November 2006 Archives

By combining coherent X-ray scattering with a method of direct phase recovery called over-sampling, lens-free microscopy in the X-ray region becomes a realistic technique. The latest hot topic is the extension of the technique from two to three dimensions. One of the most promising ways of applying this technique is the recently reported combination of (i) ab initio phase retrieval of 2D coherent diffraction patterns with a guided hybrid input-output algorithm and (ii) 3D image reconstruction with equally sloped tomography. The scheme was applied to quantitative 3D imaging of a heat-treated GaN particle with each voxel corresponding to 17×17×17 nm³. The internal GaN-Ga₂O₃ core shell structure was successfully captured in three dimensions. For more information about the analysis, see the paper, "Three-Dimensional GaN-Ga₂O₃ Core Shell Structure Revealed by X-Ray Diffraction Microscopy", J. Miao et al., Phys. Rev. Lett. 97, 215503 (2006).

A group of biologists led by Professor Guerinot (Dartmouth College, USA) has recently clarified that iron is stored in the developing vascular system of the seed of a plant called Arabidopsis. The group also found that this localization depends on a protein called VIT1, shown to transport iron to the vacuole. The experiments combined traditional mutant analysis (turning on and off the VIT1 protein) with an X-ray fluorescence micro tomography technique to obtain a map of where iron is stored in the seed. The results could help in the development of nutrient-rich seed, benefiting both human health and agricultural productivity, because iron deficiency is an area of concern in the issue of human nutrition. The experiments were done at Beamline X26A, National Synchrotron Light Source (NSLS), Brookhaven, USA. For more information, see the paper, "Localization of Iron in Arabidopsis Seed Requires the Vacuolar Membrane Transporter VIT1 ", S. A. Kim et al., Science, 314, 1295-1298 (2006).

Sodium saccharinate, NaC₇H₄NO₃S•xH₂O, listed in most catalogues as a dihydrate (x = 2), has been extensively used as a food additive and has constituted the basic component of the diabetics' diet for about 125 years. However, due to such factors as the instability of the crystal, the large unit cell and a very complex and heavily disordered structure, scientists have been unable to establish its composition with any certainty, until now. Dr. P. Naumov (Nat'l Inst for Mater. Sci., Japan) and his collaborators recently succeeded in the first determination of the crystal structure, by using special techniques for preserving unstable crystals during X-ray data collection. This crystal structure, which has as many as 16 formula units in the asymmetric unit (Z' = 16) as well as one of the largest unit cells, represents one of the most difficult cases for a small molecular species such as the saccharinate ion. It was found that, instead of being a dehydrate, the crystal is in fact a 1.875 hydrate, because of a structural misfit and the lack of two water molecules per asymmetric unit. The composition can be best described as Na₆₄(C₇H₄NO₃S)₆₄•120H₂O. At a meeting of the Asian Crystallographic Association held in Tsukuba, Japan, Dr. Naumov received the Best Presentation Award. For more information, see the paper, "Solid-state structure and temperature/evacuation-induced dehydration of sodium saccharinate 1.875 hydrate", P. Naumov et al., Angewandte Chemie, International Edition in English, 44, 1251 (2005).

Neanderthals were a species of the Homo genus who inhabited Europe and parts of western Asia approximately 24,000 ~ 350,000 years ago. It has even been suggested that Neanderthals achieved adulthood faster than modern humans do today. At the European Synchrotron Radiation Facility (ESRF), Grenoble, France, the enamel dentine junction of both a deciduous and a permanent Neanderthal molar tooth (about 130,000 years old) was studied recently by using high-resolution tomography. It was found that the dental development of Neanderthals was very similar to modern humans. The permanent molar tooth studied had completed its root growth at about 8.7 years of age, which is typical of many modern human children today. For more information on the experimental results, see the paper, "How Neanderthal molar teeth grew", R. Macchiarelli et al., Nature, published online 22 November 2006. For other recent interesting data on Neanderthals, see the paper, "Palaeoanthropology: Return of the last Neanderthal", E. Delson1et al., Nature, 443, 762-763 (2006).

