Bulk Ionic Strength Research Articles

Computational Study of the pH-Dependent Ionic Environment around β-Lactoglobulin.

Ions are involved in multiple biological processes and may exist bound to biomolecules or may be associated with their surface. Although the presence of ions in nucleic acids has traditionally gained more interest, ion-protein interactions, often with a marked dependency on pH, are beginning to gather attention. Here we present a detailed analysis on the binding and distribution of ions around β-lactoglobulin using a constant-pH MD (CpHMD) method, at a pH range 3-8, and compare it with the more traditional Poisson-Boltzmann (PB) model and the existing experimental data. Most analyses used ion concentration maps built around the protein, obtained from either the CpHMD simulations or PB calculations. The requirements of approximate charge neutrality and ionic strength equal to bulk, imposed on the MD box, imply that the absolute value of the ion excess should be half the protein charge, which is in agreement with experimental observation on other proteins ( Proc. Natl. Acad. Sci. U.S.A. 2021, 118, e2015879118) and lends support to this protocol. In addition, the protein total charge (including territorially bound ions) estimated with MD is in excellent agreement with electrophoretic measurements. Overall, the CpHMD simulations show good agreement with the nonlinear form of the PB (NLPB) model but not with its linear form, which involves a theoretical inconsistency in the calculation of the concentration maps. In several analyses, the observed pH-dependent trends for the counterions and co-ions are those generally expected, and the ion concentration maps correctly converge to the bulk ionic strength as one moves away from the protein. Despite the overall similarity, the CpHMD and NLPB approaches show some discrepancies when analyzed in more detail, which may be related to an apparent overestimation of counterion excess and underestimation of co-ion exclusion by the NLPB model, particularly at short distances from the protein.

A theoretical understanding of ionic current through a nanochannel driven by a viscosity gradient

HypothesisDifferent thermodynamic forces owing to the gradient of temperature, electrical potential, or concentration can drive ionic current through charged membranes. It has been recently shown that a viscosity gradient can drive an electrical current through a negatively charged nanochannel (Wiener and Stein, arXiv: 1807.09106). A model description of this phenomenon, based on the Maxwell–Stefan equation will help unravel the dominating physical mechanisms in so-called visco-migration. TheoryTo understand the physical mechanisms underlying this phenomenon, we employed the Maxwell–Stefan equation to develop a 1D model and obtain a relation between the flux of solvents and the driving forces. Viscosity gradients are known to drive transport, but the development of an electrical current has not been theoretically described prior to this work. FindingsOur 1D model shows that the ionic current depends on the ideality of the solvent, though both ideal and non-ideal scenarios demonstrated good agreement with experimental data. We employed the model to understand the impact of solution bulk ionic strength and pH on the drift of ionic species with same reservoirs solution properties. Our modeling results unveiled the significant impact of bulk solution properties on the drift of ions which is in agreement with the experiments. Moreover, we have shown that the diffusion gradient along the nanochannel contributes significantly into driving ionic species if we even apply a small ionic concentration gradient to both reservoirs. Our modeling results may pave the way for finding novel applications for drift of ions in a diffusion gradient, which can be induced by connecting reservoirs of different viscosity fluids.

An Experimental and Numerical Study of Polyelectrolyte Hydrogel Ionic Diodes: Towards Electrical Detection of Charged Biomolecules.

Polyelectrolyte hydrogel ionic diodes (PHIDs) have recently emerged as a unique set of iontronic devices. Such diodes are built on microfluidic chips that feature polyelectrolyte hydrogel junctions and rectify ionic currents owing to the heterogeneous distribution and transport of ions across the junctions. In this paper, we provide the first account of a study on the ion transport behavior of PHIDs through an experimental investigation and numerical simulation. The effects of bulk ionic strength and hydrogel pore confinement are experimentally investigated. The ionic current rectification (ICR) exhibits saturation in a micromolar regime and responds to hydrogel pore size, which is subsequently verified in a simulation. Furthermore, we experimentally show that the rectification is sensitive to the dose of immobilized DNA with an exhibited sensitivity of 1 ng/μL. We anticipate our findings would be beneficial to the design of PHID-based biosensors for electrical detection of charged biomolecules.

