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Query: UMLS:C0276640 (TEM)
 20,729 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The high-pressure freezing (HPF) technique was applied to the cryo-immobilization of alginate gels and the quality of the freezing analyzed on a TEM by comparison of the segregation pattern of samples of decreasing thickness. Dynamic simulations of heat transfer within an idealized slab of pure water surrounded by two walls of aluminium were performed to illustrate the effect of the heat-transfer coefficient by convection on the cooling rate of the sample. Heat-transfer coefficients in liquid nitrogen and liquid propane at ambient pressure were measured using a carefully characterized thermocouple and the values incorporated as parameters in heat-transfer simulations to compare the efficiency of the plunge-freezing technique with the high-pressure freezing technique. Values of the heat-transfer coefficient in liquid nitrogen and liquid propane, calculated between 273 K and 173 K were 670 and 18420 W/m(2)/K, respectively. Based on TEM observations and the results of heat-transfer simulations, the HPF technique was adapted to the cryo-fixation of 50-microm-thick alginate gels. The occurrence of artifacts was rejected because no differences were observed in the pattern of cryo-fixed and freeze-substituted samples of various thickness, with and without ethanol as cryo-protectant. A sample thickness of 50 microm was found to ensure an adequate preservation of structures as small as a few nanometers, as verified by TEM and SEM observations. Finally, DSC measurements on alginate solutions and alginate beads revealed that under the experimental conditions (0-3%), alginate cannot be considered to be an efficient cryo-protectant.
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PMID:Low-temperature electron microscopy for the study of polysaccharide ultrastructures in hydrogels. I. Theoretical and technical considerations. 1211 12

The silica-pillared derivatives from kanemite (NaHSi(2)O(5).3H(2)O) were prepared by intercalation of dialkyldimethylammonium (DADMA) ion and pillaring with tetraethylorthosilicate. The formation of silica pillars between the silicate sheets was demonstrated by X-ray diffraction, (29)Si CP/MAS NMR, and TEM observation. The basal spacing depended on the chain length of DADMA. Nitrogen adsorption study showed that the specific surface area was enlarged over 1000 m(2) g(-1) by the pillaring and that the pore size was in the micropore region. Water and benzene adsorption isotherms revealed that the surface properties of the pillared derivatives show hydrophobic character.
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PMID:Preparation and characterization of silica-pillared derivatives from kanemite. 1270 82

Manganese oxide hollow nanospheres were prepared using a straightforward, template-free synthesis. The resulting material was mesoporous, crystalline, and of uniform diameter. The nanospheres were characterized by XRD, HR-SEM, and HR-TEM, and pore size distributions were calculated from nitrogen desorption. Unlike previous synthesis methods that use an inorganic template, this procedure requires no separation after synthesis to remove the template. The nanospheres are composed of hexagonal gamma-manganese oxide flakes and are approximately 400 nm in diameter. gamma-MnO2 is composed of a ramsdellite matrix (1 x 2 tunnels) with randomly distributed microdomains of pyrolusite (1 x 1 tunnels). These materials could have applications as cathodic battery materials, oxidation catalysts, catalyst supports, and adsorbents for pollutants.
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PMID:Self-assembly of microporous manganese oxide octahedral molecular sieve hexagonal flakes into mesoporous hollow nanospheres. 1270 32

For the first time, nanoporous ceria and ceria-zirconia solid solutions with high surface area have been successfully synthesized directly via high-intensity ultrasound irradiation without thermal post-treatment. The ceria and the solid solutions were characterized by XRD, TEM, and nitrogen adsorption. It was found that the nanoporous structures of the materials obtained were formed by the agglomeration of monodispersed nanoparticles under high-intensity ultrasound irradiation.
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PMID:Direct sonochemical preparation of high-surface-area nanoporous ceria and ceria-zirconia solid solutions. 1274 56

