Mercury (Hg), a notorious heavy metal with detrimental impacts on human health and the environment, necessitates the development of precise measurement methods. This study introduces an expeditious and straightforward photochemical approach to synthesize thioglycolic acid (TGA)-stabilized CdTe/CdS/ZnS core/multi-shell quantum dots (QDs). The synthesized CdTe/CdS/ZnS QDs were comprehensively characterized using fluorescence spectroscopy, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), Field Emission Scanning Electron Microscopy (FESEM), and X-Ray diffraction (XRD). XRD and EDS results confirmed the successful formation of CdTe/CdS/ZnS structure. Also, FESEM and TEM results showed that CdTe/CdS/ZnS QDs were spherical. Results showed that synthesized Exhibiting vibrant green fluorescence and notable quenching in the presence of Hg ions, these QDs emerge as promising candidates for fabricating a fluorescent sensor. The proposed sensor demonstrates notable sensitivity to Hg, featuring a detection limit of 16.32 nM and a linear range from 20 nM to 70 nM. The sensor's selectivity was confirmed by analyzing various anions and cations. Moreover, when tested with tap water, river water, and agricultural samples, the sensor exhibited reliable performance, validated by Inductively Coupled Plasma (ICP) analysis. Additionally, CdTe/CdS/ZnS QDs immobilized on micro pads proved effective for on-site water sample analysis, presenting a versatile solution for environmental monitoring.