9WWW.CEN-ONLINE.ORG AUGUST 25, 2008

THE OSCILLATING Belousov-Zhabotinsky (BZ) reaction, famous for producing dazzling color-changing patterns, is now also finding use as a

novel way to move matter.Shingo Maeda of Waseda University, in Tokyo,

and colleagues designed a polymer gel that undulates with a peristaltic motion simi-lar to the way cilia or intestinal muscles pulse with rhythmical, directional contractions. As the gel undergoes the BZ reaction, a ruthenium catalyst periodically changes its oxidation state back and forth, generating chemical waves that propagate outward, causing the gel to shrink and swell (Angew. Chem. Int. Ed. 2008,47, 6690).

The authors write that they envision use of the phenomenon to develop lifelike systems—“biomimetic devices such as

autonomous matter transporters or self-beating micro-pumps that could function without external control.”

The new gel, composed of a cross-linked polymer and ruthenium, is activated by dunking it in a solution of cit-ric acid, which sets off the BZ reaction. Eventually, the re-action medium gets spent, but with the addition of more citric acid, the process starts over again, Maeda explains.

The discovery comes on the heels of work from Ken-ichi Koshigawa at the University of Kyoto and colleagues, who also studied the BZ reaction’s potential to move matter ( J. Phys. Chem. C2007, 112, 3032). Rather than the swelling and shrinking of gels, their system harnesses a solution that undergoes the BZ reaction. They were able to use that to propel a small piece of paper along the chemical wave fronts.

Koshigawa notes that the transport speed of the Waseda group’s gel is still slow (about 1 mm per minute) compared with that of their solution (about 10 mm per minute). “The re-maining problem is how to speed up the deformation rate,” Koshigawa says referring to the polymer system.

Maeda points out that the composition of the gels actually changes during the reaction, making possible a wider range of applications than Koshigawa’s strategy. —ELIZABETH WILSON

SHRINK GOLD nanoparticles down to a mere 1.4 nm in diameter and they can catalyze se-lective oxidation reactions without the help

of support materials or additives that are normally required, according to a new study.

Gold has long been known to have size-dependent catalytic activity: The metal is inert in bulk, whereas gold nanoparticles 3 to 5 nm in diameter can catalyze a variety of reactions. But those larger nanoparticles require the addition of H2 or peroxide species to drive the reaction, or an electronic interaction with a support material such as titanium dioxide.

A group led by Richard M. Lambert, a chemistry professor at Cambridge University, has now reported that 55-atom gold clusters supported on inert material can catalyze the reaction of O2 with styrene to produce

styrene oxide without the need for additives (Nature 2008, 454, 981).

Getting rid of additives makes the system more en-vironmentally friendly. The particles likely adsorb and dissociate O2 into individual O atoms on the surface of the clusters. Then the O atoms initiate the reaction with styrene, Lambert says. Oxygenated hydrocarbons, in particular epoxides such as styrene oxide, are used in a variety of commercial chemical applications.

The use of cluster chemistry to prepare the catalysts seems to be “a convenient and direct route to produce particles too small to be easily synthesized by conven-tional preparation methods,” says D. Wayne Goodman, a chemistry professor at Texas A&M University, in a commentary accompanying the report. He adds that the cluster particles conform to a very narrow and reproducible size distribution, unlike the products of traditional catalyst preparation.

The cluster-based system could also be extended to other metals or other classes of reactions, including the industrially important epoxidation of propylene, Lambert says. “Going down to such small particle sizes could also trigger other kinds of unexpected chem-istry,” he adds, referencing single-crystal studies in which reactions are sensitive to the geometry of crystal surfaces. —JYLLIAN KEMSLEY

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PULSATING Polymer gel propels an object along chemical waves.

NEWS OF THE WEEK

NANOGOLD OXIDIZES ALONECATALYSIS: Small gold clusters add oxygen to styrene without

help from additives

POLYMER SHRINKS AND SWELLS

GEL PROPULSION: Exotic oscillating reaction moves matter

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