Tailoring the surface chemistry of lignin nanoparticles using fungal laccases: Insights on the oxidation mechanisms

The functionalization of lignin nanoparticles (LNPs) can increase the application of such systems by grafting different ligands and tailoring the surface chemistry. However, these modifications are conventionally performed using harsh and energy-demanding methods. Laccase-induced reactions represent a green synthetic approach to modify the lignin, either to depolymerize or induce crosslinking of lignin monomeric units, usually assisted by redox mediators. In this study, we aim to characterize the oxidation mechanism and its effect on the surface chemistry and polymerization of LNPs that exhibit different proportions of structural units and S/G ratios in their composition, using five fungal laccases produced in house and two commercial laccases, without the presence of mediators. Our results suggest that the in house produced laccases induce about a 5-fold increase in the particle size and absorbance values of the LNPs with low S/G ratio, compared to the untreated LNPs, indicating the formation of dimeric product through C C type of linkages and consequent polymerization and crosslinking of these LNPs. Contrarily, LNPs presenting high S/G ratio show a decrease in the phenolic content along with an increase in the percentage of oxidized syringyl-type units after laccase treatment, suggesting a preferred Cα–OH type of oxidation. Overall, this study provides new insights on the mechanism of laccase-assisted oxidation and polymerization of LNPs presenting different monomeric composition, which can greatly increase the application of further tailored LNPs in different sectors, including biomedical and food sciences.