Cation disorder and phase transitions in the four-layer ferroelectric Aurivillius phases ABi4Ti4O15 (A=Ca, Sr, Ba, Pb)

Crystal structures of a series of bi-layered compounds ABi₄Ti₄O₁₅ (A=Ca, Sr, Ba, Pb) have been investigated using a combination of synchrotron X-ray and neutron powder diffraction data. All four oxides adopt an orthorhombic structure at room temperature and the structures have been refined in space group A2₁am. This orthorhombic structure is a consequence of a combination of rotation of the TiO₆, resulting from the less than optimal size of the A-type cation, and displacement of the Ti atoms towards the Bi₂O₂ layers. There is partial disorder of the Bi and A-type cations over two of the three available sites, which increases in the order Ca<Sr and Pb<Ba.     

Crystal structure of the gold nanoparticle [N(C8H17)4][Au25(SCH2CH2Ph)18

We report the crystal structure of the thiolate gold nanoparticle [TOA+][Au25(SCH2CH2Ph)18-], where TOA+ = N(C8H17)4+. The crystal structure reveals three types of gold atoms: (a) one central gold atom whose coordination number is 12 (12 bonds to gold atoms); (b) 12 gold atoms that form the vertices of an icosahedron around the central atom, whose coordination number is 6 (five bonds to gold atoms and one to a sulfur atom), and (c) 12 gold atoms that are stellated on 12 of the 20 faces of the Au13 icosahedron. The arrangement of the latter gold atoms may be influenced by aurophilic bonding. Together they form six orthogonal semirings, or staples, of -Au2(SCH2CH2Ph)3- in an octahedral arrangement around the Au13 core.     

Reactivity of electrochemically generated rhenium (II) Tricarbonyl alpha-diimine complexes: a reinvestigation of the oxidation of luminescent Re(CO)3(alpha-Diimine)Cl and related compounds

The oxidative electrochemistry of luminescent rhenium (I) complexes of the type Re(CO) 3(LL)Cl, 1, and Re(CO) 3(LL)Br, 2, where LL is an alpha-diimine, was re-examined in acetonitrile. These compounds undergo metal-based one-electron oxidations, the products of which undergo rapid chemical reaction. Cyclic voltammetry results imply that the electrogenerated rhenium (II) species 1 ( + ) and 2 ( + ) disproportionate, yielding [Re(CO) 3(LL)(CH 3CN)] (+), 7, and additional products. Double potential step chronocoulometry experiments confirm that 1 ( + ) and 2 ( + ) react via second-order processes and, furthermore, indicate that the rate of disproportionation is influenced by the basicity and steric requirements of the alpha-diimine ligands. The simultaneous generation of rhenium (I) and (III) carbonyl products was detected upon the bulk oxidation of 1 using infrared spectroelectrochemistry. The rhenium (III) products are assigned as [Re(CO) 3(LL)Cl 2] (+), 5; an inner-sphere electron-transfer mechanism of the disproportionation is proposed on the basis of the apparent chloride transfer. Chemically irreversible two-electron reduction of 5 yields 1 and Cl (-). No direct spectroscopic evidence was obtained for the generation of rhenium (III) tricarbonyl bromide disproportionation products, [Re(CO) 3(LL)Br 2] (+), 6; this is attributed to their relatively rapid decomposition to 7 and dibromine. In addition, the 17-electron radical cations, 7 ( + ), were successfully characterized using infrared spectroelectrochemistry.     

Modifications to the AOAC use-dilution test for quaternary ammonium compound-based disinfectants that significantly improve method reliability

The AOAC use-dilution test (UDT) for bactericidal disinfectant efficacy (Method 964.02) has often been criticized for its extreme variability in test results, particularly for quaternary ammonium compound (QAC)-based disinfectants against Pseudomonas aeruginosa. While efforts are under way to develop a new and better test method for hospital disinfectant products that is globally acceptable, U.S. manufacturers and formulators of QAC products must continue in the interim to measure their product performance against the current UDT method. Therefore, continued variability in the UDT places an unnecessary and unfair burden on U.S. QAC product manufacturers to ensure that their products perform against an, at best, unreliable test method. This article reports on evaluations that were conducted to attempt to identify key sources of UDT method variability and to find ways to mitigate their impact on test outcomes for the method. The results of testing across 4 laboratories, involving over 6015 carriers, determined that operator error was a key factor in test variability. This variability was found to be significantly minimized by the inclusion of a simple culture dilution step. The findings from this study suggest possible refinements to the current AOAC UDT method that would serve to improve the overall ruggedness and reliability of the method and to optimize recovery of cells from the carrier surface, thereby further improving the accuracy and reproducibility of counts and test outcomes until such time as a replacement method is implemented.     

The use of phase-inverting emulsions to show the phenomenon of interfacial crystallization on both heating and cooling

Highly anomalous crystallization behavior has been achieved in phase-inverting emulsion systems by using nonionic surfactants that induce nucleation. In particular, nucleation can be inhibited at the phase inversion, allowing systems held at, or near, this temperature to undergo crystallization either on heating or cooling. This new phenomenon is demonstrated for 27.4 wt % aqueous glycine solutions emulsified in decane using Span 20 Tween 20 blends. The inhibitory effect on interfacial nucleation at/near the phase inversion is readily shown by the maximum in the induction time for crystallization found in systems at/near the phase-inversion temperature. These findings are unprecedented. An extremely rapid rise in nucleation rate is expected on cooling glycine solutions, owing to the associated increase in supersaturation, the driving force for crystallization. The origin of this highly anomalous behavior is thought to be the low droplet interfacial tension, gammaow, that occurs at the phase-inversion temperature, which results primarily in a substantially increased contact angle between the glycine critical nucleus and the droplet interface. This may present a paradigm shift in crystallization strategies through the use of tunable contact-angle nucleators.     

Nitrofurazone accumulates in avian eyes--a replacement for semicarbazide as a marker of abuse

Intact nitrofurazone is present in whole eyes of chickens fed varying levels of this banned antibiotic and may therefore be used as an alternative to the controversial marker residue, semicarbazide, to monitor for abuse of this drug in primary production.     

Combinatorial approach to study enzyme/surface interactions

A fast combinatorial approach to access information about the immobilization behavior and kinetics of enzymes on a variation of surfaces is presented. As a test system, Candida Antarctica Lipase B was immobilized on a self-assembled monolayer bearing a gradient of surface energy. The respective immobilization behavior was monitored by Fourier transform infrared micro-spectroscopy. In addition, the activity of the immobilized enzyme was monitored over the entire film in real time with a specially developed fluorescence activity assay embedded into a siloxane gel. It was found that the highest amount of active protein was immobilized on the hydrophilic end of the gradient surface. This effect is associated with a higher surface roughness of this area resulting in hydrophobic micro-environments in which the enzyme gets immobilized.     

Selective electrochemical sensing of acidic organic molecules via a novel guest-to-host proton transfer reaction

Ferrocene-containing amidopyridine receptors bind carboxylic acids and the amino acid phenylalanine in acetonitrile via a novel proton transfer process that enables guests to be electrochemically sensed by positive shifts in the ferrocene-centred redox potentials.