|Posted on February 8, 2016 at 11:55 AM|
Nanoparticles are special in their ability to change their properties as their size (diameter) shrinks. This includes optical properties, which can give rise to brilliant colors. When metals like silver and gold are shrunk down sufficiently smaller (to nanometer scales), their electrons become more and more confined in space. These confined electrons are called plasmons; they can be thought of as ripples of electrons on an electrically conducting surface. Then light shines on a metal, its plasmons reradiate specific colors. Stained-glass windows in cathedrals like Notre Dame are created with tiny particles of gold. When these particles absorb and then reflect the light they produce beautifully vivid colors.
(Photo credit: Copyrighted some rights reserved by photographer, Chris Robinson, “The Rose Window,” downloaded from his Flickr account).
The colors they produce depend on the type of metal, the size of the nanoparticle, and the way they interact with other particles. Switching between colors quickly and reversibly is difficult. Or at least it was, until Christy Landes and her team at Rice and their collaborators at the University of Melbourne Australia developed a tunable and relatively straightforward electrochemical method to create suitable nanostructures that make the switching easier.
“We’re changing the distance between two nanoparticles, changing the absorption on the surface and changing the combination of metals,” says Christy Landes, an associate professor of chemistry and electrical and computer engineering at Rice University. “Doing all of these things simultaneously is what’s special about this set-up.” The way this works is fascinating: They tethered pairs of gold nanoparticles onto a transparent piece of glass covered with an indium-tin-oxide (ITO) semiconductor. A silver electrode in an electrochemical cell with a commonly used sodium chloride electrolyte allowed them to electroplate a thin silver shell onto the gold nanoparticles. A potentiostat allowed them to control how much charge went in and out of the cell. This provided control over the growth of the silver shells, and it allowed reversible switching of the shell between silver and silver chloride. This innovative idea allowed them to produce large color changes rapidly and reversibly. Their work does not only produce pretty colors, it sheds light on quantum mechanical tunneling. As Christy Landes says: “we should be able to control the growth of nanoparticles and test predications about quantum mechanical effects.” This creative piece of work was published in Science Advances in December 2015. Vol. 1, no. 11, e1500988.
- Written by Paulette Clancy