Noise and Transport Characterization of Single Molecular Break Junctions with Individual Molecule

We studied the noise spectra of molecule-free and molecule-containing mechanically controllable break junctions. Both types of junctions revealed typical 1/ f noise characteristics at different distances between the contacts with square dependence of current noise power spectral density on current. Additional Lorentzian-shape (1/ f 2) noise components were recorded only when nanoelectrodes were bridged by individual 1,4 benzenediamine molecule. The characteristic frequency of the revealed 1/ f 2 noise related to a single bridging molecule correlates with the lock-in current amplitudes. The recorded behavior of Lorentzian-shape noise component as a function of current is interpreted as the manifestation of a dynamic reconfiguration of molecular coupling to the metal electrodes. We propose a phenomenological model that correlates the charge transport via a single molecule with the reconfiguration of its coupling to the metal electrodes. Experimentally obtained results are in good agreement with theoretical ones and indicate that coupling between the molecule metal electrodes is important aspect that should be taken into account.Comment: 15 pages, 7 figure

Measurement of the Permanent Electric Dipole Moment of the 129^{129}Xe Atom

We report on a new measurement of the CP-violating permanent Electric Dipole Moment (EDM) of the neutral $^{129}$Xe atom. Our experimental approach is based on the detection of the free precession of co-located nuclear spin-polarized $^3$He and $^{129}$Xe samples. The EDM measurement sensitivity benefits strongly from long spin coherence times of several hours achieved in diluted gases and homogeneous weak magnetic fields of about 400~nT. A finite EDM is indicated by a change in the precession frequency, as an electric field is periodically reversed with respect to the magnetic guiding field. Our result, $\left(-4.7\pm6.4\right)\cdot 10^{-28}$ ecm, is consistent with zero and is used to place a new upper limit on the $^{129}$Xe EDM: $|d_\text{Xe}|<1.5 \cdot 10^{-27}$ ecm (95% C.L.). We also discuss the implications of this result for various CP-violating observables as they relate to theories of physics beyond the standard model

Hacia un nuevo socialismo

Nos encontramos ante el testamento político de uno de los líderes norteamericanos del socialismo democrático. M. Harrington murió en agosto de 1989 poco después de haber aparecido este libro. Intuyendo su próximo final hace Un balance histórico del movimiento socialista y apunta hacia su potencial futuro. Es el testamento de un militante y un intelectual que reflexiona sobre lo que es el socialismo desde Estados Unidos, la sociedad occidental del capitalismo más desarrollada y más refractaria a la tradición socialista.Peer reviewe

Precise Measurement of Magnetic Field Gradients from Free Spin Precession Signals of 3^{3}He and 129^{129}Xe Magnetometers

We report on precise measurements of magnetic field gradients extracted from transverse relaxation rates of precessing spin samples. The experimental approach is based on the free precession of gaseous, nuclear spin polarized $^3$He and $^{129}$Xe atoms in a spherical cell inside a magnetic guiding field of about 400 nT using LT$_C$ SQUIDs as low-noise magnetic flux detectors. The transverse relaxation rates of both spin species are simultaneously monitored as magnetic field gradients are varied. For transverse relaxation times reaching 100 h, the residual longitudinal field gradient across the spin sample could be deduced to be$|\vec{\nabla}B_z|=(5.6 \pm 0.4)$ pT/cm. The method takes advantage of the high signal-to-noise ratio with which the decaying spin precession signal can be monitored that finally leads to the exceptional accuracy to determine magnetic field gradients at the sub pT/cm scale

Optimized Continuous Application of Hyperpolarized Xenon to Liquids

International audienceIn recent years, NMR with hyperpolarized (HP) xenon inside functionalized host structures (e.g. cryptophanes) have become a potential candidate for the direct observation of metabolic processes (i.e. molecular imaging). A critical issue for real applications is the dissolution of the HP-gas in the liquid which contains the host. In this work, we present recent developments for an improved and controlled dissolution of HP-Xe in liquids using hollow fiber membranes and different compressor systems. The designed apparatus consists of a compressor and a membrane unit. The compressor provides HP-129 Xe continuously at small adjustable pressures and in a polarization-preserving way. The membrane unit enables a molecular solution of the HP-gas in aqueous liquids, avoiding the formation of bubbles or even foams. Two different types of compressors were tested in terms of function and useful materials. Special emphasis was put on a systematic reduction of transfer losses in the gas and liquid phase. In order to optimize the system parameters, several physical models were developed to describe the transport and the losses of nuclear polarization. Finally, the successful implementation was demonstrated in several experiments. HP-Xe was dissolved in an aqueous cryptophane-A-(OCH 2 COOH) 6 solution, and stable Xe signals could be measured over 35 min, only limited by the size of the gas reservoir. Such long and stable Version 6 21.10.2019 BN experimental conditions enabled the study of chemical exchange of xenon between cryptophane and water environments even for a time-consuming 2D NMR-experiment. The good signal stability over the measurement time allowed an exact determination of the residence time of the Xe-atom inside the cryptophane, resulting in an average residence time of 42.9 ± 3.3 ms

Photoresponsive Porphyrin Nanotubes of Meso-tetra(4-Sulfonatophenyl)Porphyrin and Sn(IV) meso-tetra(4-pyridyl)porphyrin

