Amplified electrokinetic response by concentration polarization near nanofluidic channel

Investigation of Operational Parameters for Nanoelectrokinetic Purification and Preconcentration

This work reports on experimental investigations into the operational parameters of nanoelectrokinetic purification and preconcentration, especially utilizing on ion concentration polarization (ICP). ICP as a nanoscale electrokinetic phenomenon has demonstrated promising advances in various fields utilizing an ion depletion zone (IDZ) with a steep electric field gradient inside the ICP layer. However, the inevitable electrokinetic instability occurring within the IDZ has posed a challenge in operating the ICP system stably. To address the need for a stable and efficient ICP operation in various devices and applications, we propose an operational strategy along with conducted research to determine optimal operating ranges. In order to investigate the operational parameters, a unit voltage (VTH) is introduced as the threshold for initiating ICP. We examined the applicability of VTH across various operating ranges to ensure its effectiveness and versatility. In ICP purification, we categorize three modes (steady, burst, and unsteady) based on IDZ expansion and stability under varying VTH and flow rate conditions, presenting optimal operational conditions that minimize the voltage margin. In ICP preconcentration, a systematic investigation is conducted to observe the influence of background electrolyte concentration and voltage conditions on preconcentration efficiency, offering insights into the correlation between preconcentration factor, electrical conditions, and preconcentration time. Therefore, this research would contribute to the practical understanding of nanoelectrokinetics, providing insight into experimental designs. These findings are expected to offer valuable guidance to researchers aiming to utilize ICP's potential across a spectrum of applications, from purification to preconcentration, in the realm of micro/nanofluidic systems.

Differential Impact of Surface Conduction and Electroosmotic Flow on Ion Transport Enhancement by Microscale Auxiliary Structures

Our research investigates the impact of auxiliary structures on ion transport in electrochemical systems such as batteries and microscale desalination units, whose importance for sustainable development has increased dramatically in recent decades. The electrochemical systems typically feature ion-selective surfaces, such as electrodes and ion exchange membranes, where ion depletion can cause performance issues including metal dendrite formation and flow instability. Recent research has shown that auxiliary structures in these electrochemical systems can enhance ion transfer near ion-selective surfaces, thereby resolving the instability problem and improving the energy conversion efficiency of the system. Our study leverages recent advancements in nanoscale electrokinetics to model these auxiliary structures as pillar arrays near an ion exchange membrane in a microchannel. We examine how these structures enhance ion transports relative to the characteristic length scale of microchannel depth and pillars' proximity to the ion-selective surface. Results show that the effect of the pillars varies significantly with their placement. Specifically, in deeper microchannels, where electrokinetic convection is stronger, the closer the auxiliary structure is to the ion-selective membrane, the better the ion transfer. However, in the thinner microchannel, the proximity of the auxiliary structure to the ion selective membrane has a less significant correlation with the ion transfer. Therefore, this finding highlights the importance of spatial arrangement of the auxiliary structures in improving the performance of electrochemical devices. Conclusively, this study can help to better understand energy conversion systems such as fuel cells, salinity gradient power generation systems, and electrochemical desalination systems, where auxiliary structures can be used in the vicinity of ion-selective surfaces. Especially, our fundamental electrokinetic study provides an effective means for designing the efficient electrochemical platforms utilizing micro/nanofluidics.

Modulating selective ionic enrichment and depletion zones in straight nanochannels via the interplay of surface charge modulation and electric field mediated fluid-thickening

We report the possibilities of achieving highly controlled segregation of ion-enriched and ion-depleted regions in straight nanochannels. This is achieved via harnessing the interplay of an axial gradient of the induced transverse electric field on account of electrical double layer phenomenon and the localized thickening of the fluid because of intensified electric fields due to the large spatial gradients of the electrical potential in extreme confinements. By considering alternate surface patches of different charge densities over pre-designed axial spans, we illustrate how these effects can be exploited to realize selectively ion-enriched and ion-depleted zones. Physically, this is attributed to setting up of an axial concentration gradient that delves on the ionic advection due to the combined effect of an externally applied electric field and induced back-pressure gradient along the channel axis and electro-migration due to the combinatorial influences of the applied and the induced electrostatic fields. With an explicit handle on the pertinent parameters, our results offer insights on the possible means of imposing delicate controls on the solute-enrichment and depletion phenomena, a paradigm that remained unexplored thus far.

