Cells Tagged by Quantum Dots

Quantum dots are all the rage among physicists and chemists. Now these versatile flecks of semiconductor, which can serve as components in tiny transistors and emit light in rainbow hues, could catch biologists' eyes as well. In tomorrow's issue of Science, two research teams report using quantum dots as fluorescent tags capable of tracing specific proteins within cells. The new tags may soon allow researchers to simultaneously track numerous molecules inside cells with ease.

The current generation of fluorescent tags, made from small organic dye molecules, are widely used to do everything from decoding DNA to helping diagnose infections. But they can also be toxic, burn out quickly, and be difficult to use in tandem, since each dye must typically be excited with photons at a different wavelength. Quantum dots come in a similar rainbow of hues, with smaller dots giving off shorter wavelength--or bluer--light. They're also nontoxic and fluoresce up to 100 times longer than organic dyes, and can all be stimulated to emit light by a single laser, allowing researchers to observe many compounds within a cell at the same time. But these tiny semiconducting grains aren't equipped to float around inside cells and latch onto specific cellular targets.

Paul Alivisatos and his colleagues at the University of California, Berkeley, and Lawrence Berkeley National Laboratory, along with another team led by Shuming Nie at the University of Indiana, Bloomington, did away with that drawback. They chemically altered the surfaces of 1 to 5 micrometer dots so they would dissolve in water, enabling them to diffuse throughout cells. Next they then linked the light emitters to molecules that would guide them to specific cellular targets, such as nuclei.

"It's quite likely these particles will replace conventional organic dyes," says Louis Brus, a quantum dot expert at Columbia University. The Berkeley and Indiana teams are already gearing up to use their dots to improve DNA sequencing and diagnostic tests for the AIDS virus.

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