Ion Surface Charge Research Articles

Interactions between ions and water-DMSO mixed solvent

As a classical cryoprotectant, the cryoprotective mechanism of DMSO remains elusive. In this study, we explore the capability of DMSO to hinder the formation of salt hydrates in the water-DMSO-salt ternary system, given that, at low temperatures, salt hydrates are more prone to attach to surfaces than ice, potentially preventing mechanical damage to cell membranes. Using THz-TDS, we validate the ability of DMSO to suppress salt hydrate formation. Combining IR spectroscopy, we conclude that this inhibitory effect partly arises from the interaction between cations and the S=O groups in DMSO molecules. Semi-quantitative DR spectroscopy provides similar insights from the perspective of molecular reorientation dynamics. Taking into account the intensified ion-dipole interactions due to a higher ion surface charge density, we focus our discussion on MgCl2, while NaCl, more akin to biological processes, serves as a reference. Delving into the specific molecular mechanisms, we put forth conjectures regarding the direct interaction between cations and DMSO or the indirect influence of cations on DMSO through the modulation of hydrogen bonds in surrounding water molecules. These research findings contribute to a better understanding and mastery of cryoprotectants.

Synthetic beidellite clay as nanocarrier for delivery of antitumor oxindolimine-metal complexes

Studies on the immobilization of oxindolimine‑copper(II) or zinc(II) complexes [ML] in synthetic beidellite (BDL) clay were developed to obtain a suitable inorganic carrier capable of promoting the modified-release of metallopharmaceuticals. Previous investigations have shown that the studied metal complexes are promising antitumor agents, targeting DNA, mitochondria, and some proteins. They can bind to DNA, causing oxidative damage via formation of reactive oxygen species (ROS). In mitochondria they lead to a decrease in membrane potential, acting as decoupling agents, and therefore efficiently inducing apoptosis. Additionally, they inhibit human topoisomerase IB and cyclin dependent kinases, proteins involved in the cell cycle. BDL clays in the sodium form were synthesized under hydrothermal conditions and characterized by a set of physicochemical techniques while the BDL-[ML] hybrid materials were prepared by ion exchange method. The characterization of pristine clay and the obtained hybrids were performed by Infrared, Raman, electron paramagnetic resonance and energy dispersive X-ray spectroscopies, thermogravimetric analysis, scanning electron microscopy, X-ray powder diffraction, specific surface area, zeta potential and surface ionic charge measurements. The [ML] release assays under the same cell incubation conditions were performed monitoring metals by X-ray fluorescence. The BDL-[CuL] hybrid materials were stable and able to derail tumor HeLa cells, with corresponding IC50 values in the 0.11–0.41 mg mL−1 range. By contrast, the analogous hybrid samples of zinc(II) and the pristine BDL proved to be non-toxic facing the same cells. These results indicate a promising possibility of using synthetic beidellite as a carrier of such antitumor metal complexes.

Time-resolved terahertz–Raman spectroscopy reveals that cations and anions distinctly modify intermolecular interactions of water

The solvation of ions changes the physical, chemical and thermodynamic properties of water, and the microscopic origin of this behaviour is believed to be ion-induced perturbation of water’s hydrogen-bonding network. Here we provide microscopic insights into this process by monitoring the dissipation of energy in salt solutions using time-resolved terahertz–Raman spectroscopy. We resonantly drive the low-frequency rotational dynamics of water molecules using intense terahertz pulses and probe the Raman response of their intermolecular translational motions. We find that the intermolecular rotational-to-translational energy transfer is enhanced by highly charged cations and is drastically reduced by highly charged anions, scaling with the ion surface charge density and ion concentration. Our molecular dynamics simulations reveal that the water–water hydrogen-bond strength between the first and second solvation shells of cations increases, while it decreases around anions. The opposite effects of cations and anions on the intermolecular interactions of water resemble the effects of ions on the stabilization and denaturation of proteins.

