Xps Studies of the Interactions of Biocells with Various Silicon Based Surfaces
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1996 Materials Research Society
select sheet silicates (e.g., chrysotile, obtained from the Geosciences Department of UWM). For ESCA analysis, small quantities of silica and silicate particulates were pressed into wafers. All of the samples were sterilized by autoclaving before the cell culturing (Ehrlich cells) experiments [12]. In the present case, cells, suspended in MEM (Minimum Essential Medium, [13]) supplemented with 4% fetal bovine serum, were grown on the wafers and finally released by trypsinization. All samples were kept for 144-216 hours (6-9 days) in a Forma scientific incubator at 37°C. Some of the wafers, were separated from the cells [3], washed and dried at 70-80'C, and then subjected to XPS analysis. The resulting cell suspension were subjected to AAS analysis [3]. In a few cases cell treated Si0 , silica and silicate systems were also freeze dried [3,4] in a Labconco freeze drier and subsequently the composite system was subjected to ESCA analysis. ESCA (or XPS): ESCA analysis was performed with a Hewlett-Packard(HP) 5950A using a high
resolution X-ray monochromator, employing an AlKca anode, with optimal resolution for Au (4f72) of 83.95 eV with a linewidth of ca. 1.0 eV. Charging effects were removed and the binding energy
scale established by fixing the C(ls) peaks for any CxHy-type hydrocarbons at 284.6 eV [14,15]. AAS: Atomic absorption spectroscopy was carried out in an IL model 357 AA/AE spectrophotometer to analyze the cellular iron and magnesium content. Standard 1000 ppm Fe and Mg solutions, obtained from Fisher Scientific, were used for calibration. The resulting absorbance values were used to create a standard -ra've from which the concentrations were determined [3]. RESULTS AND DISCUSSION Silicon is a group IVA atom which in the solid phase displays characteristics between those of a metal and nonmetal by virtue of its valence electron structure. This structure is built on an FCC Bravais lattice with two atoms associated with each lattice point and eight atoms per unit cell. A key feature of this structure is that it accommodates the tetrahedral bonding configuration. In the case of the oxides of silicon, when all four comers of the Si0 44 - tetrahedra are only shared by other silicate units, a Si0 2 network called silica is produced. Crystalline silica, a regular and ordered array of these tetrahedra exists in several polymorphic forms, such as quartz, tridymite, and crystoballite. Two-dimensional silicate sheets or layered structures can also be produced by the sharing of only three oxygen ions in each of the tetrahedra. For this structure, the repeating tetrahedral unit formula may be represented by (Si 20 5)2". Thus the pivotal structure of a serpentine [16] (e.g., chrysotile, Mg 3[Si 2O5 ](OH) 4) exhibits a 1:1 structure with the three octahedrally oriented Mg containing units linked to a single layer of silica tetrahedra. Apical oxygens connect these subsheets. Unshared anions on the octahedra are exclusively OH units. Certain variations of chrysotile-like species are kn
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