A Golden Glow

Fluorescing "artificial atoms" could act as biological labels and nanoscale optoelectronics according to researchers speaking at the 228th national meeting of the American Chemical Society at the end of August. A new class of quantum dots made from small clusters of gold atoms is at the heart of the new technology and could have many advantages over quantum dots based on semiconductors.


Robert Dickson

Robert Dickson of the Georgia Institute of Technology and his colleagues have created a new class of water-soluble quantum dots made from clusters of gold atoms encapsulated in poly-amid amine (PAMAM) dendrimers. They have found that these tiny devices fluoresce with much narrower excitation spectra than their semiconductor counterparts and so might find use in highly specific labeling for molecular biology experiments. The clusters also offer a "missing link" between atomic and nanoparticle behavior in noble metals, and so could be used as minute light sources in nanoscale optoelectronics and in energy transfer pairs for coupling different optoelectronic devices together.

"We have discovered a new class of quantum dots that are water soluble, strongly fluorescent, and display discrete excitation and emission spectra that make them potentially very useful for biological labeling," explains Dickson. "Their potential applications are really complementary to those of semiconductor quantum dots."


	Three vials of gold quantum dot solutions, each fluorescing in a different color.

Dickson and his colleagues, Yih Ling Tzeng of Emory University; Jie Zheng, Lynn Capadona, and Caiwei Zhang of Georgia Tech; and Jeffrey Petty of Furman University, created gold nanodots using 5, 8, 13, 23, or 31 atoms. Each different size of cluster glows at a different wavelength when excited with light to produce ultraviolet, blue, green, red, and infrared emissions, respectively. The fluorescence energy varies according to the radius of the quantum dot, with the smallest structures being the most efficient at light emission.

The gold quantum dots could be particularly useful in fluorescence resonance energy transfer (FRET) systems, in which emission from one nanodot would be used to excite another as a means of measuring how close together they are. Such measurements can reveal important clues about the interactions between receptors, enzymes, and genetic material.

The nanodot solutions are stable, lasting for months either in solution or as dried powders, says Dickson. Indeed, solutions from re-dissolved nanodot powders have the same properties as when they were originally created. Semiconductor quantum dots made from cadmium selenide, for instance, are far larger and contain hundreds or thousands of atoms, making them bulkier labels of biological systems.

The next step is to find a way to attach the gold quantum dots to the proteins in which biologists are interested. "We are continuing to investigate these quantum dots, to probe their fundamental photophysical and spectroscopic properties, and to develop different chemistries for functionalizing the scaffolding that encapsulates the nanoclusters so we can attach them to other molecules," Dickson adds.