Engineering Lignin Nanoparticles: A Tandem Fractionation and Enzymatic Oxidation Approach to Control Size and Hydrophobicity

Lignin nanoparticles (LNPs) are pivotal for the advancement of modern and sustainable biorefineries. Yet, optimizing their performance requires precise control over size and hydrophobicity. This study investigates a tandem approach involving fractionation and laccase-mediated oxidation of hardwood kraft lignin prior to nanosizing. Two solvent precipitation methods for LNP synthesis are compared: a sodium tosylate system (yielding hLNPs) and an ethanol:water mixture (yielding eLNPs). Results indicate that biocatalytic modifications, primarily the reduction of hydroxyl groups and increased molecular weight through radical coupling, are key determinants of the yield, dimensions, and surface functionalities of the resulting LNPs. These properties are governed by an interplay of chain-folding capability, supramolecular interactions, and specific solvation mechanisms. Notably, hLNPs exhibit diameters of 500–750 nm, whereas eLNPs are significantly smaller (<230 nm). Lastly, XPS analysis confirms successful hydrophobization, revealing up to a 45% reduction in surface hydroxyl functionalities and a 2.6-fold increase in ketone groups compared to pristine counterparts. This research provides a robust and eco-friendly strategy for engineering tailored LNPs with enhanced applicability potential.