Local Surface Charge Research Articles

Nanoscale surface charge detection in epoxy resin materials using electrostatic force spectroscopy

Electrostatic force spectroscopy (EFS) operated in a conventional force gradient detection method allows determining local surface charges in epoxy samples. This is made possible through a detailed analysis of gradient versus DC voltage curves. The parabolic dependence of these curves is closely related to the charge density. Both maximum and origin-ordinate are key data from which it is possible to extract quantitative information on the detected charge. The study is based on the combined use of numerical and analytical simulations of the probe sample interaction. Excellent sensitivities to very low surface charge densities are reported.

Contributions of Coulombic and Hofmeister Effects to the Osmotic Activation of Escherichia coli Transporter ProP.

Osmosensing transporters mediate osmolyte accumulation to forestall cellular dehydration as the extracellular osmolality increases. ProP is a bacterial osmolyte-H(+) symporter, a major facilitator superfamily member, and a paradigm for osmosensing. ProP activity is a sigmoid function of the osmolality. It is determined by the osmolality, not the magnitude or direction of the osmotic shift, in cells and salt-loaded proteoliposomes. The activation threshold varies directly with the proportion of anionic phospholipid in cells and proteoliposomes. The osmosensory mechanism was probed by varying the salt composition and concentration outside and inside proteoliposomes. Data analysis was based on the hypothesis that the fraction of maximal transporter activity at a particular luminal salt concentration reflects the proportion of ProP molecules in an active conformation. ProP attained the same activity at the same osmolality when diverse, membrane-impermeant salts were added to the external medium. Contributions of Coulombic and/or Hofmeister salt effects to ProP activation were examined by varying the luminal salt cation (K(+) and Na(+)) and anion (chloride, phosphate, and sulfate) composition and then systematically increasing the luminal salt concentration by increasing the external osmolality. ProP activity increased with the sixth power of the univalent cation concentration, independent of the type of anion. This indicates that salt activation of ProP is a Coulombic, cation effect resulting from salt cation accumulation and not site-specific cation binding. Possible origins of this Coulombic effect include folding or assembly of anionic cytoplasmic ProP domains, an increase in local membrane surface charge density, and/or the juxtaposition of anionic protein and membrane surfaces during activation.

Surface Charge Visualization at Viable Living Cells.

Scanning ion conductance microscopy (SICM) is demonstrated to be a powerful technique for quantitative nanoscale surface charge mapping of living cells. Utilizing a bias modulated (BM) scheme, in which the potential between a quasi-reference counter electrode (QRCE) in an electrolyte-filled nanopipette and a QRCE in bulk solution is modulated, it is shown that both the cell topography and the surface charge present at cellular interfaces can be measured simultaneously at high spatial resolution with dynamic potential measurements. Surface charge is elucidated by probing the properties of the diffuse double layer (DDL) at the cellular interface, and the technique is sensitive at both low-ionic strength and under typical physiological (high-ionic strength) conditions. The combination of experiments that incorporate pixel-level self-referencing (calibration) with a robust theoretical model allows for the analysis of local surface charge variations across cellular interfaces, as demonstrated on two important living systems. First, charge mapping at Zea mays root hairs shows that there is a high negative surface charge at the tip of the cell. Second, it is shown that there are distinct surface charge distributions across the surface of human adipocyte cells, whose role is the storage and regulation of lipids in mammalian systems. These are new features, not previously recognized, and their implications for the functioning of these cells are highlighted.

Modeling the electrostatic contribution to the line tension between lipid membrane domains using Poisson–Boltzmann theory

The line tension, which characterizes the excess free energy per unit length of the boundary between different lipid membrane domains, is one of the factors that determines domain size and dynamics. Consequently, experimental methods and corresponding modeling studies related to the line tension continue to attract significant interest. Considering a planar binary lipid layer with two domains consisting of neutral and anionic lipids, one with a higher and one with a lower average surface charge density, we calculate the electrostatic contribution to the line tension at the domain boundary using mean-field electrostatics. The influence of lipid mobility in each phase is studied through solutions of the Poisson–Boltzmann equation for different sets of boundary conditions that include the limiting cases of fixing the local surface charge density or surface potential, and the intermediate case of allowing the lipids to migrate subject to a demixing entropy penalty. In addition to our numerical results, we derive simple analytic expressions for the electrostatic contribution to the line tension in the linearized Debye–Huckel limit. We find the electrostatic contribution to the line tension to be negative with magnitudes on the order of piconewton close to physiological conditions. Because this is comparable to experimentally reported values of the total line tension, we conclude that electrostatic interactions generally provide an important contribution to the total line tension between differently charged domains in lipid bilayers.

