Ion Current Measurements Research Articles

Conduction processes in gas-insulated HVDC equipment: from saturated ion currents to micro-discharges

Charge generation in gas-insulated high-voltage direct current (HVDC) equipment has been investigated since surface charge accumulation was detected as the main challenge to insulation coordination. Over the years, researchers have made enormous progress using continuous ion current measurements or surface charging experiments with model spacers downscaled from gas-insulated components. Whereas the low sensitivity of the former method limits its measurement range, the latter method is time-consuming and demands comprehensive modeling. In this paper, more than 3,000 hours of surface charge measurements are presented, covering the full range of charge generation processes from ion currents owing to natural ionization to the inception of micro-discharges. In accordance with gas-insulated equipment, Al 2 O 3 -filled epoxy resin was used for the solids, and sulfur hexafluoride (SF 6 ) was used for the gaseous insulation. It has been demonstrated that the ion pair generation from natural ionization in the gas is significantly influenced by the surrounding solid materials, and therefore, scales with the insulation volume. From the onset of micro-discharges, sharp and locally limited charge patterns are formed, as surface conductivity does not appear to influence the investigated charge distributions. Whereas the primary charge dissipation from surface areas centered at the distribution could be explained in terms of the gas volume size, the discharge currents could not be reproduced with common ion-current models. At low electric fields, the observed charge decay indicates the presence of low-field conduction processes that cannot be explained by charge generation from natural ionization in the gas.

Excimer laser ablation of graphite: The enhancement of carbon dimer formation

Carbon plasmas produced by laser ablation can have important applications in the synthesis of nanostructured materials of high current interest (nanotubes, nanowires, graphene) or the deposition of diamond-like thin films. Understanding the fundamental aspects of such transient plasmas in various experimental configurations is useful for optimization of these synthesis/deposition processes. We report here a comprehensive study on the dynamics of an excimer laser produced carbon plasma, including fast photography, space- and time-resolved optical emission spectroscopy, Faraday cup ion current measurements, ablated crater depth profiling. A peculiar V-like shape of the emitting plume is evidenced and explained by the interaction of three plasma structures originating in distinct irradiance areas of the laser spot on the target. The interaction of these structures is also thought to favor an enhanced carbon dimer production, mainly through three-body recombination, at distances significantly higher than previously reported in the literature, which can find technological applications for the efficient deposition of high quality carbon-based nanostructures.

Chitosugar translocation by an unexpressed monomeric protein channel.

The outer membrane protein channel EcChiP, associated with a silent gene in E. coli, is a monomeric chitoporin. In a glucose-deficient environment, E. coli can express the ChiP gene to exploit chitin degradation products. Single-channel small ion current measurements, which reveal the dynamics of single sugar molecules trapped in channel, are used here to study the exotic transport of chitosugars by E. coli. Molecules escape from the channel on multiple timescales. Voltage-dependent trapping rates observed for charged chitosan molecules, as well as model calculations, indicate that the rapid escape processes are those in which the molecule escapes back to the side of the membrane from which it originated. The probability that a sugar molecule is translocated through the membrane is thus estimated from the current data and the dependence of this translocation probability on the length of the chitosugar molecule and the applied voltage analyzed. The described method for obtaining the translocation probability and related molecular translocation current is applicable to other transport channels.

Multi-channel retarding field analyzer for EAST

A multi-channel retarding field analyzer (MC-RFA) including two RFA modules and two Langmuir probes to measure the ion and electron temperature profiles within the scrape-off layer was developed for investigations of the interplay between magnetic topology and plasma transport at the plasma boundary. The MC-RFA probe for the stellarator W7-X and first measurements at the tokamak EAST was designed. The probe head allows simultaneous multi-channel ion temperature as well as for electron temperature measurements. The usability for radial correlation measurements of the measured ion currents is also given.

Modulation of Ionic Conductivity of Lipid Bilayer-Based Nanoscopic Channels by Pre-adsorbed Charged Macromolecules as a Tool for their Detection and Quantification

