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 ± 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.