High-performance cation electrokinetic concentrator based on a γ-CD/QCS/PVA composite and microchip for evaluating the activity of P-glycoprotein without any interference from serum albumin

The development of cation electrokinetic concentrators (CECs) has been hindered by the lack of commercial anion-exchange membranes (AEMs). This paper introduces a γ-cyclodextrin-modified quaternized chitosan/polyvinyl alcohol (γ-CD/QCS/PVA) composite as an AEM, which is combined with a microchip to fabricate a CEC. Remarkably, the CEC only concentrates cationic species, thereby overcoming the interference of the highly abundant, negatively charged serum albumin in the blood sample. P-Glycoprotein (P-gp) is recognized as an efflux transporter protein that influences the pharmacokinetics (PK) of various compounds. The CEC was used to evaluate the activity of P-gp by detecting the positively charged rhodamine 123 (Rho123 is a classical substrate of P-gp) with no interference from serum albumin in the serum sample. Using the CEC, the enrichment factor (EF) of Rho123 exceeded 105-fold under DC voltage application. The minimal sample consumption of the CEC (<10 μL) enables reduction of animal sacrifice in animal experiments. Here, the CEC has been applied to evaluate the transport activity of P-gp in in vitro and in vivo experiments by detecting Rho123 in the presence of P-gp inhibitors or agonists. The results are in good agreement with those reported in previous reports. Therefore, the CEC presents a promising application potential, owing to its simple fabrication process, high sensitivity, minimal sample consumption, lack of interference from serum albumin and low cost.

Molecularly Selective Polymer Interfaces for Electrochemical Separations

The molecular design of polymer interfaces has been key for advancing electrochemical separation processes. Precise control of molecular interactions at electrochemical interfaces has enabled the removal or recovery of charged species with enhanced selectivity, capacity, and stability. In this Perspective, we provide an overview of recent developments in polymer interfaces applied to liquid-phase electrochemical separations, with a focus on their role as electrosorbents as well as membranes in electrodialysis systems. In particular, we delve into both the single-site and macromolecular design of redox polymers and their use in heterogeneous electrochemical separation platforms. We highlight the significance of incorporating both redox-active and non-redox-active moieties to tune binding toward ever more challenging separations, including structurally similar species and even isomers. Furthermore, we discuss recent advances in the development of selective ion-exchange membranes for electrodialysis and the critical need to control the physicochemical properties of the polymer. Finally, we share perspectives on the challenges and opportunities in electrochemical separations, ranging from the need for a comprehensive understanding of binding mechanisms to the continued innovation of electrochemical architectures for polymer electrodes.

dCas9-Mediated PCR-Free Detection of Oncogenic Mutation by Nonequilibrium Nanoelectrokinetic Selective Preconcentration

Cutting-edge nanoelectrokinetic technology in this work provides a breakthrough for the present clinical demands of molecular diagnosis to detect a trace amount of oncogenic mutation of DNA in a short time without an erroneous PCR procedure. In this work, we combined the sequence-specific labeling scheme of CRISPR/dCas9 and ion concentration polarization (ICP) mechanism to separately preconcentrate target DNA molecules for rapid detection. Using the mobility shift caused by dCas9's specific binding to the mutant, the mutated DNA and normal DNA were distinguished in the microchip. Based on this technique, we successfully demonstrated the dCas9-mediated 1-min detection of single base substitution (SBS) in EGFR DNA, a carcinogenesis indicator. Moreover, the presence/absence of target DNA was identified at a glance like a commercial pregnancy test kit (two lines for positive and one line for negative) by the distinct preconcentration mechanisms of ICP, even at the 0.1% concentration of the target mutant.

Electrochemical Methods for Water Purification, Ion Separations, and Energy Conversion

Agricultural development, extensive industrialization, and rapid growth of the global population have inadvertently been accompanied by environmental pollution. Water pollution is exacerbated by the decreasing ability of traditional treatment methods to comply with tightening environmental standards. This review provides a comprehensive description of the principles and applications of electrochemical methods for water purification, ion separations, and energy conversion. Electrochemical methods have attractive features such as compact size, chemical selectivity, broad applicability, and reduced generation of secondary waste. Perhaps the greatest advantage of electrochemical methods, however, is that they remove contaminants directly from the water, while other technologies extract the water from the contaminants, which enables efficient removal of trace pollutants. The review begins with an overview of conventional electrochemical methods, which drive chemical or physical transformations via Faradaic reactions at electrodes, and proceeds to a detailed examination of the two primary mechanisms by which contaminants are separated in nondestructive electrochemical processes, namely electrokinetics and electrosorption. In these sections, special attention is given to emerging methods, such as shock electrodialysis and Faradaic electrosorption. Given the importance of generating clean, renewable energy, which may sometimes be combined with water purification, the review also discusses inverse methods of electrochemical energy conversion based on reverse electrosorption, electrowetting, and electrokinetic phenomena. The review concludes with a discussion of technology comparisons, remaining challenges, and potential innovations for the field such as process intensification and technoeconomic optimization.

