Green and Facile Preparation of Regular Lignin Nanoparticles with High Yield and Their Natural Broad-Spectrum Sunscreens

Lignin nanoparticles offer a way for a high value use of the renewable resources. However, there are few green and facile methods so far for producing lignin nanoparticles with both high yield and regular shape. In this study, lignin was first modified through a microwave acetylation process without any catalysts and solvents other than acetic anhydride, which acted as both reaction reagent and dispersion solvent. Subsequently, the regular lignin nanoparticles with a high yield were prepared by a solvent shifting combined ultrasound process. The lignin nanoparticles were rapidly formed without dialysis and easily separated by centrifugation; meanwhile, the used THE could be recycled and reused, which would simplify the process, reduce the cost, and realize the industrial scale-up production. The highest yield of lignin nanoparticles reached to 82.3% as the lignin initial concentration and the ultrasound intensity increased. Meanwhile, the ultrasound treatment improved the uniformity and dispersion of the nanoparticles. The structure transformation and the forming mechanism of lignin nanoparticles were investigated through various techniques. Furthermore, the UV absorbing ability of lignin nanoparticles was examined. In a more general plan, it was confirmed that the green chemistry principles could be realized in the development of more sustainable lignin nanomaterials with various potential applications.