NEWS OF THE WEEK
SENSORS: At low levels, bacterial lipid causes visible reordering of liquid crystals T MIGHT one day be possible to visually detect endotoxin, a molecular indicator of bacterial infection, with a little bit of liquid crystal, thanks to researchers at the University of Wisconsin, Madison, and the University of California, Davis (Science, DOI: 10.1126/science.1195639). The research team reports that the bacterial lipopolysaccharide, when added at concentrations less than 1 pg/mL to micrometer-sized droplets of liquid-crystal 4´-pentyl4-cyanobiphenyl, causes a structural change in the droplets. This change is visible with a light mi cro scope. The standard assay 5 µm for endotoxin is based on horseshoe crab blood,
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which clots in the molecule’s presence. But the heavily armored crabs have to be caught and bled to give up their blue blood for the assay material. The sensitivity of the liquid-crystal method rivals that of the standard assay. That observation hints that this new method may offer the basis of an assay for endotoxin, says UW Madison chemical engineer Nicholas L. Abbott, the leader of the study. And it doesn’t involve any crab catching. Abbott and coworkers show that endotoxin from Escherichia coli reorders the liquid-crystal droplets, causing a visible defect to form at their centers. By doing calculations and using confocal fluorescence microscopy, the team determined that each droplet’s structure changes because endotoxin, a six-tail lipid from the outer membrane of gram-negative bacteria, aggregates at a point defect in the liquid crystals. These defects—nanoscopic melted areas in the liquid crystals—stem from the droplet’s spherical geometry. Other lipids also change liquid-crystal orientation on surfaces, says Noel A. Clark, a physicist at the University of Colorado, Boulder, but “the striking observation here, and key to the method’s potential utility, is the extremely small concentration of endotoxin required to produce the change—six orders of magnitude lower than other lipids.” Although it’s “fairly straightforward” to assess the optical appearance of pure endotoxin in droplets of liquid crystal dispersed in water, Abbott says, many issues must be addressed before this new method can be used with complicated samples such as blood. Those studies, he adds, are under way.—LAUREN WOLF
CRYSTALLIZATION Stunning structures spring from humble table salt The spectacular spherulite seen in this colorized scanning electron micrograph is none other than table salt, or sodium chloride. The structure is something of a departure for NaCl, which almost always forms cubic crystals. This new ability to grow salts in unusual shapes could help researchers figure out how to improve control of the crystallization of water-soluble compounds. Researchers led by Zhongping Zhang and Suhua Wang, of the Chinese Academy of Sciences’ Institute of Intelligent Machines discovered they could coax NaCl and its chemical cousin KCl into the surprising structures when they grew the crystals at the interface of metastable water microdroplets and an organic solution of cyclohexane and acetone (Angew. Chem.
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Int. Ed., DOI: 10.1002/anie.201101704). Each hollow sphere is composed of dozens of single crystals shaped like hoppers, or square funnels. The spherulite pictured here is made up of crystals 10 µm across. Cyclohexane appears to play a critical role in the crystallization because only cubic crystals form when it is absent from the solution. Zhang, Wang, and colleagues think that cyclohexane stabilizes the water microdroplets by reducing diffusion of acetone into the water. Once the acetone does diffuse into the water, the salt solution becomes supersaturated and crystallization begins. But cyclohexane outside the droplets curtails crystal growth. As a result, the hoppershaped crystals form, arranged in a hollow microsphere.—BETHANY HALFORD
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When added to liquid-crystal droplets (top panels), endotoxin induces a structural change (bottom panels) visible with both a bright-field microscope (left panels) and a polarized microscope (right panels).
A NEW DETECTOR FOR ENDOTOXIN