Semi-Continuous Desalination and Concentration of Small-Volume Samples

Electrodialysis is an electric-field-mediated process separating ions exploiting selective properties of ion-exchange membranes. The ion-exchange membranes create an ion-depleted zone in an electrolyte solution adjacent to the membrane under DC polarization. We constructed a microfluidic system that uses the ion-depleted zone to separate ions from the processed water solution. We tested the separation performance by desalting a model KCl solution spiked with fluorescein for direct observation. We showed both visually and by measuring the conductivity of the output solutions that the system can work in three modes of operation referred to as continuous desalination, desalination by accumulation, and unsuccessful desalination. The mode of operation can easily be set by changing the control parameters. The desalination factors for the model KCl solution reached values from 80 to 100%, depending on the mode of operation. The concentration factor, given as a ratio of concentrate-to-feed concentrations, reached zero for desalination by accumulation when only diluate was produced. The water recovery, therefore, was infinite at these conditions. Independent control of the diluate and concentrate flow rates and the DC voltage turned our system into a versatile platform, enabling us to set proper conditions to process various samples.

Numerical Simulation of Continuous Extraction of Li+ from High Mg2+/Li+ Ratio Brines Based on Free Flow Ion Concentration Polarization Microfluidic System

Ion concentration polarization (ICP) is a promising mechanism for concentrating and/or separating charged molecules. This work simulates the extraction of Li⁺ ions in a diluted high Mg²⁺/Li⁺ ratio salt lake brines based on free flow ICP focusing (FF-ICPF). The model solution of diluted brine continuously flows through the system with Li⁺ slightly concentrated and Mg²⁺ significantly removed by ICP driven by external pressure and perpendicular electric field. In a typical case, our results showed that this system could focus Li⁺ concentration by ~1.28 times while decreasing the Mg²⁺/Li⁺ ratio by about 85% (from 40 to 5.85). Although Li⁺ and Mg²⁺ ions are not separated as an end product, which is preferably required by the lithium industry, this method is capable of decreasing the Mg²⁺/Li⁺ ratio significantly and has great potential as a preprocessing technology for lithium extraction from salt lake brines.

Overlimiting current near a nanochannel a new insight using molecular dynamics simulations

In this paper, we report for the first time overlimiting current near a nanochannel using all-atom molecular dynamics (MD) simulations. Here, the simulated system consists of a silicon nitride nanochannel integrated with two reservoirs. The reservoirs are filled with [Formula: see text] potassium chloride (KCl) solution. A total of [Formula: see text] million atoms are simulated with a total simulation time of [Formula: see text] over [Formula: see text] 30000 CPU hours using 128 core processors (Intel(R) E5-2670 2.6 GHz Processor). The origin of overlimiting current is found to be due to an increase in chloride ([Formula: see text]) ion concentration inside the nanochannel leading to an increase in ionic conductivity. Such effects are seen due to charge redistribution and focusing of the electric field near the interface of the nanochannel and source reservoir. Also, from the MD simulations, we observe that the earlier theoretical and experimental postulations of strong convective vortices resulting in overlimiting current are not the true origin for overlimiting current. Our study may open up new theories for the mechanism of overlimiting current near the nanochannel interconnect devices.

Overlimiting Current in Nonuniform Arrays of Microchannels: Recirculating Flow and Anticrystallization

Overlimiting current (OLC) through electrolytes interfaced with perm-selective membranes has been extensively researched for understanding fundamental nano-electrokinetics and developing efficient engineering applications. This work studies how a network of microchannels in a nonuniform array, which mimics a natural pore configuration, can contribute to OLC. Here, micro/nanofluidic devices are fabricated with arrays of parallel microchannels with nonuniform size distributions, which are faced with a perm-selective membrane. All cases maintain the same surface and bulk conduction to allow probing of the sensitivity only by the nonuniformity. Rigorous experimental and theoretical investigation demonstrates that overlimiting conductance has a maximum value depending on the nonuniformity. Furthermore, in operando visualization reveals that the nonuniform arrays induce flow loops across the microchannel network enhancing advective transport. This recirculating flow eliminates local salt accumulations so that it can effectively suppress undesirable salt crystallization. Therefore, these results can significantly advance not only the fundamental understanding of the driving mechanism of the OLC but also the design rule of electrochemical membrane applications.

Eco friendly nanofluidic platforms using biodegradable nanoporous materials

Splendid advancement of micro/nanofluidic researches in the field of bio- and chemical-analysis enables various ubiquitous applications such as bio-medical diagnostics and environmental monitoring, etc. In such devices, nanostructures are the essential elements so that the nanofabrication methods have been major issues since the last couple of decades. However, most of nanofabrication methods are sophisticated and expensive due to the requirement of high-class cleanroom facilities, while low-cost and biocompatible materials have been already introduced in the microfluidic platforms. Thus, an off-the-shelf and biodegradable material for those nanostructures can complete the concept of an eco-friendly micro/nanofluidic platform. In this work, biodegradable materials originated from well-known organisms such as human nail plate and denatured hen egg (albumen and yolk) were rigorously investigated as a perm-selective nanoporous membrane. A simple micro/nanofluidic device integrated with such materials was fabricated to demonstrate nanofluidic phenomena. These distinctive evidences (the visualization of ion concentration polarization phenomenon, ohmic/limiting/over-limiting current behavior and surface charge-governed conductance) can fulfill the requirements of functional nanostructures for the nanofluidic applications. Therefore, while these materials were less robust than nano-lithographically fabricated structures, bio-oriented perm-selective materials would be utilized as a one of key elements of the biodegradable and eco friendly micro/nanofluidic applications.

