Concentration gradient focusing and separation in a silica nanofluidic channel with a non-uniform electroosmotic flow

Ion transport through short nanopores modulated by charged exterior surfaces

Short nanopores find extensive applications, capitalizing on their high throughput and detection resolution. Ionic behaviors through long nanopores are mainly determined by charged inner-pore walls. When pore lengths decrease to sub-200 nm, charged exterior surfaces provide considerable modulation to ion current. We find that the charge status of inner-pore walls affects the modulation of ion current from charged exterior surfaces. For 50-nm-long nanopores with neutral inner-pore walls, the charged exterior surfaces on the voltage (surfaceV) and ground (surfaceG) sides enhance and inhibit the ion transport by forming ion enrichment and depletion zones inside nanopores, respectively. For nanopores with both charged inner-pore and exterior surfaces, continuous electric double layers enhance the ion transport through nanopores significantly. The charged surfaceV results in higher ion current by simultaneously weakening the ion depletion at pore entrances and enhancing the intra-pore ion enrichment. The charged surfaceG expedites the exit of ions from nanopores, resulting in a decrease in ion enrichment at pore exits. Through adjustment in the width of charged-ring regions near pore boundaries, the effective charged width of the charged exterior is explored at ∼20 nm. Our results may provide a theoretical guide for further optimizing the performance of nanopore-based applications, such as seawater desalination, biosensing, and osmotic energy conversion.

Trends and challenges in microfluidic methods for protein manipulation-A review

Proteins are important molecules involved in an immensely large number of biological processes. Being capable of manipulating proteins is critical for developing reliable and affordable techniques to analyze and/or detect them. Such techniques would enable the production of therapeutic agents for the treatment of diseases or other biotechnological applications (e.g., bioreactors or biocatalysis). Microfluidic technology represents a potential solution to protein manipulation challenges because of the diverse phenomena that can be exploited to achieve micro- and nanoparticle manipulation. In this review, we discuss recent contributions made in the field of protein manipulation in microfluidic systems using different physicochemical principles and techniques, some of which are miniaturized versions of already established macro-scale techniques.

Modulation mechanism of ionic transport through short nanopores by charged exterior surfaces

Short nanopores have various applications in biosensing, desalination, and energy conversion. Here, the modulation of ionic transport by charged exterior surfaces is investigated through simulations with sub-200 nm long nanopores under applied voltages. Detailed analysis of the ionic current, electric field strength, and fluid flow inside and outside nanopores reveals that charged exterior surfaces can increase ionic conductance by increasing both the concentration and migration speed of charge carriers. The electric double layers near charged exterior surfaces provide an ion pool and an additional passageway for counterions, which lead to enhanced exterior surface conductance and ionic concentrations at pore entrances and inside the nanopores. We also report that charges on the membrane surfaces increase the electric field strength inside nanopores. The effective width of a ring with surface charges placed at pore entrances (Lcs) is considered as well by studying the dependence of the current on Lcs. We find a linear relationship between the effective Lcs and the surface charge density and voltage, and an inverse relationship between the geometrical pore length and salt concentration. Our results elucidate the modulation mechanism of ionic transport through short nanopores by charged exterior surfaces, which is important for the design and fabrication of porous membranes.

Functionalized nanoparticles: Tailoring properties through surface energetics and coordination chemistry for advanced biomedical applications

Significant advances in nanoparticle-related research have been made in the past decade, and amelioration of properties is considered of utmost importance for improving nanoparticle bioavailability, specificity, and catalytic performance. Nanoparticle properties can be tuned through in-synthesis and post-synthesis functionalization operations, with thermodynamic and kinetic parameters playing a crucial role. In spite of robust functionalization techniques based on surface chemistry, scalable technologies have not been explored well. The coordination enhancement via surface functionalization through organic/inorganic/biomolecules material has attracted much attention with morphology modification and shape tuning, which are indispensable aspects in the colloidal phase during biomedical applications. It is envisioned that surface amelioration influences the anchoring properties of nano interfaces for the immobilization of functional groups and biomolecules. In this work, various nanostructure and anchoring methodologies have been discussed, aiming to exploit their full potential in precision engineering applications. Simultaneous discussions on emerging characterization strategies for functionalized assemblies have been made to gain insights into functionalization chemistry. An overview of current advances and prospects of functionalized nanoparticles has been presented, with an emphasis on controllable attributes such as size, shape, morphology, functionality, surface features, Debye and Casimir interactions.

