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Ultrafast electron crystallography
Subangstrom imaging
Sample preparation chamber Diffraction chamber Delay
Using ultrafast electron crystallography (UEC), Ahmed Zewail and colleagues at the California Institute of Technology have isolated and studied the structure and dySample transfer namics of self-assembled Ultrashort monolayers (SAMs) on pulses Load lock chamber metal surfaces. The technique allowed the reApparatus used in ultrafast electron crystallography. searchers to achieve unprecedented atomic-scale spatial and temporal resolution. In UEC, a beam of ultrashort electron pulses hits a crystal surface, producing diffraction patterns that can be resolved on femtosecond-to-picosecond time scales and in picometer spaces. This spatial and temporal resolution makes it possible to study nanoscale adsorbates and surface atoms. Zewail and colleagues used UEC to study single-crystal surfaces of Au(III) with and without a SAM. The SAMs included alkanethiols and thio-derivatized iron hemes. After heating the surface with near-IR pulses, the researchers observed the structural dynamics of the gold substrate and adsorbate. The presence of adsorbates was found to alter the diffraction pattern, depending on the angle of incidence. Currently, the researchers are applying the technique to longer-chain SAMs and biological networks. (J. Am. Chem. Soc. 2004, 126, 12,797–12,799)
Stephen Pennycook and colleagues at the Oak Ridge National Laboratory and the Nion Co. have pushed the resolution limits of electron microscopy. Using a scanning transmission electron microscope fitted with a new imaging technology called the Nion aberration corrector, the investigators imaged columns of atoms in a silicon crystal lattice that were only 0.78 Å apart. The ability to image atoms with subangstrom resolution will have an impact on materials, chemical, and nanoscale sciences. For example, the positions of dopant atoms can be detected by this technology, which could lead to a better understanding of materials at the atomic scale. In addition, more types of materials can be imaged in numerous orientations. (Science 2004, 305, 1741) A direct image of a silicon crystal shows pairs of atom columns 0.78 Å apart. (Adapted with permission. Copyright 2004 American Association for the Advancement of Science.)
Enzymatic nanoassay David Klenerman and colleagues at the University of Cambridge (U.K.) have demonstrated an enzymatic assay confined to the tip of a nanopipette. The work could lead to the development of miniaturized, sensitive enzymatic assays with reduced mixing times, lower reagent consumption, and improved S/N. Klenerman and colleagues studied the activity of the enzyme alkaline phosphatase in a volume of ~100 aL. A nanopipette was filled with the substrate fluorescein diphosphate. Fluorescein diphosphate does not fluoresce, but when it is cleaved, its product, fluorescein, does. The nanopipette was introduced into a buffered aqueous solution containing alkaline phosphatase. The enzyme flowed from the solution into the tip and physisorbed to the tip walls. The amount of physisorbed
as a detection region. enzyme was directly Detection Klenerman and colproportional to its region leagues calibrated the fluoconcentration in the rescence intensity of the solution. fluorescein molecules. The investigators They found that the amount controlled the flow of of cleaved substrate inthe substrate into the creased linearly as a functip of the nanopipette tion of both the applied voltby applying a voltage Substrate: Fluorescein diphosphate age and the concentration across the tip. They Enzyme: of alkaline phosphatase in monitored the activiAlkaline phosphatase the solution. Because the ty of the physisorbed Product: Fluorescein assay is limited to a tiny alkaline phosphatase Schematic of a nanopipette, in volume and has high spatial by measuring the which an enzymatic reaction occurs resolution, Klenerman and fluorescence intenin ~100 aL. colleagues suggest that the sity of the cleaved nanopipette can be scanned over cells or fluorescein. The cleaved fluorescein flowed out of the tip into a small, localized area just nanostructures for localized enzymatic asoutside the tip, which the investigators used says. (Nano Lett. 2004, 4, 1859–1862)
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