Raman probing carbon & aqueous electrolytes interfaces and molecular dynamics simulations towards understanding electrochemical properties under polarization conditions in supercapacitors

Raman probing of carbon electrode and electrolyte under dynamic conditions is performed here using different aqueous electrolytes to elucidate the fundamental events occurring in electrochemical supercapacitor during charge–discharge processes. The areal capacitance ranges from 1.54 to 2.31 mF cm−2 and it is determined using different techniques. These findings indicate that the Helmholtz capacitance governs the overall charge-storage process instead of the space charge (quantum) capacitance commonly verified for HOPG electrodes in the range of ~ 3 to 7 µF cm−2. Molecular dynamics simulations are employed to elucidate the origin of the reversible Raman spectral changes during the charge–discharge processes. A correlation is verified between the reversible Raman shift and the surface excesses of the different ionic species. A theoretical framework is presented to relate the effect of the applied potential on the Raman shift and its correlation with the surface ionic charge. It is proposed that the Raman shift is governed by the interaction of solvated cations with graphite promoted by polarization conditions. It is the first time that a comparative study on different aqueous electrolyte pH and cation ion size has been performed tracking the Raman spectra change under dynamic polarization conditions and contrasting with comprehensive electrochemistry and dynamic molecular simulations studies. This study shines lights onto the charge-storage mechanism with evidence of Kohn anomaly reduction in the carbon electrode during the reversible adsorption/desorption and insertion/extraction of ionic species.

Interpretation of photocurrent transients at semiconductor electrodes: Effects of band-edge unpinning

The transient photocurrent response of semiconductor electrodes to chopped illumination often shows spikes and overshoots that are usually interpreted as evidence that surface recombination is occurring. In the case of the high intensities used for light-driven water splitting, the interpretation is less straightforward since the electron transfer reactions are so slow that the minority carrier concentration at or near the surface increases to high values that modify the potential drop across the Helmholtz layer in the electrolyte, leading to ‘band edge unpinning’. In addition, changes in chemical composition of the surface or local changes in pH may also alter the potential distribution across the semiconductor/electrolyte junction. A quantitative theory of band edge unpinning due to minority carrier build up is presented, and numerical calculations of transient photocurrent responses are compared with experimental examples for n-type Fe2O3 and p-type lithium-doped CuO electrodes. It is shown that the apparently high reaction orders (up to third order) with respect to hole concentration reported for hematite photoanodes can be explained as arising from an acceleration of hole transfer by the increased voltage drop across the Helmholtz layer associated with band edge unpinning. The limitations of the band edge unpinning model are discussed considering additional effects associated with modification of the potential distribution brought about by light-induced changes in surface composition, surface dipoles and surface ionic charge.

Polymer-Assisted In Situ Synthesis of Silver Nanoparticles with Epigallocatechin Gallate (EGCG) Impregnated Wound Patch Potentiate Controlled Inflammatory Responses for Brisk Wound Healing.

IntroductionAn ideal wound dressing material needs to be predisposed with desirable attributes like anti-infective effect, skin hydration balance, adequate porosity and elasticity, high mechanical strength, low wound surface adherence, and enhanced tissue regeneration capability. In this work, we have synthesized hydrogel-based wound patches having antibacterial silver nanoparticles and antioxidant epigallocatechin gallate (EGCG) and showed fast wound closure through their synergistic interaction without any inherent toxicity.Methods and resultsWound patches were synthesized from modified guar gum polymer and assessed to determine accelerated wound healing. The modified polymer beget chemical-free in-situ synthesis of monodispersed silver NPs (~12 nm), an antimicrobial agent, besides lending ionic surface charges. EGCG impregnated during ionotropic gelation process amplified the efficacy of wound patches that possess apt tensile strength, porosity, and swellability for absorbing wound exudates. Further, in vitro studies endorsed them as non-cytotoxic and the post agent effect following exposure to the patch showed an unbiased response to E coli K12 and B. subtilis. In vivo study using sub-cutaneous wounds in Wistar rats validated its accelerated healing properties when compared to a commercially available wound dressing material (skin graft; Neuskin-F®) through better wound contraction, promoted collagen deposition and enhanced vascularization of wound region by modulating growth factors and inflammatory cytokines.ConclusionSynthesized wound patches showed all the desired attributes of a clinically effective dressing material and the results were validated in various in vitro and in vivo assays.

