Polycondensats entre la l-lysine et le disulfochlorure-1,3 benzene. Mise en evidence d'interactions hydrophobes en solution aqueuse

The influence of alkaline or quaternary ammonium counter-ions on the conformation in aqueous solution of the polycondensate between l-lysine and 1,3-benzene disulphonyl chloride has been shown by potentiometric titrations and viscometric studies without added salt. The polyelectrolyte has a compact structure at low degrees of neutralization, attributed to hydrophobic interactions (which are more important than the polymer-solvent interactions) between hydrocarbon groups of a same chain. The transition to random coil formation takes place for degrees of neutralization (ᾱ) between 0·2 and 0·3 for the alkaline ions and the first alkyl ammonium terms when the charge repulsions become sufficiently high. If the length of the alkyl chain increases, this transition is displaced towards much higher values of ᾱ. This effect is due to the stabilization of the hydrophobic interactions by the counter-ions retained near to the polyelectrolyte chains.     

Atomic structure of highly strained BiFeO3 thin films

We determine the atomic structure of the pseudotetragonal T phase and the pseudorhombohedral R phase in highly strained multiferroic BiFeO(3) thin films by using a combination of atomic-resolution scanning transmission electron microscopy and electron energy-loss spectroscopy. The coordination of the Fe atoms and their displacement relative to the O and Bi positions are assessed by direct imaging. These observations allow us to interpret the electronic structure data derived from electron energy-loss spectroscopy and provide evidence for the giant spontaneous polarization in strained BiFeO(3) thin films.     

The effect of parasitic boron doping on P-type piezoresistors

Absolute piezoresistive pressure sensors often use direct wafer bonding of SOI with the etched bulk silicon to provide us with sensitive membranes and sealed cavities. The advantage of such a process is that we can easily obtain a thin, monocrystalline silicon membrane with controlled thickness and a vacuum sealed cavity. However, few works have been reported on the electrical quality of this N-type membrane and its influence on performance of the P-type gauges. The problem of parasitic Boron appearance at the Si/SiO₂ interface was only recently shown in BESOI wafers, but it becomes much more significant when creating the component at the bottom side of the device layer, like in the case of an absolute piezoresistive pressure sensor.     

A neutral redox-switchable [2]rotaxane

A limited range of redox-active, rotaxane-based, molecular switches exist, despite numerous potential applications for them as components of nanoscale devices. We have designed and synthesised a neutral, redox-active [2]rotaxane, which incorporates an electron-deficient pyromellitic diimide (PmI)-containing ring encircling two electron-rich recognition sites in the form of dioxynaphthalene (DNP) and tetrathiafulvalene (TTF) units positioned along the rod section of its dumbbell component. Molecular modeling using MacroModel guided the design of the mechanically interlocked molecular switch. The binding affinities in CH(2)Cl(2) at 298 K between the free ring and two electron-rich guests--one (K(a) = 5.8 × 10(2) M(-1)) containing a DNP unit and the other (K(a) = 6.3 × 10(3) M(-1)) containing a TTF unit--are strong: the one order of magnitude difference in their affinities favouring the TTF unit suggested to us the feasibility of integrating these three building blocks into a bistable [2]rotaxane switch. The [2]rotaxane was obtained in 34% yield by relying on neutral donor-acceptor templation and a double copper-catalysed azide-alkyne cycloaddition (CuAAC). Cyclic voltammetry (CV) and spectroelectrochemistry (SEC) were employed to stimulate and observe switching by this neutral bistable rotaxane in solution at 298 K, while (1)H NMR spectroscopy was enlisted to investigate switching upon chemical oxidation. The neutral [2]rotaxane is a chemically robust and functional switch with potential for applications in device settings.     

High-Capacity Ion Trap Coupled to a Time-of-Flight Mass Spectrometer for Comprehensive Linked Scans with no Scanning Losses

A high-capacity ion trap coupled to a time-of-flight (TOF) mass spectrometer has been developed to carry out comprehensive linked scan analysis of all stored ions in the ion trap. The approach involves a novel tapered geometry high-capacity ion trap that can store more than 10(6) ions (range 800-4000 m/z) without degrading its performance. Ions are stored and scanned out from the high-capacity ion trap as a function of m/z, collisionally fragmented and analyzed by TOF. Accurate mass analysis is achieved on both the precursor and fragment ions of all species ejected from the ion trap. We demonstrate the approach for comprehensive linked-scan identification of phosphopeptides in mixtures with their corresponding unphosphorylated peptides.     

