Reversible Modification of CdSe–CdS/ZnS Quantum Dot Fluorescence by Surrounding Ca²⁺ Ions

It has been known for a long time that the fluorescence intensity of colloidal quantum dots (QDs) becomes modified when free ions are added to the QD solution. The consequences of removing free ions from the QD solution, however, have not been closely investigated. In this work we studied fluorescence from 3-mercaptopropionic acid (3-MPA) coated CdSe–CdS/ZnS core–multishell QDs when free Ca 2+ ions were added to and subsequently removed from the QD solution. It was found that QD fluorescence intensity was reduced when Ca 2+ ions were added to the QD solution, while the wavelength of the QD fluorescence peak remained unchanged. QD fluorescence recovered when the concentration of free Ca 2+ ions in the QD solution was reduced by adding Ca 2+ chelator (ethylene glycol tetraacetic acid, EGTA). It was further observed that the time of single QD fluorescence at on-state and QD fluorescence lifetimes were also reduced after adding Ca 2+ and then recovered when EGTA was added. Theoretical study shows that a free Ca 2+ ion can attach stably to the system of [QD + surface ligand], attract the photoexcited electron, and repel the photoexcited hole inside the QD core, leading to the reduction of the radiative recombination between the electron and hole, thereafter decreasing the QD fluorescence intensity, on-state time, and fluorescence lifetimes, as observed experimentally. To the best of our knowledge, this is a first study to show that the changes of QD optical properties are reversible under the influence of Ca 2+ ions. We further estimated the equilibrium association constant p K a of our QDs with Ca 2+, which is much larger than QDs with Mg 2+, Na +, and K +, indicating the feasibility of developing a QD-based Ca 2+ sensor.