Temperature-regulated surface charge manipulates ionic current rectification in tapered nanofluidic channel

Diverse ionic current rectification methods for nanofluidic chips have recently emerged. Herein, we theoretically demonstrate that by applying a temperature gradient to the aqueous solution, the ionic rectification property of a tapered nanochannel can be manipulated by applying temperature gradients from the tip to base and vice versa. Our modeling results reveal that the rectification ratio can be significantly enhanced by applying a temperature increment from the base to tip, whereas the rectification ratio is significantly suppressed by applying a reverse temperature gradient. In addition to the solution temperature, we also investigated the influence of bulk ionic strength and tip height on the rectification ratio, thereby providing overlapping and non-overlapping regimes of electrical double layers. We demonstrate that the rectification behavior of a tapered nanochannel is determined by the overlapping regime of the electrical double layer at the tip of the nanochannel. Moreover, we propose a semi-analytical solution that can capture numerical results with the same order of magnitude. We expect that the modeling results of this contribution can provide a direction for understanding ionic transport across geometrically and thermally asymmetrical media, which could find applications from energy conversion to logical nanofluidic chip components.

Wavelength-scanning second-harmonic generation for determining absolute charge density at aqueous interfaces.

We develop a new, to the best of our knowledge, optical scheme based on second-harmonic generation (SHG) at multiple wavelengths for unequivocal separation of the second-order and the electric-field-induced third-order nonlinear optical contributions from aqueous interfaces. The third-order SHG originating from the field-induced reorientation order of water molecules in the electrical double layer offers an optical label-free and inherent probe to the surface charge density and surface potential in the absolute scales. We verify this wavelength-scanning SHG scheme both theoretically and experimentally, and show that the approach is applicable to water interfaces with bulk ionic strength below 500 µM and can achieve a detection sensitivity for a surface charge density of ∼10-4C/m2.

Polyelectrolyte Conformation Controlled by a Trivalent-Rich Ion Jacket.

The configuration of charged polymers is heavily dependent on interactions with surrounding salt ions, typically manifesting as a sensitivity to the bulk ionic strength. Here, we use single-molecule mechanical measurements to show that a charged polysaccharide, hyaluronic acid, shows a surprising regime of insensitivity to ionic strength in the presence of trivalent ions. Using simulations and theory, we propose that this is caused by the formation of a "jacket" of ions, tightly associated with the polymer, whose charge (and thus effect on configuration) is robust against changes in solution composition.

Human Serum Albumin Adsorption Kinetics on Silica: Influence of Protein Solution Stability.

Adsorption kinetics of human serum albumin (HSA) on silica substrates was studied using optical waveguide lightmode spectroscopy (OWLS) and quartz microbalance (QCM) techniques. Measurements were performed at pH 3.5, 5.6, and 7.4 for various bulk suspension concentrations and ionic strengths. The diffusion coefficient measurements showed that for pH 3.5 the HSA molecules are stable for NaCl concentrations from 10-3 to 0.15 M. This allowed us to precisely determine the mass transfer rate coefficients for the OWLS and QCM cells. The experimental data were adequately interpreted in terms of a hybrid random sequential adsorption model. The OWLS maximum coverage of HSA at pH 3.5, which is equal to 1.3 mg m-2, agrees with the QCM result and with previous results derived from streaming potential measurements. Thus, the results obtained at pH 3.5 served as reference data for the analysis of adsorption kinetics at higher pHs. In this way, it was confirmed that the adsorption kinetics of HSA molecules at pH 5.6 and 7.4 was considerably slower than at pH 3.5. This effect was attributed to aggregation of HSA solutions and interpreted in terms of a theoretical model combining the Smoluchowski aggregation theory with the convective diffusion mass transfer theory. New analytical equations were derived that can be used for the interpretation of other protein adsorption from unstable solutions.