Polysaccharide films built by simultaneous or alternate spray: a rapid way to engineer biomaterial surfaces

We investigated polysaccharide films obtained by simultaneous and alternate spraying of a chitosan (CHI) solution as polycation and hyaluronic acid (HA), alginate (ALG), and chondroitin sulfate (CS) solutions as polyanions. For simultaneous spraying, the film thickness increases linearly with the cumulative spraying time and passes through a maximum for polyanion/CHI molar charge ratios lying between 0.6 and 1.2. The size of polyanion/CHI complexes formed in solution was compared with the simultaneously sprayed film growth rate as a function of the polyanion/CHI molar charge ratio. A good correlation was found. This suggests the importance of polyanion/polycation complexation in the simultaneous spraying process. Depending on the system, the film topography is either liquid-like or granular. Film biocompatibility was evaluated using human gingival fibroblasts. A small or no difference is observed in cell viability and adhesion between the two deposition processes. The CHI/HA system appears to be the best for cell adhesion inducing the clustering of CD44, a cell surface HA receptor, at the membrane of cells. Simultaneous or alternate spraying of CHI/HA appears thus to be a convenient and fast procedure for biomaterial surface modifications.     

Ammine magnesium borohydride complex as a new material for hydrogen storage: structure and properties of Mg(BH4)2.2NH3

The ammonia complex of magnesium borohydride Mg(BH4)2.2NH3 (I), which contains 16.0 wt % hydrogen, is a potentially promising material for hydrogen storage. This complex was synthesized by thermal decomposition of a hexaaammine complex Mg(BH4)2.6NH3 (II), which crystallizes in the cubic space group Fm3 m with unit cell parameter a=10.82(1) A and is isostructural to Mg(NH3) 6Cl2. We solved the structure of I that crystallizes in the orthorhombic space group Pcab with unit cell parameters a=17.4872(4) A, b=9.4132(2) A, c=8.7304(2) A, and Z=8. This structure is built from individual pseudotetrahedral molecules Mg(BH4)2.2NH3 containing one bidentate BH4 group and one tridentate BH4 group that pack into a layered crystal structure mediated by N-H...H-B dihydrogen bonds. Complex I decomposes endothermically starting at 150 degrees C, with a maximum hydrogen release rate at 205 degrees C, which makes it competitive with ammonia borane BH 3NH3 as a hydrogen storage material.     

A scanning tunneling microscopy study of distyrylbenzene on Ag/Ge(111)-(sqr rt of 3 x sqr rt of 3)R30 degrees

The adsorption and self-organized monolayers of trans,trans-distyrylbenzene (tt-DSB) and cis,cis-distyrylbenzene (cc-DSB) on Ag/Ge(111)-(sqr rt of 3 x sqr rt of 3)R30 degrees (Ag/Ge(111)-sqr rt of 3) were studied by low-temperature scanning tunneling microscopy (STM) in ultrahigh vacuum. tt-DSB and cc-DSB overlayers were prepared by vapor deposition at a substrate temperature of 200 K and imaged after the samples were cooled to 100 K. High-resolution images allow identification of the internal structure of individual tt-DSB molecules with three phenyl rings and their molecular arrangements on the Ag/Ge(111)-sqr rt of 3 surface. It is found that the intermolecular distance between two terminal phenyl rings in tt-DSB is about twice the lattice constant of Ag/Ge(111)-sqr rt of 3. Such a lattice match makes Ag/Ge(111)-sqr rt of 3 an ideal substrate for tt-DSB self-organization and the formation of a (3 x 1) overlayer unit cell. The structural model and the molecule registry corresponding to STM images for the adlayers of tt-DSB on Ag/Ge(111)-sqr rt of 3 are proposed and discussed. For cc-DSB adsorption on Ag/Ge(111)-sqr rt of 3, uniform molecular overlayers with two discernible molecular images corresponding to two major types of cc-DSB conformers were observed. The coexistence of multiple conformers and the mismatch of molecular dimension of cc-DSB with the substrate unit cell length limit the growth of large cc-DSB domains.     

Dipping versus spraying: exploring the deposition conditions for speeding up layer-by-layer assembly

Polyelectrolyte film fabrication by successive spraying of polycation and polyanion solutions is described and compared to classic dipping. The poly(styrenesulfonate)/poly(allylamine) system is examined in detail. The influence of various parameters such as spraying time, polyelectrolyte concentration, and effect of film drying during multilayer construction is investigated. It is found that film deposition by spraying is easily controlled and very reliable. The thickness of the multilayers grows linearly with the number of deposition cycles similarly to what is observed when dipping substrates or when polyelectrolyte solutions flow over a surface. The assembly of films is very fast and leads to films with small surface roughness as estimated by atomic force microscopy and X-ray reflectometry. Spray deposition allows achieving regular multilayer growth even under conditions for which dipping fails to produce homogeneous films (e.g., extremely short contact times). Moreover, because drainage constantly removes a certain quantity of the excess material arriving at the surface, one can even skip the rinsing step and, thus, speed up even further the whole buildup process.