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Here, we examine electromembrane systems for low-concentration desalination applicable to ultrapure water production. In addition to electrodialysis and ion concentration polarization (ICP) desalination, we propose a recovery-reduced ICP strategy for reducing the width of the desalted outlet for a higher salt removal ratio (SRR). The correlation between conductivity changes and thickness of the ion depletion zone is identified for electrodialysis, ICPH (1:1), and ICPQ (3:1) with a low-concentration feed solution (10 mM, 1 mM, 0.1 mM NaCl). Based on the experimental results, the scaling law and SRR for the electroconvection zone are summarized, and current efficiency (CE) and energy per ion removal (EPIR) depending on SRR are also discussed. As a result, the SRR of electrodialysis is mostly around 50%, but that of recovery-reduced ICP desalination is observed up to 99% under similar operating conditions. Moreover, at the same SRR, the CE of recovery-reduced ICP is similar to that of electrodialysis, but the EPIR is calculated to be lower than that of electrodialysis. Considering that forming an ion depletion zone up to half the channel width in the electromembrane system typically requires much power consumption, an ICP strategy that can adjust the width of the desalted outlet for high SRR can be preferable.

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The evolution of porous membranes has revitalized their potential application in sustainable osmotic-energy conversion. However, the performance of multiporous membranes deviates significantly from the linear extrapolation of single-pore membranes, primarily due to the occurrence of ion-concentration polarization (ICP). This study proposes a robust strategy to overcome this challenge by incorporating photoelectric responsiveness into permselective membranes. By introducing light-induced electric fields within the membrane, the transport of ions is accelerated, leading to a reduction in the diffusion boundary layer and effectively mitigating the detrimental effects of ICP. The developed photoelectric-responsive covalent-organic-framework membranes exhibit an impressive output power density of 69.6 W m-2 under illumination, surpassing the commercial viability threshold by ≈14-fold. This research uncovers a previously unexplored benefit of integrating optical electric conversion with reverse electrodialysis, thereby enhancing energy conversion efficiency.

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Strategies for rapid, effective nucleic acid processing hold tremendous significance to the clinical analysis of circulating tumor DNA (ctDNA), a family of important markers indicating tumorigenesis and metastasis. However, traditional techniques remain challenging to achieve efficient DNA enrichment, further bringing about complicated operation and limited detection sensitivity. Here, we developed an ion concentration polarization microplatform that enabled highly rapid, efficient enrichment and purification of ctDNA from a variety of clinical samples, including serum, urine, and feces. The platform demonstrated efficiently separating and enriching ctDNA within 30 s, with a 100-fold improvement over traditional methods. Integrating an on-chip isothermal amplification module, the platform further achieved 100-fold enhanced sensitivity in ctDNA detection, which significantly eliminated false-negative results in the serum or urine samples due to the low abundance of ctDNA. Such a simple-designed platform offers a user-friendly yet powerful diagnosis technique with a wide applicability, ranging from early tumor diagnosis to infection screening.

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Ion concentration polarization (ICP) is an electrical transport phenomenon that occurs at the micro-nano interface under the action of an applied electric field, and the ICP phenomenon can be used to enrich charged particles with high efficiency. The microfluidic chip has the advantages of high precision, high efficiency, easy integration and miniaturization in biochemical analysis, which provides a new solution and technical way for biochemical analysis. In response to the demand for the detection of trace charged target analytes in sample solution, the advantages of high enrichment multiplicity, convenient operation and easy integration of ICP are utilized to provide an effective way for microfluidic biochemical detection. The combination of ICP phenomenon and microfluidic analysis technology has been widely used in the fields of pre-enrichment of charged particles, separation of targets, and detection of target analytes in biochemical analysis. In this paper, the principle of ICP and the microfluidic ICP chip are briefly introduced. Under the action of external electric field, the co-ions pass through the ion-selective nanochannel, the counterions are rejected at the boundary of nanochannel to form a depletion zone, and the charged samples will be enriched at the boundary of the depletion zone. Then the preparation techniques and methods of ICP chips are summarized. Among them, the design of microfluidic channel structure and the preparation and design of nanostructures are emphasized. The basic single-channel structure is analyzed, and the parallel-channel structure as well as the integrated multi-functional microfluidic ICP chip are sorted out and summarized. The preparation methods of nanostructures in ICP chips and their respective advantages and disadvantages are listed, and it is summarized that the current mainstream means are the embedding method and the self-assembly method, and attention is paid to the design of nanostructures preparation methods by both of them. In addition, this paper also discusses how to optimize the enrichment efficiency of ICP chip, through the introduction of multi-field coupling, valve control and other means to achieve the optimization of the enrichment efficiency of target substances. Meanwhile, this paper provides a classified overview of the progress of application of ICP chips in biochemical analysis and detection. ICP chips have been widely used in the research and development of biosensors, which can be used for the enrichment and separation of a variety of analytes including small molecules, nucleic acids, proteins, and cells, etc. By changing the design of microfluidic structures, integrating detection methods and modifying specific antibodies, ICP chips have shown great potential in the fields of rapid enrichment and pre-processing of targets, separation of targets and highly sensitive detection. Finally, it is pointed out that ICP chips are facing challenges in improving enrichment efficiency and selectivity, and solving the problems of fluid control, mixing and transport to match the biological properties of target assay, and that microfluidic ICP chips have been continuously promoting the development of ICP chips through the improvement of materials, chip design and integration of multifunctional units, opening up new possibilities in the field of biochemical analysis methods and applications. It can be seen that microfluidic ICP chips have the advantages of low sample flow rate, good separation and enrichment, high detection efficiency, and easy integration and miniaturization, which have shown good research significance and practical prospects in the field of biochemical detection.

