Deciphering ion concentration polarization-based electrokinetic molecular concentration at the micro-nanofluidic interface: theoretical limits and scaling laws

Multiphysics analytical and numerical studies of biomolecule preconcentration utilizing ion concentration polarization: a case study of convergent microchannels

A convergent sector in microfluidic devices utilizing ion concentration polarization (ICP) can help increase the preconcentration rate and the concentration enhancement factor (CEF) of biomolecules. In this work, we present a detailed study of the nozzle-like-squeeze effect of a convergent channel on the preconcentration of biomolecules. By numerically solving coupled Nernst-Planck-Poisson-Navier-Stokes governing equations for the 2D channel model, we report the first study on the critical width of a convergent region in the channel to retain the advantage of the nozzle-like-squeeze effect in speeding up preconcentration and augmenting CEF. In addition, we investigated the impact of the location and the dimensions of a convergent sector on the mechanism of biomolecule preconcentration. The location of an ion-selective membrane was also determined to ensure that biomolecules are focused on the convergent region of the channel. Moreover, we introduce analytical studies to compare and verify simulation findings. Specifically, the formulas for the critical dimensions of a convergent channel, location of a preconcentrated biomolecule plug, and position of an ion-selective membrane are presented. Furthermore, important working parameters, including electric potentials and hydrostatic pressures, were examined to scrutinize their effect on convergent concentrators. These detailed analytical solutions and numerical simulation results are consistent with experimental observations, providing deep insights into the ICP phenomenon and the preconcentration mechanism of biomolecules in convergent microfluidic concentration devices.

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.

Determination of Critical Dimensions of Microchannels to Ensure the Electrokinetic Biomolecule Preconcentration: Analytical and Numerical Studies

Preconcentration of biomolecules based on ion concentration polarization (ICP) has been splendidly applied to various biomedical and chemical processes. However, in many circumstances, biomolecule preconcentration could not occur due to the lack of full studies on the preconcentration mechanism, especially on the effect of microchannel dimensions. In this work, we provide analytical studies on the critical dimensions (minimum and maximum) of microchannels for the preconcentration of biomolecules. These formulas are verified with the numerical results by fully solving the coupled governing equations: Poisson-Nernst-Planck and Navier-Stokes experiments with appropriate boundary conditions and assumptions. In addition, we examine the impact of operational parameters, such as electric potentials and critical external pressures, on the formation of the preconcentration. Moreover, two important results are provided for the first time, including the position of the preconcentrated biomolecule region and the concentration enhancement factor of the biomolecules. These analytical and numerical results are consistent with experimental observations and, therefore, could provide sharp insight into the mechanism of biomolecule preconcentration and give useful guidelines to better design and optimize ICP-based microfluidic preconcentration devices.

An Ion Concentration Polarization Microplatform for Efficient Enrichment and Analysis of ctDNA

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.

Rapid, Ultrasensitive, and Visual Detection of Pathogens Based on Cation Dye-Triggered Gold Nanoparticle Electrokinetic Agglutination Analysis

Rapid prescribing of the right antibiotic is the key to treat infectious diseases and decelerate the challenge of bacterial antibiotic resistance. Herein, by targeting the 16S rRNA of bacteria, we developed a cation dye-triggered electrokinetic gold nanoparticle (AuNP) agglutination (CD-TEAA) method, which is rapid, visual, ultrasensitive, culture-independent, and low in cost. The limit of detection (LOD) is as low as 1 CFU mL-1 Escherichia coli. The infection identifications of aseptic fluid samples (n = 11) and urine samples with a clinically suspected urinary tract infection (UTI, n = 78) were accomplished within 50 and 30 min for each sample, respectively. The antimicrobial susceptibility testing (AST) of UTI urine samples was achieved within 2.5 h. In ROC analysis of urine, the sensitivity and specificity were 100 and 96% for infection identification, and 100 and 98% for AST, respectively. Moreover, the overall cost of materials for each test is about US$0.69. Therefore, the CD-TEAA method is a superior approach to existing, time-consuming, and expensive methods, especially in less developed areas.

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.