Nanofluidic devices for the separation of biomolecules

Over the last 30-years, microchip electrophoresis and its applications have expanded due to the benefits it offers. Nanochip electrophoresis, on the other hand, is viewed as an evolving area of electrophoresis because it offers some unique advantages not associated with microchip electrophoresis. These advantages arise from unique phenomena that occur in the nanometer domain not readily apparent in the microscale domain due to scale-dependent effects. Scale-dependent effects associated with nanochip electrophoresis includes high surface area-to-volume ratio, electrical double layer overlap generating parabolic flow even for electrokinetic pumping, concentration polarization, transverse electromigration, surface charge dominating flow, and surface roughness. Nanochip electrophoresis devices consist of channels with dimensions ranging from 1 to 1000 nm including classical (1-100 nm) and extended (100 nm - 1000 nm) nanoscale devices. In this review, we highlight scale-dependent phenomena associated with nanochip electrophoresis and the utilization of those phenomena to provide unique biomolecular separations that are not possible with microchip electrophoresis. We will also review the range of materials used for nanoscale separations and the implication of material choice for the top-down fabrication and operation of these devices. We will also provide application examples of nanochip electrophoresis for biomolecule separations with an emphasis on nano-electrophoresis (nEP) and nano-electrochromatography (nEC).

Effective Charged Exterior Surfaces for Enhanced Ionic Diffusion through Nanopores under Salt Gradients

High-performance osmotic energy conversion requires both large ionic throughput and high ionic selectivity, which can be significantly promoted by exterior surface charges simultaneously, especially for short nanopores. Here, we investigate the enhancement of ionic diffusion by charged exterior surfaces under various conditions and explore corresponding effective charged areas. From simulations, ionic diffusion is promoted more significantly by exterior surface charges through nanopores with a shorter length, wider diameter, and larger surface charge density or under higher salt gradients. Effective widths of the charged ring regions near nanopores are reversely proportional to the pore length and linearly dependent on the pore diameter, salt gradient, and surface charge density. Due to the important role of effective charged areas in the propagation of ionic diffusion through single nanopores to cases with porous membranes, our results may provide useful guidance to the design and fabrication of porous membranes for practical high-performance osmotic energy harvesting.

Electroosmotic flow: From microfluidics to nanofluidics

Electroosmotic flow (EOF), a consequence of an imposed electric field onto an electrolyte solution in the tangential direction of a charged surface, has emerged as an important phenomenon in electrokinetic transport at the micro/nanoscale. Because of their ability to efficiently pump liquids in miniaturized systems without incorporating any mechanical parts, electroosmotic methods for fluid pumping have been adopted in versatile applications—from biotechnology to environmental science. To understand the electrokinetic pumping mechanism, it is crucial to identify the role of an ionically polarized layer, the so‐called electrical double layer (EDL), which forms in the vicinity of a charged solid–liquid interface, as well as the characteristic length scale of the conducting media. Therefore, in this tutorial review, we summarize the development of electrical double layer models from a historical point of view to elucidate the interplay and configuration of water molecules and ions in the vicinity of a solid–liquid interface. Moreover, we discuss the physicochemical phenomena owing to the interaction of electrical double layer when the characteristic length of the conducting media is decreased from the microscale to the nanoscale. Finally, we highlight the pioneering studies and the most recent works on electro osmotic flow devoted to both theoretical and experimental aspects.

