Microslit Electrokinetic Setup Research Articles

Electrostatic Interactions Modulate the Conformation of Collagen I

The pH- and electrolyte-dependent charging of collagen I fibrils was analyzed by streaming potential/streaming current experiments using the Microslit Electrokinetic Setup. Differential scanning calorimetry and circular dichroism spectroscopy were applied in similar electrolyte solutions to characterize the influence of electrostatic interactions on the conformational stability of the protein. The acid base behavior of collagen I was found to be strongly influenced by the ionic strength in KCl as well as in CaCl 2 solutions. An increase of the ionic strength with KCl from 10 −4 M to 10 −2 M shifts the isoelectric point (IEP) of the protein from pH 7.5 to 5.3. However, a similar increase of the ionic strength in CaCl 2 solutions shifts the IEP from 7.5 to above pH 9. Enhanced thermal stability with increasing ionic strength was observed by differential scanning calorimetry in both electrolyte systems. In line with this, circular dichroism spectroscopy results show an increase of the helicity with increasing ionic strength. Better screening of charged residues and the formation of salt bridges are assumed to cause the stabilization of collagen I with increasing ionic strength in both electrolyte systems. Preferential adsorption of hydroxide ions onto intrinsically uncharged sites in KCl solutions and calcium binding to negatively charged carboxylic acid moieties in CaCl 2 solutions are concluded to shift the IEP and influence the conformational stability of the protein.

Hydrophobic and Electrostatic Interactions in the Adsorption of Fibronectin at Maleic Acid Copolymer Films

Adsorption and desorption of fibronectin (FN) were investigated at thin films of alternating maleic acid copolymers with octadecene (POMA) and with propene (PPMA). The hydrophobicity and charge density of the polymers were modulated by the choice of the comonomer. In consequence, the dominant forces between the substrate and the protein were specified as hydrophobic interaction for POMA and electrostatic interaction for PPMA. The adsorption kinetics were investigated in situ as variations of the optical thickness, adsorbed mass, and viscoelastic properties (detected by reflectometric interference spectroscopy and quartz crystal microbalance technique, respectively) while alterations of the electrosurface properties were derived from surface conductivity data and isoelectric points (by streaming potential/current measurements using a microslit electrokinetic setup). The results demonstrate that the interfacial mode of adsorbed FN depends on the predominant interactions: large amounts of FN were tightly bound to POMA by hydrophobic interactions. In contrast, FN adsorbed on PPMA was concluded to attain an unfolded structure allowing for the "electrostatic matching" of positively charged residues on FN with the maleic acid groups. This conclusion was supported by the acidic IEP of 3.2 found for FN on PPMA and a significant reduction of the surface conductivity of the FN-covered polymer film, whereas FN on POMA showed an IEP of 4.2 (close to the intrinsic IEP of FN), indicating a stochastic orientation of the adsorbed protein.

Electrokinetic microslit experiments to analyse the charge formation at solid/liquid interfaces

Electrokinetic effects play an important role in microfluidics and nanofluidics. Although the related phenomena are often utilized to control fluid flow and sample transport in lab-on-a-chip devices, their dependency on the surface charges on the channel walls often remain enigmatic. This is mainly due to the lack of adequate experimental methods to analyse the electrical charging of solid/liquid interfaces of interest. To address this need, an experimental set-up—designated as microslit electrokinetic set-up (MES)—has been recently developed and applied for the investigation of charge formation processes at planar solid/liquid interfaces. The device permits to perform streaming potential and streaming current measurements across a rectangular streaming channel formed by two parallel sample carriers (20×10×3 mm3) at variable distance allowing for the determination of the surface conductivity. Utilizing the MES, charge characteristics can be determined for a wide variety of materials prepared as thin films on top of planar glass substrates. Streaming potential and streaming current data permit to investigate the mechanisms of charge formation while surface conductivity data provide information about mobile charge carriers located in different zones at the interface. The applicability of this advanced experimental approach is demonstrated with examples obtained for surfaces with different levels of complexity: 1. Preferential ion adsorption onto unpolar fluoropolymer (Teflon® AF) films was characterized in simple electrolyte solutions; the results were quantitatively evaluated with respect to interfacial ion concentrations. 2. Interrelation of charge density and conformation of grafted poly(L-glutamic acid layers) were unravelled from the determination of pH-depended variations of surface conductivity and layer thickness. 3. The impact of spatial confinements of surface functional groups on their acid–base behaviour was studied with self-assembled monomolecular films of alkanethiols chemisorbed on gold. 4. Charging of and ion mobility within poly(acrylic acid) (PAA) brushes prepared by a Langmuir–Blodgett technique were analysed at varied pH and ionic strength. 5. Interfacial modes of adsorbed proteins were distinguished at two polymer surfaces with varied hydrophobicity/charge density.