Professor Weckhuysen (Utrecht University, Netherlands) and his colleagues have recently solved the molecular mechanism for the organic-base-mediated synthesis of zeolites. AlPO₄-5 is a typical zeolite, which can be constructed from aluminium-based tetrahedra (AlO₄) and phosphorus-based tetrahedra (PO₄). The research group compared the formation of the chargeless AlPO₄-5 framework with the negatively charged framework (known as ZnAPO-34) that is formed by replacing Al³⁺ in AlPO4-5 with Zn²⁺. The former contains one-dimensional channels, but the latter spherical cavities rather than channels. By employing not only small and wide angle X-ray scattering (SAXS and WAXS), but also X-ray absorption spectroscopy, it was possible to observe in real time both the structural changes in the aluminophosphate gel and the conformational features of the organic base (tetraethylammonium hydroxide) used as a template for the crystallization of zeolite. The tetraethylammonium ion was found to form a complex with developing zeolite subunits in the gel, adopting a molecular structure close to that found in the final crystal. This molecular recognition process determines which type of crystal lattice is formed. The principal point here is that molecular organization takes place before crystallization. The experiments were done at BM26A, ESRF (Grenoble, France). For more information, see the paper, "A Combined SAXS/WAXS/XAFS Setup Capable of Observing Concurrent Changes Across the Nano-to-Micrometer Size Range in Inorganic Solid Crystallization Processes", A. M. Beale et al., J. Am. Chem. Soc., 128, 12386 (2006). Another interesting account can also be found in "Physical chemistry: Porous solids get organized", R. A. van Santen1, Nature, 444, 46 (2006).

A group at the Max Planck Institute led by Professor H. Dosch recently performed detailed studies on the gap between water and a water-repelling surface. Silicon wafers, functionalized by a self-assembled monolayer of octadecyl-trichlorosilane (OTS), provide strongly hydrophobic substrates. The main interest here is what happens when water comes onto the OTS layer. The experiment was not easy, because the liquid water-solid interface is deeply buried in this case. In this research, X-ray reflectivity measurements using an unusually high-energy beam (72.5 keV) were carried out. The results indicate the existence of a hydrophobic gap on a molecular scale at the solid-water interface. For more information, see the paper, "High-resolution in situ X-ray study of the hydrophobic gap at the water-octadecyl-trichlorosilane interface", M. Mezger et al., Proc. Natl. Acad. Sci. USA, published online before print November 20, 2006

At the FLASH free-electron laser facility at DESY in Hamburg, an international team of scientists recently published the first data on diffraction imaging of a non-crystalline sample. Theoretically, a single X-ray pulse, if it is extremely bright and perfectly coherent, can produce a diffraction pattern from a large macromolecule, a virus or a cell (for example, see, "Potential for biomolecular imaging with femtosecond X-ray pulses", R. Neutze et al., Nature, 406, 752-757 (2000)). In the present experiment, the team tested a laser pulse with 25 fs, 41013 W/cm²/pulse, containing 1012 photons at 32 nm wavelength, and obtained a coherent diffraction pattern from a nanostructured non-periodic object before this exploded into a plasma at ca. 60,000 K. They employed a novel X-ray camera assured of single-photon detection sensitivity by filtering out parasitic scattering and plasma radiation. For more information, see the paper, "Femtosecond diffractive imaging with a soft-X-ray free-electron laser", H. N. Chapman et al., Nature Physics, published online 12 November 2006.

Argonne National Laboratory researchers in collaboration with Xradia, Inc. have developed a novel X-ray surface topography technique by combining X-ray reflection, which is sensitive to height or depth on a sub nanometer scale, and full-field X-ray microscopy with condenser and objective Fresnel zone plates. Recent rapid progress in X-ray microscopy now allows scientists to obtain X-ray images with ca. 10 nm spatial resolution. However, so far, almost all full-filed imaging has employed transmission geometry. The present research has extended the technique to reflection geometry. It has become possible to image the distribution of molecular-scale interfacial features directly and non-invasively with full-field imaging. Interfacial phase contrast from elementary defect structures allows direct observation of 0.6-nm-high monomolecular steps at a solid surface. For more information, see the paper, "Observation of subnanometre-high surface topography with X-ray reflection phase-contrast microscopy", P. Fenter et al., Nature Physics, 2, 700-704 (2006).

Artists in ancient Pompeii painted the town red 2,000 years ago with a brilliant crimson pigment made of cinnabar (HgS) that dominated many of the doomed city's wall paintings. The eruption of the volcano Vesuvius showered the neighbouring towns in pumice and ash, and the Villa Sora, in Torre del Greco, remained buried until just 20 years ago, which is when excavation work started. In the remains of the house, the distinctive red colour of the wall frescoes has turned black in many places. The origins of this darkening degradation have not been clearly identified yet and remain a major issue for curators. At ESRF, by aid of micro X-ray fluorescence and absorption spectroscopy, scientists analyzed red cinnabar paintings coated on a sparry calcite (CaCO₃) mortar exhibiting different levels of degradation. The results indicate two possible degradation mechanisms; formation of HgCl₂ and CaSO₄ through reaction with NaCl and SO₂ from the environment, respectively. For more information, see the paper, "Blackening of Pompeian Cinnabar Paintings: X-ray Microspectroscopy Analysis", M. Cotte et al., Anal. Chem., 78, 7484-7492, (2006).