Periodic concentration-polarization-based formation of a biomolecule preconcentrate for enhanced biosensing

Ionic concentration-polarization (CP)-based biomolecule preconcentration is an established method for enhancing the detection sensitivity of target biomolecules. However, the formed preconcentrated biomolecule plug rapidly sweeps over the surface-immobilized antibodies, resulting in a short-term overlap between the capture agent and the analyte, and subsequently suboptimal binding. To overcome this, we designed a setup allowing for the periodic formation of a preconcentrated biomolecule plug by activating the CP for predetermined on/off intervals. This work demonstrated the feasibility of cyclic CP actuation and optimized the sweeping conditions required to obtain the maximum retention time of a preconcentrated plug over a desired sensing region and enhanced detection sensitivity. The ability of this method to efficiently preconcentrate different analytes and to successfully increase immunoassay sensitivity underscore its potential in immunoassays serving the clinical and food testing industries.

Novel ionic separation mechanisms in electrically driven membrane processes

Electromembrane processes including electrodialysis (ED) and related processes are usually limited by diffusion transport of ions from a bulk solution to ion exchange membranes. The diffusion limited current (DLC) occurs when the concentration at membrane surfaces vanishes and approaches zero. Increasing the applied potential difference above this point has no substantial effect on ion transport and causes operational problems such as low current efficiency, high energy consumption, and mineral scaling. However, it is evident from numerous studies that operating at overlimiting current (OLC) is possible and allows one to enhance the mass transfer of an electromembrane process. While OLC is sometimes possible by electrochemical means, such as water splitting or current induced membrane discharge, it has been found that exotic ion transport mechanisms, such as ion concentration polarization in micro/nanofluidic system, deionization shock waves, and ionic bridges, can provide novel electrokinetic means of achieving OLC. In this paper, these novel ionic separation mechanisms and their role in enhanced current transfer are reviewed in the context of emerging electromembrane processes, such as shock ED and electrodeionization (EDI).

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.

Focusing, sorting, and separating microplastics by serial faradaic ion concentration polarization

In this article, we report continuous sorting of two microplastics in a trifurcated microfluidic channel using a new method called serial faradaic ion concentration polarization (fICP). fICP is an electrochemical method for forming ion depletion zones and their corresponding locally elevated electric fields in microchannels. By tuning the interplay between the forces of electromigration and convection during a fICP experiment, it is possible to control the flow of charged objects in microfluidic channels. The key findings of this report are threefold. First, fICP at two bipolar electrodes, configured in series and operated with a single power supply, yields two electric field gradients within a single microfluidic channel (i.e., serial fICP). Second, complex flow variations that adversely impact separations during fICP can be mitigated by minimizing convection by electroosmotic flow in favor of pressure-driven flow. Finally, serial fICP within a trifurcated microchannel is able to continuously and quantitatively focus, sort, and separate microplastics. These findings demonstrate that multiple local electric field gradients can be generated within a single microfluidic channel by simply placing metal wires at strategic locations. This approach opens a vast range of new possibilities for implementing membrane-free separations.

A review: applications of ion transport in micro‐nanofluidic systems based on ion concentration polarization

Lab‐on‐a‐chip has been used widely in rapid, high‐throughput and low‐consumption analysis of samples in biochemistry. The ion concentration polarization (ICP) produced by ion‐selective transport of nanochannels provides a novel solution for problems in ultra‐low concentration sample detection, systems biology and desalination. This paper reviews the applications of ion transport based on the principle of ICP in micro‐nanofluidic systems. First, the fundamental governing equations of ICP are described. Then, the applications of nano‐electrokinetic ion enrichment and ion current rectification (ICR) are introduced. Nano‐electrokinetic ion enrichment is used mainly in the fields of molecular enrichment, ultra‐low concentration sample detection and seawater desalination. ICR is applied mainly to the sensitive detection of analytical substances such as proteins, nucleic acids and small molecules. The application of ion transport based on ICP principle is summarized and the possible directions worthy of further research are proposed. © 2019 Society of Chemical Industry

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.

Free Flow Ion Concentration Polarization Focusing (FF-ICPF)

Electrokinetic separation techniques in microfluidics are a powerful analytical chemistry tool, although an inherent limitation of microfluidics is their low sample throughput. In this article we report a free-flow variant of an electrokinetic focusing method, namely ion concentration polarization focusing (ICPF). The analytes flow continuously through the system via pressure driven flow while they separate and concentrate perpendicularly to the flow by ICPF. We demonstrate free flow ion concentration polarization focusing (FF-ICPF) in two operating modes, namely peak and plateau modes. Additionally, we showed the separation resolution could be improved by the use of an electrophoretic spacer. We report a concentration factor of 10 in human blood plasma in continuous flow at a flow rate of 15 μL min^-1.

