Asymmetric Substrate Research Articles

Oxygen permeation through perovskite membranes and the improvement of oxygen flux by surface modification

BSCF hollow fiber membranes possessing an asymmetric layered structure were prepared using a modified phase inversion process followed by subsequent sintering at temperatures from 1100 to 1175 ˚C. The fibers were characterized by SEM, and tested for air separation at ambient pressure and temperatures between 650 and 950 ˚C. Although the prepared hollow fibers resulted in self-supported asymmetric substrate with a very thin densified perovskite layer for mixed conduction, O2 permeation was controlled by surface O2 exchange kinetics rather than bulk diffusion. In order to improve O2 flux, surface modification was carried out by the attachment of Pt particles on the surface of the hollow fiber. The maximum O2 flux measured for pure perovskite hollow fiber was 0.0268 mol m−2 s-1 at 950 ˚C whilst O2 fluxes increased up to 25% after the surface modification using Pt micro-particles.

X-ray Structure of the EmrE Multidrug Transporter in Complex with a Substrate

EmrE is a prototype of the Small Multidrug Resistance family of efflux transporters and actively expels positively charged hydrophobic drugs across the inner membrane of Escherichia coli. Here, we report the x-ray crystal structure, at 3.7 angstrom resolution, of one conformational state of the EmrE transporter in complex with a translocation substrate, tetraphenylphosphonium. Two EmrE polypeptides form a homodimeric transporter that binds substrate at the dimerization interface. The two subunits have opposite orientations in the membrane and adopt slightly different folds, forming an asymmetric antiparallel dimer. This unusual architecture likely confers unidirectionality to transport by creating an asymmetric substrate translocation pathway. On the basis of available structural data, we propose a model for the proton-dependent drug efflux mechanism of EmrE.

Dynamical mode locking in commensurate structures with an asymmetric deformable substrate potential

The overdamped dynamics in the commensurate structures of the one-dimensional Frenkel-Kontorova model subjected to a parametrized deformable periodic substrate potential and driven by a periodic force is examined. It was found that when the shape of the substrate potential starts to deviate from the standard one, new subharmonic steps appear in the response function even in the structures with an integer value of average interparticle distance while the critical depinning force can even decrease for some values of system parameters. These novel phenomena could be particularly relevant for the charge-density wave systems, vortex lattices, and systems of Josephson-junction arrays.

Thin Defect-Free Pd Membrane Deposited on Asymmetric Porous Stainless Steel Substrate

Commercially available asymmetric porous stainless steel (APSS) tube filters with a smooth top layer were employed as substrates for preparing thin Pd-based composite membranes. The simple electroless plating and electroplating methods cannot successfully prepare defect-free thin (less than 6 μm) Pd-based membranes on the APSS substrates. Using the improved electroless plating technique based on a multidimensional plating mechanism, it is easy to prepare thin defect-free (less than 6 μm) Pd/APSS composite membranes. The heat treatment of the APSS substrates has an important effect on membrane performance. The as-prepared 3-μm Pd/APSS composite membrane under optimum conditions has a hydrogen flux as high as 0.409 mol/(m2 s) at 773 K with a pressure difference of 100 kPa while keeping the complete hydrogen selectivity versus helium. The stability of the hydrogen flux and the hydrogen selectivity were also proven by operating times as long as 200 h and gas-exchanging cycles as many as 40.

Ca-doped YBa2Cu3O7-δ bicrystal junctions fabricated on asymmetric SrTiO3 substrates

Bicrystal grain–boundary Josephson junctions of Ca–doped YBa2Cu3O7–δ that is Y0.7Ca0.3Ba2Cu3O7–δ were fabricated on three bicrystal SrTiO3 (001) substrates with asymmetric 30°, 40° and 45° orientations. An enhancement of the critical current density in these Ca–doped junctions was observed when compared with normal YBCO grain–boundary junctions with similar angular orientations. The observed increase in the critical current density is large for the junctions fabricated on the asymmetric 30° bicrystal substrate and small or negligible, for those on the asymmetric 45° bicrystal substrate. The critical current was modulated by a magnetic field applied in the plane of the junctions. However, the Fraunhofer pattern observed due to the applied magnetic field deviates from the ideal one.

