Improvement of fluorescence properties by surface modification of CdS-ZnS quantum dots by thiol compounds and its application as a sensitive fluorescence probe for copper ion detection

The capped CdS-ZnS quantum dots (QDs) were synthesized with various thiol capping agents of glycolic acid (TGA), mercaptosuccinic acid (MSA), and L-cysteine (LCY) and used as fluorescence probe for determination of Cu (II) ions. The method of two-level three-factor full-factorial experiment design was used to achieve the best optical fluorescence emission. Results revealed that Cu (II) ions can effectively quench the emission of QDs, and the fluorescence intensity is linearly decreased with increasing Cu (II) ion concentration. The limit of detection for CdS-ZnS@ QDs capped with TGA, MSA, and LCY was obtained at 1.15 × 10⁻⁷, 1.32 × 10⁻⁷, and 2.19 × 10⁻⁷ mol L⁻¹, respectively, with linear dynamic range of 3.13 × 10⁻⁶ to 1.41 × 10⁻⁴ mol L⁻¹. Luminescence quantum yields of CdS-ZnS@LCY, CdS-ZnS@MSA, and CdS-ZnS@TGA were obtained at 4.17, 1.92, and 2.47, respectively. Results indicated that no significant quenching occurred in the presence of the other metal ions. The binding constant (K_b) of capped CdS-ZnS@ QDs with Cu²⁺ and the other metal ions was also investigated and discussed. The K_b value for Cu²⁺ was obtained considerably more than that the other ions. This work presents a new and sensitive method for determination of Cu²⁺ ion.