Effect Of Surface Potential Research Articles

Chiral Electrokinetic Phenomena in Single Nanopores

AbstractThe arrangement of solvent molecules and ions at solid–liquid interfaces determines electrochemical properties that are important in separations platforms, sensing technologies, and energy‐storage systems. Here we show that single glass and polymer pores in contact with propylene carbonate (PC) solutions of LiClO4 exhibit an effective surface potential that is modulated by the enantiomeric excess of the solvent. In particular, electrochemical and electrokinetic measurements of ionic transport through glass pipettes and polymer pores reveal that the effective surface potential is significantly lower in solutions prepared using enantiomerically pure PC than in solutions prepared using racemic PC. Both pore systems became positively charged in all racemic solutions examined in the range of LiClO4 concentrations between 1 mM and 100 mM, whereas solutions in (R)‐(+)‐PC induced a positive surface potential only at concentrations above ~5 mM. The effective surface potential is quantified through asymmetry in current–voltage curves and zeta‐potential measurements. Vibrational sum‐frequency‐generation experiments on LiClO4 solutions in racemic and enantiomerically pure PC indicate that the surface lipid‐bilayer‐like region in the former is more strongly organized than in the latter, dictating the favorable positions for lithium and perchlorate ions in each case. The more ordered molecular packing in the racemic liquid leads to accumulation of lithium ions on the outside of the bilayer, creating a higher effective positive charge. Our results highlight the extreme sensitivity of the interfacial potential on molecular organization of the solvent, and the relatively unexplored role that chirality can play in electrokinetic phenomena.

Static vector solitons in a topological mechanical lattice

Topological solitons, renowned for their stability and particle-like collision behaviors, have sparked interest in developing macroscopic-scale information processing devices. However, the exploration of interactions between multiple topological solitons in mechanical systems remains elusive. In this study, we construct a topological mechanical lattice supporting static vector solitons that represent quantized degrees of freedom and can freely propagate across the system. Drawing inspiration from coupled double atomic chains with sublattice symmetry breaking, we design a mechanical analogue featuring topologically protected boundary modes and induce independent modes to finite motions along branched motion pathways. Through a continuum theory, we describe the evolution of boundary modes with vector solitons composed of superposed kink solutions, identifying them as minimum energy pathways on the rugged effective potential surface with multiple degenerate ground states. Our results reveal the connection between transformable topological lattices and multistable systems, providing insight into nonlinear topological mechanics.

Superior device characteristics of needle-contact Ge Schottky barrier diodes for low-power applications

We measured and compared the I–V characteristics of needle- and junction-contact Ge Schottky barrier diodes (SBDs). The threshold voltage (V th) of I–V characteristics in needle-contact SBDs was revealed to be lower than that of junction-contact SBDs, which resulted in higher current in the low-voltage region. This indicates that needle-contact SBDs are more suitable for low-power applications. The lower V th in needle-contact SBDs indicates the lower Schottky barrier height. We proposed a band diagram of needle-contact SBDs, where the lower Schottky barrier height is assumed considering the effect of surface potential. We confirmed the validity of the model by fitting analysis.

Effective electrostatic potential surface and aromatic affinity as quantitative guide for deep extraction denitrification from aromatic-rich oils

High polar and reactive nitrogen-containing compounds greatly hinder the application of slurry oil (SO) in carbon-based materials. Compared with the hydrodenitrification process, the extraction process avoids aromatic saturation. However, pure experiments could not effectively screen extractants because of the structural similarity of polycyclic aromatic hydrocarbon (PAH) and nitrogen-containing PAH (NC-PAH). This work focused on searching for deep eutectic solvents (DESs) through sequential theoretical screening methods. The contradiction between denitrification efficiency (DN) and PAH loss (PL) could be weakened. Firstly, column chromatography, GC–MS, etc. were used to determine the molecular structure. The σ-profile, which performs the hydrogen bond acceptors (HBAs) property of molecules, was employed to crudely screen HBAs. Then, the interaction mechanism was obtained by multiple wave function analyses. An index named effective electrostatic potential (ESP) surface area, coined EES, was defined as the area of the region that apparently affected the interaction to predict DN. Additionally, the interaction energy (IE) of the cation of HBAs and non-basic PAH (nNC-PAH) was used to assess aromatic affinity and PL. The IE of HBAs and nNC-PAH linearly increased with EES, R2 = 0.9558. Meanwhile, DN was positively and linearly correlated with IE. Moreover, PL decreased slightly, then sharply, with the increase in aromatic affinity. The DN could reach about 65.1%, and the PL was limited to 3.8% after searching for high DN by EES and excluding high PL by aromatic affinity. These denitrification performances are crucially beneficial to the further preparation of carbon materials.

