Separation of lignin from bamboo using type IV deep eutectic solvents for enhancing the antioxidant properties of chitosan films

To promote the functionalization of chitosan-based films, lignin separated from Moso bamboo using type IV deep eutectic solvents (DESs; FeCl 3 /glycerol (Gly), AlCl 3 /Gly, and CuCl 2 /Gly) was incorporated into chitosan to prepare lignin/chitosan composite films. For comparison, lignin pretreated with an aqueous NaOH solution was prepared and used for forming a film. The systematic characterization of lignin samples and lignin/chitosan composite films was performed using advanced analytical techniques. Gel permeation chromatography showed that the type-IV-DES-pretreated lignin sample possessed a lower molecular weight and more uniform structure than the NaOH-pretreated lignin sample, which was more conducive to increasing the compatibility between lignin and chitosan. 31 P nuclear magnetic resonance spectroscopy and two-dimensional heteronuclear correlation results revealed that the excellent antioxidant activity of lignin following AlCl 3 /Gly pretreatment was due to the high phenolic hydroxyl group content (1.16 mmol/g). Moreover, the free-radical scavenging capacity of the resulting DES-pretreated lignin/chitosan composite film reached 83.12 %, which was 3.4-fold higher than that of the pure chitosan film. In general, type-IV-DES-pretreated lignin was superior to NaOH-pretreated lignin in terms of enhancing the ultraviolet-shielding ability, antioxidant capacity, and antibacterial properties of the chitosan films. Notably, these films have great potential for use in food packaging applications.