Ion Current Rectification Behavior of Bioinspired Nanopores Having a pH-Tunable Zwitterionic Surface

Electroosmotic flow modulation and dispersion of uncharged solutes in soft nanochannel

We perform a systematic study on the modulation of electroosmotic flow (EOF), tuning the selectivity using electrolyte ions and hydrodynamic dispersion of the solute band across the soft nanochannel. The supporting walls of the channel are considered to be hydrophobic and bear non-zero surface charge. For such a channel, the inner side of the supporting rigid walls of the channel are coated with a soft polyelectrolyte layer (PEL). The inhomogeneous distribution of monomers and accompanying volume charge within the PEL is modelled via soft-step function. The dielectric permittivity of the PEL and electrolyte solution are in general different, which in turn leads to the ion partitioning effect. The impact of ion steric effects due to finite sized ions is further accounted through the modified ion activity coefficient. To model the EOF modulation considering the combined impact of the ion steric and ion partitioning effects as well as inhomogeneous distribution of monomers across the PEL, we adopt the modified Poisson-Boltzmann equation as the governing equation for electrostatic potential. The Debye-Bueche model is adopted to study the flow field across the PEL and the Stokes equation governs the EOF outside the PEL. In order to study the impact of the modulated EOF field on the dispersion of uncharged solution, we adopt three different models, i.e., a general 2D convective-diffusion model as well as cross-sectional averaged dispersion models due to Gill and late-time Taylor and Aris. Going beyond the widely employed Debye-Hückel approximation and uniform distribution of the monomer as well as accompanying volume charge, we find the results for the electric double layer (EDL) potential, EOF field and averaged throughput, by tuning the ion selectivity, etc., which is sufficient to analyze the transport of ionized liquid across the channel. The numerical results are supplemented with analytical results for the EDL potential as well as the EOF field under various limiting situations. Besides, we have further shown the impact of the modulated EOF field on the solute dispersion process. We have presented results that highlight the impact of parameters related to EOF field modulation, on solute dispersion governed by a convective-diffusive process, as well as obtaining the results for an effective dispersion coefficient. The dispersion models under the modulated EOF field adopted in the present study can thus be applied to study the dispersion process in engineered microdevices.

pH-Dependent Effects in Nanofluidic Memristors

Multipore membranes with nanofluidic diodes show memristive and current rectifying effects that can be controlled by the nanostructure asymmetry and ionic solution characteristics in addition to the frequency and amplitude of the electrical driving signal. Here, we show that the electrical conduction phenomena, which are modulated by the interaction between the pore surface charges and the solution mobile ions, allow for a pH-dependent neuromorphic-like potentiation of the membrane conductance by voltage pulses. Also, we demonstrate that arrangements of memristors can be employed in the design of electrochemical circuits for implementing logic functions and information processing in iontronics.

Zwitterions and betaines as highly soluble materials for sustainable corrosion protection: Interfacial chemistry and bonding with metal surfaces

The primary requirements for interfacial adsorption and corrosion inhibition are solubility and the existence of polar functional groups, particularly charges. Traditional organic inhibitors have a solubility issue due to the hydrophobic moieties they incorporate. Most documented organic inhibitors have aromatic rings, hydrocarbon chains, and a few functional groups. The excellent solubility and high efficacy of zwitterions and betaines make them the perfect replacements for insoluble corrosion inhibitors. Zwitterions and betaines are more easily soluble because of interactions between their positive and negative charges (-COO-, -PO3-, -NH3, -NHR2, -NH2R, -SO3- etc.) and the polar solvents. The positive and negative charges also aid these molecules' physical and chemical adsorption at the metal-electrolyte interfaces. They develop a corrosion-inhibiting layer through their adsorption. After becoming adsorbed at the metal-electrolyte interface, they act as mixed-type inhibitors, slowing both cathodic and anodic processes. They usually adsorb according to the Langmuir adsorption isotherm. In this article, the corrosion inhibition potential of zwitterions and betaines in the aqueous phase, as well as their mode of action, are reviewed. This article details the advantages and disadvantages of utilizing zwitterions and betaines for sustainable corrosion protection.

