Ion Concentration Polarization Research Articles

Impact of surface hydrophobicity and ion steric effects on the electroosmotic flow and ion selectivity of a conical nanopore

The electroosmotic flow (EOF) and ion transport within a hydrophobic conical nanopore connecting reservoirs are studied numerically. The ion transport equation is modified to consider the ion steric effect for a finite ion size. The continuum-based model compiled with the modified Poisson–Nernst–Planck and Navier–Stokes equations are adopted for analyzing the hydrodynamics and electric field. The governing equations in their full form are solved in a coupled manner through a control volume approach over a nonuniform staggered grid arrangement. Analytic solutions for the EOF in the hydrophobic nanopore with large membrane thickness is obtained by including the volume exclusion effects in the electrochemical potential of the ionic species. The analytic solutions are obtained by considering the steric interactions based on either the Bikerman model or the Carnahan-Starling (CS) model. However, the numerical solutions are presented based on the CS model for steric interactions. The present numerical results for the hydrophobic nanopore with large pore length are validated with these analytic solutions and existing correlations based on experimental data. The fluid slip at the pore surface modifies the nanopores electrokinetic characteristics by modifying the convective transport of fluid and ions. We have elucidated the impact of pore hydrophobicity and ion steric interactions on the EOF, ionic current, ion concentration polarization (ICP) and ion selectivity when the pore length is considered to be in the order of the pore radius. The effect of pore length on the electrokinetics in the hydrophobic nanopore is also analyzed. Our numerical solution for large pore length merged with the analytic expression for the EOF in the hydrophobic nanopore of length much higher than the radius. Hydrophobicity creates significant enhancement in EOF and the rate of EOF augmentation is higher for the thinner Debye length. The perm-selectivity performance of the nanopore is enhanced when the wall is considered to be hydrophobic. The steric interactions of ions create an enhancement in EOF, ion selectivity, and ICP. The pore hydrophobicity diminishes the impact of these steric interactions.

Temperature-depended ion concentration polarization in electrokinetic energy conversion

Previous studies on the electrokinetic energy conversion (EKEC) are limited to the isothermal condition at the environmental temperature. Here effects of temperature and membrane thermal conductivity are systematically investigated. Under isothermal conditions, elevated temperature can improve the electric power while the energy efficiency stays unchanged. Under non-isothermal conditions, at small membrane thermal conductivities, a negative temperature difference contributes to the electric power for dramatically enhanced streaming current as enhanced ion mobility along the streaming direction induces an internal ion concentration polarization (IICP) that generates a co-flow concentration gradient in the nanopore interior. At large membrane thermal conductivities, the positive temperature difference reverses the external ion concentration polarization (EICP) in the solution reservoirs due to the Soret effect, resulting in more obvious electric power improvement. Furthermore, a criterion to enhance the EKEC performance via employing asymmetric temperatures is proposed, and an alternative way to construct the tunable ionic current source is presented. Present study provides guidance for enhancing the EKEC performance by employing waste heat, and fabricating nanofluidic functional devices.

Concentration Polarization of High Concentration Solution in Sub-nm Nanopore

The behavior of ion transport through the sub-nm nanopores on the film is is different from the behavior of bulk behavior. Many intriguing phenomena in ionic transport are the key to the design and fabrication of solid-state nanofluidic devices. However, ion transport through the sub-nm nanopores is not yet clearly understood. We investigate ionic transport of sub-nm nanopore from the perspective of conductance by the method of MD. The results show that the ion concentration polarization phenomenon is heavily dependent on the external electric field and the size constraints of nanopores. At the same time, ion concentration polarization also has a profound effect on ion conductance. These conclusions indicate that ion concentration polarization has an important influence on ion transport, and help a new understanding of the design of nanofluidic devices.

Origami paper-based sample preconcentration using sequentially driven ion concentration polarization.