A New Tangential Streaming Potential Setup for the Electrokinetic Characterization of Tubular Membranes

ABSTRACTA new electrokinetic setup was developed for assessing the active layer ζ-potential of tubular membranes based on tangential streaming potential and electrical resistance measurements. Although the flow was not wholly laminar (because of the large hydraulic diameter of channels), the electrokinetics theory could be used to convert the streaming potential data into ζ-potentials because the electrical double layer lay within a laminar sublayer near the channel walls. Electrical resistance data allowed for the account of the conduction phenomenon through the membrane porous body. The new device was tested over a range of pH with a tubular ceramic membrane composed of three channels with a titania active layer. The isoelectric point was found to be in good agreement with that determined from salt retention data. The ζ-potential value determined at pH = 3.5 using the present device was compared with that obtained on a flat membrane made of the same material using the traditional microslit electrokinetic setup. A good agreement between the two measurements was observed. It was shown that neglecting the electric conduction phenomenon through the membrane porous body leads to a low underestimation of the ζ-potential (less than ~20%). This is related to the large size of channels. The contribution of the membrane porous body was found to be independent of the pH of solution. This suggests that the support layer of the membrane would make a decisive contribution to the electric conductivity of membrane porous body.

Ionic Strength-Dependent pK Shift in the Helix−Coil Transition of Grafted Poly(l-glutamic acid) Layers Analyzed by Electrokinetic and Ellipsometric Measurements

Surface-bound layers of poly(L-glutamic acid) prepared by a recently described "grafting-from" method were analyzed with respect to electrical charging and structural alterations upon variation of pH and concentration of the background electrolyte in aqueous solutions. The microslit electrokinetic setup (MES) was utilized for the combined determination of zeta potential and surface conductivity on the basis of streaming potential and streaming current measurements at polypeptide layers in contact with aqueous electrolyte solutions of varied composition. In situ ellipsometry was applied at similar samples immersed in identical aqueous solutions to investigate the influence of the solution pH on the structure of the polypeptide layers. Zeta potential and Dukhin number versus pH plots revealed the dissociation behavior of the surface-bound polypeptides indicating a significant shift of the pK of their acidic side chains correlating with the concentration of the background electrolyte potassium chloride and the related variation of the Debye screening length. Surface conductivity data pointed at a more expanded structure of the polypeptide layer in the fully dissociated state as an increased ion conductance in this part of the interface was determined. The occurrence of a strong increase of the thickness and a corresponding decrease of the refractive index for the coil state of the layer strongly supports the findings of the electrokinetic measurements. This fully reversible "switching" of the layer structure was attributed to helix-coil transitions within the grafted polypeptides induced by the dissociation of carboxylic acid functions of the polypeptide side chains. The shift of the "switching pH" of the surface-bound poly(L-glutamic acid) layers at varied concentrations of the background electrolyte was interpreted as a result of the pK shift of the carboxylic acid groups of the polypeptide side chains. The observed patterns prove that the electrostatic interactions causing this shift occur within but not between the grafted chains.

Electrokinetic Characterization of Oligo- and Poly(ethylene glycol)-Terminated Self-Assembled Monolayers on Gold and Glass Surfaces