Ion Transport in Intelligent Nanochannels: A Comparative Analysis of the Role of Electric Field

This research delves into investigating ion transport behavior within nanochannels, enhanced through modification with a negatively charged polyelectrolyte layer (PEL), aimed at achieving superior control. The study examines two types of electric fields─direct current and alternating current with square, sinusoidal, triangular, and sawtooth waveforms─to understand their impact on ion transport. Furthermore, the study compares symmetric (cylindrical) and asymmetric (conical) nanochannel geometries to assess the influence of overlapping electrical double layers (EDLs) in generating specific electrokinetic behaviors such as ionic current rectification (ICR) and ion selectivity. The research employs the finite element method to solve the coupled Poisson-Nernst-Planck and Navier-Stokes equations under unsteady-state conditions. By considering factors such as electrolyte concentration, soft layer charge density, and electric field type, the study evaluates ion transport performance in charged nanochannels, investigating effects on concentration polarization, electroosmotic flow (EOF), ion current, rectification, and ion selectivity. Notably, the study accounts for ion partitioning between the PEL and electrolyte to simulate real conditions. Findings reveal that conical nanochannels, due to improved EDL overlap, significantly enhance ion transport and related characteristics compared to cylindrical ones. For instance, under ηε = ηD = 0.8, ημ = 2, C0 = 20 mM, and NPEL/NA = 80 mol m-3 conditions, the average EOF for conical and cylindrical geometries is 0.1 and 0.008 m/s, respectively. Additionally, the study explores ion selectivity and rectification based on the electric field type, unveiling the potential of nanochannels as ion gates or diodes. In cylindrical nanochannels, the ICR remains at unity, with lower ion selectivity across waveforms compared to conical channels. Furthermore, rectification and ion selectivity trends are identified as Rf,square > Rf,DC > Rf,triangular > Rf,sinusoidal > Rf,sawtooth and Ssawtooth > Ssinusoidal > Striangular > SDC > Ssquare for conical nanochannels. Our study of ion transport control in nanochannels, guided by tailored electric fields and unique geometries, offers versatile applications in the field of Analytical Chemistry. This includes enhanced sample separation, controlled drug delivery, optimized pharmaceutical analysis, and the development of advanced biosensing technologies for precise chemical analysis and detection. These applications highlight the diverse analytical contributions of our methodology, providing innovative solutions to challenges in chemical analysis and biosensing.

Enhancing the performance of a cylindrical nanopore in osmotic power generation through designing the waveform of its inner surface

Recently, nanofluidic osmotic power, a promising technology converting the salinity difference between brine and fresh water into electricity using nanopores, has drawn the attention of researchers. Previous studies in this field were based mainly on nanopores having a smooth inner surface. To enhance the performance of nanofluidic osmotic power, we investigated four types of cylindrical nanopores, each with a unique waveform wall design (square, saw-tooth, triangle, and sine waves). This study focused on elucidating the influence of bulk salt concentration and geometric characteristics at the solid-liquid interface. We demonstrated that the presence of a waveform wall introduces new variables that have a significant impact on the overall performance of a nanofluidic osmotic power system. At the optimal amplitude of the waveform wall, raising waveform frequency can remarkably improve the osmotic current, diffusion potential, maximum power, and maximum efficiency. The present study provides a novel aspect of osmotic power, where the geometric nature of the nanopore reveals profound and intriguing phenomena primarily attributed to the distribution of ions within its interior.

Effect of membrane thermal conductivity on ion current rectification in conical nanochannels under asymmetric temperature

Nowadays, there have been extensively theoretical studies on the phenomenon of ion current rectification (ICR) induced by the asymmetric electrical double layer (EDL). As a key factor influencing the behavior of ion transport, temperature is given high priority by researchers. The thermal conductivity of the material commonly employed to prepare nanopores is 2-3 times higher than that of liquid solutions, which may affect ion transport within the nanochannel. However, it is often neglected in previous studies. Thus, we investigate the effect of membrane thermal conductivity on the ICR in conical nanochannels under asymmetric temperature. Based on the PNP-NS theoretical model, the ion current, the rectification ratio, as well as the temperature and ion concentration distributions along the nanochannel are calculated. It is found that the thermal conductivity of the solid membrane noticeably affects the temperature distribution across the nanochannel, altering the ion transport behavior. Larger membrane thermal conductivity tends to homogenize the temperature distribution in the nanochannel, leading to a decline of ionic thermal down-diffusion by a positive temperature difference and ionic thermal up-diffusion by a negative temperature difference, with the former promoting and the latter inhibiting ion current. As a result, the rectification ratio decreases under the positive temperature difference and increases under the negative temperature difference as the thermal conductivity of the membrane increases. These studies will be instructive for the design of nanofluidic diodes and biosensors.

Smart nanochannels: tailoring ion transport properties through variation in nanochannel geometry