Ion concentration polarization (ICP) is one of the preconcentration techniques which can acquire a high preconcentration factor. Still, the main hurdles of ICP are its instability and low efficiency under physiological conditions with high ionic strength and abundant biomolecules. Here, we suggested a sequentially driven ICP process, which enhanced the electrokinetic force required for preconcentration, enabling enrichment of highly ionic raw samples without increasing the electric field. We acquired a 13-fold preconcentration factor (PF) in human serum using a paper-based origami structure consisting of multiple layers for three-dimensional sequential ICP (3D seq-ICP). Moreover, we demonstrated a paper-based enzyme-linked immunosorbent assay (ELISA) by 3D seq-ICP using tau protein, showing a 6-fold increase in ELISA signals.

Exceeding ohmic scaling by more than one order of magnitude with a 3D ion concentration polarization system.

We report an ion concentration polarization (CP) system that exceeds ohmic scaling, a barrier that has stood for more than four decades, by more than one order of magnitude. CP is used in many important applications, including the enrichment of trace analytes in microfluidic systems and water purification by electrodialysis. The mechanisms that control the current through these systems have been largely discovered, but the reduced currents and loss of efficiency imparted by the high resistance of the CP ion depleted zone have not been overcome. To obtain high currents, an ion permselective element with a microscale cross-section is interfaced with a macroscale reservoir. Confocal fluorescence microscopy and microparticle tracking velocimetry (μ-PTV) are used to characterize the depleted zone that emanates vertically from the CP inducing nanoporous gel into the macroscale reservoir. The shape and growth of the depleted zone and velocity in the surrounding bulk solution are consistent with natural convection being the driver of the depleted zone morphology and eliminating the high resistance created by the depleted zone in 1D and 2D systems. Once the resistance of the depleted zone is negated, the high currents are hypothesized to result from enhancement of counter-ion concentration in the nanoporous gel-filled microchannel. In contrast with conventional systems, the current increases monotonically and remains stable at a high quasi-steady level in the reported systems. These results may be used to increase the efficiency and performance of future devices that utilize CP, while the ability to collect purified water with this geometry is demonstrated.

Electro-osmo-dialysis through nanoporous layers physically conjugated to micro-perforated ion-exchange membranes: Highly selective accumulation of trace coions

This study explores theoretically and numerically the phenomenon of counter-flow accumulation of trace coions during electro-osmo-dialysis through composite membranes containing nanoporous and perforated ion-exchange layers. The combination of charged nanopores and perforations allows electroosmosis, and ion concentration polarization appears near the ion-exchange layer. Accumulation of trace coions occurs because their advective and electromigration flow velocity components have opposite signs and are locally compensating in depletion regions where electric fields are high. This local compensation of flow components can give rise to high accumulation factors (>1000 in some cases) for trace coions at some distance from the interface between the nanoporous and ion-exchange layers. Moreover, the extent of this accumulation depends on the trace-coion diffusion coefficient. The resulting selectivity between different coions might serve as a basis for environmentally friendly separation of ionic species such as synthetic peptides or rare-earth elements. For example, a periodic process might include accumulation followed by species release into smaller volumes of solution. For both rare-earth ions and peptide-like species, the accumulation selectivity between ions with small differences in mobilities depends on system parameters such as nanoporous-layer thickness, distance between the perforations, and the zeta potential of the nanopore surface. The use of optimal values of these membrane parameters may enable tuning of the electro-osmo-dialysis system for separation of specific ions.Modelling reveals that despite inhomogeneous patterns of fluid and ion flows close to the nanoporous/perforated ion-exchange interface, at relatively short distances from this interface (less than half the distance between the perforations) all the flows become practically one-dimensional (1D). Similarly, the concentration profiles of trace coions also become 1D. The flow one-dimensionality at short distances from the current-polarized interface enables a simple 1D approximation that is useful for an approximate analysis of trends in this multi-parameter system.

Highly efficient and scalable biomarker preconcentrator based on nanoelectrokinetics

Micro/nanofluidics are excellent candidates for biological sample preparation. However, the limited process volume in micro/nanofluidics is the main hurdle limiting their practical applications. To date, most micro/nanofluidics have processed sample volumes of several microliters and have rarely been used to handle large-volume samples. Herein, we propose a microfluidic paper-based large-volume preconcentrator (u-LVP) for enrichment and purification of biomarkers (e.g., miRNA) using ion concentration polarization. A Nafion (ion-selective nanoporous membrane)-functionalized multilayer cellulose paper enables microscale division of milliliter-scale samples, thus electrokinetically separating and preconcentrating the biomarker in different locations within the u-LVP. By inserting collecting discs at optimal positions in the u-LVP, the enriched biomarker is simply recovered with high efficiency. With this approach, as an exemplary biomarker, miRNA-21 in human serum was separated from proteins and preconcentrated with an effective preconcentration factor exceeding 6.63 and a recovery rate above 84%. Thus, our platform offers new opportunities and benefits for biomarker, diagnostic, prognostic, and therapeutic research.