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.

Surface conduction and electroosmotic flow around charged dielectric pillar arrays in microchannels

Dielectric microstructures have been reported to have a negative influence on permselective ion transportation because ions do not migrate in areas where the structures are located. However, the structure can promote the transportation if the membrane is confined to a microscopic scale. In such a scale where the area to volume ratio is significantly large, the primary driving mechanisms of the ion transportation transition from electro-convective instability (EOI) to surface conduction (SC) and electroosmotic flow (EOF). Here, we provide rigorous evidence on how the SC and EOF around the dielectric microstructures can accelerate the ion transportation by multi-physics simulations and experimental visualizations. The microstructures further polarize the ion distribution by SC and EOF so that ion carriers can travel to the membrane more efficiently. Furthermore, we verified, for the first time, that the arrangements of microstructures have a critical impact on the ion transportation. While convective flows are isolated in the crystal pillar configuration, the flows show an elongated pattern and create an additional path for ion current in the aligned pillar configuration. Therefore, the fundamental findings of the electrokinetic effects on the dielectric microstructures suggest an innovative application in micro/nanofluidic devices with high mass transport efficiency.

Effect of channel geometry on ion-concentration polarization-based preconcentration and desalination

Polarization of the ion-selective systems results in the formation of ion-depleted and ion-concentrated zones in the electrolyte layers adjacent to the system. One can employ ion-concentration polarization for the removal of charged large molecules and small ions from the flowing liquid. Removal of large molecules from the flowing solution and their local accumulation is often referred to as preconcentration, removal of small ions as desalination. Here, we study the effect of the channel geometry on the removal of charged species from their water solutions experimentally. Straight, converging, and diverging channels equipped with a pair of heterogeneous cation-exchange membranes are compared in terms of their effect on preconcentration of an observable fluorescein dye and on desalination of water solution of potassium chloride. Our results show that preconcentration of the dye is not significantly affected by the channel geometry. The distance of the preconcentration band from one of the membranes was approximately the same in all tested channel geometries. The major difference was in the location of the band within the channel, when the conical channels localized the band at one of the channel walls. The straight channel showed a slightly broader range of applicable flow rates. The semibatch desalination of 0.01M KCl solution turned out to be more efficient in conical channels, which was associated with a larger volume of the channel available for the accumulation of the concentrated solution. Our results suggest that conical channels can be advantageously used in transforming the ion-concentration-polarization-based semibatch desalination into a fully continuous one.

Dynamics of driftless preconcentration using ion concentration polarization leveraged by convection and diffusion

Over the past several decades, separation and preconcentration methods of (bio)molecules have been actively developed for various biomedical and chemical processes such as disease diagnostics, point of care test and environmental monitoring. Among the great developments of the electrokinetic method in a micro/nanofluidic platform is the ion concentration polarization (ICP) phenomenon, in which a target molecule is accumulated near a permselective nanoporous membrane under an applied electric field. ICP method has been actively studied due to its easy implementation and high preconcentration/separation efficiency. However, the dynamic behavior of preconcentrated analytes has not yet been fully studied, especially driftless migration, where the applied electric field is orthogonal to the direction of the drift migration. Here, we demonstrate anomalous shapes of preconcentrated analytes (either plug or dumbbell shape) and the morphologies were analytically modeled by the leverage of convection and diffusion migration. This model was experimentally verified with various lengths of DNA and the limiting cases (convection-free environment in paper-based microfluidic device and extremely low diffusivity of red blood cells) were also shown to confirm the model. Thus, this study not only provides an insight into the fundamental electrokinetic dynamics of molecules in an ICP platform but also plays a guiding role for the design of a nanofluidic preconcentrator for a lab on a chip application.

Continuous focusing, fractionation and extraction of anionic analytes in a microfluidic chip

Electrokinetic focusing and separation methods, specifically ion concentration polarization focusing (ICPF), provide a very powerful and easy to use analytical tool for several scientific fields. Nevertheless, the concentrated and separated analytes are effectively trapped inside the chip in picoliter volumes. In this article we propose an ICPF device that allows continuous and selective extraction of the focused analytes. A theoretical background is presented to understand the dynamics of the system and a 1D model was developed that describes the general behavior of the system. We demonstrate the selective extraction of three fluorescent model anionic analytes and we report selective extraction of the analytes at a 300-fold increased concentration.

