Science Concentrates

Premelting on the inside

Making candy that melts in your mouth and not in your hands--as the old ad slogan claims--may soon become less art and more science, thanks to a new study. Arjun G. Yodh and Ahmed M. Alsayed of the University of Pennsylvania and their coworkers have shown that premelting, an early stage in the mechanism of melting in which short-range crystalline order is lost below a material's usual melting point, can occur in the bulk of a crystal--not just at its surface, as was shown previously (Science, published online June 30, dx.doi.org/10.1126/science1112399). Using acrylamide-based microgels (colloidal crystals) and video microscopy techniques, the team tracked the motions of individual particles in the interior of the temperature-controlled gels. They observed increased fluctuations in the positions of particles bordering crystal irregularities, such as dislocations and grain boundaries (shown). They note that the extent of disorder depends on the defect type and distance from the crystal defect.

Thiol-modified silica grabs Pd

Palladium is a highly useful catalyst for assembling complex molecules in organic synthesis, but it suffers from a flaw: Palladium can contaminate the product, even when the metal is immobilized on a solid support. Now, Cathleen M. Crudden and coworkers at Queen's University, in Kingston, Ontario, have developed a mesoporous silica modified with mercaptopropyl silyloxy groups (­O₃Si(CH₂)₃SH) that can scavenge loose palladium to less than 1 ppb in reaction solutions (J. Am. Chem. Soc. 2005, 127, 10045). That corresponds to removal of 99.9998% of the palladium originally in solution, which is more efficient than commonly used scavengers and could be a benefit to the pharmaceutical industry. In fact, the material grabs palladium and holds onto it so well that it can in turn be used repeatedly as a catalyst for Suzuki-Miyaura and Mizoroki-Heck coupling reactions, leaving as little as 3 ppb of palladium in solution. Validation tests show that the catalytic activity of the new material occurs on the surface of the silica, rather than by the metal dissociating out into solution as many heterogeneous catalysts do, Crudden says.

Oceans turning dangerously acidic

If current CO₂ emission trends continue, the oceans will become so acidic that corals will cease to thrive, says a report issued on June 30 by the U.K.'s Royal Society. It explains that, in the past 200 years, the average pH of the surface seawater has declined from 8.3 to 8.2, which represents a 30% increase in hydrogen ion concentration. Unless fossil fuel burning is cut back sharply, the pH could fall an additional 0.5 units by 2100, making the oceans more acidic than they have been in millions of years, the report says. The oceans are a sink for CO₂ from fossil fuel, absorbing about half of emissions. When CO₂ dissolves, it produces carbonic acid, which is corrosive to shells of marine organisms and can interfere with the oxygen supply of marine animals. It is relatively easy to predict how much the oceans will acidify under present trends, but difficult to predict exactly what acidic oceans will mean to ocean ecology and to Earth's climate, the report explains.

NO-releasing gold nanoparticles

Gold nanoparticles designed to release nitric oxide in controlled amounts have been developed by chemists at the University of North Carolina, Chapel Hill (J. Am. Chem. Soc. 2005, 127, 9362). The particles may prove useful for a range of biomedical and pharmaceutical applications, including creams that promote wound healing or dilate blood vessels below the skin, say Mark H. Schoenfisch and coworkers. The chemists prepared the NO-releasing nanoparticles from cluster nanoparticles consisting of a core of gold atoms and a protective monolayer of alkanethiol ligands. They functionalized the ends of some of the ligands with bromine and converted these bromo-terminated alkanethiols to amine-terminated alkanethiols. Diazeniumdiolate NO donors (shown) were synthesized by exposing the amine-derivatized particles to NO at high pressure. The amount of NO released from diamine-derivatized nanoparticles under physiological conditions increases with increasing concentration of diamine ligands and with increasing alkyl chain length (x) between the diamine nitrogen atoms.

Growing cells on a microchip

Using microchip-based bioreactors with microfluidic plumbing, researchers have been able to grow and monitor small populations of cells with single-cell resolution for more than hundreds of hours without interference from biofilm formation (Science 2005, 309, 137). As a demonstration, the researchers monitored bacterial cell count and cell morphology of Escherichia coli grown with and without a population control mechanism. They point out that "measurements can be readily extended to dynamic properties--for example, gene expression dynamics and distributions reported by fluorescence or luminescence." Such capability will facilitate high-throughput screening in chemical genetics and drug discovery, they add. The researchers prevent biofilm formation by alternating the operation of the miniature bioreactors between continuous circulation and cleaning and dilution. Otherwise, biofilms form within the channels within 48 hours. A chip that's as wide as a dime and about twice as long accommodates six 16-nanoliter reactors, which can be run independently and simultaneously. The device was designed and tested at Caltech by Frederick K. Balagaddé, Stephen R. Quake, and coworkers.