In situ Fe XAFS of reversible lithium insertion in a flexible metal organic framework material

X-ray absorption fine structure (XAFS) analysis of the Fe K-edge during lithium insertion and extraction into the metal organic framework material MIL-53 (Fe^III(OH, F)bdc; bdc = benzene-1,4-dicarboxylate) reveals changes in local atomic environment about iron during the process. The average oxidation state of iron is reduced upon lithium insertion, as evidenced by the edge shift of the XANES spectra, and this is accompanied by a lengthening of Fe–O bonds, seen in the EXAFS. In contrast, the EXAFS analysis shows that the closest Fe–Fe distance remains approximately constant during the insertion and extraction of lithium, consistent with a distortion of the structure due to its flexible nature. The process is reversible upon lithium extraction, proving the redox-active flexibility of the framework.     

Electron paramagnetic resonance and computational studies of radicals derived from boron-substituted N-heterocyclic carbene boranes

Fifteen second-generation NHC-ligated boranes with aryl and alkyl substituents on boron were prepared, and their radical chemistry was explored by electron paramagnetic resonance (EPR) spectroscopy and calculations. Hydrogen atom abstraction from NHC-BH(2)Ar groups produced boryl radicals akin to diphenylmethyl with spin extensively delocalized across the NHC, BH, and aryl units. All of the NHC-B·HAr radicals studied abstracted Br-atoms from alkyl bromides. Radicals with bulky N,N'-dipp substituents underwent dimerization about 2 orders of magnitude more slowly than first-generation NHC-ligated trihydroborates. The evidence favored head-to-head coupling yielding ligated diboranes. The first ligated diboranyl radical, with a structure intermediate between that of ligated diboranes and diborenes, was spectroscopically characterized during photolysis of di-t-butyl peroxide with N,N'-di-t-butyl-imidazol-2-ylidene phenylborane. The reactive site of B-alkyl-substituted NHC-boranes switched from the boron center to the alkyl substituent for both linear and branched alkyl groups. The β-borylalkyl radicals obtained from N,N'-dipp-substituted boranes underwent exothermic β-scissions with production of dipp-Imd-BH(2)· radicals and alkenes. The reverse additions of NHC-boryl radicals to alkenes are probably endothermic for alkyl-substituted alkenes, but exothermic for conjugated alkenes (addition of an NHC-boryl radical to 1,1-diphenylethene was observed). A cyclopropylboryl radical was observed, but, unlike other α-cyclopropyl-substituted radicals, this showed no propensity for ring-opening.     

A fluorescent dipyrrinone oxime for the detection of pesticides and other organophosphates

An N,N-carbonyl-bridged dipyrrinone oxime has been synthesized and studied as a potential sensor for organophosphates. The molecular sensor underwent a drastic colorimetric response upon formation of the adduct. The pesticide dimethoate was found to produce the biggest spectral response, with a limit of detection equal to 4.0 ppm using UV-visible spectroscopy. Minimal fluorescence "turn on" via a PET mechanism was seen, and molecular modeling studies were used to explain the lower than expected PET response. The X-ray crystal structure of the fluorescent dipyrrinone oxime was also obtained.     

High conductivity in hydrothermally grown AgCuO(2) single crystals verified using focused-ion-beam-deposited nanocontacts

The silver-copper mixed oxide AgCuO(2) (also formulated as Ag(2)Cu(2)O(4)) possesses a peculiar electronic structure in which both Ag and Cu are partially oxidized, with the charge being delocalized among the three elements in the oxide. Accordingly, a quasi-metallic behavior should be expected for this oxide, and indeed bulk transport measurements show conductivity values that are orders of magnitude higher than for other members of this novel oxide family. The presence of silver makes thermal sintering an inadequate method to evaluate true conductivity, and thus such measurements were performed on low density pellets, giving an underestimated value for the conductivity. In the present work we present a new synthetic route for AgCuO(2) based on mild hydrothermal reactions that has yielded unprecedented large AgCuO(2) single-crystals well over 1 μm in size using temperatures as low as 88 °C. We have used a dual beam instrument to apply nanocontacts to those crystals, allowing the in situ measurement of transport properties of AgCuO(2) single crystals. The results show a linear relationship between applied current and measured voltage. The conductivity values obtained are 50 to 300 times higher than those obtained for bulk low density AgCuO(2) pellets, thus confirming the high conductivity of this oxide and therefore supporting the delocalized charge observed by spectroscopic techniques.     

