Spatially resolved characterization of PEFCs using simultaneously neutron radiography and locally resolved impedance spectroscopy

A method for performing neutron radiography and locally resolved impedance spectroscopy simultaneously in situ in an operating polymer electrolyte fuel cell (PEFC) is presented. The new method provides concurrently spatially resolved information about the local cell performance, the locally limiting processes, and the liquid water distribution. Information about the impact of water on cell performance and limiting processes can be gained in situ on a local scale in an operating PEFC. The method was applied to a PEFC operated on pure H₂/O₂ in co-flow mode under low humidity operating conditions. The results show that in co-flow mode strong flooding and severe drying can occur at the very same time in different sections of a PEFC.     

Complete structural characterization of metallacyclic complexes in solution-phase using simultaneously X-ray diffraction and molecular dynamics simulation

Wide-angle X-ray diffraction and molecular dynamics simulation has been used to perform complete structural characterization of nitromethane solution of a 16-membered gold(I) ring. The joint application of these two methods was an adequate tool to describe not only the structure of the complex but also the solvation properties of the complex in nitromethane and the effect of the solvation on the bulk structure. It has been found that a relatively diffuse slightly distorted solvation shell is formed around the complex, following the shape of the molecule. Nitromethane molecules in the solvation sphere are distributed randomly; no special orientation can be detected. The interaction energy of the complex with nitromethane molecules is attractive. In bulk, besides the antiparallel orientation of the nitromethane molecules, T-shape orientation and long-range order in antidipole orientation can also be detected.     

Time and frequency resolved dynamics of ArBr2

We report the first spectroscopic observation of and vibrational predissociation dynamics for ArBr(2). Measurements are reported for the linear and T-shaped isomers with time and frequency resolution near the Fourier limit of a 15 ps pulse. For the T-shaped isomer, the Ar-Br(2) bond energy D(0) for the B state, nu(')=19, is found to be 200 cm(-1), yielding a D(0)(X) value of 213 cm(-1). Product appearance rates are determined for nu(')=16-25 of the B state of ArBr(2). While the rate generally increases with increasing vibrational quantum number, the trend is not monotonic. Also, obtaining reproducible rates for any given vibrational level requires very careful control of the experimental conditions. The data suggest that ArBr(2) undergoes vibrational predissociation (VP) in the sparse intramolecular vibrational relaxation regime. These observations are consistent with theoretical calculations that show that T-shaped ArBr(2) undergoes VP in the sparse regime, such that lifetimes are strongly dependent on both the vibrational and rotational quantum numbers. As for ArI(2), a linear isomer of ArBr(2) is found to contribute a quasicontinuous background to the excitation spectra. Direct excitation of this continuum results in a very broad product vibrational distribution.     

Angle resolved XPS characterization of cationic polyacrylamides

In many applications surfaces are modified using polymer films and the polymers used are often complex copolymers. In biomedical applications it is critical to determine the surface properties of a substrate as it is these that mediate the cellular interactions. The surface structure of copolymer films can only rarely be established from their bulk composition alone. In this study angle resolved XPS was used to build a model of the structure of copolymer films produced on glass substrates from a family of poly(acrylamide) copolymers containing cationic blocks. The thickness of the copolymer films was demonstrated to be dependent on the concentration of the polymer solution and the ratio of non‐cationic to cationic blocks in the copolymer. The data demonstrated that the cationic blocks of the copolymer preferentially segregated to the glass surface and the non‐cationic poly(acrylamide) blocks preferentially segregated to the air–vacuum interface. A low concentration of the cationic functional groups was present throughout the poly(acrylamide) layer and it was suggested that this resulted from a small fraction of the cationic blocks being pulled into the poly(acrylamide) layer at points along the polymer chain where the two blocks are connected. Evidence of a thin surface hydrocarbon contamination layer was also observed. Copyright © 2009 John Wiley & Sons, Ltd.     

