A gold skin forms on the silver cubes as the cubes are hollowed out from within. The silver atoms enter solution through pores that form in the clipped corners of the cubes.
"But the really cool part," says Xia, "and the cool part of nanotechnology generally, is that the tiny gold cages have very different properties than bulk gold." In particular, they respond differently to light.
The physicist Michael Faraday was the first to realize that a suspension of gold particles glowed ruby-red because the particles were extremely small. "His original sample of a gold colloid is still in the Faraday Museum in London," says Xia, Ph.D., the James M. McKelvey Professor in the Department of Biomedical Engineering. "Isn't that amazing? It's over 150 years later and it's still there."
The color is caused by a physical effect called surface plasmon resonance. Some of the electrons in the gold particles are not anchored to individual atoms but instead form a free-floating electron gas. 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 is the color we see.
The strong response at a particular wavelength, called resonance, is what makes a violin string vibrate at a particular pitch or lets a kid pump a swing high in the sky by kicking at just the right moment.
What's more, the surface plasmon resonance is tunable in much the same sense that a violin is tunable.
"Faraday used solid particles to make his colloid," comments Xia. "You can tune the resonant wavelength by changing the particles' size, but only within nar
|Contact: Diana Lutz|
Washington University in St. Louis