Modulation of Ionic Current Rectification in Ultrashort Conical Nanopores.

Nanopores that exhibit ionic current rectification (ICR) behave like diodes such that they transport ions more efficiently in one direction than in the other. Conical nanopores have been shown to rectify ionic current, but only those with at least 500 nm in length exhibit significant ICR. Here, through the finite element method, we show how ICR of conical nanopores with lengths below 200 nm can be tuned by controlling individual charged surfaces, that is, the inner pore surface (surfaceinner) and exterior pore surfaces on the tip and base side (surfacetip and surfacebase). The charged surfaceinner and surfacetip can induce obvious ICR individually, while the effects of the charged surfacebase on ICR can be ignored. The fully charged surfaceinner alone could render the nanopore counterion-selective and induces significant ion concentration polarization in the tip region, which causes reverse ICR compared to nanopores with all surfaces charged. In addition, the direction and degree of rectification can be further tuned by the depth of the charged surfaceinner. When considering the exterior membrane surface only, the charged surfacetip causes intrapore ionic enrichment and depletion under opposite biases, which results in significant ICR. Its effective region is within ∼40 nm beyond the tip orifice. We also found that individual charged parts of the pore system contributed to ICR in an additive way because of the additive effect on the ion concentration regulation along the pore axis. With various combinations of fully/partially charged surfaceinner and surfacetip, diverse ICR ratios from ∼2 to ∼170 can be achieved. Our findings shed light on the mechanism of ICR in ultrashort conical nanopores and provide a useful guide to the design and modification of ultrashort conical nanopores in ionic circuits and nanofluidic sensors.

Concentric ion concentration polarization desalination for efficient En-bloc preconcentration and desalination

The techniques of focusing or separating test samples in water prior to usage, known as preconcentration or filtration (desalination) depending on whether the samples are being enriched in or removed from the solution, are basic yet essential processes in the fields of mechanical/biological/chemical engineering applications. Over the years, many strategies have been introduced to improve the technique, yet overcoming the dilemma of the degree of preconcentration/filtration versus the efficiency still proves to be challenging. Here, as a viable solution to mitigate this trade-off issue, we introduce a preconcentration/filtration technique utilizing ion concentration polarization (ICP) desalination, performed within a two-dimensional concentric geometry. Designed to provide the compensation for the pressure imbalance and minimizing undesirable solvent/solute fluxes between channels, this system is capable of operating under a large flow rate difference between preconcentrate and filtrate flows, and therefore is able to produce highly concentrated solution (up to 425-fold) and desalted solution with high water recovery (up to 99.765%), in addition to securing a high salt removal (>96.8%). Such achievement of the concentric ICP desalination is a clear outperformance of the previous one-dimensional ICP desalination systems (with juxtaposed membranes in one direction), which can obtain less than only 60-fold of preconcentration and 98% of water recovery. This structural modification strategy not only has enhanced the performance of the selected system for our experiment, but also suggests a new approach to improve the preconcentration/filtration/desalination capability of other techniques, mainly those of which are based on continuous channel flow.

Tutorial review: Enrichment and separation of neutral and charged species by ion concentration polarization focusing

Ion concentration polarization focusing (ICPF) is an electrokinetic technique, in which analytes are enriched and separated along a localized electric field gradient in the presence of a counter flow. This field gradient is generated by depletion of ions of the background electrolyte at an ion permselective junction. In this tutorial review, we summarize the fundamental principles and experimental parameters that govern selective ion transport and the stability of the enriched analyte plug. We also examine faradaic ICP (fICP), in which local ion concentration is modulated via electrochemical reactions as an attractive alternative to ICP that achieves similar performance with a decrease in both power consumption and Joule heating. The tutorial covers important challenges to the broad application of ICPF including undesired pH gradients, low volumetric throughput, samples that induce biofouling or are highly conductive, and limited approaches to on- or off-chip analysis. Recent developments in the field that seek to address these challenges are reviewed along with new approaches to maximize enrichment, focus uncharged analytes, and achieve enrichment and separation in water-in-oil droplets. For new practitioners, we discuss practical aspects of ICPF, such as strategies for device design and fabrication and the relative advantages of several types of ion selective junctions and electrodes. Lastly, we summarize tips and tricks for tackling common experimental challenges in ICPF.