How Ion Condensation Occurs at a Charged Surface: A Molecular Dynamics Investigation of the Stern Layer for Water–Silica Interfaces

We investigate the Stern layer of charged silica–water interfaces by calculating the ion–surface interaction from molecular dynamics simulations. The McMillan–Mayer potentials of mean force between a charged oxygen site and a lithium or cesium cation have been calculated. Contact ion pairs (CIPs) are important for the adsorption and desorption of ions, especially for lithium. An activation energy appears, which can result in a large estimated relaxation time. In the case of lithium, time scales needed to bind or unbind ions to and from the surface are found to be very long (up to the order of seconds for some surfaces), which implies that molecular dynamics cannot always be fully equilibrated. This work provides a new image of the Stern layer: it is not a continuous layer but a set of Bjerrum pairs. As a matter of fact, quantitative (macroscopic) treatments of such systems with localized surface charges require a three-dimensional model, contrary to the more commonly used one- or two-dimensional theoretic...

Influence of Localized Surface Charge Distributions on the Dielectric Relaxation Spectra of Spherical Colloidal Particles in Aqueous Solution

The influence of localized charge distributions at the interfaces of spherical colloidal particles in aqueous suspension is discussed in the light of the effective medium approximation theory of heterogeneous systems. The approach is shown to give analytical results in the case of shelled particles with the presence of two distinct interfaces. The whole dielectric response of the system gives rise to different relaxation contributions, falling in different frequency regions and attributed to different polarization mechanisms, at a molecular level. In particular, the number and the strength of the dielectric relaxations associated with the presence of localized charges is discussed.

Modeling of Charging on Unconventional Surface Morphologies of PMMA Substrates During Ar Plasma Etching

The charging on unconventional, rough, surface morphologies of polymeric substrates is investigated. The case study is Ar plasma etching of PMMA surfaces with sinusoidal profiles resembling rough profiles. A modeling framework for the description of charging on plasma “wetted” two-dimensional (2d) surface morphologies is developed. It consists of models for the calculation of the ion and electron trajectories, the local surface charge density, the potential induced by the surface charge, and a surface etching model. The latter is devised by combining experimental measurements and calculations by TRIM code. Etching rate calculations with and without charging are performed; it is shown that charging affects the etching rate mainly due to the decrease of the ion energy. Calculations are performed for sinusoidal profiles of different amplitude (roughness); as the amplitude increases, the ion energy decreases and the angle of ion incidence increases contributing competitively to the etching rate.

Graphene–Inorganic Hybrids with Cobalt Oxide Polymorphs for Electrochemical Energy Systems and Electrocatalysis: Synthesis, Processing and Properties

We report on the synthesis and physical property characterization of graphene–inorganic ‘hybrid’ nanomaterials coupled with nano-/microscale transition metal oxide polymorphs namely, cobalt oxides, i.e. CoO [Co(II)] and Co3O4 [Co(II, III)]), for alternative energy storage and conversion devices. Their demand is owed to higher specific capacitance, wide operational potential window, stability through charge–discharge cycling, environmentally benignity, easily processability, reproducibility and manufacturability. To accomplish this, we strategically designed these hybrids by direct anchoring or physisorption of CoO and CO3O4 on two different variants of graphene: graphene oxide which is semiconducting, and its reduced form showing conducting behavior via mixing dispersions of the constituents under mild ultrasonication and drop-cast (or spray-cast) resulting in different combinations. This facile approach affords strong chemical/physical attachment and is expected to have coupling between the pseudocapacitive transition metal oxides and supercapacitive graphene showing enhanced surface activity/reactivity and reasonable areal density of tailored interfaces. We used a range of complementary tools to establish microscopic structure–property-function correlations including scanning electron microscopy combined with energy dispersive x-ray spectroscopy, atomic force microscopy, x-ray diffraction, transmission electron microscopy in conjunction with selected-area electron diffraction, and resonance Raman spectroscopy combined with elemental Raman mapping. They reveal surface morphology, local (lattice dynamical) and average structure and surface charge transfer/doping due to physically (or chemically) adsorbed cobalt oxide and highlight the surface structure and interfaces. This lays the groundwork to further investigate the electrochemical properties as high-performance supercapacitor cathodes, rechargeable secondary battery anodes and electrocatalytical platforms.

