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

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.