Bulk conductivity of soft surface layers: experimental measurement and electrokinetic implications

Conductivity measurements were carried out on a family of polyacrylamide-co-sodium acrylate gels cross-linked with N,N'-methylenebisacrylamide in a homemade electrokinetic cell. The conductivity data allowed the equilibrium Donnan potential difference between the bulk gel and the bulk electrolyte solution to be estimated at various ionic strengths. The data were fit to a simple model assuming full dissociation of functional groups as well as to a more complete model (Dukhin, S. S.; Zimmerman, R.; Werner, C. J. Colloid Interface Sci. 2004, 274, 309) that accounts for the weak electrolyte nature of the acrylate groups fixed within the gel structure. The conductivity of the gel layers was observed to be significantly larger than the conductivity of the bulk electrolyte solutions at low ionic strengths. The increased conductivity reflects the enhanced concentration of ions within the gel structure due to Donnan equilibrium and the mobility of ions within the high water content gel layers. The electrokinetic implications of the bulk conductivity of gel-like soft surface layers are discussed in terms of the influence of the gel conductance on the resulting streaming potential.     

Kinetic adhesion of bacterial cells to sand: cell surface properties and adhesion rate

Correlation between microbial surface thermodynamics using the extended DLVO (XDLVO) theory and kinetic adhesion of various bacterial cells to sand was investigated. Two experimental setups were utilized. Adhesion tests were conducted in batch reactors with slow agitation. Also, bacteria were circulated through small sand columns in a closed loop and the results were analyzed with a simple model which accounted for the rate of the adhesion phenomena (omega in h(-1)) and adhesion percentage. Cells surface properties were derived from contact angle measurements. The wicking method was utilized to characterize the sand. Zeta potentials were measured for the sand and the cells. Kinetic of bacterial retention by the porous media was largely influenced by the electrostatic interactions which are correlated with omega from the model (R(2)=0.71). Negative zeta potentials resulted in electrostatic repulsions occurring between the sand and the bacterial cells which in result delayed bacterial adhesion. While no correlation was found between the adhesion percentage and the total interaction energy calculated with the XDLVO theory the respective behavior of hydrophobic and hydrophilic bacteria as well as the importance of electrostatic interactions was evidenced. All the bacterial strains studied adhered more in the column experiments than in the adhesion tests, presumably due to enhanced collision efficiency and wedging in porous media, while filtration could be ignored except for the larger Bacillus strains. Approximate XDLVO calculations due to solid surface nanoscale roughness, retention in a secondary minimum and population heterogeneity are discussed. Our results obtained with a large variety of different physicochemical bacterial strains highlights the influence of both surface thermodynamics and porous media related effects as well as the limits of using the XDLVO theory for evaluating bacterial retention through porous media.     

Parameter identifiability in application of soft particle electrokinetic theory to determine polymer and polyelectrolyte coating thicknesses on colloids

Soft particle electrokinetic models have been used to determine adsorbed nonionic polymer and polyelectrolyte layer properties on nanoparticles or colloids by fitting electrophoretic mobility data. Ohshima first established the formalism for these models and provided analytical approximations ( Ohshima, H. Adv. Colloid Interface Sci.1995, 62, 189 ). More recently, exact numerical solutions have been developed, which account for polarization and relaxation effects and require fewer assumptions on the particle and soft layer properties. This paper characterizes statistical uncertainty in the polyelectrolyte layer charge density, layer thickness, and permeability (Brinkman screening length) obtained from fitting data to either the analytical or numerical electrokinetic models. Various combinations of particle core and polymer layer properties are investigated to determine the range of systems for which this analysis can provide a solution with reasonably small uncertainty bounds, particularly for layer thickness. Identifiability of layer thickness in the analytical model ranges from poor confidence for cases with thick, highly charged coatings, to good confidence for cases with thin, low-charged coatings. Identifiability is similar for the numerical model, except that sensitivity is improved at very high charge and permeability, where polarization and relaxation effects are significant. For some poorly identifiable cases, parameter reduction can reduce collinearity to improve identifiability. Analysis of experimental data yielded results consistent with expectations from the simulated theoretical cases. Identifiability of layer charge density and permeability is also evaluated. Guidelines are suggested for evaluation of statistical confidence in polymer and polyelectrolyte layer parameters determined by application of the soft particle electrokinetic theory.     

