Pore Charge Research Articles

Nanopore charge inversion and current-voltage curves in mixtures of asymmetric electrolytes

We consider the screening of the negative charges (carboxylic acid groups) fixed on the surface of a conical-shaped track-etched nanopore by divalent magnesium (Mg2+) and trivalent lanthanum (La3+). The experimental current (I)–voltage (V) curves and current rectification ratios allow discussing fundamental questions about the overcompensation of spatially-fixed charges by multivalent ions over nanoscale volumes. The effects of charge inversion or reversal on nanopore transport are discussed in mixtures of asymmetric electrolytes (LaCl3 and MgCl2 with KCl). In particular, pore charge inversion is demonstrated for La3+ as well as for mixtures of this trivalent ion at low concentrations with monovalent potassium (K+) and divalent Mg2+ ions at biologically relevant concentrations. It is found that small concentrations of multivalent ions can modulate the nanopore rectification and the transport of other majority ions in the solution. We study also the kinetics of the nanopore electrical recovery when the electrolyte solutions bathing the single-pore membrane are changed and show the hysteretic effects observed in the I–V curves. Finally, we describe the hysteresis observed in the I–V curves of CaCl2, MgCl2, and BaCl2 and mixtures. We also give a qualitative description of the effects of charge reversal on the pore rectification using the Nernst-Planck flux equations for multivalent ion mixtures.

Scaling Laws for Ionic Transport in Nanochannels: Bulk, Surface and Interfacial Effects

The usual description of ion transport in membrane channels is based on dual model describing the channel conductance as the addition of bulk and surface contributions. This vision constitutes an idealization that it is extremely useful for modelling purposes. However, there are no surface- and bulk-labelled counterions in real solutions, but only ions that due to thermal agitation continuously interchange their role. Furthermore, ion transport in confined geometries may differ significantly from that in bulk conditions. Besides direct electrostatic interactions between the permeating ions and pore charges, other phenomena like interfacial access resistance or entropic effects due to obstacles and irregularities of the boundaries may play a role. We investigate here the limitations of the abovementioned two-state model by assessing experimentally the scaling behavior of channel conductance (G) with salt concentration (c) in structurally different protein and proteolipidic pores. Previous studies in both nanochannels have suggested a power law dependence G ∼ cα, where α is an exponent that has been reported to attain a variety of values depending of the system and the concentration regime. We hypothesize here that scaling exponents found in a specific system arise from a particular interplay between bulk and surface effects, being the distinction between them so subtle that the two-state model faints. In the case of biological pores, we show also that the presence of interfacial effects could give rise to an apparent universal scaling that does not reflect the channel actual characteristics.

Facilitated polymer capture by charge inverted electroosmotic flow in voltage-driven polymer translocation.

The optimal functioning of nanopore-based biosensing tools necessitates rapid polymer capture from the ion reservoir. We identify an ionic correlation-induced transport mechanism that provides this condition without the chemical modification of the polymer or the pore surface. In the typical experimental configuration where a negatively charged silicon-based pore confines a 1 : 1 electrolyte solution, anionic polymer capture is limited by electrostatic polymer-membrane repulsion and the electroosmotic (EO) flow. Added multivalent cations suppress the electrostatic barrier and reverse the pore charge, inverting the direction of the EO flow that drags the polymer to the trans side. This inverted EO flow can be used to speed up polymer capture from the reservoir and to transport weakly or non-uniformly charged polymers that cannot be controlled by electrophoresis.

Pore Polarity and Charge Determine Differential Block of Kir1.1 and Kir7.1 Potassium Channels by Small-Molecule Inhibitor VU590.

