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PubMedDate not supplied

A differentially pumped secondary electron detector for low-vacuum scanning electron microscopy.

A new design of secondary electron (SE) detector is described for use in low-vacuum scanning electron microscopes. Its distinguishing feature is a separate detector chamber, which can be maintained at a pressure independent of the pressure in the specimen chamber. The two chambers are separated by a perforated membrane or mesh across which an electric field is applied, making it relatively transparent to low-energy electrons but considerably less so to the gas molecules. The benefits of this arrangement are discussed. The final means of detecting the electrons can be a conventional scintillator and photomultiplier arrangement or any of the methods using the ambient gas as an amplifying medium. Images obtained with the detector show good SE contrast and low backscattered electron contribution.

Animals↗
PubMed1994

An electron microscope and electron diffraction study of the effect of calcofluor and congo red on the biosynthesis of chitin in vitro.

The structure of chitin made in vitro by chitin synthetase was studied by electron microscopy and electron diffraction. Two different forms of chitin synthetase from the fungus Mucor rouxii were tested: chitosomes and 16 S particles. The long chitin fibrils produced by chitosomes had a high degree of crystallinity as revealed by electron diffraction. Specimen regions made of largely parallel microfibril bundles produced distinct fiber diagrams, an indication that the chitin chains were aligned along the fibril axis. Microdiffractometry of the shorter chitin crystals synthesized by 16 S particles also showed a similar alignment of chitin chains. Calcofluor and Congo red were powerful inhibitors of chitin synthetase and had profound effects on the color, macroscopic texture, electron microscopic morphology, and crystal structure of the biosynthesized chitin. Chitin made in the presence of Congo red had a bright red color; the one made in the presence of Calcofluor was strongly fluorescent and had a distinctly blue hue when illuminated by daylight. The dyes were tightly bound to the chitin and could not be removed by washing with water or ethanol. At low dye concentration, a mixture of two kinds of crystals was produced by 16 S particles: some were of the same dimensions as those made in the absence of dyes, but others were much thinner. At high dye concentration, there were only thin crystals. With increasing concentrations of Calcofluor or Congo red, the typical electron diffraction reflections of alpha-chitin, particularly the strong equatorial band at 0.466 nm became fainter and a new additional reflection centered at 0.40 nm arose as the dominant feature of the patterns. We regard the gel-like material, synthesized at highly inhibitory dye concentrations, as an apposition complex where the dye does not form part of the crystal structure of chitin and the bulk of the complex consists of molecular stacks of dye associated with nascent chitin chains or narrow chitin microfibrils.

Benzenesulfonates↗
PubMed2000

Investigation of titanium leak to bone tissue surrounding dental titanium implant: electron microscopic findings and analysis by electron diffraction.

This study investigated the tissue response associated with dental titanium implants. The mandibular third and fourth premolars and first molar of three adult beagle dogs were extracted bilaterally. Healing was then allowed for 3 months. Six titanium implants were placed in the mandibles of a dog. Three weeks after the implantation, mandibular sections containing the implants were retrieved with the use of a bone saw and investigated by light and electron microscopy, X-ray microanalyzer, and electron diffraction. Scanning electron microscopic observation showed titanium particles on the implant-bone interface, and investigation by microanalyzer revealed titanium not only on the implant-bone interface but also in the bone tissue. Transmission electron microscopic observation and investigation by electron diffraction showed titanium in the bone matrix and cells other than macrophages. In this study, titanium particles from the dental implant were recognized morphologically in the surrounding bone tissue. Thus, study of the influence of titanium particles on the human body is needed.

Animals↗
PubMed2004

Gated electron transfers and electron pathways in azurin: a NMR dynamic study at multiple fields and temperatures.

