Flavonoids-in-Lignin Coextraction Strategy for Developing Long-Lasting UVA-Enhanced Sunscreens

Abstract

Lignin contains benzene rings, carbonyl groups, and phenolic groups that serve as natural macromolecular UV absorber. However, lignin provides insufficient protection in the UVA region because it lacks a long conjugated structure. In this study, we propose a flavonoids-in-lignin coextraction strategy to improve lignin’s UVA protection performance. Flavonoids-containing lignin was isolated from Musa basjoo (Musa basjoo Siebold) and Moso bamboo (Phyllostachys edulis) leaves using l-cysteine (l-cys)-assisted extraction, and the contents of flavonoids in lignin could be further enhanced by employing non-dewaxed sources. Lignin’s UV-protective performance and antioxidant activity were improved as more flavonoids were reserved. Particularly, lignin isolated from non-dewaxed bamboo leaves (NBL) exhibited the best UV protection performance, achieving a sun protection factor (SPF) value of 40.99 with the addition of 5 wt % lignin into SPF 15 commercial sunscreen, representing a 143% enhancement, and a UVA protection factor (PFA) value of 12.17, reflecting a 109% increment. The sunscreens prepared from flavonoid-rich lignin maintained SPF values exceeding 30 and PFA values exceeding 9 (PA+++) even after 8 h of UV irradiation, indicating their long-term UV-protective properties. Furthermore, flavonoids-in-lignin coextraction demonstrated superior UV-protective performance compared to alkali lignin with adscititious flavonoid extracts. This study leverages the inherent advantages of natural plant cell wall components through a straightforward coextraction strategy, further unlocking the potential of lignin for sunscreen applications.