Capacitive Properties of<tex>$rm YBa_2rm Cu_3rm O_7$</tex>Grain Boundary Josephson Junctions Fabricated on 45<tex>$^circ$</tex>[100] Tilt Asymmetric and 24<tex>$^circ$</tex>[001] Tilt Symmetric Bicrystals

We present a comparative study of the electromagnetic parameters, mainly focused in the analysis of the capacitance (C) associated to the barrier, of YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) grain boundary Josephson junctions (GBJJs) fabricated on 24/spl deg/ [001] symmetric and 45/spl deg/ [100] asymmetric tilt bicrystalline substrates. The Swihart velocity (c~) is slightly affected by the nature of the barrier and mainly fixed by the junction width. However, a notable influence of the barrier structure, so of the geometry of the bicrystal substrate, on the relative dielectric constant to the barrier thickness ratio (/spl epsiv//sub r//t) values, so on C, has been found. We have observed that the capacitance values deduced by means of Fiske steps in YBCO 24/spl deg/ [001] symmetric and 45/spl deg/ [100] asymmetric GBJJs scale with junction resistance in opposite direction. This result could reveal the presence of a tunnel barrier in the YBCO 45/spl deg/ [100] asymmetric GBJJs. On the other hand, different capacitance values have been obtained by means of Fiske steps and hysteresis observed in the current-voltage characteristics in 45/spl deg/ [100] asymmetric GBJJs. An interpretation of this result can be made taking into account the contribution of the depleted YBCO layers close to the crystallographic grain boundary.

Controlling the motion of interacting particles: Homogeneous systems and binary mixtures

We elaborate on recent results on the transport of interacting particles for both single-species and binary mixtures subject to an external driving on a ratchetlike asymmetric substrate. Moreover, we also briefly review motion control without any spatial asymmetric potential (i.e., no ratchet). Our results are obtained using an analytical approach based on a nonlinear Fokker-Planck equation as well as via numerical simulations. By increasing the particle density, the net dc ratchet current in our alternating (ac)-driven systems can either increase or decrease depending on the temperature, the drive amplitude, and the nature of the inter-particle interactions. This provides an effective control of particle motion by just changing the particle density. At low temperatures, attracting particles can condense at some potential minima, thus breaking the discrete translational symmetry of the substrate. Depending on the drive amplitude, an agglomeration or condensation results either in a drop to zero or in a saturation of the net particle velocity at densities above the condensation density-the latter case producing a very efficient rectification mechanism. For binary mixtures we find three ways of controlling the particle motion of one (passive) B species by means of another (active) A species: (i) Dragging the target particles B by driving the auxiliary particles A, (ii) rectifying the motion of the B particles on the asymmetric potential created by the A-B interactions, and (iii) dynamically modifying (pulsating) this potential by controlling the motion of the A particles. This allows to easily control the magnitude and direction of the velocity of the target particles by changing either the frequency, phase and/or amplitude of the applied ac drive(s).

Interacting particles on a rocked ratchet: Rectification by condensation

The transport of interacting particles subject to an external low-frequency ac force on a ratchetlike asymmetric substrate is studied via a nonlinear Fokker-Planck equation as well as via numerical simulations. With increasing the particle density, the ratchet current can either increase or decrease depending on the temperature, the drive amplitude, and the nature of the interparticle interaction. At low temperatures, attracting particles can condense randomly at some potential minima, thus breaking the discrete translational symmetry of the substrate. Depending on the drive amplitude, condensation results either in a drop to zero or in the saturation of the net particle velocity at densities above the condensation density-the latter case producing a very efficient rectification mechanism.

