"The key to photothermal therapy," says Xia, "is the cages' ability to efficiently absorb light and convert it to heat. "
Suspensions of the gold nanocages, which are roughly the same size as a virus particle, are not always yellow, as one would expect, but instead can be any color in the rainbow.
They are colored by something called a surface plasmon resonance. Some of the electrons in the gold are not anchored to individual atoms but instead form a free-floating electron gas, Xia explains. Light falling on these electrons can drive them to oscillate as one. This collective oscillation, the surface plasmon, picks a particular wavelength, or color, out of the incident light, and this determines the color we see.
Medieval artisans made ruby-red stained glass by mixing gold chloride into molten glass, a process that left tiny gold particles suspended in the glass, says Xia.
The resonance and the color can be tuned over a wide range of wavelengths by altering the thickness of the cages' walls. For biomedical applications, Xia's lab tunes the cages to 800 nanometers, a wavelength that falls in a window of tissue transparency that lies between 750 and 900 nanometers, in the near-infrared part of the spectrum.
Light in this sweet spot can penetrate as deep as several inches in the body (either from the skin or the interior of the gastrointestinal tract or other organ systems).
The conversion of light to heat arises from the same physical effect as the color. The resonance has two parts. At the resonant frequency, light is typically both scattered off the cages and absorbed by them.
By controlling the cages' size, Xia's lab tailors them to achieve maximum absorption.
"If we put bare nanoparticles into your body," says Xia, "proteins would deposit on the particles, and they would be captured by the immune system and dragged out of the bloodstream into
|Contact: Diana Lutz|
Washington University in St. Louis