Electrokinetic and Electroconvective Effects in Ternary Electrolyte Near Ion-Selective Microsphere

The paper presents theoretical and experimental investigations of the behavior of an electrolyte solution with three types of ions near an ion-selective microparticle with electrokinetically and pressure-driven flow. A special experimental cell has been developed for the investigations. An anion-selective spherical particle composed of ion-exchange resin is fixed in the center of the cell. An enriched region with a high salt concentration appears at the anode side of the particle when an electric field is turned on, according to the nonequilibrium electrosmosis behavior. A similar region exists near a flat anion-selective membrane. However, the enriched region near the particle produces a concentration jet that spreads downstream akin to a wake behind an axisymmetrical body. The fluorescent cations of Rhodamine-6G dye are chosen as the third species in the experiments. The ions of Rhodamine-6G have a 10-fold lower diffusion coefficient than the ions of potassium while bearing the same valency. This paper shows that the concentration jet behavior is described accurately enough with the mathematical model of a far axisymmetric wake behind a body in a fluid flow. The third species also forms an enriched jet, but its distribution turns out to be more complex. The concentration of the third species increases in the jet with an increase in pressure gradient. The pressure-driven flow stabilizes the jet, yet electroconvection has been observed near the microparticle for sufficiently strong electric fields. The electrokinetic instability and the electroconvection partially destroy the concentration jet of salt and the third species. The conducted experiments show good qualitative agreement with the numerical simulations. The presented results could be used in future for implementing microdevices based on membrane technology for solving problems of detection and preconcentration, and thus simplifying chemical and medical analyses utilizing the superconcentration phenomenon. Such devices are called membrane sensors, and are actively being studied.

Continuous Submicron Particle Separation Via Vortex-Enhanced Ionic Concentration Polarization: A Numerical Investigation

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.

Digital microfluidics-like manipulation of electrokinetically preconcentrated bioparticle plugs in continuous-flow

Herein, we demonstrate digital microfluidics-like manipulations of preconcentrated biomolecule plugs within a continuous flow that is different from the commonly known digital microfluidics involving discrete (i.e. droplets) media. This is realized using one- and two-dimensional arrays of individually addressable ion-permselective membranes with interconnecting microfluidic channels. The location of powered electrodes, dictates which of the membranes are active and generates either enrichment/depletion diffusion layers, which, in turn, control the location of the preconcentrated plug. An array of such powered membranes enables formation of multiple preconcentrated plugs of the same biosample as well as of preconcentrated plugs of multiple biosample types introduced via different inlets in a selective manner. Moreover, digital-microfluidics operations such as up-down and left-right translation, merging, and splitting, can be realized, but on preconcentrated biomolecule plugs instead of on discrete droplets. This technology, based on nanoscale electrokinetics of ion transport through permselective medium, opens future opportunities for smart and programmable digital-like manipulations of preconcentrated biological particle plugs for various on-chip biological applications.

Out-of-plane faradaic ion concentration polarization: stable focusing of charged analytes at a three-dimensional porous electrode

Ion concentration polarization (ICP) accomplishes preconcentration for bioanalysis by localized depletion of electrolyte ions, thereby generating a gradient in electric field strength that facilitates electrokinetic focusing of charged analytes by their electromigration against opposing fluid flow. Such ICP focusing has been shown to accomplish up to a million-fold enrichment of nucleic acids and proteins in single-stage preconcentrators. However, the rate at which the sample volume is swept is limited, requiring several hours to achieve these high enrichment factors. This limitation is caused by two factors. First, an ion depleted zone (IDZ) formed at a planar membrane or electrode may not extend across the full channel cross section under the flow rate employed for focusing, thereby allowing the analyte to "leak" past the IDZ. Second, within the IDZ, large fluid vortices lead to mixing, which decreases the efficiency of analyte enrichment and worsens with increased channel dimensions. Here, we address these challenges with faradaic ICP (fICP) at a three-dimensional (3D) electrode comprising metallic microbeads. This 3D-electrode distributes the IDZ, and therefore, the electric field gradient utilized for counter-flow focusing across the full height of the fluidic channel, and its large area, microstructured surface supports smaller vortices. An additional bed of insulating microbeads restricts flow patterns and supplies a large area for surface conduction of ions through the IDZ. Finally, the resistance of this secondary bed enhances focusing by locally strengthening sequestering forces. This easy-to-build platform lays a foundation for the integration of enrichment with user-defined packed bed and electrode materials.

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.

Simulation and Experimental Study of Ion Concentration Polarization Induced Electroconvective Vortex and Particle Movement

Ion concentration polarization (ICP) has been widely applied in microfluidic systems in pre-concentration, particle separation, and desalination applications. General ICP microfluidic systems have three components (i.e., source, ion-exchange, and buffer), which allow selective ion transport. Recently developed trials to eliminate one of the three components to simplify the system have suffered from decreased performance by the accumulation of unwanted ions. In this paper, we presented a new ICP microfluidic system with only an ion-exchange membrane-coated channel. Numerical investigation on hydrodynamic flow and electric fields with a series of coupled governing equations enabled a strong correlation to experimental investigations on electroconvective vortices and the trajectory of charged particles. This study has significant implications for the development and optimization of ICP microfluidic and electrochemical systems for biomarker concentration and separation to improve sensing reliability and detection limits in analytic chemistry.