This research explores ion transport behavior and functionality in a hybrid nanochannel that consists of two conical and cylindrical parts. The numerical investigation focuses on analyzing the length of each part in the nanochannel. The nanochannels are hybrid cavities embedded in a membrane, where the size of the conical part varies as equal to, larger than, or smaller than the cylindrical part. The nanochannel is coated with a polyelectrolyte layer that exhibits a dense charge density distribution. The charge density of the soft layer is described using the soft step distribution function. We study the electroosmotic flow, ionic current, rectification, and selectivity of the nanochannel versus bulk electrolyte concentration, the charge density of the polyelectrolyte layer, and decay length, while considering the effect of ionic partitioning. The steady-state Poisson-Nernst-Planck and Navier-Stokes equations are solved using the finite element method. The findings reveal that the nanochannel with a more extensive conical section demonstrates increased rectification, with the rectification factor rising from 1.4 to 2 at a bulk concentration of 100 mM. Additionally, the nanochannel with a longer cylindrical part exhibits improved selectivity under negative voltage conditions, while positive voltage introduces a different situation. The nanochannel with equal cylindrical and conical parts significantly affects conductivity by modifying the charge density in the soft layer, resulting in a 3.125-fold increase in conductivity under positive voltage when the charge density in the polyelectrolyte layer is raised from 25 to 100 mol m-3. This research focuses on creating intelligent nanochannels by controlling mass concentration, charge density, and collapse length, improving system performance, and optimizing properties. It also offers valuable insights into ion transport mechanisms in nanochannel systems, advancing our understanding in this field.

Layer-by-Layer Nanofluidic Membranes for Promoting Blue Energy Conversion

Access to and use of energy resources are now crucial components of modern human existence thanks to the exponential growth of technology. Traditional energy sources provide significant challenges, such as pollution, scarcity, and excessive prices. As a result, there is more need than ever before to replace depleting resources with brand-new, reliable, and environmentally friendly ones. With the aid of reverse electrodialysis, the salinity gradient between rivers and seawater as a clean supply with easy and infinite availability is a viable choice for energy generation. The development of nanofluidic-based reverse electrodialysis (NRED) as a novel high-efficiency technology is attributable to the progress of nanoscience. However, understanding the predominant mechanisms of this process at the nanoscale is necessary to develop and disseminate this technology. One viable option to gain insight into these systems while saving expenses is to employ simulation tools. In this study, we looked at how a layer-by-layer (LBL) soft layer influences ion transport and energy production in charged nanochannels. We solved the steady-state Poisson-Nernst-Planck (PNP) and Navier-Stokes (NS) equations for three different types of nanochannels with a trumpet geometry, where the narrow part is covered with a built-up LbL soft layer and the rest is a hard wall with a surface charge density of σ = -10, 0, or +10 mC/m2. The findings show that in type (I) nanochannels, at NPEL/NA = 100 mol/m3 and pH = 7, the maximum power output rises 675-fold as the concentration ratio rises from 10 to 1000. The results of this study can aid in a better understanding of energy harvesting processes using nanofluidic-based reverse electrodialysis in order to identify optimal conditions for the design of an intelligent route with great controllability and minimal pollution.

Quantitative model for predicting the electroosmotic flow in dual-pole nanochannels

Developing and assessing nanofluidic systems is time-consuming and costly owing to the method's novelty; hence, modeling is essential to determine the optimal areas for implementation and to grasp its workings. In this work, we examined the influence of dual-pole surface and nanopore configuration on ion transfer simultaneously. To achieve this, the two trumpet and cigarette configuration were coated with a dual-pole soft surface so that the negative charge could be positioned in the nanopore's small aperture. Subsequently, the Poisson-Nernst-Planck and Navier-Stokes equations were simultaneously solved under steady-state circumstances using varied values physicochemical properties for the soft surface and electrolyte. The pore's selectivity was S Trumpet > S Cigarette ${S}_{{\rm{Trumpet}}} &gt; {S}_{{\rm{Cigarette}}}$ , and the rectification factor, on the other hand, was R f Cigarette < R f Trumpet ${R}_{{f}_{{\rm{Cigarette}}}} &lt; {R}_{{f}_{{\rm{Trumpet}}}}$ , when the overall concentration was very low. When the ion partitioning effect is taken into account, we clearly show that the rectifying variables for the cigarette configuration and the trumpet configuration can reach values of 45 and 49.2, when the charge density and mass concentration were 100 mol/m³ and 1 mM, respectively. By using dual-pole surfaces, the controllability of nanopores' rectifying behavior may be modified to produce superior separation performance.

Understanding and modelling the magnitude of the change in current of nanopore sensors

Nanopores are promising sensing devices that can be used for the detection of analytes at the single molecule level. It is of importance to understand and model the current response of a nanopore sensor for improving the sensitivity of the sensor, a better interpretation of the behaviours of different analytes in confined nanoscale spaces, and quantitative analysis of the properties of the targets. The current response of a nanopore sensor, usually called a resistive pulse, results from the change in nanopore resistance when an analyte translocates through the nanopore. This article reviews the theoretical models used for the calculation of the resistance of the nanopore, and the corresponding change in nanopore resistance due to a translocation event. Models focus on the resistance of the pore cavity region and the access region of the nanopore. The influence of the sizes, shapes and surface charges of the translocating species and the nanopore, as well as the trajectory that the analyte follows are also discussed. This review aims to give a general guidance to the audience for understanding the current response of a nanopore sensor and the application of this class of sensor to a broad range of species with the theoretical models.

