The progress of modern society is closely tied to expanding cities, industrial growth, transportation networks, energy production, and intensive agriculture. While these advances improve quality of life, they also increase the release of harmful contaminants into air, water, and soil. Mercury is especially concerning because it is toxic, and capable of accumulating in aquatic systems, where it can threaten environmental quality and human health. This makes the development of simple, sensitive, and selective materials for both mercury detection and removal an important goal in water-quality monitoring and remediation. Nanozymes are promising for this purpose because they combine enzyme-like activity with improved stability, tunable surface chemistry, ease of synthesis, and multifunctional performance. In this study, a fluorescent nanozyme was synthesized using a one-pot hydrothermal method with CuCl2, polyethyleneimine, and polyethylene glycol for Hg2+ detection and removal from aqueous environments. The nanozyme was characterized using multiple analytical techniques, including UV–vis spectroscopy, fluorescence spectroscopy, zeta potential analysis, Fourier-transform infrared spectroscopy, transmission electron microscopy, and X-ray diffraction. The material showed mercury dependent fluorescence quenching with good selectivity over common interfering ions, demonstrating its potential for sensitive Hg2+ detection. Batch adsorption studies conducted in Hg (II)-spiked deionized water, Red River water, and secondary-treated wastewater effluent showed strong Hg2+ removal performance, with 80% removal achieved after 24 h. Overall, this work presents a dual-function fluorescent nanozyme platform for sensitive and selective Hg2+ detection and efficient removal, with potential application in water-quality monitoring and heavy-metal remediation.