Direct Numerical Simulation of Seawater Desalination Based on Ion Concentration Polarization

The problem of water shortage needs to be solved urgently. The membrane-embedded microchannel structure based on the ion concentration polarization (ICP) desalination effect is a potential portable desalination device with low energy consumption and high efficiency. The electroosmotic flow in the microchannel of the cation exchange membrane and the desalination effect of the system are numerically analyzed. The results show that when the horizontal electric field intensity is 2 kV/m and the transmembrane voltage is 400 mV, the desalting efficiency reaches 97.3%. When the electric field strength increases to 20 kV/m, the desalination efficiency is reduced by 2%. In terms of fluid motion, under the action of the transmembrane voltage, two reverse eddy currents are formed on the surface of the membrane due to the opposite electric field and pressure difference on both sides of the membrane, forming a pumping effect. The electromotive force in the channel exhibits significant pressure-flow characteristics with a slip boundary at a speed approximately six times that of a non-membrane microchannel.

Ion Concentration Polarization for Microparticle Mesoporosity Differentiation

Microparticle porosity is normally determined in bulk manner providing an ensemble average that hinders establishing the individual role of each microparticle. On the other hand, single particle characterization implies expensive technology. We propose to use ion concentration polarization to measure differences in mesoporosity at the single particle level. Ion concentration polarization occurs at the interface between an electrolyte and a porous particle when an electric field is applied. The extent of ion concentration polarization depends, among others, on the mesopore size and density. By using a fluorescence marker, we could measure differences in concentration polarization between particles with 3 and 13 nm average mesopore diameters. A qualitative model was developed in order to understand and interpret the phenomena. We believe that this inexpensive method could be used to measure differences in mesoporous particle materials such as catalysts.

Novel Electrochemical Flow Sensor Based on Sensing the Convective-Diffusive Ionic Concentration Layer

Presented is a novel flow sensor based on electrochemical sensing of the ionic concentration-polarization (CP) layer developed within a microchannel-ion permselective membrane device. To demonstrate the working principle of the electrochemical flow sensor, the effect of advection on the transient and steady-state ionic concentration-polarization (CP) phenomenon in microchannel-Nafion membrane systems is studied. In particular, we focused on the local impedance, measured using an array of electrode pairs embedded at the bottom of the microchannel, as well as the total current across the permselective medium, as two approaches for estimating the flow. We examined both a stepwise application of CP under steady-state flow and a stepwise application of flow under steady-state CP.

Nanoelectrokinetic bufferchannel-less radial preconcentrator and online extractor by tunable ion depletion layer

Among various preconcentration strategies using nanofluidic platforms, a nanoscale electrokinetic phenomenon called ion concentration polarization (ICP) has been extensively utilized due to several advantages such as high preconcentration factor and no need of complex buffer exchange process. However, conventional ICP preconcentrator had difficulties in the recovery of preconcentrated sample and complicated buffer channels. To overcome these, bufferchannel-less radial micro/nanofluidic preconcentrator was developed in this work. Radially arranged microchannel can maximize the micro/nano membrane interface so that the samples were preconcentrated from each microchannel. All of preconcentrated plugs moved toward the center pipette tip and can be easily collected by just pulling out the tip installed at the center reservoir. For a simple and cost-effective fabrication, a commercial printer was used to print the nanoporous membrane as "Nafion-junction device." Various analytes such as polystyrene particle, fluorescent dye, and dsDNA were preconcentrated and extracted with the recovery ratio of 85.5%, 79.0%, and 51.3%, respectively. Furthermore, we used a super inkjet printer to print the silver electrode instead of nanoporous membrane to preconcentrate either type of charged analytes as "printed-electrode device." A Faradaic reaction was used as the main mechanism, and we successfully demonstrated the preconcentration of either negatively or positively charged analytes. The presented bufferchannel-less radial preconcentrator would be utilized as a practical and handy platform for analyzing low-abundant molecules.

High-performance bioanalysis based on ion concentration polarization of micro-/nanofluidic devices

Micro-/nanofluidics has received considerable attention over the past two decades, which allows efficient biomolecule trapping and preconcentration due to ion concentration polarization (ICP) within nanostructures. The rich scientific content related to ICP has been widely exploited in different applications including protein concentration, biomolecules sensing and detection, cell analysis, and water purification. Compared to pure microfluidic devices, micro-/nanofluidic devices show a highly efficient sample enrichment capacity and nonlinear electrokinetic flow feature. These two unique characterizations make the micro-/nanofluidic systems promising in high-performance bioanalysis. This review provides a comprehensive description of the ICP phenomenon and its applications in bioanalysis. Perspectives are also provided for future developments and directions of this research field.

dCas9-mediated Nanoelectrokinetic Direct Detection of Target Gene for Liquid Biopsy

The-state-of-the-art bio- and nanotechnology have opened up an avenue to noninvasive liquid biopsy for identifying diseases from biomolecules in bloodstream, especially DNA. In this work, we combined sequence-specific-labeling scheme using mutated clustered regularly interspaced short palindromic repeats associated protein 9 without endonuclease activity (CRISPR/dCas9) and ion concentration polarization (ICP) phenomenon as a mechanism to selectively preconcentrate targeted DNA molecules for rapid and direct detection. Theoretical analysis on ICP phenomenon figured out a critical mobility, elucidating two distinguishable concentrating behaviors near a nanojunction, a stacking and a propagating behavior. Through the modulation of the critical mobility to shift those behaviors, the C-C chemokine receptor type 5 ( CCR5) sequences were optically detected without PCR amplification. Conclusively, the proposed dCas9-mediated genetic detection methodology based on ICP would provide rapid and accurate micro/nanofluidic platform of liquid biopsies for disease diagnostics.