Porphyrin macrocycles and their supramolecular nanoassemblies are being widely explored in energy harvesting, sensor development, catalysis, and medicine because of a good tunability of their light-induced charge separation and electron/energy transfer properties. In the present work, we prepared and studied photoresponsive porphyrin nanotubes formed by the self-assembly of meso-tetrakis(4-sulfonatophenyl)porphyrin and Sn(IV) meso-tetra(4-pyridyl)porphyrin. Scanning electron microscopy and transmission electron microscopy showed that these tubular nanostructures were hollow with open ends and their length was 0.4–0.8 μm, the inner diameter was 7–15 nm, and the outer diameter was 30–70 nm. Porphyrin tectons, H4TPPS42- : Sn(IV)TPyP4+, self-assemble into the nanotubes in a ratio of 2:1, respectively, as determined by the elemental analysis. The photoconductivity of the porphyrin nanotubes was determined to be as high as 3.1 × 10−4 S m−1, and the dependence of the photoconductance on distance and temperature was investigated. Excitation of the Q-band region with a Q-band of SnTPyP4+ (550–552 nm) and the band at 714 nm, which is associated with J-aggregation, was responsible for about 34 % of the photoconductive activity of the H4TPPS42--Sn(IV)TPyP4+ porphyrin nanotubes. The sensor properties of the H4TPPS42-- Sn(IV)TPyP4+ nanotubes in the presence of iodine vapor and salicylate anions down to millimolar range were examined in a chemiresistor sensing mode. We have shown that the porphyrin nanotubes advantageously combine the characteristics of a sensor and a transducer, thus demonstrating their great potential as efficient functional layers for sensing devices and biomimetic nanoarchitectures

Establishing a sensitive fluorescence-based quantification method for cyclic nucleotides

BACKGROUND: Approximately 40% of prescribed drugs exert their activity via GTP-binding protein-coupled receptors (GPCRs). Once activated, these receptors cause transient changes in the concentration of second messengers, e.g., cyclic adenosine 3′,5′-monophosphate (cAMP). Specific and efficacious genetically encoded biosensors have been developed to monitor cAMP fluctuations with high spatial and temporal resolution in living cells or tissue. A well characterized biosensor for cAMP is the Förster resonance energy transfer (FRET)-based Epac1-camps protein. Pharmacological characterization of newly developed ligands acting at GPCRs often includes numerical quantification of the second messenger amount that was produced. RESULTS: To quantify cellular cAMP concentrations, we bacterially over-expressed and purified Epac1-camps and applied the purified protein in a cell-free detection assay for cAMP in a multi-well format. We found that the biosensor can detect as little as 0.15 pmol of cAMP, and that the sensitivity is not impaired by non-physiological salt concentrations or pH values. Notably, the assay tolerated desiccation and storage of the protein without affecting Epac1-camps cyclic nucleotide sensitivity. CONCLUSIONS: We found that determination cAMP in lysates obtained from cell assays or tissue samples by purified Epac1-camps is a robust, fast, and sensitive assay suitable for routine and high throughput analyses

Spreading of lipid monolayers on hydrophilic substrates at increased relative humidities

A self-repairing ability of phospholipid monolayers deposited onto thin hydrogel films was observed by fluorescence microscopy, in terms of the expansion and lateral spreading of the monolayer into film defects. The spreading was quantitatively analyzed by covering half of a substrate supporting a thin hydrogel film with a lipid monolayer by Langmuir-Blodgett transfer and observing the spreading of the initially confined monolayer at increased relative humidities due to the hydration of both lipid headgroups and the polymer support. During the observation time of a typical spreading experiment, a constant spreading velocity was observed. A strong influence of monolayer pressure on the spreading velocity was observed. A nonlinear relationship between spreading velocity and monolayer pressure was found, which can possibly be explained by a dependence of the disjoining pressure in the monolayer/substrate interface on the monolayer pressure. The spreading velocity was additionally influenced by the kind of support and by the nature (e.g., phase state) of the deposited monolayer

Polysaccharide-supported planar bilayer lipid model membranes

Bilayer lipid membranes were deposited onto two different thin water swellable polymer cushions predominantly by Langmuir-Blodgett trough methods. Membranes consisting of zwitterionic lipids supported by agarose films were shown to be unstable, as observed by fluorescence microscopy, reflection interference contrast microscopy, and the impossibility of bilayer spreading (Radler, J.; Strey, H.; Sackmann, E. Langmuir 1995, 11, 4539-4548) on the agarose surface. Chitosan, formerly observed to permanently coat liposome surfaces of zwitterionic lipid membranes (Henriksen, I.; Smistad, G.; Karlsen, J. Int. J. Pharm. 1994,101, 227-236), was used to prepare thin films by spin-coating and was found to sufficiently adsorb and thereby stabilize planar bilayer lipid membranes. Bilayer spreading on a chitosan surface was observed, indicating the self-healing ability of chitosan-supported lipid membranes

Surface plasmon microscopy measurements of lipid vesicle adsorption on a micropatterned self-assembled monolayer

This paper presents a novel method of measuring the adsorption of egg-phosphatidylcholine (egg-PC) vesicles onto a micropatterned heterogeneous self-assembled monolayer (SAM). Microcontact printing (mu CP) was used to make an array of mercaptoethanol patches surrounded by an octadecanethiol (ODT) SAM. Surface plasmon microscopy (SPM) followed the change in the different plasmon minima of the mercaptoethanol and ODT SAMs with respect to lipid adsorption time. Gray scale analysis (GSA) of the two domains showed clear differences in the kinetics of lipid adsorption, which were equated to a very different mechanism of lipid vesicle fusion on microcontact printed (mu CP) SAMs compared with homogeneous SAMs. Surface plasmon microscopy combined with GSA has allowed us to spatially deconvolute the adsorption of lipid on a patterned surface, giving a unique insight into the lipid adsorption process. Conventional surface plasmon resonance measurements were made on the homogeneous SAM moieties in order to quantify the gray scale analysis in terms of adsorbed lipid thickness