STED Imaging Works With Quantum Dots

Most classes of fluorescent labels have been successfully used in the superresolution microscopy method known as stimulated emission depletion, or STED. The glaring exception has been quantum dots. In STED, one laser beam excites fluorescence and a second laser beam turns off the fluorescence in all but a small defined region. Quantum dots haven’t worked because their broad excitation spectra overlap the emission band, making it hard to identify an appropriate wavelength for stimulated emission shutdown of the fluorescence. Stefan W. Hell of the German Cancer Research Center and the Max Planck Institute for Biophysical Chemistry and coworkers now report successful STED microscopy with commercially available red-emitting quantum dots (Nat. Commun. 2015, DOI: 10.1038/ncomms8127). The researchers excite ZnS-coated CdSe quantum dots with 628-nm light and shut down the fluorescence with 775-nm light. By repeating the process in different spots, they build up images with spatial resolution of about 100 nm. The quantum dots survive more than 1,000 scans, which suggests they would be suitable for time-lapse STED imaging, according to the researchers.