Jeffrey M. Davis

Pilot estuarine mesocosm study on the environmental fate of Silver nanomaterials leached from consumer products.

Although nanosilver consumer products (CPs) enjoy widespread availability, the environmental fate, leaching, and bioaccumulation behaviors of silver nanoparticles (AgNPs) from these products are not well understood. In this work, three nanosilver CPs, two AgNP standards, and an ionic silver (Ag(+)) standard were studied in estuarine mesocosms. The CPs exhibited long-term release of significant amounts of silver over a 60d residence time in the mesocosms, and ultimately released 82 - 99% of their total silver loads. Measurements of total silver as a function of time, by inductively coupled plasma mass spectrometry (ICP-MS), indicated that the silver was transferred from the water column and accumulated in the estuarine biota, including hard clams, grass shrimp, mud snails, cordgrass stalks and leaves, biofilms, intertidal sediment, and sand. The ICP-MS results and calculations of bioconcentration and trophic transfer factors indicated that significant amounts of silver were taken up by the organisms through trophic transfer. Silver was also adsorbed from the seawater into the biofilms, sediment, and sand, and from the sand into the clams.

EDS measurements of X-ray intensity at WDS precision and accuracy using a silicon drift detector.

The accuracy and precision of X-ray intensity measurements with a silicon drift detector (SDD) are compared with the same measurements performed on a wavelength dispersive spectrometer (WDS) for a variety of elements in a variety of materials. In cases of major (>0.10 mass fraction) and minor (>0.01 mass fraction) elements, the SDD is demonstrated to perform as well or better than the WDS. This is demonstrated both for simple cases in which the spectral peaks do not interfere (SRM-481, SRM-482, and SRM-479a), and for more difficult cases in which the spectral peaks have significant interferences (the Ba L/Ti K lines in a series of Ba/Ti glasses and minerals). We demonstrate that even in the case of significant interference high count SDD spectra are capable of accurately measuring Ti in glasses with Ba:Ti mass fraction ratios from 2.7:1 to 23.8:1. The results suggest that for many measurements wavelength spectrometry can be replaced with an SDD with improved accuracy and precision.

Fluorinated Copolymer Nanoparticles for Multimodal Imaging Applications

Nanomaterials have emerged as valuable tools in biomedical imaging techniques. Here, the synthesis and characterization of a novel fluorinated nanoparticle with potential applications as an MRI contrast agent is reported. Particles were synthesized using a free radical polymerization technique. Secondary ion mass spectrometry analysis showed that the particles surface contained fluorinated groups and nitrogen-containing groups. Solid-state NMR spectroscopy suggested the presence of two distinct fluorine resonances, which conforms to the structure of the fluorinated monomer. Ongoing studies aim to evaluate the performance of the nanoparticles as MRI contrast agents both in vitro and in vivo.

Bridging the micro-to-macro gap: a new application for micro X-ray fluorescence.

X-ray elemental mapping and X-ray spectrum imaging are powerful microanalytical tools. However, their scope is often limited spatially by the raster area of a scanning electron microscope or microprobe. Limited sampling size becomes a significant issue when large area (>10 cm²), heterogeneous materials such as concrete samples or others must be examined. In such specimens, macro-scale structures, inclusions, and concentration gradients are often of interest, yet microbeam methods are insufficient or at least inefficient for analyzing them. Such requirements largely exclude the samples of interest presented in this article from electron probe microanalysis. Micro X-ray fluorescence-X-ray spectrum imaging (μXRF-XSI) provides a solution to the problem of macro-scale X-ray imaging through an X-ray excitation source, which can be used to analyze a variety of large specimens without many of the limitations found in electron-excitation sources. Using a mid-sized beam coupled with an X-ray excitation source has a number of advantages, such as the ability to work at atmospheric pressure and lower limits of detection owing to the absence of electron-induced bremsstrahlung. μXRF-XSI also acts as a complement, where applicable, to electron microbeam X-ray output, highlighting areas of interest for follow-up microanalysis at a finer length scale.

