Scientists who work in the hot new field of nanotechnology—creating very small-scale materials with many potential applications—often employ tiny particles of gold, silver, or other metals. 

Now an SIUC undergraduate, Sandie Cheung, has come up with a way to make metal nanoparticles with more-uniform sizes and shapes by "growing" them inside a special kind of glass. The technique gives the glass new optical and electronic properties.

Cheung's achievement started with an SIUC chemist’s twist on glassy materials called sol-gels.

Sol-gels are made of silica but, unlike ordinary glass, they have a porous matrix. By adding organic molecules to the matrix, Bakul Dave, an assistant professor of chemistry, has created sol-gels whose pores are chemically active—able to attract and bind various substances.

Dave (pronounced da-VAY) explains it this way: these little cavities are "sticky," unlike the cavities in ordinary sol-gels. He calls his modified sol-gels "smart glass" because it can react to its environment.

Perspectives magazine has previously reported on efforts in Dave’s lab to make smart glass that can function as a tiny drug delivery device, that can convert carbon dioxide to methanol, and that can be used as a biosensor to detect glucose concentrations, among other uses.

Cheung, a junior who plans to specialize in materials science engineering, has added to these applications by focusing on precious metals. 

In an earlier project with Dave, funded by an SIUC Undergraduate Research/Creative Activity Award, Cheung worked to adapt smart-glass technology for cleaning up metal contaminants in water. The research sparked her interest in materials science. So she and Dave came up with a related idea: using smart glass to encapsulate metal nanoparticles for possible use in sensors and other applications. 

The pores of Dave’s smart glass, they realized, could serve as miniature templates for "growing" and embedding nanosized metal spheres. The pores would physically limit the particles to a certain size range, generally 4 to 8 nanometers. (A nanometer is one-millionth of a millimeter—almost down to the molecular scale.)

"People are trying to find new ways to make controlled nanostructures," Dave says. 

In December 2001, Cheung received an Undergraduate Materials Research Initiative Award from the Materials Research Society to fund her efforts. Her ultimate goal was to synthesize gold particles (often used in nanotech research because of their stability) in smart glass.

She began by adding methanol when making the smart glass. Methanol slows down the process by which the glass matrix forms and solidifies. The result is pores that are smaller, more uniform in size, and more evenly distributed. 

She also had to incorporate ligands in the glass—organic molecules that could attract and bind the metal ions she wanted to work with. "A ligand is like a magnet that you place into the matrix," she explains.

Then she was ready to synthesize nanoparticles in the glass’s pores. Surprisingly, her process for making gold particles starts with copper. But the chemistry is straightforward, and its progress can be tracked through color.

First, a film of the smart glass is immersed in a solution of copper ions. The ligands bind the ions inside the glass’s pores, and the ions turn the glass bright blue.

Then the glass is treated with a chemical solution that "donates" electrons to the copper ions, changing them to copper particles. The color of the glass is now a dull brownish-gray.

Next, the glass is immersed in a solution of silver ions. The copper gives up electrons to the silver ions, which change into encapsulated silver particles. The glass’s color now is amber.

Finally, the glass is exposed to a solution of gold ions. The silver gives up electrons to the gold ions, which become gold particles. They turn the glass not gold, but a rich maroon.

Why go through this metal exchange? Why not create gold particles from the start?

That can be done, Dave says, but the results aren’t as good. By itself, he explains, "Gold has a tendency to make big particles very quickly." Instead of uniform spheres, you get clumps.

By starting with copper and then converting it to other metals, Cheung was able to produce similar-size spheres evenly distributed throughout the glass. "You want to make the particles uniform so that the glass is conductive," she says.

Such control will be crucial to the functioning of nanomaterials when they are eventually employed in commercial devices.

Cheung used x-ray analysis and electron microscopy to confirm the composition, size, and distribution of the particles. (Most are about 5 nanometers in diameter, far too small for ordinary microscopes to image.) A bonus, she says, is that the metal-infused smart glass is easy to produce: "You don't have to work in a controlled or hazardous environment. It can be done safely and efficiently at room temperature."

Cheung reported her findings at the national conference of the Materials Research Society in April 2002. "Very rarely do undergraduates have enough results to present at national meetings," says Dave. "It’s quite impressive."

The alchemists of yore would have been green with envy.

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