Protein adsorption mechanisms at rough surfaces: Serum albumin at a gold substrate

Adsorption kinetics of human serum albumin (HSA) at a gold substrate was studied using the quartz microbalance (QCM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Measurements were performed at pH 3.5 for various bulk suspension concentrations and ionic strengths. The QCM experimental data were compared with the dry coverage of HSA derived from AFM and from the solution of the mass transfer equation acquired using a hybrid random sequential adsorption model. In this way, the water factor and the dynamic hydration function for HSA monolayers were quantitatively evaluated as a function of dry coverage for various ionic strengths. A comparison of these results with previously known for a silica sensor confirmed that the QCM kinetic measurements are sensitive to the roughness of the sensor characterized in terms of the rms parameter. For the more rough gold sensor (rms = 2.5 nm, average surface feature size 6 nm) the QCM mass transfer rate constant was 2.6 times lower than for the silica sensor characterized by rms = 0.86 nm. This gives for the gold sensor the apparent water factor equal to one and zero hydration function. Moreover, the hydration function increased for larger HSA coverage and was dependent on ionic strength in contrast to the silica substrate. This unexpected behavior was interpreted in terms of the buoyancy effect where the HSA molecules adsorbing in cavities existing at rough surfaces replace the stagnant (hydrodynamically bound) water. Hence, these results confirm that the sensor roughness of the size comparable with protein molecule dimensions exerts a decisive influence on their adsorption kinetic derived from QCM measurements.

Kinetics of human serum albumin adsorption at silica sensor: Unveiling dynamic hydration function.

Adsorption kinetics of human serum albumin (HSA) at a silica substrate was studied using the QCM-D and AFM methods. Measurements were performed at pH 3.5 for various bulk suspension concentrations and ionic strengths. The QCM experimental data were compared with the dry coverage of HSA derived from AFM and from the solution of the mass transfer equation. In this way, the dynamic hydration functions and water factors of HSA monolayers were quantitatively evaluated as a function of dry coverage for various ionic strengths. Using the hydration functions, the HSA adsorption runs derived from QCM-D measurements were converted to the dry coverage vs. the time relationships. In this way, the maximum coverage of irreversibly bound HSA molecules was determined. It was equal to 0.35 and 1.4 mg m-2 for NaCl concentration of 0.001 and 0.15 M, respectively. These results agree with previous experimental data derived by streaming potential measurements for mica and with theoretical modeling. Therefore, the side-on mechanism of HSA adsorption at silica sensor at pH 3.5 was confirmed. Also, a quantitative analysis of the desorption runs allowed one to calculate the binding energy of the reversibly bound HSA fraction. Beside significance to basic science, these results enable to develop a robust technique of preparing HSA monolayers at silica sensor of well-controlled coverage and molecule orientation.

STUDY OF ORGANOLEPTIC AND RHEOLOGICAL PROPERTIES OF CHOCOLATE MOUSSE INCORPORATING SODIUM CASEINATE AND GELATIN

در این پژوهش اثر مقادیر مختلف ژلاتین (1، 2 و 3گرم) و سدیم کازئینات (1، 2 و 3 گرم) بر ویژگی‌های حسی و رئولوژیکی نمونه‌های موس شکلات بررسی شد. همچنین از روش تجزیه به مولفه‌های اصلی جهت تعیین روابط پارامترهای حسی و بدست آوردن مولفه‌های اصلی استفاده گردید. مشاهدات نشان داد نمونه موس شکلات حاوی دو گرم ژلاتین و سه گرم سدیم کازئینات بهترین ویژگی‌های حسی را داشت. نتایج حاصل از برازش داده نشان داد که مدل پاورلا با داشتن R2 بالا (99/0R2>) و RMSE پایین بهترین مدل جهت توصیف رفتار جریان می‌باشد. ضریب قوام در نمونه‌ای از موس شکلات که سه گرم ژلاتین و سه گرم سدیم کازئینات را دارا بود، بیشتر بود. دانسیته نیز با افزایش مقدار پروتئین و کاهش مقدار ژلاتین، کاهش یافت. همچنین با استفاده از روش تجزیه به مولفه‌های اصلی مشخص شد پذیرش کلی نمونه‌ها همبستگی بالایی با نرمی، پوشش دهانی و ژله‌ای بودن داشت.

