Similarity solutions of radially symmetric two-phase flow

When both the diffusivityD and fractional flow functionf have a power law dependence on the water content , i.e.D=D _o andf=⁺¹, the nonlinear transport equation for radially symmetric two phase flow can, in certain circumstances, be reduced to a weakly coupled system of two first order nonlinear ordinary differential equations. Numerical solutions of these equations for a constant flux boundary conditionV _wo and comparison with experimental data are given. In particular, when the fluxV _wo and a are related byV _wo( + 1)/D _o=2, a new fully explicit analytical solution is found as (r, t)=(1 – r ²/4D _ot)^1/ forr ² < 4D _ot/ and (r, t)=0 forr ² 4D _ot/ We show that the existence of this exact soution is due to the presence of a Lagrangian symmetry.     

Fluorogenic resolution of ligand binding by a nucleic acid aptamer

An ATP-binding nucleic acid aptamer was converted into a solution-phase fluorescence-detected sensor by incorporating a 2'-amine group in a structure that is sensitive to a ligand-induced conformational change in the aptamer. The 2'-amine substitution in the free aptamer reacts efficiently with fluorescamine to yield a fluorescent product detectable by energy transfer to a tethered fluor emitting at 615 nm. In the presence of ATP, the fluorogenic reaction is substantially reduced, both under simplified conditions and in a urine background. Fluorogenic chemistry represents a potentially general approach for creating sensors from aptamers that bind their ligands via induced fit.     

Voltammetric measurement of arsenic in natural waters

There are several U.S. EPA approved methodologies for the determination of arsenic in ground water. Such technologies are lab-based, time intensive and can lead to a large capital cost for multi-sample analysis. In light of the number of sites found to contain arsenic at levels higher than the maximum contaminant level (MCL), on-site screening and monitoring systems are an attractive alternative. This review article summarizes several examples in the recent literature to illustrate the breadth of work in voltammetric analysis of arsenic in environmental samples. Also, included are recent voltammetric results, obtained with a microfabricated gold array and a field portable potentiostat, at an arsenic contaminated site in southern New Jersey.     

Solvothermal synthesis of silver nanoparticles from thiolates

A new solvothermal method for the synthesis of thiol-protected silver nanoparticles starting from silver thiolates is reported. The method has been tried with thiols of different chain length, such as octane and octadecane thiols, and the particle size was found to be nearly the same for both molecules. The synthesis was dependent on heating conditions and the best results were obtained when the temperature was between 180 and 200 degrees C. Addition of complexation agents such as acetyl acetone or triethylamine to the solvent did not change the product distribution significantly.     

Reversible hydrogen gas uptake in nanoporous Prussian Blue analogues

The family of dehydrated nanoporous Prussian Blue analogues, M(II)3[Co(III)(CN)6]2 (M(II) = Mn, Fe, Co, Ni, Cu, Zn, Cd), which contain coordinatively unsaturated divalent metal cations, undergoes reversible sorption of hydrogen gas up to 1.2 wt% (at 77 K, 101.3 kPa), the capacity of which depends on the metal ion.     

Phase behavior and 3D structure of strongly attractive microsphere-nanoparticle mixtures

We investigate the phase behavior and 3D structure of strongly attractive mixtures of silica microspheres and polystyrene nanoparticles. These binary mixtures are electrostatically tuned to promote a repulsion between like-charged (microsphere-microsphere and nanoparticle-nanoparticle) species and a strong attraction between oppositely charged (microsphere-nanoparticle) species. Using confocal fluorescence scanning microscopy, we directly observe the 3D structure of colloidal phases assembled from these mixtures as a function of varying composition. In the absence of nanoparticle additions, the charged-stabilized microspheres assemble into a polycrystalline array upon sedimentation. With increasing nanoparticle volume fraction, nanoparticle bridges form between microspheres, inducing their flocculation. At even higher nanoparticle volume fractions, the microspheres become well coated with nanoparticles, leading to their charge reversal and subsequent restabilization. We demonstrate how this fluid-gel-fluid transition can be utilized to control the morphology of the colloidal phases formed under gravity-driven sedimentation.     

Behavior of M[Al2Me6N3] (M=K,Rb, Cs) with aromatic solvents and the crystal structures of Cs[Al2Me6N3]·2p-xylene and [K·dibenzo-18-crown-6] [Al2Me6N3]· 1.5(1-methylnaphthalene)

The title compounds were synthesized by the addition of AlMe₃ to the corresponding azide suspended in an aromatic solvent. Both products were obtained as air-sensitive colorless crystals. Cs[Al₂Me₆N₃]·2p-xylene crystallizes in the monoclinic space groupC2/m witha=19.143(6),b=16.227(6),c=10.392(5) Å, =114.06(2)^o, and _calc = 1.20 g cm^–3 forZ=4. Refinement led to a conventionalR value of 0.037 for 2179 observed reflections. The cesium atom resides on a mirror plane, and the anion is disordered about a twofold axis. Thep-xylene molecules sandwich the cesium ion.[K·dibenzo-18-crown-6] [Al_Me6N₃]·1.5(1-methylnaphthalene) crystallizes in the monoclinic space groupP2₁/c witha=14.176(5),b=13.021(5),c=25.324(8) Å, =98.23(4)⁰, and _calc = 1.08 g cm^–3 forZ=4. The finalR value was 0.132 for 1402 observed reflections. One of the 1-methylnaphthalene molecules is disordered about a center of inversion and interacts with the potassium ion. The other solvent molecule is found roughly in layers in the lattice and also exhibits disorder of the methyl substituent. For both title compounds the AlMe₃ groups of the anion exhibit a staggered (C _s) conformation. Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82015 (32 pages).