Ionic current rectification in asymmetric nanofluidic devices

In recent decades, the properties and behaviors of nanofluidic devices have been widely explored in varied subjects such as engineering, physics, chemistry, and biology. Among the rich properties of nanofluidics, ionic current rectification (ICR) is a unique phenomenon arising from asymmetric nanofluidic devices with electric double layer (EDL) overlapped. The ICR property is especially useful in applications including energy conversion, mass separation, sea water purification and bioanalysis. In this review, the ICR property in nanofluidics as well as the underlying mechanism is demonstrated. The influencing factors concerning to the ICR property are systematically summarized. The asymmetric geometry as well as the charge distribution is in charge of the ICR behavior occurring in nanofluidic devices. This review is aimed at readers who are interested in the fundamentals of mass transport in nanofluidics in general, as well as those who are willing to apply nanofluidics in various research fields.     

Electrokinetic transport through nanochannels

This article presents a numerical study of the electrokinetic transport phenomena (electroosmosis and electrophoresis) in a three-dimensional nanochannel with a circular cross-section. Due to the nanometer dimensions, the Boltzmann distribution of the ions is not valid in the nanochannels. Therefore, the conventional theories of electrokinetic flow through the microchannels such as Poisson-Boltzmann equation and Helmholtz-Smoluchowski slip velocity approach are no longer applicable. In the current study, a set of coupled partial differential equations including Poisson-Nernst-Plank equation, Navier-Stokes, and continuity equations is solved to find the electric potential field, ionic concentration field, and the velocity field in the three-dimensional nanochannel. The effects of surface electric charge and the radius of nanochannel on the electric potential, liquid flow, and ionic transport are investigated. Unlike the microchannels, the electric potential field, ionic concentration field, and velocity field are strongly size-dependent in nanochannels. The electric potential gradient along the nanochannel also depends on the surface electric charge of the nanochannel. More counter ions than the coions are transported through the nanochannel. The ionic concentration enrichment at the entrance and the exit of the nanochannel is completely evident from the simulation results. The study also shows that the flow velocity in the nanochannel is higher when the surface electric charge is stronger or the radius of the nanochannel is larger.     

电动泵技术研究进展

电液动力学（EHD）研究的是在流体上施加电场后流体的流动。在微分析系统中,EHD的主要应用是电动泵（EKP）技术即电泳泵和电渗泵两个主要泵技术。这些独特的泵技术被广泛应用在推动极小截面的管道中化学和生物流体流动,管道尺寸从平方毫米到平方微米量级,甚至纳米通道量级。近年来,电渗泵出现了填充床、整体柱、平行多通道、纳米通道和微孔膜等各种新的形式,显示了其在微分析系统中的集成化和在毛细管液相色谱、流动注射分析和药物输送等应用研究中的潜力。本文对电动泵技术进行评述。     

纳通道的物质传输特性及应用

近年来,随着材料科学、微纳加工技术和微纳尺度物质传输理论的发展,纳通道技术得到了越来越多的研究和关注。纳通道包括生物纳通道和人工纳通道,其孔径通常为1~100 nm。在这一尺度下,通道表面与通道内物质之间的作用概率大大增强,使得纳通道表现出许多与宏观体系不同的物质传输特性,例如通道表面电荷与通道内离子之间的静电作用产生了离子选择性,通道内电化学势的不对称分布产生了离子整流特性,物质传输过程中占据通道产生了阻塞脉冲特性等。纳通道中的这些物质传输特性在传感、分离、能源等领域具有广泛应用,例如通过对纳通道进行功能化修饰可以实现门控离子传输;利用亚纳米尺度的通道可以实现单分子传感;利用通道与传输物质之间的相互作用可以实现离子、分子、纳米粒子的分离;利用纳通道的离子选择性可以在通道内实现电荷分离,将不同形式的能量（如光、热、压力、盐差等）高效转化为电能。纳通道技术是化学、材料科学、纳米技术等多学科的交叉集合,在解决生物、环境、能源等基本问题方面具有良好的前景。该文综述了近10年来与纳通道物质传输理论以及纳通道技术应用相关的前沿研究,梳理了纳通道技术的发展过程,并对其在各个领域的应用进行了总结与展望。     

Predicting ion concentration polarization and analyte stacking/focusing at nanofluidic interfaces

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.     