VU590 was the first publicly disclosed, submicromolar-affinity (IC50 = 0.2 μM), small-molecule inhibitor of the inward rectifier potassium (Kir) channel and diuretic target, Kir1.1. VU590 also inhibits Kir7.1 (IC50 ∼ 8 μM), and has been used to reveal new roles for Kir7.1 in regulation of myometrial contractility and melanocortin signaling. Here, we employed molecular modeling, mutagenesis, and patch clamp electrophysiology to elucidate the molecular mechanisms underlying VU590 inhibition of Kir1.1 and Kir7.1. Block of both channels is voltage- and K+-dependent, suggesting the VU590 binding site is located within the pore. Mutagenesis analysis in Kir1.1 revealed that asparagine 171 (N171) is the only pore-lining residue required for high-affinity block, and that substituting negatively charged residues (N171D, N171E) at this position dramatically weakens block. In contrast, substituting a negatively charged residue at the equivalent position in Kir7.1 enhances block by VU590, suggesting the VU590 binding mode is different. Interestingly, mutations of threonine 153 (T153) in Kir7.1 that reduce constrained polarity at this site (T153C, T153V, T153S) make wild-type and binding-site mutants (E149Q, A150S) more sensitive to block by VU590. The Kir7.1-T153C mutation enhances block by the structurally unrelated inhibitor VU714 but not by a higher-affinity analog ML418, suggesting that the polar side chain of T153 creates a barrier to low-affinity ligands that interact with E149 and A150. Reverse mutations in Kir1.1 suggest that this mechanism is conserved in other Kir channels. This study reveals a previously unappreciated role of membrane pore polarity in determination of Kir channel inhibitor pharmacology.

Counter-ion transport number and membrane potential in working membrane systems

In this work we use the general space-charge (SC) theory for a combined transport model of fluid and ion through cylindrical nanopores to derive equations for the membrane potential and counter-ion transport numbers. We discuss this approach for ion exchange membranes assuming aqueous domains as interconnected network of cylindrical pores. The transport number calculations from the SC theory are compared with the corresponding ones from the uniform potential (UP) and Teorell-Meyer-Sievers (TMS) models in the case of both zero and non-zero concentration gradient across the membrane and with an applied current density. By using this approach we suggest the optimal conditions for performing membrane potential experiments (i.e. choice of electrolyte and concentration difference) depending on an easily accessible membrane property, namely the volumetric charge density. We also theoretically describe a novel dynamic method to determine in a single experiment the membrane potential and membrane conductivity. To exemplify the use of the dynamic method we report the calculations based on typical operating conditions of the reverse electrodialysis process. The numerical results are presented in terms of the electrical potential difference versus the average pore radius and charge density. The resulting map is a useful tool for a rational design of an effective membrane morphology for a specific electrochemical application.

Modulation of Asymmetric Flux in Heterotypic Gap Junctions by Pore Shape, Particle Size and Charge.

Gap junction channels play a vital role in intercellular communication by connecting cytoplasm of adjoined cells through arrays of channel-pores formed at the common membrane junction. Their structure and properties vary depending on the connexin isoform(s) involved in forming the full gap junction channel. Lack of information on the molecular structure of gap junction channels has limited the development of computational tools for single channel studies. Currently, we rely on cumbersome experimental techniques that have limited capabilities. We have earlier reported a simplified Brownian dynamics gap junction pore model and demonstrated that variations in pore shape at the single channel level can explain some of the differences in permeability of heterotypic channels observed in in vitro experiments. Based on this computational model, we designed simulations to study the influence of pore shape, particle size and charge in homotypic and heterotypic pores. We simulated dye diffusion under whole cell voltage clamping. Our simulation studies with pore shape variations revealed a pore shape with maximal flux asymmetry in a heterotypic pore. We identified pore shape profiles that match the in silico flux asymmetry results to the in vitro results of homotypic and heterotypic gap junction formed out of Cx43 and Cx45. Our simulation results indicate that the channel's pore-shape established flux asymmetry and that flux asymmetry is primarily regulated by the sizes of the conical and/or cylindrical mouths at each end of the pore. Within the set range of particle size and charge, flux asymmetry was found to be independent of particle size and directly proportional to charge magnitude. While particle charge was vital to creating flux asymmetry, charge magnitude only scaled the observed flux asymmetry. Our studies identified the key factors that help predict asymmetry. Finally, we suggest the role of such flux asymmetry in creating concentration imbalances of messenger molecules in cardiomyocytes. We also assess the potency of fibroblasts in aggravating such imbalances through Cx43-Cx45 heterotypic channels in fibrotic heart tissue.

