Michael P. Eastman

Mode of incorporation of Sr2+ in calcite: Determination by X-ray absorption spectroscopy

By probing the local atomic environment of strontium coprecipitated with natural and synthetic calcites, X-ray absorption spectroscopy (XAS) reveals that the strontium is incorporated in the calcite by substitution at Ca2+ structural sites, forming a dilute solid solution. Both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) demonstrated that the local structural environment of Sr2+ in natural and synthetic calcites was similar to that of Ca2+ in calcite and quite different from that of Sr2+ in strontianite. The Sr2+-O2− distance derived from EXAFS, 2.58 ± 0.03A, is consistent with the sum of the radii of these two ions in six- and three-fold coordination, respectively, of 2.57 A. The XANES and EXAFS further eliminate such modes of incorporation as adsorption, occlusion, and the presence of trace amounts of strontianite or other Sr2+-rich phase. However, evidence of a possible relict Sr2+-rich aragonite was encountered in a diagenetic calcite, indicating that XAS may prove a sensitive tool for detecting incomplete mineralogical alteration in carbonates. Because Sr2+ is bound at Ca2+ lattice sites, the record of biomineralization, diagenesis and age encoded in Sr2+Ca2+ ratios, and strontium isotopes in geologic calcites is secure; likewise, 90Sr sequestered in natural calcite should not undergo significant preferential leaching.

Investigation of the antigen antibody reaction between anti-bovine serum albumin (a-BSA) and bovine serum albumin (BSA) using piezoresistive microcantilever based sensors

A new microsensor application based on piezoresistive microcantilever technology has been used to study the interaction of anti-bovine serum albumin (a-BSA) with bovine serum albumin (BSA). A thin layer of BSA attached to a glass slide was used as the active sensing layer for the detection of a-BSA in solution. This design produced a large, consistent cantilever deflection when exposed to the analyte. In this system, the cantilever deflection is measured as a simple resistance change in the piezoresistive channel within the cantilever. In a second set of experiments, 3:1 BSA:PEO protein/polymer blended substrates were used as the active sensing layer for the detection of a-BSA in an aerosol delivery. A distinct signature for the analyte, separate from the water vapor carrier, is obtained for this system.

Inorganic/organic host–guest materials: Surface and interclay reactions of styrene with copper(II)-exchanged hectorite

Many important layered silicate–polymer nanocomposite materials may be synthesized using an in-situ polymerization process. Using this technique, organic monomers are intercalated into the interlayer regions of the hosts, where subsequent polymerization may then occur. In this paper, we report on the in-situ polymerization of styrene in Cu(II)-exchanged hectorite thin films. Scanning force microscopy (SFM) images of the polymer surface reveal that the surface polystyrene is generally aggregated into groups of elongated strands. SFM imaging of the interclay regions, in conjunction with X-ray diffraction (XRD) and electron spin resonance (ESR) data, indicates that approximately 20–30% of these regions contain polystyrene, with minimal reduction in the majority of Cu2+ sites observed. XRD data shows little or no intercalation of the monomer into the true intergallery regions. Instead, the polymer likely forms in intercrystallite or planar defect regions. In addition, two distinct phases of polymeric material are found within these defect regions, a highly polymerized polystyrene in addition to a polystyrene form exhibiting greater material stiffness.

Site-Specific Prebiotic Oligomerization Reactions of Glycine on the Surface of Hectorite

Abstract. Condensation reactions of the amino acid glycine on the surface of Cu(II)-exchanged hectorite are investigated using the technique of scanning force microscopy. Prebiotic conditions are simulated using alternate wetting and heating cycles. Concentration, immobilization, and subsequent polymerization resulting in glycine oligomers are seen to occur primarily at step edges or faults in the topmost layer. Condensation reactions also occur within tiny micropores or defects in the topmost layer. These reactions are facilitated by the availability of intergallery metal cations at the step edges or pores in the surface region.

XAS study of lanthanide ion speciation in borosilicate glass

Abstract The site environment of selected lanthanide ions (Ce, Pr, Gd and Lu) representative of 4f element fission products has been investigated in a typical borosilicate nuclear waste glass. X-ray absorption spectroscopy (XAS) in combination with other techniques showed that under air-firing conditions the lanthanide ions, except for Ce, enter the glass predominantly in the trivalent state and have well-defined first coordination shells. Approximate coordination numbers of 8 ± 2 (Pr) and 6 ± 2 (Gd, Lu) and metal-oxygen distances of 2.49 ± 0.05 A ( Pr ), 2.29 ± 0.05 A ( Lu ) and 2.20 ± 0.05 A ( Lu ) were deduced.

