Introduction: Organic selenium plays vital roles in antioxidant defense, immune regulation, thyroid balance, and reproductive health. Selenium nanoparticles (SeNPs) have emerged as safer, more effective alternatives to bulk selenium due to their reduced toxicity and enhanced bioactivity arising from a high surface-to-volume ratio. While most studies focus on conventional synthesis, this work introduces a novel approach by applying ultraviolet (UV) irradiation, UVC (254 nm), and UVA (365 nm) to investigate how photonic treatment modulates SeNP characteristics.

Methods: SeNPs were synthesized by reducing 400 µL of 0.001 M selenomethionine with 30 mL of 0.001 M NaBH₄ under ice-bath stirring (500 rpm, 20 min), followed by dropwise addition (~1 drop/s). The suspensions were divided into three groups: non-irradiated control (S1), UVC-exposed (S2), and UVA-exposed (S3), each irradiated for 20 min at a distance of 10–15 cm. Characterisation was performed immediately using UV–Vis spectroscopy (200–800 nm, Thermo INSIGHT™ 2 software, Thermo Fisher Scientific, USA), diffuse reflectance analysis, and dynamic light scattering (DLS) with zeta potential (25 °C, Zetasizer Nano ZS).

Results: UV–Vis spectra showed broad SeNP absorption between 220 and 400 nm. Baseline absorbance increased from ~0.10 (control) to ~0.28 A.U. after UVC exposure and decreased to ~0.05 A.U. after UVA. Diffuse reflectance declined from ~95–100% in controls to ~60–70% with UVC and ~80–85% with UVA, confirming wavelength-dependent optical modulation. DLS revealed particle sizes of 40–90 nm for S1, 50–100 nm for S2 (mild aggregation), and 45–110 nm for S3 (greater polydispersity).

Conclusion: UV irradiation significantly modulated the optical and size properties of SeNPs without compromising colloidal stability. UVC exposure promoted mild aggregation, whereas UVA increased polydispersity, highlighting photonic modulation as a tunable strategy to optimise SeNPs for biomedical applications.