A Swiss Army Knife for Nanomedicine

July 28, 2009 – 12:35 pm

Nanoparticles are being developed to perform an array of medical uses — imaging tumors, carrying drugs, delivering pulses of heat. Rather than settling for just one of these, researchers at the University of Washington (UH) have combined two nanoparticles in one tiny package by combining gold nanoparticles with fluorescing quantum dots, allowing for detection and heat delivery.

The structure is reportedly the first multipurpose nanotechnology tool suited for both medical imaging and therapy. The structure is described in a paper published online this week in 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,” says lead author Xiaohu Gao, a UW assistant professor of bioengineering.

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 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 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 says. “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 says. “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.

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.

More information is available from University of Washington News.

Brian Buntz

Tags: gold shell, nanomedicine, nanotechnology, Nature Nanotechnology, quantum dot, UH, University of Washington