Simplified Fabrication for Ion-Selective Optical Emulsion Sensor with Hydrophobic Solvatochromic Dye Transducer: A Cautionary Tale.

It has recently been reported that polystyrene microbeads may be modified to realize plasticizer-free ion-selective optical sensors (optodes) on the basis of solvatochromic dye transducers. We show here that the functionalized microbeads, individually isolated by flow cytometry, exhibit unexpectedly poor fluorescent properties and that the sensor response is instead attributed to the supernatant. A more thorough study reveals that such optical microemulsion sensors can be made operationally functional and chemically selective, seemingly in the absence of any solvent matrix or added surfactant. Instead, it is shown that residual THF used in the fabrication of the emulsified sensors may solubilize the sensing components and give a functional optode response. To evaluate this further, the number of sensing components was stepwise simplified to assess their need. Variation of residual THF levels has no effect on the ion optode response when plasticizer is present, in support of established results. Lipophilic solvatochromic dye transducers are also shown not to require an added surfactant as their nature already endows the emulsified sensors with a stabilizing ionic surface charge. The ionophores are shown to exhibit much larger stability constants in the surfactant-free formulations than surfactant-based ones (valionomycin, log β > 9.2 compared to 6.1; Na+-ionophore X, 6.7 vs 4.7), which is attributed to a less polar solvent environment for the ionophore. Potassium-, sodium-, and calcium-selective sensors were used as model systems in this study.

Elucidating Surface and Bulk Emission in 3D Hybrid Organic–Inorganic Lead Bromide Perovskites

AbstractThree dimensional (3D) hybrid organic–inorganic lead halide perovskites (HOIP) have emerged in recent years as promising materials for a wide variety of optoelectronic applications. However, the photoluminescence energies in bromide‐based HOIP have been reported to vary in the range from 2.16 to 2.35 eV. The occurrence of surface reconstructions due to uncompensated surface ionic charges may change the photo‐physical properties of the surface regions, but this has not been studied in detail. Herein, by performing angle‐dependent photoluminescence (PL) and spectroscopic ellipsometry of single‐crystal as well as polycrystalline HOIP crystals, the intrinsic excitonic emissions from the surface and bulk regions are clearly identified. It is verified that the high energy PL at 2.31 eV originates from a phase‐modified surface region. The large absorption coefficient of perovskite results in signal depletion of the lower energy PL at 2.16 eV, which originates from the bulk. High resolution synchrotron X‐ray diffraction reveals that air‐exposed HOIP crystals form a multilayer structure consisting of PbBr2, and an interfacial layer of orthorhombic phase, while the bulk crystal remains cubic phase. This study provides the unambiguous identification of a phase‐modified surface region with a larger band gap than the bulk and which dominates the excitonic emission in HOIP crystals.

Surface ionic charge dependence on the molecular mobility and self-assembly behavior of ionomers produced from carboxylic acid-terminated dendrimers

A carboxylic acid-terminated dendrimer was synthesized via a Steglich esterification of a hyperbranched polyol precursor. Progressive neutralization of the carboxylic groups was carried out, yielding a series of ionic dendrimers. In the ionic clusters, the anion–cation electrostatic interactions induced by the neutralization process dramatically affected the chain mobility of the polymer chains and their order–disorder transition temperatures. The self-assembly of the ionic dendrimers from aqueous milieu into 2D tree morphologies is demonstrated, highlighting their potential use as a highly versatile template for multifunctional semiconductor applications.

