Rapid determination of 226Ra in drinking water samples using dispersive liquid-liquid microextraction coupled with liquid scintillation counting

A new radioanalytical method was developed for rapid determination of ²²⁶Ra in drinking water samples. The method is based on extraction and preconcentration of ²²⁶Ra from a water sample to an organic solvent using a dispersive liquid-liquid microextraction (DLLME) technique followed by radiometric measurement using liquid scintillation counting. In DLLME for ²²⁶Ra, a mixture of an organic extractant (toluene doped with dibenzo-21-crown-7 and 2-theonyltrifluoroacetone) and a disperser solvent (acetonitrile) is rapidly injected into the water sample resulting in the formation of an emulsion. Within the emulsion, ²²⁶Ra reacts with dibenzo-21-crown-7 and 2-theonyltrifluoroacetone and partitions into the fine droplets of toluene. The water/toluene phases were separated by addition of acetonitrile as a de-emulsifier solvent. The toluene phase containing ²²⁶Ra was then measured by liquid scintillation counting. Several parameters were studied to optimize the extraction efficiency of ²²⁶Ra, including water immiscible organic solvent, disperser and de-emulsifier solvent type and their volume, chelating ligands for ²²⁶Ra and their concentrations, inorganic salt additive and its concentration, and equilibrium pH. With the optimized DLLME conditions, the accuracy (expressed as relative bias, B _r ) and method repeatability (expressed as relative precision, S _B ) were determined by spiking ²²⁶Ra at the maximum acceptable concentration level (0.5 Bq L⁻¹) according to the Guidelines for Canadian Drinking Water Quality. Accuracy and repeatability were found to be less than −5% (B _r ) and less than 6% (S _B ), respectively, for both tap water and bottled natural spring water samples. The minimum detectable activity and sample turnaround time for determination of ²²⁶Ra was 33 mBq L⁻¹ and less than 3 h, respectively. The DLLME technique is selective for extraction of ²²⁶Ra from its decay progenies.     

NCIPLOT: a program for plotting non-covalent interaction regions

Non-covalent interactions hold the key to understanding many chemical, biological, and technological problems. Describing these non-covalent interactions accurately, including their positions in real space, constitutes a first step in the process of decoupling the complex balance of forces that define non-covalent interactions. Because of the size of macromolecules, the most common approach has been to assign van der Waals interactions (vdW), steric clashes (SC), and hydrogen bonds (HBs) based on pairwise distances between atoms according to their van der Waals radii. We recently developed an alternative perspective, derived from the electronic density: the Non-Covalent Interactions (NCI) index [J. Am. Chem. Soc. 2010, 132, 6498]. This index has the dual advantages of being generally transferable to diverse chemical applications and being very fast to compute, since it can be calculated from promolecular densities. Thus, NCI analysis is applicable to large systems, including proteins and DNA, where analysis of non-covalent interactions is of great potential value. Here, we describe the NCI computational algorithms and their implementation for the analysis and visualization of weak interactions, using both self-consistent fully quantum-mechanical, as well as promolecular, densities. A wide range of options for tuning the range of interactions to be plotted is also presented. To demonstrate the capabilities of our approach, several examples are given from organic, inorganic, solid state, and macromolecular chemistry, including cases where NCI analysis gives insight into unconventional chemical bonding. The NCI code and its manual are available for download at http://www.chem.duke.edu/~yang/software.htm.     

Enhanced gauge symmetry in 6D F‐theory models and tuned elliptic Calabi‐Yau threefolds

We systematically analyze the local combinations of gauge groups and matter that can arise in 6D F‐theory models over a fixed base. We compare the low‐energy constraints of anomaly cancellation to explicit F‐theory constructions using Weierstrass and Tate forms, and identify some new local structures in the “swampland” of 6D supergravity and SCFT models that appear consistent from low‐energy considerations but do not have known F‐theory realizations. In particular, we classify and carry out a local analysis of all enhancements of the irreducible gauge and matter contributions from “non‐Higgsable clusters,” and on isolated curves and pairs of intersecting rational curves of arbitrary self‐intersection. Such enhancements correspond physically to unHiggsings, and mathematically to tunings of the Weierstrass model of an elliptic CY threefold. We determine the shift in Hodge numbers of the elliptic threefold associated with each enhancement. We also consider local tunings on curves that have higher genus or intersect multiple other curves, codimension two tunings that give transitions in the F‐theory matter content, tunings of abelian factors in the gauge group, and generalizations of the “E₈” rule to include tunings and curves of self‐intersection zero. These tools can be combined into an algorithm that in principle enables a finite and systematic classification of all elliptic CY threefolds and corresponding 6D F‐theory SUGRA models over a given compact base (modulo some technical caveats in various special circumstances), and are also relevant to the classification of 6D SCFT's. To illustrate the utility of these results, we identify some large example classes of known CY threefolds in the Kreuzer‐Skarke database as Weierstrass models over complex surface bases with specific simple tunings, and we survey the range of tunings possible over one specific base.     

