CdSe/ZnS quantum dots capped with oleic acid and L-glutathione: Structural properties and application in detection of Hg2+

Two structures of CdSe/ZnS quantum dots (QDs), with different solubility properties, were prepared, characterized, and applied as fluorescent-based probes to detect Hg2+ ions. From oleic acid-capped CdSe/ZnS QDs (CdSe/ZnS/OA), glutathione-capped CdSe/ZnS QDs (CdSe/ZnS/GSH) were obtained by a ligand exchange reaction upon microwave radiation. X-ray diffraction (XRD), Fourier-Transform Infrared (FT-IR), and X-ray Photoelectron Spectroscopy (XPS) were employed to determine crystallinity, structural characteristics, and chemical composition according to the capping agent. Synthesis methodology produced nanocrystals with cubic zinc blende structure. The coordination mode between the carboxylate group of oleic acid (OA) and the thiol group of glutathione (GSH), and the Zn2+ ions of QDs surface was discussed. Ultraviolet-visible (UV–Vis) spectroscopy and photoluminescence (PL) measurements characterized the optical properties of QDs. The replacement of OA by GSH ligands on QDs surface produced a red-shift of the first absorption maximum and the emission band. Photoluminiscence quantum yield (PL QY) improved from 20% (for CdSe/ZnS/OA) to 26% (for CdSe/ZnS/GSH). The fluorescence (FL) quenching of Hg2+ induction was studied considering the solubility properties of the two structures of QDs. Measurements for hydrophobic CdSe/ZnS/OA QDs were realized in a heterogeneous medium composed of toluene and water. While for CdSe/ZnS/GSH, experiments were carried out in an aqueous solution. Hg2+ ions quenched 100% fluorescence of both structures of QDs, displaying high selectivity, rapid response time. The limit of detection (LOD) for CdSe/ZnS/OA was 74.8 nM and for CdSe/ZnS/GSH was 54.8 nM. Results indicate that the two CdSe/ZnS core/shell QDs are candidates for Hg2+ and other metal ions sensing.