Modeling of the effect of temperature and field-induced electron emission from the cathode with a thin insulating film on the Townsend discharge ignition voltage in argon–mercury mixture

A model of the low-current (Townsend) discharge in argon–mercury mixture in the presence of a thin insulating oxide film on the cathode is developed. It takes into account the cathode ion-electron emission and the electron emission from the cathode metal substrate under the strong electric field generated in the film by the ion surface charge. An influence of the mixture temperature and the oxide film on the discharge ignition voltage is estimated and it is shown that formation of a thin insulating film on the cathode surface can facilitate the discharge ignition in mercury-containing gas discharge devices at low temperatures.

Influence of a cathode surface oxide film on the energy distributions of ions and fast atoms in a glow discharge

A model of the cathode sheath of a glow discharge in the presence of a thin dielectric oxide film on the cathode is formulated. It takes into account cathode ion-electron emission and the emission of electrons from the cathode metal substrate under a strong electric field generated in the film by the ion surface charge. The influence of the oxide film on the discharge parameters, the ion and fast atom energy spectra near the cathode surface, and the intensity of its sputtering is estimated.

Optimization of Cell Growth on Bacterial Cellulose by Adsorption of Collagen and Poly-L-Lysine

Poly-L-lysine and collagen were separately added to bacterial cellulose (BC) nanofibers. The ionic surface charge had been previously modified in order to promote the adsorption of poly-L-lysine and collagen. Cell adhesion of Chinese hamster ovary (CHO) cells on BC surfaces was confirmed by removing unattached cells from the BC substrates. Cell viability was calculated and it was determined that both poly-L-lysine-BC and collagen-BC substrates are viable for cell growth. The results showed that the cell viability in poly-L-lysine modified BC substrate is similar to the one observed in polystyrene tissue culture plates.

Mechanism behind self-sustained oscillations in direct current glow discharges and dusty plasmas

An alternative explanation to the mechanism behind self-sustained oscillations of ions in direct current (DC) glow discharges is provided. Such description is distinguished from the one provided by the fluid models, where oscillations are attributed to the positive feedback mechanism associated with photoionization of particles and photoemission of electrons from the cathode. Here, oscillations arise as consequence of interaction between an ion and the surface charges induced by it at the bounding electrodes. Such mechanism provides an elegant explanation to why self-sustained oscillations occur only in the negative resistance region of the voltage-current characteristic curve in the DC glow discharges. Furthermore, this alternative description provides an elegant explanation to the formation of plasma fireballs in the laboratory plasma. It has been found that oscillation frequencies increase with ion's surface charge density, but at the rate which is significantly slower than it does with the electric field. The presented mechanism also describes self-sustained oscillations of ions in dusty plasmas, which demonstrates that self-sustained oscillations in dusty plasmas and DC glow discharges involve common physical processes.

First Evidence of Singlet Oxygen Species Mechanism in Silicate Clay for Antimicrobial Behavior

ABSTRACTThin silicate nanoplatelets, derived from the exfoliation of natural Sodium montmorillonite (Na+-MMT) clays, show an unexpected antimicrobial property. A physical trapping mechanism has been proposed because the clay nanoplatelets can indiscriminately inhibit the growth of a broad spectrum of bacteria, including drug-resistant species such as methicillin-resistance S. aureus (MRSA) and silver ion-resistant E. coli. The ability to generate singlet oxygen species was first observed for the clay platelets that showed a high-aspect-ratio geometric shape and the presence of surface ionic charges. By comparison, the pristine clay with a multilayered structure failed to generate any singlet oxygen species. The ability to emit singlet oxygen species provides direct evidence for the antimicrobial ability of clay through a non-chemical mechanism, which opens the potential for medical use.

