The wavelengths he wants are the ones at which human tissue is relatively transparent, so that cages in the bloodstream can be opened by laser light shone on the skin.
The color of nanocages can be tuned over a wider range than solid particles by altering the thickness of the cages' walls, says Xia. As more gold is deposited and the shells thicken, a suspension of nanocages shifts from red, to purple, to bright blue, to dark blue, to the wavelengths in the near-infrared.
Xia's team wants to hit a narrow window of tissue transparency that lies between 750 and 900 nanometers, in the near-infrared. This window is bordered on one side by wavelengths strongly absorbed by blood and on the other by those strongly absorbed by water.
Light in this sweet spot can penetrate as deep as several inches in the body.
"People used to do a demonstration at talks," Xia says, laughing. "They'd put a red diode laser in their mouths, and the audience could see it from outside, because the diode's wavelength is 780 nanometers, a wavelength at which flesh is pretty transparent."
Here things get even trickier and yet more amazing. The resonance actually has two parts. At the resonant frequency, light can be scattered off the cages, absorbed by them, or a combination of these two processes.
Just as they can tune the surface plasmon resonance, the scientists can adjust how much energy is absorbed rather than scattered by manipulating the size and porosity of the nanocages.
Xia illustrates the difference between scattering and absorption with a marvelous Roman artifact, the 4th-century Lycurgus Cup. The cup looks jade-green from the outside but turns pink when lit from the inside.
Modern analysis shows the ancient glass contains nanoparticles of a silver-gold alloy that scatters light strongly at a wavelength in the green part of the spectrum. When th
|Contact: Diana Lutz|
Washington University in St. Louis