Multifactorial analysis of ion concentration polarization for microfluidic preconcentrating applications using response surface method

Ion concentration polarization (ICP) in a microfluidic device requires a precise balance of forces on charged molecules to achieve high concentrating efficiency. It is, thus, of considerable interest to study the impact of all governing parameters on ICP performance. Experimental study of the ICP multifactorial phenomenon seems impractical and costly. We report a systematic approach to understand the impacts of governing parameters on the ICP phenomenon using a robust numerical model established in COMSOL Multiphysics®. We varied the buffer concentration, applied voltage, and microchannel length to study their impacts on the ICP phenomenon. Then, we developed a statistical model via the response surface method (RSM) for the numerical results to study the direct and interactive effects of the mentioned parameters on ICP optimization. It was found that the buffer concentration (Cbuffer) plays a key role in the enrichment factor (EF); however, simultaneous impacts of the applied voltage and channel length must be considered as well to enhance EF. For low buffer concentrations, Cbuffer < 0.1 mM, the ionic conductivity was found to be independent of Cbuffer, while for high buffer concentrations, Cbuffer > 1 mM, the ionic conductivity was directly linked to Cbuffer. In addition, the RSM-based model prediction for a certain buffer concentration (∼1 mM) highlighted that an electric field of 20 V/cm–30 V/cm is suitable for the initial design of experiments in ICP microdevices.

Scaling relations in shear electroconvective vortices

This paper is devoted to the quantitative understanding of the electroosmotic slip velocity, which is the most essential physical quantity of shear electroconvective (SEC) microfluidics. It is well known that SEC instability caused by the electroosmotic slip velocity is triggered near the permselective membranes. Here, we present for the first time the unifying scaling relations of the electroosmotic slip velocity and overlimiting transport in SEC flow under the moderate voltage. The interplaying effects of the salt flux gradient and voltage result in a slip velocity that loses the pressure flow effect. Determined by both the applied potential and the electrolyte physical properties, the slip velocity is shown to scale as V4/3κ2/3, which deviates significantly from the relation of V2 reported in classical theory [I. Rubinstein and B. Zaltzman, “Equilibrium electroconvective instability,” Phys. Rev. Lett. 114(11), 114502 (2015)]. Since the convection flux and the electromigration flux reached an asymptotic equilibrium, a universal scaling κVPe1/3 was obtained for the overlimiting transport. Detailed direct numerical simulations in conjunction with existing experimental data [R. Kwak et al., “Shear flow of an electrically charged fluid by ion concentration polarization: Scaling laws for electroconvective vortices,” Phys. Rev. Lett. 110, 114501 (2013)] corroborate this novel scaling. Our theory provides a unified view and a perfect interpretation of the existing SEC microfluidics.

Boosting power generation from salinity gradient on high-density nanoporous membrane using thermal effect

The potential for harvesting energy from salinity gradients using nanoporous membranes has attracted significant attention in the blue energy field. To maximize the harvesting efficiency, it is necessary to both enlarge the effective area of the nanopores and control the nanopore density in such a way as to suppress the ion concentration polarization (ICP) effect. However, this is not easily achieved in large-scale manufacturing due to technology limitations. Accordingly, this study proposes a method for minimizing the ICP in conventional high-nanopore-density membranes by imposing a temperature gradient across the low- and high-concentration salt reservoirs. The feasibility of the proposed method is demonstrated experimentally by increasing the temperature of the low salt concentration reservoir by 25 K compared to that of the high concentration reservoir. It is shown that the application of an asymmetric heating effect increases the power generation by around 64% compared to the isothermal case. The experimental results are validated by means of COMSOL multiphysics simulations based on the Poisson-Nernst-Planck, Navier-Stokes and heat transfer equations. The simulation results indicate that the higher power generation obtained under asymmetric thermal heating is the result of a lower ICP at the nanopore-reservoir interface, which enhances the ion transport through the membrane.