Force−distance studies suggest that electrostatic forces play a dominant role in the resistance of oligo(ethylene glycol) (OEG) terminated self-assembled monolayers (SAMs) to adsorption from aqueous solution, whereas for end grafted poly(ethylene glycol) (PEG) coated surfaces inertness is explained by a steric repulsion effect. To study the role of electrostatics in the repulsive forces measured on methoxytri(ethylene glycol) (EG3OMe) terminated alkanethiolate SAMs on gold surfaces, their electrokinetic potentials and charging properties were determined with the microslit electrokinetic setup (MES) using rectangular microchannels formed by SAM-covered sample carriers. The zeta potentials measured on the (EG3OMe) SAMs, both on gold and on glass substrate, indicate preferential hydroxide ion adsorption from the aqueous phase resulting in a net negative surface charge at pH >4. Calculated charge densities agree well with the results from force−distance studies. We also studied the electrokinetic properties of poly(ethylene glycol) (PEG) SAMs (HS(CH2)11(OCH2CH2)44−OCH3). All these films show a negative zeta potential in neutral and alkaline solutions which decreases with increasing packing density and optical film thickness, reflecting the changes in frictional properties of the PEG chains with increasing coverage. An explanation why the negatively charged (EG3OMe) surfaces exhibit repulsive electrostatic forces in aqueous solution, but other negatively charged surfaces do not, is proposed.

Ionization Characteristics and Structural Transitions of Alternating Maleic Acid Copolymer Films

Alternating maleic acid copolymers dissolved in aqueous solutions exhibit characteristic two-step dissociation profiles and pH-induced structural transitions in dependence on the comonomer unit. In extension of these findings, we analyzed a set of thin films of maleic acid copolymers (poly(octadecene-alt-maleic acid), POMA; poly(propylene-alt-maleic acid), PPMA; poly(styrene-alt-maleic acid), PSMA) randomly attached to planar glass surfaces by covalent binding. Streaming potential/streaming current measurements with a microslit electrokinetic setup (MES) were applied to determine zeta potential (ζ) and surface conductivity (Kσ) data of the polymer layers in aqueous solutions of varied pH. The results confirmed the two-step dissociation behavior for all immobilized copolymers. A dramatic increase of Kσ was observed for PPMA and PSMAbut not for POMAlayers at alkaline pH values, indicating that electrostatic repulsion of ionized groups controls the extension of the confined layers as long as hydrophobic inte...

Electrosurface phenomena at polymer films for biosensor applications.

Electrosurface phenomena at thin polymer films utilized in the preparation of biosensors have been studied by use of the Microslit Electrokinetic set-up. For the investigated polymer layers (aminodextran, carboxylated dextran, diaminopoly(ethylene glycol), dicarboxypoly(ethylene glycol), biotinylated poly(ethylene glycol), and streptavidin on biotinylated poly(ethylene glycol), the charge formation in aqueous electrolyte solutions was found to depend on the pH value, that is, OH- and H3O+ are the charge determining ions. The isoelectric points obtained from zeta potential versus pH plots could be utilized to draw conclusions on the introduction of acidic or basic groups and on the degree of molecular surface coverage, respectively. The hydrodynamically mobile charge reflected by the magnitude of the zeta potential contributed to only about 6% or less of the total surface conductivity of the polymer layers. The experimental determination of the total surface conductivity was found to provide valuable information on structural features of biosensor interfaces in aqueous environments.

Characterization of oxide layers on Ti6Al4V and titanium by streaming potential and streaming current measurements

Titanium and titanium alloys (e.g. Ti6Al4V) are increasingly used as medical implant materials in a wide variety of applications. So far, many surface properties of the passive layer considered to explain interactions with biological tissues are deduced from those of the crystalline phases of titanium dioxide (anatase, rutile, brookite), but do not necessarily correspond to those of naturally formed amorphous passive layers. We report on streaming potential and streaming current measurements on oxide layers on Ti6Al4V and Ti, carried out using a microslit electrokinetic set-up (MES) and a commercial electrokinetic analyzer (EKA, PAAR). Passive and anodic oxide layers on Ti6Al4V, as well as passive layers on titanium sputtered on glass, were investigated in this study. Isoelectric points (IEP) of ≈4.4 were found for all oxide layers. The IEP of the air-formed passive layer on Ti6Al4V did not depend on the KCl concentration. Hence, it was concluded that IEP is here identical to the point of zero charge (pzc). Controversially, the charge formation process seems to depend on the chloride ion concentration in the neutral and basic pH region.