Investigation of transport phenomena in the nanoscopic channels (pores) having the characteristic cross dimension of less than 100 nm is of utmost importance for diverse areas of applied biology, medicine and technology. One of the mainstream lines of development of nanofluidic systems and applications today is creation of biosensors capable of sensing single molecules and manipulating them in a controllable manner. Molecules can be detected based on the measurements of ionic currents through appropriately sized channels: entry into a channel of a molecule with the effective cross-dimension comparable to that of the channel lumen is accompanied by decrease of the ionic current recorded at a given transmembrane potential. The transport properties of such channels can be modulated by coating their walls with lipid bilayers, two-dimensional fluids capable of sustaining transport processes within them. In our present work, we made use of this property of the membranes to develop a method for detecting and controllably transporting single-stranded DNA molecules through channels formed by lipid membrane cylinders with the luminal radius of 5-7 nm. Entry of a DNA molecule into such a channel in the conditions of low (∼10 mM) ion strength proved to be accompanied by detectable increase of its ionic conductivity in a manner dependent on the direction of the electric field gradient. The amplitude of the conductivity increment can be credibly used to quantify the number the DNA molecules within the channel. Besides that, adsorption of DNA molecules on the lipid bilayer surface was shown to render the membrane cylinder the properties of a voltage-dependent channel with ion selectivity. The work was financially supported by the Russian Science Fund No 17-75-30064.

Glass Chip for Nanopore Based Low Noise Resistive Pulse Sensing

Resistive-pulse sensing provides structural insights on individual proteins when translocated through nanopores. The substrate capacitance, however, induces noise in the ionic current signal, which increases strongly with increasing bandwidth of the recording. For this reason, minimizing substrate capacitance is critical for characterizing single molecules through resistive-pulse sensing. Inspired by recent reports on low-noise nanopore substrates, we engineered a glass chip for low noise nanopore resistive-pulse measurements using microfabrication. Replacing silicon substrate material with fused silica gives an approximately 5-fold reduction of noise at a bandwidth of ∼57 kHz. For fabrication, we deposited a 30 nm thick silicon nitride (SiNx) layer on glass by low-pressure chemical vapor deposition. After etching the glass a SiNx membrane with a diameter of ∼20 µm remained and prepared a ∼20 nm diameter nanopore in the membrane by controlled dielectric breakdown. For a glass chip, the standard deviation of the ionic current during resistive-pulse measurements was 8 pA at 15 kHz recording bandwidth and 19 pA at 57 kHz and for a silicon chip, the standard deviation of the ionic current was 29 pA at 15 kHz and 95 pA at 57 kHz. We demonstrate improved sensitivity for resistive-pulse sensing by using IgG protein as test particle translocating through the glass-based nanopore. The low noise from the glass chip at high bandwidth improves the shape characterization of the protein and potentially allows the detection of subtle changes in protein dynamics within the nanopore. Additionally, we have shown low laser-induced noise from glass chips during ionic current measurements, in contrast to silicon-based chips, which respond to laser by significantly increased noise. The absence of light-induced noise from electrically insulating and optically transparent chips for nanopore recordings makes them compelling for synchronized electrical and optical measurements.

Chasing a Protein’s Tail: Detection of Polypeptide Translocation through Nanopores

Chasing a Protein’s Tail: Detection of Polypeptide Translocation through Nanopores

Signal and Noise in FET-Nanopore Devices.

The combination of a nanopore with a local field-effect transistor (FET-nanopore), like a nanoribbon, nanotube, or nanowire, in order to sense single molecules translocating through the pore is promising for DNA sequencing at megahertz bandwidths. Previously, it was experimentally determined that the detection mechanism was due to local potential fluctuations that arise when an analyte enters a nanopore and constricts ion flow through it, rather than the theoretically proposed mechanism of direct charge coupling between the DNA and nanowire. However, there has been little discussion on the experimentally observed detection mechanism and its relation to the operation of real devices. We model the intrinsic signal and noise in such an FET-nanopore device and compare the results to the ionic current signal. The physical dimensions of DNA molecules limit the change in gate voltage on the FET to below 40 mV. We discuss the low-frequency flicker noise (<10 kHz), medium-frequency thermal noise (<100 kHz), and high-frequency capacitive noise (>100 kHz) in FET-nanopore devices. At bandwidths dominated by thermal noise, the signal-to-noise ratio in FET-nanopore devices is lower than in the ionic current signal. At high frequencies, where noise due to parasitic capacitances in the amplifier and chip is the dominant source of noise in ionic current measurements, high-transconductance FET-nanopore devices can outperform ionic current measurements.

Role of outer surface probes for regulating ion gating of nanochannels

Nanochannels with functional elements have shown promise for DNA sequencing, single-molecule sensing, and ion gating. Ionic current measurement is currently a benchmark, but is focused solely on the contribution from nanochannels’ inner-wall functional elements (NIWFE); the attributes of functional elements at nanochannels’ outer surface (NOSFE) are nearly ignored, and remain elusive. Here we show that the role of NOSFE and NIWFE for ion gating can be distinguished by constructing DNA architectures using dual-current readout. The established molecular switches have continuously tunable and reversible ion-gating ability. We find that NOSFE exhibits negligible ion-gating behavior, but it can produce a synergistic effect in alliance with NIWFE. Moreover, the high-efficiency gating systems display more noticeable synergistic effect than the low-efficiency ones. We also reveal that the probe amount of NOSFE and NIWFE is almost equally distributed in our biomimetic nanochannels, which is potentially a premise for the synergistic ion-gating phenomena.

