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SCIENCE
The semiconductor nanoparticles known as quantum dots have moved closer to biological applications, according to two recent reports.
Watt W. Webb, professor of applied physics at Cornell University, and his coworkers injected amphiphilic-coated quantum dots made of CdSe-ZnS intravenously into mice [Science, 300, 1434 (2003)]. Using multiphoton microscopy, they obtained images of blood vessels through skin and fat--tissues that are challenging for imaging because they scatter and absorb radiation. They could easily measure blood flow and detect heart rate directly through the skin. The combination of quantum dots and multiphoton microscopy (in which multiple lower energy photons are used to excite a higher energy electron transition) lessens potential tissue damage in two ways. First, the quantum dots require less excitation intensity because they have a large excitation cross-section. Second, the multiphoton microscopy limits any tissue damage to the focal volume and avoids scattering and absorption problems. In contrast to earlier quantum dot studies, Webb and his colleagues don't observe blinking on a millisecond or faster timescale, which Webb attributes to the dots being in solution rather than stuck on a surface. If the dots don't blink at longer timescales as well, they could be appropriate for tracking individual biomolecules in living cells for long periods of time. First, however, quantum dots with only a single binding site need to be made. However, Webb predicts that quantum dots won't be used for human applications anytime soon. "The best quantum dots are made of cadmium selenide," Webb says. "Cadmium itself is toxic. There's always going to be an issue of whether the quantum dots are sufficiently stable to avoid becoming toxic." The Japanese researchers, led by Takuzo Aida, professor of chemistry and biotechnology at the University of Tokyo, formed inclusion complexes between CdS quantum dots and the chaperone protein GroEL from Escherichia coli [Nature, 423, 628 (2003)]. GroEL has a cavity that usually helps other proteins fold properly but can also be made to hold a single nanoparticle. When adenosine triphosphate (ATP) binds to the chaperone protein in the presence of Mg2+ and K+, it causes a conformation change that releases the nanoparticle. "We wanted to fabricate semiconductor nanoclusters that are highly stabilized but maintain high sensitivities," Aida says. "Other proteins may result in the formation of stable nanoclusters, which are dormant to chemical and biological stimuli." Aida suggests that such protein-nanoparticle combinations could be used as biosensors. Shuming Nie, a quantum dot expert at Georgia Institute of Technology and Emory Medical School, says the Japanese work "could have implications in designing hybrid nanoparticle-biomolecular systems, opening new ways in using nanoparticles in biology. From an application point of view, new biosensors might be designed, but the types of analytes will be limited to those ligands that can trigger a dramatic protein conformational change upon binding."
IN A BARREL The chaperone protein GroEL forms an inclusion complex with a CdS nanoparticle. In the presence of ATP, Mg2+, and K+, the protein changes conformation and releases the nanoparticle, which can coagulate with other nanoparticles. NATURE © 2003 |
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