Volume 2, Issue 15 October 2005
Catalyzing Nanotechnology
As scientists and engineers continue to make progress in the realm of nanotechnology, new tools become necessary to synthesize more complicated structures on such tiny scales. UC Berkeley chemical engineer Alexander Katz is developing several techniques to fashion structures that spur specific chemical reactions but are as small as a single nanometer. His processes range from a cookie-cutter templating technique to methods directly inspired by Mother Nature. Eventually, the materials that Katz and his collaborators discover could speed the development of nanoscale electronic components for future computers and related memory systems.
Alexander Katz and his colleagues have a U.S. patent on one of their methods to bind functional groups on surfaces discovered at Berkeley, with two others currently pending.
"Standard lithographic etching used to make microprocessors is certainly able to create mechanical features of the right size and shape," Katz explains. "But as these features become smaller in the future, what becomes as important as their size and shape is local arrangement of chemical functional groups. How can we organize these groups and the environment surrounding them in solids?"
Because of their small size, the structures that Katz's research group synthesizes can be used as active catalytic sites for causing chemical transformations to occur. Chemists use catalysts to speed the rate of chemical reactions. The catalyst acts as a pathway between the reactants and the end product that requires less of an energetic barrier than the transformation would take otherwise. Because the nanoscale order in Katz's sites can interact with a reactant molecule specifically, these sites can induce chemical reactions with great selectivity. For instance, some of Katz's sites can steer the product of a chemical reaction to be one or another molecule, depending on the functional group arrangement. The most proficient examples of how elaborate organization of functional groups can affect catalysis can be found inside of each of us.
After immobilizing gold nanoparticle imprints in silica, the gold core is etched to expose the chemical functional group organization on the gold surface that is tethered off of the silica network. The three images depict the shrinking core of colloidal gold (black) over a period of 22 hours. (courtesy the researchers)
"The functional groups that keep us alive consist of relatively simple building blocks," Katz says. "But the way they're assembled is intricate. It's that assembly that imparts elaborate catalytic properties."
Molecular imprinting in silica is a method Katz and his colleagues developed to achieve nanoscale functional group organization in solids. The researchers take a particular molecule and mold silica around it. The molecule is then removed but chemical functional groups are left attached to the inside of the mold. The end result is a solid, visually not unlike an ordinary piece of glass, but actually riddled with miniscule imprinted pores. Organic molecules bind inside these pores where the imprinted functional groups promote a chemical reaction.
The researchers have also explored a method to imprint bulk silica with particle templates as large as 15 nanometers. Rather than organize several functional groups at a time, the synthesis of nanoparticle building blocks for bulk silica imprinting is ideal for organizing thousands of functional groups at once, Katz says.
This slide depicts the synthetic and biological catalysts consisting of similar organic and organometallic active sites. The confined environment surrounding both biological catalysts results from the hydrophobic interior of the enzyme. The researchers successfully replicated this confinement in the synthetic equivalents of the biological active sites shown on the right side of this figure. (courtesy the researchers)
The process is similar to the single-molecule imprinting, but in this case a nanoparticle with a functional group organized on its surface is bound in the silica. After the nanoparticle core is removed, the organized functional groups remain immobilized in the structure.
In another technique that Katz and his coworkers discovered, bowl-shaped functional groups are grafted to the surface of a piece of silica. The functional groups act as one nanometer-sized "pocket" that only allows certain catalytic reactions to occur. The rim of the pocket and the surface of the silica can also be altered to affect the catalyst properties.
"The mechanism and the selectivity of these reactions, in addition to catalyst activity, can be dictated by our ability to organize chemical functional groups in solids," Katz explains. "All of our efforts are about taking something ordinary, like these functional groups, and enabling them to do extraordinary things when arranged cooperatively within a nanoscale site."