Ion Transport in pH-Regulated Double-Barreled Nanopores

Single-molecule detection and characterization with nanopores is a powerful technique that does not require labeling. Multinanopore systems, especially double nanopores, have attracted wide attention and have been applied in many fields. However, theoretical studies of electrokinetic ion transport in nanopores mainly focus on single nanopores. In this paper, for the first time, a theoretical study of pH-regulated double-barreled nanopores is conducted using three-dimensional Poisson-Nernst-Planck equations and Navier-Stokes equations. Four ionic species and the surface chemistry on the walls of the nanopores are included. The results demonstrate that the properties of the bulk salt solution significantly affect nanopore conductivity and ion transport phenomena in nanopores. There are two ion-enriched zones and two ion-depleted zones in double-barreled nanopores. Due to the symmetry of the double-barreled nanopore structure and surface charge density, there is no ionic rectification effect in double-barreled nanopores. The ion selectivity is similar to that of conventional single pH-regulated nanopores.

Nanosensing of Acetylcholine Molecules: Influence of the Association Mechanism

A bullet-shaped nanopore surface modified by two polyelectrolyte (PE) layers, an inner polyethyleneimine (PEI) layer and an outer p-sulfonatocalix[4]-arene (SCX4) layer, is applied to sense trace levels of acetylcholine (Ach) molecules. We show that the higher the order of the association reaction of Ach with SCX4, the smaller the difference between the ionic current when Ach is present and that when it is absent, and so is the difference in the space charge density. In addition, the larger the binding constant K of that reaction, the lower the detection limit but narrower the detection range. Choosing pH 7 is most appropriate because if the pH is low, the concentration polarization of H⁺ is significant, and as it gets high, both PE layers become uncharged. At pH 7 and K = 2 × 10⁷ L/mol, the detection limit of the nanopore ranges from 1 to 10 nM, which is orders of magnitude lower than that of the other approaches.

Origin of Ultrahigh Rectification in Polyelectrolyte Bilayers Modified Conical Nanopores

The switching of "ON" and "OFF" states of an ionic diode is investigated by considering a conical nanopore partially functionalized two polyelectrolyte (PE) layers via layer-by-layer deposition. Through observing the inversion of its rectification behavior, we demonstrate the function of the PE bilayers in ionic transport regulation. The ionic diode exhibits an ultrahigh ion rectification at a low level of pH. In an aqueous NaCl solution at pH 2, for example, the ratio of the current at "ON" state and that at "OFF" state can be about 800 and 200 for 1 and 100 mM, respectively. This remarkable gating behavior can be explained by the anion-pump-induced ion accumulation in the neutral region as well as the depletion zone at the interface. Our results further demonstrate the possibility of achieving an ultrahigh rectification in an ionic diode having a unipolar-like configuration.

A surface charge governed nanofluidic diode based on a single polydimethylsiloxane (PDMS) nanochannel

HYPOTHESIS

Nanofluidic diodes have attracted intense attention recently. Commonly used materials to design these devices are membrane-based short nanopores and aligned Carbon nanotube bundles. It is highly desirable and very challenging to develop a nanofluidic diode based on a single PDMS nanochannel which is easier to be introduced into an integrated electronic system on a chip. Layer-by-layer (LBL) deposition of charged polyelectrolytes can change the size and surface properties of PDMS nanochannels that provides new possibilities to develop high-performance nanofluidic based on PDMS nanochannels.

EXPERIMENTS

A novel design of nanofluidic diode is presented by controlling the surface charges and sizes of single PDMS nanochannels by surface modification using polyelectrolytes. Polybrene (PB) and Dextran sulfate (DS) are used to reduce the PDMS nanochannel size to meet the requirement of ion gating by LBL method and generate opposite surface charges at the ends of nanochannels. The parameters of such a nanofluidic diode are investigated systematically.

FINDINGS

This nanofluidic diode developed in this work has high effective current rectification performance. The rectification ratio can be as high as 218 which is the best ever reported in PB/DS modified nanochannels. This rectification ratio reduces with high voltage frequency and ionic concentration whereas increases in shorter nanochannels.

Understanding Carbon Nanotube-Based Ionic Diodes: Design and Mechanism

The rectification of ion transport through biological ion channels has attracted much attention and inspired the thriving invention and applications of ionic diodes. However, the development of high-performance ionic diodes is still challenging, and the working mechanisms of ionic diodes constructed by 1D ionic nanochannels have not been fully understood. This work reports the systematic investigation of the design and mechanism of a new type of ionic diode constructed from horizontally aligned multi-walled carbon nanotubes (MWCNTs) with oppositely charged polyelectrolytes decorated at their two entrances. The major design and working parameters of the MWCNT-based ionic diode, including the ion channel size, the driven voltage, the properties of working fluids, and the quantity and length of charge modification, are extensively investigated through numerical simulations and/or experiments. An optimized ionic current rectification (ICR) ratio of 1481.5 is experimentally achieved on the MWCNT-based ionic diode. These results promise potential applications of the MWCNT-based ionic diode in biosensing and biocomputing. As a proof-of-concept, DNA detection and HIV-1 diagnosis is demonstrated on the ionic diode. This work provides a comprehensive understanding of the working principle of the MWCNT-based ionic diodes and will allow rational device design and optimization.