When Salt-Rejecting Polymers Meet Protons: An Electrochemical Impedance Spectroscopy Investigation.

Polymeric membranes are widely used for salt removal, but mechanism of ion permeation is still insufficiently understood. Here we analyze ion transport in polymers relevant to desalination, dense aromatic polyamide Nomex and cellulose acetate (CA), using impedance spectroscopy, focusing on the effects of the salt type, concentration and pH. The results highlight the role of proton uptake in ion permeation. For Nomex the exceptionally high affinity to proton results in a power-low scaling of conductivity with salt concentrations with an unusual exponent 1/2. The results for CA suggest dominance of pore transport, with pore charge increasing with decreasing pH, which contradicts previous view of CA as a weakly acidic polymer and points to proton uptake as possible pore-charging mechanism. The observed effects may have far-reaching consequences in desalination, as even at neutral pH they may both enhance and suppress salt permeation and affect pH changes.

Effects of Membrane Tension on Snare-Mediated Single Fusion Pores

Fast and temporally controlled release is imperative for neurotransmitter release via evoked synaptic vesicle exocytosis. This process is driven by the interaction between vesicle v-SNAREs with target t-SNAREs on the plasma membrane. The initial, nanometer-sized connection between two fusing lipid bilayers is called the fusion pore. This structure only lasts for a short time before it either dilates completely (resulting in full fusion), or reseals again (kiss-and-run fusion). Fusion pore dynamics determine the amount and size of cargo released, as well as vesicle recycling. Thus they constitute a fundamental mechanism that controls the extent of postsynaptic neuronal activity. Due to its small size and transient nature, the fusion pore has been an elusive object to study. We have recently developed an assay using biochemically well-defined components to look at current flow through single SNARE-mediated fusion pores. The components of the assay include HeLa cells with “flipped” t-SNARE proteins expressed on the outside of the cell (t-cells), and nanodiscs with v-SNARE proteins embedded within them (v-discs). When we fill a patch pipette with v-discs and establish a cell-attached patch onto a t-cell, the v- and t-SNAREs interact, leading to the formation of a fusion pore connecting the cytosol to the pipette solution. Currents through these fusion pores report single fusion pore dynamics.Membrane tension has been shown to affect fusion pore dynamics in neuroendocrine cells. In our assay, we can quickly and accurately change the pressure in the patch pipette, thereby modulate membrane tension, using a high-speed pressure clamp. We will show how an increase in membrane tension causes extended pore lifetimes, as well as increased pore conductance and charge influx through the pore.

Relationships between pore size and charge transfer resistance of carbon aerogels for organic electric double-layer capacitor electrodes

Carbon aerogels are prepared from polycondensation of resorcinol and formaldehyde using sodium carbonate as a base catalyst while controlling the ratio of resorcinol to catalyst (R/C). Pore size of the carbon aerogel shows a tendency to increase with the R/C ratio because of the low addition reaction rate between two starting materials with a small amount of catalyst. In addition, the pore size of the carbon aerogels strongly affects their charge transfer resistance and their potential use as organic electric double-layer capacitor electrodes. Ionic resistance to facile mobility of electrolyte ions decreases with increasing pore size of the carbon aerogels, while their electronic resistance exhibited the opposite trend with respect to pore size. These compensation effects between ionic resistance and electronic resistance lead to a different optimum pore size of carbon aerogels for EDLC electrodes depending on the applied charge-discharge rate. A carbon aerogel with large pore size is favorable for use in a low-rate charge-discharge process, while one with small pore size have good performance in a high-rate charge-discharge process. Therefore, we suggest that designing an efficient mesoporous carbon material for an EDLC electrode should include significant consideration of its end use, especially the rate employed for the charge-discharge process.

