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.     

Ionic rectification properties of a bipolar interface consisting of a cationic surfactant and cation-exchange membrane

A novel bipolar interface that consists of cationic surfactant and cation-exchange membrane was successfully prepared in an aqueous electrolyte system. This bipolar interface shows a ionic rectification behavior similar to that observed in bipolar membranes. However, different from bipolar membranes, this system has a total rectification behavior, where we cannot observe the occurrence of a water-splitting phenomenon, which always occurs in the bipolar membrane process under reverse bias conditions.     

Metal ion coordination with an asymmetric fan-shaped dendrimer at the air-water interface

Metal coordination to monolayers of 4-{10-[4-(3,5-bis-benzyloxy)-phenyl]-anthracen-9-yl}-benzoic acid ([G1-An]-CO(2)H, G1) and 4-(10-{4-[3,5-bis-(3,5-bis-benzyloxy)-benzyloxy]-phenyl}-anthracen-9-yl)-benzoic acid ([G2-An]-CO(2)H, G2) at the air-water interface and to Langmuir-Blodgett (LB) films was investigated using surface pressure-area isotherms, ultraviolet-visible (UV-vis) spectroscopy, atomic force microscopy (AFM), and X-ray reflectivity (XRR). Surface pressure-area isotherms show that G1 and G2 have different limiting areas according to the type of subphase. The limiting area of G1 and G2 increased more with Al(3+) than with Eu(3+) in the subphase. This result indicates that the hydrophilic core group is anchored to ions in the water via bidentate chelates with the carboxylate oxygen atoms of G1 and G2. Circular domains and aggregates were observed for the LB film. The different behavior of Eu(3+) and Al(3+) complexes is originated from the intrinsic nature of the ion, i.e., coordination number.     

A pH-tunable nanofluidic diode: electrochemical rectification in a reconstituted single ion channel

We report pH-dependent electrochemical rectification in a protein ion channel (the bacterial porin OmpF) reconstituted on a planar phospholipid membrane. The measurements performed at single-channel level show that the electric current is controlled by the protein fixed charge and it can be tuned by adjusting the local pH. Under highly asymmetric pH conditions, the channel behaves like a liquid diode. Unlike other nanofluidic devices that display also asymmetric conductance, here the microscopic charge distribution of the system can be explored by using the available high-resolution (2.4 A) channel crystallographic structure. Continuum electrostatics calculations confirm the hypothesized bipolar structure of the system. The selective titration of the channel residues is identified as the underlying physicochemical mechanism responsible for current rectification.     

Ion current rectification at nanopores in glass membranes

The origin of ion current rectification observed at conical-shaped nanopores in glass membranes immersed in KCl solutions has been investigated using finite-element simulations. The ion concentrations and fluxes (due to diffusion, migration, and electroosmotic convection) were determined by the simultaneous solution of the Nernst-Planck, Poisson, and Navier-Stokes equations for the two-ion (K+ and Cl-) system. Fixed surface charge on both the internal and external glass surfaces that define the pore structure was included to account for electric fields and nonuniform ion conductivity within the nanopores and electric fields in the external solution near the pore mouth. We demonstrate that previous observations of ion current rectification in conical-shaped glass nanopores are a consequence of the voltage-dependent solution conductivity in the vicinity of the pore mouth, both inside and outside of the pore. The simulations also demonstrate that current rectification is maximized at intermediate bulk ion concentrations, a combination of (i) the electrical screening of surface charge at high concentrations and (ii) a fixed number of charge-carrying ions in the pore at lower concentration, which are physical conditions where the voltage dependence of the conductivity disappears. In addition, we have quantitatively shown that electroosmotic flow gives rise to a significant but small contribution to current rectification.     

A hydrophobic entrance enhances ion current rectification and induces dewetting in asymmetric nanopores

Hydrophobic interactions and local dewetting of hydrophobic cavities have been identified as a key mechanism for ionic gating in biological voltage-gated channels in a cell membrane. Hydrophobic interactions are responsible for rectification of the channels, i.e. the ability to transport ions more efficiently in one direction compared to the other. We designed single polymer nanopores with a hydrophobic gate on one side in the form of a single layer of C10 or C18 thiols. This nanoporous system behaves like an ionic diode whose direction of rectification is regulated by the pH of the electrolyte. In addition, reversible dewetting of the hydrophobic region of the pore was observed as voltage-dependent ion current fluctuations in time between conducting and non-conducting states. The observations are in accordance with earlier molecular dynamics simulations, which predicted the possibility of spontaneous and reversible dewetting of hydrophobic pores.     