Thermally responsive ionic transport system reinforced by aligned functional carbon nanotubes backbone

Ion transport plays an important role in energy conversion, biosensors, and a variety of biological processes. Carbon nanotubes, especially for the carbon nanotubes arrays with controlled vertically aligned structures, have displayed great potential as a promising material for regulating ion transport behaviors in the applications of the nanofluidic devices and osmotic energy conversion. Herein, we demonstrate the thermo-controlled ion transport system through the vertically aligned multiwall carbon nanotubes arrays membrane modified by the thermo-responsive hydrogel in a simple and reliable way. The functional carbon nanotubes backbone with the inherent surface charge and interstitial channels structure renders the system improved ion transport behaviors and well controlled switching property by thermo. Based on the integrated properties, the energy output from osmotic power in this system could be regulated by the reversible temperature switches. Moreover, it can realize a higher osmotic energy conversion property regulated by the thermos, which may extend the practical application in the future. The system that combines intelligent response with controlled ion transport behaviors and potential osmotic energy utilizations presents a valuable paradigm for the use of carbon nanotubes and hydrogel composite materials and provides a promising way for applications of nanofluidic devices.     

Design and Fabrication of a Biomimetic Nanochannel for Highly Sensitive Arginine Response in Serum Samples

Inspired from their biological counterparts, chemical modification of the interior surface of nanochannels with functional molecules may provide a highly efficient means to control ionic or molecular transport through nanochannels. Herein, we have designed and prepared a aldehyde calix[4]arene (C4AH), which was attached to the interior surface of a single nanochannel by using a click reaction, and that showed a high response for arginine (Arg). Furthermore, the nanofluidic sensing system has been challenged with complex matrices containing a high concentration of interfering sequences and serum. Based on this finding, we believe that the artificial nanochannel can be used for practical Arg‐sensing devices, and be applied in a biological environment.     

Nanofluidic channels by anodic bonding of amorphous silicon to glass to study ion-accumulation and ion-depletion effect

A unique phenomenon, ion-enrichment and ion-depletion effect, exists in nanofluidic channels and is observed in amorphous silicon (α-Si) nanochannels as shallow as 50 nm. As a voltage is applied across a nanochannel, ions are rapidly enriched at one end and depleted at the other end of the nanochannel. α-Si is deposited on glass by plasma enhanced chemical vapor deposition and is selectively etched to form nanochannels. The depth of nanochannels is defined by the thickness of the α-Si layer. Low temperature anodic bonding of α-Si to glass was used to seal the channel with a second glass wafer. The strength of the anodic bond was optimized by the introduction of a silicon nitride adhesion promoting layer and double-sided bonding resulting from the electric field reversal. Completed channels, 50 nm in depth, 5 micron wide, and 1 mm long were completely and reliably sealed. Structures based on nanochannels 50-300 nm deep were successfully incorporated into nanofluidic devices to investigate ionic accumulation and depletion effect due to overlapping of electric double layer.     

Scaling of electrokinetic transport in nanometer channels

Electrokinetic transport is a popular transport mechanism used in nanofluidic systems, and understanding its scaling behavior is important for the design and optimization of nanofluidic devices. In this article, we report on the scaling of electroosmotic flow and ionic conductivity in positively charged slit nanochannels by using continuum and atomistic simulations. The effects of confinement and surface charge are discussed in detail. In particular, we found that the viscosity of the interfacial water increases substantially as the surface charge density increases and the electrophoretic mobility of the interfacial ions decreases. We show that such effects can influence the scaling of the electrokinetic transport in confined nanochannels significantly.     