Charge storage, electrocatalytic and sensing activities of nest-like nanostructured Co3O4

We synthesized nanostructured Co3O4 samples using anionic (SDS), cationic (CTAB) and nonionic (Triton X-100) surfactant molecules in hydrothermal conditions and subsequent calcination. This approach facilitates the synthesis of porous Co3O4 material with bundle-like-sheet, nest-like and flake-like morphologies with specific surface areas in the range of 50–77m2g−1. Among these materials, the nest-like nanostructured Co3O4 material has unique pore architecture, larger pore volume, low solution and charge transfer resistance, and found to be an active material for charge storage, electrocatalytic and sensing applications. The specific capacitance value of the nest-like Co3O4 is 404Fg−1 at a current density of 2Ag−1 with 80% specific capacitance retention. The electrocatalytic oxidation of methanol occurs at lower onset potential on this material with good electrochemical stability. It has good sensing ability for glucose with high sensitivity of 929μAcm−2mM−1, fast response time of ∼0.5s and detection limit as low as ∼1μM. These results show that the nest-like nanostructured Co3O4 material is a versatile candidate for various applications.

Fabrication, Characterization and Antimicrobial property of natural TTOLs/CS composite sponges

The essential oil liposomes, a kind of ecological friendly natural antibacterial agents, have good bactericidal effect. In the present study, tea tree oil liposomes (TTOLs) were prepared by the thin-membrane hydration method with sonication, and then were blended with chitosan (CS) to successfully fabricate novel TTOLs/CS composite sponges by freeze-dried method. Through the scanning electron microscope (SEM), fourier transform infrared spectroscopy (FTIR) and performance tests, it was found that the material had good water absorption, water retention and water vapor permeability due to the high porosity. Furthermore, the incorporation of TTOLs in the CS-based sponges significantly improved the microbicidel effect of the sponges against Staphylococcus aureus (S. aureus), Escherichia coli (E. coli) and Candida albicans (C. albicans). Killing log values of TTOLs/CS composite sponges against bacteria and fungi reached over 3. According to the microbial clearance test, propidium iodide (PI) fluorescence test and transmission electron microscope (TEM) observation, the results indicated on one hand that TTOLs/CS composite sponges adsorbed and intercepted microbial cells through the internal pore and surface charge, and on the other hand that they could destroy bacterial intercellular substance, disperse cell colony and damage the integrity of cell membrane, finally leading to the death of microbial cells. In summary, TTOLs/CS composite sponges had great potential to be used as antimicrobial materials in the field of food, cosmetics, medicine, biomedical and biochemical engineering.

Novel adsorptive ultrafiltration membranes derived from polyvinyltetrazole-co-polyacrylonitrile for Cu(II) ions removal

Novel adsorptive ultrafiltration membranes were manufactured from synthesized polyvinyltetrazole-co-polyacrylonitrile (PVT-co-PAN) by nonsolvent induced phase separation (NIPS). PVT-co-PAN with various degree of functionalization (DF) was synthesized via a [3+2] cycloaddition reaction at 60°C using a commercial PAN. PVT-co-PAN with varied DF was then explored to prepare adsorptive membranes. The membranes were characterized by surface zeta potential and static water contact angle measurements, scanning electron microscopy as well as atomic force microscopy (AFM) techniques. It was shown that PVT segments contributed to alter the pore size, charge and hydrophilic behavior of the membranes. The membranes became more negatively charged and hydrophilic after addition of PVT segments. The PVT segments in the membranes served as the major binding sites for adsorption of Cu(II) ions from aqueous solution. The maximum adsorption of Cu(II) ions by the membranes in static condition and in a continuous ultrafiltration of 10ppm solution was attained at pH=5. The adsorption data suggest that the Freundlich isotherm model describes well Cu(II) ions adsorption on the membranes from aqueous solution. The adsorption capacity obtained from the Freundlich isotherm model was 44.3mgg−1; this value is higher than other membrane adsorption data reported in the literature. Overall, the membranes fabricated from PVT-co-PAN are attractive for efficient removal of heavy metal ions under the optimized conditions.

Tailoring Membrane Nanostructure and Charge Density for High Electrokinetic Energy Conversion Efficiency.