The copper-tellurolate cluster [(Cu(6)(TePh)(6)(PPh(2)Et)(5)] has been loaded into the pores of MCM-41 by solid-state impregnation techniques. It was found that the best loading conditions are 110 degrees C and 10(-)(3) Torr static vacuum. The resulting material was analyzed by powder X-ray diffraction (PXRD), nitrogen adsorption isotherms, thermogravimetric analysis (TGA), (31)P CP MAS NMR spectroscopy, and TEM. It was observed that loading is accompanied by loss of the phosphine shell, with retention of the copper-tellurium core. Condensation of the impregnated material may proceed thermally or photochemically. Thermal condensation results in the formation of Cu(2)Te nanoparticles as demonstrated by PXRD, and TEM data suggests that the process has taken place inside the pores of MCM-41. Photochemical condensation yields larger metal-chalcogen clusters in the pores as suggested by the result of UV-vis diffuse reflectance spectroscopy and TEM measurements.
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PMID:Preparation, characterization, and condensation of copper tellurolate clusters in the pores of periodic mesoporous silica MCM-41. 1470 65

Gold-nanoparticle/organic films were prepared via layer-by-layer self-assembly using dodecylamine-stabilised Au-nanoparticles and poly(propyleneimine) (PPI) dendrimers of generation one to five (G1-G5) or hexadecanedithiol (HDT) as linker compounds. TEM and FE-SEM images revealed that the bulk of the films consisted of nanoparticles with diameters of about 4 nm. XPS was used to study the chemical composition of the films. The C 1s and N 1s signals of an AuPPI-G4 film were interpreted qualitatively according to the dendrimer structure. The absence of the nitrogen signal in case of an AuHDT film indicated that the dodecylamine ligands were quantitatively exchanged during film assembly. About 76% of the sulfur atoms were bound to the nanoparticles. the remainder being present as free thiol (S H) groups. All films displayed linear current voltage characteristics and Arrhenius-type activation of charge transport. The conductivities of the AuPPI films decreased exponentially over approximately two orders of magnitude (6.8 x 10(-2) to 1.0 x 10(-3) ohms(-1) cm(-1)) with a five-fold increase of the dendrimer generation number. Dosing the films with solvent vapours caused their resistances to increase. Using different solvent vapours demonstrated that the sensitivity of this response was determined by the solubility properties of the linker compounds. Microgravimetric measurements showed that absorption of analyte was consistent with a Langmuir adsorption model. These measurements also revealed a linear correlation between the electrical response (deltaR/Rini) and the concentration of absorbed analyte. The absorption of d4-methanol from a saturated vapour atmosphere was studied by neutron reflectometry with an AuPPI-G4 film. This measurement indicated condensation of methanol on top of the film and a uniform distribution of the analyte across the film thickness.
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PMID:Gold-nanoparticle/organic linker films: self-assembly, electronic and structural characterisation, composition and vapour sensitivity. 1475 Jun 66

Trisethylene-imino-s-triazine (triethylene melamine or TEM) produced minimal effects in inhibiting transplantable lymphoma and mammary adenocarcinoma in mice. In strain A mice, injection of the compound induced pulmonary tumors.TEM was tried on 32 patients with neoplastic disease, including nine patients with Hodgkin's disease and five with lymphosarcoma and lymphatic leukemia. The therapeutic and toxic effects were similar to those observed with nitrogen mustard (HN2). Satisfactory remissions of up to three months were observed in Hodgkin's disease and lymphosarcoma following parenteral administration of TEM. It is the authors' impression that the remissions obtained with TEM were not as complete and did not last as long as those obtained with HN2.TEM is effective by the oral route as well as parenterally, and produces much less emetic reaction than HN2. On the other hand, the chemotherapeutic range is narrower than that of HN2. Patients who do not respond to HN2 show no response to TEM.TEM is a drug of some clinical usefulness in the same conditions and with the same general limitations and toxic effects as HN2. The ease of administration of TEM increases its hazards, and close clinical and hematologic observations are essential on patients receiving the agent.
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PMID:Trisethylene-imino-s-triazine (triethylene melamine or TEM) in the treatment of neoplastic diseases. 1484 18

