By now, we’ve documented numerous scientific efforts aiming to develop nanoparticles for all sorts of medical purposes, like cancer detection, drug delivery, and redirection of heat to try to destroy tumor tissue. Researchers at the University of Washington now managed to combine gold nanoparticles with fluorescing quantum dots, allowing for both detection and heat delivery in one package.
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, quantum dots’ unique optical properties 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 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 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.
Press release: All-in-one nanoparticle: A Swiss Army knife for nanomedicine…
Abstract in Nature Nanotechnology: Plasmonic fluorescent quantum dots…