Decreasing the Limits of Detection and Analysis Time of Ion Current Rectification Biosensing Measurements via a Mechanically Applied Pressure Differential

Improving on the analytical capabilities of a measurement is a fundamental challenge with all assays, particularly decreasing the limit of detection while maintaining a practical associated analysis time. Of late, ion current rectification (ICR) biosensing measurements have received a great deal of attention as an analyte-specific, label-free assay. In ICR biosensing, a nanopore coated with an analyte specific binding molecule (e.g., an antibody, an aptamer, etc.) is used to detect a target analyte based on the ability of the target analyte to alter the ICR response of the nanopore upon it binding to the aperture interior. This binding changes the local surface charge and/or size of the nanopore aperture, thus altering its ICR response in a time dependent manner. Here, we report the ability to enhance the transport of a target analyte molecule to and through the aperture of an antibody modified glass nanopore membrane (AMGNM) with the application of a mechanically applied pressure differential. We demonstrate that there is an optimal pressure that balances the flux of the target analyte through the AMGNM aperture with its ability to be bound and detected. Applying the optimal pressure differential allows for picomolar concentrations of the cleaved form of synaptosomal-associated protein 25 (cSNAP-25) to be detected within the same analysis time as micromolar concentrations detected without the use of the pressure differential. The methodology presented here significantly expands the utility of ICR biosensing measurements for detecting low-abundance biomolecules by lowering the limit of detection and reducing the associated analysis time.

ChemInform Abstract: Ion Specific Effects: Decoupling Ion‐Ion and Ion‐Water Interactions

AbstractReview: 110 refs.

Local surface charge dissipation studied using force spectroscopy method of atomic force microscopy

We propose herein a method to study local surface charge dissipation in dielectric films using force spectroscopy technique of atomic force microscopy. By using a normalization procedure and considering an analytical expression of the tip-sample interaction force, we could estimate the characteristic time decay of the dissipation process. This approach is completely independent of the atomic force microscopy tip geometry and considerably reduces the amount of experimental data needed for the calculation compared with other techniques. The feasibility of the method was demonstrated in a freshly cleaved mica surface, in which the local charge dissipation after cleavage followed approximately a first-order exponential law with the characteristic time decay of approximately 7–8 min at 30% relative humidity (RH) and 2–3.5 min at 48% RH. Copyright © 2015 John Wiley & Sons, Ltd.

Spin initialization of a p-doped quantum dot coupled to a bowtie nanoantenna

The spin initialization of a hybrid system consisting of a p-doped semiconductor quantum dot coupled to a gold bowtie nanoantenna is analyzed. The quantum dot is described as a four-level atom-like system using the density matrix formalism. The two lower levels are Zeeman-split hole spin states and the two upper levels correspond to positively charged excitons with spin-up, spin-down hole pair and opposite spin electron. The gold bowtie nanoantenna is placed in close proximity to the quantum dot. A linearly polarized laser field drives two of the optical transitions of the quantum dot and produces localized surface charge oscillations in the nanoantenna which act back upon the quantum dot thus changing the effective field felt by it. The angular frequencies of those charge oscillations are very different along its two principal axes, resulting in an anisotropic modification of the spontaneous emission rates of the allowed optical transitions of the quantum dot. These changes are accounted for by using the Green tensor method, and result in a faster spin state initialization than that of the isolated quantum dot. We also show that the presence of the nanoantenna dramatically modifies the optical properties of the fluorescent photons, either in the spectral or in the time domain.

Presence of electrolyte promotes wetting and hydrophobic gating in nanopores with residual surface charges.