The porosity and the available surface area of a lignocellulosic fiber can influence the accessibility and reactivity in derivatization and modification reactions because the porous cell-wall network determines the upper size limit for molecules that can penetrate and react with the interior of the wall. To obtain information concerning the accessibility of the porous cell wall of wood fibers, surfactant-templated sol-gel mineralization has been examined. Wood and kraft pulp samples of Norway spruce were impregnated with a silica sol-gel and subsequently heated (calcined) and transformed into structured mesoporous silica. Microscopy studies (environmental scanning electron microscopy, transmission electron microsopy, TEM) on the silica casts showed that the three-dimensional architecture of the wood and pulp fiber cell wall was revealed down to the nanometer level. Image analysis of TEM micrographs of silica fragments from the never-dried pulp revealed complete infiltration of the cell-wall voids and microcavities (mean pore width 4.7 +/- 2 nm) by the sol-gel and the presence of cellulose fibrils with a width of 3.6 +/- 1 nm. Cellulose fibrils of the same width as that shown by image analysis were also identified by nitrogen adsorption measurements of the pore size distribution in the replicas.
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PMID:Silica nanocasts of wood fibers: a study of cell-wall accessibility and structure. 1513 4

Seventeen arginine auxotrophic mutants of Sinorhizobium meliloti Rmd201 were isolated by random transposon Tn5 mutagenesis using Tn5 delivery vector pGS9. Based on intermediate feeding studies, these mutants were designated as argA/argB/argC/argD/argE (ornithine auxotrophs), argF/argI, argG and argH mutants. The ornithine auxotrophs induced ineffective nodules whereas all other arginine auxotrophs induced fully effective nodules on alfalfa plants. In comparison to the parental strain induced nodule, only a few nodule cells infected with rhizobia were seen in the nitrogen fixation zone of the nodule induced by the ornithine auxotroph. TEM studies showed that the bacteroids in the nitrogen fixation zone of ornithine auxotroph induced nodule were mostly spherical or oval unlike the elongated bacteroids in the nitrogen fixation zone of the parental strain induced nodule. These results indicate that ornithine or an intermediate of ornithine biosynthesis, or a chemical factor derived from one of these compounds is required for the normal development of nitrogen fixation zone and transformation of rhizobial bacteria into bacteroids during symbiosis of S. meliloti with alfalfa plants.
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PMID:Isolation and symbiotic characterization of transposon Tn5-induced arginine auxotrophs of Sinorhizobium meliloti. 1524 85

Light harvesting in photosynthetic antenna proteins involves a series of highly efficient ultrafast energy transfers between spectroscopically different populations of chlorophylls. Several strategies have recently been employed to mimic this natural energy transfer process, including polymers, dendrimers, and oligomeric porphyrin arrays linked by covalent bonds or by self-assembly. In all of these systems, excitation energy transfer occurs from one molecule to another, while very few of them involve energy transfer from one very strongly interacting chromophore aggregate to another such aggregate. Here we report the synthesis and characterization of a covalent zinc phthalocyanine-2,3,9,10,16,17,23,24-octacarboxytetraimide in which all four imide nitrogen atoms are substituted with N-octyl-N'-(4-aminophenyl)-1,7(3',5'di-tert-butylphenoxy)perylene-3,4:9,10-bis(dicarboximide) (ZnPcIm4-PDI4). The individual molecules self-assemble into stacked heptamers in solution as evidenced by small-angle X-ray scattering and form long fibrous structures in the solid as evidenced by TEM. The ZnPcIm4 and PDI molecules both stack in register with the same components in an adjacent covalent building block. Ultrafast energy transfer occurs with tau = 1.3 ps from the aggregated peripheral PDI chromophores to the core ZnPcIm4 chromophore aggregate. Exciton hopping between the ZnPcIm4 chromophores occurs with tau = 160 fs.
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PMID:Ultrafast aggregate-to-aggregate energy transfer within self-assembled light-harvesting columns of zinc phthalocyanine tetrakis(perylenediimide). 1533 43


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