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This study presents a theoretical investigation into the voltammetric behavior of bipolar interfacial nanopores due to the effect of potential scan rate (1–1000 V/s). Finite element method (FEM) is utilized to explore the current–voltage (I–V) properties of bipolar interfacial nanopores at different bulk salt concentrations. The results demonstrate a strong impact of the scan rate on the I–V response of bipolar interfacial nanopores, particularly at relatively low concentrations. Hysteresis loops are observed in bipolar interfacial nanopores under specific scan rates and potential ranges and divided by a cross-point potential that remains unaffected by the scan rate employed. This indicates that the current in bipolar interfacial nanopores is not just reliant on the bias potential that is imposed but also on the previous conditions within the nanopore, exhibiting history-dependent or memory effects. This scan-rate-dependent current–voltage response is found to be significantly influenced by the length of the nanopore (membrane thickness). Thicker membranes exhibit a more pronounced scan-rate-dependent phenomenon, as the mass transfer of ionic species is slower relative to the potential scan rate. Additionally, unlike conventional bipolar nanopores, the ion current passing through bipolar interfacial nanopores is minimally affected by the membrane thickness, making it easier to detect. [ABSTRACT FROM AUTHOR]

The limited analgesic efficiency of magnesium restricts its application in pain management. Here, we report boron hydride (BH) with ion currents rectification activity that can enhance the analgesic efficiency of magnesium without the risks of drug tolerance or addiction. We synthesize MgB2, comprising hexagonal boron sheets alternating with Mg2+. In pathological environment, Mg2+ is exchanged by H+, forming two‐dimensional borophene‐analogue BH sheets. BH interacts with the charged cations via cation‐pi interaction, leading to dynamic modulation of sodium and potassium ion currents around neurons. Additionally, released Mg2+ competes Ca2+ to inhibit its influx and neuronal excitation. In vitro cultured dorsal root neurons show a remarkable increase in threshold potential from the normal −35.9 mV to −5.9 mV after the addition of MgB2, indicating potent analgesic effect. In three typical pain models, including CFA‐induced inflammatory pain, CINP‐ or CCI‐induced neuropathic pain, MgB2 exhibits analgesic efficiency approximately 2.23, 3.20, and 2.0 times higher than clinical MgSO4, respectively, and even about 1.04, 1.66, and 1.95 times higher than morphine, respectively. The development of magnesium based intermetallic compounds holds promise in addressing the non‐opioid medical need for pain relief. [ABSTRACT FROM AUTHOR]

Saccharides, being one of the fundamental molecules of life, play essential roles in the physiological and pathological functions of cells. However, their intricate structures pose challenges for detection. Nanopore technology, with its high sensitivity and capability for single-molecule-level analysis, has revolutionized the identification and structural analysis of saccharide molecules. This review focuses on recent advancements in nanopore technology for carbohydrate detection, presenting an array of methods that leverage the molecular complexity of saccharides. Biological nanopore techniques utilize specific protein binding or pore modifications to trigger typical resistive pulses, enabling the high-sensitivity detection of monosaccharides and oligosaccharides. In solid-state nanopore sensing, boronic acid modification and pH gating mechanisms are employed for the specific recognition and quantitative analysis of polysaccharides. The integration of artificial intelligence algorithms can further enhance the accuracy and reliability of analyses. Serving as a crucial tool in carbohydrate detection, we foresee significant potential in the application of nanopore technology for the detection of carbohydrate molecules in disease diagnosis, drug screening, and biosensing, fostering innovative progress in related research domains. [ABSTRACT FROM AUTHOR]

The properties of nanopipettes largely rely on the materials introduced onto their inner walls, which allow for a vast extension of their sensing capabilities. The challenge of simultaneously enhancing the sensitivity and selectivity of nanopipettes for pH sensing remains, hindering their practical applications. Herein, we report insulin-modified nanopipettes with excellent pH response performances, which were prepared by introducing insulin onto their inner walls via a two-step reaction involving silanization and amidation. The pH response intensity based on ion current rectification was significantly enhanced by approximately 4.29 times when utilizing insulin-modified nanopipettes compared with bare ones, demonstrating a linear response within the pH range of 2.50 to 7.80. In addition, insulin-modified nanopipettes featured good reversibility and selectivity. The modification processes were monitored using the I-V curves, and the relevant mechanisms were discussed. The effects of solution pH and insulin concentration on the modification results were investigated to achieve optimal insulin introduction. This study showed that the pH response behavior of nanopipettes can be greatly improved by introducing versatile molecules onto the inner walls, thereby contributing to the development and utilization of pH-responsive nanopipettes. [ABSTRACT FROM AUTHOR]

This study presents a theoretical investigation into the voltammetric behavior of bipolar interfacial nanopores due to the effect of potential scan rate (1–1000 V/s). Finite element method (FEM) is utilized to explore the current–voltage (I–V) properties of bipolar interfacial nanopores at different bulk salt concentrations. The results demonstrate a strong impact of the scan rate on the I–V response of bipolar interfacial nanopores, particularly at relatively low concentrations. Hysteresis loops are observed in bipolar interfacial nanopores under specific scan rates and potential ranges and divided by a cross-point potential that remains unaffected by the scan rate employed. This indicates that the current in bipolar interfacial nanopores is not just reliant on the bias potential that is imposed but also on the previous conditions within the nanopore, exhibiting history-dependent or memory effects. This scan-rate-dependent current–voltage response is found to be significantly influenced by the length of the nanopore (membrane thickness). Thicker membranes exhibit a more pronounced scan-rate-dependent phenomenon, as the mass transfer of ionic species is slower relative to the potential scan rate. Additionally, unlike conventional bipolar nanopores, the ion current passing through bipolar interfacial nanopores is minimally affected by the membrane thickness, making it easier to detect.