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.

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.

Scaling laws of electroconvective flow with finite vortex height near permselective membranes

In a steady state, the linear scaling laws are confirmed between the intensity characteristics of electroconvective (EC) vortex (including the vortex height and electroosmotic slip velocity) and the applied voltage for the nonshear EC flow with finite vortex height near permselective membranes. This finding in the nonshear EC flow is different from the shear EC flow [Kwak et al., Phys. Rev. Lett. 110, 114501 (2013)10.1103/PhysRevLett.110.114501] and indicates that the local concentration gradient has a significant improvement in the analysis of slip velocity. Further, our study reveals that the EC vortex is mainly driven by the second peak effect of the Coulomb thrust in the extended space-charge layer, and the linear scaling law exhibited by the Coulomb thrust is an essential reason for the linear scaling laws of vortex intensity. The scaling laws proposed in this paper are supported by our direct numerical simulation data and previous experimental observations [Rubinstein et al., Phys. Rev. Lett. 101, 236101 (2008)10.1103/PhysRevLett.101.236101].

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.

One-Step Nucleic Acid Purification and Noise-Resistant Polymerase Chain Reaction by Electrokinetic Concentration for Ultralow-Abundance Nucleic Acid Detection

Nucleic acid amplification tests (NAATs)integrated on a chip hold great promise for point-of-care diagnostics. Currently, nucleic acid (NA) purification remains time-consuming and labor-intensive, and it takes extensive efforts to optimize the amplification chemistry. Using selective electrokinetic concentration, we report one-step, liquid-phase NA purification that is simpler and faster than conventional solid-phase extraction. By further re-concentrating NAs and performing polymerase chain reaction (PCR) in a microfluidic chamber, our platform suppresses non-specific amplification caused by non-optimal PCR designs. We achieved the detection of 5 copies of M. tuberculosis genomic DNA (equaling 0.3 cell) in real biofluids using both optimized and non-optimal PCR designs, which is 10- and 1000-fold fewer than those of the standard bench-top method, respectively. By simplifying the workflow and shortening the development cycle of NAATs, our platform may find use in point-of-care diagnosis.

Droplet encapsulation of electrokinetically-focused analytes without loss of resolution

Lab-on-chip electrokinetic focusing and separation techniques are widely used in several scientific fields. In a number of cases, these techniques have been combined with a selective analyte extraction for off-chip analysis. Nevertheless, the usability of the extracts is limited by diffusion which reduces the separation resolution. In this paper we propose the integration of a droplet generator capable of continuous or on-demand generation and extraction of electrokinetically separated and focused analytes. We demonstrate the selective droplet extraction of model analytes separated and concentrated via ion concentration polarization focusing (ICPF). We report extracted droplets with 1000-fold increased concentration. Importantly, the droplet generator does not interrupt the ICPF process making it suitable for integration with the majority of electrokinetic separation techniques.

Ion Current Rectification in Extra-Long Nanofunnels

Nanofluidic systems offer new functionalities for the development of high sensitivity biosensors, but many of the interesting electrokinetic phenomena taking place inside or in the proximity of nanostructures are still not fully characterized. Here, to better understand the accumulation phenomena observed in fluidic systems with asymmetric nanostructures, we study the distribution of the ion concentration inside a long (more than 90 µm) micrometric funnel terminating with a nanochannel. We show numerical simulations, based on the finite element method, and analyze how the ion distribution changes depending on the average concentration of the working solutions. We also report on the effect of surface charge on the ion distribution inside a long funnel and analyze how the phenomena of ion current rectification depend on the applied voltage and on the working solution concentration. Our results can be used in the design and implementation of high-performance concentrators, which, if combined with high sensitivity detectors, could drive the development of a new class of miniaturized biosensors characterized by an improved sensitivity.

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.

