Electrokinetic transport through nanochannels

Electrokinetic transport phenomena in nanofluidics and their applications

Much progress has been made in the electrokinetic phenomena inside nanochannels in the last decades. As the dimensions of the nanochannels are compatible to that of the electric double layer (EDL), the electrokinetics inside nanochannels indicate many unexpected behaviors, which show great potential in the fields of material science, biology, and chemistry. This review summarizes the recent development of nanofluidic electrokinetics in both fundamental and applied research. First, the techniques for constructing nanochannels are introduced to give a guideline for choosing the optimal fabrication technique based on the specific feature of the nanochannel. Then, the theories and experimental investigations of the EDL, electroosmotic flow, and electrophoresis of nanoparticles inside the nanochannels are discussed. Furthermore, the applications of nanofluidic electrokinetics in iontronics, sensing, and biomolecule separation fields are summarized. In Section 5, some critical challenges and the perspective on the future development of nanofluidic electrokinetics are briefly proposed.

Ion transport properties in the pH-dependent bipolar nanochannels

In recent years, researchers have made significant strides in understanding the ion transport characteristics of nanochannels, resulting in the development of various materials, modifications, and shapes of nano ion channel membranes. The aim is to create a nanochannel membrane with optimal ion transport properties and high stability by adjusting factors, such as channel size, surface charge, and wettability. However, during the nanochannel film fabrication process, controlling the geometric structures of nanochannels can be challenging. Therefore, exploring the stability of nanochannel performance under different geometric structures has become an essential aspect of nanochannel design. This article focuses on the study of cylindrical nanochannel structures, which are categorized based on the different methods for generating bipolar surface charges on the channel's inner surface, either through pH gradient effects or different material types. Through these two approaches, the study designed and analyzed the stability of ion transport characteristics in two nanochannel models under varying geometric structures. Our findings indicate that nanochannels with bipolar properties generated through pH gradients demonstrate more stable ion selection, whereas nanochannels with bipolar properties generated through different materials show stronger stability in ion rectification. This conclusion provides a theoretical foundation for future nanochannel designs.

Local Electric Field and Electrical Conductivity Analysis Using a Glass Microelectrode

Transport phenomena in microfluidic chips are induced by electric fields and electrolyte concentrations. Liquid flows are often affected by ionic currents driven by electric fields in narrow channels, which are applied in microelectromechanical systems, microreactors, lab-on-a-chip, and so forth. Even though numerical studies to evaluate those local fields have been reported, measurement methods seem to be under construction. To deeply understand the dynamics of ions at the microscale, measurement techniques are necessary to be developed. In this study, we propose a novel method to directly measure electrical potential differences in liquids, local electric fields, and electrical conductivities, using a glass microelectrode. Scanning an electrolyte solution, for example, KCl solutions, with a 1 μm tip under constant ionic current conditions, a potential difference in liquids is locally measured with a micrometer-scale resolution. The conductivity of KCl solutions ranging from 0.56 to 100 mM is evaluated from electric fields locally measured, and errors are within 5% compared with the reference values. It is found that the present method enables us to directly measure local electric fields under constant current and that the electrical conductivity is quantitatively evaluated. Furthermore, it is suggested that the present method is available for various electrical analyses without calibration procedures before measurements.

Mechanism and performance of ionic diodes fabricated from 2D trapezoidal-shaped nanochannels

Bioinspired asymmetric two-dimensional (2D) nanochannels with ionic diode behavior are highly desirable, as they can be constructed and modified easily. However, the knowledge about the rectification mechanism of the nanochannels is still very limited. In this paper, the ionic current rectification (ICR) of the 2D trapezoidal-shaped nanochannels was studied both numerically and experimentally. A multi-physics model, considering the electric field, the ion concentration field, and the flow field, was built for simulating the ion transportation inside the nanochannels. With a limited channel height, the 2D nanochannels are counter-ion selective; therefore, under an external electric field, the accumulation of co-ions takes place at one end of the nanochannels. By introducing shape asymmetry to the nanochannels, the ICR was achieved due to the asymmetric ion concentration polarization at two ends of the nanochannels under opposite electric fields. The structure of the nanochannels, the surface charge density of the nanochannel walls, and the ionic strength of the working fluids affect the ICR of the ionic diodes by changing the ion concentration polarization at two ends of the nanochannels. In the experiment, the current-voltage curves of the nanochannel arrays fabricated by assembling graphene oxide nanosheets were measured, which are in accordance with the numerical results. This paper provides a comprehensive understanding of the mechanism of the 2D trapezoidal-shaped ionic diodes, which may act as a guideline for the design and optimization of ionic diodes.