Stochastic transport of interacting particles in periodically driven ratchets

An open system of overdamped, interacting Brownian particles diffusing on a periodic substrate potential U(x+l)=U(x) is studied in terms of an infinite set of coupled partial differential equations describing the time evolution of the relevant many-particle distribution functions. In the mean-field approximation, this hierarchy of equations can be replaced by a nonlinear integro-differential Fokker-Planck equation. This is applicable when the distance a between particles is much less than the interaction length lambda , i.e., a particle interacts with many others, resulting in averaging out fluctuations. The equation obtained in the mean-field approximation is applied to an ensemble of locally (a<<lambda<<l) interacting (either repelling or attracting) particles placed in an asymmetric one-dimensional substrate potential, either with an oscillating temperature (temperature rachet) or driven by an ac force (rocked ratchet). In both cases we focus on the high-frequency limit. For the temperature ratchet, we find that the net current is typically suppressed (or can even be inverted) with increasing density of the repelling particles. In contrast, the net current through a rocked ratchet can be enhanced by increasing the density of the repelling particles. In the case of attracting particles, our perturbation technique is valid up to a critical value of the particle density, above which a finite fraction of the particles starts condensing in a liquidlike state near the substrate minima. The dependence of the net transport current on the particle density and the interparticle potential is analyzed in detail for different values of the ratchet parameters.

Epicardial but not endocardial premature stimulation initiates ventricular tachyarrhythmia in canine in vitro model of long QT syndrome

To explore the mechanism, we tested the hypothesis that premature epicardial stimulation transiently increased the dispersion of repolarization leading to VT. Premature stimulation initiated ventricular tachycardia (VT) when applied to the epicardium but not to the endocardium in a canine model of long QT syndrome (LOTS). We optically mapped action potentials (APs) on the cut-exposed transmural surfaces of isolated wedges of canine ventricular walls perfused with anemone toxin II (ATX-II), which produced type 3 LQTS with an asymmetrical transmural profile of repolarization that was earliest in the epicardium and latest in deep subendocardium. Earliest excitable epicardial stimulation triggered VT in 5 of 18 wedges receiving > or =5 nmol/L ATX-II by direct activation of epicardium, which delayed repolarization in the still refractory midmyocardium and further enhanced the dispersion of repolarization. These VTs were initiated 197 +/- 72 ms (n = 10) after the premature stimulation, from focal regions of earliest repolarization downstream to the steepest local spatial gradients of repolarization, and maintained by new focal activation and reentry. Transmural differences in the cycle lengths of activations altered conduction pathways and resulted in torsades de pointes-like polymorphic VT. In contrast, VTs were not initiated by endocardial stimulation at the same premature intervals or when ATX-II was < or =2.5 nmol/L. Failed VT initiation was associated with significantly lower maximum local gradient of repolarization. Heterogeneic repolarization in LQTS provides a transmural asymmetrical substrate for the earliest excitable epicardial, but not endocardial, stimulation to further delay midmyocardial repolarization and produce a steep spatial gradient of repolarization potential initiating torsades de pointes-like polymorphic VT.

Asymmetrical neural substrates of tactile discrimination in humans: a functional magnetic resonance imaging study.

The left-hand advantage seen during tactile discrimination tasks suggests hemispheric-processing asymmetry, although its neural substrates are not well known. We used functional magnetic resonance imaging to evaluate the laterality of the neural substrates involved in tactile discrimination in 19 normal volunteers. Passive tactile discrimination tasks, along with appropriate control tasks, were performed with both the right and left hands to evaluate the effects of the hand used and hemispheric effects (i.e., laterality of the activation pattern). Regardless of the hand used, the right dorsolateral prefrontal cortex, posterior parietal cortex, pre-supplementary motor area, and rostral portion of the dorsal premotor cortex (PMdr) were activated asymmetrically during tactile discrimination. This confirms the previous finding of a right-sided asymmetry for tactile shape discrimination. Hand effects were found in the left caudal portion of PMd (PMdc) adjacent to the central sulcus, which showed prominent activation during right-handed but not left-handed discrimination tasks. This asymmetric activation in the left PMdc might be related to the asymmetric interhemispheric interaction during right-handed tactile discrimination.