Tripling the reverse electrodialysis power generation in conical nanochannels utilizing soft surfaces

We theoretically investigate the feasibility of enhancing the reverse electrodialysis power generation in nanochannels by covering the surface with a polyelectrolyte layer (PEL). Along these lines, two conical nanochannels are considered that differ in the extent of the covering. Each nanochannel connects two large reservoirs filled with KCl electrolytes of different ionic concentrations. Considering the Poisson-Nernst-Planck and Navier-Brinkman equations, finite-element-based numerical simulations are performed under a steady-state. The influences of the PEL properties and the salinity gradient on the reverse electrodialysis characteristics are examined in detail via a thorough parametric study. It is shown that the maximum power generated is an increasing function of the charge density and the thickness of the PEL. This means that the maximum power generated may be theoretically increased to any desired degree by covering the nanochannel surface with a sufficiently dense and thick PEL. Considering a typical PEL with a charge density of 100 mol m-3 and a thickness of 8 nm along with a high-to-low concentration ratio of 1000, we demonstrate that it is possible to extract a power density of 51.5 W m-2, which is nearly three times the maximum achievable value employing bare conical nanochannels at the same salinity gradient.

Electrokinetic behavior of bullet-shaped nanopores modified by functional groups: Influence of finite thickness of modified layer

We examined theoretically the electrokinetic behavior of a bullet-shaped nanopore modified by a functional layer, focusing on the influence of its thickness. The nanopore contains both fixed surface charge coming from the original bare surface, and space fixed charge from the modified layer. The results of numerical simulation reveal that the presence of this layer is crucial to the electrokinetic behavior of the nanopore. In particular, its softness is capable of influencing ionic profiles through electroosmotic flow (EOF). Unlike a conical nanopore where its surface normal vector is constant, that of the present bullet-shaped nanopore varies along the pore axis, thereby affecting the degree of EOF, which in turn, can make the ionic profile inside the modified layer more uniform. This is crucial to the applications of the nanopore, for example, in mimicking biological membranes and sensing metal ions.

Improved Rectification and Osmotic Power in Polyelectrolyte-Filled Mesopores

Ample studies have shown the use of nanofluidics in the ionic diode and osmotic power generation, but similar ionic devices performed with large-sized mesopores are still poorly understood. In this study, we model and realize the mesoscale ionic diode and osmotic power generator, composed of an asymmetric cone-shaped mesopore with its narrow opening filled with a polyelectrolyte (PE) layer with high space charges. We show that, only when the space charge density of a PE layer is sufficiently large (>1×106 C/m3), the considered mesopore system is able to create an asymmetric ionic distributions in the pore and then rectify ionic current. As a result, the output osmotic power performance can be improved when the filled PE carries sufficiently high space charges. For example, the considered PE-filled mesopore system can show an amplification of the osmotic power of up to 35.1-fold, compared to the bare solid-state mesopore. The findings provide necessary information for the development of large-sized ionic diode and osmotic power harvesting device.

Anomalies of Ionic/Molecular Transport in Nano and Sub-Nano Confinement

Understanding and exploring the transport behaviors of ions and molecules in the nano and sub-nano confinement has great meaning in the fields of nanofluidics and basic transport physics. With the rapid progress in nanofabrication technology and effective characterization protocols, more and more anomalous transport behaviors have been observed and the ions/molecules inside small confinement can behave dramatically differently from bulk systems and present new mechanisms. In this Mini Review, we summarize the recent advances in the anomalous ionic/molecular transport behaviors in nano and sub-nano confinement. Our discussion includes the ionic/molecular transport of various confinement with different surface properties, static structures, and dynamic structures. Furthermore, we provide a brief overview of the latest applications of nanofluidics in membrane separation and energy conversion.