Electroosmotic Flow Behavior of Viscoelastic LPTT Fluid in a Microchannel

In many research works, the fluid medium in electroosmosis is considered to be a Newtonian fluid, while the polymer solutions and biological fluids used in biomedical fields mostly belong to the non-Newtonian category. Based on the finite volume method (FVM), the electroosmotic flow (EOF) of viscoelastic fluids in near-neutral (pH = 7.5) solution considering four ions (K+, Cl-, H+, OH-) is numerically studied, as well as the viscoelastic fluids' flow characteristics in a microchannel described by the Linear Phan-Thien-Tanner (LPTT) constitutive model under different conditions, including the electrical double layer (EDL) thickness, the Weissenberg number (Wi), the viscosity ratio and the polymer extensibility parameters. When the EDL does not overlap, the velocity profiles for both Newtonian and viscoelastic fluids are plug-like and increase sharply near the charged wall. Compared with Newtonian fluid at Wi = 3, the viscoelastic fluid velocity increases by 5 times and 9 times, respectively, under the EDL conditions of kH = 15 and kH = 250, indicating the shear thinning behavior of LPTT fluid. Shear stress obviously depends on the viscosity ratio and different Wi number conditions. The EOF is also enhanced by the increase (decrease) in polymer extensibility parameters (viscosity ratio). When the extensibility parameters are large, the contribution to velocity is gradually weakened.

Ion concentration polarization (ICP) of proteins at silicon micropillar nanogaps

Fast detection of low-abundance protein remains a challenge because detection speed is limited by analyte transport to the detection site of a biosensor. In this paper, we demonstrate a scalable fabrication process for producing vertical nanogaps between micropillars which enable ion concentration polarization (ICP) enrichment for fast analyte detection. Compared to horizontal nanochannels, massively paralleled vertical nanogaps not only provide comparable electrokinetics, but also significantly reduce fluid resistance, enabling microbead-based assays. The channels on the device are straightforward to fabricate and scalable using conventional lithography tools. The device is capable of enriching protein molecules by >1000 fold in 10 min. We demonstrate fast detection of IL6 down to 7.4 pg/ml with only a 10 min enrichment period followed by a 5 min incubation. This is a 162-fold enhancement in sensitivity compared to that without enrichment. Our results demonstrate the possibility of using silicon/silica based vertical nanogaps to mimic the function of polymer membranes for the purpose of protein enrichment.

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.

Universal amplification-free molecular diagnostics by billion-fold hierarchical nanofluidic concentration

Rapid and reliable detection of ultralow-abundance nucleic acids and proteins in complex biological media may greatly advance clinical diagnostics and biotechnology development. Currently, nucleic acid tests rely on enzymatic processes for target amplification (e.g., PCR), which have many inherent issues restricting their implementation in diagnostics. On the other hand, there exist no protein amplification techniques, greatly limiting the development of protein-based diagnosis. We report a universal biomolecule enrichment technique termed hierarchical nanofluidic molecular enrichment system (HOLMES) for amplification-free molecular diagnostics using massively paralleled and hierarchically cascaded nanofluidic concentrators. HOLMES achieves billion-fold enrichment of both nucleic acids and proteins within 30 min, which not only overcomes many inherent issues of nucleic acid amplification but also provides unprecedented enrichment performance for protein analysis. HOLMES features the ability to selectively enrich target biomolecules and simultaneously deplete nontargets directly in complex crude samples, thereby enormously enhancing the signal-to-noise ratio of detection. We demonstrate the direct detection of attomolar nucleic acids in urine and serum within 35 min and HIV p24 protein in serum within 60 min. The performance of HOLMES is comparable to that of nucleic acid amplification tests and near million-fold improvement over standard enzyme-linked immunosorbent assay (ELISA) for protein detection, being much simpler and faster in both applications. We additionally measured human cardiac troponin I protein in 9 human plasma samples, and showed excellent agreement with ELISA and detection below the limit of ELISA. HOLMES is in an unparalleled position to unleash the potential of protein-based diagnosis.

Continuous micellar electrokinetic focusing of neutral species driven by ion concentration polarization

Ion concentration polarization (ICP) has been broadly applied to accomplish electrokinetic focusing of charged species. However, ICP-based extraction and enrichment of uncharged (neutral) compounds, important for pharmaceutical, biological, and environmental applications, has not yet been reported. Here, we report the ICP-based continuous extraction of two neutral compounds from aqueous solution, by their partition into an ionic micellar phase. Our initial results show that the efficiency of the extraction increases with the concentration of the surfactant comprising the micellar phase, reaching 98 ± 2%, and drops precipitously when the concentration of the target compound exceeds the capacity of the micelles. As a key feature relevant to the practical application of this method, we show that focusing occurs even an order of magnitude below the critical micelle concentration through the local enrichment and assembly of surfactants into micelles, thus minimizing their consumption. To underscore the relevance of this approach to water purification, this method is applied to the extraction of pyrene, a model for polyaromatic hydrocarbons. This approach provides access to a broad range of strategies for selective separation that have been developed in micellar electrokinetic chromatography.