Controlling the dynamics of molecular encapsulation and gating

This critical review describes mechanisms by which guest molecules enter and depart molecular capsules. The discussion focuses on presenting gated molecular encapsulation, i.e., trapping and releasing of guest molecules at rates that are controlled by conformational changes in the host's structure. Developing quantitative rules that describe the gating are, at present, a matter of scientific curiosity but could play an important role in building more effective catalysts, drug-delivery devices or membranes (105 references).     

Photocurrent measurements for laterally resolved interface characterization

A miniaturized optical set-up based on a CD-ROM player optic was developed for LAPS (light addressable potentiometric sensors). A focus of 2.6 microm was achieved using this easy to handle device. The lateral resolution of LAPS measurements can be improved by using GaAs as the semiconductor material instead of Si. The diffusion length of the minority charge carriers was determined to be smaller than 3.1 microm. A new method called SPIM (scanning photo-induced impedance microscopy) is described. Using this technique, the impedance of thin films can be measured with lateral resolution.     

Dielectric characterization of Babesia bovis using the dielectrophoretic crossover frequency

Coinfection with the tick-transmitted pathogen Babesia spp. is becoming a serious health problem because of the erythrocyte invasion through Ixodes scapularis tick. The transmission of this protozoan by blood transfusion often results in high morbidity and mortality in recipients. A novel way to detect parasitized erythrocytes is by utilizing dielectrophoresis, an electrokinetic technique on a microfluidic platform, to improve the diagnostics of Babesia spp. The differences in the dielectric properties of Babesia spp.–infected erythrocytes versus healthy erythrocytes were exploited to design a fast and cost-effective diagnostic tool. One crucial factor for a successful diagnostic platform via dielectrophoretic separation is the dielectric characterization of Babesia-infected erythrocytes, which is investigated in this paper. The influence of medium conductivity and erythrocytes phenotype and genotype over the first crossover frequency (f_co1) are used to quantify the dielectric properties of the infected cells. A sigmoidal curve was plotted via curve fitting of the single-shell model, which has been proven appropriate for parasitized cell populations where considerable cell geometry variation occurs. The difference in these curves is relevant for the separation of cells population. Microliters of sample and reagent were used throughout this experiment; the scale, results obtained, and simplicity of the system often make it very suitable for point-of-care babesiosis disease diagnostics.     

Photocurrent measurements for laterally resolved interface characterization

A miniaturized optical set-up based on a CD-ROM player optic was developed for LAPS (light addressable potentiometric sensors). A focus of 2.6 μm was achieved using this easy to handle device. The lateral resolution of LAPS measurements can be improved by using GaAs as the semiconductor material instead of Si. The diffusion length of the minority charge carriers was determined to be smaller than 3.1 μm. A new method called SPIM (scanning photo-induced impedance microscopy) is described. Using this technique, the impedance of thin films can be measured with lateral resolution.     

Spatially resolved vibrational energy transfer in molecular monolayers

We have shown that it is possible to input heat to one location of a molecule and simultaneously measure its arrival in real time at two other locations, using an ultrafast flash-thermal conductance technique. A femtosecond laser pulse heats an Au layer to approximately 800 degrees C, while vibrational sum-frequency generation spectroscopy (SFG) monitors heat flow into self-assembled monolayers (SAMs) of organic thiolates. Heat flow into the SAM creates thermally induced disorder, which decreases the coherent SFG signal from the CH-stretching transitions. Recent improvements in the technique are described, including the use of nonresonant background-suppressed SFG. The improved apparatus was characterized using alkanethiolate and benzenethiolate SAMs. In the asymmetric 2-methyl benzenethiolate SAM, SFG can simultaneously monitor CH-stretching transitions of both phenyl and methyl groups. The phenyl response to flash-heating occurs at least as fast as the 1 ps time for the Au surface to heat. The methyl response has a faster portion similar to the phenyl response and a slower portion characterized by an 8 ps time constant. The faster portions are attributed to disordering of the methyl-substituted phenyl rings due to thermal excitation of the Au-S adbonds. The slower portion, seen only in the methyl SFG signal, is attributed to heat flow from the metal surface into the phenyl rings and then to the methyl groups.     