Dynamics of Ion Channels via Non-Hermitian Quantum Mechanics

We study dynamics and thermodynamics of ion transport in narrow, water-filled channels, considered as effective 1D Coulomb systems. The long range nature of the inter-ion interactions comes about due to the dielectric constants mismatch between the water and the surrounding medium, confining the electric filed to stay mostly within the water-filled channel. Statistical mechanics of such Coulomb systems is dominated by entropic effects which may be accurately accounted for by mapping onto an effective quantum mechanics. In presence of multivalent ions the corresponding quantum mechanics appears to be non-Hermitian. In this review we discuss a framework for semiclassical calculations for the effective non-Hermitian Hamiltonians. Non-Hermiticity elevates WKB action integrals from the real line to closed cycles on a complex Riemann surfaces where direct calculations are not attainable. We circumvent this issue by applying tools from algebraic topology, such as the Picard-Fuchs equation. We discuss how its solutions relate to the thermodynamics and correlation functions of multivalent solutions within narrow, water-filled channels.

Direct observation of confinement-induced diffusophoresis

Nanofluidic devices have channel dimensions which come to within one order of magnitude of the Debye length of common aqueous solutions. Conventionally, external driving is used to create concentration polarization of ions and biomolecules in nanofluidic devices. Here we show that long-range ionic strength gradients intrinsic to all nanofluidic devices, even at equilibrium, also drive a drift of macromolecules. To demonstrate the effect, we confine long DNA to straight nanochannels of constant, rectangular cross-section (100 × 100 nm²) which are connected to large microfluidic reservoirs. The motion of DNA is observed in absence of any driving. We find that at low ionic strengths, molecules in nanochannels migrate toward the nano-micro interface, while they are undergoing purely diffusive motion in high salt. Using numerical models, we demonstrate that the motion is consistent with the ionic strength gradient at the micro-nano interface even at equilibrium, and that the dominant cause of the drift is diffusophoresis.

An improved hybrid continuum-atomistic four-way coupled model for electrokinetics in nanofluidics

In this study, an efficient hybrid continuum-atomistic method is proposed to study electrokinetic transport of aqueous solutions in nanofluidics. The aqueous phase is considered as a continuous phase containing immersed ion particles. The behavior of the system is then simulated through utilization of an improved hybrid continuum-atomistic four-way coupled approach, including the MultiPhase Particle-In-Cell method for the short-ranged interaction between the ion particles, the Brownian force for the collision between the aqueous phase molecules and the ion particles, and a wall force accounting for the short-ranged interaction of ions and walls. The validation of the proposed model with the results of Molecular Dynamics simulations suggests that this model can be a promising approach for studying the electrokinetic phenomena in more complicated geometries where the Molecular Dynamics approach is computationally prohibitive. Finally, the effects of electrokinetic parameters, such as the height of the channel, the external electric field, and bulk ionic concentration, on the electroosmotic flow in a nanochannel are investigated and discussed.

Fabrication of polydimethylsiloxane nanofluidic chips under AFM tip-based nanomilling process

In current research realm, polydimethylsiloxane (PDMS)-based nanofluidic devices are widely used in medical, chemical, and biological applications. In the present paper, a novel nanomilling technique (consisting of an AFM system and a piezoelectric actuator) was proposed to fabricate nanochannels (with controllable sizes) on PDMS chips, and nanochannel size was controlled by the driving voltage and frequency inputted to the piezoelectric actuator. Moreover, microchannel and nanochannel molds were respectively fabricated by UV lithography and AFM tip-based nanomilling, and finally, PDMS slabs with micro/nanochannels were obtained by transfer process. The influences of PDMS weight ratio on nanochannel size were also investigated. The bonding process of microchannel and nanochannel slabs was conducted on a homemade alignment system consisted of an optical monocular microscope and precision stages. Furthermore, the effects of nanochannel size on electrical characteristics of KCl solution (concentration of 1 mM) were analyzed. Therefore, it can be concluded that PDMS nanofluidic devices with multiple nanochannels of sub-100-nm depth can be efficiently and economically fabricated by the proposed method.

