This research focuses on designing and developing a new chemo-sensor tailored explicitly for detecting Hg(II) ions, one of the most hazardous heavy metal ions. The sensor probe, (2-((4-chlorophenyl)amino)-N-(1,3-dioxoisoindolin-2-yl)acetamide (CADA), was successfully synthesized and demonstrated a "turn-off" fluorescence response upon interaction with Hg(II) ions, providing extreme sensitivity. The Isoindole structure of the CADA sensor plays a crucial role in this quenching mechanism. The CADA sensor exhibits unique optical properties, including a significant Stokes shift of approximately 135 nm, which enhances its detection capabilities. Moreover, the sensor demonstrates remarkable selectivity for Hg(II) ions over other cations, making it highly effective for targeted detection. The studied mechanism revealed that the luminescence quenching observed in the CADA-Hg(II) complex is primarily due to an internal charge transfer (ICT) mechanism. This ICT-dependent selectivity is a key factor in the sensor's ability to precisely sense Hg(II) ions. Experimental investigations were carried out in a 1:9 (v/v) ethanol-HEPES buffer solution (20 mM, pH = 7.4) to study the luminescence and UV-Vis characteristics of the CADA probe in the presence of Hg(II) ions. The potential interference from different cations was also examined using a mixture having various metal cations, confirming the sensor's high selectivity. The CADA molecule demonstrated outstanding sensitivity with a low limit of detection (LOD) of 1.025 x10-7 M, enabling the detection of Hg(II) ions within a narrow dynamic range of 3.416 x 10-7 M to 18 µM. Additionally, the sensor's reversibility was achieved by using ethylenediaminetetraacetic acid (EDTA) as a potent chelating agent, which effectively restored the sensor's fluorescence. This reversibility adds to the CADA sensor's practicality for repeated use in environmental and biological applications. Overall, the CADA sensor's distinctive properties, including high selectivity, sensitivity, and reversibility, make it a promising tool for detecting Hg(II) ions in various real-world scenarios.