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December 10, 2001
Volume 79, Number 50
CENEAR 79 50 pp. 45-55
ISSN 0009-2347
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SENSORS AND ALL. New clinical sensors based on microcantilevers and quartz crystal microbalances (QCMs) were among this year's sensor developments. Professor of mechanical engineering Arun Majumdar at the University of California, Berkeley, and coworkers, including Thomas Thundat of Oak Ridge National Laboratory, used microcantilevers to detect prostate-specific antigen (PSA), a marker for early detection of prostate cancer [Nat. Biotechnol., 19, 856 (2001); C&EN, Sept. 10, page 13]. The researchers attached PSA antibodies to a gold-coated microcantilever, a microscopic diving-board-like structure. When a PSA sample was flowed over the microcantilever surface, PSA-antibody binding caused a cantilever deflection proportional to concentration, and the deflection was measured with a laser beam. The microcantilever assay is as sensitive as the enzyme-linked immunosorbent assay procedure currently used to quantify PSA.

ENCODED When coated with specific reagents such as antibodies or DNA, bar-coded microrods developed by Keating, Natan, and coworkers can be used to capture and identify specific proteins and DNA sequences in biological samples.
David Klenerman, Matthew A. Cooper, and colleagues in the department of chemistry at the University of Cambridge developed rupture event scanning, a technique in which a QCM is used to detect viruses [Nat. Biotechnol., 19, 833 (2001)]. Viruses in a sample bound to antibodies on the surface of the QCM, and subsequent QCM oscillations caused the antibody-virus bonds to break. The QCM then acted as a microphone to convert acoustic emissions from the bond ruptures to electrical signals. The researchers found the QCM method was nearly as sensitive as polymerase chain reaction for virus detection.

Chemistry professor Reginald M. Penner and coworkers at the University of California, Irvine, developed a miniaturized H2 sensor based on palladium nanowires that works much better than conventional H2 sensors [Science, 293, 2227 (2001); C&EN, Sept. 24, page 14]. The conventional sensors have macroscopic palladium wires, and adsorption of H2 to those wires increases their resistance. In the new sensors, H2 adsorption to the nanowires lowers their resistance. Not only is this sensing mechanism unique, but the nanowire sensors also exhibit faster response, lower sensitivity to other gases, and lower power consumption than the conventional ones.

Also in the sensor realm in 2001 were "bar-coded" metal microrods for DNA and protein bioassays developed by assistant professor of chemistry Christine D. Keating of Pennsylvania State University, Chief Technology Officer Michael J. Natan of SurroMed (Mountain View, Calif.), and coworkers [Science, 294, 137 (2001); C&EN, Oct. 8, page 13]. Bar-coded microrods were synthesized with bands of up to five different metals by sequential electrochemical reduction of metal ions. Sequence and width variations in these bar codes were distinguishable by light microscopy. Microrods coated with specific reagents, such as antibodies and DNA, can capture particular antigens and complementary DNA sequences, respectively, from biological samples. The DNA fragments and proteins can then be identified using the bar codes and quantitated by other means, such as fluorescence imaging. Keating, Natan, and coworkers demonstrated the technology by using it in a DNA hybridization experiment and an immunoassay protocol, and SurroMed hopes to develop it for a wide variety of bioanalytical applications.

Meanwhile, a porous silicon sensor capable of distinguishing between gram-negative and gram-positive bacteria was developed by assistant professor of chemistry Benjamin L. Miller and coworkers at the University of Rochester [J. Am. Chem. Soc., 123, 11797 (2001); C&EN, Nov. 26, page 23]. An organic receptor compound covalently attached to porous silicon specifically binds lipid A, a key component of gram-negative bacteria cell membranes. This binding interaction causes the photoluminescence spectrum of the biosensor to shift toward red upon exposure to gram-negative bacteria, but not gram-positive ones. The sensor was designed to detect bacteria in wounds and foods, but it could also prove applicable to biological weapons detection.

In fuel-cell research this year, the smallest power source ever built--an enzyme-based fuel cell with sufficient power to run a tiny biosensor-transmitter system--was developed by chemical engineering professor Adam Heller, graduate student Ting Chen, and coworkers at the University of Texas, Austin [J. Am. Chem. Soc., 123, 8630 (2001); C&EN, Sept. 3, page 10]. The device's electrodes were 1/180th the size of the smallest previous biofuel cell, and its power density was five times that of the best previous biofuel cell. Such a device could potentially be implanted in tissue.

In an inorganic chemistry study, microporous titanosilicates suitable for separating gas mixtures were developed by a group including research fellow and manager Steven M. Kuznicki and research associate Valerie A. Bell at Engelhard Corp., Iselin, N.J., and associate professor of chemical engineering Michael Tsapatsis at the University of Massachusetts, Amherst [Nature, 412, 720 (2001); C&EN, Aug. 20, page 15]. The materials' pores can be systematically adjusted to allow access to molecules of one gas while shutting out slightly larger molecules of another. The work "promises to be a new departure in molecular sieve technology and an intriguing new area for academic investigation," a researcher commented.

Self-cleaning glass was developed by two groups. Titanium dioxide on the surface of Activ glass acts as a catalyst in the presence of UV light to reduce organic dust and grime to water and carbon dioxide (C&EN, July 2, page 8). Titanium dioxide also reduces the glass's surface tension, permitting rainwater to easily wash dirt away easily. Activ glass is marketed by British glassmaker Pilkington, Toledo, Ohio. A transparent coating of titanium dioxide on SunClean glass, developed by Pittsburgh-based PPG Industries, also combines with UV light to help loosen and dissolve dirt and other organic material photocatalytically.

And in an environmental study with possible policy implications, biogeochemistry professor William H. Schlesinger of Duke University and coworkers found that forests are not the insatiable sinks for carbon dioxide that some global-change researchers had assumed them to be [Nature, 411, 469 (2001), C&EN; May 28, page 10]. Some scientists had proposed that greater rates of plant and tree growth would sequester sufficient carbon dioxide from the atmosphere to compensate for growing atmospheric levels of CO2, which acts as a greenhouse gas to promote global warming. But Schlesinger's group discovered that after an initial surge, the growth rate of pine trees in a CO2-enriched atmosphere tended to slow down. The slowdown is significant enough that prospects for carbon sequestration by such trees is "unduly optimistic," the researchers concluded.


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