Catalysis resolved using scanning tunnelling microscopy

The technique of scanning tunnelling microscopy has revolutionised our understanding of surface chemistry, due to its ability to image at the atomic and molecular scale, the very realm at which chemistry operates. This critical review focuses on its contribution to the resolution of various problems in heterogeneous catalysis, including surface structure, surface intermediates, active sites and spillover. In the article a number of images of surfaces are shown, many at atomic resolution, and the insights which these give into surface reactivity are invaluable. The article should be of interest to catalytic chemists, surface and materials scientists and those involved with nanotechnology/nanoscience. (129 references.)The graphical abstract shows the reaction between gas phase methanol and oxygen islands on Cu(110), courtesy of Philip Davies of Cardiff University. The added-row island is shown as silver-coloured spheres (copper) and red (oxygen) on the copper surface. Methanol preferentially reacts with the terminal oxygen atoms in the island forming adsorbed methoxy and OH groups. Only the terminal oxygen atoms in the island are active sites for the reaction.     

Nanoscale characterization of semiconductor surfaces by spatially resolved photocurrent measurements

A method based on scanning tunneling microscopy has been developed to measure spatially resolved primary photocurrents together with the topographical image. As model semiconductors n- and p-type WSe₂-monocrystals have been used. The system consisting of metal tip, gap and semiconductor sample behaves as a classical metal-insulator-semiconductor (MIS) solar cell. The resolution achieved in the photocurrent image is at least 1 nm. In order to demonstrate the technique, the surfaces of clean n- and p-type WSe₂ samples have been investigated. The method has been extended to analyze metal modified semiconductor surfaces and some of these results are shown here.     

Thermodynamics and selectivity of two-dimensional metallo-supramolecular self-assembly resolved at molecular scale

We investigated the thermodynamic processes of two-dimensional (2D) metallo-supramolecular self-assembly at molecular resolution using scanning tunneling microscopy and variable-temperature low-energy electron diffraction. On a Au(111) substrate, tripyridyl ligands coordinated with Cu in a twofold Cu-pyridyl binding mode or with Fe in a threefold Fe-pyridyl binding mode, forming a 2D open network structure in each case. The network structures exhibited remarkable thermal stability (600 K for the Cu-coordinated network and 680 K for the Fe-coordinated network). The Fe-pyridyl binding was selected thermodynamically as well as kinetically in self-assembly involving both modes. The selectivity can be effectively suppressed in a specifically designed self-assembly route.     

Spatially resolved electrical impedance methods for cell and particle characterization

Electrical impedance is an established technique used for cell and particle characterization. The temporal and spectral resolution of electrical impedance have been used to resolve basic cell characteristics like size and type, as well as to determine cell viability and activity. Such electrical impedance measurements are typically performed across the entire sample volume and can only provide an overall indication concerning the properties and state of that sample. For the study of heterogeneous structures such as cell layers, biological tissue, or polydisperse particle mixtures, an overall measured impedance value can only provide limited information and can lead to data misinterpretation. For the investigation of localized sample properties in complex heterogeneous structures/mixtures, the addition of spatial resolution to impedance measurements is necessary. Several spatially resolved impedance measurement techniques have been developed and applied to cell and particle research, including electrical impedance tomography, scanning electrochemical microscopy, and microelectrode arrays. This review provides an overview of spatially resolved impedance measurement methods and assesses their applicability for cell and particle characterization.     

Structural characterization of interfacial n-octanol and 3-octanol using molecular dynamic simulations