Dynamic properties of electron transfer pathways in a small blue copper cupredoxin are explored using an extensive 15N NMR relaxation study of reduced Pseudomonas aeruginosa azurin at four magnetic fields (500-900 MHz) and at two temperatures chosen well below the melting point of the protein. Following a careful model-free analysis, several protein regions with different dynamic regimes are identified. Nanosecond time-scale mobility characterizes various residues of the hydrophobic surface patch believed to mark the natural entry point for electrons, notably the surface-exposed copper-ligand His117. These findings are consistent with a gated electron transfer process according to the "dynamic docking" model. Residues 47-49 along intramolecular pathways of electrons show rigidity that is remarkably conserved when increasing the temperature. Three different conformational exchange processes were observed in the millisecond range, one near the only disulfide bridge in the molecule and two near the copper ion. The latter two processes are consistent with previous data such as crystal structures at various pH values and NMR relaxation dispersion experiments; they may indicate an additional gated electron transfer mechanism at slower time-scales.

Azurin↗
PubMed2004

Evidence for asymmetric electron transfer in cyanobacterial photosystem I: analysis of a methionine-to-leucine mutation of the ligand to the primary electron acceptor A0.

The X-ray crystal structure of photosystem I (PS I) depicts six chlorophyll a molecules (in three pairs), two phylloquinones, and a [4Fe-4S] cluster arranged in two pseudo C2-symmetric branches that diverge at the P700 special pair and reconverge at the interpolypeptide FX cluster. At present, there is agreement that light-induced electron transfer proceeds via the PsaA branch, but there is conflicting evidence whether, and to what extent, the PsaB branch is active. This problem is addressed in cyanobacterial PS I by changing Met688(PsaA) and Met668(PsaB), which provide the axial ligands to the Mg2+ of the eC-A3 and eC-B3-chlorophylls, to Leu. The premise of the experiment is that alteration or removal of the ligand should alter the midpoint potential of the A0-/A0 redox pair and thereby result in a change in the forward electron-transfer kinetics from A0- to A1. In comparison with the wild type, the PsaA-branch mutant shows: (i) slower growth rates, higher light sensitivity, and reduced amounts of PS I; (ii) a reduced yield of electron transfer from P700 to the FA/FB iron-sulfur clusters at room temperature; (iii) an increased formation of the 3P700 triplet state due to P700(+)A0- recombination; and (iv) a change in the intensity and shape of the polarization patterns of the consecutive radical pair states P700(+)A1- and P700(+)FX-. The latter changes are temperature dependent and most pronounced at 298 K. These results are interpreted as being due to disorder in the A0 binding site, which leads to a distribution of lifetimes for A0- in the PsaA branch of cofactors. This allows a greater degree of singlet-triplet mixing during the lifetime of the radical pair P700(+)A0-, which changes the polarization patterns of P700(+)A1- and P700(+)FX-. The lower quantum yield of electron transfer is also the likely cause of the physiological changes in this mutant. In contrast, the PsaB-branch mutant showed only minor changes in its physiological and spectroscopic properties. Because the environments of eC-A3 and eC-B3 are nearly identical, these results provide evidence for asymmetric electron-transfer activity primarily along the PsaA branch in cyanobacterial PS I.

Binding Sites↗
PubMed2005

Probing the contribution of electronic coupling to the directionality of electron transfer in photosynthetic reaction centers.