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A nanoelectrokinetic phenomenon called ion concentration polarization (ICP) has been recently applied to microfluidic paper-based devices for the high fold preconcentration of low-abundant analytes. The inherent microstructural characteristics of cellulose papers can sufficiently stabilize the chaotic electroconvection of ICP, which is a significant annoyance for typical engineered microfluidic channels. However, a high electrical voltage to induce ICP in a paper-fluidic channel can increase unavoidable electrophoretic forces over drag forces so that the preconcentrated plug is rapidly receded with severe dispersion. In order to enhance the hydraulic drag force that helps the preconcentration of analytes, here we introduce a multilayered paper structure into paper-fluidic channel. We theoretically and experimentally demonstrate that a hierarchical capillary structure in a multilayered paper-fluidic channel can effectively increase the hydraulic drag force. For the practical utility in the field of diagnostics, the mechanism is verified by a simple example of the immunoassay using biotin-streptavidin complexation.

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The unique ion-transport properties in nanoconfined pores enable nanofluidic devices with great potential in harvesting osmotic energy. The energy conversion performance could be significantly improved by the precise regulation of the "permeability-selectivity" trade-off and the ion concentration polarization effect. Here, we take the advantage of electrodeposition technique to fabricate a Janus metal-organic framework (J-MOF) membrane that possesses rapid ion-transport capability and impeccable ion selectivity. The asymmetric structure and asymmetric surface charge distribution of the J-MOF device can suppress the ion concentration polarization effect and enhance the ion charge separation, exhibiting an improved energy harvesting performance. An output power density of 3.44 W/m2 has been achieved with the J-MOF membrane at a 1000-fold concentration gradient. This work provides a new strategy for fabricating high-performance energy-harvesting devices.

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In this paper, we report a method to integrate the electrokinetic pre-enrichment of nucleic acids within a bed of probe-modified microbeads with their label-free electrochemical detection. In this detection scheme, hybridization of locally enriched target nucleic acids to the beads modulates the conduction of ions along the bead surfaces. This is a fundamental advancement in that this mechanism is similar to that observed in nanopore sensors, yet occurs in a bed of microbeads with microscale interstices. In application, this approach has several distinct advantages. First, electrokinetic enrichment requires only a simple DC power supply, and in combination with nonoptical detection, it makes this method amenable to point-of-care applications. Second, the sensor is easy to fabricate and comprises a packed bed of commercially available microbeads, which can be readily modified with a wide range of probe types, thereby making this a versatile platform. Finally, the sensor is highly sensitive (picomolar) despite the modest 100-fold pre-enrichment we employ here by faradaic ion concentration polarization (fICP). Further gains are anticipated under conditions for fICP focusing that are known to yield higher enrichment factors (up to 100,000-fold enrichment). Here, we demonstrate the detection of 3.7 pM single-stranded DNA complementary to the bead-bound oligoprobe, following a 30 min single step of enrichment and hybridization. Our results indicate that a shift in the slope of a current-voltage curve occurs upon hybridization and that this shift is proportional to the logarithm of the concentration of target DNA. Finally, we investigate the proposed mechanism of sensing by developing a numerical simulation that shows an increase in ion flux through the bed of insulating beads, given the changes in surface charge and zeta potential, consistent with our experimental conditions.