Theory for the Charge Dependence of POPG:POPC Liposome Repulsions in Deionized Water

Previously we analyzed the shear moduli of a series of deionized suspensions of electrostatically interacting 100 nm POPG:POPC liposomes. The liposome charge was varied by altering the POPG content from 0 to 100% in 10 increments. We used the theory of Joanny (1979), which relates the shear modulus of a deionized bcc-ordered suspension of charged particles to an “effective charge” Z∗ on each particle The Joanny treatment employs a Debye-Huckel (DH) screened Coulomb potential with an effective bulk ionic strength equal to the average dissociated [H+]∗ in the sample. The validity of this approximation is questionable. Moreover, static light scattering shows our samples to be a semi-ordered glass (probably due to surface-charge polydispersity) rather than a bcc crystal.Calculated plots of shear modulus vs Z∗ using the Joanny formalism are non-monotonic with pronounced maxima at Z∗ ∼2000. Below this value, direct Coulombic charge repulsions dominate the interaction, and above it, H+ screening dominates.Experimental plots of shear modulus vs. POPG content have similar maxima, at POPG:POPC ∼ 20:80, corresponding to a chemical liposome charge Z ∼10,000. Z∗ and Z differ due to: 1) failure of the DH approximation near the liposome surface where the potentials are large, and 2) partial protonation of PG- headgroups,which reduces Z below its chemical fully-deprotonated value.We refine the Joanny formulation for our experiment by 1) calculating the shear modulus for a glass geometry, and 2) replacing the DH approximations with a counterion-only solution to the Poisson-Boltzmann equation, thereby replacing Z∗ with Z. The calculated value of Z at the shear-modulus maximum, when compared to the experimental Z ∼10,000 at the same maximum, yields the degree of protonation or charge regulation occurring in this strongly interacting system.

Kinetics of silver nanoparticle deposition at PAH monolayers: reference QCM results.

The deposition kinetics of silver nanoparticles on Au/SiO2 /PAH substrate was studied under in situ conditions using the QCM method and the ex situ SEM imaging. Because of low dissipation, the Sauerbrey equation was used for calculating the mass per unit area (coverage). Measurements were done for various bulk suspension concentrations, flow rates, and ionic strengths. It was shown that particle deposition for the low coverage regime is governed by the bulk mass transfer step that results in a linear increase of the coverage with the time. A comparison of QCM and SEM results showed that the hydration of the silver monolayers was negligible. This allowed one to derive a universal kinetic equation that describes the mass transfer rates in the cell as a function of the bulk concentration, flow rate, and diffusion coefficient. Measurements were also performed for longer times and for various ionic strengths where the deposition kinetics and the maximum coverage of particles were determined. The experimental data confirmed a significant increase in the maximum coverage with ionic strength. This was interpreted as due to the decreasing range of the electrostatic interactions among deposited particles. These results were adequately interpreted in terms of the extended random sequential adsorption (eRSA) model. Additionally, it was shown that the QCM data matched the ex situ SEM results, indicating that the monolayer hydration was also negligible for higher coverage range. These results derived for the model silver nanoparticle system can be exploited as reference data for the interpretation of protein adsorption kinetics where the dry mass is needed in order to assess the extent of hydration.

The influence of 1-butanol and electrolytic background on the properties of CTAB micelles as examined using a set of indicator dyes

The ionic equilibrium of six indicator dyes, bromophenol blue, thymol blue, sulfonefluorescein, n-decyl ester and methyl ether of fluorescein, and N,N′-di-n-octadecylrhodamine was studied in CTAB micellar solutions. It was revealed that the values of the apparent ionization constants, Kaapp, of the indicators are substantially influenced by additions of 1-butanol either at low or high bulk ionic strength created by NaBr additions. Whereas the pKaapp values at ionic strength of 0.05M as a rule decrease to more or less extent as compared to the corresponding pKas in water, the penetration of the alcohol into the micellar pseudophase results in either increase or decrease in pKaapp. The dependence of pKaapp vs. log[Br−] at 0.80M n-C4H9OH allows estimating the degree of counter-ion binding under such conditions: β=0.40±0.01. The distinct deviation of the last quantity from the corresponding value, β=0.76±0.02 registered in butanol-free CTAB micellar solution is caused by the decrease in the interfacial concentration of the cationic head groups within the Stern layer. The molar fraction of 1-butanol in micelles and the partition constant of this alcohol between the aqueous phase and micelles were estimated.