Surface Charge Density-Dependent DNA Capture through Polymer Planar Nanopores

Surface charge density of nanopore walls plays a critical role in DNA capture in nanopore-based sensing platforms. This paper studied the effect of surface charge density on the capture of double-stranded (ds) DNA molecules into a polymer planar nanopore numerically and experimentally. First, we simulated the effective driving force ( F_eff) for the translocation of a dsDNA through a planar nanopore with different sizes and surface charge densities. Focus was given on the capture stage from the nanopore mouth into the nanopore by placing a rodlike DNA at the nanopore mouth rather than inside the nanopore. For negatively charged DNA and nanopore walls, electrophoretic driving force ( F_EP) under an electric field is opposed by the viscous drag force by electroosmotic flow ( F_EOF). As the surface charge density of the nanopore wall becomes more negative, F_EOF exceeds F_EP beyond a threshold surface charge density, σ_threshold, where DNA molecules cannot be driven through the nanopore via electrophoretic motion. For a 10 nm diameter nanopore filled with 1× TE buffer, σ_threshold was determined to be -50 mC/m². The simulation results were verified by performing dsDNA translocation experiments using a planar nanopore with 10 nm equivalent diameter imprinted on three polymer substrates with different surface charge densities. Both fluorescence observation and ionic current measurement confirmed that only nanopore devices with the surface charge density less negative than σ_threshold allowed DNA translocation, indicating that the simulated σ_threshold value can be used as a parameter to estimate the translocation of biopolymers in the design of nanopore devices.

Thermoplastic nanofluidic devices for biomedical applications

Microfluidics is now moving into a developmental stage where basic discoveries are being transitioned into the commercial sector so that these discoveries can affect, for example, healthcare. Thus, high production rate microfabrication technologies, such as thermal embossing and/or injection molding, are being used to produce low-cost consumables appropriate for commercial applications. Based on recent reports, it is clear that nanofluidics offers some attractive process capabilities that may provide unique venues for biomolecular analyses that cannot be realized at the microscale. Thus, it would be attractive to consider early in the developmental cycle of nanofluidics production pipelines that can generate devices possessing sub-150 nm dimensions in a high production mode and at low-cost to accommodate the commercialization of this exciting technology. Recently, functional sub-150 nm thermoplastic nanofluidic devices have been reported that can provide high process yield rates, which can enable commercial translation of nanofluidics. This review presents an overview of recent advancements in the fabrication, assembly, surface modification and the characterization of thermoplastic nanofluidic devices. Also, several examples in which nanoscale phenomena have been exploited for the analysis of biomolecules are highlighted. Lastly, some general conclusions and future outlooks are presented.

Influence of temperature gradients on charge transport in asymmetric nanochannels

Imposing a temperature gradient over a charge-selective asymmetric nanochannel enhances the selectivity, total current and rectification of the nanochannel.

Electroosmotic flow in single PDMS nanochannels

The electroosmotic flow (EOF) velocity in single PDMS nanochannels with dimensions as small as 20 nm is investigated systematically by the current slope method in this paper. A novel method for the fabrication of single nanochannels on PDMS surfaces is developed. The effects of channel size, ionic concentration of the electrolyte solution and electric field on the EOF velocity in single nanochannels are investigated. The results show that the EOF velocity in smaller nanochannels with overlapped electric double layers (EDL) is proportional to the applied electric field but is smaller than the EOF velocity in microchannels under the same applied electric field. The EOF velocity in relatively large nanochannels without the overlap of EDLs is independent of the channel size and is the same as that in microchannels under the same applied electric field. Furthermore, in smaller nanochannels with overlapped EDLs, the EOF velocity depends on the ionic concentration and also on the channel size. The experimental results reported in this paper are valuable for the future studies of electrokinetic nanofluidics.

Thermoelectricity in Confined Liquid Electrolytes

The electric field in an extended phase of a liquid electrolyte exposed to a temperature gradient is attributed to different thermophoretic mobilities of the ion species. As shown herein, such Soret-type ion thermodiffusion is not required to induce thermoelectricity even in the simplest electrolyte if it is confined between charged walls. The space charge of the electric double layer leads to selective ion diffusion driven by a temperature-dependent electrophoretic ion mobility, which-for narrow channels-may cause thermovoltages larger in magnitude than for the classical Soret equilibrium.

Temperature effects on the electrohydrodynamic and electrokinetic behaviour of ion-selective nanochannels

A non-isothermal formulation of the Poisson-Nernst-Planck with Navier-Stokes equations is used to study the influence of heating effects in the form of Joule heating and viscous dissipation and imposed temperature gradients on a microchannel/nanochannel system. The system is solved numerically under various cases in order to determine the influence of temperature-related effects on ion-selectivity, flux and fluid flow profiles, as well as coupling between these phenomena. It is demonstrated that for a larger reservoir system, the effects of Joule heating and viscous dissipation only become relevant for higher salt concentrations and electric field strengths than are compatible with ion-selectivity due to Debye layer overlap. More interestingly, it is shown that using different temperature reservoirs can have a strong influence on ion-selectivity, as well as the induced electrohydrodynamic flows.