Subpicosecond transient absorption studies are reported for a set of Rhodobacter (R.) capsulatus bacterial photosynthetic reaction centers (RCs) designed to probe the origins of the unidirectionality of charge separation via one of two electron transport chains in the native pigment-protein complex. All of the RCs have been engineered to contain a heterodimeric primary electron donor (D) consisting of a bacteriochlorophyll (BChl) and a bacteriopheophytin (BPh). The BPh component of the M heterodimer (Mhd) or L heterodimer (Lhd) is introduced by substituting a Leu for His M200 or His L173, respectively. Previous work on primary charge separation in heterodimer mutants has not included the Lhd RC from R. capsulatus, which we report for the first time. The Lhd and Mhd RCs are used as controls against which we assess RCs that combine the heterodimer mutations with a second mutation (His substituted for Leu at M212) that results in replacement of the native L-side BPh acceptor with a BChl (beta). The transient absorption spectra reveal clear evidence for charge separation to the normally inactive M-side BPh acceptor (H(M)) in Lhd-beta RCs to form D+H(M)- with a yield of approximately 6%. This state also forms in Mhd-beta RCs but with about one-quarter the yield. In both RCs, deactivation to the ground state is the predominant pathway of D decay, as it is in the Mhd and Lhd single mutants. Analysis of the results indicates an upper limit ofV2L/V2m < or = 4 for the contribution of the electronic coupling elements to the relative rates of electron transfer to the L versus M sides of the wild-type RC. In comparison to the L/M rate ratio (kL/kM) approximately 30 for wild-type RCs, our findings indicate that electronic factors contribute approximately 35% at most to directionality with the other 65% deriving from energetic considerations, which includes differences in free energies, reorganization energies, and contributions of one- and two-step mechanisms on the two sides of the RC.

Bacteriochlorophylls↗
PubMed2006

Electron transport and redox reactions in carbon-based molecular electronic junctions.

A unique molecular junction design is described, consisting of a molecular mono- or multilayer oriented between a conducting carbon substrate and a metallic top contact. The sp2 hybridized graphitic carbon substrate (pyrolyzed photoresist film, PPF) is flat on the scale of the molecular dimensions, and the molecular layer is bonded to the substrate via diazonium ion reduction to yield a strong, conjugated C-C bond. Molecular junctions were completed by electron-beam deposition of copper, titanium oxide, or aluminium oxide followed by a final conducting layer of gold. Vibrational spectroscopy and XPS of completed junctions showed minimal damage to the molecular layer by metal deposition, although some electron transfer to the molecular layer resulted in partial reduction in some cases. Device yield was high (>80%), and the standard deviations of junction electronic properties such as low voltage resistance were typically in the range of 10-20%. The resistance of PPF/molecule/Cu/Au junctions exhibited a strong dependence on the structure and thickness of the molecular layer, ranging from 0.13 ohms cm2 for a nitrobiphenyl monolayer, to 4.46 ohms cm2 for a biphenyl monolayer, and 160 ohms cm2 for a 4.3 nm thick nitrobiphenyl multilayer. Junctions containing titanium or aluminium oxide had dramatically lower conductance than their PPF/molecule/Cu counterparts, with aluminium oxide junctions exhibiting essentially insulating behavior. However, in situ Raman spectroscopy of PPF/nitroazobenzene/AlO(x)/Au junctions with partially transparent metal contacts revealed that redox reactions occurred under bias, with nitroazobenzene (NAB) reduction occurring when the PPF was biased negative relative to the Au. Similar redox reactions were observed in PPF/NAB/TiO(x)/Au molecular junctions, but they were accompanied by major effects on electronic behavior, such as rectification and persistent conductance switching. Such switching was evident following polarization of PPF/molecule/TiO2/Au junctions by positive or negative potential pulses, and the resulting conductance changes persisted for several minutes at room temperature. The "memory" effect implied by these observations is attributed to a combination of the molecular layer and the TiO2 properties, namely metastable "trapping" of electrons in the TiO2 when the Au is negatively biased.

Carbon↗
PubMed2004

L358P mutation on cytochrome P450cam simulates structural changes upon putidaredoxin binding: the structural changes trigger electron transfer to oxy-P450cam from electron donors.