High-throughput, multi-platform metabolomics on very small volumes: 1H NMR metabolite identification in an unadulterated tube-in-tube system

As small animal models of disease become more widely used, there is increasing importance and potential for characterizing their metabolomes. However, as the animal becomes smaller, the amounts of biofluids such as urine and cerebral spinal fluid available for metabolomic studies are more limited. Further, in multi-platform systems biology when the same small sample must be used for several analyses, it is a frequent requirement that no additions are made to the sample (even as simple as D2O or an NMR chemical shift reference) to maintain sample integrity. Herein we describe a method for high-throughput 1H-NMR studies using ~30 µl volumes, suitable for biofluid matrices. The compartmentalization of the sample and NMR standards, however, requires chemical shift corrections due to bulk magnetic susceptibility and ionic strength changes for metabolite profiling using a reference library or data-binning of the chemical shift axis. This set-up minimizes the cost of individual data collection per small animal and is suitable for high-throughput, longitudinal, multimodal metabolomic studies of biofluids available in limited quantities.

Deposition of gold nanoparticles on mica modified by poly(allylamine hydrochloride) monolayers

Bulk characteristics and deposition of gold nanoparticles on mica modified by (PAH) was studied by UV–visible spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS) and atomic force microscopy (AFM). The size of gold nanoparticles (AuNPs) was 15nm as determined by TEM and AFM. The electrophoretic mobilities and electrokinetic charge of particles were quantitatively evaluated for a broad range of pH and ionic strength. Using Henry's model, it was calculated that the zeta potential of particles varied between −50mV and −80mV for pH 2 and pH 11, respectively (at 10−2M of NaCl). Measurements of nanoparticle deposition kinetics were performed under diffusion-controlled transport conditions using AFM and SEM imaging of particle monolayers. The influence of the bulk suspension concentration was systematically studied. Additionally, the maximum coverage of particle monolayers, which monotonically increased with ionic strength, was determined by SEM. The obtained data were in agreement with theoretical predictions derived from the random sequential adsorption (RSA) model. It was also confirmed that by varying the bulk suspension concentration and ionic strength one can prepare homogeneous gold particle monolayers of controlled coverage.

Phase diagram of mixed monolayers of stearic acid and dimyristoylphosphatidylcholine. Effect of the acid ionization

The aim of this work is to study the phase diagram of mixed monolayers composed of dimyristoylphosphatidylcholine (DMPC) and stearic acid (SA) at different ionic strength and bulk pH of the aqueous subphase. In this way, the effect of ionization of SA on the interaction and thus on phase separation with the DMPC matrix can be analyzed. To this purpose, we first determined the ionization state of pure SA monolayers as a function of the bulk subphase pH. The SA monolayers are nearly fully ionized at pH 10 and essentially neutral at pH 4 and the mixture of DMPC and SA was studied at those two pHs. We found that the DMPC-enriched phase admits more SA if the SA monolayer is in a liquid-expanded state, which is highly related to the acid ionization state, and thus to the bulk pH and ionic strength. At pH 4 the molecules hardly mix while at pH 10 the mixed monolayer with DMPC can admit between 30 and 100% of SA (depending on the lateral pressure) before phase separation is established. The addition of calcium ions to the subphase has a condensing effect on SA monolayers at all pHs and the solubility of SA in the DMPC matrix does not depend on the bulk pH in these conditions. The observed phase diagrams are independent on the manner in which the state of the mixed film is reached and may thus be considered states of apparent equilibrium.

Transport and re-entrainment of soil colloids in saturated packed column: effects of pH and ionic strength

Purpose Colloid migration in subsurface environments has attracted considerable attention in recent years because of its suspected role in facilitating transport of strongly adsorbed contaminants to groundwater. The influence of bulk solution pH or ionic strength on model colloid (i.e., latex microsphere, amorphous silica colloids) transport is well established, while little attention has been paid to water-dispersible soil colloids. In this study, saturated packed columns were conducted to explore the mechanism of transport and fate of water-dispersible soil colloids and facilitating transport of Cu during transients in solution chemistry.

