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.

This paper describes the synthesis of highly water-soluble and fluorescent ZnSe(S)-alloyed quantum dots (QDs). We used zinc perchlorate hexahydrate, sodium hydrogen selenide as precursors and mercaptopropionic acid as stabilizer to synthesize ZnSe QDs in aqueous solution at 160 °C for 9 h. The as-prepared ZnSe QDs possess a quantum yield (QY) of 8.1% and high trapped emission. After UV irradiation using a 100 W Hg–Xe lamp for 0.5 h, ZnSe(S) QDs having a QY of 19.0% are formed from ZnSe QDs. However, aggregation of ZnSe(S) QDs under longer UV irradiation (> 0.5 h) takes place, leading to instability and irreproducibility. To overcome this, additional thiol compounds (mercaptopropionic acid, mercaptosuccinic acid, 11-mercaptoundecanoic acid, and thioglycolic acid) were separately added to ZnSe QD solutions during UV irradiation. UV irradiation and oxygen accelerate the release of S²⁻ from the thiol compounds, leading to the formation of ZnSe(S) QDs. Among the thiol compounds, mercaptosuccinic acid is the most suitable in terms of stability and photoluminescence intensity. We suggest that the size and functional group of the thiol compounds play an important role in determining the optical properties and stability of ZnSe(S) QDs. The as-prepared ZnSe(S) QDs fluoresce strongly (QY up to 44.0%) at 407 nm with a narrow bandwidth (W_1/2 < 25 nm) when excited at 325 nm.