Investigation of Lignin Content in the Fabrication of ZnO NPs and their Antioxidant, Antibiofilm, and Antimicrobial Properties

Lignin is a remarkable biopolymer with significant potential for valorization. As the most abundant aromatic renewable biopolymer, its intricate structure endows it with notable antioxidant and antimicrobial properties. In this study, the effect of lignin content on the synthesis and biological properties of zinc oxide nanoparticles (ZnO NPs) was investigated. Three distinct types of NPs were synthesized: pure ZnO, ZnO@Lignin 0.1%, and ZnO@Lignin 1%. The synthesized NPs were characterized using various techniques. Fourier transform infrared (FT-IR) spectroscopy revealed that higher lignin content enhanced lignin-related functional groups. X-ray diffraction (XRD) confirmed a hexagonal wurtzite structure with lignin influencing crystal orientation. Energy dispersive spectroscopy (EDS) and elemental mapping demonstrated successful lignin coating, which was thicker at higher concentrations. Dynamic light scattering (DLS) and zeta potential analyses indicated that ZnO@Lignin 1% NPs had better colloidal stability, with a more negative zeta potential and smaller particle size. Field emission transmission electron microscopy (FE-TEM) showed a core-shell structure, with a more pronounced lignin coating and reduced particle size in ZnO@Lignin 1% NPs compared to pure ZnO NPs. The NPs’ antimicrobial efficacy was tested against various bacterial and fungal pathogens, with minimum inhibitory concentrations (MIC) ranging from 256 to > 2048 µg/mL. ZnO@Lignin 1% NPs demonstrated superior antimicrobial and anti-biofilm properties, significantly inhibiting biofilm formation by 84.3% in Pseudomonas aeruginosa and 76.1% in Staphylococcus aureus at 1024 µg/mL. In addition, incorporating 1% lignin imparted antioxidant properties to the NPs, with an IC₅₀ value of 363.88 µg/mL in the ABTS assay. These findings highlight the potential of ZnO@lignin NPs, particularly at a 1% lignin concentration, as multifunctional materials with enhanced colloidal stability and improved biological properties.