Functionalized, carbon nanotube material for the catalytic degradation of organophosphate nerve agents

AbstractRecent world events have emphasized the need to develop innovative, functional materials that will safely neutralize chemical warfare (CW) agents in situ to protect military personnel and civilians from dermal exposure. Here, we demonstrate the efficacy of a novel, proof-of-concept design for a Cu-containing catalyst, chemically bonded to a single-wall carbon nanotube (SWCNT) structural support, to effectively degrade an organophosphate simulant. SWCNTs have high tensile strength and are flexible and light-weight, which make them a desirable structural component for unique, fabric-like materials. This study aims to develop a self-decontaminating, carbon nanotube-derived material that can ultimately be incorporated into a wearable fabric or protective material to minimize dermal exposure to organophosphate nerve agents and to prevent accidental exposure during decontamination procedures. Carboxylated SWCNTs were functionalized with a polymer, which contained Cu-chelating bipyridine groups, and their catalytic activity against an organophosphate simulant was measured over time. The catalytically active, functionalized nanomaterial was characterized using X-ray fluorescence and Raman spectroscopy. Assuming zeroth-order reaction kinetics, the hydrolysis rate of the organophosphate simulant, as monitored by UV-vis absorption in the presence of the catalytically active nanomaterial, was 63 times faster than the uncatalyzed hydrolysis rate for a sample containing only carboxylated SWCNTs or a control sample containing no added nanotube materials.n

Using Viscosity Modifiers to Reduce Effective Diffusivity in Mortars

AbstractThree viscosity modifiers (a commercial shrinkage-reducing admixture, a polypropylene glycol, and a cellulose ether) are used to reduce the effective diffusivity of chloride ions through mortars during a 1-year exposure. Two delivery mechanisms were studied: (1)xa0adding a viscosity modifier to the mix water and (2)xa0diluting the viscosity modifier in water, prewetting fine lightweight aggregate (LWA) with the solution, and replacing a portion of the sand with the prewetted LWA, which is equivalent to the practice of using LWA for internal curing. After a 28-day curing period, the cylinders were submerged in a 1-u2009u2009mol/L chloride solution. After 24 and 52xa0weeks of exposure, micro X-ray fluorescence analysis was used to profile the radial chloride concentration under ambient air pressure. The effective diffusivity was estimated by regression, assuming ideal Fickian radial diffusion. Compared with the control mortar (no admixture, no LWA), the addition of the viscosity modifier to the mix water reduced ...

Detection and speciation of brominated flame retardants in high-impact polystyrene (HIPS) polymers

Polymeric materials have been suggested as possible environmental sources of persistent organic pollutants such as flame retardants. In situ, micrometre‐scale characterization techniques for polymer matrix containing flame retardants may provide some insight into the dominant environmental transfer mechanism(s) of these brominated compounds. In this work, we demonstrate that micro X‐ray fluorescence spectroscopy (μXRF), focused ion beam scanning electron microscopy (FIB‐SEM) combined with energy dispersive X‐ray spectroscopy (EDS), and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) are promising techniques for the elemental and chemical identification of brominated fire retardant compounds (such as the deca‐congener of polybrominated diphenyl ether, BDE‐209) within polymeric materials (e.g. high‐impact polystyrene or HIPS). Data from μXRF demonstrated that bromine (Br) inclusions were evenly distributed throughout the HIPS samples, whereas FIB SEM‐EDS analysis revealed that small antimony (Sb) and Br inclusions are present, and regionally higher concentrations of Br surround the Sb inclusions (compared to the bulk material). Four prominent mass‐to‐charge ratio peaks (m/z 485, 487, 489 and 491) that correspond to BDE‐209 were identified by ToF‐SIMS and can be used to chemically distinguish this molecule on the surface of polymeric materials with respect to other brominated organic molecules. These techniques can be important in any study that investigates the route of entry to the environmental surroundings of BDE‐containing materials.