Covering the conical nanochannels with dense polyelectrolyte layers significantly improves the ionic current rectification

Because of their asymmetry, conical nanochannels/nanopores exhibit various attractive electrokinetic features, including ion selectivity, ionic concentration polarization, and ionic current rectification. The polyelectrolyte layer (PEL)-covered (soft) conical nanochannels have recently attracted significant attention because of their unique rectification characteristics. In the modeling of soft nanochannels, it is usually assumed that the properties of the PEL and the electrolyte are the same, an assumption that is not true, especially for dense PELs. In the present work, the influence of the PEL-electrolyte property difference on the ionic current rectification in conical soft nanochannels is studied. To this end, adopting a finite-element approach, the Poisson-Nernst-Planck and Navier-Stokes equations are numerically solved for a steady-state by considering different values of permittivity, diffusivity, and dynamic viscosity for the PEL and the electrolyte. The model is validated by comparing the results with the available theoretical and experimental data. The results show that the PEL-electrolyte property difference leads to a significant improvement of the rectification behavior, especially at low and moderate salt concentrations. This not only highlights the importance of considering different properties for the PEL and the electrolyte but also implies that the rectification behavior of soft nanochannels/nanopores may be improved considerably by utilizing denser PELs.

Ion current rectification behavior of a nanochannel having nonuniform cross-section

Due to its versatile applications in biotechnology, ion current rectification (ICR), which arises from the asymmetric nature of the ion transport in a nanochannel, has drawn much attention, recently. Here, the ICR behavior of a pH-regulated nanochannel comprising two series connected cylindrical nanochannels of different radii is examined theoretically, focusing on the influences of the radii ratio, the length ratio, the bulk concentration, and the solution pH. The results of numerical simulation reveal that the rectification factor exhibits a local maximum with respect to both the radii ratio and the length ratio. The values of the radii ratio and the length ratio at which the local maximum in the rectification factor occur depend upon the level of the bulk salt concentration. The rectification factor also shows a local maximum as the solution pH varies. Among the factors examined, the solution pH influences the ICR behavior of the nanochannel most significantly.

Electrodiffusioosmosis-Induced Negative Differential Resistance in pH-Regulated Mesopores Containing Purely Monovalent Solutions

Negative differential resistance (NDR) refers to a unique electrical property where current decreases with increasing voltage. Herein, we report experimental evidence showing that the NDR effect can be observed in mesopores that feature charged pore walls and are subjected to a KCl concentration gradient. NDR in our system originates from the solution and ion flows driven by the synergistic effects of electroosmosis [electroosmotic flow (EOF)] and diffusioosmosis, the so-called electrodiffusioosmosis. Experiments reveal that in addition to the ion current rectification, the mesopores considered here exhibit the NDR phenomenon that is dependent on the magnitude and direction of the salinity gradient and on pH. The NDR behavior can be observed only at conditions at which the EOF and diffusioosmosis occur in the opposite directions: diffusioosmosis fills the tip opening with a high concentration solution, while EOF brings a low concentration solution to the pore. All experimental findings are supported by our numerical model, which takes into account the interfacial site reactions of acidic and basic functional groups on the entire pore membrane surfaces. Our results provide an important insight into how liquid pH, salinity gradients, interfacial site reactions, and pore geometries can influence the current-voltage characteristics of mesopores, enriching transport modes that can be induced by voltage.

Regulating the ionic current rectification behavior of branched nanochannels by filling polyelectrolytes

The overlapping of the electric double layer (EDL) in a nanochannel yields many interesting and significant electrokinetic phenomena such as ionic current rectification (ICR), which occurs only at a relatively low bulk salt concentration (∼1 mM) where the EDL thickness is comparable to the nanochannel size. In an attempt to raise this concentration to higher levels and the ICR performance improved appreciably, a branched nanochannel filled with polyelectrolytes (PEs) is proposed in this study. We show that these objectives can be achieved by choosing appropriate PE. For example, if the stem side of an anodic aluminun oxide nanochannel is filled with polystyrene sulfonate (PSS) an ICR ratio up to 850 can be obtained at 1 mM, which was not reported in previous studies. Taking account of the effect of electroosmotic flow, the underlying mechanisms of the ICR phenomena observed are discussed and the influences of the solution pH, the bulk salt concentration, and how the region(s) of a nanochannel is filled with PE examined. We show that the ICR behavior of a branched nanochannel can be modulated satisfactorily by filling highly charged PE and the solution pH.

Unraveling the Anomalous Surface-Charge-Dependent Osmotic Power Using a Single Funnel-Shaped Nanochannel

Nanofluidic osmotic power, which converts a difference in salinity between brine and fresh water into electricity with nanoscale channels, has received more and more attention in recent years. It is long believed that to gain high-performance osmotic power, highly charged channel materials should be exploited so as to enhance the ion selectivity. In this paper, we report counterintuitive surface-charge-density-dependent osmotic power in a single funnel-shaped nanochannel (FSN), violating the previous viewpoint. For the highly charged nanochannel, the performance of osmotic power decreases with a further increase in its surface charge density. With increasing pH (surface charge density), the FSN enables a local maximum power density as high as ∼3.5 kW/m² in a 500 mM/1 mM KCl gradient. This observation is strongly supported by our rigorous model where the equilibrium chemical reaction between functional carboxylate ion groups on the channel wall and protons is taken into account. The modeling reveals that for a highly charged nanochannel, a significant increase in the surface charge density amplifies the ion concentration polarization effect, thus weakening the effective salinity ratio across the channel and undermining the osmotic power generated.