The electrokinetic energy conversion (EKEC) of hydraulic work directly into electrical energy has been investigated in charged polymeric membranes with different pore charge densities and characteristic diameters of the nanoporous network. The membranes were synthesized from blends of nitrocellulose and sulfonated polystyrene (SPS) and were comprehensively characterized with respect to structure, composition, and transport properties. It is shown that the SPS can be used as a sacrificial pore generation medium to tune the pore size and membrane porosity, which in turn highly affects the transport properties of the membranes. Furthermore, it is shown that very high EKEC efficiencies (>35%) are encountered in a rather narrow window of the properties of the nanoporous membrane network, that is, with pore diameters of ca. 10 nm and pore charge densities of 4.6 × 10(2) to 1.5 × 10(3) mol SO3(-) m(-3) for dilute solutions (0.03 M LiCl). The high absolute value of the efficiency combined with the determination of the optimal membrane morphology makes membrane-based EKEC devices a step closer to practical applications and high-performance membrane design less empirical.

Molecular separation by poly (N-vinyl imidazole) gel-filled membranes

Here we demonstrate the tunable molecular selectivities and corresponding mechanisms of poly(N-vinyl imidazole) (PVI) gel-filled membranes prepared by UV initiated graft polymerization of N-vinyl imidazole from ultraporous polysulfone (PSf) substrate. The separation performance was investigated in detail by extensive filtration tests with various organic molecules in water of varying pH and also in salt or ethanol aqueous solution. The variation of membrane water permeability and solute rejection with the feed pH was attributed to the sensitive pore size and charge characteristics of the membranes upon reversible protonation of the confined PVI gel. The separation results discerned three main mechanisms, i.e. size exclusion (sieving), electrostatic repulsion (Donnan exclusion) and adsorption. The rejection of the neutral molecules and the large dye molecules was mostly ascribed to size exclusion, while that of the small charged solutes was dominated by the electrostatic interactions including repulsion for cationic ones and attraction for anionic ones (causing specific adsorption). Typically, at pH 3 the protonated membranes rejected ~90% of the small cationic Vitamin B1 (VB1) and 100% of the anionic Sunset Yellow (SY) before adsorption saturation, in contrast to less than ~20% rejection of the neutral Vitamin B2 (VB2) having similar molecule size. In the high concentration salt or ethanol solution the membranes still maintained molecular sieving capability. Most notably, the separations of solute mixtures were indeed achieved, depending on the membrane parameters, the solute characteristics as well as the filtration conditions. The PVI gel-filled membranes developed in this work are promising in some specified or environment-sensitive molecular level separations.

Severe acute respiratory syndrome coronavirus E protein transports calcium ions and activates the NLRP3 inflammasome

Severe acute respiratory syndrome coronavirus (SARS-CoV) envelope (E) protein is a viroporin involved in virulence. E protein ion channel (IC) activity is specifically correlated with enhanced pulmonary damage, edema accumulation and death. IL-1β driven proinflammation is associated with those pathological signatures, however its link to IC activity remains unknown. In this report, we demonstrate that SARS-CoV E protein forms protein–lipid channels in ERGIC/Golgi membranes that are permeable to calcium ions, a highly relevant feature never reported before. Calcium ions together with pH modulated E protein pore charge and selectivity. Interestingly, E protein IC activity boosted the activation of the NLRP3 inflammasome, leading to IL-1β overproduction. Calcium transport through the E protein IC was the main trigger of this process. These findings strikingly link SARS-CoV E protein IC induced ionic disturbances at the cell level to immunopathological consequences and disease worsening in the infected organism.

Long-term corrosion behavior of HVOF sprayed FeCrSiBMn amorphous/nanocrystalline coating

A FeCrSiBMn amorphous/nanocrystalline coating with 700 μm in thickness and 0.65% in porosity, was prepared by high velocity oxygen fuel (HVOF) spraying process. The long-term corrosion behavior of the FeCrSiBMn coating was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests in a 3.5% NaCl solution with a hard chromium coating as a reference. The FeCrSiBMn coating exhibited higher corrosion potential and lower corrosion current density than the hard chromium coating. The pore resistance (Rp) and charge transfer resistance (Rct) of FeCrSiBMn coating were higher than those of the hard chromium coating. In addition, after immersion in the NaCl solution for 28 d, only small pores in the FeCrSiBMn coating were observed. All the results indicated that the FeCrSiBMn coating held superior corrosion resistance to the hard chromium coating. This could be attributed to the dense structure, low porosity and amorphous/nanocrystalline phases of the FeCrSiBMn coating.

