Ion channel characterization using single cell impedance spectroscopy

A micro electrical impedance spectroscopy system (microEIS) for single cell analysis has been developed and used to differentiate ion channel activities of bovine chromaffin cells. K+ and Ca2+ channels were blocked and their electrical impedances were measured over a frequency range of 100 Hz to 5.0 MHz and compared to that of unblocked chromaffin cells. When ion channels were blocked, an increase in magnitude and decrease in phase of the measured impedances were observed. This result demonstrates that ion channel activities can be distinguished using the developed microsystem and it is expected that this system can be used to provide positive/negative information of ion channel blockage in a high throughput screening setup.     

Quantitative characterization of DNA films by X-ray photoelectron spectroscopy

We describe the use of self-assembled films of thiolated (dT)25 single-stranded DNA (ssDNA) on gold as a model system for quantitative characterization of DNA films by X-ray photoelectron spectroscopy (XPS). We evaluate the applicability of a uniform and homogeneous overlayer-substrate model for data analysis, examine model parameters used to describe DNA films (e.g., density and electron attenuation length), and validate the results. The model is used to obtain quantitative composition and coverage information as a function of immobilization time. We find that when the electron attenuation effects are properly included in the XPS data analysis, excellent agreement is obtained with Fourier transform infrared (FTIR) measurements for relative values of the DNA coverage, and the calculated absolute coverage is consistent with a previous radiolabeling study. Based on the effectiveness of the analysis procedure for model (dT)25 ssDNA films, it should be generally valid for direct quantitative comparison of DNA films prepared under widely varying conditions.     

Quantitative characterization of ion pairing and cluster formation in strong 1:1 electrolytes

Aqueous solutions of 1:1 strong electrolytes are considered to be the prototype for complete ionic dissociation. Nonetheless, clustering of strong 1:1 electrolytes has been widely reported in all atom molecular dynamics simulations, and their presence is indirectly implicated in a diverse range of experimental results. Is there a physical basis for nonidealities such as ion pairing and cluster formation in aqueous solutions of strong 1:1 electrolytes? We attempt to answer this question by direct comparison of results from detailed molecular dynamics simulations to experimentally observed properties of 1:1 electrolytes. We report the analysis of a series of lengthy molecular dynamics simulations of alkali-halide solutions carried out over a wide range of physiologically relevant concentrations using explicit representations of water molecules. We find evidence for pronounced nonideal behavior of ions at all concentrations in the form of ion pairs and clusters which are in rapid equilibrium with dissociated ions. The phenomenology for ion pairing seen in these simulations is congruent with the multistep scheme proposed by Eigen and Tamm based on data from ultrasonic absorption experiments. For a given electrolyte, we show that the dependence of cluster populations on concentration can be described through a single set of equilibrium constants. We assess the accuracy of calculated ion pairing constants by favorable comparison to estimates obtained by Fuoss and co-workers and based on conductometric experiments. Ion pairs and clusters form on length scales where the size of individual water molecules is as important as the hard core radius of ions. Ion pairing results as a balance between the favorable Coulomb interactions and the unfavorable partial desolvation of ions needed to form a pair.     

Characterization of nanostructured hybrid and organic solar cells by impedance spectroscopy

We review the application of impedance spectroscopy in dye-sensitized solar cells, quantum dot-sensitized solar cells and organic bulk heterojunction solar cells. We emphasize the interpretation of the impedance parameters for determining the internal features of the device, concerning the carrier distribution, materials properties such as the density of states and/or doping of the semiconductors, and the match of energy levels for photoinduced charge generation and separation. Another central task is the determination of recombination mechanisms from the measured resistances, and the factors governing the device performance by combined analysis of resistances as a function of voltage and current-voltage curves.     

