| Summary: | The detection of heavy metals in aqueous solutions has always attracted much attention from all over the world. A fluorescent probe of CdSe/CdS core-shell quantum dots (QDs) was designed to detect trace Cd<sup>2+</sup> in aqueous solutions using the OFF–ON mode rapidly and efficiently, likely based on adsorption and desorption reactions between ethylenediaminetetraacetic acid disodium salt (EDTA) and CdSe/CdS QDs. In the OFF mode, the optical shielding function of EDTA results in fluorescence quenching owing to the strong adsorption ability of EDTA with Cd<sup>2+</sup> on the sites of CdSe/CdS QDs surface. In the ON mode, the introduction of Cd<sup>2+</sup> promotes the desorption of EDTA from the EDTA-CdSe/CdS QDs and restores the fluorescence intensity. There were two linear response ranges which were 0.1–20 µmol/L and 20–90 µmol/L for the EDTA-CdSe/CdS system to detect Cd<sup>2+</sup>. The detection limit was 6 nmol/L, and the standard deviation was below 4% for the detection of Cd<sup>2+</sup> concentration in tap water.
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