Nanoscale molecular traps and dams for ultrafast protein enrichment in high-conductivity buffers

We report a new approach, molecular dam, to enhance mass transport for protein enrichment in nanofluidic channels by nanoscale electrodeless dielectrophoresis under physiological buffer conditions. Dielectric nanoconstrictions down to 30 nm embedded in nanofluidic devices serve as field-focusing lenses capable of magnifying the applied field to 10(5)-fold when combined with a micro- to nanofluidic step interface. With this strong field and the associated field gradient at the nanoconstrictions, proteins are enriched by the molecular damming effect faster than the trapping effect, to >10(5)-fold in 20 s, orders of magnitude faster than most reported methods. Our study opens further possibilities of using nanoscale molecular dams in miniaturized sensing platforms for rapid and sensitive protein analysis and biomarker discovery, with potential applications in precipitation studies and protein crystallization and possible extensions to small-molecules enrichment or screening.     

Complete plastic nanofluidic devices for DNA analysis via direct imprinting with polymer stamps

Development of all polymer-based nanofluidic devices using replication technologies, which is a prerequisite for providing devices for a larger user base, is hampered by undesired substrate deformation associated with the replication of multi-scale structures. Therefore, most nanofluidic devices have been fabricated in glass-like substrates or in a polymer resist layer coated on a substrate. This letter presents a rapid, high fidelity direct imprinting process to build polymer nanofluidic devices in a single step. Undesired substrate deformation during imprinting was significantly reduced through the use of a polymer stamp made from a UV-curable resin. The integrity of the enclosed all polymer-based nanofluidic system was verified by a fluorescein filling experiment and translocation/stretching of λ-DNA molecules through the nanochannels. It was also found that the funnel-like design of the nanochannel inlet significantly improved the entrance of DNA molecules into nanochannels compared to an abrupt nanochannel/microfluidic network interface.     

Numerical investigation of the current transition regimes in nanochannels

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

A simple polysilsesquioxane sealing of nanofluidic channels below 10 nm at room temperature

We present a simple sealing method to fabricate nanofluidic channels, where plasma treated polysilsesquioxane (PSQ) thin film on a rigid support is used to bond to a hydrophilic glass surface permanently at room temperature. This method shows precise dimension control below 10 nm with easy experimental setup. Using this method, one dimensional confined shallow nanochannels with a depth as small as 8 nm and an aspect ratio of <4 x 10(-5), two dimensional confined nanochannel arrays, and integrated nano/microchannel devices with a micro-to-nano interface have been demonstrated. Smooth transfer of DNA fragments from microchannel to nanochannel through the interface area was observed.     

Effects of Stern layer conductance on electrokinetic energy conversion in nanofluidic channels

A thermo-electro-hydro-dynamic model is developed to analytically account for the effects of Stern layer conductance on electrokinetic energy conversion in nanofluidic channels. The optimum electrokinetic devices performance is dependent on a figure of merit, in which the Stern layer conductance appears as a nondimensional Dukhin number. Such surface conductance is found to significantly reduce the figure of merit and thus the efficiency and power output. This finding may explain why the recently measured electrokinetic devices performances are far below the theoretical predictions where the effects of Stern layer conductance have been ignored.     

A Bioinspired Switchable and Tunable Carbonate‐Activated Nanofluidic Diode Based on a Single Nanochannel

A smart nanofluidic diode that exhibits both ion gating and ion current rectification has been developed using a 1‐(4‐amino‐phenyl)‐2,2,2‐trifluoro‐ethanone‐functionalized, conical nanochannel in a polyimide (PI) membrane. The switch‐like property can be tuned by controlling the wettability and charge distribution with carbonate ions. Such a nanodevice is advantageous for precisely controlling conductive states with an ultrahigh gating ratio of up to 5000, and a high rectification ratio of 27. By virtue of the high selectivity and sensitivity for carbonate ions, this nanofluidic diode may find applications in carbonate or carbon dioxide detection.     