Synthetic ion channel activity documented by electrophysiological methods in living cells

Hydraphiles are synthetic ion channels that use crown ethers as entry portals and that span phospholipid bilayer membranes. Proton and sodium cation transport by these compounds has been demonstrated in liposomes and planar bilayers. In the present work, whole cell patch clamp experiments show that hydraphiles integrate into the membranes of human embryonic kidney (HEK 293) cells and significantly increase membrane conductance. The altered membrane permeability is reversible, and the cells under study remain vital during the experiment. Control compounds that are too short (C(8)-benzyl channel) to span the bilayer or are inactive owing to a deficiency in the central relay do not induce similar conductance increases. Control experiments confirm that the inactive channel analogues do not show nonspecific effects such as activation of native channels. These studies show that the combination of structural features that have been designed into the hydraphiles afford true, albeit simple, channel function in live cells.     

Electrochemical impedance spectroscopy fingerprints the ion selectivity of microgel functionalized ion-exchange membranes

Surface modification methods are applied to alter interfacial phenomena and improve ion transport through membranes. In this work we present a novel method for tailoring the surface of cation-exchange membranes based on the deposition of thin microgel monolayers. The charge of such layers exerts a strong influence on the monovalent-ion-selectivity, and this is reflected in the electrochemical impedance responses. Membranes coated with uncharged microgels show similar behavior to that of unmodified ones, with impedance spectra dominated by low-frequency diffusional arcs. However, membranes modified with positively charged microgels exhibit an increased resistance due to the hindered transport of cations through the modification. An additional high-frequency capacitive arc is obtained with the monovalent-ion-selective membranes, which is attributed to concentration polarization effects at the membrane/modification interface. The characteristic frequency of this arc decreases with the valency of the ion, thus proving that multivalent ions pass through the modification layer at rates much slower than monovalent ones. Accordingly, electrochemical impedance spectroscopy has been used to feature monovalent-ion-selective properties of layered membranes.     

Simulating ion channel activation mechanisms using swarms of trajectories

Atomic-level studies of protein activity represent a significant challenge as a result of the complexity of conformational changes occurring on wide-ranging timescales, often greatly exceeding that of even the longest simulations. A prime example is the elucidation of protein allosteric mechanisms, where localized perturbations transmit throughout a large macromolecule to generate a response signal. For example, the conversion of chemical to electrical signals during synaptic neurotransmission in the brain is achieved by specialized membrane proteins called pentameric ligand-gated ion channels. Here, the binding of a neurotransmitter results in a global conformational change to open an ion-conducting pore across the nerve cell membrane. X-ray crystallography has produced static structures of the open and closed states of the proton-gated GLIC pentameric ligand-gated ion channel protein, allowing for atomistic simulations that can uncover changes related to activation. We discuss a range of enhanced sampling approaches that could be used to explore activation mechanisms. In particular, we describe recent application of an atomistic string method, based on Roux's “swarms of trajectories” approach, to elucidate the sequence and interdependence of conformational changes during activation. We illustrate how this can be combined with transition analysis and Brownian dynamics to extract thermodynamic and kinetic information, leading to understanding of what controls ion channel function. © 2019 Wiley Periodicals, Inc.     

Application of electrochemical impedance spectroscopy in characterization of structural changes of printing plates

The porous structure of the aluminium oxide surface of lithographic printing plate (PP) has a most significant influence on the quality of the imprints. This study presents the results of application of electrochemical impedance spectroscopy (EIS) in characterization of PPs' porous structures and their changes during chemical processing. Two common PP types—thermal and conventional—were investigated. The influence of the processing solution’s working age on topographical changes of PP surface and associated change in the impedance spectra are investigated and discussed. The equivalent electrical circuit models reproducing the observed EIS spectra are proposed. Based on these models two mechanisms of surface’s topography changes responsible for degradation of PP performance due to the processing are identified and discussed.     

Electrochemical impedance spectroscopy for characterization of coatings with intrinsically conducting polymers

The method of electrochemical impedance spectroscopy (EIS) was applied to investigate the behaviour of a thin intrinsically conducting polymer film (ICP) deposited on a metal substrate. Especially the conductivity, the redox properties, the anion release properties, and the corrosion protection of a coating with and without ICP film on an iron or steel substrate were studied. Combined with other electrochemical methods, the reactions taking place at an injured surface area of the coated iron were studied. The corrosion protection mechanism of polythiophene could be explained.     