Hydrophobic nanopores provide a model system to study hydrophobic interactions at the nanoscale. Such nanopores could also function as a valve since they halt the transport of water and all dissolved species. It has recently been found that a hydrophobic pore can become wetted i.e. filled with condensed water or an aqueous solution of salt when a sufficiently high electric field is applied across the membrane. The wetting process is reversible thus when the voltage is lowered or switched off, the pore comes back to a closed state due to water evaporation in the pore. In this manuscript we present experimental studies on how the switching between conducting and non-conducting states can be regulated by the electrolyte concentration. Transport properties of single nanopores modified with alkyl chains of different lengths were recorded in salt concentrations between 10 mM and 1 M KCl. Nanopores modified with propyl chains exhibited gating in 10 mM KCl and were open for ionic transport for all voltages at higher salt concentrations. Nanopores modified with decyl chains did not conduct current in 10 mM and exhibited repeatable hydrophobic gating in 100 mM and 1 M KCl. The results are explained in the context of Maxwell stress in confined geometry with local surface charges, which change the shape of the water-vapor interface and promote wetting.

Extracting local surface charges and charge regulation behavior from atomic force microscopy measurements at heterogeneous solid-electrolyte interfaces.

We present a method to determine the local surface charge of solid-liquid interfaces from Atomic Force Microscopy (AFM) measurements that takes into account shifts of the adsorption/desorption equilibria of protons and ions as the cantilever tip approaches the sample. We recorded AFM force distance curves in dynamic mode with sharp tips on heterogeneous silica surfaces partially covered by gibbsite nano-particles immersed in an aqueous electrolyte with variable concentrations of dissolved NaCl and KCl at pH 5.8. Forces are analyzed in the framework of Derjaguin-Landau-Verwey-Overbeek (DLVO) theory in combination with a charge regulation boundary that describes adsorption and desorption reactions of protons and ions. A systematic method to extract the equilibrium constants of these reactions by simultaneous least-squared fitting to experimental data for various salt concentrations is developed and is shown to yield highly consistent results for silica-electrolyte interfaces. For gibbsite-electrolyte interfaces, the surface charge can be determined, yet, an unambiguous identification of the relevant surface speciation reactions is not possible, presumably due to a combination of intrinsic chemical complexity and heterogeneity of the nano-particle surfaces.

Practical aspects of Kelvin-probe force microscopy at solid/liquid interfaces in various liquid media

The distributions of surface charges or surface potentials on biological molecules and electrodes are directly related to various biological functions and ionic adsorptions, respectively. Electrostatic force microscopy and Kelvin-probe force microscopy (KFM) are useful scanning probe techniques that can map local surface charges and potentials. Here, we report the measurement and analysis of the electrostatic and capacitive forces on the cantilever tip induced by application of an alternating voltage in order to discuss the feasibility of measuring the surface charge or potential distribution at solid/liquid interfaces in various liquid media. The results presented here suggest that a nanometer-scale surface charge or potential measurement by the conventional voltage modulation techniques is only possible under ambient conditions and in a non-polar medium and is difficult in an aqueous solution. Practically, the electrostatic force versus dc voltage curve in water does not include the minimum, which is used for the surface potential compensation. This is because the cantilever oscillation induced by the electrostatic force acting on the tip apex is overwhelmed by the parasitic oscillation induced by the electrostatic force acting on the entire cantilever as well as the surface stress effect. We both experimentally and theoretically discuss the factors which cause difficulties in application of the voltage modulation techniques in the aqueous solutions and present some criteria for local surface charge and potential measurements by circumventing these problems.

Effects of discreteness of surface charges on the effective electrostatic interactions.

In the present work, we study effects of relative arrangement of discrete surface charges on electrostatic potential of mean force (EPMF) between two surfaces immersed in a primitive model electrolyte solution. To help in exploring and ascertaining the relevant mechanism, different combinations of values for electrostatic parameters (such as ion concentration, diameter, and valence, solution temperature and relative permittivity) are investigated by solving classical density functional theory in three dimensional space, in combination with different discrete surface charge configurations and surface charge homogeneity configuration as the effects may alter along with change of the surface charge arrangement. Our findings include: (i) if counter-ion is univalent, the EPMF of charge homogeneity surface is always more repulsive than those of charge heterogeneity surfaces whether the discrete surface charges are distributed on the two face-face surfaces commensurately or non-commensurately; (ii) in presence of high valence counter-ion well depth of like charge attraction between two charge homogeneity surfaces approximately takes an averaged value of those for various charge heterogeneity surfaces whether the discrete surface charges are distributed on the two face-face surfaces commensurately or non-commensurately; (iii) the EPMF becomes ever more attractive or less repulsive with a centralization of the discrete surface charges whether the counter-ion valence is univalent, divalent or trivalent; and (iv) the EPMF in the case of charge heterogeneity surfaces is influenced by bulk electrolyte concentration, salt ion diameter and valence, solution temperature, and relative permittivity in the same manner as in the case of charge homogeneity surfaces. All of these findings can be explained self-consistently by a recently proposed hydrogen-bonding style mechanism (for dealing with the EPMF between two charge homogeneity surfaces) reinforced by two new concepts: counter-ion effective adsorption spaces belonging to different localized surface charges and hydrogen bond directivity.

