For all the hype about nanotechnology, sometimes small isn't quite small enough. Quantum dots enable imaging advances in fields from oncology to neuroscience, yet at a whopping dozen nanometers or more, sometimes they're just too big. "They're the size of proteins," says Marcel P. Bruchez, cofounding scientist at Quantum Dots Corp. "Anything you can do to minimize the size will minimize the impact on the biological system."
A quantum dot's size governs the color of light it emits, but the size that determines the optical properties is only the core-shell. The problem is that for biological applications, quantum dots must be changed from being hydrophobic as grown, to hydrophilic, without a loss in fluorescence or stability.
The solution is to create the high-tech equivalent of a peanut M&M: a semiconductor core (usually cadmium sulfide, selenide, or telluride), coated by an insulating shell, which is then given a ligand coating, sometimes ...