Numerical investigation of the current transition regimes in nanochannels

The concentration polarization phenomena and its effects represent one of the main challenges for the optimal operation of many nanofluidic systems. A numerical investigation of the different electric current transition regimes observed during the concentration polarization phenomena in nanochannels is performed. This included a 2D-axisymmetric simulation of the nanofluidic system (reservoir-nanochannel-reservoir). From these simulations, a novel mechanism is discovered that explains that different current transition regimes. This driving mechanism involves the applied electric field penetration while the convective flow mechanism is found to be negligible. This differs with the classical statement that the mixing process with less depleted areas initiated by an electrokinetic vortex instability starts the overlimiting regime. Additionally, the numerical approach allows us to identify new characteristics of the linear-limiting transition such as source-like and saddle-like points of the electric field streamlines. The three voltage-current regimes (linear, limiting and overlimiting) are explained by observing and quantifying changes in electric field, potential, ion concentration and ion concentration gradients within the system.

Micro/nano acoustofluidics: materials, phenomena, design, devices, and applications

Acoustic actuation of fluids at small scales may finally enable a comprehensive lab-on-a-chip revolution in microfluidics, overcoming long-standing difficulties in fluid and particle manipulation on-chip. In this comprehensive review, we examine the fundamentals of piezoelectricity, piezoelectric materials, and transducers; revisit the basics of acoustofluidics; and give the reader a detailed look at recent technological advances and current scientific discussions in the discipline. Recent achievements are placed in the context of classic reports for the actuation of fluid and particles via acoustic waves, both within sessile drops and closed channels. Other aspects of micro/nano acoustofluidics are examined: atomization, translation, mixing, jetting, and particle manipulation in the context of sessile drops and fluid mixing and pumping, particle manipulation, and formation of droplets in the context of closed channels, plus the most recent results at the nanoscale. These achievements will enable applications across the disciplines of chemistry, biology, medicine, energy, manufacturing, and we suspect a number of others yet unimagined. Basic design concepts and illustrative applications are highlighted in each section, with an emphasis on lab-on-a-chip applications.

Direct numerical simulation of continuous lithium extraction from high Mg2+/Li+ ratio brines using microfluidic channels with ion concentration polarization

A novel ion concentration polarization-based microfluidic device is proposed for continuous extraction of Li+ from high Mg2+/Li+ ratio brines. With simultaneous application of the cross-channel voltage that drives electroosmotic flow and the cross-membrane voltage that induces ion depletion, Li+ is concentrated much more than other cations in front of the membrane in the microchannel. The application of external pressure produces a fluid flow that drags a portion of Li+ (and Na+) to flow through the microchannel, while keeping most of Mg2+ (and K+) blocked, thus implementing continuous Li+ extraction. Two-dimensional numerical simulation using a microchannel of 120 µm length and 4 µm height and a model, highly concentrated brine, shows that the system may produce a continuous flow rate of 1.72 mm/s, extracting 25.6% of Li+, with a Li+/Mg2+ flux ratio of 2.81×103, at a pressure of 100 Pa and cross-membrane voltage of 100 times of thermal voltages (25.8 mV). Fundamental mechanisms of the system are elaborated and effects of the cross-membrane voltage and the external pressure are analyzed. These results and findings provide clear guidance for the understanding and designing of microfluidic devices not only for Li+ extraction, but also for other ionic or molecular separations.

On-line microchip electrophoresis-mediated preconcentration of cationic compounds utilizing cationic polyacrylamide gels fabricated by in situ photopolymerization

A simple and efficient method was developed for the fabrication of a cationic sample preconcentrator on a channel of a commercial poly(methyl methacrylate) (PMMA) microchip.