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The osmotic energy from a salinity gradient (i. e. blue energy) is identified as a promising non-intermittent renewable energy source for a sustainable technology. However, this membrane-based technology is facing major limitations for large-scale viability, primarily due to the poor membrane performance. An atomically thin 2D nanoporous material with high surface charge density resolves the bottleneck and leads to a new class of membrane material the salinity gradient energy. Although 2D nanoporous membranes show extremely high performance in terms of energy generation through the single pore, the fabrication and technical challenges such as ion concentration polarization make the nanoporous membrane a non-viable solution. On the other hand, the mesoporous and micro porous structures in the 2D membrane result in improved energy generation with very low fabrication complexity. In the present work, we report femtosecond (fs) laser-assisted scalable fabrication of µm to mm size pores on Graphene membrane for blue energy generation for the first time. A remarkable osmotic power in the order of µW has been achieved using mm size pores, which is about six orders of magnitudes higher compared to nanoporous membranes, which is mainly due to the diffusion-osmosis driven large ionic flux. Our work paves the way towards fs laser-assisted scalable pore creation in the 2D membrane for large-scale osmotic power generation.

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Separation and isolation of suspended submicron particles is fundamental to a wide range of applications, including desalination, chemical processing, and medical diagnostics. Ion concentration polarization (ICP), an electrokinetic phenomenon in micro-nano interfaces, has gained attention due to its unique ability to manipulate molecules or particles in suspension and solution. Less well understood, though, is the ability of this phenomenon to generate circulatory fluid flow, and how this enables and enhances continuous particle capture. Here, we perform a comprehensive study of a low-voltage ICP, demonstrating a new electrokinetic method for extracting submicron particles via flow-enhanced particle redirection. To do so, a 2D-FEM model solves the Poisson-Nernst-Planck equation coupled with the Navier-Stokes and continuity equations. Four distinct operational modes (Allowed, Blocked, Captured, and Dodged) were recognized as a function of the particle's charges and sizes, resulting in the capture or release from ICP-induced vortices, with the critical particle dimensions determined by appropriately tuning inlet flow rates (200-800 [µm/s]) and applied voltages (0-2.5 [V]). It is found that vortices are generated above a non-dimensional ICP-induced velocity of U*=1, which represents an equilibrium between ICP velocity and lateral flow velocity. It was also found that in the case of multi-target separation, the surface charge of the particle, rather than a particle's size, is the primary determinant of particle trajectory. These findings contribute to a better understanding of ICP-based particle separation and isolation, as well as laying the foundations for the rational design and optimization of ICP-based sorting systems.

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Nucleic acid detection is widely used in disease diagnosis, food safety, environmental monitoring and many other research fields. The continuous development of rapid and sensitive new methods to detective nucleic acid is very important for practical application. In this study, we developed a rapid nucleic-acid detection method using polymerase chain reaction (PCR) combined with electrokinetic preconcentration based on ion concentration polarization (ICP). Using a Nafion film, the proposed ICP microfluidic chip is utilized to enrich the nucleic acid molecules amplified by PCR thermal cycles. To demonstrate the capability of the microfluidic device and the hybrid nucleic-acid detection method, we present an animal-derived component detection experiment for meat product identification applications. With the reduced cycle numbers of 24 cycles, the detection can be completed in about 35 min. The experimental results show that this work can provide a microfluidic device and straightforward method for rapid detection of nucleic acids with reduced cycle numbers.

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In this study, we developed a microfluidic device for a dual work of protein preconcentration and subsequent capture by an immunoassay system. The fabrication of nano-interstices (nanochannels) to generate ion concentration polarization effect (ICP) for the preconcentration was simply performed by exploiting the loose association of glass-on-modified AuNPs to the polydimethylsiloxane (PDMS) channel pads as well as spaces between the substrate and the PDMS pad caused by these AuNPs. As a result, 65-fold concentration enhancement was achieved when performed on a sample of fluorescein isothiocyanate-labeled bovine serum albumin (FITC-BSA). Furthermore, a modification procedure of antibodies responsible for capturing target proteins was performed on gold electrodes integrated into the proposed chip. After preconcentration, the immunoassay system was worked, and showed a good performance in capturing targets. Through this study, we demonstrated that the device can work efficiently for the dual purpose, has the potential to apply widely for the analysis and capture of various targets.

