Phytochemical-Mediated Redox Synthesis of Selenium Nanoparticles Under Sunlight: Mechanistic and Bioactivity Insights

Selenium nanoparticles are recognized for their significant biomedical potential; however, conventional synthesis methods often rely on toxic reagents and energy- intensive processes. Green, plant-mediated synthesis offers a sustainable alternative, although controlling particle size and maintaining high colloidal stability remain challenging. This study aimed to establish a green, sunlight-assisted biosynthesis route for SeNPs using dual extracts of Achillea millefolium and Echinops spp., optimized through response surface methodology.

Synthesis conditions were optimized using a central composite design. The optimized SeNPs were characterized by UV-Vis spectroscopy, DLS, FT-IR, XRD, and SEM-EDX analyses. They were further evaluated for antioxidant and antibacterial activities.

The quadratic RSM models demonstrated good predictive capability (R² = 0.97-0.99). Optimal conditions (1000 μL extract, 30 mL Na₂SeO₃ and 5 minutes of sunlight exposure) produced spherical SeNPs with a mean hydrodynamic diameter of 40.6 nm (range: 38-80 nm), a PDI of 0.36-0.55 and a strongly negative zeta potential (-31.4 mV). A characteristic absorption peak at 550 nm confirmed nanoparticle formation. The synthesized SeNPs exhibited considerable antioxidant activity (DPPH inhibition: 72% at 100 μg/mL; IC₅₀ = 71.9 μg/mL) as well as antibacterial activity against Staphylococcus aureus (MIC = 62.5 μg/mL) and Escherichia coli (MIC = 125 μg/mL).

These findings demonstrate that the dual-plant, sunlight-assisted synthesis strategy provides an effective green approach to producing stable SeNPs with favorable physico-chemical characteristics and promising biological activities.

This dual-extract, sunlight-assisted approach provides a rapid (5 min), environmentally benign, and cost-effective method for producing stable and biofunctional SeNPs with promising antioxidant and antibacterial properties.