Force fields of charged particles in micro-nanofluidic preconcentration systems

Electrokinetic concentration devices based on the ion concentration polarization (ICP) phenomenon have drawn much attention due to their simple setup, high enrichment factor, and easy integration with many subsequent processes, such as separation, reaction, and extraction etc. Despite significant progress in the experimental research, fundamental understanding and detailed modeling of the preconcentration systems is still lacking. The mechanism of the electrokinetic trapping of charged particles is currently limited to the force balance analysis between the electric force and fluid drag force in an over-simplified one-dimensional (1D) model, which misses many signatures of the actual system. This letter studies the particle trapping phenomena that are not explainable in the 1D model through the calculation of the two-dimensional (2D) force fields. The trapping of charged particles is shown to significantly distort the electric field and fluid flow pattern, which in turn leads to the different trapping behaviors of particles of different sizes. The mechanisms behind the protrusions and instability of the focused band, which are important factors determining overall preconcentration efficiency, are revealed through analyzing the rotating fluxes of particles in the vicinity of the ion-selective membrane. The differences in the enrichment factors of differently sized particles are understood through the interplay between the electric force and convective fluid flow. These results provide insights into the electrokinetic concentration effect, which could facilitate the design and optimization of ICP-based preconcentration systems.

Enhancing the sensitivity of DNA detection by structurally modified solid-state nanopore

Solid-state nanopore is an ionic current-based biosensing platform, which would be a top candidate for next-generation DNA sequencing and a high-throughput drug-screening tool at single-molecular-scale resolution. There have been several approaches to enhance the sensitivity and reliability of biomolecule detection using the nanopores particularly in two aspects: signal-to-noise ratio (SNR) and translocation dwell time. In this study, an additional nano-well of 100-150 nm diameter and the aspect ratio of ∼5 called 'guide structure' was inserted in conventional silicon-substrate nanopore device to increase both SNR and dwell time. First, the magnitude of signals (conductance drop (ΔG)) increased 2.5 times under applied voltage of 300 mV through the guide-inserted nanopore compared to the conventional SiN/Si nanopore in the same condition. Finite element simulation was conducted to figure out the origin of ΔG modification, which showed that the guide structure produced high ΔG due to the compartmental limitation of ion transports through the guide to the sensing nanopore. Second, the translocation velocity decreased in the guide-inserted structure to a maximum of 20% of the velocity in the conventional device at 300 mV. Electroosmotic drag formed inside the guide structure, when directly applied to the remaining segment of translocating DNA molecules in cis chamber, affected the DNA translocation velocity. This study is the first experimental report on the effect of the geometrical confinement to a remnant DNA on both SNR and dwell time of nanopore translocations.

Experimental verification of simultaneous desalting and molecular preconcentration by ion concentration polarization

While the ion concentration polarization (ICP) phenomenon has been intensively researched for the last decade, a complete picture of ion and analyte distributions near nanoporous membranes is strongly desired, not only for fundamental nano-electrokinetic studies but also for the development of lab-on-a-chip applications. Since direct concentration measurements, using either time-consuming collection or microelectrodes, are limited due to low throughput (<nL min^-1 in typical micro/nanofluidic device) and Faradaic reactions, respectively, we measured the concentration changes of prefilled solutions in individual reservoirs in this work. As a result, analytes larger than the size of nanopores were completely repelled by the ICP layer, 65% of cations were transported through the nanoporous membrane to sustain the ICP phenomenon, and the remaining anions were consumed by electrode reactions for electro-neutrality requirements. These combined effects would enable the perfect recovery of a target analyte and the removal of unnecessary salts simultaneously. Using this scenario, the novel concept of an ink recycler was also demonstrated in this work. We showed that 40% of unnecessary salt, which causes serious deterioration of inkjet heads, was removed, while the concentration of ink molecules was doubled in a single-step operation. This simultaneous desalting and molecular preconcentration mechanism would be a key operational strategy of various refinery/purification applications for drug discovery and the chemical industry, etc.

Selectivity enhancements in gel-based DNA-nanoparticle assays by membrane-induced isotachophoresis: thermodynamics versus kinetics

Selectivity against mutant nontargets with a few mismatches remains challenging in nucleic acid sensing. Sensitivity enhancement by analyte concentration does not improve selectivity because it affects targets and nontargets equally. Hydrodynamic or electrical shear enhanced selectivity is often accompanied by substantial losses in target signals, thereby leading to poor limits of detection. We introduce a platform based on depletion isotachophoresis in agarose gel generated by an ion-selective membrane that allows both selectivity and sensitivity enhancement with a two-step assay involving concentration polarization at an ion-selective membrane. By concentrating both the targets and probe-functionalized nanoparticles by ion enrichment at the membrane, the effective thermodynamic dissociation constant is lowered from 40 nM to below 500 pM, and the detection limit is 10 pM as reported previously. A dynamically optimized ion depletion front is then generated from the membrane with a high electrical shear force to selectively and irreversibly dehybridize nontargets. The optimized selectivity against a two-mismatch nontarget (in a 35-base pairing sequence) is shown to be better than the thermodynamic equilibrium selectivity by more than a hundred-fold, such that there is no detectable signal from the two-mismatch nontarget. We offer empirical evidence that irreversible cooperative dehybridization plays an important role in this kinetic selectivity enhancement and that mismatch location controls the optimum selectivity even when there is little change in the corresponding thermodynamic dissociation constant.