Light-switchable lignin-based nanospheres for photodynamic eradication of drug-resistant pathogens and ROS scavenging

The increasing prevalence of drug-resistant bacteria and the ecological burden of synthetic antimicrobials demand sustainable biomaterials with superior antibacterial properties. Natural lignin, an amorphous polymer, presents considerable potential as a structural building block for sustainable biomaterial. Herein, inspired by antimicrobial photodynamic inactivation, kraft lignin was first reacted with formaldehyde and ethylenediamine to synthesize aminated lignin, the aminated lignin was further reacted with acetic anhydride, resulting in modified lignin. Thereafter, the modified lignin functions as a carrier, encapsulating the hydrophobic photosensitizer chlorin e6 to improve its dispersibility and aqueous solubility. Chitosan was further utilized to coat the nanoparticles, improving bacterial uptake, ultimately obtaining the desired final nanospheres (CS-ML-Ce6). Upon laser irradiation, CS-ML-Ce6 achieved >99.99 % bactericidal efficiency against methicillin-resistant Staphylococcus aureus. Additionally, it demonstrated a minimum inhibitory concentration of modified lignin of 67.71 μg/mL when exposed to light. It could result in membrane disruption and intracellular component leaking, while bacterial adsorption increased to 47.36 %. The dead/live rate of CS-ML-Ce6 against drug-resistant pathogens was almost 8-fold to modified lignin control. Concurrently, hemolysis assays with rabbit red blood cells confirmed exceptional biocompatibility, with hemolysis rates maintained below 5 % even at 6 × MIC concentrations. Considerable reactive oxygen species scavenging effectiveness at both the solution and cellular levels was also observed of CS-ML- Ce6, this dual-function approach uniquely combines light-activated bactericidal properties and dark-phase antioxidant repair. Utilizing lignin, an industrial byproduct, provides an efficient and promising antimicrobial alternative to prevent overuse of antibiotics and has significant potential for human healthcare.