Analysis of peptides synthesized in the presence of SAz-1 montmorillonite and Cu2+ exchanged hectorite

We have investigated the synthesis of oligopeptides containing glycine and tyrosine in the presence of the clay minerals montmorillonite (non-exchanged, SAz-1) and Cu(2+) exchanged hectorite. In both cases, homopolymers of the two amino acids are formed, as are mixed peptides. In the case of Cu(2+) hectorite, mixed oligopeptides up to trimers are detected in small amounts. For montmorillonite, heterogeneous oligopeptides up to hexamers are detected. Our experiments indicate montmorillonite is more effective in promoting oligopeptide formation than Cu(2+) hectorite. Analysis of the oligopeptide sequences formed on the montmorillonite surfaces indicates preferential synthesis of certain Gly-Tyr sequences over others.

The formation of poly(methyl-methacrylate) on transition metal-exchanged hectorite

Abstract Hectorite clay films exchanged with Cu 2+ or Fe 3+ react with methyl-methacrylate monomer in a solventless process to form poly(methyl-methacrylate) (PMMA). Scanning force microscopy (SFM), electron spin resonance (ESR), and X-ray diffraction (XRD) are useful in studying the resulting clay/polymer composite. SFM shows that PMMA forms on the surface of both the Cu 2+ - and Fe 3+ -exchanged films. In addition, XRD shows PMMA formation in the interlayer region of the Fe 3+ -exchanged hectorite but not the interlayer region of Cu 2+ -exchanged hectorite. SFM shows that the morphology of the polymer formed on the film surface depends on: (1) the type of transition metal exchanged into the interlayer region. (2) The mode of delivery (vapor vs. liquid) of methyl-methacrylate monomer to the surface of the clay. PMMA does not form to a significant extent on or within Ca 2+ -exchanged hectorite, and ESR shows no evidence of free radical formation or reduction of the transition metals during any of the polymerization reactions. The results suggest the possibility of an unusual cationic polymerization of methyl-methacrylate to form PMMA with organic cations being stabilized by interaction with the silicate surface. The experimental results have implications for “solventless synthesis” of PMMA and other polymers.

Studies on the reaction of the 5′-phosphorimidazolide of adenosine with Cu(II)-exchanged hectorite

The role of clay minerals in the prebiotic synthesis of nucleotide oligomers has received considerable attention in recent years. Scanning force microscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry are used to identify oligomers of adenylic acid formed on the clay mineral Cu(II)-exchanged hectorite in simulated prebiotic cycling experiments. Electron-spin resonance and x-ray diffraction data indicate that the monomer (5′-phosphorimidazolide of adenosine, or ImpA) penetrates into the intergallery regions of the mineral substrate, and complexes the gallery Cu(II) cations. It is postulated that polymerization of the monomer is initiated in the clay intergallery regions, producing oligomers of adenylic acid up to 8 units in length or more.

Chemical Sensing Through Measurement of Thickness/Impedance Characteristics of Ion-Conducting Polymer Films

A chemical microsensor system capable of simultaneously measuring the impedance and thickness of thin polymer films exposed to analyte vapors is described. The system is constructed from a quartz substrate with interdigitated electrodes and a piezoresistive microcantilever. Measurements of the thickness and impedance of 10-30 μm thick films of LiClO 4 doped polyethyleneoxide (PEO) exposed to water, ethanol, acetone, and n-octane are reported. Experiments show that the thickness and impedance measurements are complementary and that thickness/impedance profiles allow the identification and quantification of the analytes tested. Scanning force microscopy (SFM) shows that PEO lamellae become smaller as the level of LiClO 4 doping increases. SFM also shows an increase in the amount of amorphous material increases when doped PEO is exposed to water vapor. The uniqueness of the thickness/impedance profiles for the analytes is discussed in terms of the SFM results.

Electrical and surface properties of clay-conducting polymer composites

Organic guests such as aniline, pyrrole and thiophene polymerize on the surface and in the intergallery regions of smectite clays which contain exchangeable transition metal cations such as Cu2+ and Fe3+. We monitor these reactions in thin films of smectite clays using electron spin resonance (ESR) and impedance spectroscopies. Polymers that form on the surface and in the interlayer region are studied by scanning force microscopy (SFM). ESR studies have shown that the transition metal ions are reduced during the polymerization process. Impedance measurements indicate that the formation of conducting polymer in the interlayer region of dry, Cu2+ exchanged hectorite thin films results in a dramatic reduction in observed impedance. SFM scans indicate that the conducting polymers can adopt a variety of morphologies on the surfaces of the films and within the intergalleries of the host framework. These studies have applications in the development of advanced materials including microsensors and novel nanocomposites.