Immobilization of lignin nano/microparticles on plasma-modified polymer nanofibers

Lignin, one of the most abundant renewable phenolic biopolymers, exhibits antibacterial, antioxidant, and UV-blocking properties. Synthesizing lignin nanoparticles (NPs) opens up new applications by offering advantages over bulk lignin, providing a high specific surface area, increased reactivity, and water solubility. We discovered that lignin sizes and concentrations sufficient to elicit an antimicrobial effect in suspension against E. coli were simultaneously cytotoxic in vitro, underscoring the need for careful dose optimization in biomedical applications. Since polymer nanofibers (NFs) serve as excellent carriers for NPs, we aimed to immobilize four types of lignin nano/microparticles (15nm-2μm in diameter) onto polymer nanofibrous mats. Dip-coating of lignin particles from a water colloid/suspension onto the NF mats proved to be the most reproducible and homogeneous method. To address the issues related to the hydrophobicity of synthetic polymers and to mediate particle attachment, we explored plasma-based surface modifications of NFs, namely O₂ plasma treatment and deposition of carboxyl and amine plasma polymers (PPs). Besides particle immobilization, plasma modification improves the biocompatibility and bioactivity of NFs. Overall, up to 15% of lignin initial mass was bound to NFs, resulting in a high surface coverage (∼50%). Among the tested modifications, positively charged amine PP coatings demonstrated the highest efficiency and versatility, yielding stable attachment of negatively charged lignin nano/microparticles even during immersion into water. Since the water contact angle of pristine and carboxyl-PP-coated NFs was similar, 119-121°, we proposed that these mats immobilize lignin particles through hydrophobic interactions, resulting in partial particle release upon immersion in water.