Tunable Fano resonance in hybrid graphene-metal gratings

Hybrid graphene-metal gratings with tunable Fano resonance are proposed and theoretically investigated in THz band. The grating contains alternately aligned metal and graphene stripes, which could be viewed as the superposition of two kinds of gratings with the same period. Due to different material properties, the resonance coupling between the metal and graphene parts forms typical Fano-type transmitting spectra. The related physical mechanism is studied by inspecting the induced dipole moment and local surface charge distributions at different wavelengths. Both of the resonance amplitude and frequency of the structure thus are adjustable by tuning graphene's Fermi energy and the grating's geometrical parameters. Furthermore, the Fano-type spectra are also quite sensitive to environmental indices, which supply another kind of tunability. All these features should have promising applications in tunable THz filters, switches, and modulators.

Dielectric response of shelled toroidal particles carrying localized surface charge distributions. The effect of concentric and confocal shells

Dielectric models of biological cells are generally based on spherical or ellipsoidal geometries, where the different adjoining dielectric media are arranged as distinct core and shells, representing the cytosol and the cell membrane. For ellipsoidal particles, this approach implies the assumption of confocal shells that, in turn, means a cell membrane of ill-defined thickness. A quantitative analysis of the influence of a non-uniform thickness of the cell membrane has been not considered so far.In the case of a toroidal particle, this problem can be conveniently addressed by considering the solution of the Laplace equation in two different coordinate systems, i.e., toroidal coordinates (confocal shells and hence non-uniform thickness of the shell membrane) and toroidal polar coordinate, (concentric shells and hence a uniform thickness of the shell membrane).In the present paper, we compare the dielectric spectra of a toroidal particle aqueous suspension obtained from the two above stated solutions of the Laplace equation and we furnish a first quantitative estimate of the differences arising from considering the presence of confocal or concentric shells.This approach offers a complete view of the influence of the membrane thickness on the whole dielectric spectrum of a biological particle suspension, at least as far as toroidal objects are concerned.

Size Dependent Surface Charge Properties of Silica Nanoparticles

Nanoparticle surface charge density plays an important role in many applications, such as drug delivery and cellular uptake. In this study, surface charge properties of silica nanoparticles with different sizes are studied using a multi-ion surface charge-regulation model. In contrast to most previous studies utilizing constant surface charge, protonation and deprotonation surface reactions are used to obtain the local surface charge, which depends on the particle size and electrolyte solution properties, including the salt concentration and pH. For a fixed particle size, the magnitude of the surface charge typically increases with an increase in pH or background salt concentration. For fixed background salt concentration and pH, the magnitude of surface charge decreases with an increase in the particle size and reaches a constant when the particle size exceeds a critical value. Size dependent surface charge is further characterized by the ratio of electrical double layer thickness to the particle diameter, and the surface charge varies significantly when this dimensionless ratio is above 0.2.

Hydrothermally formed functional niobium oxide doped tungsten nanorods

Nanorod forms of metal oxides are recognized as one of the most remarkable morphologies. Their structure and functionality have driven important advancements in a vast range of electronic devices and applications. In this work, we postulate a novel concept to explain how numerous localized surface states can be engineered into the bandgap of niobium oxide nanorods using tungsten. We discuss their contributions as local state surface charges for the modulation of a Schottky barrier height, the relative dielectric constant and their respective conduction mechanisms. Their effects on hydrogen gas molecule interaction mechanisms are also examined herein. We synthesized niobium tungsten oxide (Nb17W2O25) nanorods via a hydrothermal growth method and evaluated the Schottky barrier height, ideality factor, dielectric constant and trap energy level from the measured I–V versus temperature characteristics in the presence of air and hydrogen to show the validity of our postulations.