Support Vector Machines for Classification and Quantitative Analysis

Support vector machines (SVMs) are widely used and broadly applicable machine learning algorithms [1]. Their principal use is for classification algorithms, although they can also be used to develop regressions for predictive modeling. This talk will present the basic theory behind SVMs and show their applications to X-ray microanalysis in the classification of images and in the production of calibration curves for quantitative analysis.

Quantifying CD4 receptor protein in two human CD4+ lymphocyte preparations for quantitative flow cytometry

BackgroundIn our previous study that characterized different human CD4+ lymphocyte preparations, it was found that both commercially available cryopreserved peripheral blood mononuclear cells (PBMC) and a commercially available lyophilized PBMC (Cyto-Trol™) preparation fulfilled a set of criteria for serving as biological calibrators for quantitative flow cytometry. However, the biomarker CD4 protein expression level measured for T helper cells from Cyto-Trol was about 16% lower than those for cryopreserved PBMC and fresh whole blood using flow cytometry and mass cytometry. A primary reason was hypothesized to be due to steric interference in anti- CD4 antibody binding to the smaller sized lyophilized control cells.MethodTargeted multiple reaction monitoring (MRM) mass spectrometry (MS) is used to quantify the copy number of CD4 receptor protein per CD4+ lymphocyte. Scanning electron microscopy (SEM) is utilized to assist searching the underlying reasons for the observed difference in CD4 receptor copy number per cell determined by MRM MS and CD4 expression measured previously by flow cytometry.ResultsThe copy number of CD4 receptor proteins on the surface of the CD4+ lymphocyte in cryopreserved PBMCs and in lyophilized control cells is determined to be (1.45u2009±u20090.09)u2009×u2009105 and (0.85u2009±u20090.11)u2009×u2009105, respectively, averaged over four signature peptides using MRM MS. In comparison with cryopreserved PBMCs, there are more variations in the CD4 copy number in lyophilized control cells determined based on each signature peptide. SEM images of CD4+ lymphocytes from lyophilized control cells are very different when compared to the CD4+ T cells from whole blood and cryopreserved PBMC.ConclusionBecause of the lyophilization process applied to Cyto-Trol control cells, a lower CD4 density value, defined as the copy number of CD4 receptors per CD4+ lymphocyte, averaged over three different production lots is most likely explained by the loss of the CD4 receptors on damaged and/or broken microvilli where CD4 receptors reside. Steric hindrance of antibody binding and the association of CD4 receptors with other biomolecules likely contribute significantly to the nearly 50% lower CD4 receptor density value for cryopreserved PBMC determined from flow cytometry compared to the value obtained from MRM MS.

Solving the Micro-to-Macro Spatial Scale Problem with Milliprobe X-ray Fluorescence/X-ray Spectrum Imaging

Milliprobe x-ray fluorescence (mXRF) with x-ray spectrum imaging (XSI) enables elemental mapping over centimeter lateral distances with a resolution of 40-150 μm. While highly complementary to classic elemental mapping scanning electron microscopy/energy dispersive x-ray spectrometry (SEM/EDS), mXRF has several advantages: (1) The lack of electron bremsstrahlung in the XRF spectrum, except for the elastic scattering of the primary continuum, means that the inherent detection sensitivity of mXRF is better. (2) The broad continuum excitation of mXRF enables sensitive access to secondary x-rays with photon energies in the range 10 keV to 40 keV, which are either not efficiently excited or are completely inaccessible with SEM. (3) The range of penetration of x-rays (with minimal sideways scatter) is typically 10 to 100 times the range of an electron beam, enabling deeper probing into the specimen or even viewing the specimen through protective covering such as glass or plastic. (4) The vacuum requirements of mXRF are much less than even environmental SEM, and primary excitation and secondary detection can occur through an atmospheric gas path if required. The scale of mXRF-XSI mapping is particularly useful for attacking problems that require following compositional structures over a wide spatial scale, the classic micro-to-macro spatial scale challenge.