Green synthesis of selenium nanoparticles using Bacillus sp. strain STG-83: optimization, characterization, and prospects for cancer radiosensitization.

Selenium nanoparticles (SeNPs) have gained increasing attention due to their favorable biological properties and potential applications in cancer research. In this study, the ability of Bacillus sp. strain STG-83 to biosynthesize SeNPs was systematically investigated. This study comprehensively investigated how Bacillus sp. strain STG-83 can biosynthesize SeNPs. Response surface methodology (RSM) was utilized to optimize the bioreduction of selenite and to identify the key process parameters that play a significant role. The developed quadratic model showed a strong correlation with experimental data (R²=0.92). Statistical analysis demonstrated that time and selenium concentration significantly affected selenite reduction efficiency (P < 0.05), whereas bacterial inoculum percentage were not significant (P > 0.05). Increasing selenium concentration from 0.5 to 25 mM reduced the bioreduction efficiency from 100% to 29.37%, while extending time from 8 to 96 h increased efficiency from 42.03% to 61.51%. The biosynthesized SeNPs were characterized using UV-Vis's spectroscopy, FTIR, EDX, SEM, TEM, and XRD analyses. The nanoparticles were predominantly spherical, with sizes ranging from 80 to 140 nm, and were coated with a bioorganic surface layer. Biological evaluation revealed that SeNPs induced dose-dependent cytotoxicity in U-87 line while exerting lower toxicity toward normal fibroblast cells. Flow cytometry analysis further demonstrated a significant increase in intracellular reactive oxygen species (ROS) levels following SeNP exposure, suggesting that oxidative stress plays a central role in the observed anticancer effects. The ROS generation triggered by SeNPs suggests they might serve as effective radiosensitizing agents. Future studies that combine radiation and in vivo approaches should confirm this potential.