Citric Acid Capped CdS Quantum Dots for Fluorescence Detection of Copper Ions (II) in Aqueous Solution

Citric acid capped CdS quantum dots (CA-CdS QDs), a new assembled fluorescent probe for copper ions (Cu²⁺), was synthesized successfully by a simple hydrothermal method. In this work, the fluorescence sensor for the detection of heavy and transition metal (HTM) ions has been extensively studied in aqueous solution. The results of the present study indicate that the obtained CA-CdS QDs could detect Cu²⁺ with high sensitivity and selectivity. It found that the existence of Cu²⁺ has a significant fluorescence quenching with a large red shifted (from greenish-yellow to yellowish-orange), but not in the presence of 17 other HTM ions. As a result, Cu₂S, the energy level below the CdS conduction band, could be formed at the surface of the CA-CdS QDs and leads to the quenching of fluorescence of CA-CdS QDs. Under optimal conditions, the copper ions detection range using the synthesized fluorescence sensor was 1.0 × 10^‒8 M to 5.0 × 10^‒5 M and the limit of detection (LOD) is 9.2 × 10^‒9 M. Besides, the as-synthesized CA-CdS QDs sensor exhibited good selectivity toward Cu²⁺ relative to other common metal ions. Thus, the CA-CdS QDs has potential applications for detecting Cu²⁺ in real water samples.