Catalysts are prized for their ability to speed chemical reactions by grabbing molecular building blocks and knitting them together. But most catalysts are either on or off—and there hasn't been much scientists could do to flip the switch. Now, however, researchers have created a sandwich-shaped scaffold for turning on and off nearly any catalyst at will. If developed further, the new design could allow researchers to detect minute amounts of a wide range of small molecules—from explosives such as TNT to neurotransmitters that carry messages in the nervous system.
Unlike industrial catalysts, many enzymes—biological catalysts made from proteins—rely on small molecules to turn them on and off. In a process known as allosteric regulation, the binding of a particular small molecule changes the shape of the protein's catalytic site, allowing it to function. A related process can also disrupt a catalyst, turning the enzyme off.
In recent years, several groups have tried to mimic this switching to control synthetic catalysts used in a variety of industrial processes, such as creating plastics and other polymers. Five years ago, Chad Mirkin, a chemist at Northwestern University in Evanston, Illinois, and colleagues created a system that could activate a catalyst that has two metal atoms at its core. Although the scheme worked, only a few catalysts operate with two metal atoms. Moreover, the scaffold can't be easily adapted to work with the much larger number of catalysts that harbor just a single metal atom in their core. So Mirkin and his colleagues wanted to determine whether they could design a new system to work with single-metal catalysts.
They report  doing so in today's issue of Science. To make the system work, Mirkin's team designed the catalyst to be like a three-layer sandwich. The "meat" at the center is an aluminum-containing catalyst designed to break open ring shaped compounds and stitch them together into a polymer known as polycaprolactone. When the catalyst is off, two flat organic compounds hide the meat like pieces of bread, preventing the catalyst from interacting with other molecules. But when the researchers add chloride ions, the ions bind at the edges of the outside bread layers, kicking out key nitrogen atoms. The outside layers swing open, allowing the catalyst to do its work. Removing the chloride ions snaps the outside layers shut, and the catalyst is turned off.
Wenbin Lin, a chemist at the University of North Carolina, Chapel Hill, says he's impressed with the work. "It's a very general approach that can be applicable to many different things," Lin says. That could make it useful for detecting a wide variety of small molecules, such as environmental contaminants or compounds present in particular diseases. By triggering the catalyst's production of large amounts of a substance, the method makes it easy to spot when the chosen small molecules are present. The new technique could also prove important industrially to control the work of multiple catalysts that are often used in tandem to build polymers and other complex structures. In cases such as these, researchers would be able to turn catalysts on and off at specific times, ensuring that the production process proceeds in the correct order.
And that's not the end, says Lin. "I think there's likely to be other new [applications] that we do not realize yet."