Nishimura et al. describe silicon nanocrystals that overcome some of the failings of fluorescent proteins and similar molecular labels.
Fluorescence microscopy has been illuminating for cell biologists, but probes such as GFP and fluorescent organic compounds that are used to highlight molecules have some drawbacks. The tags photobleach, or fade when exposed to light, and they often blink. These limitations make single-molecule tracking difficult. Quantum dots are possible replacements, but they are bulky. Silicon nanocrystals seem like an ideal alternative because they can be smaller than GFP, rarely photobleach, and can emit several different wavelengths of light. However, researchers have had difficulty making uniform particles and attaching them to individual molecules.
Nishimura et al. used a novel technique that involves chemically etching silicon wafers. The researchers crafted uniform silicon nanocrystals that emitted red light and were 4.1 nm in diameter—smaller than a single GFP molecule. During the five hours the team observed them, the crystals didn’t photobleach or blink.
After coupling transferrin molecules to single nanocrystals, the researchers tracked individual transferrin receptors in the plasma membrane of kidney cells. Because fluorescent tags blink, researchers are often forced to interpolate a molecule’s trajectory during the outage. With this new system, the team was able to follow nanocrystal-tagged molecules continuously for two minutes, compared with less than a second for GFP-labeled molecules. The researchers are now working to develop crystals that emit different wavelengths of light.
Text by Mitch Leslie