Photonic Lignin with Tunable and Stimuli-Responsive Structural Color.

Due to the growing sustainability and health requirements, structural color materials fabricated with functional natural polymers have attracted increasing attention in advanced optical and biomedical fields. Lignin has many attractive features such as great biocompatibility, ultraviolet resistance, antioxidant property, and thermostability, making it a promising natural resource to be fabricated as functional structural color materials. However, to date, the utilization of lignin as the building block for structural color materials is still a challenge due to its disordered structure. Herein, we present a strategy to transform disordered lignin into ordered "photonic lignin", in which monodisperse lignin colloidal spheres are prepared via solvent/antisolvent self-assembly, and then the periodic structure is constructed by centrifugal effect. The photonic lignin exhibits structural colors that are tunable by modulating the diameter of lignin colloidal spheres. We further demonstrate the application of photonic lignin as a natural polymer-based coating that shows bright, angle-independent, and stimuli-responsive structural colors. Moreover, the cytotoxicity assay indicates the excellent biocompatibility of photonic lignin with human skin, blood vessels, digestive systems, and other tissues, which demonstrates the great potential of photonic lignin in the applications such as implanted/wearable optical devices, advanced cosmetics, and smart food packaging.