JULY 29, 2009--Researchers at the University of Washington (Seattle, WA) have produced what they say is the first structure to create a multipurpose nanotechnology tool for medical imaging and disease therapy. The structure, which combines a quantum dot and a gold nanoparticle, is described in a paper published this week by the journal Nature Nanotechnology. "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," said lead author Xiaohu Gao, a UW assistant professor of bioengineering.
Gold nanoparticles are being developed for delivering drugs and treating arthritis, and in combination with infrared light for medical imaging and therapies that "cook" diseased cells. Quantum dots have proven helpful for biomedical imaging. But while the two have been successfully combined on a surface, they have never before been paired in a single particle, the researchers say, because their individual electric fields interfere with one another and so neither behaves as it would on its own.
The paper describes a manufacturing technique that uses proteins to surround a quantum dot core with a thin gold shell held at 3 nanometers distance, so the two components' optical and electrical fields do not clash. 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 co-author Yongdong Jin, a UW postdoctoral researcher, is general and could apply to other nanoparticle combinations, they said.
"We picked a tough case," Gao said. "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 polymer, polyethylene glycol. The distance between the quantum dot core and charged gold ion is determined by the length of the polymer chain and can be increased with nanometer precision by adding links to the chain. On the outside layer they added short amino acids called polyhistidines, which bind to charged gold atoms.
Gao compares the completed structure to a golden egg, where the quantum dot is the yolk, the gold is the shell, and polymers fill up the space of the egg white. Using ions allowed the researchers to build a 2- to 3-nanometer gold shell that's thin enough to allow about half of the quantum dot's fluorescence to pass through.
"All the traditional techniques use premade gold nanoparticles instead of gold ions," Gao said. "Gold nanoparticles are 3 to 5 nanometers in diameter, and with factoring in roughness the thinnest coating you can build is 5-6 nanometers. Gold ions are much, much smaller."
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.
Also, gold is biocompatible, 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 said.
For more information see the paper, Plasmonic fluorescent quantum dots, published by Nature Nanotechnology.