Structure and electrical properties of DNA nanotubes embedded in lipid bilayer membranes.

Engineering the synthetic nanopores through lipid bilayer membrane to access the interior of a cell is a long persisting challenge in biotechnology. Here, we demonstrate the stability and dynamics of a tile-based 6-helix DNA nanotube (DNT) embedded in POPC lipid bilayer using the analysis of 0.2 μs long equilibrium MD simulation trajectories. We observe that the head groups of the lipid molecules close to the lumen cooperatively tilt towards the hydrophilic sugar-phosphate backbone of DNA and form a toroidal structure around the patch of DNT protruding in the membrane. Further, we explore the effect of ionic concentrations to the in-solution structure and stability of the lipid-DNT complex. Transmembrane ionic current measurements for the constant electric field MD simulation provide the I-V characteristics of the water filled DNT lumen in lipid membrane. With increasing salt concentrations, the measured values of transmembrane ionic conductance of the porous DNT lumen vary from 4.3 to 20.6 nS. Simulations of the DNTs with ssDNA and dsDNA overhangs at the mouth of the pore show gating effect with remarkable difference in the transmembrane ionic conductivities for open and close state nanopores.

Orthogonal time-of-flight mass spectrometry of an ion beam with a broad kinetic energy profile.

A combined experimental and modeling effort is undertaken to assess a detection system composed of an orthogonal extraction time-of-flight (TOF) mass spectrometer coupled to a continuous ion source emitting an ion beam with kinetic energy of several hundred eV. The continuous ion source comprises an electrospray capillary system employing an undiluted ionic liquid emitting directly into vacuum. The resulting ion beam consists of ions with kinetic energy distributions of width greater than a hundred of eV and mass-to-charge (m/q) ratios ranging from 111 to 500 000 amu/q. In particular, the investigation aims to demonstrate the kinetic energy resolution along the ion beam axis (axial) of orthogonally extracted ions in measurements of the axial kinetic energy-specific mass spectrum, mass flow rate, and total ion current. The described instrument is capable of simultaneous measurement of a broad m/q range in a single acquisition cycle with approximately 25 eV/q axial kinetic energy resolution. Mass resolutions of ∼340 (M/ΔM, FWHM) were obtained for ions at m/q = 1974. Comparison of the orthogonally extracted TOF mass spectrum to mass flow and ion current measurements obtained with a quartz-crystal microbalance and Faraday cup, respectively, shows reasonable numeric agreement and qualitative agreement in the trend as a function of energy defect.

HPPMS deposition from composite targets: Effect of two orders of magnitude target power density changes on the composition of sputtered Cr-Al-C thin films

The effect of target power density, substrate bias potential and substrate temperature on the thin film composition was studied. A Cr-Al-C composite target was sputtered utilizing direct current (DCMS: 2.3 W/cm2) and high power pulsed magnetron sputtering (HPPMS: 373 W/cm2) generators. At floating potential, all Cr-Al-C thin films showed similar compositions, independently of the applied target power density. However, as substrate bias potential was increased to −400 V, aluminum deficiencies by a factor of up to 1.6 for DCMS and 4.1 for HPPMS were obtained. Based on the measured ion currents at the substrate, preferential re-sputtering of Al is suggested to cause the dramatic Al depletion. As the substrate temperature was increased to 560 °C, the Al concentration was reduced by a factor of up to 1.9 compared to the room temperature deposition. This additional reduction may be rationalized by thermally induced desorption being active in addition to re-sputtering.

Brownian dynamics of a protein-polymer chain complex in a solid-state nanopore.

We study the movement of a polymer attached to a large protein inside a nanopore in a thin silicon dioxide membrane submerged in an electrolyte solution. We use Brownian dynamics to describe the motion of a negatively charged polymer chain of varying lengths attached to a neutral protein modeled as a spherical bead with a radius larger than that of the nanopore, allowing the chain to thread the nanopore but preventing it from translocating. The motion of the protein-polymer complex within the pore is also compared to that of a freely translocating polymer. Our results show that the free polymer's standard deviations in the direction normal to the pore axis is greater than that of the protein-polymer complex. We find that restrictions imposed by the protein, bias, and neighboring chain segments aid in controlling the position of the chain in the pore. Understanding the behavior of the protein-polymer chain complex may lead to methods that improve molecule identification by increasing the resolution of ionic current measurements.