Joule Heating Effects on Transport-Induced-Charge Phenomena in an Ultrathin Nanopore

Transport-induced-charge (TIC) phenomena, in which the concentration imbalance between cations and anions occurs when more than two chemical potential gradients coexist within an ultrathin dimension, entail numerous nanofluidic systems. Evidence has indicated that the presence of TIC produces a nonlinear response of electroosmotic flow to the applied voltage, resulting in complex fluid behavior. In this study, we theoretically investigate thermal effects due to Joule heating on TIC phenomena in an ultrathin nanopore by computational fluid dynamics simulation. Our modeling results show that the rise of local temperature inside the nanopore significantly enhances TIC effects and thus has a significant influence on electroosmotic behavior. A local maximum of the solution conductivity occurs near the entrance of the nanopore at the high salt concentration end, resulting in a reversal of TIC across the nanopore. The Joule heating effects increase the reversal of TIC with the synergy of the negatively charged nanopore, and they also enhance the electroosmotic flow regardless of whether the nanopore is charged. These theoretical observations will improve our knowledge of nonclassical electrokinetic phenomena for flow control in nanopore systems.

Electropreconcentration diagrams to optimize molecular enrichment with low counter pressure in a nanofluidic device

Concentration polarization (CP)-based focusing electrokinetics nanofluidic devices have been developed in order to simultaneously detect and enrich highly diluted analytes on-a-chip. However, stabilization of focal points over long time under the application of the electric field remains as a technical bottleneck. If pressure-assisted preconcentration methods have been proposed to stabilize propagating modes at low inverse Dukhin number ( 1 / D u ≪ 1 ) , these recent protocols remain laborious for optimizing experimental parameters. In this paper, "electric field E/counter-pressure P" diagrams have been established during pressure-assisted electro-preconcentration of fluorescein as a model molecule. Such E/P diagram allows direct observation of the region for which the optimal counter-pressure P leads to a stable focusing regime. This region of stable focusing is shown to vary depending of the nanoslit length (100 μm < L_nanoslit < 500 μm) and the nature of the background electrolyte (KCl and NaCl). Longer nanoslits (500 μm) produce stabilization at low counter-pressure P, whereas NaCl offers a narrower region of stable focusing in the E/P diagram compared to KCl. Finally, the ability of such pressure-assisted protocol to concentrate negatively charged proteins has been tested with a more applicative protein, i.e., ovalbumin. The corresponding E/P diagram confirms the existence of the stable focusing regime at both low electric field E (≤20 V) and counter-pressure P (≤0.4 bar). With an enrichment factor as high as 70 after 2 min for ovalbumin at a concentration of 10 μM, such pressure-assisted nanofluidic electro-preconcentration protocol appears very promising to concentrate and detect biomolecules.

Size and surface charge characterization of nanoparticles with a salt gradient

Exosomes are nanometer-sized lipid vesicles present in liquid biopsies and used as biomarkers for several diseases including cancer, Alzheimer’s, and central nervous system diseases. Purification and subsequent size and surface characterization are essential to exosome-based diagnostics. Sample purification is, however, time consuming and potentially damaging, and no current method gives the size and zeta potential from a single measurement. Here, we concentrate exosomes from a dilute solution and measure their size and zeta potential in a one-step measurement with a salt gradient in a capillary channel. The salt gradient causes oppositely directed particle and fluid transport that trap particles. Within minutes, the particle concentration increases more than two orders of magnitude. A fit to the spatial distribution of a single or an ensemble of exosomes returns both their size and surface charge. Our method is applicable for other types of nanoparticles. The capillary is fabricated in a low-cost polymer device.

Exosomes are used as disease biomarkers, but their characterization in biological samples is challenging. Here the authors achieve simultaneous characterization of size and zeta potential of individual nanoparticles and particle mixtures at physiological salinity conditions, exploiting a salt gradient in a capillary channel.

Nanochannel Gradient Separations

Gradient-based electrophoretic separations enable simultaneous separation and concentration of molecules. Compared with conventional injection-based separations, they enable enrichment of low-concentration analytes from larger sample volumes that are not limited by an injection volume. We have demonstrated that a nanochannel, connecting two chemically different reservoirs, can maintain a stationary chemical gradient while trapping biomolecules and effectively averaging out many of the complex physicochemical hydrodynamics that would broaden the bands in a meso- or microscale capillary. Here we describe chemical and physical methods that enable this work.