To investigate the functional and structural characterization of a crucial cytochrome P450cam (P450cam)-putidaredoxin (Pdx) complex, we utilized a mutant whose spectroscopic property corresponds to the properties of the wild type P450cam in the presence of Pdx. The 1H NMR spectrum of the carbonmonoxy adduct of the mutant, the Leu-358 --> Pro mutant (L358P), in the absence of Pdx showed that the ring current-shifted signals arising from d-camphor were upfield-shifted and observed as resolved signals, which are typical for the wild type enzyme in the presence of Pdx. Signals from the beta-proton of the axial cysteine and the gamma-methyl group of Thr-252 were also shifted upfield and down-field, respectively, in the L358P mutant as observed for Pdx-bound wild type P450cam. The close similarity in the NMR spectra suggests that the heme environment of the L358P mutant mimics that of the Pdx-bound enzyme. The functional analysis of the L358P mutant has revealed that the oxygen adduct of the L358P mutant can promote the oxygenation reaction for d-camphor with nonphysiological electron donors such as dithionite and ascorbic acid, showing that oxygenated L358P is "activated" to receive electron from the donor. Based on the structural and functional characterization of the L358P mutant, we conclude that the Pdx-induced structural changes in P450cam would facilitate the electron transfer from the electron donor, and the Pdx binding to P450cam would be a trigger for the electron transfer to oxygenated P450cam.

Ascorbic Acid↗
PubMed1988

Kinetics and mechanism of electron transfer between purines and pyrimidines, their dinucleotides and polynucleotides after reaction with hydrated electrons; a pulse radiolysis study.

The radical spectra of mixtures of thymidine 5'-monophosphate (TMP) or uridine 5'-monophosphate (UMP) with adenine 5'-monophosphate (AMP) after hydrated electron attack, measured from 5 to 3000 microsecond after pulse radiolysis, can only be described in terms of the radical spectra of the nucleotides if an electron transfer is taken into account from the purine radical anion to the pyrimidine, resulting in the formation of a pyrimidine radical anion. From analysis of the spectra of the dinucleoside phosphates ApU, dApT and dCpdA after eaq- attack it follows that the electron-donating species is the purine radical anion (A-.) rather than the protonated purine radical. The electron transfer competes with the fast protonation of the purine radical anion: A-. + py----A + py.- and A-. + H2O in equilibrium AH. respectively. The electron transfer is found to have a diffusion-controlled reaction rate constant of approximately 1.2 X 10(10) for TMP and 3.5 X 10(9) dm3 mol-1 s-1 for UMP.

Electron Transport↗
PubMed1974

Electron microscopy and electron probe analysis of mitochondrial cation accumulation in smooth muscle.

The contractile responses to barium and the ultrastructure and ionic composition of mitochondria were studied in vascular smooth muscle. In normal rabbit portal anterior mesenteric vein (PAMV) and main pulmonary artery (MPA) smooth muscle mitochondria were frequently associated with the surface vesicles. The average distance between the outer mitochondrial and inner surface vesicle membrane was 4-5 nm. Ba contractures of MPA were tonic and of PAMV were phasic. Incubation of MPA and PAMV with Ba resulted in the accumulation of mitochondrial granules, followed in the MPA by massive mitochondrial swelling. Oligomycin and anoxia inhibited the appearance of mitochondrial electron-opaque granules and prevented the Ba-induced mitochondrial swelling in the MPA. Electron probe analysis of mitochondria in PAMV incubated with Ba and containing granules showed characteristic Ba signals over the mitochondria. Electron probe X-ray microanalysis also showed a highly significant (P < 0.001) correlation of P with mitochondrial Ba, in an estimated elemental ratio of approximately 3 Ba/4 P. Mitochondrial granules were still prominent after block staining of the osmium-fixed, Ba-loaded PAMV, but electron probe microanalysis showed no Ba, but only U, emissions. Tissues incubated with strontium had electron-opaque mitochondrial granules and deposits in the sarcoplasmic reticulum. X-ray microanalysis of mitochondria containing granules showed the presence of characteristic Sr and Ca emissions. The presence of Sr was similarly verified in the sarcoplasmic reticulum. These findings indicate the energy dependent uptake of divalent cations, in association with phosphate, by mitochondria in vascular smooth muscle in situ and the possibility that mitochondria may contribute to the regulation of intracellular divalent cation levels in smooth muscle.