New Site-Specific Endonucleases F-TflI, F-TflII, and F-TflIV Encoded by Bacteriophage T5

Novel site-specific H-N-H endonucleases F-TflI, F-TflII and F-TflIV were identified. These endonucleases are encoded by open reading frames localized in the bacteriophage T5 tRNA gene region. The endonuclease F-TflIV was shown to introduce double-strand break into pseudo palindromic 17 bp DNA sequence yielding 1 bp extensions with 3'-overhangs. In contrast to F-TflIV, endonucleases F-TflI and F-TflII cleave only one strand of their asymmetric divergent DNA substrates. Each of these endonucleases introduces interruptions into only the particular strand (template or coding). Amino acid sequences of the endonucleases under study are highly homologous in the H-N-H motif regions and C-terminal sequences, forming putative catalytic domain. Endonuclease F-TflIV N-terminal region is homologous to the amino acid sequences representing H-T-H recognition domain found in LuxR family transcription regulators. Putative recognition NUMOD4 motif characteristic for a number of H-N-H endonucleases was shown to be also present in the endonuclease F-TflI and F-TflII N-terminal sequences. Two-domain structure was proposed for endonucleases F-TflI, F-TflII and F-TflIV with N-terminal recognition domain and C-terminal catalytic domain. A hypothesis of evolutionary origin of these endonucleases as a result of catalytic and recognition domains recombination was suggested.

Growth of uniformly aligned nanorod arrays by oblique angle deposition with two-phasesubstrate rotation

Due to the shadowing effect, the oblique angle deposition technique can produce nanorodstilted toward the incident deposition flux. Periodic posts serving as seeds on a substrateallow the fabrication of nanorod arrays with controllable separations. However, in aconventional oblique angle deposition with no substrate rotation, nanorods grow fasteralong their widths in the direction perpendicular to the plane of incident flux. Thisanisotropic growth can result in ‘fan-out’ shapes of nanorods that touch eachother due to the faster growing widths. Asymmetric two-phase substrate rotationwas designed to eliminate the side growth in oblique angle deposition. In thismethod, the growing rods are exposed to the deposition flux from all angles withsome portion of a rod surface receiving more flux than the rest. We fabricatedwell-aligned Si nanorod arrays with uniform sizes from templates arranged insquare and triangular lattices using this two-phase substrate rotation method.

Kink compactons in models with parametrized periodic double-well and asymmetric substrate potentials

We derive kink solutions with compact support (kink compactons) in a nonlinear Klein–Gordon system with anharmonic coupling. The model is characterized by the double-well Remoissenet–Peyrard (RP) substrate potential V RP ( θ, r) whose shape can be moved as a function of the parameter r in the range 0⩽ r<1. The phase trajectories, as well as an analytical analysis, provide information on a disintegration of kink compactons upon reaching some critical values of the lattice parameters. Exact analytic expressions for the dependence of this threshold value on the nonlinear parameter, on the velocity of the kink compactons and on the shape parameter are derived. The dependence of the four classes of kink compacton solutions, on the shape parameter r is obtained, and the total energies of each class of kink compactons are exactly calculated.

Fabrication and characterization of (Pd/Ag)–La0.2Sr0.8CoO3−δ composite membrane on porous asymmetric substrates

