Isolation, properties, and recent advancements of lignin nanoparticles as green antioxidants

Over 220 million tonnes of biomass waste are generated annually around the world. One of the biomasses generated is lignocellulosic biomass which consists of lignin, cellulose, and hemicellulose. Since lignin possesses numerous unique qualities, researchers ought to substitute lignin for high value-added applications. However, lignin's properties such as irregular shape, heterogeneity, low dispersity, and large particle size have limited its capability for high value-added applications. Thus, the conversion of lignin into lignin nanoparticles (LNPs) has gained much attention in the research community. The present review aims to educate readers about the techniques employed to isolate LNPs, including physical and chemical approaches which influence the properties of LNPs. Characterization methods for LNPs, including structural, thermal, molecular weight, microstructural, and macromolecular properties are further discussed. Various determination methods pertaining to antioxidant activity of LNPs such as 1,1-diphenyl-2-picryl-hydrazyl (DPPH), ferric-reducing antioxidant power (FRAP), TEAC (Trolox equivalent antioxidant capacity) assay, and oxygen uptake inhibition (OUI) are reviewed. This review also details the recent advancements of LNPs as antioxidants. Prospects and challenges in the development of LNPs are also highlighted.