An AC electrokinetic technique for collection and concentration of particles and cells on patterned electrodes

We report an electrohydrodynamic effect arising from the application of alternating electric fields to patterned electrode surfaces. The AC fields were applied to dilute suspensions of latex microspheres enclosed between a patterned silicon wafer and an ITO-coated glass slide in a small chamber. The latex particles became collected in the center of the conductive "corrals" on the silicon wafer acting as bottom electrode. The particle collection efficiency and speed depended only on the frequency and strength of the field and were independent of the material properties of the particles or the electrodes. The leading effect in the particle collection process is AC electrohydrodynamics. We discuss how the electrohydrodynamic flows emerge from the spatially nonuniform field and interpret the experimental results by means of electrostatic and hydrodynamic simulations. The technique allows three-dimensional microfluidic pumping and transport by the use of two-dimensional patterns. We demonstrate on-chip collection of latex particles, yeast cells, and microbes.     

Significance of cell electrokinetic properties determined by soft-particle analysis in bacterial adhesion onto a solid surface

The influence of extracellular polymeric substances (EPSs) on bacterial cell electrokinetic properties and on cell adhesion onto glass beads in connection with bacterial cell electrokinetic properties was investigated using 12 heterotrophic bacterial strains. Bacterial cell surface properties such as the softness 1/lambda and charge density ZN were determined by Ohshima's soft-particle analysis using the measured electrophoretic mobility as a function of ionic strength. In 10 of 12 strains, when EPSs covering the cell surface were removed, the softness of the cell decreased, indicating that EPS adsorption enhanced the ease of liquid fluid in the ion-penetrable layer on the cell surface. On the other hand, the negative charge density of the cell surface increased for 9 of 12 strains, suggesting that EPSs covering the cell surface decreased the negative charge density of the cell surface layer. In addition, the characteristics of bacterial cell adhesion onto glass beads were evaluated by the packed-bed method and the data were interpreted to indicate cell adhesiveness. As a result, the efficiency of cell adhesion onto glass beads increased as negative cell surface potential psi0 decreased, whereas there seemed to be no correlation between zeta potential and cell adhesiveness. Cell surface potential psi0, which was derived by taking the bacterial polymer layer with EPSs into consideration, provided a more detailed understanding of the electrokinetic properties of bacterial cells.     

Focused electrophoretic motion and selected electrokinetic dispensing of particles and cells in cross-microchannels

The electrokinetic focusing and the resultant accelerated electrophoretic motion of polystyrene particles and red blood cells were visualized in microfluidic cross-channels. The experimentally measured width of the focused stream and the measured velocity increase of particles and cells at different voltage ratios follow the proposed analytical formula within the experimental error. The attained velocity increase is insensitive to the particle size, particle property (i.e., particle or cell), and particle trajectory. By solving the electrical potential field in the cross-channel at the experimental conditions, we demonstrate that the squeezed electrical field lines in the channel intersection determine the shape of the focused stream, and the nonuniform distribution of axial electrical field strength underlies the variation of particle/cell electrophoretic velocity through the focusing region. However, the dielectrophoretic force resulting from the nonuniform electrical field in the intersection seems to push the acceleration region of particles and cells slightly in the downstream direction. We have also achieved the single particle/cell dispensing by instantly triggering an electrical pulse perpendicular to the focused particulate flow in a double-cross microchannel. The electrokinetic manipulation of particle/cell in microchannels demonstrated in this work can be used for developing integrated lab-on-a-chip devices for studies of cells.     

Electrophoresis of soft particles at high electrolyte concentrations: an interpretation by the Henry theory

The existence of electrophoretic mobility at high electrolyte concentrations defines a remarkable peculiarity in the electrosurface characteristics of soft particles. According to Ohshima [H. Ohshima, Colloids Surf. 103 (1995) 249], this effect is caused by the electroosmotic flow within the soft particle shell. An explanation supporting Ohshima's conclusion can be derived from classic electrokinetic theories. Based on the Henry theory [D.C. Henry, Proc. R. Soc. London Ser. A 133 (1931) 106], we demonstrate that the electrophoretic mobility of soft particles does not disappear at decinormal concentration.     