Trends in Ln(III) Sorption to Quartz Assessed by Molecular Dynamics Simulations and Laser-Induced Fluorescence Studies

Molecular dynamics simulations were performed to examine trends in trivalent lanthanide [Ln(III)] sorption to ≡SiOH0 and ≡SiO– sites on the 001 surface of α-quartz across the 4f period. Complementary laser-induced fluorescence studies examined Eu(III) sorption to α-quartz at a series of ionic strengths from 1 × 10–4 M to 0.5 M such that properties of the surface-sorbed species could be extrapolated to zero ionic strength, the conditions under which the simulations are performed. Such extrapolation allows for a more direct comparison of the data and enables a molecular understanding of the surface-sorbed species and the role of the ion surface charge density upon the interfacial reactivity. Potential of mean force molecular dynamics as well as simulations of presorbed Ln(III) species agrees with the spectroscopic study of Eu(III) sorption, indicating that strongly bound inner-sphere complexes are formed upon sorption to an ≡SiO– site. The coordination shell of the ion contains 6–7 waters of hydration, and it is predicted that surface silanol OH groups transfer from the quartz to the inner coordination shell of Eu(III). Molecular simulations predict less-strongly bound inner-sphere species in early lanthanides and more strongly bound species in late lanthanides, following trends in the surface charge density of the 4f ions. Hydroxyl ligands that derive from the surface silanol groups are consistently observed to bind in the inner coordination shell of surface-sorbed inner-sphere Ln(III) ions, provided that the ion is able to migrate within 2.0–3.0 Å of the plane formed by the silanol O atoms (∼3.5 Å from an individual ≡SiO– group). Sorption to a fully protonated quartz surface is not predicted to be favorable by any Ln(III), except perhaps Lu. The present work demonstrates a combined theoretical and experimental approach in the prediction of the fate of trivalent radioactive contaminants at temporary and permanent nuclear waste storage sites.

Domain structures of ferroelectric thin film controlled by oxidizing atmosphere

Evolutions of domain morphology in the ferroelectric thin film subjected to the oxidizing atmosphere were predicted by using the phase field simulations, which incorporate the long-range electrostatic interactions and ionic surface charges. Due to effect of the oxidizing atmosphere, it is found that the ionic surface charges carried by oxygen can effectively change the internal electric filed, control the polarization orientation, and drive the domain wall motion of the ferroelectric thin film. Domain structures were simulated and also reveal that domain morphology of the ferroelectric thin film can be adjusted from a multi-domain to a mono-domain with increasing of the ionic charge density.

Effect of Cation Size and Charge on the Interaction between Silica Surfaces in 1:1, 2:1, and 3:1 Aqueous Electrolytes

Application of two complementary AFM measurements, force vs separation and adhesion force, reveals the combined effects of cation size and charge (valency) on the interaction between silica surfaces in three 1:1, three 2:1, and three 3:1 metal chloride aqueous solutions of different concentrations. The interaction between the silica surfaces in 1:1 and 2:1 salt solutions is fully accounted for by ion-independent van der Waals (vdW) attraction and electric double-layer repulsion modified by cation specific adsorption to the silica surfaces. The deduced ranking of mono- and divalent cation adsorption capacity (adsorbability) to silica, Mg(2+) < Ca(2+) < Na(+) < Sr(2+) < K(+) < Cs(+), follows cation bare size as well as cation solvation energy but does not correlate with hydrated ionic radius or with volume or surface ionic charge density. In the presence of 3:1 salts, the coarse phenomenology of the force between the silica surfaces as a function of salt concentration resembles that in 1:1 and 2:1 electrolytes. Nevertheless, two fundamental differences should be noticed. First, the attraction between the silica surfaces is too large to be attributed solely to vdW force, hence implying an additional attraction mechanism or gross modification of the conventional vdW attraction. Second, neutralization of the silica surfaces occurs at trivalent cation concentrations that are 3 orders of magnitude smaller than those characterizing surface neutralization by mono- and divalent cations. Consequently, when trivalent cations are added to our cation adsorbability series the correlation with bare ion size breaks down abruptly. The strong adsorbability of trivalent cations to silica contrasts straightforward expectations based on ranking of the cationic solvation energies, thus suggesting a different adsorption mechanism which is inoperative or weak for mono- and divalent cations.

