Electrokinetics of soft polymeric interphases with layered distribution of anionic and cationic charges

Soft surface coatings attract increasing attention due to the versatile options they provide in numerous applications e.g. in the flourishing nanomedicine and nanobiotechnology areas. Optimisation of the performance of such ion- and solvent-permeable polyelectrolytic materials requires a detailed understanding of their electrostatic properties. This task is rendered difficult by the inherent non-uniform distribution of their structural charges. In this article, we review recent advances made in the measurement and theory of the electrokinetics (electrophoresis/streaming current) of soft surface coatings that carry spatially-separated cationic and anionic charges. Examples of such charge-stratified systems are polyelectrolyte-coated particles, polyelectrolyte multilayers, particles with zwitterionic interfacial functionality, microbial cells or hard–soft composite interfaces. It is shown here that the electrokinetic features of such colloidal systems are remarkably different from those of their counterparts with homogeneously distributed cationic and anionic charges. In particular, the interplay between electrostatic and hydrodynamic flow fields developed under electrokinetic conditions in the bulk and interfacial compartments of charge-stratified colloids/films are shown to induce a reversal of their electrokinetic response (electrophoretic mobility/streaming current) that depends on the concentration of monovalent electrolyte in solution. The prerequisites for occurrence of such spectacular behaviour are theoretically identified in terms of the Debye length, the spatial length scales defining charge layering, and the typical length for flow penetration within the colloids/films. Electrophoresis and streaming current results recently reported for poly(amidoamine) carboxylated nanodendrimers, natural rubber colloids and poly(ethyleneimine)-supported lipid bilayers are further discussed to illustrate the generic electrokinetic properties of soft interfaces defined by a given stratification of their anionic and cationic structural charges.     

Electrokinetics with blood

Microfluidics based lab‐on‐a‐chip technology holds tremendous promises towards point‐of‐care diagnosis of diseases as well as for developing engineered devices aimed towards replicating the intrinsic functionalities of human bodies as mediated by blood vessel mimicking circulatory networks. While the analysis of transport of blood including its unique cellular constituents has remained to be the focus of many reported studies, a progressive interest on understanding the interplay between electric field and blood flow dynamics has paved a new way towards further developments from scientific engineering as well as clinical viewpoint. Here, we briefly outline the interconnection between electrokinetics and blood flow through micro‐capillaries, in an effort to address several challenging propositions in a wide variety of applications encompassing biophysical transport to medical diagnostics. We first present the fundamentals of interaction of electric field with cellular components. In conjunction with the unique rheological features of blood, we show that this interaction may turn out to be compelling for the use of electric fields for transporting blood samples through microfluidic conduits. We discuss the perspectives of both direct current and alternating current electrokinetics in the context of blood flow. In addition, we provide a brief outline of the concerned theoretical developments. We also bring out the relevant biophysical perspectives and focus on applications such as blood plasma separation and separation of circulatory tumor cells. Finally, we attempt to provide a futuristic outlook and envisage the potential of combining electrokinetics with blood microcirculation towards developing futuristic biomimetic microdevices that can replicate a novel control mechanism over micro‐circulatory transport in the entire connective network of human bodies. This may effectively pave the way towards the realization of a next‐generation medical simulation device, significantly advanced from what is available under the ambit of the state of art technology in the field.     

Electrokinetics in extremely bimodal suspensions

Prompted by the results obtained by Mantegazza et al. [Nature Physics 1 (2005) 103], where the electric birefringence of suspensions of elongated particles was strikingly affected by the presence of a sea of very small (size ratio lower than 10:1) colloidal spheres, we have undertaken an investigation of other electrokinetic phenomena in suspensions containing various relative concentrations of large (Teflon or polystyrene latex) and small (nanometer-sized silica spheres) colloids. We have determined the quantities that might be greatly affected by the size distribution of the particles, mainly in the presence of ac electric fields, since the response of the suspensions will show very characteristic relaxations, dominated in principle by the size of the particles. In this work, we report on measurements of the dielectric dispersion of mixed particles as a function of the concentration, ionic strength, and field frequency. The results indicate that the response is not just a simple combination of those obtained with suspensions of the individual particles, and in fact the presence of even small amounts of the small particles affects considerably the frequency response of the suspensions.     

Electrokinetics of soft particles

Theories of electrokinetics of soft particles, which are particles covered with an ion-penetrable surface layer of polyelectrolytes, are reviewed. Approximate analytic expressions are given, which describe various electrokinetics of soft particles both in dilute and concentrated suspensions, that is, electrophoretic mobility, electrical conductivity, sedimentation velocity and potential, dynamic electrophoretic mobility, colloid vibration potential, and electrophoretic mobility under salt-free condition.     

