A DNA–Au NP probe for sensing Hg²⁺ using the formation of DNA–Hg²⁺ complexes through thymidine (T)–Hg²⁺–T coordination to control the negative charge density of the DNA strands—thereby varying their structures—adsorbed onto Au NPs.

Monitoring the levels of potentially toxic metal (PTM) ions (e.g., Hg²⁺, Pb²⁺, Cu²⁺) in aquatic ecosystems is important because these ions can have severe effects on human health and the environment. Gold (Au) nanomaterials are attractive sensing materials because of their unique size- and shape-dependent optical properties. This review focuses on optical assays for Hg²⁺, Pb²⁺, and Cu²⁺ ions using functionalized Au nanomaterials. The syntheses of functionalized Au nanomaterials are discussed. We briefly review sensing approaches based on changes in absorbance resulting from metal ion-induced aggregation of Au nanoparticles (NPs) or direct deposition of metal ions onto Au NPs. The super-quenching properties of Au NPs allow them to be employed in ‘turn on’ and ‘turn off’ fluorescence approaches for the sensitive and selective detection of Hg²⁺, Pb²⁺, and Cu²⁺ ions. We highlight approaches based on fluorescence quenching through analyte-induced aggregation or the formation of metallophilic complexes of Au nanodots (NDs). We discuss the roles of several factors affecting the selectivity and sensitivity of the nanosensors toward the analytes: the size of the Au nanomaterial, the length and sequence of the DNA or the nature of the thiol, the surface density of the recognition ligand, and the ionic strength and pH of the buffer solution. In addition, we emphasize the potential of using new nanomaterials (e.g., fluorescent silver nanoclusters) for the detection of PTM ions.