Theory of Transport-Induced-Charge Electroosmotic Pumping toward Alternating Current Resistive Pulse Sensing

In this work, we study transport-induced-charge electroosmosis toward alternating current resistive pulse sensing for the next generation of biomedical applications. Transport-induced-charge electroosmosis, being a new class of electrokinetic phenomenon, occurs as a salt concentration gradient works in synergy with an electric field in ultrathin nanopores. Apart from the conventional electric double layer-governed electroosmotic flow in which the flow behavior is subject to the surface charge, it is found that the transport-induced-charge electroosmotic flow behaves independently of surface charge magnitude but can be linearly regulated by the bulk salt concentration bias. The reversal of the electric field simultaneously inverses the induced charge allowing the establishment of a unidirectional flow under the application of a periodic alternating current field. This unique phenomenon permits continuous water and nanoparticles pumping through a two-dimensional material nanopore in spite of the reversal of the electric field. Built upon this mechanism, we propose a theoretical prototype of alternating current resistive pulse sensing in a two-dimensional nanopore system.

Manipulation of Protein Translocation through Nanopores by Flow Field Control and Application to Nanopore Sensors

The control of biomolecule translocation through nanopores is important in nanopore protein detection. Improvement in current nanopore molecule control is desired to enhance capture rates, extend translocation times, and ensure the effective detection of various proteins in the same solutions. We present a method that simultaneously resolves these issues through the use of a gate-modulated conical nanopore coupled with solutions of varying salt concentration. Simulation results show that the presence of an induced reverse electroosmotic flow (IREOF) results in inlet flows from the two ends of the nanopore centerline entering into the nanopore in opposite directions, which simultaneously elevates the capture rate and immobilizes the protein in the nanopore, thus enabling steady current blockage measurements for a range of proteins. In addition, it is shown that proteins with different size/charge ratios can be trapped by a gate modulation intensified flow field at a similar location in the nanopore in the same solution conditions.

A mobility shift assay for DNA detection using nanochannel gradient electrophoresis

Conventional detection of pathogenic or other biological contamination relies on amplification of DNA using sequence-specific primers. Recent work in nanofluidics has shown very high concentration enhancement of biomolecules with some degree of simultaneous separation. This work demonstrates the combination of these two approaches by selectively concentrating a mobility-shifted hybridization product, potentially enabling rapid detection of rare DNA fragments such as highly specific 16S ribosomal DNA. We have performed conductivity gradient electrofocusing within nanofluidic channels and have shown concentration of hybridized peptide nucleic acids and DNA oligomers. We also show selectivity to single base-pair mismatch on 18-mer oligos. This approach may enable sensitive optical detection of small amounts of DNA.

Electrokinetics of the silica and aqueous electrolyte solution interface: Viscoelectric effects

The manipulation of biomolecules, fluid and ionic current in a new breed of integrated nanofluidic devices requires a quantitative understanding of electrokinetics at the silica/water interface. The conventional capacitor-based electrokinetic Electric Double Layer (EDL) models for this interface have some known shortcomings, as evidenced by a lack of consistency within the literature for the (i) equilibrium constants of surface silanol groups, (ii) Stern layer capacitance, (iii) zeta (ζ) potential measured by various electrokinetic methods, and (iv) surface conductivity. In this study, we consider how the experimentally observable viscoelectric effect - that is, the increase of the local viscosity due to the polarisation of polar solvents - affects electrokinetcs at the silica/water interface. Specifically we consider how a model that considers viscoelectric effects (the VE model) performs against two conventional electrokinetic models, namely the Gouy-Chapman (GC) and Basic Stern capacitance (BS) models, in predicting four fundamental electrokinetic phenomena: electrophoresis, electroosmosis, streaming current and streaming potential. It is found that at moderate to high salt concentrations (>5×10(-3)M) predictions from the VE model are in quantitative agreement with experimental electrokinetic measurements when the sole additional adjustable parameter, the viscoelectric coefficient, is set equal to a value given by a previous independent measurement. In contrast neither the GS nor BS models is able to reproduce all experimental data over the same concentration range using a single, robust set of parameters. Significantly, we also show that the streaming current and potential in the moderate to high surface charge range are insensitive to surface charge behaviour (including capacitances) when viscoelectric effects are considered, in difference to models that do not consider these effects. This strongly questions the validity of using pressure based electrokinetic experiments to measure surface charge characteristics within this experimentally relevant high pH and moderate to high salt concentration range. At low salt concentrations (<5×10(-3)M) we find that there is a lack of consistency in previously measured channel conductivities conducted under similar solution conditions (pH, salt concentration), preventing a conclusive assessment of any model suitability in this regime.

