Photosensitizer-conjugated lignin nanoparticles for photodynamic antimicrobial inactivation

Towards the goal of developing broad-spectrum, self-disinfecting, and cost-effective renewable antimicrobial materials for the prevention of pathogen transmission relevant to healthcare-associated infections (HAIs), a facile cyanuric chloride coupling strategy was employed to covalently attach a commercially available phenothiazine-based photosensitizer, thionine (Th), onto lignin nanoparticles (LNPs). The resultant photodynamic Th-LNPs conjugate was characterized by a variety of techniques, including elemental analysis, diffuse reflectance UV-visible and energy-dispersive X-ray spectroscopies, scanning electron microscopy, and thermal gravimetric analysis. Antibacterial photodynamic inactivation studies demonstrated high levels of inactivation (99.999+%) by the Th-LNPs against Gram-positive pathogens methicillin-resistant Staphylococcus aureus (MRSA; ATCC-44) and vancomycin-resistant Enterococcus faecium (VRE; ATCC-2320), and Gram-negative multi-drug-resistant Acinetobacter baumannii (MDRAB; ATCC-1605), upon visible light illumination (60 min, 400-700 nm, 65 ± 5 mW/cm²). Vesicular stomatitis virus (VSV), employed as a model enveloped virus, exhibited similarly high levels of inactivation (99.996%) under the same illumination conditions. Taken together, these results suggest that applications that are agnostic to the chemical and structural heterogeneity of the polymer, such as photodynamic inactivation, can enable the valorization of lignin from a low-value industrial byproduct into a potent, scalable and renewable antimicrobial material via a facile two-step synthesis.