Semiconductor (CdSe) QDs 1-10 nm in diameter have been synthesized [27, 28]. The methods for the synthesis of the QDs have been continuously improved . Water-soluble QDs are now commercially available and have been prepared by capping CdSe QDs with ZnS and silica-coating the CdSe/ZnS [10, 11, 30-32]. These QDs offer narrow, tunable, symmetric fluorescence emission spectra and are relatively photochemically stable. The emission wavelengths of the QDs show size dependence [27-34]. The sizes of these QDs are comparable with the sizes of proteins. In addition, the fluorescence emission of the QDs allows them to be visualized inside live cells more easily and clearly than that of the surface plasmon resonance of Au and Ag nanopar-ticles. Nevertheless, QDs are not as photochemically stable as noble metal nanoparticles. It is evident that the emission intensity of the QDs decreases substantially after labeling with biological molecules (e.g., antibodies) . This affects the sensitivity of the QDs for bioanalysis. Furthermore, QDs suffer an on-off blinking phenomenon that may an obstacle for their use as effective probes for real-time dynamics measurements in live cells. Moreover, the toxicity of the QDs may become a major obstacle to be studied and overcome before they become popular nanoprobes for the study of live-cell dynamics and disease diagnosis in humans.
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