Picture of the Week: Quantum Dot Sphere

What looks like a fuchsia olive in the picture above is hardly edible. You’re actually seeing a magnified collection of quantum dots—tiny luminescing particles—that have assembled to form a hollow sphere between 100 and 1000 microns across. If you could zoom in even closer on the “olive,” you’d notice that each individual dot is itself a solid sphere, measuring in the nanometer range.

The hollow orb in the picture took shape at the Pacific Northwest National Laboratory (PNNL), where researchers are studying how to coax quantum dots to aggregate into various shapes. The long-term goal is to see if those shapes can be used as probes in cellular imaging. How? Quantum dots are made out of semiconducting material—in this case, cadmium sulfide. When light hits them, their electrons jump into an excited energetic state. When these excited electrons return to the resting ground state, energy can be released as luminescent light. Now, say a researcher wants to see the mitochondria in a cell. Problem is, there’s a lot of other stuff in a tissue sample (such as blood, cell proteins, and salts) that can obscure the view from a prying scientist’s eyes. A glowing quantum dot aggregate whose shape fits nicely around mitochondria could help researchers detect those organelles better.

Morphology is just one quality that can influence how well a quantum dot probe works, however, says Sayandev Chatterjee, one of the scientists on the team that created the hollow sphere and others like it. The quantum dots must also emit long-lasting light in a wavelength that won’t fight with light that other biological structures might emit. The probes must survive under the physiological conditions of a biological medium, too. And they need some sort of molecular marker—an ID tag, in other words—that will help them find their target in the first place.

For now, though, Chatterjee and his colleagues are focusing on the different types of quantum dot morphologies they can concoct by experimenting with various reagents, temperatures, and reaction times. Although, they did make room for an art project—the image above, taken by teammate Paul MacFarlan with an electron microscope and colored later by graphic designer Jeff London, was part of the 2011 PNNL Science As Art contest.