International Journal of Biological Macromolecules 223 (2022) 1287–1296 Available online 14 November 2022 0141-8130/© 2022 Elsevier B.V. All rights reserved. Revealing the relationship between molecular weight of lignin and its color, UV-protecting property Yarong Li a, b , Siyu Zhao a, c , Yihan Li a, b , Arthur J. Ragauskas d, e, f , Xueping Song a, c, * , Kai Li a, b, ** a College of Light Industry and Food Engineering, Guangxi University, Nanning 530004, PR China b Engineering Research Center for Sugar Industry and Comprehensive Utilization, Ministry of Education, Nanning 530004, PR China c Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, Nanning 530004, PR China d Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, TN 37996, USA e Joint Institute for Biological Sciences, Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37771, USA f Center for Renewable Carbon, Department of Forestry, Wildlife and Fisheries, The University of Tennessee, Knoxville, TN 37996, USA A R T I C L E INFO Keywords: Lignin Molecular weight Sunscreen UV-protecting property Color ABSTRACT Lignin has great potential as a natural, green, and sustainable broad-spectrum sunscreen active ingredient. However, the coexistence of dark color and sunscreen properties hinders its application in cosmetics. In this study, we focus on the effects of the molecular weight of lignin on tis UV-protecting property and color in order to prepare lignin-based sunscreen with high performance. A prepared sunscreen containing low molecular weight lignin (F5, <1000 g/mol) exhibits good UV-protecting property (sun protection factor (SPF) = 7.14) and light color advantages (ΔE = 46.2). Moreover, a strong synergistic effect on UV-protecting property exists be- tween low molecular weight lignin and ethylhexyl methoxycinnamate (EHMC), resulting in high SPF of F5@EHMC-based sunscreen (55.56). Additionally, added TiO 2 can effciently mitigate the dark color of lignin- based sunscreens due to prominent covering power of TiO 2 . Moreover, lignin-based sunscreens have good biocompatibility with HaCaT cells. This work is useful for understanding the mechanism of the UV-protecting property and dark color of lignin, and for designing an effcient and safe lignin-based sunscreen. 1. Introduction Lignin is a major component of plant cell walls and a biopolymer with natural reserves second to cellulose [1–3]. The global pulp and paper, and bioethanol refning industries produce nearly 70 million tons of lignin every year, but approximately 98 % of this is treated as low- value fuel or waste; essentially, the maximum value of lignin is not being used [4,5]. High-value utilization of lignin resources, by taking advantage of its unique properties, can effectively improve this situa- tion. Notably, lignin has natural UV-protecting property; thus, the application of lignin as a green sunscreen active ingredient in cosmetics is promising and has attracted considerable attention in research [6–9]. At present, commercial sunscreen active ingredients primarily include chemical and physical ingredients, of which chemical ingredients ac- count for 80 % [10]. Generally, chemical sunscreen active ingredients are small molecular aromatic compounds derived from unsustainable petrochemical resources, and their safety to humans is controversial [11–13]. In addition, chemical sunscreen active ingredients are increasingly polluting marine resources [14–17]. Thus, using lignin in sunscreen products not only produces natural, green, and safe sunscreen products but also reduces the pollution load of chemical sunscreens on marine resources. Previous studies have reported that all types of lignin (such as alkali lignin, kraft lignin, and milled wood lignin) exhibit UV-protecting property [18]. Moreover, the conjugation system in lignin formed by its benzene ring and functional groups (such as carbonyl, carbon‑carbon double bond, phenolic hydroxyl, and methoxy) can reduce the energy required for electronic transition; thus, lignin exhibits a broad absorp- tion in the UV spectrum range [6,19,20]. However, lignin has a dark color owing to the existence of a conjugation system, and its dark color renders lignin-based sunscreen unpopular in cosmetics [21]. Previously researchers in order to develop an excellent lignin dye dispersant, have reported that lignin with high molecular weight (M w = 18,632, 14,870 g/mol) has a darker color than lignin with a low molecular weight (M w * Correspondence to: X. Song, Guangxi Key Laboratory of Clean Pulp & Papermaking and Pollution Control, Nanning 530004, PR China. ** Correspondence to: K. Li, Engineering Research Center for Sugar Industry and Comprehensive Utilization, Ministry of Education, Nanning 530004, PR China. E-mail addresses: sx_ping@gxu.edu.cn (X. Song), gxlikai@gxu.edu.cn (K. Li). Contents lists available at ScienceDirect International Journal of Biological Macromolecules journal homepage: www.elsevier.com/locate/ijbiomac https://doi.org/10.1016/j.ijbiomac.2022.11.067 Received 12 September 2022; Received in revised form 28 October 2022; Accepted 8 November 2022