Simulation of Pore Width and Pore Charge Effects on Selectivities of CO2 vs. H2 from a Syngas-like Mixture in Carbon Mesopores

Classical molecular dynamics simulations were performed to study the effect of pore width and surface charge in carbon mesoporous materials on adsorption and diffusion selectivities of CO2/H2 in a syngas-like mixture (mole fraction of CO2 = 0.30). The pore width of the graphite slit varied from 2.5 to 5.0 nm, while the temperature varied from 300K to 400 K. Both selectivities were relatively insensitive to the pore width. Metal contamination, mimicked with localized charges within an electroneutral pore surface, was found to increase the adsorption selectivity ratio for CO2 vs H2 and decrease the diffusion selectivity for CO2 vs H2. Such surface charges interact with CO2 molecules only and can enhance the separation of H2 from the syngas mixture, retaining CO2. Rising temperature will enhance diffusion selectivity, however reduce the adsorption selectivity of CO2. The results imply that surface charges can be used to favor the enrichment process of CO2.

Passive and Iontophoretic Transport of Fluorides across Enamel In Vitro

Passive and iontophoretic transport of fluoride from three fluoride sources, NaF, sodium monofluorophosphate (MFP), and SnF2 solutions, across bovine enamel was investigated to (1) determine the characteristics of the intrinsic barrier of enamel for ion transport, (2) examine the feasibility of iontophoretically enhanced transport of fluoride across enamel, and (3) identify the transport mechanisms involved in enamel iontophoresis. Conductivity experiments were performed with bovine enamel specimens in side-by-side diffusion cells to evaluate the electrical and barrier properties of the enamel with electrolytes of different ion sizes and under different ion concentrations and pH conditions in vitro. Transport experiments of the enamel were performed in the diffusion cells with the NaF, MFP, and SnF2 solutions. The conductivity results showed that the enamel specimens behaved as a neutral membrane or that of low pore charge density. Cathodal iontophoresis significantly enhanced the delivery of fluoride ions across the enamel from the solutions over passive transport, consistent with Nernst-Planck theory and the direct field effect (i.e., electrophoresis) as the dominant flux-enhancing mechanism. The enamel demonstrated significant transport hindrance for the ions, and the effective pore radii of the transport pathways in the enamel were found to be approximately 0.7-0.9 nm.

Nanoparticle-induced rectification in a single cylindrical nanopore: Net currents from zero time-average potentials

Rectification in nanopores is usually achieved by a fixed asymmetry in the pore geometry and charge distribution. We show here that nanoparticle blocking of a cylindrical pore induces rectifying properties that can support significant net currents with zero time-average potentials. To describe experimentally this effect, the steady-state current-voltage curves of a single nanopore are obtained for different charge states and relative sizes of the pore and the charged nanoparticles, which are present only on one side. The rectification phenomena observed can find applications in the area of nanofluidics and involves physical concepts that are also characteristic of the blocking of protein ion channels by ionic drugs.

BARRIER PROPERTY DETERMINATION AND LIFETIME PREDICTION BY ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY OF A HIGH PERFORMANCE ORGANIC COATING

The anticorrosion performance of an Epoxy-Mastic organic coating was evaluated during continuous immersion in saline solution using electrochemical impedance spectroscopy (EIS). The typical parameters of pore resistance and charge transfer resistance were determined employing an equivalent electric circuit. Constant phase elements (CPE) were used in order to determine fraction of water absorbed, mass diffusion, solubility and the swelling coefficients, as well as to predict the failure times of the coating. The results found by EIS measurements match very well with the high resistance to deterioration exhibited by the coating. It was also found that the excellent protection performance of the coating was mainly due to low water solubility and low permeability.