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Analytical Chemistry

Science Concentrates

March 1, 2004 | A version of this story appeared in Volume 82, Issue 9

'Liquid Teflon' used to fabricate microfluidic devices

A fluoropolymer that is liquid at room temperature and resistant to organic solvents has been used to fabricate microfluidic devices, which have micrometer-scale fluid channels [J. Am. Chem. Soc., 126, 2322 (2004)]. Chemistry professor Joseph M. DeSimone at the University of North Carolina, Chapel Hill; physics professor Stephen R. Quake at Caltech; and coworkers relied on a photocurable perfluoropolyether-based elastomer to make the devices. Unlike poly(dimethylsiloxane), the material of choice for many microfluidic applications, perfluoropolyethers are resistant to swelling in common organic solvents. The researchers prepared the fluoropolymer by methacrylate functionalization of a commercially available polymer (shown). They then blended the product with a phenylacetophenone and fabricated the devices by exposing layers of the blends to UV radiation. Until now, microfluidic devices for use with organic solvents have been fabricated from silicon and glass using photolithography and etching techniques. The processes are costly, require clean-room conditions, and are labor intensive. "Our developments allow, for the first time, the use of organic solvents in microfluidic devices made from easy-to-use soft-lithographic techniques," DeSimone says.


Nanocarriers made in one pot

Using three simple chemical building blocks and a one-pot procedure, University of California, Irvine, chemistry professor Zhibin Guan and graduate student Guanghui Chen report a general and facile synthesis of water-soluble, dendritic molecular nanocarriers [J. Am. Chem. Soc., 126, 2662 (2004)]. These polymeric structures, which have a hydrophobic core and a hydrophilic shell, could be used for drug delivery. Guan and Chen make the structures by copolymerizing ethylene and an a-olefin using a palladium-based chain walking catalyst, which introduces branching into mostly linear polymers via an isomerization mechanism. Ethylene polymerized at low pressures forms the dendritic hydrophobic core. And the a-olefin--an alkene with a poly(ethylene glycol) tail--forms the hydrophilic shell and gives the structure its water solubility. Spectroscopic studies confirm that the polymeric structures exist as individual molecular nanocarriers and behave as unimolecular micelles in water.


Carbon anomaly from deep space


Organic material that existed prior to the formation of the solar system has been discovered in an interplanetary dust particle [Science, 303, 1355 (2004)]. Christine Floss, a geochemist at Washington University, St. Louis, led the group that measured the isotopic fractionation of carbon and nitrogen in a dust particle (like the one shown) recovered from Earth's stratosphere. Using a sensitive ion microprobe, they found pockets in which both nitrogen and carbon showed a significantly altered isotopic composition from terrestrial compositions. (The carbon, for example, had a 12C/13C ratio of 96.6, as opposed to the normal ratio of 87.3.) The results suggest that the material formed in cold and radiation-rich interstellar molecular clouds. Researchers previously have found dust particles containing both hydrogen and nitrogen having isotopic anomalies. But Floss and coworkers' discovery of a carbon anomaly indicates that the organic material formed in deep space, survived for some 4.5 billion years or more, and eventually made its way to Earth, Floss says. "You have 8 tons of interplanetary dust coming [to] Earth every day, some of which contains interstellar organic material that may have contributed to the organic matter that formed life on Earth."


Tiny 'scale' allows attogram mass detection

A nanometer-sized silicon cantilever "scale" that can weigh objects at the attogram level (10­18 g) has been developed by graduate student B. Rob Ilic and physics professor Harold G. Craighead of Cornell University and their coworkers. The cantilever, made by standard lithography procedures, oscillates when irradiated with a laser. When molecules or particles adsorb onto the silicon surface, a shift in the oscillation frequency can be measured and correlated to the mass of the adsorbed species. The researchers tested the device by weighing alkanethiol monolayers adsorbed on gold dots deposited on the cantilever surface. The detection limit is 0.39 attograms--three orders of magnitude better than a femtogram cantilever oscillator reported last year by researchers at Oak Ridge National Laboratory. Reducing the cantilever size and altering the oscillation frequency could increase the sensitivity to the zeptogram range (10­21 g), Ilic and coworkers note. Their goal is to develop sensors to detect chemical or biological agents. For example, using the oscillating cantilever method, Craighead's group has previously weighed a bacterial cell following its immunospecific binding to a surface. The new results will be published in an upcoming issue of the Journal of Applied Physics.


H2O2 in martian atmosphere

After years of frustrated searches, astronomers have finally detected the long-postulated existence of hydrogen peroxide in Mars's atmosphere. Astronomer R. Todd Clancy at the Space Sciences Institute in Boulder, Colo., and colleagues used the James Clerk Maxwell telescope atop 14,000-foot Mauna Kea, in Hawaii, to find the first radio spectral evidence of the molecule, which presumably acts as a catalyst in martian atmospheric chemistry. The results will be published in the March issue of Icarus. The observed abundance of H2O2 agrees with that predicted by models. The H2O2 is produced when ultraviolet light photolyzes water vapor. The molecule likely regulates the chemistry of CO, N2, and O2 in the martian atmosphere. The researchers note that H2O2 is hazardous to microorganisms and so would discourage the existence of life on the surface of Mars.



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