Biomass-Based Synthesis of Tunable Photoactive Bimetallic Nanoparticles for Antibacterial and Catalytic Applications

The green synthesis of noble metal nanoparticles has gained increasing attention due to cost-effectiveness and environmental friendliness. This research aims to develop an efficient method to synthesize lignin-capped gold–silver nanoparticles (Li-GS NPs). Lignin has recently been reported to function both as a reducing agent and as a capping agent in the synthesis of metal nanoparticles. These Li-GS NPs were successfully synthesized via galvanic replacement by using lignin-capped silver nanoparticles (Li-Ag NPs) as a precursor. Lignin extracted from various sources, bagasse (BG), pararubber woodchip (PRW), and palm kernel shells (PKS) were used to investigate the effect of different functional group compositions on the nanoparticle formation. Li-Ag NPs were uniformly synthesized and transformed into well-dispersed Li-GS NPs via galvanic replacement with KAuCl 4, as indicated by an LSPR shift from 420 nm to 550–600 nm. Li-Ag NPs ranged from 8 nm to 28 nm, while Li-GS NPs exhibited a broader size distribution, reflecting gold-induced growth and morphological variation. Li-GS NPs demonstrated stronger antibacterial activity against both Gram-positive and Gram-negative bacteria, with Ag contributing significantly to bactericidal effects. Additionally, Li-GS NPs exhibited high catalytic efficiency in 4-nitrophenol reduction, completing the reaction within 1 min. Importantly, using lignin as both a reducing and a stabilizing agent is expected to significantly reduce costs and minimize the use of hazardous chemicals.