Animals↗
PubMed2001

Electronic structure studies of quinones and semiquinones: accurate calculation of spin densities and electron paramagnetic resonance parameters.

The application of electronic structure methods to the prediction of geometries, spin densities, and hyperfine couplings for biologically relevant quinones and semiquinones is reviewed. It is demonstrated that hybrid-type density functional methods are particularly suitable for such studies. Hydrogen bonding to the semiquinone oxygen by appropriate donors is shown to lead to a redistribution of spin density in the carbonyl group of the semiquinone. Experimental trends are well reproduced by the calculated values. Symmetric and asymmetric models of hydrogen bonding are modelled. It is shown that the symmetric models give good agreement with solution studies in vitro. The asymmetric models of hydrogen bonding give quite good agreement with values measured for in vivo semiquinones generated in the reaction centres of the purple photosynthetic bacterium, Rb sphaeroides, and also for the phyllosemiquinone free radical formed during electron transfer in Photosystem I of green plants. These recent advances in electronic structure calculations, in particular the applicability of density functional methods to the study of free radical properties, have opened up an exciting avenue for the complete characterisation of their electronic properties. In particular, the combination of experimental methods of electron paramagnetic resonance and such calculations should in future provide a clearer understanding of free radical chemistry in many areas of biology.

Electron Spin Resonance Spectroscopy↗
PubMed1985

Partial purification and characterization of glutaryl-coenzyme A dehydrogenase, electron transfer flavoprotein, and electron transfer flavoprotein-Q oxidoreductase from Paracoccus denitrificans.

Glutaryl-coenzyme A (CoA) dehydrogenase and the electron transfer flavoprotein (ETF) of Paracoccus denitrificans were purified to homogeneity from cells grown with glutaric acid as the carbon source. Glutaryl-CoA dehydrogenase had a molecular weight of 180,000 and was made up of four identical subunits with molecular weights of about 43,000 each of which contained one flavin adenine dinucleotide molecule. The enzyme catalyzed an oxidative decarboxylation of glutaryl-CoA to crotonyl-CoA, was maximally stable at pH 5.0, and lost activity readily at pH values above 7.0. The enzyme had a pH optimum in the range of 8.0 to 8.5, a catalytic center activity of about 960 min-1, and apparent Michaelis constants for glutaryl-CoA and pig liver ETF of about 1.2 and 2.5 microM, respectively. P. denitrificans ETF had a visible spectrum identical to that of pig liver ETF and was made up of two subunits, only one of which contained a flavin adenine dinucleotide molecule. The isoelectric point of P. denitrificans ETF was 4.45 compared with 6.8 for pig liver ETF. P. denitrificans ETF accepted electrons not only from P. denitrificans glutaryl-CoA dehydrogenase, but also from the pig liver butyryl-CoA and octanoyl-CoA dehydrogenases. The apparent Vmax was of similar magnitude with either pig liver or P. denitrificans ETF as an electron acceptor for these dehydrogenases. P. denitrificans glutaryl-CoA dehydrogenase and ETF were used to assay for the reduction of ubiquinone 1 by ETF-Q oxidoreductase in cholate extracts of P. denitrificans membranes. The ETF-Q oxidoreductase from P. denitrificans could accept electrons from either the bacterial or the pig liver ETF. In either case, the apparent Km for ETF was infinitely high. P. denitrificans ETF-Q oxidoreductase was purified from contaminating paramagnets, and the resultant preparation had electron paramagnetic resonance signals at 2.081, 1.938, and 1.879 G, similar to those of the mitochondrial enzyme.

Amino Acids↗
PubMed2006

Palatine mucosa of aging and experimentally induced diabetic aging rabbits: light-microscopic, scanning electron microscopic and transmission electron microscopic qualitative study.