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Increased antimicrobial resistance presents a major threat to public health, and it is a global health problem due to the rapid globalization and transmission of infectious diseases. However, fast and precise diagnosis tool is lacking, and inappropriate antibiotic prescription leads to the unforeseen production of drug-resistant bacteria. Here, we report a Rapid and Accurate Nanoelectrokinetic Diagnostic System (RANDx) for detecting drug-resistant bacteria, which cause a common infectious disease called Urinary Tract Infection (UTI), within 7 min. We develop nanoelectrokinetic paper-based analytic device (NEK-PAD) as a sample prep module of RANDx and obtain >100-fold post-wetting preconcentration by balancing between ion concentration polarization (ICP) and radial imbibition for a constant flow rate. Simultaneously with preconcentration, our cathodic nanochannel design enables NEK-PAD to extract drug-resistant enzymes without denaturation and accelerate enzyme-linked reactions under electrical spontaneous heating at approximately 37 °C. Finally, using a cell phone camera, we detect label-free drug-resistant bacteria as low as 104 cfu/mL, which is higher than clinically required threshold (>105 cfu/mL) by enhancing 1000 times of the limit of detection (LOD) of colorimetric nitrocefin assay. We believe that the RANDx will be an innovative precision medicine tool for UTI and other infectious diseases in limited remote settings.

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In this paper, simultaneous enrichment and separation of ions and amphoteric components were successfully demonstrated by using electric field (E) and pH gradient (double gradient) in the ion depletion zone of anion concentration polarization interface established on a paper fluid channel. Experimental results were visualized with colored ions (bright blue and amaranth) and protein probes (phycocyanin and cytochrome C). With optimization, colored phycocyanin and bovine hemoglobin with similar pI as that of albumin and immunoglobulin respectively were well separated in 900 s with 10-fold enrichment effect. Based on the separation and enrichment function of this paper-based analytical device (PAD) and rapid selective staining of human serum albumin (HSA) with bromophenol blue, a rapid colorimetric detection of HSA was implemented with smartphone camera. A limit of detection (LOD) of 5.2 mg·L-1 was achieved in the clinically significant range of 10-300 mg·L-1 (R2 = 0.99). This method was applied to real human urine samples with good agreement (ɑ = 0.01) to clinical detection method (immunoturbidimetry). With the separation and enrichment functions of PAD, both the specificity and sensitivity were enhanced, which provides a solid basis for point-of-care test of microalbuminuria. Therefore, this PAD device is potential for sample pretreatment and detection of target components from complex physiological samples.

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A portable seawater desalination system would be highly desirable to solve water challenges in rural areas and disaster situations. While many reverse osmosis-based portable desalination systems are already available commercially, they are not adequate for providing reliable drinking water in remote locations due to the requirement of high-pressure pumping and repeated maintenance. We demonstrate a field-deployable desalination system with multistage electromembrane processes, composed of two-stage ion concentration polarization and one-stage electrodialysis, to convert brackish water and seawater to drinkable water. A data-driven predictive model is used to optimize the multistage configuration, and the model predictions show good agreement with the experimental results. The portable system desalinates brackish water and seawater (2.5-45 g/L) into drinkable water (defined by WHO guideline), with the energy consumptions of 0.4-4 (brackish water) and 15.6-26.6 W h/L (seawater), respectively. In addition, the process can also reduce suspended solids by at least a factor of 10 from the source water, resulting in crystal clear water (<1 NTU) even from the source water with turbidity higher than 30 NTU (i.e., cloudy seawater by the tide). We built a fully integrated prototype (controller, pumps, and battery) packaged into a portable unit (42 × 33.5 × 19 cm3, 9.25 kg, and 0.33 L/h production rate) controlled by a smartphone, tested for battery-powered field operation. The demonstrated portable desalination system is unprecedented in size, efficiency, and operational flexibility. Therefore, it could address unique water challenges in remote, resource-limited regions of the world.

