Structural changes occurring during the stripping of thin films of silver, copper, gold and nickel from their substrates
Abstract
Thin films of silver, copper, gold and nickel (island, coalescence and channel stage) were evaporated in high and ultrahigh vacuum onto cleavage faces of NaCl, KCl, MgO and mica and were backed with a supporting layer of SiO or carbon. The films were stripped in water or acidic media and the resulting morphology and structure were examined by transmission electron microscopy and transmission electron diffraction. A close examination of the microstructure of the replicating film and a comparison with over-growths that had been prepared simultaneously but shadowed with platinum or Pt-C revealed that the original film structure is generally not retained during chemical detachment from the substrate. Inter alia, the contraction of particles, coalescence, agglomeration and a loss of particles were observed. These artefactual changes were examined in detail for different substrate-overgrowth combinations and were found to be particularly significant on silver films. The results are compared with those of other workers, and possible mechanisms for the formation of artefacts as well as the means of their prevention are discussed.
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Cited by (5)
Formation of metal nanoclusters on specific surface sites of protein molecules
2002, Journal of Molecular BiologyDuring vacuum condensation of metals on frozen proteins, nanoclusters are preferentially formed at specific surface sites (decoration). Understanding the nature of metal/protein interaction is of interest for structure analysis and is also important in the fields of biocompatibility and sensor development. Studies on the interaction between metal and distinct areas on the protein which enhance or impede the probability for cluster formation require information on the structural details of the protein's surface underlying the metal clusters. On three enzyme complexes, lumazine synthase from Bacillus subtilis, proteasome from Thermoplasma acidophilum and GTP cyclohydrolase I from Escherichia coli, the decoration sites as determined by electron microscopy (EM) were correlated with their atomic surface structures as obtained by X-ray crystallography. In all three cases, decoration of the same protein results in different cluster distributions for gold and silver. Gold decorates surface areas consisting of polar but uncharged residues and with rough relief whereas silver clusters are preferentially formed on top of protein pores outlined by charged and hydrophilic residues and filled with frozen buffer under the experimental conditions. A common quality of both metals is that they strictly avoid condensation on hydrophobic sites lacking polar and charged residues. The results open ways to analyse the binding mechanism of nanoclusters to small specific sites on the surface of hydrated biomacromolecules by non-microscopic, physical-chemical methods. Understanding the mechanism may lead to advanced decoration techniques resulting in fewer background clusters. This would improve the analysis of single molecules with regard to their symmetries and their orientation in the adsorbed state and in precrystalline assemblies as well as facilitate the detection of point defects in crystals caused by misorientation or by impurities.
Epitaxially grown model catalyst particles of platinum, rhodium, iridium, palladium and rhenium studied by electron microscopy
1995, Thin Solid FilmsIn order to obtain oriented thin film model catalysts, small particles of Pt, Rh, Ir, Pd and Re (2–20 nm in size) were grown by high vacuum evaporation on NaCl cleavage faces or on in situ deposited NaCl films at 523–673 K. The particles were covered with a supporting film of Al2O3 or carbon and removed from the substrate. High resolution electron microscopy, selected area electron diffraction and weak-beam dark-field imaging were applied to determine the particular morphology, microstructure and orientation of the observed particles. Special attention was paid to Rh particles which appear in a variety of shapes. Pt, Ir and Pd model catalysts consist mainly of (001) oriented half octahedra which may exhibit truncations at the corners or on the top. This was also the dominant shape of Rh particles but in addition half tetrahedra in (011) epitaxy and multiply-twinned particles like decahedra in (001), (011) and (111) orientation were evident. These habits provide a definite “initial state” for study of the changes in structure and morphology of the particles during activating heat treatments necessary to induce catalytic activity of the Al2O3 supported metal films. Although Re films consisted of irregularly shaped particles, electron diffraction revealed a partial epitaxial alignment of both c.p.h. and f.c.c. Re.
Metal decoration of biomacromolecules and molecular assemblies: a review
1992, UltramicroscopyApplications of metal decoration for the study of molecular assemblies and macromolecules of biological interest are reviewed. The principles of thin film formation and preferred cluster stabilization are discussed. On inorganic crystals decoration occurs on imperfections of the crystal surface. On proteins the metal distribution is related to the topochemistry of the molecule's surface. Two- or three-dimensional crystals of the molecules are needed to obtain an averaged decoration pattern but the possibility of applying decoration to non-crystalline specimens has been discussed. Decoration techniques have been used for the observation of periodic structures, e.g. lipid phases, and for the distinction of the two faces of bacterial membranes and bacterial surface layers. On frozen-hydrated crystals of enzymes molecular symmetries have been observed and first attempts to correlate decoration sites of different metals to molecular structure have been reported. The technique has been applied to support the packing analysis of protein crystals.
Electron microscopy of subnanometer surface features on metal-decorated protein crystals
1989, Journal of Molecular BiologyCrystals of heavy riboflavin synthase from Bacillus subtilis were freeze-etched and vacuumcoated at normal incidence with 0.1 to 0.4 nm of gold and silver, respectively. This decoration technique was applied to probe the protein surface for preferential nucleation sites. Image processing of the electron micrographs revealed two particular decoration sites for silver and a different one for gold. According to X-ray crystallography, the riboflavin synthase molecules are spherical and smooth except for a surface corrugation of less than 1 nm, which can not be depicted by heavy-metal shadowing. Thus the decoration sites represent sites of specific physical-chemical interactions between the condensing metal and the protein. The decoration pattern correctly reflects the icosahedral symmetry of the almost spherical protein molecules. Owing to the molecule's symmetry, the position of these topochemical sites with respect to the symmetry axes can be localized within 5 Å. The packing of the molecules in the crystal can be directly observed on shadowed replicas. Only decoration, however, makes it possible to observe the exact orientation of the molecules within the crystal planes and to derive the true lattice constant along the 6-fold screw axis. This proves decoration to be a technique suitable for studying crystal packing and the molecular symmetry of protein complexes at high resolution. The technique can be applied to crystals that are not large enough or insufficiently ordered for X-ray crystallography.
Decoration and shadowing of freeze-etched catalase crystals
1985, UltramicroscopyAqueous suspensions of catalase crystals were freeze-cleaved, deep-etched and either shadowed with Ta/W at 45° or decorated with 0.1–0.9 nm thick deposits of Au and Pt at normal incidence. The electron micrographs of the decorated specimens were processed by correlation averaging and compared with a relief reconstruction obtained from shadowed specimens. Pronounced decoration was observed on the catalase crystals at temperatures between 130 and 180 K. Disregarding the difficulties in interpretation, the averages of 0.1–0.2 nm thick Au and Pt films reveal more structural detail than the relief reconstruction. Perfect shadowing provides information on surface topography and is relatively easy to comprehend; decoration renders variations in physico-chemical affinity visible. Problems of the interference of decoration and shadowing effects in the intuitive interpretation of freeze-etch replicas and in relief reconstruction are discussed as well as the disturbing effects of a non-ideal carbon backing. The perspectives of using decoration intentionally as a positive staining technique for the investigation of frozen-hydrated surfaces are evaluated and quality criteria are defined.