BACKGROUND: Data associating palatine mucosa, microvascular architecture, aging and diabetes mellitus are few, so the aim of the present study was to examine these conditions structurally and ultrastructurally. METHODS: We used 18 female rabbits, aged about 2 years at the beginning of the experiment, distributed into 2 groups: aging diabetic and aging animals, prepared by three different methodologies: light microscopy, scanning electron microscopy and transmission electron microscopy. The samples were stained with hematoxylin-eosin for light microscopy, and for scanning electron microscopy, we used Mercox Cl-2B vascular corrosion casts. For transmission electron microscopy, the specimens were fixed and embedded in Epon 812 resin. RESULTS: Under light microscopy, we noted small amounts of underdeveloped connective papillae and significant flattened areas through the epithelium-connective tissue interface in the aging diabetic group. Larger blood vessels were deeply located in the palatine mucosa, branching off in the direction of the epithelium-connective tissue interface. Capillaries followed the interface contour. In both groups, vascular corrosion casts revealed capillaries sprouting out in disorganized rows but parallel between themselves. 'Hair-pin' capillary loops and convoluted capillary loops were noted. CONCLUSION: The aging diabetic group showed rarefying microvasculature areas with complex tortuous capillary loops. Transmission electron microscopy results showed that the aging diabetic group presented small cytoplasmatic projections directed to the vessel lumen and micropinocytic vesicle, i.e. caveolas. Epithelium-connective tissue interface, connective papillae, microangioarchitecture, and information about endothelial cells alterations were observed in the aging diabetic and aging animals.

Age Factors↗
PubMed1987

Periodontally diseased vs. normal roots as evaluated by scanning electron microscopy and electron probe analysis.

A scanning electron microscope and electron probe study was carried out to compare root structure from deep within periodontal pockets with roots exhibiting no periodontal disease. To eliminate the possibility of extraneous ion introduction or deletion, no attempt was made to subject the roots to fixation, embedding, or dehydration prior to sectioning and viewing in the electron microscope. Mineral content and concentration were determined with an electron probe on two specimens per tooth. On one specimen, only the external cemental surface was analyzed. On an adjacent cross section, readings were taken from the surface and at incremental depths into the root. A nonparametric statistical analysis compared diseased with nondiseased roots. The following conclusions can be drawn from the study: (1) Consistent and repeatable qualitative electron probe analyses can be performed on human teeth with minimal specimen preparation. This gives a more accurate assessment, since the integrity of the crystalline structure is not disrupted. (2) Minerals consistently found were P, Ca, Cu, Zn, Mg and Na. They were in similar concentrations throughout the area analyzed. (3) Mg and Cu showed higher values in the nondiseased teeth. (4) There were no differences in concentrations for Ca, P, Zn, and Na between roots exposed to a periodontal pocket and nondiseased roots.

Copper↗
PubMedDate not supplied

[Role of the electron acceptor properties of lysozyme and its substrate inhibitors in photosensitized electron transport in their complexes by the EPR method].

The nature of electron-acceptor groups in the system of lysozyme with its substrate-inhibitors has been studied in a wide range of pH values by the method of photosensitized electron transfer. In the lysozyme molecule disulphide bonds and peptide groups are the electron--acceptor groups. The nature of radicals in irradiated lysozyme depends on pH. At complex-formation of lysozyme with oligomeres of N-acetylglucosamine the electron transfer from the enzyme molecule to N-acetyl group of the substrate-inhibitor molecule is realized. Under conditions ruling out complex-formation of lysozyme with the inhibitors (N-acetylglucosamine and its dimer) the electrons are localized on disulphide bonds of the protein molecules at alkaline pH and at pH less than or equal to 3 the radicals are observed which are due to the remove of hydrogen atom from the Calpha-atom of the protein polypeptide chain.