Forty Years of Sodium Channels: Structure, Function, Pharmacology, and Epilepsy.

Voltage-gated sodium channels initiate action potentials in brain neurons. In the 1970s, much was known about the function of sodium channels from measurements of ionic currents using the voltage clamp method, but there was no information about the sodium channel molecules themselves. As a postdoctoral fellow and staff scientist at the National Institutes of Health, I developed neurotoxins as molecular probes of sodium channels in cultured neuroblastoma cells. During those years, Bruce Ransom and I crossed paths as members of the laboratories of Marshall Nirenberg and Philip Nelson and shared insights about sodium channels in neuroblastoma cells from my work and electrical excitability and synaptic transmission in cultured spinal cord neurons from Bruce's pioneering electrophysiological studies. When I established my laboratory at the University of Washington in 1977, my colleagues and I used those neurotoxins to identify the protein subunits of sodium channels, purify them, and reconstitute their ion conductance activity in pure form. Subsequent studies identified the molecular basis for the main functions of sodium channels-voltage-dependent activation, rapid and selective ion conductance, and fast inactivation. Bruce Ransom and I re-connected in the 1990s, as ski buddies at the Winter Conference on Brain Research and as faculty colleagues at the University of Washington when Bruce became our founding Chair of Neurology and provided visionary leadership of that department. In the past decade my work on sodium channels has evolved into structural biology. Molecular modeling and X-ray crystallographic studies have given new views of sodium channel function at atomic resolution. Sodium channels are also the molecular targets for genetic diseases, including Dravet Syndrome, an intractable pediatric epilepsy disorder with major co-morbidities of cognitive deficit, autistic-like behaviors, and premature death that is caused by loss-of-function mutations in the brain sodium channel NaV1.1. Our work on a mouse genetic model of this disease has shown that its multi-faceted pathophysiology and co-morbidities derive from selective loss of electrical excitability and action potential firing in GABAergic inhibitory neurons, which disinhibits neural circuits throughout the brain and leads directly to the epilepsy, premature death and complex co-morbidities of this disease. It has been rewarding for me to use our developing knowledge of sodium channels to help understand the pathophysiology and to suggest potential therapeutic approaches for this devastating childhood disease.

Bacterial Outer Membrane Porins as Electrostatic Nanosieves: Exploring Transport Rules of Small Polar Molecules.

Transport of molecules through biological membranes is a fundamental process in biology, facilitated by selective channels and general pores. The architecture of some outer membrane pores in Gram-negative bacteria, common to other eukaryotic pores, suggests them as prototypes of electrostatically regulated nanosieve devices. In this study, we sensed the internal electrostatics of the two most abundant outer membrane channels of Escherichia coli, using norfloxacin as a dipolar probe in single molecule electrophysiology. The voltage dependence of the association rate constant of norfloxacin interacting with these nanochannels follows an exponential trend, unexpected for neutral molecules. We combined electrophysiology, channel mutagenesis, and enhanced sampling molecular dynamics simulations to explain this molecular mechanism. Voltage and temperature dependent ion current measurements allowed us to quantify the transversal electric field inside the channel as well as the distance where the applied potential drops. Finally, we proposed a general model for transport of polar molecules through these electrostatic nanosieves. Our model helps to further understand the basis for permeability in Gram-negative pathogens, contributing to fill in the innovation gap that has limited the discovery of effective antibiotics in the last 20 years.

Nanopore-Based Measurements of Protein Size, Fluctuations, and Conformational Changes.

Proteins are structurally dynamic macromolecules, and it is challenging to quantify the conformational properties of their native state in solution. Nanopores can be efficient tools to study proteins in a solution environment. In this method, an electric field induces electrophoretic and/or electro-osmotic transport of protein molecules through a nanopore slightly larger than the protein molecule. High-bandwidth ion current measurement is used to detect the transit of each protein molecule. First, our measurements reveal a correlation between the mean current blockade amplitude and the radius of gyration for each protein. Next, we find a correlation between the shape of the current signal amplitude distributions and the protein fluctuation as obtained from molecular dynamics simulations. Further, the magnitude of the structural fluctuations, as probed by experiments and simulations, correlates with the ratio of α-helix to β-sheet content. We highlight the resolution of our measurements by resolving two states of calmodulin, a canonical protein that undergoes a conformational change in response to calcium binding.

