Innovative antimicrobial lignins: Extraction and characterization for advanced hydrogel applications

The antioxidant and antimicrobial activities of lignin are often emphasized; however, not every type exhibits these properties. In this work, water-soluble fractions of alkali lignin (AL), poly-(caffeyl alcohol) lignin (PCFA), pyrolytic lignin (PL) and grape seed lignin (GSL) were prepared. The original and water-soluble lignin fractions were comprehensively characterized using high-resolution 2D NMR spectroscopy. Notably, water-soluble fractions of PCFA, PL and GSL lignins exhibited 3.6 to 3.9 higher antioxidant activities than the original lignins despite having a phenolic content of approximately 12 % to 55 % lower. Additionally, these fractions demonstrated antimicrobial activities against Micrococcus luteus, Serratia marcescens and Escherichia coli. The potential of water-soluble lignin fractions as active modifiers for physically crosslinked hydrogels was also investigated. Specifically, PL/F lignin served as an antioxidant and antimicrobial agent for modifying carrageenan without disrupting its viscoelastic and swelling behaviour. Carrageenan hydrogels with 6 % PL/F lignin showed an antioxidant activity of 219.6 mg TE g^-1 hydrogel and reduction rates of 43.9 % against M. luteus, 31.6 % against S. marcescens and 20.6 % against E. coli.