Acetylglucosamine↗
PubMed1988

An electron nuclear double resonance investigation of redox-induced electronic structural change at CuA2+ in cytochrome c oxidase.

We measured an electronic change at cysteine ligand(s) of the CuA2+ center brought on by reduction of other metal centers within cytochrome c oxidase, notably cytochrome a. This change specifically manifested itself as a modification in magnetic hyperfine coupling to the beta-protons of the beta-carbons adjacent to the cysteine sulfur in the CuA2+ coordination sphere. The electron nuclear double resonance ENDOR signals of these beta-protons had previously been assigned through study of selectively deuterated yeast oxidase. In the present study the ENDOR signals of the CuA2+ center were compared from the following forms of oxidase: resting (a3+.CuA2+.a3+3.CuB2+); mixed valence, 2-electron-reduced CO-ligated oxidase (a3+.CuA2+.a2+3CO.CuB+), and a more completely reduced mixed-valence CO-ligated oxidase. In agreement with previous studies on 3-electron-reduced oxidase, the latter more completely reduced oxidase showed cytochrome a preferentially reduced with respect to CuA, implying that the majority of paramagnetic CuA2+ centers had reduced cytochrome a partners. The ENDOR-resolved splitting of the beta-proton hyperfine features substantially decreased in going from the first two more oxidized forms to the more fully reduced latter form. Thus, the electronic structure of the CuA2+ center specifically monitored by hyperfine couplings to cysteine protons changed in response to a reductive event elsewhere in the protein. This structural change may correlate with the anticooperative redox interaction recently reported between cytochrome a and CuA.

Animals↗
PubMed1982

A new method of magnifying photographic images using the scanning electron microscope in the backscattered electron detection mode.

This paper describes a new method of magnifying small images in photographic film by means of a scanning electron microscope (SEM) operated in the backscattered electron detection mode. The study included tests of several types of radiographic film, transmission electron microscopy film, and black and white 35 mm film. The electron optical enlargement method is particularly useful in situations where the film sample is opaque to light and for generating enlarged images at magnifications beyond the reach of light optical enlargement methods, i.e. up to approximately 2000X with ease and rapidity in a single step. The electron optical enlargements compare favorably in contrast and detail with the enlargements made with a light microscope and with a darkroom enlarger.

Breast Diseases↗
PubMed1982

Conditions required for high quality high magnification images in secondary electron-I scanning electron microscopy.

High quality of secondary electron (SE) images, taken at useful magnifications of 100,000 to 200,000, require new signal generation and collection methods and new metal coating procedures. High quality is defined as the condition under which image contrast describes accurately the topographic features of the specimen in a size range that approximates the beam diameter. Such high resolution contrasts are produced by the SE (SE-I) generated by a small electron probe on the specimen surface. Tobacco mosiac virus and ferritin molecules deposited on bulk substrates were introduced as test specimens to check the image quality obtained. The SE-I signal contrast could be imaged when SE (SE-III), produced by backscattered electrons (BSE) at the pole piece of the final lens, were eliminated with an electron absorption device attached to the pole piece. This signal collection procedure will be referred to as "Secondary Electron-I Image" (SE-I image) mode. In addition to the SE-III, BSE generate SE-II in the specimen itself. On specimens deposited on bulk gold or platinum, and coated with the same metals SE-II produced a microroughness contrast that limited particle resolution in the SE-I image mode to approximately 10 nm. Reduction of SE-II and enrichment of the signal in SE-I was achieved by using continuous fine crystalline coatings of tantalum, niobium and chromium. By applying these metals in films of approximately 2.0 nm thickness, the SE-I contrast generation was found to be indepedent of the atomic number of the metal. Edge sharpness was improved when the specimens were coated with low atomic number metals. Under these conditions, the quality of images obtained in SE-I image mode equals that of images obtained in TEM from identically coated specimens and was limited only by the size of the topographic details, beam diameter and beam current.

Ferritins↗