Effects of Iran Light Crude Oil and Corexit EC9500A on the UV-Induced Formation, Colloidal Stability, and Bacterial Toxicity of Gold Nanoparticles in Simulated Seawater

Oil-derived organic matter and dispersant components can generate reactive oxygen species under irradiation, but their role in mediating Au(III) reduction and gold nanoparticle formation remains unclear. Here, laboratory-prepared WAO, WAD, and DWAO systems in simulated seawater were used as model matrices for Au(III) photoreduction. EPR analysis revealed the generation of O2•–, •OH, and 1O2 during nanoparticle formation. Scavenger experiments suggested that O2•– was a major contributor to Au(III) reduction, while •OH and 1O2 appeared to influence the process indirectly. Stability assessments showed that DWAO-AuNPs exhibited the highest colloidal stability, as evidenced by the highest critical coagulation concentration (CCC) values in NaCl, Na2SO4, and CaCl2 solutions. DLVO and E-DLVO analyses suggested that the enhanced stability of DWAO-AuNPs was associated mainly with larger hydrodynamic diameters and stronger repulsive interaction energy. Toxicological assays against Escherichia coli and Staphylococcus aureus revealed that WAO-AuNPs showed comparatively stronger bactericidal effects, primarily due to cell membrane damage and greater intracellular internalization, without the release of Au(III) ions. Conversely, DWAO-AuNPs showed lower antibacterial activity under the tested conditions, possibly due to their larger particle size and reduced cellular uptake and the surface coating-mediated “stealth” effect.