Electrophoresis of soft particles: analytic approximations

An approximate analytic expression is derived for the electrophoretic mobility of a weakly charged spherical soft particle (i.e., a hard particle covered with a weakly charged polyelectrolyte layer) on the basis of the general mobility expression for soft particles (Ohshima, H., J. Colloid Interface Sci. 2000, 228, 190-193). The obtained mobility expression, which reproduces various approximate results so far derived and gives some new mobility formulas, covers all types of weakly charged soft particles with arbitrary values of the thickness of polymer layer, the radius of the particle core, the electrophoretic softness, and the Debye length, including spherical polyelectrolytes with no particle core as well as spherical hard particles with no polyelectrolyte layer.     

Origin and control of adhesion between emulsion drops stabilized by thermally sensitive soft colloidal particles

We used soft microgels made of poly(N-isopropylacrylamide) (pNIPAM) of variable cross-linking degrees and the same colloidal size to stabilize oil-in-water Pickering emulsions. The extent of droplet flocculation increased and the resistance of the emulsions to mechanical stresses decreased as the cross-linking density was augmented. Large flat films were separating the droplets, and we could measure the adhesion angle at the junction with the free interfaces through several microscopy methods. The size of the flat films and the values of the angles were reflecting strong adhesive interactions between the interfaces as a result of microgel bridging. In parallel, cryo-SEM imaging of the thin films allowed a precise determination of their structure. The evolution of the adhesion angle and of the film structure as a function of microgels cross-linking density provided interesting insights into the impact of particle softness on film adhesiveness and emulsion stability. We exploited our main findings to propose a novel route for controlling the emulsions end-use properties (flocculation and stability). Owing to particle softness and thermal sensitivity, the interfacial coverage was a path function (it depended on the sample "history"). As a consequence, by adapting the emulsification conditions, the interfacial monolayer could be trapped in a very dense and rigid configuration, providing improved resistance to bridging flocculation and to flow-induced coalescence.     

Surface modification of hydroxyapatite to avoid bacterial adhesion

Hydroxyapatite was surface-modified by adsorption of nonionic polymers carrying phosphate groups as anchoring groups. A combination of alcohol ethoxylate and alkyl phosphate was also used. The possibility of interfering with early microbial colonization on apatite, mimicking the tooth surface, was investigated using radiolabelledStreptococcus mutans as model bacteria. The polymers, a nonionic cellulose ether and an EO/PO block copolymer based on polyglycerol as starting alcohol, were effective in buffer but gave only limited reduction of bacterial adhesion when the apatite had been pretreated with saliva. A 11 molar mixture of alcohol ethoxylate and alkyl phosphate was effective both with and without saliva, however. Studies with¹⁴C-labeled compounds, as well as microelectrophoresis experiments, indicate that an unsymmetrical double layer is formed on the apatite surface with predominantly alkyl phosphate in the inner layer and with alcohol ethoxylate pointing towards the water phase.     

Quantitative analysis of bacterial adhesion to fish tissue

Adhesion to host tissue represents a first crucial step in most bacterial infections. Both specific adhesion-ligand as well as hydrophobic interactions may be involved. The adhesion of Aeromonas salmonicida subsp. salmonicida, Lactococcus garvieae, and Yersinia ruckeri strains to fish tissue cells was assessed. To determine whether the observed bacterial adhesion to fish tissue cells was caused by non-specific interactions, adhesion to bovine serum albumin (BSA) and polystyrene was also tested. Our results demonstrated that non-specific adhesion such as hydrophobic interactions are only partially involved in the binding process since adhesion to BSA was low, and there was no correlation between adhesion to polystyrene and adhesion to fish tissue cells.     

Effects of dielectric gradients-mediated ions partitioning on the electrophoresis of composite soft particles: An analytical theory

In this work, we report original analytical expressions defining the electrophoretic mobility of composite soft particles comprising an inner core and a surrounding polymer shell with differentiated permeabilities to ions from aqueous background electrolyte and to fluid flow developed under applied DC field conditions. The existence of dielectric permittivity gradients operational at the core/shell and shell/solution interfaces is accounted for within the Debye–Hückel approximation and flat plate configuration valid in the thin double layer regime. The proposed electrophoretic mobility expressions, applicable to weakly to moderately charged particles with size well exceeding the Debye layer thickness, involve the relevant parameters describing the particle core/shell structure and the electrohydrodynamic features of the core and shell particle components. It is shown that the analytical expressions reported so far in literature for the mobility of hard (impermeable) or porous particles correspond to asymptotic limits of the more generic results detailed here. The impacts of dielectric-mediated effects of ions partitioning between bulk solution and particle body on the electrophoretic response are further discussed. The obtained expressions pave the way for a refined quantitative, analytical interpretation of electrophoretic mobility data collected on soft (nano)particles (e.g., functionalized dendrimers and multilayered polyelectrolytic particles) or biological cells (e.g., viruses) for which the classical hard core-soft shell representation is not appropriate.     

