Multiplex Detection of Cd²⁺, Hg²⁺, Glutathione, and Ascorbic Acid Using NaYF₄:Yb,Er@NaYF₄ Core–Shell Upconversion Nanoparticles and Trithiocyanuric Acid-Modified Gold Nanoparticles

High-efficiency detection of cadmium ions (Cd2+) and mercury ions (Hg2+) is crucial due to their harmful effects on the environment and human health. Here, we developed a ratiometric upconversion fluorescence (UCF)-based nanosensor by incorporating lanthanide-doped upconversion nanoparticles (UCNPs) and gold nanoparticles modified with trithiocyanuric acid (TMT-AuNPs) as specific recognition elements for the highly sensitive and selective real-time detection of Cd2+ and Hg2+. The developed nanosensor exhibited selective absorbance enhancement and UCF quenching behavior with the addition of Cd2+ and Hg2+ based on the inner filter effect (IFE). The sensitivity of the nanosensor was significantly enhanced by converting the change in absorbance into an exponential variation in UCF intensity. Therefore, the quantitative detection of Cd2+ and Hg2+ was achieved by measuring the UCF ratio I654/I540. In order to distinguish between Cd2+ and Hg2+, two biomolecules, glutathione (GSH) and ascorbic acid (AA), were used based on their different interactions with Cd2+ and Hg2+. GSH formed a specific tetrahedral complex (GSH)4Cd with Cd2+, thereby preventing Cd2+ from inducing changes in UCF intensity. In contrast, AA specifically reduced Hg2+, thereby preventing Hg2+-induced UCF changes. Thus, GSH and AA could also be detected. The limits of detection (LODs) of Cd2+, Hg2+, GSH, and AA were 2.8, 5.2, 5.1, and 7.2 nM, respectively. The designed detection platform provides a sensitive and straightforward sensing strategy for the multiplex detection of heavy metal ions and biological small molecules.