Advances in Coordination Chemistry–Based Sensors for Monitoring Toxic Metal Pollutants in Aquatic Environments

Toxic metal contamination of surface water, groundwater and wastewater threatens ecosystems and the communities that depend on them. Conventional methods such as atomic absorption spectrometry and inductively coupled plasma-based techniques are reliable but need laboratory facilities, which restricts routine monitoring in remote and resource-limited settings. Coordination chemistry links metal-ion recognition to measurable optical or electrochemical responses. This review summarises the principles, major platforms and environmental applications of coordination-based sensors for lead(II), mercury(II), cadmium(II), copper(II) and chromium species, including fluorescent and colorimetric probes, coordination polymers, metal–organic frameworks and electrochemical systems. Representative studies are compared using their reported detection limits, working ranges, selectivity and real-water evidence, and the detection limits are set against WHO drinking-water guideline values. The comparison shows that a low detection limit alone does not establish practical performance: matrix interference, chemical speciation, pH dependence, material stability and validation against reference methods remain decisive. Future development should prioritise water-compatible receptors, realistic sample validation, portable instrumentation and responsible management of sensing materials.