Combining dielectrophoresis and concentration polarization-based preconcentration to enhance bead-based immunoassay sensitivity

Ionic concentration-polarization (CP)-based biomolecule preconcentration is an established method for enhancing the detection sensitivity of a target biomolecule immunoassay. However, its main drawback lies in its inability to directly control the spatial overlap between the preconcentrated plug of biomolecules and the surface immobilized antibodies. To overcome this, we simultaneously preconcentrated freely suspended, surface functionalized nanoparticles and target molecules along the edge of a depletion layer, thus, increasing the binding kinetics and avoiding the need to tune their relative locations to ensure their spatial overlap. After the desired incubation time, the nanoparticles were dielectrophoretically trapped for postprocessing analysis of the binding signal. This novel combination of CP-based preconcentration and dielectrophoresis (DEP) was demonstrated through binding of avidin and biotin-conjugated particles as a model bead-based immunoassay, wherein increased detection sensitivity was demonstrated compared to an immunoassay without CP-based preconcentration. The DEP trapping of the beads following binding is important not only for an enhanced detection signal due to the preconcentration of the beads at the electrode edges but also for controlling their location for future applications integrating localized sensors. In addition, DEP may be important also as a preprocessing step for controlling the number of beads participating in the immunoassay.

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.

Femtomolar Detection of Lipopolysaccharide in Injectables and Serum Samples Using Aptamer-Coupled Reduced Graphene Oxide in a Continuous Injection-Electrostacking Biochip

A method for microfluidic sample preconcentration to detect femtomolar level of lipopolysaccharide (LPS) is introduced, enabled by 6-carboxyfluorescein (6-FAM) labeled aptamer-LPS binding along with reduced graphene oxide (rGO). The free FAM-aptamers can be adsorbed onto the surface of rGO, resulting in fluorescence quenching of background signals. Conversely, the aptamer-LPS complex cannot be adsorbed by rGO, so the fluorescence is maintained and detected. When an electric field is applied across the microchannel with Nafion membrane in the chip, only the fluorescence of aptamer-LPS complex can be detected and stacked by continuous injection-electrostacking (CI-ES). The method shows a high selectivity (in the presence of pyrophosphate, FAD⁺, NAD⁺, AMP, ADP, ATP, phosphatidylcholine, LTA, and β-d-glucans which respond positively to LAL) to LPS and an extreme sensitivity with the limit of detection (LOD) at 7.9 fM (7.9 × 10^-4 EU/mL) and 8.3 fM (8.3 × 10^-4 EU/mL) for water sample and serum sample, respectively. As a practical application, this method can detect LPS in injections and serum samples of human and sepsis model mouse and quickly distinguish Gram-negative bacteria Escherichia coli ( E. coli) from Gram-positive bacteria Staphylococcus aureus ( S. aureus) and fungus Candida albicans ( C. albicans). More importantly, by changing the aptamers based on different targets, we can detect different analytes. Therefore, aptamer-coupled rGO in a CI-ES biochip is a universal, sensitive, and specific method. For TOC only.

Pressure-Modulated Selective Electrokinetic Trapping for Direct Enrichment, Purification, and Detection of Nucleic Acids in Human Serum

Micro total-analysis systems (μTAS) have been extensively developed for the detection of nucleic acids (NAs) in resource-limited settings in recent years, yet the sample-preparation steps that interface real-world samples with on-chip analytics remain as the technical bottleneck. We report pressure-modulated selective electrokinetic trapping (PM-SET) for the direct enrichment, purification, and detection of NAs in human serum in one step without involving tedious solid-phase extraction, chemical amplification, and surface-hybridization-based assays. Under appropriately modulated hydrostatic pressures, NAs in human serum were selectively enriched in an electrokinetic concentrator with the majority of background proteins removed, achieving an enrichment factor of >4800 in 15 min. A sequence-specific NA was detected simultaneously during the enrichment process using a complementary morpholino (MO) probe, realizing a limit of detection of 3 pM in 15 min. PM-SET greatly reduces the cost, time, and complexity of sample preparation for NA detection and could be easily interfaced with existing NA-detection devices to achieve true sample-to-answer biomolecular analytics.