secondary label reagent. Microtubules were clearly labeled with QD
630–streptavidin (Fig. 3A). When cells were incubated with QD-streptavidin alone, very weak or no apparent signals were detected
(Fig. 3B), indicating that QD-streptavidin labeling of the micro-tubules was specific.
Bruchez et al. reported labeling of actin filaments with QD-biotin2. The fluorescent signal was weak even after two rounds of
biotin-avidin amplification. In addition to the specific staining of
the actin filaments, the probe also showed some nonspecific binding
to the nuclear membrane2. To investigate whether our QD probes
could label the fine cellular structure of actin filaments, we stained
fibroblasts with QD 535–streptavidin after the cells were incubated
with biotinylated phalloidin, a reagent that specifically binds to filamentous F-actin but not to globular G-actin17. F-actin filaments
were clearly labeled with QD 535–streptavidin with only one round
of biotin-streptavidin interaction (Fig. 3C); moreover, there was no
nonspecific staining of other parts of the cell. When cells were incubated with QD-streptavidin alone, no or very weak signals were
detected (Fig. 3D). These results indicate that the QD-based probes
can be bright enough and specific enough for effective labeling of
fine cellular structures, a considerable improvement in performance
over previously reported QD probes.
Although there is a report of “staining” the nucleus with QDs
coated with urea and acetate groups2, it remained unclear whether
QDs conjugated with biomolecules could label specific antigens
inside the nucleus. To investigate the labeling efficiency of QD-based
bioprobes for nuclear targets, we incubated fixed human epithelial