Gold nanoparticles (AuNPs) have gained attention due to their unique optical and electronic properties. Their exceptional stability, high conductivity, and strong light interaction also make them ideal for sensing applications. In this study, we developed a green microwave-assisted approach for synthesizing uniform (~ 10 nm) AuNPs using Acacia concinna fruit extract as a bio-stabilizing agent. Synthesized AuNPs were employed for a rapid and straightforward AuNPs-based sensing probe for the detection of mercury ions (Hg) in aqueous samples and merbromin medicine, exhibiting fluorescence at 793 nm. The sensing probe was established using a fluorescence quenching approach for selective detection of Hg⁺. Results indicate that the AuNPs demonstrate high selectivity for Hg⁺ compared to other cations, including Pb⁺, Cd⁺, Fe⁺, Ni⁺, Ba⁺, Zn⁺, Cu, Ag and Cr. A strong correlation between Hg⁺ concentration and the observed fluorescence intensity ratio (F₀/F) enables precise quantitative detection, with a limit of detection (LOD) of 0.60 µg/mL in water samples. This study highlights the effectiveness of the fluorescence quenching method as a cost-effective, rapid, and simple tool for Hg⁺ detection in both environmental water samples and medicinal applications.