CdTe and CdTe/CdS quantum dots (QDs) were prepared in aqueous solutions using thioglycolic acid as a stabilizing agent. The photoluminescence (PL) wavelength of the QDs depended strongly on the size of CdTe cores and the thickness of CdS shells. Being kept at room temperature for 130 days, the PL wavelength of CdTe and CdTe/CdS QDs was red-shifted. However the red-shifted degree of CdTe QDs is larger than that of CdTe/CdS QDs. The size of CdTe QDs and the thickness of CdS play important roles for the red-shift of PL spectra of CdTe/CdS QDs. In contrast, the full width at half maximum of PL spectra of CdTe and CdTe/CdS QDs almost remained unchanged. This is ascribed to the effects of Cd2+ and TGA in solutions on the growth of CdTe and CdTe/CdS QDs. This associated with the variation of surface state of the QDs during store. The results demonstrate that CdTe/CdS core/shell QDs have high stability compared with CdTe QDs due to a CdS shell. 1. Introduction Quantum dots (QDs) have attracted considerable interest in the past two decades because of their excellent properties such as narrower and symmetric emission spectra, broad absorption spectra, and better photostability than traditional fluorescent labels [1–5]. An organic synthetic approach in trioctylphosphine or trioctylphosphine oxide at high temperature has been well developed to prepare highly luminescent II–VI QDs [6]. However, the QDs are insoluble in an aqueous solution and not compatible for biological applications. Compared with organic strategies, an alternative approach to produce water soluble QDs is to directly synthesize QDs in an aqueous solvent. Such approach is less expensive, highly reproducible, less toxic, and more biocompatible [7]. It is easy to control QD size in an aqueous approach because of a low nucleation and growth rate. Early aqueous CdTe QDs with a low stability were prepared in the presence of thioglycolic acid (TGA) as the capping agent by using a reaction between Cd2+ and NaHTe solution [8]. These QDs were easily subjected to the photooxidation and photobleaching when used to labeling in living cells, resulting in significant PL quenching [9]. To improve the photostability, epitaxial growth of an inorganic shell on the surface of CdTe QDs in aqueous solution was explored in a few research groups, similar to growing a shell of ZnS, CdS, or ZnSe on CdSe cores in organic solutions [6, 10–13]. It has been illustrated that epitaxial growth of an inorganic shell with a broader band gap on the surface of luminescent QDs is an ideal approach to improve the properties of QDs
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