Mechanism of Strong Luminescence Photoactivation of Citrate-Stabilized Water-Soluble Nanoparticles with CdSe Cores

Abstract

CdSe and CdSe@CdS semiconductor nanocrystals have been synthesized in aqueous solutions, using sodium citrate as a stabilizer. Although initially these quantum dots display photoluminescence with very low quantum yields, upon prolonged illumination with visible light, enhancements up to 5000% have been measured. This leads to aqueous quantum dots with high luminescence, which can have important implications in biological and other applications. A distinct correlation between the photocorrosion process and the photoactivation process is observed. The primary reason for luminescence enhancement is considered to be the smoothing of the CdSe core surface. Importantly, even stronger activation was observed in silica- and CdS-coated nanocolloids where the CdSe core was expected to be shielded from photocorrosion. Preferential adsorption of oxygen molecules in the porous silicate shell accelerates the photocorrosion process. In CdS-coated particles, incomplete coating of the original particles is postulated, which is accompanied by the reforming of the CdS coat because of ionic diffusion at the interface on the newly opening areas with smoother surfaces.

Entrapment in phospholipid vesicles quenches photoactivity of quantum dots

“…52 Furthermore, photo-enhancement of QD fluorescence was observed both in air and in nitrogen saturated cells 53 and solutions. 54 This presumably can be attributed to residual oxygen adsorbed on the QD surface 54 and the oxygen passivation effect. 55 Passivation is the process of making a material passive in relation to another material.…”

Determination of Silver Ion with Cadmium Sulfide Quantum Dots Modified by Bismuthiol II as Fluorescence Probe

“…It may adsorb on the surface of CdS particle by Langmuir adsorption 35 . The adsorption makes the surface of CdS particle passivate and improves the stability and luminescent efficiency of the particle 36,37 . At a monomolecular layer, each particle can be adsorbed by more than one molecule of Bismuthiol II.…”

Nonlinear optical properties of CdTe nanocrystals synthesized by a green room temperature solution method

“…This capping agent provides the nanoparticles with sufficient electrostatic stabilization to avoid agglomeration for long periods of time. It also plays a key role in the growth process and the photoluminescence quantum yields of aqueous CdTe nanocrystals [12,13]. In this work, a green room temperature method using the sodium citrate as a capping agent for environmentally friendly synthesis of CdTe nanocrystals is introduced.…”

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These materials exhibit quantum confinement effects when their sizes are comparable with the diameter of the bulk exciton, leading to split the continuum of electronic energy levels into discrete states with an effective band gap blue shifted from that of the bulk [7][8][9].There are two different synthetic methods for producing CdTe nanostructures, the organometallic route and synthesis in an aqueous solution [10][11][12][13]. The organometallic method usually requires hard conditions such as high temperature, toxic reagents and solvents, and a rigorous process for protecting synthesized nanostructures from oxidation, while the aqueous synthesis method requires mild conditions and is a simpler one [14,15].

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“…There are two different synthetic methods for producing CdTe nanostructures, the organometallic route and synthesis in an aqueous solution [10][11][12][13]. The organometallic method usually requires hard conditions such as high temperature, toxic reagents and solvents, and a rigorous process for protecting synthesized nanostructures from oxidation, while the aqueous synthesis method requires mild conditions and is a simpler one [14,15].…”