Lawrence F. Allard

Observations on Normal and Degenerating Human Otoconia

Specimens of human otoconia obtained from autopsy material and representing various stages from fetal to advanced old age, were studied by microdissection, scanning electron microscopy, electron microprobe analysis, and x-ray powder diffraction. The typical adult otoconial configuration is a cylindrical, finely serrated body with pointed ends; crystallographically, it corresponds to a single crystal of calcite. Other, less numerous types include joined otoconia, pure rhombohedrons and multifaceted, presumably immature forms. Many otoconia achieve the adult configuration during fetal development. The multifaceted otoconia are most numerous, and the rhombohedrons proliferate, during childhood in the utricle. Crystals from both end organs are virtually identical in composition in the young adult, but saccular otoconia are the larger. In middle and advanced age the otoconia decrease in number, especially in the saccule. Saccular otoconia degenerate progressively in a posteroanterior direction across the macula; they assume a specific, fibrous, hollowed-out appearance, which is not duplicated by either chemical etching or autolysis. Neogenesis and growth of otoconia appear to occur postnatally, with different characteristic growth potentials for those of the saccule and the utricle. Age-related saccular otoconial degeneration appears to involve the organic material, which disappears either before or simultaneously with the mineral substance.

Alumina-supported triosmium clusters and ensembles: Characterization by high-resolution transmission electron microscopy

Structurally defined supported osmium clusters [HOs3(CO)10OAl<] were prepared by the reaction of [Os3(CO)12] with the -OH groups of γ-Al2O3. The supported clusters were heated to 200 °C in argon and then reduced in hydrogen at 400 °C, being characterized at different stages of the treatment by high-resolution transmission electron microscopy. After the sample had been heated to 200 °C, scattering centers about 6 A in diameter were evident in the micrographs, their size being consistent with the expected geometry of three-atom ensembles of Os(II) complexes formed by cluster breakup. The ensembles were remarkably stable, and even prolonged exposure of the same area to the 100 keV electron beam caused neither disintegration nor agglomeration. After reduction of the osmium in hydrogen at 400 °C, aggregates of similar size were observed. In contrast to the ensembles of Os(II) complexes, the aggregates seemed to disintegrate slowly under the impact of the electron beam. The difference in stability of the ensembles and the reduced osmium aggregates is attributed to the difference between the strong ionic bonds in the former sample and the weaker interaction between the reduced metal and the Al2O3 surface.

A Multichannel Depth Probe Fabricated Using Electron-Beam Lithography

A multielectrode probe structure is described in which several thin-film metal electrodes are defined on the outer surface of a glass micropipette using electron-beam lithography. Electrode geometries are controlled to within one micron, resulting in electrode recording characteristics which are extremely well matched. Recording sites are 5 , um wide rings spaced 100 , um apart in depth. Analysis and characterization show the structure to be capable of accurately recording tissue potentials with a minimum of tissue damage. Use of these probes in current source-density (CSD) analysis of extraceliular current flow is described.

ULTRASTRUCTURAL AND MICROANALYTICAL RESULTS FROM ECHINODERM CALCITE: IMPLICATIONS FOR BIOMINERALIZATION AND DIAGENESIS OF SKELETAL MATERIAL

Abstract Magnesian calcite skeletal elements of the modern crinoid echinoderm Neocrinus blakei were studied using high resolution TEM, high voltage TEM and STEM microanalysis. Unlike inorganic magnesian calcites which are compositionally heterogeneous, magnesium in these skeletal calcites is homogeneous to at least the 0.1 μm level. While a mosaic structure exists in echinoderm calcite, high voltage TEM reveals the absence of defects or dislocation features which should exist as a consequence of the structure. By comparison, inorganic magnesian calcites show a plethora of defects and dislocation features. High resolution lattice fringe images of the echinoderm calcite exhibit a kinking of fringes between mosaic domains, the boundaries of which are largely coherent. Large scale dislocation structures are not observed. Such a ‘stressed’ lattice structure, if pervasive, explains conflicting observations concerning the ‘single crystal’ or ‘polycrystalline aggregate’ nature of echinoderm calcite. The microstructural and microchemical data demonstrate strong organismal control of skeletal deposition in Echinodermata. Both ultrastructural and compositional heterogeneity/homogeneity should be assessed when determining the susceptibility of skeletal material to diagenetic change.

Silicification of developing internodes in the perennial scouring rush (Equisetum hyemale var. affine)

Abstract An electron microprobe (EMP) analysis of silica (SiO 2 ) deposition in the epidermis of developing internodes of the perennial scouring rush ( Equisetum hyemale var. affine ) indicates that SiO 2 is first detected in the stomatal apparatus beginning with internode 3, then the epidermal papillae (internode 8), and finally in radial cell walls of the long epidermal cells (internode 10). This process is initiated in the intercalary growth regions at the bases of the elongating internodes. The deposition of SiO 2 in long epidermal cell walls occurs after internodal extension has ceased and should therefore be considered as one of the final stages in internodal differentiation that involves strengthening the cellulosic framework of the cell wall. EMP measurements indicate that SiO 2 in stomata is equivalent to 30% of a pure SiO 2 standard and that SiO 2 in the radial walls of long epidermal cells averages twice that measured on the tangential walls of these same cells. This study supports the view that silicification plays a major role in strengthening the developing perennial scouring rush internodal system and that regulation of this process in this and other species of Equisetum , whose SiO 2 deposition patterns are markedly different, deserves further study.