Electrokinetics over hydrophobic surfaces

Segregation of phases in the vicinity of hydrophobic surfaces may turn out to be immensely consequential towards altering the coupling of electrostatics and hydrodynamics over interfacial scales. Here, we review the fundamental advances towards bringing out the various facets of electrokinetic transport over hydrophobic interfaces. We lay significant emphasis on the developments in understanding the slippery electrohydrodynamics over such interfaces, by appealing to the considerations across various spatio‐temporal scales as unveiled by molecular dynamics as well as mesoscopic modelling paradigms (such as phase field and lattice Boltzmann). We envisage that despite significant advancements being achieved towards relating the macroscopic slip‐length with the underlying molecular or mesoscopic phenomena, future efforts could be directed towards developing more robust statistically based models that may connect rarefied gas dynamics in the segregated phase with bulk electrokinetic transport and possible giant augmentations in the consequent fluid flow.     

Deformation effects on ionic transport in polymeric films

When a periodical deformation is applied to a cellophane film through which an ion flux is established by a concentration gradient, an electrical signal, synchronous with the deformation, can be detected on electrodes placed on both sides of the membrane. This signal was studied as a function of the amplitude and frequency of the deformation and the ionic concentration. Ionic resistance, permeability, water content and ionic concentration measurements were carried out in order to explain the particular dependence of the signal with the ionic concentration. © 1994 John Wiley & Sons, Inc.     

Ibuprofen induced drug loaded polymeric micelles

Three model drugs with different function groups were chosen to dialyze with dextran-graft-poly(N-isopropylacrylamide).Only ibuprofen could induce the formation of drug loaded micelles,which was confirmed with dynamic light scattering and transmission electron microscope.Hydrogen-bonding between the amide groups of poly(N-isopropylacrylamide) and the carboxyl groups of ibuprofen was driving force for the drug-loaded micelle.It was also found that the diameter of the ibuprofen-loaded micelles changed reversibly against temperature.     

Electrokinetics in antimicrobial resistance analysis: A review

Infections caused by antimicrobial resistance are a serious problem in the world. Currently, commercial devices for antimicrobial susceptibility testing and resistant bacteria identification are time-consuming. There is an urgent need to develop fast and accurate methods, especially in the process of sample pretreatment. Electrokinetic (EK) is a family of electric-field-based kinetic phenomena of fluid or embedded objects, and EK applications have been found in various fields. In this paper, EK bacteria manipulation, including enrichment and separation, is reviewed. Focus is given to the rapid electric-based minimum inhibitory concentration measurement. The future directions and major challenges in this field are also outlined.     

Gas transport through homogeneous and asymmetric polyethersulfone membranes

Both homogeneous and asymmetric polyethersulfone (PES) membranes were prepared by solvent casting. The sorption and permeation behavior of CO₂, O₂, and N₂ using these two kinds of cast PES membranes and commercially available homogeneous PES film was investigated to extract the pressure dependence of gas permeability and the permselectivity for CO₂ relative to N₂, and to confirm the validity of the working assumption that a skin layer in an asymmetric membrane can be essentially replaced by a thick homogeneous dense membrane. The pressure dependence of the mean permeability coefficient to CO₂ in homogeneous membranes obeys the dual-mode mobility model. The ideal separation factor for CO₂ relative to N₂ at an upstream pressure of 0.5 MPa attains ca. 40, while the permeability to CO₂ is about 2.7 Barrer at the same upstream pressure. The same separation factor in asymmetric membranes amounts to 35. The diffusion behavior for the skin layer in an asymmetric membrane with a thin skin layer can be simulated approximately by that in a homogeneous dense membrane. © 1993 John Wiley & Sons, Inc.     

Electrokinetics in microfluidic channels containing a floating electrode

Electrokinetic transport within a buffer-filled microchannel incorporating a flat bipolar electrode is investigated. The key finding is that the presence of the electrode disrupts the passage of electrical current through the microchannel and thereby alters the uniformity of the local electric field. Electroosmotic flow further modulates the local field gradient. These dynamics are demonstrated experimentally by utilizing the field gradient for concentration enrichment of negatively charged tracer molecules, and a set of computer simulations is presented to interpret the underlying electrokinetics.     

Functional polymers. 6. Unusual catalysis of polymeric cinchona alkaloids in asymmetric reaction

Acrylonitrile-cinchona alkaloid copolymers catalyzed the asymmetric addition of benzyl mercaptan to 2-nitrostyrene to give always (+)-enantiomer in excess, indicating the participation of C(3)-chirality in the enantioface differentiating step.     