Excellent passivation of thin silicon wafers by HF‐free hydrogen plasma etching using an industrial ICPECVD tool

In this work, hydrogen plasma etching of surface oxides was successfully accomplished on thin (～100 µm) planar n‐type Czochralski silicon wafers prior to intrinsic hydrogenated amorphous silicon [a‐Si:H(i)] deposition for heterojunction solar cells, using an industrial inductively coupled plasma‐enhanced chemical vapour deposition (ICPECVD) platform. The plasma etching process is intended as a dry alternative to the conventional wet‐chemical hydrofluoric acid (HF) dip for solar cell processing. After symmetrical deposition of an a‐Si:H(i) passivation layer, high effective carrier lifetimes of up to 3.7 ms are obtained, which are equivalent to effective surface recombination velocities of 1.3 cm s^–1 and an implied open‐circuit voltage (V_oc) of 741 mV. The passivation quality is excellent and comparable to other high quality a‐Si:H(i) passivation. High‐resolution transmission electron microscopy shows evidence of plasma‐silicon interactions and a sub‐nanometre interfacial layer. Using electron energy‐loss spectroscopy, this layer is further investigated and confirmed to be hydrogenated suboxide layers. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Microchip-based electrochemical detection for monitoring cellular systems

The use of microchip devices to study cellular systems is a rapidly growing research area. There are numerous advantages of using on-chip integrated electrodes to monitor various cellular processes. The purpose of this review is to give examples of advancements in microchip-based cellular analysis, specifically where electrochemistry is used for the detection scheme. These examples include on-chip detection of single-cell quantal exocytosis, electrochemical analysis of intracellular contents, the ability to integrate cell culture/immobilization with electrochemistry, and the use of integrated electrodes to ensure cell confluency in longer-term cell culture experiments. A perspective on future trends in this area is also given.     

A Review of the Status of Polymerization in Thermotropic Liquid Crystal Media and Liquid Crystalline Monomers

Thermotropic liquid crystals offer uniquely ordered media for intermolecular reactions such as polymerization. Unlike the recently investigated topochemical polymerizations where molecules are rigidly constrained on lattice points [l], the liquid crystalline state permits full two-dimensional (nematic and cholesteric) and one-dimensional (smectic) movement. In addition, various degrees of translational freedom are attainable depending on the class of mesogen. Such freedom of choice, plus the ability to orient nematic and cholesteric mesogens on a molecular scale in an electric or magnetic field or on certain surfaces, makes the liquid crystal state an attractive polymerization medium.     

Multi-functionality of fluorescent nanocrystals in glass ceramics

Thermal processing of as-made fluorozirconate glasses, which were additionally doped with rare-earth and chlorine ions, results in the formation of fluorescent nanocrystals therein. For medical applications, the glasses were doped with divalent europium ions as the fluorescent rare-earth ion, while trivalent neodymium was used to develop up-conversion systems. The samples were annealed up to 290 °C to initiate the growth of hexagonal or orthorhombic phase BaCl₂ nanocrystals therein. Upon annealing some of the rare-earth ions were incorporated into the BaCl₂ nanocrystals leading to enhanced fluorescence properties. The particle diameters were in the range of a few nanometers to several tens of nanometers.     

Efficient implementation of the proper outlet flow conditions in blood flow simulations through asymmetric arterial bifurcations

We present an efficient implementation of the proper (in vivo) outlet boundary conditions in detailed, three‐dimensional (3D) and time‐periodic simulations of blood flow through arteries. This is achieved through the intermediate use of an approximate ‘simulant’ model of the outlet pressure/flow relationship corresponding to the full 3D and time‐dependent numerical simulation. This model allows us to efficiently couple the 3D outlet pressure/flow conditions to the equivalent relations due to the downstream arterial network, as obtained from a one‐dimensional approximate model in the form of Fourier frequency impedance coefficients. An adjustable time‐periodic function correction term in the simulant model requires input from the full 3D model that has to run iteratively until convergence. The advantage of the proposed numerical scheme is that it decouples the upstream detailed simulation from the downstream approximate network model offering exceptional versatility. This approach is demonstrated here in a series of detailed 3D simulations of blood flow, performed using the commercial software FLUENT?, through an asymmetric arterial bifurcation. Two cases are considered: first a healthy system patterned after the left main coronary arterial bifurcation, and second a diseased case where an occlusion has developed in one of the daughter vessels, resulting in strengthening the asymmetry of the bifurcation. Rapid convergence of the iterative process was achieved in both cases. Subtle changes occur in the shear patterns of the daughter vessels, whereas the flow distribution is quite different. In the presence of a stenosis additional regions of low shear develop due to inertial effects. Copyright © 2010 John Wiley & Sons, Ltd.