CO hydrogenation catalyzed by alumina-supported osmium: Particle size effects

Alumina-supported catalysts were prepared by conventional aqueous impregnation with [H2OsCl6] and by reaction of organoosmium clusters {[Os3(CO)12], [H4Os4(CO)12], and [Os6(CO)18]} with the support. The catalysts were tested for CO hydrogenation at 250–325 °C and 10 atm, the products being Schulz-Flory distributions of hydrocarbons with small yields of dimethyl ether. The fresh and used catalysts were characterized by infrared spectroscopy and high-resolution transmission electron microscopy. The catalyst prepared from [H2OsCl6] had larger particles of Os (~70 A). The cluster-derived catalysts initially consisted of molecular clusters on the support; the used catalysts contained small Os aggregates (typically 10–20 A in diameter). The catalytic activity for hydrocarbon formation increased with increasing Os aggregate size, but the activity for dimethyl ether formation was almost independent of aggregate size. The hydrocarbon synthesis was evidently catalyzed by the Os aggregates, and the ether synthesis was perhaps catalyzed by mononuclear Os Complexes.

Water Vapor in Closed-Cell In Situ Gas Reactions: Initial Experiments

Modern technology for in situ closed-cell gas-flow allows reactions to be conducted at elevated temperatures and at gas pressures up to one atmosphere [1]. However, introducing water vapor into the cell at percentages above the ~2% possible with room-temperature air at 100% relative humidity (e.g. to mimic bench-top reaction conditions) remains challenging due to the impracticality of heating the gas feed lines into the cell that is necessary to prevent water condensation. Water-vapor experiments are being performed using as a trial specimen MgO nanoparticles (from burning MgO ribbon and collecting the smoke). MgO is a hygroscopic mineral that converts to Mg(OH)2 (brucite) when exposed to water. Because the MgO smoke nanoparticles form perfect cubes, it was hypothesized that they could serve as a sensitive indicator for the presence of water vapor in the closed-cell gas-reactor holder. In situ results will ultimately be correlated to similar experiments performed in an ex situ reactor to confirm the presence of H2O vapor in the cell. Results of preliminary experiments are described here.

Model “Alloy” Specimens for MEMS-Based Closed-Cell Gas-Reactions

MEMS-based closed-cell gas-reaction specimen holders offer unique opportunities for in situ TEM/STEM studies of the behavior of samples such as catalyst powders at elevated temperatures and in gas environments up to a full atmosphere pressure [1,2]. Preparing a cell for catalyst (or nanoparticle) studies typically requires deposition onto the heater (or “E-chip) of the material either from the dry powder, or from a liquid suspension. Comparable studies of “bulk” materials, however, requires preparation of the bulk sample so that a thin slice of suitable geometry (e.g. electron transparent by a few microns in lateral extent) can be secured in some fashion to the E-chip membrane. This process can be conducted using focused-ion-beam (FIB)-milling procedures [3], with the caveat that the milling process typically leaves residual Ga on the slice surface, thereby complicating the reaction process. Nano-sized particles of alloy compositions are not readily available, and crushing alloy powders has also been problematical [4]. We describe here a potential new method for producing alloy specimens of controlled composition and geometry onto gas-cell heater membranes.

From Atoms To Functional Nanomaterials; Structural Modifications As Observed Using Aberration-Corrected STEM

For over a decade aberration-corrected scanning transmission electron microscopy (AC-STEM) has proven essential in the study of nanomaterials. Nanomaterials are essentially assemblies of atoms arranged in such a way that they can serve a myriad of applications. Considering their small size, it is not surprising that subtle changes in the atomic structure can significantly impact functionality. In such systems AC-STEM is indispensable, and unrivalled, being able to probe structural modifications with atomistic detail. The current work aims to track functional nanostructure formation starting from dispersed atomic states; this is followed by case studies documenting how modified atomic structure can impact functionality.

Practical considerations for EDS analysis in an AEM

EDS analysis of thin specimens in an analytical electron microscope is a powerful technique uniquely suited to a wide variety of analytical problems. It is a true “microanalytical” technique, even when compared to electron microprobe analysis, because AEM techniques easily permit the analysis of i0 4 to 10-6 the volumes analysed by EMPA techniques on bulk samples. However, under the conditions and constraints of operation of an AEM, additional analytical problems arise which are not in general encountered in EMPA work. These problems are often subtle, and prove hazardous to EDS analysis in the AEM. This paper will review the practical considerations attendant to the collection and interpretation of both qualitative and quantitative EDS data with which the analyst must be concerned before AEM results can be obtained which are as accurate as the technique theorectically permits. The theoretical aspects of EDS quantification schemes have been given in several excellent review articles (1,2,3), and will not be discussed here.