Defect induced asymmetric pit formation on hydroxyapatite

Defect sites on bone minerals play a critical role in bone remodeling processes. We investigated single crystal hydroxyapatite (100) surfaces bearing crystal defects under acidic dissolution conditions using real-time in situ atomic force microscopy. At defect sites, surface structure-dependent asymmetric hexagonal etch pits were formed, which dominated the overall dissolution rate. Meanwhile, dissolution from the flat terraces proceeded by stochastic formation of flat bottom etch pits. The resulting pit shapes were intrinsically dictated by the HAP crystal structure. Computational modeling also predicted different step energies associated with different facets of the asymmetric etch pits. Our microscopic observations of HAP dissolution are significant for understanding the effects of local surface structure on the bone mineral remodeling process and provide useful insights for the design of novel therapies for treating osteoporosis and dental caries.     

2-Chlorotetrafluoroethanesulfinamide induced asymmetric vinylogous Mannich reaction

The generality of (S)-2-chlorotetrafluoroethanesulfinamide (CTFSA) induced asymmetric vinylogous Mannich (AVM) addition reaction has been investigated. The reaction of aldimines derived from (S)-CTFSA with 2-(tert-butyldimethylsilyloxy) furan (TBSOF) took place regioselectively at the γ-site at room temperature in DMSO under air atmosphere to give the desired addition products in syn-configuration with high diastereoselectivities (up to 98:2 dr). However, anti-configuration product as major isomer was obtained in the presence of tetra-butyl ammonium fluoride (TBAF) at −78 °C. Facile removal of the auxiliary group without epimerization demonstrated their synthetic potential for piperidone derivatives.     

Alkaloid induced asymmetric electrocarboxylation of 4-methylpropiophenone

The alkaloid-induced electrocarboxylation of 4-methylpropiophenone is examined in mild conditions. Comparative studies with several inductors indicate that the efficient enantiodiscrimination of the electrocarboxylation depends on the nucleophilic quinuclidine nitrogen atom and the OH group of the inductors.     

Continuous-Flow Nanoparticle Trapping Driven by Hybrid Electrokinetics in Microfluidics

We introduce herein an efficient microfluidic approach for continuous transport and localized collection of nanoparticles via hybrid electrokinetics, which delicately combines linear and nonlinear electrokinetics driven by a composite DC-biased AC voltage signal. The proposed technique utilizes a simple geometrical structure, in which one or a series of metal strips serving as floating electrode (FE) are attached to the substrate surface and arranged in parallel between a pair of coplanar driving electrodes (DE) in a straight microchannel. On application of a DC-biased AC electric field across the channel, nanoparticles can be transported continuously by DC bulk electroosmotic flow, and then trapped selectively onto the metal strips due to AC-field induced-charge electrokinetic (ICEK) phenomenon, which behaves as counter-rotating micro-vortices around the ideally polarizable surfaces of FE. Finite-element simulation is carried out by coupling the dual-frequency electric field, flow field and sample mass transfer in sequence, for guiding a practical design of the microfluidic nanoparticle concentrator. With the optimal device geometry, the actual performance of the technique is investigated with respect to DC bias, AC voltage amplitude, and field frequency by using both latex nanospheres (∼500 nm) and BSA molecules (∼10 nm). Our experimental observation indicates nanoparticles are always enriched into a narrow bright band on the surface of each FE, and a horizontal concentration gradient even emerges in the presence of multiple metal strips, which therefore permits localized analyte enrichment. The proposed trapping method is supposed to guide an elaborate design of flexible electrokinetic frameworks embedding FE for continuous-flow analyte manipulation in modern microfluidic systems.     

Polyelectrolyte entry and transport through an asymmetric alpha-hemolysin channel

We study the entry and transport of a polyelectrolyte, dextran sulfate (DS), through an asymmetric alpha-hemolysin protein channel inserted into a planar lipid bilayer. We compare the dynamics of the DS chains as they enter the channel at the opposite stem or vestibule sides. Experiments are performed at the single-molecule level by using an electrical method. The frequency of current blockades varies exponentially as a function of applied voltage. This frequency is smaller for the stem entrance than for the vestibule one, due to a smaller coupling with the electric field and a larger activation energy for entry. The value of the activation energy is quantitatively interpreted as an entropic effect of chain confinement. The translocation time decreases when the applied voltage increases and displays an exponential variation which is independent of the stem or vestibule sides.