A bit like modern-day alchemists, researchers have made "artificial atoms" from gold that shine in a whole host of different colors.
Robert Dickson from the Georgia Institute of Technology and his colleagues have synthesized a new class of quantum dots from clusters of gold atoms. Their small size and water-solubility make these tiny gold nuggets ideal candidates for a biological labeling system that could track multiple molecules in living cells.
A quantum dot is a type of atomic configuration that corrals electrons in a potential well. The electrons bounce around in this small box, but only at specific energies - similar to what happens to the electrons buzzing around an atom.
Dickson and his collaborators selected gold clusters that were strongly fluorescent, i.e. they absorbed light at one frequency and emitted at another.
In a recent issue of the journal Physical Review Letters, the scientists showcased dots with 5, 8, 13, 23, and 31 gold atoms, which emit, respectively, in ultraviolet, blue, green, red, and infrared light.
Typically, quantum dots are made of semiconductor material, but gold is a metal and a good conductor. Metal quantum dots are vastly smaller than their semiconductor counterparts, which tend to have hundreds, up to thousands, of atoms.
Because of their unique optical properties, quantum dots may find future applications as lasers and detectors. Research is also exploring the possibility of tagging proteins and other molecules with dots to follow them in biological processes.
Currently, most of these so-called "biological labels" are made from organic dyes and proteins from jellyfish and fireflies.
"Organic dyes are a much more developed technology," Dickson said.
However, natural molecules emit light over a broad range of frequencies, which can make it difficult to distinguish different labels in a single setting. Quantum dots emit over a much narrower band, so several labels can be used simultaneously. They also tend to fluoresce longer than natural molecules.
The advantage that metal dots have over semiconductor ones is their smaller size.
"Semiconductor dots are large - comparable to some of the proteins that they are used to label," Dickson said. Using such a big dot "can definitely affect the functioning of the protein," he said.