Photoactivation of CdSe/ZnS Quantum Dots Embedded in Silica Colloids

Abstract

A study of the influence of the local environment on the light-induced luminescence enhancement of CdSe/ZnS quantum dots (QD) embedded in silica colloids that are dispersed in various solvents is presented. The photoluminescence of the embedded QD is enhanced up to a factor of ten upon photoactivation by ultraviolet or visible light. This enhancement is strongly dependent on the local environment. The thickness-dependent permeability of the silica shell covering the QD controls the influence of the solvent on the QD. If foreign ions are present the activation state is stabilized after termination of the activation, whereas in their absence the process is partially reversible. A new qualitative model for the photoactivation of QD in various environments is developed. It comprises light-induced passivation and subsequent oxidation processes. The embedded QD also retain their fluorescence quantum yield inside living cells. Moreover, they can be activated for many hours in living cells by laser radiation in the visible regime.

Entrapment in phospholipid vesicles quenches photoactivity of quantum dots

“…40 Such photo-activation (photo-enhancement) of fluorescence has been observed in solutions of free QDs after exposure to sunlight, 41 UV radiation, 42 and blue light 43 or QDs embedded in silica colloids under UV radiation. 44 Normally, a decrease in QD fluorescence intensity occurs due to oxidative decay of the CdSe lattice in the presence of oxygen, while in nitrogen, photobleaching is absent. 45 Dembski et al 44 proposed that UV radiation generates excitons, which form O 2 − from the oxygen adsorbed on the surface of QDs: when irradiation stops, O 2 − dissociates, diminishing the oxygen passivation effect and thus decreasing QD fluorescence.…”

“…44 Normally, a decrease in QD fluorescence intensity occurs due to oxidative decay of the CdSe lattice in the presence of oxygen, while in nitrogen, photobleaching is absent. 45 Dembski et al 44 proposed that UV radiation generates excitons, which form O 2 − from the oxygen adsorbed on the surface of QDs: when irradiation stops, O 2 − dissociates, diminishing the oxygen passivation effect and thus decreasing QD fluorescence. However, irradiation for a longer period will lead to oxidation of the QD surface and desorption of SeO 2 and SO 4 − , 44 accompanied by the release of Cd 2+ and Zn 2+ .…”

“…45 Dembski et al 44 proposed that UV radiation generates excitons, which form O 2 − from the oxygen adsorbed on the surface of QDs: when irradiation stops, O 2 − dissociates, diminishing the oxygen passivation effect and thus decreasing QD fluorescence. However, irradiation for a longer period will lead to oxidation of the QD surface and desorption of SeO 2 and SO 4 − , 44 accompanied by the release of Cd 2+ and Zn 2+ . 35 This will smooth the QD surface, leading to elimination of surface defects, increase of radiative recombination, reduction of particle size, and permanent enhancement of fluorescence.…”

“…35 This will smooth the QD surface, leading to elimination of surface defects, increase of radiative recombination, reduction of particle size, and permanent enhancement of fluorescence. 42,44 Overall, it is reasonable to assume that fluorescence properties of QDs and efficiency of radiative electron-hole recombination depend on the QD surface and its environment. 46,47 Events responsible for such photo-reversible fluorescence of QDs have not yet been completely understood.…”

Polymersomes containing quantum dots for cellular imaging

“…Similarly, increased photo-enhancement of QDs embedded in silica colloids have been reported in living cells, due to partial degradation mediated by oxidation with minimal aggregation. 46 Results suggested that Ps protect QDs from biological interactions in cell compartments, thus improving the quality of cellular imaging.…”