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We performed a photoemission study on the (α-Si)/Si(111) homojunction with a thin intralayer of calcium fluoride (one monolayer). The intralayer and the amorphous silicon were grown in situ on cleaved Si(111) single crystals. From the core-level analysis we found a valence-band discontinuity of 0.35 eV due to the electrostatic dipole induced by the intralayer ionic bonds. Our results show that the intralayer forms an abrupt interface with Si and that the overlayer valence band has a lower binding energy than the substrate one. This implies that the most probable interface configuration is Si(111)-Ca-F-(α-Si), confirming the results of previous works on the CaF 2 /Si(111) interface

We investigate the sensitivity of x-ray absorption spectroscopy to cubic-hexagonal polytypism in which nearest-neighbor positions are virtually unchanged. Experimental spectra and multiple-scattering calculations are reported at the major absorption edges in the zinc-blende and wurtzite modifications of ZnS. We demonstrate that d-like unoccupied bands are preferentially affected, as only L-edge absorption is sensitive to this structural transition. The results allow near-edge x-ray absorption spectroscopy to be evaluated as a detection method for crystal structure modifications in nano-scale systems.

UV/ozone ashing of thin tissue sections and cell cultures is a simple technique to enhance relative elemental concentrations, while maintaining their spatial location at the sub-micron level. This approach may enhance the capability of spatially resolved analysis techniques to detect the distribution of trace elements in biological matrices. We present results from light microscopy and x-ray spectromicroscopy studies of tissues and cells demonstrating that the micro-structure is very well conserved. We show the signal enhancement resulting from the removal of carbon, which allows otherwise undetectable gadolinium to be mapped in cancer tissue for a novel neutron capture therapy.

EXPERIMENTAL evidence of the preferential uptake of aluminium by GABAergic neurones and glial cells was provided by synchrotron spectromicroscopy studies. We observed rat cerebellar cultures enriched for GABAergic neurones or glial cells exposed to aluminium ions, detecting the presence and identifying the chemical status of aluminium on cell structures.

We performed a photoemission study on the (a-Si)/Si(111) homojunction with a thin intralayer of calcium fluoride (one monolayer). The intralayer and the amorphous silicon were grown in situ on cleaved Si(111) single crystals. From the core-level analysis we found a valence-band discontinuity of 0.35 eV due to the electrostatic dipole induced by the intralayer ionic bonds. Our results show that the intralayer forms an abrupt interface with Si and that the overlayer valence band has a lower binding energy than the substrate one. This implies that the most probable interface configuration is Si(111)-Ca-F(a-Si), confirming the results of previous works on the CaF2/Si(111) interface.

Changes in the thermal behavior of DMPC (dimyristoyl-r-phosphatidylcholine) and an equimolar mixture of DMPC and DMPE (dimyristoyl-L-phosphatidylethanolamine) induced by the plant hormone abscisic acid (ABA) have been investigated using fluorescent probes. The fluorescence decay of the hydrophobic probe 1,6-diphenyl-1,3,5-hexatriene (DPH) in these vesicles has been measured using frequency-domain fluorometry, and has been analyzed using both models of discrete exponential components and continuous lifetime distributions. In the DMPC vesicles, using the distributional approach, higher center and width values were observed in the presence of abscisic acid (ABA), indicating a decrease in the dielectric constant of the lipid phase that we attribute to a decrease in the water concentration within the bilayer. Moreover, the presence of ABA in the liposomes increased the phospholipid phase transition temperature. The addition of ABA to the DMPC/DMPE mixture strongly increased the microheterogeneity of the system as reported by the FWHM (full-width at half-maximum) of the distributional approach.

157Gd is a potential agent for neutron capture cancer therapy (GdNCT). We directly observed the microdistribution of Gd in cultured human glioblastoma cells exposed to Gd-diethylenetriaminepentaacetic acid (Gd-DTPA). We demonstrated, with three independent techniques, that Gd-DTPA penetrates the plasma membrane, and we observed no deleterious effect on cell survival. A systematic microchemical analysis revealed a higher Gd accumulation in cell nuclei compared with cytoplasm. This is significant for prospective GdNCT because the proximity of Gd to DNA increases the cell-killing potential of the short-range, high-energy electrons emitted during the neutron capture reaction. We also exposed Gd-containing cells to thermal neutrons and demonstrated the GdNC reaction effectiveness in inducing cell death. These results in vitro stimulated in vivo Gd-DTPA uptake studies, currently underway, in human glioblastoma patients.

Specimen charging under X-ray illumination is a well known phenomenon that can seriously obstruct the analysis of insulating samples. Synchrotron X-PEEM spectromicroscopy can reach a lateral resolution of 20 nm, 1-2 orders of magnitude larger than electron microscopies, but has the added capacity to probe oxidation state through total yield X-ray absorption near edge structure (XANES) spectroscopy. This capability may be compromised, however, if specimen charging restricts electron emission, as was encountered in the study of silicified bacteria from an Icelandic hot spring microbial mat. Bacteria living in an environment containing a high concentration of dissolved silica provide nucleation sites for amorphous silicate precipitation, a process which may lead to the preservation of the cellular structure, i.e. fossilization. TEM studies of bacteria in progressive stages of mineralization showed that mineral formation was initiated in the extracellular sheath, reaching the cell interior after death. Spectromicroscopy at the Si L-edge of sectioned mineralized bacteria encountered major charging difficulties, which were relieved by simultaneously illuminating the specimen with 325 nm HeCd laser light during the analysis. The low energy light excites mobile free electrons below the work function threshold, which can offset surface positive charge. This approach allowed spectroscopy to be performed from microscopic areas, and may be applicable to a wider range of insulating samples. (C) 2001 Elsevier Science B.V. All rights reserved.

Boron neutron capture therapy (BNCT) is an experimental, binary treatment far brain cancer which requires as the first step that tumor tissue is targeted with a boron-10 containing compound. Subsequent exposure to a thermal neutron flux results in destructive, short range nuclear reaction within 10 mu m of the boron compound. The success of the therapy requires than the BNCT agents be well localized in tumor, rather than healthy tissue. The MEPHISTO spectromicroscope, which performs microchemical analysis by x-ray absorption near edge structure (XANES) spectroscopy from microscopic areas, has been used to study the distribution of trace quantities of boron in human brain cancer tissues surgically removed from patients first administered with the compound Na2B12H11SH (BSH). The interpretation of XANES spectra is complicated by interference from physiologically present sulfur and phosphorus, which contribute structure in the same energy range as baron. We addressed this problem with the present extensive set of spectra from S, B, and P in relevant compounds. We demonstrate that a linear combination of sulfate, phosphate and BSH XANES can be used to reproduce the spectra acquired on boron-treated human brain tumor tissues. We analyzed human glioblastoma tissue from two patients administered and one not administered with BSH. As well as weak signals attributed to BSH, re-ray absorption spectra acquired from tissue samples detected boron in a reduced chemical state with respect to baron in BSH. This chemical state was characterized by a sharp absorption peak at 188.3 eV. Complementary studies on BSH reference samples were not able to reproduce this chemical state of boron, indicating that it is not an artifact produced during sample preparation or x-ray exposure. These data demonstrate that the chemical state of BSH may be altered by in vivo metabolism.