Gold quantum dots.

Scientists have created a new type of quantum dot that could find applications in everything from biological imaging to computer displays. Like their semiconductor counterparts (SN: 2/15/03, p. 107), these tiny clusters of gold atoms fluoresce brightly and emit different wavelengths of light when scientists vary the number of atoms in each cluster.

Robert Dickson and his colleague of the Georgia Institute of Technology in Atlanta created the clusters by dissolving gold salts in a solution containing branched-polymer molecules called dendrimers. The dendrimers encapsulated the gold atoms, creating the nanometer-scale quantum dots.

By varying the concentration of the gold relative to the dendrimers, the researchers produced dots of many sizes, each of which fluoresces at a different wavelength. For instance, 5-atom dots emit ultraviolet light, whereas 31-atom clusters emit infrared light, which is at the other end of the spectrum. The three other clusters that the researchers made--containing 8, 13, or 23 atoms--fluoresced bright blue, green, or red, respectively.

Compared with this new class of quantum dots, semiconductor dots are much larger, containing hundreds to thousands of atoms, and are typically made of toxic materials such as cadmium selenide. This makes them difficult to use as labels on cells or proteins within the body, an application sought after for the sake of tracking diseases such as cancer. In contrast, gold is relatively inert and biocompatible.