New insights into the effects of structural and constituent heterogeneities in lignin on the formation of nanoparticles

Conversion of lignin into nanoparticles has garnered growing interest due to the potential for high-value applications across various fields. However, the technical grade and structural heterogeneity of lignin largely impact on the polydispersity and morphology of formed lignin nanoparticles (LNPs). Therefore, it is still needed to investigate the correlation between lignin structure and the formation of LNPs to a further understanding on the formation mechanism. This work aimed to elucidate the impact of constituents and structural heterogeneity of industrial softwood kraft lignin (ISKL) on the formation of spherical LNPs. To achieve this, LNPs derived from six fractions obtained through sequential solvent extraction of ISKL were compared: four fractions with trace amounts of carbohydrates (TCs), one fraction with the higher amount of native lignin sublinkages and carbohydrates (HC3), and a fraction consisting of extractives. Additionally, lignin-glucose complexes were synthesized via click chemistry to investigate the potential influence of carbohydrates on LNPs formation. The results showed that aliphatic hydroxyl groups (-OH) both from the side chain of lignin and covalently bonded carbohydrates result in LNPs in a smaller size but with less regular spherical morphology. Besides, heterogeneous constituents of the fatty acids remaining in parental lignin play a significant role in the generating LNPs with hollow structures. In summary, this study provided us with an in-depth understanding of the effect of structural and constituents properties on the characteristics of lignin nanoparticles.