Functionalized CdS-ZnS Quantum Dots With Thiolate Group Capping Agents and Their Application as Fluorescence Probe for Measurement of Europium Ion

CdS-ZnS core-shell quantum dots (QDs) were synthesized with three different types of capping agents. To achieve the best fluorescence emission efficiency the factors of pH, temperature, and the ratio of CdS-ZnS QDs to the capping agent, were investigated via a full factorial experimental design. The best fluorescence intensity was obtained at pH 8, temperature 80°C, and a ratio of CdS-ZnS to the capping agent of 1:1.5. CdS-ZnS QDs were fabricated with different capping agents: thioglycolic acid, mercaptosuccinic acid, and L-cysteine. The interaction of the synthesized QDs with metals ions Gd³⁺, La³⁺, Th⁴⁺, Pr³⁺, Eu³⁺, and Ce⁴⁺ was investigated. The results showed that europium (III) ions significantly reduced the fluorescence emission intensity of QDs. Additionally, the fluorescence intensity decreases linearly with increasing europium (III) ion concentration. The detection limits for measuring europium (III) ions were obtained 1.56 × 10⁻⁶ a linear dynamic range of 7.8 × 10⁻⁶ to 5.0 × 10⁻⁴ M. Also, the fluorescence quantum yields for CdS-ZnS@LCY, CdS-ZnS@MSA and CdS-ZnS@TGA were obtained 4.23, 2.32 and 2.64, respectively. The binding constants (K_b) of CdS-ZnS@TGA, CdS-ZnS@MSA, and CdS-ZnS@LCY QDs with Eu²⁺ were investigated and results were found as 194, 178, and 929, respectively. The Stern–Volmer constant and R² were investigated for the above ions with QDs.