Quantitative determination of glyphosate in human serum by 1H NMR spectroscopy

The determination and quantification of glyphosate in serum using (1)H NMR spectroscopy is reported. This method permitted serum samples to be analysed without derivatization or any other sample pre-treatment, using 3-trimethylsilyl 2,2',3,3'-tetradeuteropropionic acid (TSP-d(4)) as a qualitative and quantitative standard. Characterization of the herbicide N-(phosphonomethyl)glycine was performed by analysing chemical shifts and coupling constant patterns. Quantification was performed by relative integration of CH(2)-P protons to the TSP-d(4) resonance peak. The method was tested for repeatability (n=5) and yielded coefficients of variation of 1% and 3%, respectively: detection and quantification limits were also determined and were 0.03 and 0.1mmol/L, respectively. The method was applied to the quantification of glyphosate in a case of acute poisoning.     

Modification of polysulfone membranes with polyethylene glycol and lignosulfate: electrical characterization by impedance spectroscopy measurements

Two sets of composite membranes having an asymmetric sulfonated polysulfone membrane as support layer have been obtained and electrically characterized (membranes SPS-PEG and PA-LIGS). The skin layer of the membrane SPS-PEG contains different percentages of polyethylene glycol in the casting solution (5, 25, 40, and 60 wt%), while lignosulfonate was used for manufacturing PA-LIGS membranes (5, 10, 20, and 40 wt%). Membrane electrical characterization was done by means of impedance spectroscopy (IS) measurements, which were carried out with the membranes in contact with NaCl solutions at different concentrations (10(-3) < or = c(M) < or = 5x10(-2)). Electrical resistance and equivalent capacitance of the different membrane samples were determined from IS plots by using equivalent circuits as models. Results show a clear decrease in the membrane electrical resistance as a result of both polysulfone sulfonation and the increase of the concentration of modifying substances, although a kind of limit concentration was obtained for both polyethylene glycol and lignosulfonate (40 and 20%, respectively). Results also show a decrease of around 90% in electrical resistance due to polysulfone sulfonation, while the value of the dielectric constant (hydrated state) clearly increases.     

Label-free detection of telomerase activity in HeLa cells using electrochemical impedance spectroscopy

A simple assay based on electrochemical impedance spectroscopy (EIS) for detection of telomerase activity is developed, and it is demonstrated that the label-free EIS method is capable of detecting the telomerase activity in HeLa cells with a detection limit of 1000 HeLa cells without using any amplification technique.     

Aggregation of methyl orange probed by electrical impedance spectroscopy

We present results of an electrical impedance spectroscopy investigation of the evolution of the aggregation of methyl orange (MO) in pure aqueous solutions as the concentration of the dye is varied. By applying the constant phase element (CPE) approximation to model the electrical response of the MO solutions, we have verified that the formation of dimers and oligomers can be recognized by specific signatures in the loss and capacitive components of the dielectric response of the system. We interpret these well-defined changes in the dielectric properties of the solutions as a result of molecular rearrangements caused by the aggregation process that alter the current circulation pathways and the electric dipole distribution. The fact that these specific changes in the dielectric behavior coincide with critical concentrations where dimer and oligomer formation in pure aqueous MO solutions are known to occur suggests that electrical impedance spectroscopy can be a competitive technique for the investigation of aggregation behavior in dyes and surfactants.     

Electrical impedance spectroscopy for detection of bacterial cells in suspensions using interdigitated microelectrodes