On the different roles of anions and cations in the solvation of enzymes in ionic liquids

The solvation of the enzyme Candida antarctica lipase B (CAL-B) was studied in eight different ionic liquids (ILs). The influence of enzyme-ion interactions on the solvation of CAL-B and the structure of the enzyme-IL interface are analyzed. CAL-B and ILs are described with molecular dynamics (MD) simulations in combination with an atomistic empirical force field. The considered cations are based on imidazolium or guanidinium that are paired with nitrate, tetrafluoroborate or hexafluorophosphate anions. The interactions of CAL-B with ILs are dominated by Coulomb interactions with anions, while the second largest contribution stems from van der Waals interactions with cations. The enzyme-ion interaction strength is determined by the ion size and the magnitude of the ion surface charge. The solvation of CAL-B in ILs is unfavorable compared to water because of large formation energies for the CAL-B solute cages in ILs. The internal energy in the IL and of CAL-B increases linearly with the enzyme-ion interaction strength. The average electrostatic potential on the surface of CAL-B is larger in ILs than in water, due to a weaker screening of charged enzyme residues. Ion densities increased moderately in the vicinity of charged residues and decreased close to non-polar residues. An aggregation of long alkyl chains close to non-polar regions and the active site entrance of CAL-B are observed in one IL that involved long non-polar decyl groups. In ILs that contain 1-butyl-3-methylimidazolium cations, the diffusion of one or two cations into the active site of CAL-B occurs during MD simulations. This suggests a possible obstruction of the active site in these ILs. Overall, the results indicate that small ions lead to a stronger electrostatic screening within the solvent and stronger interactions with the enzyme. Also a large ion surface charge, when more hydrophilic ions are used, increases enzyme-IL interactions. An increase of these interactions destabilizes the enzyme and impedes enzyme solvation due to an increase in solute cage formation energies.

Clay-assisted dispersion of organic pigments in water

We reported a new method for dispersing insoluble organic pigments (OPs) in water by simply pulverizing with the selected clay. The smectite clay of synthetic fluorinated mica (Mica) enabled to homogenize the pigments due to its inherent colloidal properties of surface ionic charges and unique plate-like geometric shape in an average dimension of 300 × 300 × 1 nm 3. Five pigments of different colors were homogeneously dispersed in water and maintained their nanoscale particle sizes in ca. 70 nm–2 μm. The zeta potential data revealed a strong surface-charge interaction between the OPs and Mica with a noticeable charge measurement varying from −40 mV to −24 mV. For practical applications, the homogeneous dispersion of pigments was mixed with PVA and spin-coated into polymer films exhibiting the possibility of fabricating color filters. The process has the advantage of avoiding the conventional uses of organic surfactants and solvents.

Hydrophobic Intercalation of Layered Silicate Clays and Hierarchical Self‐Assemblies via Platelet‐Shape Directing

AbstractMost clay research in recent years has centered on the development of polymer/layered‐silicate nanocomposites besides the conventional applications such as catalyst and adsorbents for organics. Recently, we have reported the hydrophobic modification of the naturally‐occurring sodium montmorillonite clay (d spacing = 12 Å) by exchanging with hydrophobic poly(oxypropylene)‐diamine salts (POP‐salt) to afford the space‐enlarged silicates (d spacing up to 50–60 Å). Owing to the geometrically plate‐like shape and surface ionic charges, the organoclays enabled to self‐assemble into rod‐shape morphologies. For example, unique formation of lengthy rod (ca. 0.3 µm in diameter and up to 40 µm in length) was observed by using scanning electron microscopy (SEM). The self‐assembling mechanism may involve the piling of the primary stack units by face‐to‐face and edge‐to‐edge alignments. The clay primary stacks intercalated by POP‐salt and the hydrophilic analog of poly(oxyethylene)‐diamine salts were observed by AFM.