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Molecular Wheels Need Hubs to Form

Chemists catch nanometer-scale structures self-assembling around transient templates



COURTESY LEE CRONIN AND COLLEAGUES

A team of chemists has unraveled the process by which a complex nanoscale structure self-assembles, finding that a wheel-shaped molybdenum oxide molecule takes shape with the help of a transient scaffold at its center.

The researchers describe the formation process of these molybdenum oxide wheels in the January 1 issue of Science. Although the basic ingredients needed to spur the self-assembly of the nanowheels were known, the actual mechanism of their growth "was really a complete mystery," says study co-author Lee Cronin, a chemist at the University of Glasgow. "Self-assembly on the nanoscale is so intricate and so complex, we just don't understand it at a key level."

Cronin and his colleagues at Glasgow and at Bielefeld University in Germany used a controlled flow system to keep molybdate (an oxygen-containing salt of molybdenum), nitric acid and an electron-providing reducing agent out of equilibrium—that is, to prevent their resulting reaction from proceeding unabated to its final state. They were thus able to isolate a wheel, 3.6 nanometers across, in the midst of its self-assembly. (A nanometer is one billionth of a meter; for comparison, the nanowheels are about 1.5 times as wide as a strand of DNA.) Nestled inside the wheel was a molecular scaffold, in the form of a smaller molybdenum oxide cluster, that is not present in the final product—at some point, the wheel ejects its own hub. The scaffold cluster remains intact after it is ejected and can go on to form the template for other nanowheels.

The template cluster acts "like a seed crystal, if you like," Cronin says, which he and his colleagues verified by starting the reaction over, with preformed templates added to the mix. Sure enough, the nanowheel-forming reaction was accelerated, as the first step in the process—template synthesis—was already complete.

The new research explained to Cronin the order of the recipe for cooking up molybdenum oxide nanowheels. "When I was first making these, I had three things that I mixed in a pot," he says. But the results seemed to depend on the order in which the ingredients were added. "If I added the acid to molybdate, and then the reducing agent, it worked" very quickly, Cronin says, as had been established by his Bielefeld colleagues. But adding the reducing agent, or electron donor, first yielded nanowheels over a much longer time. In light of the new study, it appears that the template clusters that have long been known to form from the acid and molybdate alone then facilitate the growth of the larger wheels.*

Rice University chemist Kenton Whitmire, who wrote a commentary accompanying the research in Science, says that wheel-like molecules are interesting for a number of reasons. For one, their holes, or pores, can be used to perform size or shape exclusion functions on other molecules—those with certain physical properties will bind into their pores whereas others will not. But for many such ring-shaped structures, the center must be manually fabricated, introduced, and then forcibly ejected once its job is done. "What's cute about [this] paper is that the system creates its own template," Whitmire says.

The template and surrounding nanowheel are both negatively charged, but they are able to forestall their mutual repulsion thanks to an intermediate layer of counterbalancing positive sodium ions. As more electrons are added and the reaction proceeds, however, it seems that repulsion becomes unavoidable. "As it continues being reduced, the charge forces the inside molecule out," Whitmire says. "They're no longer compatible."

Cronin says that arresting molecules in the act of self-assembly to reveal their formation processes may even show the way to designing novel structures. "There are lots of questions that this sort of work will start to allow us to access," he says. "I think the next few years are going to be very interesting, because we might be able to really start to control things at the nanoscale, rather than discovering them by accident."

*Note (1/4/10): This paragraph was modified after publication for accuracy.

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