Ionic thermal up-diffusion in nanofluidic salinity-gradient energy harvesting

Advances in nanofabrication and materials science give a boost to the research in nanofluidic energy harvesting. Contrary to previous efforts on isothermal conditions, here a study on asymmetric temperature dependence in nanofluidic power generation is conducted. Results are somewhat counterintuitive. A negative temperature difference can significantly improve the membrane potential due to the impact of ionic thermal up-diffusion that promotes the selectivity and suppresses the ion-concentration polarization, especially at the low-concentration side, which results in dramatically enhanced electric power. A positive temperature difference lowers the membrane potential due to the impact of ionic thermal down-diffusion, although it promotes the diffusion current induced by decreased electrical resistance. Originating from the compromise of the temperature-impacted membrane potential and diffusion current, a positive temperature difference enhances the power at low transmembrane-concentration intensities and hinders the power for high transmembrane-concentration intensities. Based on the system's temperature response, we have proposed a simple and efficient way to fabricate tunable ionic voltage sources and enhance salinity-gradient energy conversion based on small nanoscale biochannels and mimetic nanochannels. These findings reveal the importance of a long-overlooked element—temperature—in nanofluidic energy harvesting and provide insights for the optimization and fabrication of high-performance nanofluidic power devices.

Selectively Enhanced Ion Transport in Graphene Oxide Membrane/PET Conical Nanopore System

Graphene oxide (GO) has become a promising 2D material in many areas, such as gas separation, seawater desalination, antibacterial materials, and so on because of its abundant oxygen-containing functional groups and excellent dispersibility in various solvents. The graphene oxide membrane (GOM), a laminar and channel-rich structure assembled by stacked GO nanosheets, served as a kind of precise and ultrafast separation material has attracted widespread attention in membrane separation field. To break the trade-off between ion permeability and ion selectivity of separation membrane based on GOM, GOM/conical nanopore system is obtained by spin-coating ultrathin GOM on PET conical nanopore, which possesses ion rectification property. Comparing to pure PET conical nanopore, the existence of GOM not only enhances the cation conductance but also makes the ion rectification ratio increase from 4.6 to 238.0 in KCl solution. Assisted by COMSOL simulation, it is proved that the GOM can absorb large amount of cations and act as cation source to improve the ion selectivity and rectification effect of GOM/conical nanopore system. Finally, the chemical stability of GOM/conical nanopore is also investigated and the corresponding results reveal that the GOM/conical nanopore system can perform the ion rectification behavior in a wider pH range than pure PET conical nanopore. The presented findings demonstrate the great potential applications of GOM/conical nanopore system in ionic logic circuits and sensor systems.

Electrokinetic ion transport in an asymmetric double-gated nanochannel with a pH-tunable zwitterionic surface

Bioinspired, artificial functional nanochannels for intelligent molecular and ionic transport control have versatile potential applications in nanofluidics, energy conversion, and controlled drug release. To simulate the gating and rectification functions of biological ion channels, we model the electrokinetic ion transport phenomenon in an asymmetric double-gated nanochannel having a pH-regulated, zwitterionic surface. Taking account of the effect of electroosmotic flow (EOF), the conductance of the nanochannel and its ion current rectification (ICR) behavior are investigated and the associated mechanisms interpreted. In particular, the influences of the solution pH, the bulk salt concentration, and the base opening radius and the surface curvature of the nanochannel on these behaviors are examined. We show that through adjusting the base opening radius and the surface curvature of a nanochannel, its ICR behavior can be tuned effectively. In addition to proposing underlying mechanisms for the phenomena observed, the results gathered in this study also provide necessary information for designing relevant devices.

Voltage-controlled ion transport and selectivity in a conical nanopore functionalized with pH-tunable polyelectrolyte brushes

Chemically functionalized bioinspired nanopores are widely adopted to control the ionic transport for various purposes. A detailed understanding of the underlying mechanisms is not only desirable but also necessary for device design and experimental data interpretation. Here, the conductance and the ion selectivity of a conical nanopore surface modified by a polyelectrolyte (PE) layer are studied through adjusting the pH, the bulk salt concentration, and the level of the applied potential bias. Possible mechanisms are proposed and discussed in detail. We show that the conductance is sensitive to the variation in the solution pH. The ion selectivity of the nanopore is influenced significantly by both the solution pH and the level of the applied potential bias. In particular, a cation-selective nanopore might become anion-selective through raising the applied potential bias. The ion transport behavior can be tuned easily by adjusting the level of pH, salt concentration, and applied potential bias, thereby providing useful information for the design of nanopore-based sensing devices.