Current rectification by asymmetric molecules: an ab initio study

Current rectification effect in an asymmetric molecule HCOO-C6H4-(CH2)n sandwiched between two aluminum electrodes has been studied using an ab initio nonequilibrium Green's function method. The conductance of the system decreases exponentially with the increasing number n of CH2. The phenomenon of current rectification is observed such that a very small current appears at negative bias and a sharp negative differential resistance at a critical positive bias when n>or=2. The rectification effect arises from the asymmetric structure of the molecule and the molecule-electrode couplings. A significant rectification ratio of approximately 38 can be achieved when n=5.     

Design and fabrication of a multilayered polymer microfluidic chip with nanofluidic interconnects via adhesive contact printing

The design and fabrication of a multilayered polymer micro-nanofluidic chip is described that consists of poly(methylmethacrylate) (PMMA) layers that contain microfluidic channels separated in the vertical direction by polycarbonate (PC) membranes that incorporate an array of nanometre diameter cylindrical pores. The materials are optically transparent to allow inspection of the fluids within the channels in the near UV and visible spectrum. The design architecture enables nanofluidic interconnections to be placed in the vertical direction between microfluidic channels. Such an architecture allows microchannel separations within the chip, as well as allowing unique operations that utilize nanocapillary interconnects: the separation of analytes based on molecular size, channel isolation, enhanced mixing, and sample concentration. Device fabrication is made possible by a transfer process of labile membranes and the development of a contact printing method for a thermally curable epoxy based adhesive. This adhesive is shown to have bond strengths that prevent leakage and delamination and channel rupture tests exceed 6 atm (0.6 MPa) under applied pressure. Channels 100 microm in width and 20 microm in depth are contact printed without the adhesive entering the microchannel. The chip is characterized in terms of resistivity measurements along the microfluidic channels, electroosmotic flow (EOF) measurements at different pH values and laser-induced-fluorescence (LIF) detection of green-fluorescent protein (GFP) plugs injected across the nanocapillary membrane and into a microfluidic channel. The results indicate that the mixed polymer micro-nanofluidic multilayer chip has electrical characteristics needed for use in microanalytical systems.     

Ionic solvation at the solid/electrolyte interface: A statistical-mechanical approach

We have studied studied the influence of the size of ions on their adsorbability at a solid surface in the presence of a molecular solvent. Ions and molecules are represented respectively by charged hard spheres and dipolar hard spheres and the surface is just a neutral hard wall. We have found that the electrostatic interaction between ions and molecules can induce the exclusion of small ions from the surface. A pure MSA (mean spherical approximation) calculation would not give any effect of the solvation on the ionic density profile. The present calculation is limited to the case of infinite ionic dilution.     

The iron/electrolyte interface at extremely large current densities

Electrochemical Machining (ECM) is a technique to shape metal parts by anodic dissolution in neutral solutions (e.g. aqueous NaNO₃) at extremely large current densities of about 100 A cm⁻². We developed a capillary based microcell to simulate this process in the laboratory and to simultaneously analyze the products by UV-Vis spectroscopy. In addition, a brief overview of electrochemical micro techniques is given.     

Continuous flow separation of particles within an asymmetric microfluidic device

A microfluidic based device has been developed for the continuous separation of polymer microspheres, taking advantage of the flow characteristics of systems. The chip consists of an asymmetric cavity with variable channel width which enables continuous amplification of the particle separation for different size particles within the laminar flow profile. The process has been examined by varying the sample inlet position, the sample to media flow rate ratio, and the total flow rate. This technique can be applied for manipulating both microscale biological and colloidal particles within microfluidic systems.     

Direct observation of chemical oscillation at a water/nitrobenzene interface with a sodium-alkyl-sulfate system.