In this study, we present a new, simple and rapid impedance method to detect bacterial cells by making use of the impedance properties of bacterial cell suspensions using interdigitated microelectrodes. It was found that bacterial cell suspensions in deionized (DI) water with different cell concentrations could generate different electrical impedance spectral responses, whereas cell suspensions in phosphate buffered saline (PBS) solution could not produce any significant differences in impedance spectra in response to different cell concentrations. In DI water suspensions, impedance at 1 kHz decreased with the increasing cell concentrations in the suspensions. The impedance of cell suspensions in DI water was discussed and found that it was resulted from the cell wall charges and the release of ions or other osmolytes from the cells. A linear relationship between the impedance and the logarithmic value of the cell concentration was found in the cell concentration range from 10⁶ to 10¹⁰ cfu/ml, which can be expressed by a regression equation of Z (kΩ) = −2.06 log C (cells/20 μl) + 5.23 with R² = 0.98. The detection limit was calculated to be 3.45 × 10⁶ cfu/ml, which is comparable with many label-free immunosensors for detection of pathogenic bacteria reported in the literature. To achieve the selectivity of this method, we also demonstrated the feasibility of integrating magnetic separation to this impedance method. This study has demonstrated that bacterial cell concentration can be inferred by measuring the impedance of cell suspensions in DI water. This new detection mechanism could be an alternative to current impedance methods that have been reported for the detection of bacterial cells, e.g. impedance microbiology and electrical/electrochemical impedance biosensors.     

Study of silica supported PtxNi1-x catalysts by ion scattering spectroscopy

As supported Pt_xNi_1–x catalysts are used for hydrogenation reactions, it seemed necessary to assess the surface composition of the reduced samples. To approach the usual reduction conditions we applied in situ reduction in a reaction chamber (1 mbar H₂ up to 500 °C) placed in ultra high vacuum recipient (UHV: 10^–9 to 2.10^–10mbar). All ion scattering spectroscopy measurements were performed in UHV. Charging of the samples was avoided by electron bombardment (5 eV). The variation of the surface composition was determined after subsequent sputtering, thermal treatment at 500 °C and during oxygen adsorption. A comparison with previous results of surface compositions of binary alloys (polycrystalline foils [1, 2] and single crystals Pt_xNi_1–x [3]) is given.Dedicated to Professor Dr. rer. nat. Dr. h. c. Hubertus Nickel on the occasion of his 65th birthday     

Probing DNA hybridization in thiolipid monolayers by means of impedance spectroscopy

A composite monolayer consisting of a thiolipid and a nucleic acid probe (1), acts as a microenvironment for probing hybridization with the complementary strand (2) by means of impedance spectroscopy. The impedance measurements indicate a significant decrease in the resistance at the electrode surface upon the formation of the (1)/(2) duplex. The impedance measurements were performed in the absence of any amplifying label or added redox label. In order to characterize the electrodes surface and to follow the immobilization processes kinetics, surface plasmon resonance measurements were performed.     

Characterization of reaction intermediates by ion spectroscopy

In the last decade, we have experienced massive progress in spectroscopic methods for mass-selected ions. The aim of this tutorial review is to present action spectroscopy as a powerful tool for the investigation of ionic reaction intermediates. Examples span from ultraviolet and infrared photodissociation spectroscopy of model reaction intermediates to applications of infrared multiphoton dissociation spectroscopy (IRMPD) to intermediates directly sampled from reaction mixtures. The first example of double resonance IR-UV spectroscopy of model intermediates in an organometallic reaction is also mentioned.     

Real-time monitoring of immobilized single yeast cells through multifrequency electrical impedance spectroscopy

We present a microfluidic device, which enables single cells to be reliably trapped and cultivated while simultaneously being monitored by means of multifrequency electrical impedance spectroscopy (EIS) in the frequency range of 10 kHz–10 MHz. Polystyrene beads were employed to characterize the EIS performance inside the microfluidic device. The results demonstrate that EIS yields a low coefficient of variation in measuring the diameters of captured beads (~0.13 %). Budding yeast, Saccharomyces cerevisiae, was afterwards used as model organism. Single yeast cells were immobilized and measured by means of EIS. The bud growth was monitored through EIS at a temporal resolution of 1 min. The size increment of the bud, which is difficult to determine optically within a short time period, can be clearly detected through EIS signals. The impedance measurements also reflect the changes in position or motion of single yeast cells in the trap. By analyzing the multifrequency EIS data, cell motion could be qualitatively discerned from bud growth. The results demonstrate that single-cell EIS can be used to monitor cell growth, while also detecting potential cell motion in real-time and label-free approach, and that EIS constitutes a sensitive tool for dynamic single-cell analysis. Figure

?