Nanoslit design for ion conductivity gradient enhanced dielectrophoresis for ultrafast biomarker enrichment in physiological media

Selective and rapid enrichment of biomolecules is of great interest for biomarker discovery, protein crystallization, and in biosensing for speeding assay kinetics and reducing signal interferences. The current state of the art is based on DC electrokinetics, wherein localized ion depletion at the microchannel to nanochannel interface is used to enhance electric fields, and the resulting biomarker electromigration is balanced against electro-osmosis in the microchannel to cause high degrees of biomarker enrichment. However, biomarker enrichment is not selective, and the levels fall off within physiological media of high conductivity, due to a reduction in ion concentration polarization and electro-osmosis effects. Herein, we present a methodology for coupling AC electrokinetics with ion concentration polarization effects in nanoslits under DC fields, for enabling ultrafast biomarker enrichment in physiological media. Using AC fields at the critical frequency necessary for negative dielectrophoresis of the biomarker of interest, along with a critical offset DC field to create proximal ion accumulation and depletion regions along the perm-selective region inside a nanoslit, we enhance the localized field and field gradient to enable biomarker enrichment over a wide spatial extent along the nanoslit length. While enrichment under DC electrokinetics relies solely on ion depletion to enhance fields, this AC electrokinetic mechanism utilizes ion depletion as well as ion accumulation regions to enhance the field and its gradient. Hence, biomarker enrichment continues to be substantial in spite of the steady drop in nanostructure perm-selectivity within physiological media.

Electro-osmotic pumping and ion-concentration polarization based on conical nanopores

A numerical investigation is performed into the characteristics of an electro-osmotic pump consisting of a negatively charged conical nanopore. It is shown that the dependence of the flow rectification effect on the bias direction is the reverse of that of the ion current rectification effect. Moreover, the nozzle mode (i.e., the bias is applied from the base side of the nanopore to the tip side) has a higher flow rate compared to the diffuser mode (i.e., the bias is applied from the tip side of the nanopore to the base side). The results showed that the ion-concentration polarization effect occurred inside the conical nanopore, resulting in surface conduction dominating in the ionic current. The ions inside the nanopore are depleted and enriched under the nozzle mode and the diffuser mode, respectively. As a result, the electro-osmotic pump yields a greater pumping pressure, flow rate, and energy conversion efficiency when operating in the nozzle mode. In addition, we also investigated the flow rate rectification behavior for the conical nanopore. The best flow rate rectification factor in this work is 2.06 for an electrolyte concentration of 10(-3) M.

Isoelectric focusing in a silica nanofluidic channel: effects of electromigration and electroosmosis

Isoelectric focusing of proteins in a silica nanofluidic channel filled with citric acid and disodium phosphate buffers is investigated via numerical simulation. Ions in the channel migrate in response to (i) the electric field acting on their charge and (ii) the bulk electroosmotic flow (which is directed toward the cathode). Proteins are focused near the low pH (anode) end when the electromigration effect is more significant and closer to the high pH (cathode) end when the electroosmotic effect dominates. We simulate the focusing behavior of Dylight labeled streptavidin (Dyl-Strep) proteins in the channel, using a relationship between the protein's charge and pH measured in a previous experiment. Protein focusing results compare well to previous experimental measurements. The effect of some key parameters, such as applied voltage, isoelectric point (pI), bulk pH, and bulk conductivity, on the protein trapping behavior in a nanofluidic channel is examined.