Are quantum nanodots 'the missing link'?

A new class of water-soluble quantum dots made from small numbers of gold atoms could be the basis for a new biological labeling system.

Oct 1st, 2004

A new class of water-soluble quantum dots made from small numbers of gold atoms could be the basis for a new biological labeling system.

When I first heard of quantum dots, ­several years ago, I was reminded of the popular view of the early laser as “a solution looking for a problem.” Well, semiconductor lasers are now commonplace and are solving many a problem. But what about those pesky ­quantum dots? Will they also prove to be problem-solvers? The answer seems to be getting clearer.

A new class of water-soluble quantum dots made from small numbers of gold atoms could be the basis for a new biological labeling system with ­narrower excitation spectra, smaller particle size, and fluorescence comparable to systems based on semiconductor quantum dots, according to researchers at Georgia Tech.

Providing the “missing link” between atomic and nanoparticle behavior in noble metals, these multi-electron “artificial atoms” could also serve as light-emitting sources in nanoscale optoelectronics and in energy transfer pairs. “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,” said Robert Dickson, associate professor at Georgia Tech. “Their potential applications are really complementary to those of semiconductor quantum dots.”

The gold nanodots are made up of 5, 8, 13, 23, or 31 atoms, each size fluorescing at a different wavelength 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 the most efficient at light emission.

Because of their narrow excitation spectra and small physical size, 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 proximity.

The broad excitation spectra of semiconductor quantum dots and their larger size make them more difficult to use in FRET-based research, Dickson noted. By using poly-amidoamine (PAMAM) dendrimers to encapsulate their gold clusters, the researchers produced quantum dots with very clean mass spectra. The eight-atom cluster, for instance, produces bright blue emission and fluorescence quantum yields of 42 percent in an aqueous solution.

Before these gold quantum dots can be useful in biological labeling, however, the researchers must develop a mechanism for attaching them to proteins that scientists wish to track in cells. “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 noted.

“We will need to determine ways to functionalize these quantum dots so they will get across cell membranes, seek out specific proteins inside a cell, and label those proteins,” explained Dickson. “We are basically developing the tools for in vivo, single-molecule sensitivity and labeling in living systems in the presence of very high backgrounds. We expect to produce a new set of probes that will be size-tunable, nontoxic, and very bright.”

Beyond the potential applications, studying the gold clusters provides information about the properties of small clusters of noble metals. “They can help us understand the very small size scale that is really not well understood for noble metals,” Dickson added. “They can provide the ‘missing link’ between atomic and nanoparticle behavior in these metals.”

Of course, there’s a long way to go before we can manufacture quantum nanodots in commercial quantities but I fully expect to see a raft of startups in the field sooner rather than later. The nano prefix is already causing Wall Streeters to salivate and press money on embryo companies staffed by researchers. Anyone for “Nanodot Solutions, Inc.?”

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