Control of intrinsic polarity for work function modulation of polyvinylidene fluoride crystalline phases

Ferroelectric polymers with high flexibility and inherent piezo- and pyro-electric properties have gained tremendous importance for next-generation wearable electronics. In this context, we investigate the intrinsic polarity mediated work function modulation in α-, γ-, and β-crystalline phases of a ferroelectric polymer, namely, polyvinylidene fluoride. A wide range of surface potentials (i.e., −5 to −70 V) were observed depending upon the crystalline polymorph and their surface morphologies. For example, upon nucleation of electroactive γ- and β-phases, a reduction in spherulite size is observed in comparison to its α-counterpart. Ultraviolet photoelectron spectroscopy was employed to realize the effect of surface potential on the valence bands spectrum. In particular, the work function of the non-electroactive α-phase (φα ∼ 5.09 eV) significantly increased when it is converted into the electroactive γ (φγ ∼ 5.99 eV) and β (φβ ∼ 7.39 eV) phases. The advantage of surface potential variation is shown by synergistic charge generation as a result of contact electrification of single active material-based polar interfaces with different work functions.

Investigation of electrostatically tunable adhesion and instability of flying head slider

The interfacial adhesion between microstructures is inevitable in a micro-electro-mechanical system (e.g., hard disk drive (HDD)), which may lead to complicated microtribodynamics problems. This research has investigated the effect of surface potential on the interfacial adhesion and microtribodynamics of the head–disk interface (HDI) in an HDD. A dynamic continuum surface force model, where the electrowetting is considered, is proposed to evaluate the interfacial interaction, and then employed into a two-degree-of-freedom (2DOF) model to theoretically analyze the potential influence mechanism on the microtribodynamics. The results confirm that the elimination of potential can effectively repress the adhesion retention, which is further proved by the measured slider response with a laser Doppler vibrometer (LDV). Moreover, the effect of the potential on the adhesion-induced instability is also analyzed through the phase portrait. It tells that the critical stable flying height can be lowered with the elimination of potential.

Effect of surface potential pinning on strain behavior of AlGaN/GaN device structures

Understanding the varied strain effects in AlGaN/GaN devices is crucial for realizing optimized flexible electronics systems and strain sensors. Here, we report on the effects of surface potential pinning, altered by the deposition of device-relevant SiNx passivation and Ni gate layers, on the strain-dependent carrier density, ns, of AlGaN/GaN two-dimensional electron gas structures. Flexible van der Pauw samples were made by separating AlGaN/GaN layers from the sapphire growth substrate using a two-dimensional boron nitride van der Waals release layer and transferring them to flexible substrates. For bare surface samples, we observed relatively large decreases in ns with tensile strain (Δns of −2 × 1011 cm−2 at 0.1% uniaxial strain), indicating an unpinned AlGaN surface potential. For the SiNx and Ni covered samples, the ns-strain trends were nearly flat, indicating a more pinned surface potential. Additionally, sub-bandgap 400 nm light is shown to effectively pin the surface potential as evidenced by flattening the ns-strain trend, the mechanism of which we explain in terms of the persistent photoconductivity effect. These observations could have important implications in tuning strain sensors and minimizing device variability in flexible electronics.

Update to the Newly Developed Expression for the Stability Ratio of Colloidal Dispersions.

The colloidal stability, one of the basic and important properties of a colloidal dispersion, is commonly evaluated in terms of the stability ratio. In this study, a recently developed expression for the stability ratio is updated, by reformulating the fraction of successful collisions leading to secondary minimum coagulation. The updated formula reinterprets the statistical meaning of the fraction of successful collisions leading to primary or secondary minimum coagulation, ensuring that the total fraction of successful collisions is always less than or equals to 1. It was shown to be superior to the available expressions in accounting for the contribution of the primary and secondary minimum coagulations on the stability ratio. It can well interpret the stability of colloidal dispersions of spherical particles; moreover, it is of great potential to be applied to colloidal dispersions of plate-like particles. In addition, this formula is found to be consistent with the concept of the critical coagulation concentration and well interpret the effects of particle size, counterion valence, surface potential, and Hamaker constant on the colloidal stability.

