Photolytic formation of silver nanoparticles under violet light and their antibacterial efficacy against Escherichia coli

Silver nitrate is a well-known antimicrobial, but its photoactivation under visible light remains underexplored. This study demonstrates that violet light (λₘₐₓ = 403 nm) illumination of Escherichia coli treated with silver nitrate leads to the formation of silver nanoparticles with a mean diameter of 4.35 nm and markedly enhances antibacterial photodynamic inactivation (aPDI). Compared with dark or green/red light conditions, violet illumination achieved up to a 4-log reduction in E. coli viability at sub-millimolar AgNO₃ concentrations. Transmission electron microscopy (TEM) confirmed nanoparticle generation, while reactive oxygen species (ROS) assays indicated intense superoxide radical ( O_2^{•−} ) production during illumination. These observations indicate that in the system of silver-nitrate‐treated E. coli exposed to violet light, Ag + ions likely act as oxidising agents by accepting electrons from the photoexcited endogenous photosensitisers within E. coli . This process results in the photoreduction of Ag + ions and nanoparticle formation and significantly reduces the survival rate of E. coli . The silver-nitrate-treated E. coli system reveals a dual-action light-activated, ROS-induced aPDI approach that kills E. coli under violet light illumination and prevents regrowth in the dark by releasing silver ions from silver nanoparticles, providing a novel strategy for wavelength-specific antimicrobial control. GRAPHICAL ABSTRACT