science/ technology concentrates
Aerogels play host to particulate guests Naval Research Laboratory scientists in Washington, D.C., are applying a new technique to incorporate a wide range of particles such as platinum colloids, fine zeolite powders, and polymer powders in silica-based aerogels [Science, 2 8 4 , 622 (1999)]. Aerogels are ultralightweight solidsriddledwith a network of tiny pores. Unlike conventional techniques, the new method, first described in Langmuir [ 15, 674 (1999)], prevents total encapsulation of "guest" particles by the silica, preserving their chemical and transport properties within the aerogel. As a result, research chemist Debra R. Rolison says, "we can now predetermine what components need to be present in the aerogel to provide desired functions for a specific application." Rolison and coworkers mix silica sol (a colloidal suspension of nanoscopic silica particles in a liquid) with a second solid when the sol is just about to gel. The mixture is then dried to form a composite aerogel. Because the pores are interconnected, molecular reactants or analytes can rapidly move into the aerogel interior to react with, or be sensed by, the guest particles.^
talline nickel and nickel aluminide (Ni3Al) become superplastic at 350 °C and 650 ° C—temperatures that are 470 ° C and 450 °C below that of their microcrystalline forms. The group says its findings suggest that "mechanistic details of superplasticity in nanocrystalline materials are fundamentally different from those in microcrystalline materials."^
Plants use H 2 0 2 to prevent sunburn
Plants can use the small, diffusible molecule hydrogen peroxide to send messages about exposure to high-intensity sunlight from one part of the plant to another. Plant molecular biologists Stanislaw Karpinski at the Swedish University of Agricultural Sciences, Umea; Philip Mullineaux at the John Innes Centre, Norwich, England; and their colleagues find that when some leaves of the shade-loving Arabidopsis plant are exposed to excessive sunlight, the plant defends itself from potential harm by activating genes that encode the antioxidant ascorbate peroxidase. These genes are turned on not only in the exposed leaves, Superplastic metals but elsewhere in the plant as well, and the at lower temperatures entire plant is then better able to withstand the harmful effects of subsequent Nanocrystalline metallic materials be- exposure to too much sun [Science, 2 8 4 , come superplastic at much lower temper- 654 (1999)]. In the image above, which atures than their microcrystalline forms, views the plant from above, only leaves according to a research team led by above the white line were exposed to Amiya K. Mukherjee, professor of mate- high-intensity light. Yet antioxidant derials science at the University of Cali- fense genes, modified to fluoresce when fornia, Davis [Nature, 3 9 8 , 684 (1999)]. activated, are turned on throughout the When materials become superplastic, plant: yellow and red indicate highest acthey behave much like chewing gum, un- tivity; blue, lower activity; and black, litdergoing exceptionally large elongations tle or no activity. The activation matches during tensile deformation. Superplastici- an increase in H 2 0 2 in the unexposed ty is of industrial interest, because it can leaves, the researchers say.^4 be used to produce components having complex shapes from materials that are hard to machine, such as metal matrix Fluorinated, chlorinated composites and intermetallics. The use of polyacetylenes superplastic forming might become more common if scientists knew how to Even at very low temperatures, difluoromake materials superplastic at lower tem- and chlorofluoroacetylene tend to experatures. Mukherjee and coworkers have plode. Dichloroacetylene explodes now shown that this can be achieved by when it reacts with air. So despite inmaking materials with grain sizes of 100 tense interest in polyacetylenes in renm or smaller. They find that nanocrys- cent years, polymers of those halogenat-
ed compounds have received little attention. Now, assistant professor of chemistry George L. Gould at the University of Illinois, Chicago, and coworkers have synthesized polydifluoroacetylene, polychlorofluoroacetylene, and polydichloroacetylene and are exploring their structures and properties [/. Am. Chem. Soc, 121,3781 (1999)].The difluoro and chlorofluoro polymers are prepared by slowly warming the monomers from -196 °C. The dichloro material is made by catalytic polymerization. X-ray photoelectron spectroscopy of the deep red, airsensitive fluorocarbon polymer reveals that it is not a simple polyene but contains a significant percentage of CF2 moieties and carbons that don't bear any fluorine substituents—the result of disproportionation reactions, according to Gould. "We think it is a complicated, heavily crosslinked material," he tells C&EN. The chlorofluoro polymer is a mixture of a soluble fraction—probably a linear polyene—and an insoluble fraction that may be cross-linked. The dichloro polymer appears to have a nonplanar polyunsaturated backbone, with the chlorine atoms on the same side of each double bond. ^
'Buckytube' electrodes for electrochemistry Carbon nanotubes have begun to be used as tips for scanning probe microscopes and as electron emitters for field emission displays. Now, researchers at Texas A&M University, College Station, have demonstrated another use for "buckytubes": as nanoelectrodes for electrochemical experiments [J. Am. Chem. Soc, 121, 3779 (1999)]. Graduate student Joseph K. Campbell and research associate l i Sun, working with chemistry professor Richard M. Crooks, make the nanoelectrodes by attaching a single carbon nanotube to the end of a platinum wire with silver epoxy. Preliminary results suggest that the great strength and high length-to-diameter ratio of these nanoelectrodes will be particularly valuable in scanning electrochemical microscopy (SECM) and bioelectrochemistry. Crooks expects that the small diameter of the nanotubetipwill allow SECM studies of corrosion and catalysis on surfaces to be extended into the nanoscale regime. In addition, he thinks it might be possible to insert a nanotubular electrode into a living cell to monitor its rate of metabolism. This has been "one of the Holy Grails of bioelectrochemistry," Crooks notes.^ APRIL 26,1999 C&EN 2 3