Ionic conductance of nanopores in microscale analysis systems: where microfluidics meets nanofluidics

In this tutorial review we illustrate the origin and dependence on various system parameters of the ionic conductance that exists in discrete nanochannels as well as in nanoporous separation and preconcentration units contained as hybrid configurations, membranes, packed beds, or monoliths in microscale liquid phase analysis systems. A particular complexity arises as external electrical fields are superimposed on internal chemical and electrical potential gradients for tailoring molecular transport. It is demonstrated that the variety of geometries in which the microfluidic/nanofluidic interfaces are realized share common, fundamental features of coupled mass and charge transport, but that phenomena behind the key steps in a particular application can be significantly tuned, depending on the morphology of a material. Thus, the understanding of morphology-related transport in internal and external electrical potential gradients is critical to the performance of a device. This addresses a variety of geometries (slits, channels, filters, membranes, random or regular networks of pores, etc.) and applications, e. g., the gating, sensing, preconcentration, and separation in multifunctional miniaturized devices. Inherently coupled mass and charge transport through ion-permselective (charge-selective) microfluidic/nanofluidic interfaces is analyzed with a stepwise-added complexity and discussed with respect to the morphology of the charge-selective spatial domains. Within this scenario, the electrostatics and electrokinetics in microfluidic and nanofluidic channels, as well as the electrohydrodynamics evolving at microfluidic/nanofluidic interfaces, where microfluidics meets nanofluidics, define the platform of central phenomena.     

Flexible fabrication and applications of polymer nanochannels and nanoslits

Fluidic devices that employ nanoscale structures (<100 nm in one or two dimensions, slits or channels, respectively) are generating great interest due to the unique properties afforded by this size domain compared to their micro-scale counterparts. Examples of interesting nanoscale phenomena include the ability to preconcentrate ionic species at extremely high levels due to ion selective migration, unique molecular separation modalities, confined environments to allow biopolymer stretching and elongation and solid-phase bioreactions that are not constrained by mass transport artifacts. Indeed, many examples in the literature have demonstrated these unique opportunities, although predominately using glass, fused silica or silicon as the substrate material. Polymer microfluidics has established itself as an alternative to glass, fused silica, or silicon-based fluidic devices. The primary advantages arising from the use of polymers are the diverse fabrication protocols that can be used to produce the desired structures, the extensive array of physiochemical properties associated with different polymeric materials, and the simple and robust modification strategies that can be employed to alter the substrate's surface chemistry. However, while the strengths of polymer microfluidics is currently being realized, the evolution of polymer-based nanofluidics has only recently been reported. In this critical review, the opportunities afforded by polymer-based nanofluidics will be discussed using both elastomeric and thermoplastic materials. In particular, various fabrication modalities will be discussed along with the nanometre size domains that they can achieve for both elastomer and thermoplastic materials. Different polymer substrates that can be used for nanofluidics will be presented along with comparisons to inorganic nanodevices and the consequences of material differences on the fabrication and operation of nanofluidic devices (257 references).     

Induced electrokinetic transport in micro-nanofluidic interconnect devices

Hybrid micro-nanofluidic interconnect devices can be used to control analyte transfer from one microchannel to the other through a nanochannel under rest, injection, and recovery stages of operation by varying the applied potential bias. Using numerical simulations based on coupled transient Poisson-Nernst-Planck and Stokes equations, we examine the electrokinetic transport in a gateable device consisting of two 100 microm long, 1 microm wide negatively charged microchannels connected by a 1 microm long, 10 nm wide positively charged nanochannel under both positive and negative bias potentials. During injection, accumulation of ions is observed at the micro-nano interface region with the positive potential and depletion of ions is observed at the other micro-nano junction region. Net space charge in the depletion region gives rise to nonlinear electrokinetic transport during the recovery stage due to induced pressure, induced electroosmotic flow of the second kind, and complex flow circulations. Ionic currents are computed as a function of time for both positive and negative bias potentials for the three stages. Analytical expressions derived for ion current variation are in agreement with the simulated results. In the presence of multiple accumulation or depletion regions, we show that a hybrid micro-nano device can be designed to function as a logic gate.     

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.