Luminescent semiconductor nanocrystals: Intermittent behavior and use as fluorescent biological probes

The temperature and intensity dependence of the intermittency of single CdSe-CdS nanocrystals has been studied using single molecule detection techniques. The temperature dependence of the intermittency revealed a thermally activated component with a barrier of 8 meV which was inconsistent with the simple Auger ionization model previously proposed in the literature. The thermally activated component provides evidence for another mechanism of nanocrystal darkening in this system. Room temperature intermittency was also studied in composite nanocrystals made of a CdSe core, a CdS shell and a further ZnS shell. The on times were found to increase with increasing shell growth while the off times were found to decrease, indicating more effective insulation of these cores by the shell materials.; These luminescent semiconductor nanocrystals were prepared for use as fluorescent biological probes by coating with a polymerized shell of silica and attaching a number of biological molecules to the surface, such as charged groups, biotin, streptavidin, and antibodies. The nanocrystals have a broad excitation spectrum, a narrow emission spectrum, and high photochemical stability These advantages have been exploited in a number of biological experiments. Two color labeling of fixed cells, hybridization detection on combed DNA fibers, and flow cytometry experiments on both latex beads and cells have been carried out using these nanocrystal based fluorescent probes. In these experiments. the nanocrystals have performed comparably to dye molecules. These nanocrystal probes are complementary and may be superior to existing fluorophores in some cases.