Improved silicon nitride surfaces for next-generation microarrays

This work reports how the use of a standard integrated circuit (IC) fabrication process can improve the potential of silicon nitride layers as substrates for microarray technology. It has been shown that chemical mechanical polishing (CMP) substantially improves the fluorescent intensity of positive control gene and test gene microarray spots on both low-pressure chemical vapor deposition (LPCVD) and plasma-enhanced chemical vapor deposition (PECVD) silicon nitride films, while maintaining a low fluorescent background. This results in the improved discrimination of low expressing genes. The results for the PECVD silicon nitride, which has been previously reported as unsuitable for microarray spotting, are particularly significant for future devices that hope to incorporate microelectronic control and analysis circuitry, due to the film's use as a final passivating layer.     

An EXAFS study of the formation of a nanoporous metal-organic framework: evidence for the retention of secondary building units during synthesis

EXAFS data measured from amorphous intermediates and crystallisation solutions provides the first evidence that trimeric iron oxide secondary building units remain intact during crystallisation of the metal-organic framework MIL-89 from starting materials to products.     

Elucidation of the Streptomyces coelicolor pathway to 4-methoxy-2,2'-bipyrrole-5-carboxaldehyde, an intermediate in prodiginine biosynthesis

The biosynthetic pathway to 4-methoxy-2,2'-bipyrrole-5-carboxaldehyde (MBC), a key intermediate in the biosynthesis of prodiginine antibiotics in Streptomyces coelicolor, has been elucidated using a combination of gene replacements and feeding experiments with chemically synthesised MBC and a synthetic analogue of a pathway intermediate.     

Syntheses of copper(I)cis-1,3,5-tri-iminocyclohexane complexes

Copper(I) complexes of the ligand cis-1,3,5-tris(cinnamylideneamino)cyclohexane (L) have been prepared from a versatile precursor complex, [Cu(I)(L)NCMe]BF4, which incorporates a labile acetonitrile ligand that can be exchanged to give a range of new Cu(L)X complexes (where X = Cl, Br, NO2, SPh). 1H NMR spectra and X-ray structures of the Cl, Br and NO2 complexes show L coordinated in a symmetric fashion about the copper centre. The complexes have been further characterised using UV/Visible spectroscopy and cyclic voltammetry. CuLCl shows an electrochemically reversible Cu(I/II) redox couple at 0.51 V (vs. Ag/AgCl) while the CuLNO2 complex shows an analogous quasi-reversible wave at 0.41 V (vs. Ag/AgCl).     

Carbon nanotube network formation from evaporating sessile drops

Fabrication of single-walled carbon nanotube (SWNT) networks using evaporation of SDS-SWNT sessile drops on a hydrophobized silicon substrate is reported. It is suggested that the organization of nanotubes during evaporation is controlled by aggregates (in the SDS-SWNT dispersion) and hydrophobicity of the substrate. On hydrophobic substrates, the evaporation of SDS-SWNT sessile drops proceeds through constant contact area. On hydrophilic substrates, nanotube aggregates in SDS-SWNT dispersion stop the contact line from moving, resulting in the formation of "coffee-stains". The (partial) removal of aggregates by centrifugation is essential for a freely moving contact line leading to the organization of nanotubes into a network of homogeneously distributed nanotubes on the most hydrophobic substrate. The evaporation of sessile drops was characterized by microscopic, spectroscopic, and topographical techniques.