Structurally isomeric octanol interfacial systems, water/vapor, 3-octanol/vapor, n-octanol/vapor, 3-octanol/water, and n-octanol/water are investigated at 298 K using molecular dynamics simulation techniques. The present study is intended to investigate strongly associated liquid/liquid interfaces and probe the atomistic structure of these interfaces. The octanol and water molecules were initially placed randomly into a box and were equilibrated using constant pressure techniques to minimize bias within the initial conditions as well as to fully sample the structural conformations of the interface. An interface formed via phase separation during equilibration and resulted in a slab geometry with a molecularly sharp interface. However, some water molecules remained within the octanol phase with a mole fraction of 0.12 after equilibration. The resulting "wet" octanol interfaces were analyzed using density profiles and orientational order parameters. Our results support the hypothesis of an ordered interface only 1 or 2 molecular layers deep before bulk properties are reached for both the 3-octanol and water systems. However, in contrast to most other interfacial systems studied by molecular dynamics simulations, the n-octanol interface extends for several molecular layers. The octanol hydroxyl groups form a hydrogen-bonding network with water which orders the surface molecules toward a preferred direction and produces a hydrophilic/hydrophobic layering. The ordered n-octanol produces an oscillating low-high density of oxygen atoms out of phase with a high-low density of carbon atoms, consistent with an oscillating dielectric. In contrast, the isomeric 3-octanol has only a single carbon-rich layer directly proximal to the interface, which is a result of the different molecular topology. Both 3-octanol and n-octanol roughen the water interface with respect to the water/vapor interface. The "wet" octanol phases, in the octanol/water systems reach bulk properties in a shorter distance than the "dry" octanol/vapor interfaces.     

Complete profiling and characterization of in vitro nefazodone metabolites using two different tandem mass spectrometric platforms

This paper describes the complete profiling and characterization of in vitro metabolites of the antidepressant agent nefazodone (NEF) generated by human liver microsome (HLM). Two new metabolic pathways (biotransformation) for NEF have been discovered by the characterization of three new metabolites, including two new metabolites (M24, M25) formed due to the N-dealkylation reaction that occurred between the triazolone and propyl units, and one new metabolite (M26) formed due to the O-dearylation reaction that occurred on the phenoxyethyl unit. These metabolites were initially detected by a 4000 Q-Trap instrument and then confirmed by exact mass measurement using an LTQ-Orbitrap. Both instruments proved to be capable of providing complete in vitro metabolite information in a single liquid chromatography/tandem mass spectrometry (LC/MS/MS) analysis, although each had its advantages and disadvantages. One noticeable disadvantage of the 4000 Q-Trap was the reduced quality of isotopic pattern in the enhanced mass scan (EMS) spectrum when it was used as survey scan to trigger multiple dependent product ion scans. The problem was especially exacerbated for minor metabolites with low signal intensity. On the other hand, the LTQ-Orbitrap maintained excellent isotopic pattern when used as a full scan survey scan. Twenty-six metabolites were detected and identified. The formation of these new metabolites was also confirmed by analyzing duplicate incubations at different time points.     

Labile zinc-assisted biological phosphate chemosensing and related molecular logic gating interpretations

Herein, molecular fluorescence 'OFF-ON' behavior with aqueous addition of biological phosphate and Zn(2+) is studied with Zn(2)(slys)(2)Cl(2) [H(2)slys = 6-amino-2-{(2-hydroxybenzylidene)amino}hexanoic acid], a fluorescent water-soluble complex, using various spectroscopic tools (e.g., (31)P NMR, UV-vis, emission, and CD spectroscopy) at the micromolar level. Adduct-dependent fluorescence intensity changes can be interpreted as a two-input (cation/anion) implication molecular logic gating system. A displacement study of PPi from the dizinc complex is also reported. Diphosphate and triphosphate addition/displacements were also studied. (31)P NMR spectroscopy shows gradual NMR peak shifts from bound ADP/GDP to free ADP/GDP with increasing [PPi]. In the emission spectrum, fluorescence quenching is shown: CD signal maxima decrease with addition of PPi. These displacement events are also tested with triphosphates (ATP, GTP), and their binding strength/displacement ability over ADP/GDP is quantified: PPi > ATP ≈ GTP (3.35 ± 0.77 × 10(4) M(-1) for PPi, 7.73 ± 1.79 × 10(3) M(-1) for ATP, 9.21 ± 2.88 × 10(3) M(-1) for GTP over 1·ADP). Many anions and cations were also screened for selectivity. Tubulin polymerization was assayed in the presence of 1 and its copper analogue which reflected a slight inhibition in polymerization.