The oscillation of the interfacial tension and electrical potential at a water/nitrobenzene interface was observed with homologous anionic surfactant molecules, sodium-alkyl-sulfates. Concerning small molecules with a short hydrophobic carbon chain, the oscillation period and amplitude decreased with a decrease of the length of the alkyl chain. On the other hand, when surfactant molecules with a long hydrophobic carbon chain were used, no remarkable periodic oscillation occurred after the first oscillation. In all systems, an interfacial flow by Marangoni convection was observed when the oscillation took place. By monitoring the movement of carbon powder scattered on the liquid/liquid interface with a CCD camera, we could observe that the liquid/liquid interface expanded outward from the area on which the surfactant molecules adsorbed when the oscillation occurred. When the small molecule was used, the speed of expansion of the interface (flow speed) was small and shrinkage followed by expansion of the interface repeatedly occurred. However, when the large molecule was used, the flow speed was large and expansion occurred only one time. These results show that hydrodynamic factors and surface activities are important in chemical oscillation systems.     

Ion current rectification and rectification inversion in conical nanopores: a perm-selective view

Ionic transport in charged conical nanopores is known to give rise to ion current rectification. The present study shows that the rectification direction can be inverted when using electrolyte solutions at very low ionic strengths. To elucidate these phenomena, electroneutral conical nanopores containing a perm-selective region at the tip have been investigated and shown to behave like classical charged nanopores. An analytical model is proposed to account for these rectification processes.     

Development of a pressure-driven nanofluidic control system and its application to an enzymatic reaction

A novel air-pressure-based nanofluidic control system was developed and its performance was examined. We found that the flow in a 100 nm scale nanochannel on a chip (called an extended nanospace channel) could be controlled within the pressure range of 0.003–0.4 MPa, flow rate range of 0.16–21.2 pL/min, and residence time range of 24 ms–32.4 s by using the developed nanofluidic control system. Furthermore, we successfully demonstrated an enzyme reaction in which the fluorogenic substrate TokyoGreen-β-galactoside (TG-β-gal) was hydrolyzed to the fluorescein derivative TokyoGreen (TG) and β-galactose by the action of β-galactosidase enzyme as a calalyst in a Y-shaped extended nanospace channel. The parameters for the reaction kinetics, such as K _m, V _max and k _cat, were estimated for the nanofluidic reaction, and these values were compared with the results of bulk and microfluidic reactions. A comparison showed that the enzyme reaction rate in the Y-shaped extended nanospace channel increased by a factor of about two compared with the rates in the bulk and micro spaces. We thought that this nanospatial property resulted from the activated protons of water molecules in the extended nanospace. This assumption was supported by the result that the pH dependence of the maximum enzyme activity in the Y-shaped extended nanospace channel was slightly different from that in the bulk and micro spaces. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Ion transport regulation through triblock copolymer/PET asymmetric nanochannel membrane:Model system establishment and rectification mapping

Controlling ions transport across the membrane at different pH environments is essential for the physiological process and artificial systems.Many efforts have been devoted to pH-responsive ion gating,while rarely systems can maintain the rectification in pH-changing environments.Here,a composite nanochannel system is fabricated,which shows unidirectional rectification with high performance in a wide pH range.In the system,block copolymer(BCP) and polyethylene te rephthalate(PET) are employed for the amphoteric nanochannels fabrication.Based on the composite system,a model is built for the theoretical simulation.Thereafter,rectification mapping is conducted on the system,which can provide abundant info rmation about the relations between charge distribution and ions transport prope rties.The proposed rectification mapping can definitely help to design new materials with special ion transport properties,such as high-performance membranes used in the salinity gradient power generation field.     

Visualization of the interfacial turbulence associated with remarkable faradaic current amplification at a polarized water/1,2-dichloroethane interface

We report the first visualization of the interfacial turbulence developed at the polarized water/1,2-dichloroethane interface in the form of rotating surface structures. This leads to a remarkable amplification of the faradaic current (polarographic maximum) associated with the transfer of the Na⁺ ion from 1,2-dichloroethane to water. Interfacial turbulence is visualized at the surface of the sessile electrolyte drop using suspended graphite microparticles as a fluid flow tracer. We show that the magnitude of the faradaic current increases as the circular surface structures rotate more rapidly. The results appear to differ from the profile of the disturbing flow near the mercury/electrolyte interface accompanying polarographic maxima of the first kind.     

