Fabrication of lignin nanoparticles with adjustable size, antioxidant, antibacterial, and hydrophobic properties by a two-step fractionation

Lignin nanoparticles (LNPs) are gaining attention for their renewability and environmental friendliness in advanced nanomaterials. To establish a new sustainable value chain, it is vital to fully utilize lignin resources and thoroughly examine the effects of LNPs size and structure on performance. Herein, a two-step fractionation scheme is engineered via combining sequential organic solvent fractionation and acid precipitation methods to obtain four lignin fractions (denoted as F1, F2, F3, and F4) with low heterogeneity, suitable hydroxyl content and the syringyl (S)/guaiacyl (G) ratio for LNPs fabrication. Up to 88.7 % of alkali lignin was collected to prepare LNPs, and the LNPs showed controllable sizes (100-500 nm, denoted as F1-LNP, F2-LNP, F3-LNP, and F4-LNP). The size gradually decreased from F1-LNP to F4-LNP with increasing specific surface area of LNPs, contributing to superior antibacterial and antioxidant properties. Notably, a higher S/G ratio with enriched p-hydroxyphenyl (H) units resulted in a smaller size of LNPs, possibly resulting from the greater attraction and larger binding energy between S-S and H-H than G-G. This work gives insights into the full utilization of technical lignin to nano-particles to meet specific performance requirements, which will particularly broaden the commercialization and high-value utilization of lignin.