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Ion current rectification (ICR) phenomena in asymmetric nanofluidic structures, such as conical-shaped nanopores and funnel-shaped nanochannels, have been widely investigated in recent decades. To date, the effect of asymmetric nanofluidic structures on electrokinetic power generation driven by the streaming current/potential has not been explored. Accordingly, this study employed a numerical model based on the Poisson equation, Nernst-Planck equation, and Navier-Stokes equation to investigate the electrokinetic energy conversion (EKEC) in a conical nanopore while considering hydrodynamic slippage. The results indicated that the asymmetric characteristics of streaming current (short-circuit current), streaming potential (open-circuit voltage), maximum power generation, maximum conversion efficiency, and flow rate were observed in conical nanopores under the forward pressure bias (tip-to-base direction) and reverse pressure bias (base-to-tip direction) once the nonequilibrium ion concentration polarization (ICP) became considerable. The rectification behaviors in the streaming current, maximum power, and maximum conversion efficiency were all shown to be opposite to those of the well-known ICR in conical nanopores. In other words, the reverse pressure bias revealed a higher EKEC performance than the forward pressure bias. It was concluded that the asymmetric behavior in EKEC is attributed to the asymmetric electrical resistance resulting from asymmetric ion depletion and ion enrichment. Particularly, it was found that the decrease in electrical resistance (i.e., the change in electrical resistance dominated by the ion enrichment) observed in the reverse pressure bias enhanced the maximum power and maximum conversion efficiency. The asymmetric EKEC characteristics became more significant with increasing slip length, surface charge density, cone angle, and pressure bias, especially at lower salt concentrations. The present findings provide useful information for the future development of EKEC in engineered membranes with asymmetric nanopores.

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We report on the investigation of electropreconcentration phenomena in micro-/nanofluidic devices integrating 100 µm long nanochannels using 2D COMSOL simulations based on the coupled Poisson-Nernst-Planck and Navier-Stokes system of equations. Our numerical model is used to demonstrate the influence of key governing parameters such as electrolyte concentration, surface charge density, and applied axial electric field on ion concentration polarization (ICP) dynamics in our system. Under sufficiently extreme surface-charge-governed transport conditions, ICP propagation is shown to enable various transient and stationary stacking and counter-flow gradient focusing mechanisms of anionic analytes. We resolve these spatiotemporal dynamics of analytes and electrolyte ICP over disparate time and length scales, and confirm previous findings that the greatest enhancement is observed when a system is tuned for analyte focusing at the charge, excluding microchannel, nanochannel electrical double layer (EDL) interface. Moreover, we demonstrate that such tuning can readily be achieved by including additional nanochannels oriented parallel to the electric field between two microchannels, effectively increasing the overall perm-selectivity and leading to enhanced focusing at the EDL interfaces. This approach shows promise in providing added control over the extent of ICP in electrokinetic systems, particularly under circumstances in which relatively weak ICP effects are observed using only a single channel.

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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.

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Microfluidic paper-based analytical devices (µPADs) have attracted much attention over the past decade. They embody many advantages, such as abundance, portability, cost-effectiveness, and ease of fabrication, making them superior for clinical diagnostics, environmental monitoring, and food safety assurance. Despite these advantages, µPADs lack the high sensitivity to detect many analytes at trace levels than other commercial analytical instruments such as mass spectrometry. Therefore, a preconcentration step is required to enhance their sensitivity. This review focuses on the techniques used to separate and preconcentrate the analytes onto the µPADs, such as ion concentration polarization, isotachophoresis, and field amplification sample stacking. Other separations and preconcentration techniques, including liquid-solid and liquid-liquid extractions coupled with µPADs, are also reviewed and discussed. In addition, the fabrication methods, advantages, disadvantages, and the performance evaluation of the µPADs concerning their precision and accuracy were highlighted and critically assessed. Finally, the challenges and future perspectives have been discussed.

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Many industries have a significant but largely unmet need for efficient and high-flux emulsion separation, particularly for nanoemulsions. Conventional separation membranes rely on size-based separation mainly utilizing a sieving mechanism plus a wetting phenomenon, resulting in a dramatic trade-off between separation efficiency and separation flux. Herein we address this challenge by adapting electrokinetics to membrane-based separation, using a charge-based mechanism capable of separating even nanoemulsions with a demonstrated separation efficiency of >99% and ultrahigh flux up to 40 000 L/H·m2. Our device arrests nano-oil droplets, allowing them to coalesce into larger droplets which are then able to be filtered by coarser membranes. This hybrid technology makes electrokinetic-assisted filtration scalable and commercially viable and allows for a better understanding of the multiphysics underlying dynamic separation.

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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.