The electrokinetic properties of latex particles: comparison of electrophoresis and dielectrophoresis

A comprehensive study of the AC and DC electrokinetic properties of submicrometre latex particles as a function of particle size and suspending medium conductivity and viscosity is presented. Electrophoretic mobility and dielectrophoretic cross-over results were measured for particle diameters ranging from 44 to 2000 nm. The zeta potentials of the particles were calculated from the electrophoretic mobility data for different suspending medium conductivities, using various models, with and without the inclusion of surface conduction. The dielectrophoretic data was analysed to derive values for the Stern layer conductance and zeta potentials.     

A high-throughput microfluidic biochip to quantify bacterial adhesion to single host cells by real-time PCR assay

A high-throughput microfluidic poly-(dimethylsiloxane) biochip was developed to quantify bacterial adhesion to single host cells by real-time PCR assay. The biochip is simply structured with a two-dimensional array of 900 micro-wells, one inlet, and one outlet micro-channels. Isolation of single infected host cells into the individual micro-wells of the biochip was achieved by one-step vacuum-driven microfluidics. The adhered bacterial cells were then quantified by direct on-chip real-time PCR assay with single-bacterium-detection sensitivity. The performance of this microfluidic platform was demonstrated through profiling of the association of a common bacterial pathogen, Pseudomonas aeruginosa, to single host human lung epithelial A549 cells, revealing an adherence distribution that has not been previously reported. This microfluidic platform offers a simple and effective tool for biologists to analyze pathogen–host interaction at the single-cell level without the necessities of fluorescence labeling. The chip can similarly be used for other PCR-based applications requiring single-cell analysis.     

Long-time self-diffusion of charged colloidal particles: electrokinetic and hydrodynamic interaction effects

The authors analyze the long-time self-diffusion of charge-stabilized colloidal macroions in nondilute suspensions using a mode-coupling scheme developed for multicomponent suspensions of interacting Brownian spheres. In this scheme, all ionic species, including counterions and electrolyte ions, are treated on an equal footing as charged hard spheres undergoing overdamped Brownian motion. Hydrodynamic interactions between all ions are accounted for on the far-field level. We show that the influence on the colloidal long-time self-diffusion coefficient arising from the relaxation of the microionic atmosphere surrounding the colloids, the so-called electrolyte friction effect, is usually insignificant in comparison with the friction contributions arising from direct and hydrodynamic interactions between the colloidal particles. This finding is true even for small colloid concentrations unless the mobility difference between colloidal particles and microions is not large. Furthermore, we observe an interesting nonmonotonic density dependence of the colloidal long-time self-diffusion coefficient in suspensions with low amount of added salt. We show that this unusual density dependence is due to colloid-colloid hydrodynamic interactions.     

Recent advances in direct current electrokinetic manipulation of particles for microfluidic applications

Microfluidic devices have been extensively used to achieve precise transport and placement of a variety of particles for numerous applications. A range of force fields have thus far been demonstrated to control the motion of particles in microchannels. Among them, electric field‐driven particle manipulation may be the most popular and versatile technique because of its general applicability and adaptability as well as the ease of operation and integration into lab‐on‐a‐chip systems. This article is aimed to review the recent advances in direct current (DC) (and as well DC‐biased alternating current) electrokinetic manipulation of particles for microfluidic applications. The electric voltages are applied through electrodes that are positioned into the distant channel‐end reservoirs for a concurrent transport of the suspending fluid and manipulation of the suspended particles. The focus of this review is upon the cross‐stream nonlinear electrokinetic motions of particles in the linear electroosmotic flow of fluids, which enable the diverse control of particle transport in microchannels via the wall‐induced electrical lift and/or the insulating structure‐induced dielectrophoretic force.     

