target was stained with QD-streptavidin, the fluorescent signal was
very stable against photobleaching (Fig. 6A). In contrast, Alexa 488
was bleached very quickly.
We compared further the photostability of QD 608–streptavidin
and Alexa 488–streptavidin with specimens mounted with glycerol or
the antifade mounting medium Vectashield (Fig. 6B). Without
antifade medium, the fluorescence intensity of Alexa 488 decreased to
50% of the initial intensity at 10 s, and to 10% at 60 s. With the protection of antifade reagents, Alexa 488 retained ∼of the initial
intensity at 60 s and 55% at the end of 3 min of illumination. For QD
608–streptavidin, the intensity was 104% of the initial intensity for
glycerol-mounted specimens and 97% for antifade medium–mounted
specimens at the end of the illumination. The results indicate that
even with anti-bleaching protection, Alexa 488–streptavidin is still
much less photostable than the QD-streptavidin. Although antifade
mounting medium can provide conventional organic dyes some protection against photobleaching, for some applications, such as live cell
imaging, antifade medium cannot be used. Therefore photostable QD
probes will be particularly valuable for quantitative fluorescent molecular detection and live cell imaging.
With the completion of the Human Genome Project and the cataloging of all gene sequences, biological and biomedical investigations
are now focusing on how the tens or hundreds of thousands of proteins in a single cell function and interact with each other. A promising
approach is fluorescence microscopy, which has exquisite sensitivity
down to the single-molecule level together with high spatial and sub-