PLLA scaffolds with controlled surface potential and piezoelectricity for enhancing cell adhesion in tissue engineering

The effect of the surface potential of biomaterials on cell attachment and development in regenerative medicine is still an unexplored area driving many regeneration processes, especially in piezoelectric bone tissue. Within this study, we electrospun poly(l-lactide) scaffolds constructed of fibers with either higher (−600 mV) or lower (−300 mV) surface potential, which is controlled by applied voltage polarity during their production. Interestingly, this way, the piezoelectric performance of PLLA fibers can be enhanced. The direct measurement of the PLLA fiber surface potential using Kelvin probe force microscopy (KPFM) showed a good correlation with the zeta potential analysis. The piezoelectricity of PLLA fibers was verified with piezoresponse force microscopy (PFM), indicating that it can be enhanced by applying the positive voltage polarity to the nozzle during electrospinning. Importantly, the cell adhesion assay showed a significant effect of the higher surface potential of PLLA fibers on osteoblasts behavior and creating a favorable bioelectric microenvironment. We observed enhanced initial adhesion of cells in the first 5 h with no additional effect of surface potential on proliferation, morphology, or collagen production. It was demonstrated that electrospun PLLA fibers with tunable surface potential and piezoelectricity are excellent for constructing bone tissue engineering scaffolds.

Gating ion and fluid transport with chiral solvent.

The development of modern membranes for ionic separations and energy-storage devices such as supercapacitors depends on the description of ions at solid interfaces, as is often provided by the electrical double layer (EDL) model. The classical EDL model ignores, however, important factors such as possible spatial organization of solvent at the interface and the influence of the solvent on the spatial dependence of the electrochemical potential; these effects in turn govern electrokinetic phenomena. Here we provide a molecular-level understanding of how solvent structure can dictate ionic distributions at interfaces using a model system of a polar, aprotic solvent, propylene carbonate, in its enantiomerically pure and racemic forms, at a silica interface. We link the interfacial structure to the tuning of ionic and fluid transport by the chirality of the solvent and the salt concentration. The results of nonlinear spectroscopic experiments and electrochemical measurements suggest that the solvent exhibits lipid-bilayer-like interfacial organization, with a structure that is dependent on the solvent chirality. The racemic form creates highly ordered layered structure that dictates local ionic concentrations, such that the effective surface potential becomes positive in a wide range of electrolyte concentrations. The enantiomerically pure form exhibits weaker ordering at the silica surface, which leads to a lower effective surface charge induced by ions partitioning into the layered structure. The surface charge in silicon nitride and polymer pores is probed through the direction of electroosmosis that the surface charges induce. Our findings add a new dimension to the nascent field of chiral electrochemistry, and emphasize the importance of including solvent molecules in descriptions of solid-liquid interfaces.

Polarized P(VDF-TrFE) film promotes skin wound healing through controllable surface potential

Surface potential of biomaterials is found to be important for wound healing. Here, poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)) films with different surface potentials and piezoelectric responses were prepared and explored for the effect of surface potential on wound healing. The crystalline state of P(VDF-TrFE) films were characterized with X-ray diffraction (XRD), differential scanning calorimetry (DSC) and Fourier-transformed infrared spectroscopy (FTIR), illustrated that the electric polarization will promote the crystallization of the β phase of P(VDF-TrFE), in which the content of β phase increased from 82.9 % to 86.8 % compared with the control. Then, Kelvin potential and piezoelectric coefficient d33 were to evaluate surface potential and polarization performance. Moreover, bovine serum albumin (BSA) adsorption and cell culture results showed that high surface potential can promote protein adsorption as well as fibroblast proliferation and macrophage polarization. Finally, in vivo experiments indicated that high voltage polarized P(VDF-TrFE) films can generate higher dynamic potential up to 2.3 V, and promoted wound healing from the phases of inflammation, proliferation and remodeling, the wound healing rate of which was 88.8 % ± 0.8 %, significantly higher than 79.1 % ± 2.5 % and 86.4 % ± 1.8 % of blank and control. In general, this work revealed that polarized P(VDF-TrFE) films can promote wound healing, shed light on designing wound healing materials with similar properties.

Surface-potential-modulated piezoresistive effect of core–shell 3C-SiC nanowires

The effect of surface potential on the carrier mobility and piezoresistance of core–shell silicon carbide nanowires (SiC NWs) was investigated to realize small and sensitive SiC-microelectromechanical systems sensors. The p-type cubic crystalline SiC (3C-SiC) NWs were synthesized via the vapor–liquid–solid method and coated with silicon dioxide (SiO2) or aluminum oxide (Al2O3) dielectric shells to form core–shell structured NWs with different surface potentials. Four-point bending devices (FBDs) with a field-effect transistor (FET) configuration integrating a single core–shell 3C-SiC NW as the FET channel were fabricated to apply an additional electric field and strain to the core–shell 3C-SiC NWs. The fixed oxide charge densities of the SiO2 and Al2O3 shells showed positive and negative values, respectively, which were equivalent to electric fields of the order of several hundred thousand volt per centimeter in absolute values. In the core–shell 3C-SiC NWs with originally low impurity concentrations, the electric field induced by the fixed oxide charge of the shells can determine not only the electrical conduction but also the charge carriers in the NWs. Bending tests using the FBDs showed that the piezoresistive effect of the SiO2-coated NW was almost the same as that of the as-grown 3C-SiC NW reported previously, regardless of the gate voltage, whereas that of the Al2O3-coated NW was considerably enhanced at negative gate voltages. The enhancement of the piezoresistive effect was attributed to the piezo-pinch effect, which was more pronounced in the NW, where the carrier density at the core–shell interface is enhanced by the electric field of the dielectric.

