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Walking Molecules Pick Up Cargo

An organic molecule moves in a straight line and can pick up and transport up to two carbon dioxides from one point to another.

Researchers have created a tiny machine made of just one molecule that can carry other molecules on a surface. The technique can be used to move atoms or molecules close to each other, controlling when they react. The new “molecule carrier,” described online last week in Science Express, could eventually lead to more-efficient catalysts and new methods for assembling molecular electronics.

Anthraquinone molecules move in a straight line on a copper surface, while carbon dioxide moves randomly. But when the two molecules get close together, the anthraquinone picks up the carbon dioxide and keeps walking. The “molecule carrier” is able to carry two carbon dioxides.

The researchers, led by Ludwig Bartels, a chemistry professor at the University of California, Riverside, use a molecule called anthraquinone, an organic compound containing a chain of three benzene rings, with two oxygen atoms attached to the central ring, one on each side. When the researchers place anthraquinone molecules on a flat copper surface at temperatures of around -223 ºC, the molecules move in a straight line. Carbon dioxide molecules, on the other hand, move randomly. But if the two molecules come close to each other, carbon dioxide attaches to the anthraquinone’s oxygen atom.

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Because each anthraquinone molecule has two oxygen atoms, it can effectively pick up two carbon dioxides and move with them, Bartels says. The researchers can manipulate the molecules with the tip of a scanning tunneling microscope probe. They can, for example, slide carbon dioxide closer to the anthraquinone to load, and they can unload the carbon dioxide by “bumping the tip into the carbon dioxide molecule,” Bartels says.

“I think [this is] a really elegant study and very key to understanding transport at a molecular scale,” says Kevin Kelly, an assistant professor of electrical and computer engineering at Rice University. The study lays down the groundwork for designing new molecules that can efficiently transport cargo molecules, says Kelly, who previously demonstrated controlled molecular motion using “nanocars.” Kelly says that work in this area could have “real technological implications in the next three to five years” in terms of developing catalysts.

An automobile’s catalytic converter, for example, uses platinum as a catalyst: carbon monoxide and oxygen attach to the platinum surface and react when they get close to each other. But moving the two closer with the help of another molecule would speed up the process and use less of the expensive platinum, says Talat Rahman, a physics professor at the University of Central Florida and coauthor of the Science paper. “We can wait for [carbon monoxide and oxygen] to jiggle around on the surface, somehow break the bond with the surface, and find each other,” she says. “Or we could find something that can carry them from one place to another.”

Experts say the work could also have implications for building molecular electronics, a fledging research area in which organic molecules, rather than silicon, serve as transistors and other electronic devices. Paul Weiss, a chemistry and physics professor at Penn State University, says that the researchers have made a significant breakthrough toward self-assembled molecular circuits.

To move along the copper surface, anthraquinone has to overcome a small energy barrier. The researchers have found that this barrier doubles and triples, respectively, when the anthraquinone picks up one and then a second carbon dioxide molecule. Before now, no one has measured energy changes as molecules assemble and disassemble, Weiss says. By understanding how the energy barrier changes, we can learn how to control it, which would be crucial for assembling molecular circuits, he says. “That is the beauty of the work.”

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