Interfacial charge of organic thin films characterized by streaming potential and streaming current measurements

Self-assembled monolayers of alkanethiol compounds chemisorbed on flat gold surfaces were characterized by streaming potential and streaming current measurements in aqueous electrolyte solutions using a novel microslit electrokinetic setup. The alkanethiols analyzed were terminated with different functional groups. Depending on the type of alkanethiol used different mechanisms were relevant for the generation of interfacial charge: dissociation for thiols bearing ionizable surface groups and preferential adsorption of ions for methyl-terminated thiols. In all investigated cases, the zeta potential calculated from the streaming potential was significantly lower than the zeta potential obtained from the streaming current. This was due to a contribution of the conductivity of the underlying gold substrate to the surface conductivity. Based on the data obtained for the zeta potential and the surface conductivity, the surface charge of acidically functionalized monolayers was concluded to be compensated almost completely in the stagnant layer whereas for methyl-terminated monolayers a considerable part of the countercharge is localized in the diffuse, hydrodynamically mobile part of the electric double layer.

Streaming potential and streaming current measurements at planar solid/liquid interfaces for simultaneous determination of zeta potential and surface conductivity

Strongly encouraged and supported by Stanislav S. Dukhin the authors of this article recently designed, built and tested a new device for the simultaneous determination of zeta potential and surface conductivity from streaming potential and streaming current measurements across rectangular slit channels formed between two planar samples. In this Microslit Electrokinetic Set-up (MES) the planar samples are adjusted in parallel to form a channel of variable height which can become as narrow as about 1 μm. Due to this key feature of the device electrokinetic measurements can be performed at conditions where surface conductivity can be neglected and at conditions where surface conductivity provides a substantial part of the total channel conductivity. Utilizing the novel set-up, zeta potential and surface conductivity data can be obtained for a wide variety of materials which can be prepared as thin films on top of planar, macroscopic glass carriers. In order to demonstrate the potentialities of the advanced experimental technique of electrokinetic surface characterization we discuss three examples reflecting different levels of complexity of the analysed solid/liquid interface: (1) The charge formation at unpolar polymers without dissociating surface functions is studied referring to an inert, plasma-deposited fluoropolymer layer (PDFP) in simple electrolyte solutions. An extended evaluation of the experimental data of zeta potential and surface conductivity is given; (2) Further, grafted polypetide chains bearing dissociating side groups (polyglutamic acid and polylysine) were characterized with regard to the pH-depended variation of zeta potential and surface conductivity to provide new insights into the interrelation of charge density and conformation; (3) Finally, adsorbed fibrinogen on top of plasma-deposited fluoropolymer was studied by zeta potential and surface conductivity measurements as an example for highly hydrated macromolecular adsorption layers.

Electrokinetic Measurements Reveal Interfacial Charge at Polymer Films Caused by Simple Electrolyte Ions

The interfacial charge on spin-coated films of poly(tetrafluoroethylene-co-2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole) (Teflon AF) was studied by streaming potential and streaming current measurements in diluted aqueous solutions of potassium chloride, potassium hydroxide, and hydrochloric acid. ζ potential and surface conductivity were derived from electrokinetic data determined at varied concentrations of the electrolytes by means of the novel microslit electrokinetic setup (ref 1: J. Colloid Interface Sci. 1998, 208, 329). The results obtained revealed the high relevance of unsymmetrical (preferential) adsorption of ions as the origin of charge formation at unpolar polymer materials in aqueous environments. The preferential adsorption of hydroxide ions (OH-) was found to predominate as compared to the adsorption of hydronium ions (H3O+) at similar concentrations, i.e., in solutions of neutral pH. No effect of preferential adsorption was induced by chloride (Cl-) and potassium (K+) ions. For the...