Electrostatic Correlation Induced Ion Condensation and Charge Inversion in Multivalent Electrolytes.

The study of the electrical double layer lies at the heart of colloidal and interfacial science. The standard mean-field Poisson-Boltzmann (PB) theory is incapable of modeling many phenomena originating from ion correlation. An important example is charge inversion or overcharging of electrical double layers in multivalent electrolyte solutions. Existing theories aiming to include correlations cannot capture the non-monotonic dependence of charge inversion on salt concentration because they have not systematically accounted for the inhomogeneous nature of correlations from surface to the bulk and the excluded volume effect of ions and solvent molecules. In this work, we modify the Gaussian renormalized fluctuation theory by including the excluded volume effect to study ion condensation and charge inversion. A boundary layer approach is developed to accurately model the giant difference in ion correlations between the condensed layer near the surface and the diffuse layer outside. The theory is used to study charge inversion in multivalent electrolytes and their mixtures. We predict a surface charge induced formation of a three-dimensional condensed layer, which is necessary but not sufficient for charge inversion. The value of the effective surface potential is found to depend non-monotonically on the bulk salt concentration. Our results also show a non-monotonic reduction in charge inversion in monovalent and multivalent electrolyte mixtures. Our work is the first to qualitatively reproduce experimental and simulation observations and explains the underlying physics.

A study involving PC-3 cancer cells and novel carbamate gemini surfactants: Is zeta potential the key to control adhesion to cells?

Liposome surface potential effect on cellular uptake and cytotoxicity is evaluated using liposomes, modified with cationic lipid DOTAP, a series of cationic gemini surfactants with two carbamate fragments, and an amphiphilic peptide SSRGD. The surfactants used are novel representatives of the gemini family with improved self-assembling activity coupled with potential biodegradable properties and displayed increasing antibacterial activity and cytotoxicity with the shortening of hydrophobic alkyl tails. The longest alkyl tail surfactant, 14-6-14(Et), was the most biocompatible of the series, which was chosen for liposome modification. Prepared liposomes of various compositions are characterized from morphological and physicochemical standpoints in order to optimize their biocompatibility and stability. The carbamate gemini surfactants were also twice as effective at providing positive charge to liposomes and less toxic compared to DOTAP. On their own, carbamate surfactants were able to increase cellular uptake of liposomes by 190%. The mixed composition of 14-6-14(Et) surfactant and SSRGD amphiphilic peptide was the most readily absorbed formulation among different tested neutral, cationic and RGD-modified liposomes. The comparison between the cellular uptake promotion is conducted as to what is the most selective and efficient approach to enhance lipid nanoparticle uptake by cancerous cells.

Effects of surface potentials on Goos-Hänchen and Imbert-Fedorov shifts in Weyl semimetals

Weyl semimetals exhibit a myriad of exotic transport responses, among which, the Goos-H\"anchen (GH) and Imbert-Fedorov (IF) effects have recently garnered substantial attention. Besides the usual parametric dependence inherited from the underlying Hamiltonian to describe a Weyl system, the IF shift particularly carries a topological identity---it depends on the chirality of the Weyl cones. Observing such signatures following the trail of theoretical predictions applied to clean systems can be severely obfuscated by surface potentials induced by localized impurities that are naturally present in real materials hosting the semimetallic phase. Classifying these potentials, we study their effects on GH and IF shifts to provide useful guidance to experiments that are tuned to the objective of characterizing Weyl semimetals and leveraging them to provide the basis for future technological advances. A transfer matrix-based approach is proposed to study the profile of Weyl wave functions scattering from the impurity potentials. As we unfold, the presence of such potentials can lead to several remarkable effects such as the complete nullification of the IF shift and valley inversion.

