<p>In this study, a microwave-assisted green synthesis method, known for being environmentally friendly, efficient, and cost-effective was utilized to synthesize selenium nanoparticles (SeNPs). RL extract was used for the first time as both a capping and reducing agent for synthesizing SeNPs from its precursor selenium salt due to its rich content of flavonoids, phenolic compounds, and antioxidants, which facilitate electron donation and stabilization of SeNPs. The time duration was optimized (30 to 50&#xa0;sec) for the formation of RL@SeNPs and then characterized using different techniques. Ultra-violet visible (UV–visible) spectroscopy revealed an absorption band at 264&#xa0;nm, indicating the successful formation of RL@SeNPs. Fourier transforms infrared (FTIR) spectroscopy showed the involvement of hydroxyl and amine functional groups from the RL extract in the reduction and capping of SeNPs. Scanning electron microscopy (SEM) indicated that nanoparticles were spherical and uniformally distibuted, while energy dispersive x-ray spectroscopy (EDX) confirmed the presence of selenium along with oxygen, sodium, and potassium. X-ray diffraction (XRD) analysis revealed that synthesized RL@SeNPs were crystalline with an average particle of 26&#xa0;nm. The synthesized RL@SeNPs were confirmed to be a highly sensitive, exceptionally selective, cost-effective, environmentally friendly, and swiftly responsive colorimetric sensor for Hg²⁺ detection, indicated by a change in the color of the solution from red to transparent. The developed sensor exhibited a linear dynamic range with an R2 value of 0.9985, operating in the 0.25–60&#xa0;μM range. The limit of detection (LOD) and limit of quantitation (LOQ) were determined to be 0.03&#xa0;μM and 0.11&#xa0;μM, respectively. Metal cations such as Be²⁺, Ca²⁺, Cr²⁺, Co²⁺, As³⁺, Mg²⁺, and Ni²⁺ showed minor interaction with the sensor. The sensor that was built was effectively utilized to identify low concentrations of Hg²⁺ in actual water samples.</p>