Golden combo yields do-it-all nanotool

U. WASHINGTON (US)—Think of it as the Swiss Army knife approach to nanotechnology. Researchers have combined two nanoparticles into one tiny package the could be used for a range of applications—from medical imaging to solar cells.

A team from the University of Washington developed the new technique. The result is the first structure that creates a multipurpose nanotechnology tool that could be use for imaging tumors, carrying drugs, and delivering pulses of heat.

“This is the first time that a semiconductor and metal nanoparticles have been combined in a way that preserves the function of each individual component,” says lead author Xiaohu Gao, an assistant professor of bioengineering.

The current focus is on medical applications, but the researchers says multifunctional nanoparticles could also be used in energy research.

Quantum dots are fluorescent balls of semiconductor material just a few nanometers across, a small fraction of the wavelength of visible light (a nanometer is 1-millionth of a centimeter). At this tiny scale, the unique optical properties of quantum dots cause them to emit light of different colors depending on their size. The dots are being developed for medical imaging, solar cells, and light-emitting diodes.

Glowing gold nanoparticles have been used since ancient times in stained glass; more recently they are being developed for delivering drugs, for treating arthritis, and for a type of medical imaging that uses infrared light. Gold also reradiates infrared heat and so could be used in medical therapies to cook nearby cells.

But combine a quantum dot and a gold nanoparticle, and the effects disappear. The electrical fields of the particles interfere with one another and so neither behaves as it would on its own. The two have been successfully combined on a surface, but never in a single particle.

The latest findings, published online in the journal Nature Nanotechnology, describe a manufacturing technique that uses proteins to surround a quantum dot core with a thin gold shell held at three nanometers distance, so the two components’ optical and electrical fields do not interfere with one another. The quantum dot likely would be used for fluorescent imaging. The gold sphere could be used for scattering-based imaging, which works better than fluorescence in some situations, as well as for delivering heat therapy.

The manufacturing technique developed by Gao and coauthor Yongdong Jin, a postdoctoral researcher, is general and could apply to other nanoparticle combinations, they say.

“We picked a tough case,” Gao notes. “It is widely known that gold or any other metal will quench quantum dot fluorescence, eliminating the quantum dot’s purpose.”

Gao and Jin avoided this problem by building a thin gold sphere that surrounds but never touches the quantum dot. They carefully controlled the separation between the gold shell and the nanoparticle core by using chains of a polymer, polyethylene glycol. On the outside layer they added short amino acids called polyhistidines, which bind to charged gold atoms.

Using ions allowed the researchers to build a two- to three-nanometer gold shell that’s thin enough to allow about half of the quantum dot’s fluorescence to pass through.

The total diameter of the combined particle is roughly 15-20 nanometers, small enough to be able to slip into a cell.

Incorporating gold provides a well-established binding site to attach biological molecules that target particular cells, such as tumor cells. Gold could also potentially amplify the quantum dot’s fluorescence by five to 10 times, as it has in other cases.

The gold sphere offers one further benefit. Gold is biocompatible, is medically approved and does not biodegrade. A gold shell could thus provide a durable non-toxic container for nanoparticles being used in the body, Gao says.

The research was supported by the National Institutes of Health, the National Science Foundation, the Seattle Foundation, and the University of Washington’s Department of Bioengineering.

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