Smartphone-assisted dual-channel discriminative detection of Hg(II) and Cu(II) ions with a simple, unique, readily available probe

Heavy metal ions are well-known atmospheric and environmental pollutants and cause long-term damage to humans. Mercury is one of the most toxic and hazardous one that induce critical motor disorders and a variety of diseases. Likewise, excess copper can lead to Alzheimer's and Parkinson's disease, as well as various other health conditions. Therefore, selective and sensitive methods for determining and visualizing mercury and copper remain necessary. In our study, we developed the first-ever optical-electrochemical method of discriminatively detecting Hg2+ and Cu2+ ions using 1-thioglycerol (TG)-modified gold nanoparticles (AuNPs) and by altering the solution's medium. Adding Hg2+ ions to AuNP-TG in ultrapure water changed the original burgundy color blue, the initial absorbance value (i.e., 520 nm) decreased, and a new absorbance peak appeared at lambda max = 670 nm. In a phosphate-buffered solution, Cu2+ ions issued a similar response. However, neither Hg2+ nor Cu2+ ions exhibited cross-reactivity under those sensing conditions. The optical mechanism for detecting both metal ions seems to have relied on the destabilization and aggregation of AuNPs. On the one hand, the electrochemical detection of Cu2+ stemmed from the reduction of the complex between Cu2+ and TG's neighboring hydroxyl groups in the probe, whereas the electrochemical detection of Hg2+ stemmed from the fact that Hg2+ broke the AuNP-S bond, after which the AuNPs clustered and significantly increased the electrode's electron transfer rate. Last, because the method involves using a smartphone to read signals without requiring professional equipment, it considerably decreases the cost of detection and shows promise for the rapid on-site detection of Hg2+ and Cu2+ ions.