Spreading Equilibria Under Mildly Singular Potentials: Pancakes Versus Droplets

We study global minimizers of a functional modeling the free energy of thin liquid layers over a solid substrate under the combined effect of surface, gravitational, and intermolecular potentials. When the latter ones have a mild repulsive singularity at short ranges, global minimizers are compactly supported and display a microscopic contact angle of pi /2. Depending on the form of the potential, the macroscopic shape can either be droplet-like or pancake-like, with a transition profile between the two at zero spreading coefficient for purely repulsive potentials. These results generalize, complete, and give mathematical rigor to de Gennes’ formal discussion of spreading equilibria. Uniqueness and non-uniqueness phenomena are also discussed.

Low-Barrier Hydrogen Bond in Fujikurin A-D: A Computational Study.

The compounds Fujikurin A, B, and D, recently isolated from Fusarium fujikuroi, possess intramolecular low-barrier hydrogen bonds (LBHBs), which are hydrogen bonds with a very low-energy barrier for proton transfer. The isolated compounds have a hydrogen-bonded proton that appears to rapidly switch between two equilibrium states via a transition state (TS). To understand the characteristics of these intramolecular LBHBs in detail, we performed path integral molecular dynamics (PIMD) simulations, which can consider nuclear quantum effects (NQEs) under a finite temperature. The PIMD simulations predicted that the NQE completely washed out the energy barrier for the proton transfer reaction. Consequently, a single-well shape emerged in the results, along with the effective free-energy potential surface for the hydrogen-bonded proton distribution. Thus, we conclude that the hydrogen-bonded proton in Fujikurin does not in fact transfer between two equilibrium structures but widely delocalizes around the global minimum structure involving the TS region.

The electrical‐double layer revisited

AbstractThe electrical‐double layer (EDL) model is fundamental to our understanding of interactions in ionic solutions, and is widely used in chemical, biological, and technological contexts, particularly in the description of aqueous electrolyte solutions. However, recent experiments have raised questions regarding the validity of this model in polar, aprotic solvents; some observations, such as a surface potential that changes sign with increasing salt concentration, are not consistent with the EDL picture. We demonstrate in a model system, acetonitrile at a silica interface, that solvent organization dictates the ionic distributions. Ion‐transport measurements in nanopores, surface‐selective spectroscopy, and molecular dynamics simulations reveal that the distribution of ions in acetonitrile at a silica interface is determined by the lipid‐bilayer‐like organization that the interface imposes upon the liquid, which accounts for the change in sign of the potential. Our findings emphasize the importance of including solvent molecules and ions explicitly in descriptions of solid/liquid interfaces.Key Points In a range of acetonitrile‐based electrolyte solutions in contact with silica, the effective surface potential is negative at low salt concentrations and positive at high salt concentrations. The long‐range, lipid‐bilayer‐like organization of acetonitrile at this interface dictates the preferred locations for cations and anions. Due to this organizational effect, the classical electrical double‐layer modal is not applicable to the acetonitrile/silica system, as likely also does not hold for other systems involving polar, aprotic liquids.

Protonation of model ionizable sidechains in transmembrane protein domains

The ionization states of individual amino acid residues of membrane proteins in lipid membrane environment are difficult to decipher or assign directly. The effective pK(a) values of protein groups are determined by a complex interplay between local polarity, Coulomb interactions, and structural reorganizations. The analysis is further complicated by the dearth of information about gradients in polarity, electric potentials, and hydration at the protein-membrane interface. In this work we report on a spin-labeling EPR method for assessing effects of membrane surface potential, local environment at the protein-membrane interface, and water penetration along this interface on effective pK(a) of membrane-buried ionizable groups.

Zinc oxide nanoparticle interface moderation with tyrosine and tryptophan reverses the pro-amyloidogenic property of the particle

Zinc oxide nanoparticle with negative surface potential (ZnONP) enhances bovine insulin fibrillation. Here, we are exploring ZnONP with positive surface potential (ZnONPUnc) and surface functionalized with tyrosine and tryptophan amino acids to observe the effects of surface potential and surface functional groups on the fibrillation. ZnONPUnc, despite of inversed surface potential, enhances the insulin fibrillation with increase in the interface concentration at physiological pH. Whereas, the interface moderation with the amino acids mitigates the surface-mediated insulin fibrillation propensity. Additionally, the study indicates that the change in interfacial functional groups at ZnONPUnc significantly reverses the interface-mediated destabilization of insulin conformation. The functional groups from the amino acids, like CO, N–H and aromatic functional groups, are anticipated to further stabilize the insulin conformation by forming hydrogen bond and van der Waals interactions with the key amyloidogenic sequences of insulin, A13-A20 from A-chain and B9–B20 from B-chain. Hence, the altered interaction profile, with change in interfacial functional groups, mitigates the interface-mediated insulin fibrillation and the ZnONPUnc-/fibril-mediated cytotoxicity.