Role of surface defects in colloidal cadmium selenide (CdSe) nanocrystals in the specificity of fluorescence quenching by metal cations.

This study aimed to answer the question as whether crystal defects at the surface of soluble capped CdSe nanocrystals (or quantum dots, QDs) in water colloidal suspension are involved in the mechanism of fluorescence quenching induced by metal cations. Nanocrystals of CdSe were synthesized by an aqueous protocol, varying the ratio between the CdSe precursors and the grafted ligand mercaptosuccinic acid (MSA). Changing the MSA/CdSe ratio during synthesis impacts the crystal nucleation growth, which plays an important role in surface construction of CdSe QDs and changes the surface state. In this way, we could modulate the crystal surface defects of CdSe, as verified by analysis of the individual bands which constitute the emission spectra and are associated with different relaxation processes. We found that the various tested metal cations, which interact in solution with the MSA ligand grafted on the QDs, quench their fluorescence differently, depending on the MSA/CdSe ratio used in synthesis. The crystal defects modulate the excitonic relaxation in CdSe and we demonstrated here that the surface defects intervene in the quenching of QDs induced by the binding of cations.

Use of MPA-capped CdS quantum dots for sensitive detection and quantification of Co2+ ions in aqueous solution.

Water soluble CdS quantum dots (QDs) were synthesized by a simple aqueous chemical route using mercaptopropionic acid (MPA) as a stabilizer. These QDs had a fluorescence emission band maximum at 540 nm with a FWHM ∼130 nm and a quantum yield of ∼12%. Transmission electronic microscopy images were used to determine the QD diameter of 8.9 ±â€¯0.4 nm. From this value we calculated the molecular mass M(QD) = 1.17 × 106 g mol-1 and the extinction coefficient at the band edge (450 nm) ε450 = 4.7 × 106 cm-1 M-1, which allowed to determine the true molar concentration of 17 nM for spectroscopic measurements in solution. The fluorescence intensity of MPA-CdS QDs was quenched only in the presence of Co2+ ions, but not in the presence of thirteen other metal cations. The fluorescence quenching of MPA-CdS QDs appeared proportional to the Co2+ concentration in the range 0.04-2 µM. Based on a fluorescence peak position and a lifetime both independent from Co2+ concentration, the quenching mechanism of MPA-CdS QDs appeared static. Because the strong electronic absorption of Co2+ overlaps the emission of QDs, our results can be explained by Förster energy transfer from QD to the bound Co2+ cations.