Electrokinetic surface characterization of biomedical polymers — a survey

The application of streaming potential measurements in support of the development of blood compatible polymers is reviewed. Current knowledge about the relevance of interfacial charge of materials with respect to thrombogenicity of and coagulation activation by medical devices is discussed. Potentialities of streaming potential measurements for the characterization of biomedical polymers comprise the distinction between interfacial charge formation processes, the derivation of acid–base characteristics of surface functions, and the estimation of the relative contribution of known molecular surface sites to the interfacial charge density. Frequently, polymer surfaces consist of different types of surface sites. Beyond sites with Bronsted-acidic or -basic character the preferential adsorption of electrolyte ions onto indifferent surface sites may substantially contribute to the interfacial charge. Examples are given for the detection of acidic and basic properties of hemodialysis membranes of different shape made of poly(sulfone)/poly(vinylpyrrolidone) and cellulose derivates. For partially amine-modified cellulose flat membranes the percentage of basic surface sites is derived from the shift of the isoelectric point. Further, the use of streaming potential measurements is demonstrated in the optimization of surface modifications of cardiovascular implant materials by low pressure plasma treatments. The introduction of basic groups into the poly(tetrafluoroethylene) implant surfaces by ammonia plasma treatments manifests itself in the shift of the isoelectric point. A correlation between hydrophobicity and charge formation of the polymer–solution interface by preferential ion adsorption is confirmed for polymers without dissociating functions. This is based on the comparison of water contact angles for a series of poly(styrene) derivates of different hydrophobicity and the maximum zeta potential of these polymer films in potassium hydroxide solutions. Advanced applications of streaming potential measurements for the characterization of biomedical polymers are comprise the in situ characterization of protein adsorption processes onto polymer solids. This approach is demonstrated for the adsorption of the blood plasma proteins albumin and fibrinogen onto a fluorocarbon polymer substrate. The potentialities of a recently developed microslit electrokinetic setup are illustrated which permits to study simultaneously zeta potential and surface conductivity of flat polymer–liquid interfaces. Data gained by the microslit electrokinetic setup reveal the dramatic increase of the surface conductivity of a fluorocarbon polymer–solution interface caused by fibrinogen adsorption.

Extended Electrokinetic Characterization of Flat Solid Surfaces

An experimental setup has been developed and applied for the combined determination of the electrokinetic potential and the surface conductivity of flat surfaces. The key feature of the new device (designated as microslit electrokinetic setup) is the variability of the distance between two parallel flat sample surfaces (10 mm × 20 mm) forming a slit channel. The setup allows us to decrease this distance down to about 1 μm keeping the surfaces parallel. In consequence, streaming potential measurements can be performed at a given solid/liquid interface both at conditions where surface conductivity is negligible and at conditions where surface conductivity significantly contributes to the total channel conductivity. The zeta potential is calculated at different channel geometries based on streaming potential and channel conductivity data and, alternatively, based on streaming current measurements and the dimensions of the cross section of the slit channel. The results obtained were found to agree well if correct conductivity values for the calculation of the zeta potential based on the streaming potential data are used. The surface conductivity is determined from the extrapolation of the channel conductance values gained at a number of sufficiently small distances between the parallel sample surfaces to the distance zero. An additional feature of the developed microslit electrokinetic setup is the assessability of the hydrodynamic thickness of adsorbed layers of macromolecules or particles at the investigated flat surface. In a series of measurements a plasma-deposited fluoropolymer (PDFP) layer on top of a glass carrier and an adsorption layer of the blood protein fibrinogen on top of the PDFP layer were characterized by zeta potential and surface conductivity measurements in different aqueous electrolyte solutions (KCl, KOH, HCl). For the PDFP/solution interfaces zeta potential up to –100 mV were obtained in solutions of neutral pH exclusively due to preferential ion adsorption. After adsorption of fibrinogen the zeta potential is considerably reduced. For the PDFP/solution interfaces surface conductivities were determined in the range of (1–2) × 10−9S. The contribution of the diffuse layer to the surface conductivity has been calculated from the zeta potential according to the approach of Bikerman (Kolloid Z.72, 100 (1935)) and compared with the experimentally determined surface conductivity. Based on this comparison ions in hydrodynamically immobile interfacial layers were concluded to contribute considerably to the surface conductivity in all investigated cases. This so-called additional surface conductivity is attributed to the accumulation of hydroxide and hydronium ions in the Stern layer. Both the high specific mobility of these ions (as compared to the potassium and the chloride ions) and the conductivity of the charge determining species may contribute to the experimental observations. After adsorption of fibrinogen onto the PDFP surface the additional surface conductivity is increased by about an order of magnitude. The latter fact is assumed to be caused by the presence of mobile ions in the interfacial volume of the adsorbed protein layer. In addition to the electrochemical characterization of the adsorbed protein layer its hydrodynamic thickness has been determined by means of liquid flow measurements with the microslit electrokinetic setup. The obtained value of 48 ± 5 nm correlates well with the protein dimensions given in the literature and is in the order of magnitude of the optical layer extension determined by ellipsometry.