Affinity adhesion of carbohydrate particles and yeast cells to boronate-containing polymer brushes grafted onto siliceous supports

Cross-linked agarose particles (Sepharose CL-6B) and baker's yeast cells were found to adhere to siliceous supports end-grafted with boronate-containing copolymers (BCCs) of N,N-dimethylacrylamide at pH> or =7.5, due to boronate interactions with surface carbohydrates of the particles and the cells. These interactions were registered both on macroscopic and on molecular levels: the BCCs spontaneously adsorbed on the agarose gel at pH> or =7.5, with adsorption increasing with pH. Agarose particles and yeast cells stained with Procion Red HE-3B formed stable, monolayer-like structures at pH 8.0, whereas at pH 7.0-7.8 the structures on the copolymer-grafted supports were less stable and more random. At pH 9.0, 50 % saturation of the surface with adhering cells was attained in 2 min. Stained cells formed denser and more stable layers on the copolymer-grafted supports than they did on supports modified with self-assembled organosilane layers derivatized with low-molecular-weight boronate, presumably due to a higher reactivity of the grafted BCCs. Quantitative detachment of adhered particles and cells could be achieved by addition of 20 mM fructose--a strong competitor for binding to boronates--at pH 7.0-9.0. Regeneration of the grafted supports allowed several sequential adhesion and detachment cycles with stained yeast cells. Affinity adhesion of micron-sized carbohydrate particles to boronate-containing polymer brushes fixed on solid supports is discussed as a possible model system suggesting a new approach to isolation and separation of living cells.     

Influence of sialic acid and bacterial sialidase on differential adhesion of Pseudomonas aeruginosa to epithelial cells

Epithelial cell lines from several tissues show a differential sensitivity to Pseudomonas aeruginosa adherence. A549 (lung), HepG2 (liver) and Caco-2 (colon) cells presented an adhesion index of about 3, 1.5 and 5 CFU/cell, respectively, whereas Mz-Ch cell lines (gallbladder cholangiocytes) presented adhesion indexes up to 35. These variations could be associated with the variable amount of sialic acid in cell surface glycoconjugates. Moreover, the presence of free sialic acid in culture media induces the secretion by P. aeruginosa of a sialidase which is able to hydrolyze glycoconjugate-linked sialic acid. As shown with A549 cells, this specific hydrolysis increases bacterial adhesion, probably by unmasking new binding sites onto the cell surface.     

Numerical study of colloidal suspensions of soft spherical particles using the network method. 2. AC electrokinetic and dielectric properties

The network simulation method is used to solve numerically the equation system that determines the dynamic electrophoretic mobility and the dielectric response of dilute suspensions of soft particles. This system was extensively studied theoretically by Ohshima (H. Ohshima, J. Colloid Interface Sci. 233 (2001) 142-152), who obtained analytical expressions for the static and dynamic electrophoretic mobility. However, the validity of his analytical result is restricted to relatively thick membranes with high drag coefficient and to relatively high electrolyte concentrations. As for the dielectric properties, there are only a few works dealing with particles without a core (ion exchange resins) and, to our knowledge, no numerical studies. Our theoretical model is basically similar to Ohshima's, except that we take into account the mechanical force acting on the surface of the core, which he neglects. The inclusion of this term is crucial when the general problem including arbitrary values of the parameters is analyzed. However, it has little bearing when the membrane is thick and the drag coefficient is high, so that our results for the electrophoretic mobility generally confirm Ohshima's equation when all the required conditions are met.     

Spherical molecularly imprinted polymer particles: a promising tool for molecular recognition in capillary electrokinetic separations

Spherical molecularly imprinted polymer particles obtained via precipitation polymerization, were introduced as a pseudostationary phase in capillary electrophoresis (CE) to study molecular recognition. Analyses were performed via a partial filling technique using (+)-ephedrine-imprinted microspheres (100-200 nm) which were polymerized from methacrylic acid and 1,1,1-Tris(hydroxymethyl)propanetrimethacrylate using acetonitrile as the solvent. The influence of pH and the modifier content on the separation was investigated. A 0.1% w/v suspension in an aqueous 10 mM phosphate buffer (pH 2.5 with 40% acetonitrile) was hydrodynamically injected into the CE system (80% of the effective capillary length) and led to full baseline separation of racemic ephedrine within 10 min.