Solvothermal and surfactant-free synthesis of crystalline Nb(2)O(5), Ta(2)O(5), HfO(2), and Co-doped HfO(2) nanoparticles

A simple route to niobium, hafnium and tantalum oxide nanocrystals using a nonaqueous sol-gel route based on the solvothermal reaction of the corresponding metal chlorides with benzyl alcohol is presented. This approach can easily be extended to the preparation of high quality Co-doped HfO(2) nanoparticles of uniform size and shape and with a homogenous distribution of the magnetic ions. The structural characterization of all these nanomaterials as well as the magnetic properties of pure and doped hafnia, with special attention to the doping efficiency, are discussed. The obtained Co-doped hafnia exhibits paramagnetic properties with very weak antiferromagnetic interactions between Co ions moments.     

Analysis of Polymer Dynamics by NMR Modulated Gradient Spin Echo

NMR modulated gradient spin echo method, which allows the quantification of polymer segmental displacement via the measurement of the velocity autocorrelation, requires the formulation of theoretical predictions in the frequency domain in order to test their validity. We worked out the velocity autocorrelation spectrum of the segmental motion according to the Rouse and the tube/reptation model to compare it to the experimental results obtained by the new NMR technique. The analysis of polybutadiene shows the segmental velocity autocorrelation spectrum typical for the reptation-like motion of polymer in a “tube”. The measurement of bulk water indicates a kind of Rouse motion in a network of hydrogen bonds.     

Delivering quantum dots into cells: strategies,progress and remaining issues

The use of semiconductor quantum dots (QDs) in biological sensing and labeling continues to grow with each year. Current and projected applications include use as fluorescent labels for cellular labeling, intracellular sensors, deep-tissue and tumor imaging agents, sensitizers for photodynamic therapy, and more recently interest has been sparked in using them as vectors for studying nanoparticle-mediated drug delivery. Many of these applications will ultimately require the QDs to undergo targeted intracellular delivery, not only to specific cells, but also to a variety of subcellular compartments and organelles. It is apparent that this issue will be critical in determining the efficacy of using QDs, and indeed a variety of other nanoparticles, for these types of applications. In this review, we provide an overview of the current methods for delivering QDs into cells. Methods that are covered include facilitated techniques such as those that utilize specific peptide sequences or polymer delivery reagents and active methods such as electroporation and microinjection. We critically examine the benefits and liabilities of each strategy and illustrate them with selected examples from the literature. Several important related issues such as QD size and surface coating, methods for QD biofunctionalization, cellular physiology and toxicity are also discussed. Finally, we conclude by providing a perspective of how this field can be expected to develop in the future.     

The red‐shift effect and radiation decay on black hole spacetimes

We consider solutions to the linear wave equation □_g? = 0 on a (maximally extended) Schwarzschild spacetime with parameter M > 0, evolving from sufficiently regular initial data prescribed on a complete Cauchy surface Σ, where the data are assumed only to decay suitably at spatial infinity. (In particular, the support of ? may contain the bifurcate event horizon.) It is shown that the energy flux F(??) of the solution (as measured by a strictly timelike T? that asymptotically matches the static Killing field) through arbitrary achronal subsets ?? of the black hole exterior region satisfies the bound F(??) ≤ C E(v + u), where v and u denote the infimum of the Eddington‐Finkelstein advanced and retarded time of ??, v₊ denotes max{1, v}, and u₊ denotes max{1, u}, where C is a constant depending only on the parameter M, and E depends on a suitable norm of the solution on the hypersurface t ? u + v = 1. (The bound applies in particular to subsets ?? of the event horizon or null infinity.) It is also shown that ? satisfies the pointwise decay estimate |?| ≤ C Ev in the entire exterior region, and the estimates |r?| ≤ C_R?E(1 + |u|)^?1/2 and |r^1/2?| ≤ C_R?Eu in the region {r ≥ R?} ∩ J⁺(Σ) for any R? > 2M. The estimates near the event horizon exploit an integral energy identity normalized to local observers. This estimate can be thought to quantify the celebrated red‐shift effect. The results in particular give an independent proof of the classical result |?| ≥ C E of Kay and Wald without recourse to the discrete isometries of spacetime. © 2009 Wiley Periodicals, Inc.     

Material properties of anisotropic materials with pores

An approach based on cavity compliance contribution tensor is utilized for evaluation of effective stiffness of anisotropic materials with irregularly shaped pores. The shapes considered in the present work were extracted from the X-ray computed microtomography of a carbon/carbon composite sample. The cavity contribution tensors of individual pores were calculated numerically and then employed in the micromechanical modeling procedure. It was shown that presence of pores with total porosity of p = 9.13% results in approximately 20% reduction in the material's Young's moduli. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Self-similar turbulent boundary layer in pressure gradient

Self-similar flows in a turbulent boundary layer when the free-stream velocity is specified as a power function of longitudinal coordinate are investigated. The self-similar formulation not only simplifies solving of the problem by reducing the equations of motion to ordinary differential equations but also provides a mean for formulating closure conditions. It is shown that for the class of flows under consideration that depend on three governing parameters the dimensionless mixing length is a function of the normalised distance from the wall and the exponent in the law specifying the free-stream velocity distribution in the outer region and a universal function of local Reynolds number in the wall region, the latter corollary being true even when the skin friction vanishes. In calculations this function is set to be independent of pressure gradient, which gives the results very close to experimental data. There exist four different self-similar flow regimes. Each regime is related to its similarity parameter, one of which is the well-known Clauser equilibrium parameter and the other three are established for the first time. In case of adverse pressure gradient when the exponent lies within certain limits, which depend on Reynolds number, the problem has two solutions with different values of the boundary layer thickness and skin friction, which points out the possibility of hysteresis in near-separating flow. Separation occurs not at the minimum value of the exponent that corresponds to the strongest adverse pressure gradient but at a higher one whose dependence on Reynolds number is calculated in the paper. The results of the theory are in good agreement with experimental data. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis, characterization, and theoretical study of stable isomers of C70(CF3)n (n = 2, 4, 6, 8, 10)