Perovskite La 0.2 Sr 0.8 CoO 3− δ (LSCO-80) was selected as a model mixed-conductive material for the fabrication of a ceramic electrolyte membrane on porous MgO and ZrO 2 disks. Aiming at overcoming mechanical cracks in the membrane caused by thermal mismatch between the membrane and the asymmetric support, Pd/Ag alloy phase was introduced into the LSCO-80 membrane through a non-powder-blending technique. The alloy phase not only offers relevant plasticity to buffer thermal stress generated in the membrane but also possess adequate chemical stability at high temperature. The metallization was carried out on the LSCO-80 membrane by silver slurry coating and followed by electroless palladium plating. Subsequently, the overlaying metal bi-layer was annealed to allow the formation of Pd/Ag–LSCO-80 composite. The metallization could satisfactorily cure mechanical cracks arisen from sintering of the LSCO-80 powder-packing layer. To pursue further promotion of mechanical stability of the composite membrane at high temperature, the LSCO-80 powder was modified by covering individual particles with a Pd/Ag thin layer, which has been proven to be an effective complement to the above coating approach. In addition to focusing on the alloy-assisted mechanical enhancement, this work has also investigated ionic conduction behavior of three different composite membranes by measuring their AC impedances. The results suggest that the metallization improves anionic oxygen conductivity in the perovskite LSCO-80 lattice at high temperatures. This phenomenon is ascribed to a fast cathodic process wherein the Pd/Ag phase plays the key role.

Controlling transport in mixtures of interacting particles using Brownian motors.

An outstanding open problem in nanoscience is how to control the motion of tiny particles. Ratchetlike devices, inspired by biological motors, have been proposed as a way to achieve this goal. However, the net directed transport is almost suppressed if the diffusing particles are weakly coupled to the underlying spatially asymmetric substrate. Here we show how adding particles of an auxiliary species, that interact with both the primary particles of interest and the substrate, provides a controlled enhancement of the flow for both species. These can move either together or in opposite directions, depending upon the strength of the interaction, and whether it is attractive or repulsive.

DETERMINISTIC RATCHETS: FROM CONFINED TO DIFFUSIVE CHAOS

An AC driven, noiseless, underdamped Brownian particle moving on an asymmetric periodic substrate provides an interesting example of deterministic ratchet: In the adiabatic regime periodic and chaotic trajectories coexist on fine tuning the amplitude of the input signal; the transition from regular to chaotic transport happens through an intermediate class of trajectories exhibiting confined chaos.

Dehydrogenation of secondary amines to imines catalyzed by an iridium PCP pincer complex: initial aliphatic or direct amino dehydrogenation?

The PCP pincer complex, IrH 2(C 6H 3-2,6-(CH 2PBu t 2) 2) catalyzes the transfer dehydrogenation of secondary amines to imines. The catalytic system is highly sensitive to both steric and electronic factors and greater than 99% regioselectivity are observed in the dehydrogenation of the asymmetric substrates, cyclohexyethylamine and benzylpropyl amine. Good to excellent yields are obtained when the reactions are carried out in toluene solution. The dehydrogenation of 2,2,2′,2′-tetramethyldibutylamine leads exclusively to the production of the corresponding imine indicating that the catalytic reaction pathway involves direct amino rather than initial aliphatic dehydrogenation.

Rapid Determination of Enantiomeric Excess Using Infrared Thermography

Infrared thermography (IRT) is presented as a novel technique to screen a potentially large number of asymmetric catalysts or substrates in a high-throughput fashion. IRT was used as a simple, rapid, and practical approach for initial screening of the substrate specificity of Candida antarctica lipase. This was carried out using a 96-well microtitre plate format. Potential advantages and limitations of IRT for the enzymatic stereoselective acylation of primary and secondary alcohols of interest are discussed.

Surface Modification of Polymeric Materials by Plasma Treatment

Low-temperature plasma treatment has been used in the last years as a useful tool to modify the surface properties of different materials, in special of polymers. In the present work low temperature plasma was used to treat the surface of asymmetric porous substrates of polysulfone (PSf) membranes. The main purpose of this work was to study the influence of the exposure time and the power supplied to argon plasma on the permeability properties of the membranes. Three rf power levels, respectively 5, 10 and 15 W were used. Treatment time ranged from 1 to 50 min. Reduction of single gas permeability was observed with Ar plasma treatments at low energy bombardment (5 W) and short exposure time (20 min). Higher power and/or higher plasma exposition time causes a degradation process begins. The chemical and structural characterization of the membranes before and after the surface modification was done by AFM, SEM and XPS.