.