Reaction of C70 with ten equivalents of silver(I) trifluoroacetate at 320-340 degrees C followed by fractional sublimation at 420-540 degrees C and HPLC processing led to the isolation of a single abundant isomer of C70(CF3)n for n = 2, 4, 6, and 10, and two abundant isomers of C70(CF3)8. These six compounds were characterized by using matrix-assisted laser desorption ionization (MALDI) mass spectrometry, 2D-COSY and/or 1D 19F NMR spectroscopy, and quantum-chemical calculations at the density functional theory (DFT) level. Some were also characterized by Raman spectroscopy. The addition patterns for the isolated compounds were unambiguously found to be C1-7,24-C70(CF3)2, C1-7,24,44,47-C70(CF3)4, C2-1,4,11,19,31,41-C70(CF3)6, Cs-1,4,11,19,31,41,51,64-C70(CF3)8, C2-1,4,11,19,31,41,51,60-C70(CF3)8, and C1-1,4,10,19,25,41,49,60,66,69-C70(CF3)10 (IUPAC numbering). Except for the last compound, which is identical to the recently reported, crystallographically characterized C70(CF3)10 derivative prepared by a different synthetic route, these compounds have not previously been shown to have the indicated addition patterns. The largest relative yield under an optimized set of reaction conditions was for the Cs isomer of C70(CF3)8 (ca. 30 mol % of the sublimed mixture of products based on HPLC integration). The results demonstrate that thermally stable C70(CF3)n isomers tend to have their CF3 groups arranged on isolated para-C6(CF3)2 hexagons and/or on a ribbon of edge-sharing meta- and/or para-C6(CF3)2 hexagons. For Cs- and C2-C70(CF3)8 and for C2-C70(CF3)6, the ribbons straddle the C70 equatorial belt; for C1-C70(CF3)4, the para-meta-para ribbon includes three polar hexagons; for C1-7,24-C70(CF3)2, the para-C6(CF3)2 hexagon includes one of the carbon atoms on a C70 polar pentagon. The 10.3-16.2 Hz 7JF,F NMR coupling constants for the end-of-ribbon CF3 groups, which are always para to their nearest-neighbor CF3 group, are consistent with through-space Fermi-contact interactions between the fluorine atoms of proximate, rapidly rotating CF3 groups.     

Electronic properties and reactivity of short-chain oligomers of 3,4-phenylenedioxythiophene (PheDOT)

The dimer and trimer of 3,4-phenylenedioxythiophene (PheDOT) have been synthesized. Unlike the parent systems based on 3,4-ethylenedioxythiophene (EDOT), these compounds are quite stable under atmospheric conditions. The electronic absorption spectra of di- and tri-PheDOT exhibit a well-resolved vibronic fine structure indicative of self-rigidification of the conjugated structure by noncovalent intramolecular sulfur-oxygen interactions. Comparison of UV-visible data for the PheDOT oligomers with those of the corresponding EDOT oligomers reveals a faster decrease of the HOMO-LUMO gap with chain length for the former. Cyclic voltammetric data show that whereas PheDOT oxidizes at a lower potential than EDOT, the PheDOT dimer and trimer exhibit much higher oxidation potentials than their EDOT-based analogues. A comparative analysis of the electropolymerization of the three PheDOT-based systems shows that although PheDOT is very difficult to polymerize, its dimer and trimer can be readily electropolymerized. This unexpected increase of reactivity with chain extension is discussed with the aid of theoretical calculations.     

Conformational study of monomeric 2,3-butanediols by matrix-isolation infrared spectroscopy and DFT calculations

The FT-IR spectra of two diastereomers of 2,3-butanediol, (R,S) and (S,S), isolated in low-temperature argon and xenon matrixes were studied, allowing the identification of two different conformers for each compound. These conformers were characterized by a +/-gauche arrangement around the O-C-C-O dihedral angle, thus enabling the establishment of a very weak intramolecular hydrogen bond of the O...H-O type. No other forms of these compounds were identified in matrixes, despite the fact that these four conformers had calculated relative energies from 0 to 5.1 kJ mol(-1) and were expected to be thermally populated from 50 to 6% in the gaseous phase of each compound. The nonobservation of additional conformers was explained in terms of low barriers to intramolecular rotation, resulting in the conformational relaxation of the compounds during deposition of the matrixes. The barriers to internal rotation of the OH groups were computed to be less than 4 kJ mol(-1) and are easily overcome in matrixes within the family of conformers with the same heavy atom backbone. The barriers for intramolecular rearrangement of the O-C-C-O dihedral angle in both diastereomers were calculated to range from 20 to 30 kJ mol(-1). Interconversions between the latter conformers were not observed in matrixes, even after annealing up to 65 K. Energy calculations, barriers, and calculated infrared spectra were carried out at the DFT(B3LYP)/6-311++G theory. Additional MP2/6-311++G calculations of energies and vibrational frequencies were performed on the most relevant conformers. Finally, independent estimations of the hydrogen-bond enthalpy in the studied molecules were also obtained based on theoretical structural data and from vibrational frequencies (using well-established empirical correlations). The obtained values for -DeltaH for both diastereomers of 2,3-butanediol amount to ca. 6-8 kJ mol(-1).