Dynamic Curvature Nanochannel-Based Membrane with Anomalous Ionic Transport Behaviors and Reversible Rectification Switch

Biological nanochannels control the movements of different ions through cell membranes depending on not only those channels' static inherent configurations, structures, inner surface's physicochemical properties but also their dynamic shape changes, which are required in various essential functions of life processes. Inspired by ion channels, many artificial nanochannel-based membranes for nanofluidics and biosensing applications have been developed to regulate ionic transport behaviors by using the functional molecular modifications at the inner surface of nanochannel to achieve a stimuli-responsive layer. Here, the concept of a dynamic nanochannel system is further developed, which is a new way to regulate ion transport in nanochannels by using the dynamic change in the curvature of channels to adjust ionic rectification in real time. The dynamic curvature nanochannel-based membrane displays the advanced features of the anomalous effect of voltage, concentration, and ionic size for applying simultaneous control over the curvature-tunable asymmetric and reversible ionic rectification switching properties. This dynamic approach can be used to build smart nanochannel-based systems, which have strong implications for flexible nanofluidics, ionic rectifiers, and power generators.

Alternating electric field-induced ion current rectification and electroosmotic pump in ultranarrow charged carbon nanocones

Pumping fluid in ultranarrow (sub-2 nm) synthetic channels, analogous to protein channels, has widespread applications in nanofluidic devices, molecular separation, and related fields. In this work, molecular dynamics simulations were performed to study a symmetrical sinusoidal electric field-induced electroosmotic pump in ultranarrow charged carbon nanocone (CNC) channels. The results show that the CNC channels could rectify the ion current because of the different ion flow rates in the positive and negative half circles of the sinusoidal electric field. Electroosmotic flow (EOF) rectification yielded by the ion current rectification is also revealed, and net water flow from the base to the tip of the CNC channels is observed. The simulations also show that the preferential ion current conduction direction in the ultranarrow CNC channels (from base to tip) is opposite to that in conical nanochannels with tip diameters larger than 5 nm (from tip to base). However, the preferential EOF direction is the same as that of large conical nanochannels (from base to tip). We also investigated the influences of ion concentration and the amplitudes and periods of the sinusoidal electric field on the EOF pump. The results show that high ion concentration, large amplitudes, and long periods are desired for high EOF pumping efficiency. Finally, through comparison with a constant electric field and a pressure-induced water pump, we prove that the EOF pump under an alternating electric field has a higher pump efficiency. The approach outlined in this work provides a general scheme for pumping fluid in ultranarrow charged conical nanochannels.

Influence of salt valence on the rectification behavior of nanochannels

Taking account of the influence of electroosmotic flow, the behavior of the ion current rectification of a charged conical nanochannel is studied theoretically focusing on the effect of ionic valence. A continuum-based model comprising coupled Poisson-Nernst-Planck (PNP) equations for the ionic mass transport and Navier-Stokes equations for the hydrodynamic field is adopted. We show that if the bulk salt concentration is fixed, the behavior of the current-voltage curve depends highly on the ionic valence, which arises from the difference in ionic strength and ion diffusivity. As the bulk salt concentration varies, the rectification factor shows a local maximum, and the bulk salt concentration at which it occurs depends upon the salt valence: the higher the valence the lower that concentration. However, regardless of the salt valence, the ionic strength at which that local maximum occurs is essentially the same, implying that the thickness of electric double layer is the key factor. Due to the difference in ionic diffusivity, the magnitude of the rectification factor depends upon the type of salt. For example, the rectification factor of KCl is larger than that of KNO₃. The qualitative behavior of the ion current rectification of a positively charged conical nanochannel is similar to that of a negatively charged nanochannel.

Anomalous pH-Dependent Nanofluidic Salinity Gradient Power

Previous studies on nanofluidic salinity gradient power (NSGP), where energy associated with the salinity gradient can be harvested with ion-selective nanopores, all suggest that nanofluidic devices having higher surface charge density should have higher performance, including osmotic power and conversion efficiency. In this manuscript, this viewpoint is challenged and anomalous counterintuitive pH-dependent NSGP behaviors are reported. For example, with equal pH deviation from its isoelectric point (IEP), the nanopore at pH < IEP is shown to have smaller surface charge density but remarkably higher NSGP performance than that at pH > IEP. Moreover, for sufficiently low pH, the NSGP performance decreases with lowering pH (increasing nanopore charge density). As a result, a maximum osmotic power density as high as 5.85 kW m^-2 can be generated along with a conversion efficiency of 26.3% achieved for a single alumina nanopore at pH 3.5 under a 1000-fold concentration ratio. Using the rigorous model with considering the surface equilibrium reactions on the pore wall, it is proved that these counterintuitive surface-charge-dependent NSGP behaviors result from the pH-dependent ion concentration polarization effect, which yields the degradation in effective concentration ratio across the nanopore. These findings provide significant insight for the design of next-generation, high-performance NSGP devices.