Interrogating Surface Functional Group Heterogeneity of Activated Thermoplastics Using Super-Resolution Fluorescence Microscopy

We present a novel approach for characterizing surfaces utilizing super-resolution fluorescence microscopy with subdiffraction limit spatial resolution. Thermoplastic surfaces were activated by UV/O3 or O2 plasma treatment under various conditions to generate pendant surface-confined carboxylic acids (-COOH). These surface functional groups were then labeled with a photoswitchable dye and interrogated using single-molecule, localization-based, super-resolution fluorescence microscopy to elucidate the surface heterogeneity of these functional groups across the activated surface. Data indicated nonuniform distributions of these functional groups for both COC and PMMA thermoplastics with the degree of heterogeneity being dose dependent. In addition, COC demonstrated relative higher surface density of functional groups compared to PMMA for both UV/O3 and O2 plasma treatment. The spatial distribution of -COOH groups secured from super-resolution imaging were used to simulate nonuniform patterns of electroosmotic flow in thermoplastic nanochannels. Simulations were compared to single-particle tracking of fluorescent nanoparticles within thermoplastic nanoslits to demonstrate the effects of surface functional group heterogeneity on the electrokinetic transport process.

Salt gradient driven ion transport in solid-state nanopores: the crucial role of reservoir geometry and size

The crucial influence of the reservoir geometry and size on the salt gradient driven ion transport in solid-state nanopores is unraveled.

Pressure-assisted selective preconcentration in a straight nanochannel

We investigate the preconcentration profiles of a fluorescein and bovine serum albumin derivatized with this fluorescent tag in a microfluidic chip bearing a nanoslit. A new preconcentration method in which a hydrodynamic pressure is added to both electroosmotic and electrophoretic contributions is proposed to monitor the location of the preconcentration frontline. A simple predictive model of this pressure-assisted electropreconcentration is proposed for the evolution of the flow profile along this micro/nano/microfluidic structure. We show with a small analyte such as fluorescein that the additional hydrostatic pressure mode enables to stabilize the concentration polarization (CP) effect, resulting in better control of the cathodic focusing (CF) peak. For BSA (bovine serum albumin), we exhibit that the variation of the hydrodynamic pressure can have an even more drastic effect on the preconcentration. We show that, depending on this hydrodynamic pressure, the preconcentration can be chosen, either in the cathodic side or in the anodic one. For the first time, we prove here that both anodic focusing (AF) and cathodic focusing (CF) regimes can be reached in the same structures. These results also open new routes for the detection and the quantification of low abundance biomarkers.

Electrokinetic ion and fluid transport in nanopores functionalized by polyelectrolyte brushes

Chemically functionalized nanopores in solid-state membranes have recently emerged as versatile tools for regulating ion transport and sensing single biomolecules. This study theoretically investigated the importance of the bulk salt concentration, the geometries of the nanopore, and both the thickness and the grafting density of the polyelectrolyte (PE) brushes on the electrokinetic ion and fluid transport in two types of PE brush functionalized nanopore: PE brushes are end-grafted to the entire membrane surface (system I), and to its inner surface only (nanopore wall) (system II). Due to a more significant ion concentration polarization (CP), the enhanced local electric field inside the nanopore, the conductance, and the electroosmotic flow (EOF) velocity in system II are remarkably smaller than those in system I. In addition to a significantly enhanced EOF inside the nanopore, the direction of the flow field near both nanopore openings in system I is opposite to that of EOF inside the nanopore. This feature can be applied to regulate the electrokinetic translocation of biomolecules through a nanopore in the nanopore-based DNA sequencing platform.

Thermal analysis of nanofluids in microfluidics using an infrared camera

We present the thermal analysis of liquid containing Al(2)O(3) nanoparticles in a microfluidic platform using an infrared camera. The small dimensions of the microchannel along with the low flow rates (less than 120 μl min(-1)) provide very low Reynolds numbers of less than 17.5, reflecting practical parameters for a microfluidic cooling platform. The heat analysis of nanofluids has never been investigated in such a regime, due to the deficiencies of conventional thermal measurement systems. The infrared camera allows non-contact, three dimensional and high resolution capability for temperature profiling. The system was studied at different w/w concentrations of thermally conductive Al(2)O(3) nanoparticles and the experiments were in excellent agreement with the computational fluid dynamics (CFD) simulations.