Techno-economic analysis of multi-stage ion concentration polarization with recirculation for treatment of oil produced water

The concept of recirculation of diluate/concentrate stream is implemented in multi-stage ion concentration polarization (ICP) desalination to deal with the issue of uncontrolled concentrate streams and deteriorated overall recovery rate to treat highly concentrated oil produce water from refineries. An improved empirical optimization model was established to calculate total energy consumption for operating cost and required membrane area for capital cost for a given set of operating parameters, feed salinity, salt removal ratio, and flow velocity. Using the empirical optimization model, a techno-economic analysis is performed to evaluate the feasibility of two-stage ICP system with recirculation loops. Brine of 160 g/kg is set as the system feed stream, whereas other operating conditions such as dilaute and concentrate streams are being controlled/fixed with 20 g/kg and ~250 g/kg respectively. Also, the system can be flexibly controlled to produce a specific concentration of product water and a recovery ratio with a corresponding water cost. With careful choices of recirculation rates, one can significantly increase the recovery ratio of two-stage ICP brine treatment process (from 25% to 39%) with only minor increase in overall cost (from $16.4–25.9/m3 to $20.6–22.54/m3), which is favourable for brine waste treatment application.

Investigation on the Stability of Random Vortices in an Ion Concentration Polarization Layer with Imposed Normal Fluid Flow.

While nanoscale electrokinetic studies based on ion concentration polarization has been actively researched recently, random vortices naturally occur, leading to significantly destabilize in laboratory experiments or practical applications. These random vortices agitate the fluid inside microchannels and let the sample molecules seriously leak out preventing them from being controlled. Therefore, several trials have been reported to regulate those uninvited fluctuations by fluid flow tangential to a nanoporous membrane. Indeed, the influence of normal flow should be studied since the mass transport happens in the normal direction to the membrane. Thus, in this work, the nonlinear influence of normal flow to the instability near ion-selective surface was investigated by fully-coupled direct numerical simulation using COMSOL Multiphysics. The investigation on the effect of normal flow revealed that a space charge layer plays a significant role in the onset and growth of instability. The normal flow from the reservoir into the ion-selective surface pushed the space charge layer and decreased the size of vortices. However, there existed a maximum point for the growth of instability. The squeeze of the space charge layer increased the gradient of ion concentration in the layer, which resulted in escalating the velocity of vortices. On the other hand, the normal flow from the ion-selective surface into the reservoir suppressed the instability by spreading ions in the expanding space charge layer, leading to the reduction of ion concentration delayed the onset of instability. These two different mechanisms rendered asymmetric transition of stability as a function of the Peclet number and applied voltage. Therefore, this investigation would help understand the growth of instability and control the inevitable random vortices for the inhibition of fluid-agitation and leakage.

Covering the conical nanochannels with dense polyelectrolyte layers significantly improves the ionic current rectification

Because of their asymmetry, conical nanochannels/nanopores exhibit various attractive electrokinetic features, including ion selectivity, ionic concentration polarization, and ionic current rectification. The polyelectrolyte layer (PEL)-covered (soft) conical nanochannels have recently attracted significant attention because of their unique rectification characteristics. In the modeling of soft nanochannels, it is usually assumed that the properties of the PEL and the electrolyte are the same, an assumption that is not true, especially for dense PELs. In the present work, the influence of the PEL-electrolyte property difference on the ionic current rectification in conical soft nanochannels is studied. To this end, adopting a finite-element approach, the Poisson-Nernst-Planck and Navier-Stokes equations are numerically solved for a steady-state by considering different values of permittivity, diffusivity, and dynamic viscosity for the PEL and the electrolyte. The model is validated by comparing the results with the available theoretical and experimental data. The results show that the PEL-electrolyte property difference leads to a significant improvement of the rectification behavior, especially at low and moderate salt concentrations. This not only highlights the importance of considering different properties for the PEL and the electrolyte but also implies that the rectification behavior of soft nanochannels/nanopores may be improved considerably by utilizing denser PELs.