Self-assembly of asymmetric fan-shaped dendrimers with different generation numbers at the air-water interface

Monolayer formation of two dendrimers containing a hydrophilic core group (COOH) and hydrophobic peripheral groups (anthracene and aryl ether tail groups), 4-{10-[4-(3,5-bis-benzyloxy)-phenyl]-anthracen-9-yl}-benzoic acid (G1) and 4-(10-{4-[3,5-bis-(3,5-bis-benzyloxy)-benzyloxy]-phenyl}-anthracen-9-yl)-benzoic acid (G2), were studied. To understand the mechanism of the self-assembly of these molecules, we measured the surface pressure-surface area (Pi- A) isotherm and investigated the surface texture of Langmuir-Blodgett monolayers transferred onto hydrophilic silicon wafers. Both dendrimers form circular domains at the onset point of surface pressure as a result of the difference in hydrophobicity between the core group and the peripheral end group. The core group has a functional group at the end of dendrimer and can be anchored on the water surface. Upon further compression, monolayer of G1 shows a domain of molecules whereas a monolayer of G2 is aligned in the direction of compression at 10 mN/m. At higher surface pressure (20 mN/m), G1 molecules have several aggregates of domains, but G2 molecules maintain their ordering. These results were confirmed by the electron density profile of G1 and G2 monolayers transferred to silicon substrates, as measured by X-ray reflectivity.     

Responsive Polyelectrolyte Multilayers Assembled at High Ionic Strength with an Unusual Collapse at Low Ionic Strength

Responsive polyelectrolyte multilayers (PEMs) of poly(diallyl dimethyl ammonium chloride) (PDADMAC) and poly(styrene sodium sulfonate) (PSS) with thicknesses between 350 and 400 nm for 11 deposited polyelectrolyte layers were fabricated assembling the polyelectrolytes at 3 M NaCl. When the 3 M NaCl bulk solution is replaced by water, the PEMs release water, approximately a 46% of the total mass, and experience a thickness reduction of more than 200 nm. Changes in thickness and water content are fully reversible. The film recovers its original thickness and water content when it is exposed again to a 3 M NaCl solution. A responsive polymer film is achieved with the capability of swelling at high ionic strength and collapsing in water with variations in thickness of hundred of nanometers.     

Ionic components dependence of the charge transfer reactions at the silicon/HF solution interface

The actual requirements for circuit miniaturization and production economy require obtaining smooth silicon surfaces using diluted chemicals, especially HF treatment. This fundamental research deals with the electrochemical corrosion of n- and p-type silicon substrates in 0.25 M dilute HF solutions, and examines the influence of fluoride ions or protons additives. All experiments were conducted both in the dark and under constant light flux, with solutions thoroughly degassed by high purity argon bubbling. Polarization resistance measurements near the open circuit potential lead to the value of the corrosion current. The kinetics of charge transfer reactions, studied by linear voltammetry, were interpreted as a function of the carrier density in the energy levels of the semiconductor and the concentration of acceptor species in the solution. The influence of these parameters on the surface roughening of the silicon samples was also studied by ex situ atomic force microscopy profile measurements. Received: 13 December 1998 / Accepted: 29 March 1999     

Continuous on-line derivatization and determination of amino acids by a microfluidic capillary electrophoresis system with a continuous sample introduction interface

An automatic, rapid and continuous on-line derivatization system coupled to microfluidic capillary electrophoresis (CE) for the determination of amino acids using o-phthaldialdehyde/N-acetyl-l-cysteine (OPA/NAC) as the derivative agents has been developed. By on-line derivatization, amino acids were automatically and reproducibly converted to the UV-absorbing derivatives, which were separated by capillary zone electrophoresis (CZE). Optimization of derivatization and separation condition was carried out to achieve both good sensitivity and separation efficiency. The separation could be achieved within 4 min and sample throughput rate can reach up to 16 h⁻¹. The repeatability (defined as relative standard deviation, R.S.D.) was 2.56, 2.85, 3.24 and 3.60% with peak area evaluation and 2.93, 3.12, 4.20 and 4.91% with peak height evaluation for arginine (Arg), phenylalanine (Phe), serine (Ser) and glycine (Gly), respectively. The limits of detection (S/N=3) were 10.46, 13.14, 34.39 and 44.79 μmol/l for Arg, Phe, Ser and Gly, respectively. Major advantages of the proposed method include improved precision and efficient automation of the derivatization by the FI system and the enhanced sampling frequencies by the combined FI-CE system.