Semiconductor quantum dots (QDs) are among the most promising emerging fluorescent labels for cellular
imaging. However, it is unclear whether QDs, which are nanoparticles rather than small molecules, can specif-
ically and effectively label molecular targets at a subcellular level. Here we have used QDs linked to
immunoglobulin G (IgG) and streptavidin to label the breast cancer marker Her2 on the surface of fixed and
live cancer cells, to stain actin and microtubule fibers in the cytoplasm, and to detect nuclear antigens inside
the nucleus. All labeling signals are specific for the intended targets and are brighter and considerably more
photostable than comparable organic dyes. Using QDs with different emission spectra conjugated to IgG and
streptavidin, we simultaneously detected two cellular targets with one excitation wavelength. The results indi-
cate that QD-based probes can be very effective in cellular imaging and offer substantial advantages over
organic dyes in multiplex target detection.