Studies on the interactions between bovine β-lactoglobulin and chitosan at the solid–liquid interface

Chitosan ultrathin films have been formed on polycrystalline Au substrates using the LbL technique with the purpose of studying its interaction with bovine β-lactoglobulin (β-LG) at the solid–liquid interface. The immobilization of chitosan was followed by Quartz Crystal Microbalance with energy dissipation (QCM-D), Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). The behavior of the chitosan films in the presence of β-LG solutions with different bulk concentrations ([β-LG]), ionic strength ( I), and pH has been investigated using the same techniques plus Atomic Force Microscopy (AFM). The results showed that for pHs lower than protein's p I, weak intermolecular forces (H bonding, Van der Waals, hydrophobic, etc.) are established between β-LG and chitosan (especially close to the p I) leading to low coverage nonspecific adsorption. On the contrary when pH > p I, strong ionic bonding through attractive electrostatic interactions lead to high coverage adsorbed phases composed of large β-LG aggregates. The adsorption process was shown to consist of a relatively fast step (in which these interactions are predominant) which is followed, once the β-LG monolayer is exceeded, by the slow formation of thicker and increasingly viscoelastic films through β-LG self-aggregation. QCM-D and AFM experiments unveiled the role of [β-LG] and I on the formation of these aggregates. The adsorption isotherm built from impedance data in the medium-low [β-LG] range (0.001–0.3 mg mL −1), showed good fitting to the Langmuir model confirming that the formation of one β-LG monolayer is achieved in this concentration range.

Ternary [Al 2O 3–electrolyte–Cu 2+] species: EPR spectroscopy and surface complexation modeling

Cu 2+ binding on γ-Al 2O 3 is modulated by common electrolyte ions such as Mg 2+, SO 4 2 - , and PO 4 3 - in a complex manner: (a) At high concentrations of electrolyte ions, Cu 2+ uptake by γ-Al 2O 3 is inhibited. This is partially due to bulk ionic strength effects and, mostly, due to direct competition between Mg 2+ and Cu 2+ ions for the SO − surface sites of γ-Al 2O 3. (b) At low concentrations of electrolyte ions, Cu 2+ uptake by γ-Al 2O 3 can be enhanced. This is due to synergistic coadsorption of Cu 2+ and electrolyte anions, SO 4 2 - and PO 4 3 - . This results in the formation of ternary surface species ( SOH 2SO 4Cu) +, ( SOH 2PO 4Cu), and ( SOH 2HPO 4Cu) + which enhance Cu 2+ uptake at pH < 6. The effect of phosphate ions may be particularly strong resulting in a 100% Cu uptake by the oxide surface. (c) EPR spectroscopy shows that at pH ≪ pH PZC, Cu 2+ coordinates to one SO − group. Phosphate anions form stronger, binary or ternary, surface species than sulfate anions. At pH ≫ pH PZC Cu 2+ may coordinate to two SO − groups. At pH ≪ pH PZC electrolyte ions SO 4 2 - and PO 4 3 - are bridging one O-atom from the γ-Al 2O 3 surface and one Cu 2+ ion forming ternary [ γ-Al 2O 3/elecrolyte/Cu 2+] species.

Natural and Engineered Nano and Colloidal Transport: Role of Zeta Potential in Prediction of Particle Deposition

This study is designed to answer the question of whether the zeta-potential measurement of natural and engineered nano and colloidal particles can be used as a general predictor for their transport and deposition within porous medium. The transport and deposition of engineered zeolite particles and natural colloids (soil and sediment colloids) was studied and evaluated considering their zeta-potential measurement. The zeta-potential of Ca(2+) or K(+) saturated zeolite particles increased (i.e., became less negative) with increasing bulk ionic strength, independent of the cation species in the bulk solution, while the zeta-potential of Ca(2+) or K(+) saturated natural colloids was dependent on the cation species in the bulk solution, unexpectedly decreasing with the bulk K(+) concentrations while increasing with bulk Ca(2+) concentrations (10(-5) to 10(-2) M). The particle deposition rate coefficient (k) was sensitive to the cation species in the bulk solution and on the cation exchangeable sites of the particle surfaces. The trend of the zeta-potential of the natural colloids, especially with simple electrolytes in bulk solution, was at odds with that of k under the same conditions, indicating that the zeta-potential of these natural colloids could not be used to adequately predict their transport and deposition. The observed anomaly is a direct result of surface heterogeneity and hydrophobicity of natural colloids, and it points out the important difference between natural and engineered nano and colloidal particles in terms of their transport and deposition prediction.