Renewable carbon-based materials for enhanced ion concentration polarization in sustainable separation devices

Abstract Renewable carbon-based materials derived from sugarcane harvest residues were used as a porous medium to study the enhancement effect of ion concentration polarization as an emerging electrolyte separation concept. A home-made cell for continuous electrolyte flow including two electrodes was used to study this process. A set of continuous flow runs using 0.1 mM KCl solution were performed under applied electric potentials between 0.0–5.0 V. A decrease of the outlet ion conductivity was obtained when the electric potential was applied to the cell. The depletion effect increased when flow velocities were lower and the phenomenon was reversible as the voltage was turned off. A 25 % conductivity drop was achieved for an electrolyte flow rate of 0.7 ccm. The conductivity drop was higher for less permeable porous medium, i.e., tighter biochar porous material. This is an example of how renewable biomass-derived materials can be integrated into the manufacturing of new separation devices.

Paper-Based Preconcentration and Isolation of Microvesicles and Exosomes.

Microvesicles and exosomes are small membranous vesicles released to the extracellular environment and circulated throughout the body. Because they contain various parental cell-derived biomolecules such as DNA, mRNA, miRNA, proteins, and lipids, their enrichment and isolation are critical steps for their exploitation as potential biomarkers for clinical applications. However, conventional isolation methods (e.g., ultracentrifugation) cause significant loss and damage to microvesicles and exosomes. These methods also require multiple repetitive steps of ultracentrifugation, loading, and wasting of reagents. This article describes a detailed method to fabricate an origami-paper-based device (Exo-PAD) designed for the effective enrichment and isolation of microvesicles and exosomes in a simple manner. The unique design of the Exo-PAD, consisting of accordion-like multifolded layers with convergent sample areas, is integrated with the ion concentration polarization technique, thereby enabling fivefold enrichment of the microvesicles and exosomes on specific layers. In addition, the enriched microvesicles and exosomes are isolated by simply unfolding the Exo-PAD.

Controllable pH Manipulations in Micro/Nanofluidic Device Using Nanoscale Electrokinetics.

Recently introduced nanoscale electrokinetic phenomenon called ion concentration polarization (ICP) has been suffered from serious pH changes to the sample fluid. A number of studies have focused on the origin of pH changes and strategies for regulating it. Instead of avoiding pH changes, in this work, we tried to demonstrate new ways to utilize this inevitable pH change. First, one can obtain a well-defined pH gradient in proton-received microchannel by applying a fixed electric current through a proton exchange membrane. Furthermore, one can tune the pH gradient on demand by adjusting the proton mass transportation (i.e., adjusting electric current). Secondly, we demonstrated that the occurrence of ICP can be examined by sensing a surrounding pH of electrolyte solution. When pH > threshold pH, patterned pH-responsive hydrogel inside a straight microchannel acted as a nanojunction to block the microchannel, while it did as a microjunction when pH < threshold pH. In case of forming a nanojunction, electrical current significantly dropped compared to the case of a microjunction. The strategies that presented in this work would be a basis for useful engineering applications such as a localized pH stimulation to biomolecules using tunable pH gradient generation and portable pH sensor with pH-sensitive hydrogel.

A numerical study on ion concentration polarization and electric circuit performance of an electrokinetic battery.

Ion concentration polarization (ICP) imposes remarkable adverse effects on the energy conversion performance of the pressure-driven electrokinetic (EK) flows through a capillary system that can be equivalently treated as a battery. An optimized dimensionless numerical method is proposed in this study to investigate the causes and the effects of the ICP. Results show that remarkable ICP phenomena are induced under certain conditions such as high applied pressure, high surface charge density, and small inversed Debye length at dimensionless values of 6000, -10, and 0.5. Meanwhile, different factors influence the ICP and the corresponding electric properties in different ways. Particularly for the overall electric resistance, the applied pressure and the surface charge density mainly affect the variation amplitude and the level of the overall electric resistance when varying the output electric potential, respectively. Differently, the Debye length affects the overall electric resistance in both aspects. Ultimately, the induced ICP leads to significant nonlinear current-potential curves.