Experimental investigations of ion current in liquid-fuelled gas turbine combustors

This work covers investigations of the static and dynamic behaviour of a confined, co-swirled and liquid-fuelled airblast injection system. The focus lies on the application of ion current sensors for the qualitative measurement of the heat release rate or for flame monitoring purposes in complex technical combustion processes. The ion current sensor is to operate in a feedback control loop in order to react on combustion dynamics in real time. The first part of the work analyses experimental data, which were obtained with different techniques, e.g. dynamic pressure, chemiluminescence, fine-wire thermocouples and ion current. The results show that the thermo-acoustic instability and the precessing vortex core generate an interaction mode. The frequency of this interaction mode is the difference of the other two modes. This has not yet been observed for partially premixed and liquid-fuelled injection systems before and also was not detected by the chemiluminescence of the flame. The ion current measurement technique is able to detect the helical mode of the precessing vortex core as well as the interaction frequency, leading to the conclusion that the chemical reactions are influenced by this helical structure. Contour maps of the frequencies reveal this influence in the outer shear layer. The second part of the study focused on the ion current probe as a method to predict static combustion instabilities, such as lean blowout. According to the results, the ion current is a fast responding method to detect lean blowout, provided that the detector is mounted at a suitable position. Measurements at different positions in the flame were compared with phase-locked chemiluminescence measurements. Precursors in the ion current signal for lean-blowout prediction were found using a statistical approach, which is based on ion peak distance. The precursor events allow for the use of this approach with a feedback control loop in future applications.

(Invited) Simultaneous Ionic Current and Potential Detection of Biomolecules and Nanoparticles By a Multifunctional Nanopipette

Nanopore sensing based technologies have made significant progress for biomedical applications. In recent years, multimode sensing by multifunctional nanopores shows the potential to greatly improve the sensitivity and selectivity of traditional resistive-pulse sensing method. In this talk, we will show that two label-free electric sensing modes can work cooperatively to detect biological entities, such as biomolecules and biological nanoparticles, in ionic solution by a multifunctional nanopipette. The multifunctional nanopipettes containing both nanopore and nanoelectrode (NE) at the tip are fabricated quickly and cheaply. We will discuss the detection mechanism and demonstrate that the ionic current and local electrical potential changes can be detected simultaneously during the translocation of individual entities. With the combination of ionic current and potential measurements, we are able to detect not only the translocation of single entity but also the motions of entities outside the nanopore entrance, revealing rich information of biological entities.

Control of the Combustion Process Parameters in ICE by the Ion Current Signals

This paper is devoted to control of the combustion processes in the internal combustion engines (ICE). It presents a method for estimation of the position of the peak pressure, the maximal pressure of the cycle and the air-fuel ratio based on the analysis of the integral characteristics of the ion current signals. The advantages of the proposed method are stability of the final result of the calculation to the influence from the cycle-to-cycle combustion parameter variations, stability to the changes in the parameters of the measuring probe during a long operation time and absence of restrictions on the wave form of the ion current. It also presents the results of the experimental testing of the developed method and its comparison with the conventional methods of analysis of the ion current and control of the combustion process parameters. The authors compared the efficiency and accuracy of the method using a combustion pressure sensor, based on the piezoelectric ceramics; the methods of the ion current analysis, based on detection of the thermal-ionization ion current peak; the method of the ion current analysis based on the integral characteristic of the ion current signal. The results of the experimental studies indicate a possibility of the use of the integral characteristic of the ion current signal for estimation of the combustion process parameters with the accuracy of the combustion pressure sensor based on the piezoelectric ceramics. The ion current measurement technology is cheaper than the piezoelectric ceramic sensors and can be widely applied in the serial engines.

Mechanism of functional interaction between potassium channel Kv1.3 and sodium channel NavBeta1 subunit

The voltage-gated potassium channel subfamily A member 3 (Kv1.3) dominantly expresses on T cells and neurons. Recently, the interaction between Kv1.3 and NavBeta1 subunits has been explored through ionic current measurements, but the molecular mechanism has not been elucidated yet. We explored the functional interaction between Kv1.3 and NavBeta1 through gating current measurements using the Cut-open Oocyte Voltage Clamp (COVC) technique. We showed that the N-terminal 1–52 sequence of hKv1.3 disrupts the channel expression on the Xenopus oocyte membrane, suggesting a potential role as regulator of hKv1.3 expression in neurons and lymphocytes. Our gating currents measurements showed that NavBeta1 interacts with the voltage sensing domain (VSD) of Kv1.3 through W172 in the transmembrane segment and modifies the gating operation. The comparison between G-V and Q-V with/without NavBeta1 indicates that NavBeta1 may strengthen the coupling between hKv1.3-VSD movement and pore opening, inducing the modification of kinetics in ionic activation and deactivation.