Heterogenous nanofluidic membranes with bi‐layer structures and ionic diode effect are shown great potential in efficiently harvesting the energy existing in a salinity gradient (or called the osmotic power conversion). However, exploitation of a heterogenous membrane with superior ion selectivity, excellent conductance, and strong ionic diode characteristics has remained a great challenge. Here, a novel heterogenous subnanochannel membrane with a tri‐continuous pore structure of a large geometry gradient ranging from sub‐nanoscale to nanoscale to sub‐microscale, which is composed of a thin and crack‐free layer of zeolitic imidazolate framework‐8 (ZIF‐8)/polystyrene sulfonate (PSS) membranes and an aligned branch‐type alumina nanochannel membrane (BANM) is reported. It is demonstrated that such a tri‐continuous pore structure can endow the exploited membrane, ZIF‐8/PSS@BANM, with enhanced ion selectivity, strong ion current rectification, and ultrafast ion transport properties, in organic electrolyte solutions. Thus, an amazingly high power of ≈50.5 W m−2 is produced by mixing a 2 m LiCl‐methanol and pure methanol solutions, which is over 45‐fold higher than the existing membranes. Realizing high ion selectivity and amplified directional ion transport at sub‐nanoconfined spaces in organic solvents paves the new way to develop ion‐channel‐mimetic membranes toward efficient ion separation and high‐performance energy harvesters for battery applications. [ABSTRACT FROM AUTHOR]

Saccharides, being one of the fundamental molecules of life, play essential roles in the physiological and pathological functions of cells. However, their intricate structures pose challenges for detection. Nanopore technology, with its high sensitivity and capability for single-molecule-level analysis, has revolutionized the identification and structural analysis of saccharide molecules. This review focuses on recent advancements in nanopore technology for carbohydrate detection, presenting an array of methods that leverage the molecular complexity of saccharides. Biological nanopore techniques utilize specific protein binding or pore modifications to trigger typical resistive pulses, enabling the high-sensitivity detection of monosaccharides and oligosaccharides. In solid-state nanopore sensing, boronic acid modification and pH gating mechanisms are employed for the specific recognition and quantitative analysis of polysaccharides. The integration of artificial intelligence algorithms can further enhance the accuracy and reliability of analyses. Serving as a crucial tool in carbohydrate detection, we foresee significant potential in the application of nanopore technology for the detection of carbohydrate molecules in disease diagnosis, drug screening, and biosensing, fostering innovative progress in related research domains.

The properties of nanopipettes largely rely on the materials introduced onto their inner walls, which allow for a vast extension of their sensing capabilities. The challenge of simultaneously enhancing the sensitivity and selectivity of nanopipettes for pH sensing remains, hindering their practical applications. Herein, we report insulin-modified nanopipettes with excellent pH response performances, which were prepared by introducing insulin onto their inner walls via a two-step reaction involving silanization and amidation. The pH response intensity based on ion current rectification was significantly enhanced by approximately 4.29 times when utilizing insulin-modified nanopipettes compared with bare ones, demonstrating a linear response within the pH range of 2.50 to 7.80. In addition, insulin-modified nanopipettes featured good reversibility and selectivity. The modification processes were monitored using the I-V curves, and the relevant mechanisms were discussed. The effects of solution pH and insulin concentration on the modification results were investigated to achieve optimal insulin introduction. This study showed that the pH response behavior of nanopipettes can be greatly improved by introducing versatile molecules onto the inner walls, thereby contributing to the development and utilization of pH-responsive nanopipettes.

Various efforts have been made to mimic the regulatory properties of biological channels that exist in cell membranes of living organisms. Such ion channels are able to tune electric signals in response to changes in surrounding environments (e.g., pH or ions). Here, we report the design and fabrication of a novel pH-switchable ionic diode membrane (IDM), TFP-EB COF@PET, composed of a thin layer of sub-3 nm covalent organic framework (COF) channels and a polyethylene terephthalate (PET) membrane having conical nanochannels with tip size of 20–40 nm. The designed IDM is found to exhibit ion current rectification (ICR) over a wide range of electrolyte concentrations, attributable to its structural and charge asymmetries. Interestingly, the TFP-EB COF@PET can exhibit pH-switchable ion transport behaviors even though the surface charge natures of TFP-EB COF and PET channels remain unchanged. The mechanisms for the pH-switchable ICR properties are proposed to explain the inversion phenomenon. This work highlights the pH-switchable ionic transport properties of the sub-3 nm-scale COF-based IDM, as well as its potential for long-term operation under high salinity conditions.

Facing the energy crisis, using the salinity gradient between seawater and freshwater for osmotic energy conversion is a direct way to obtain energy. So far, most nanofluidic membranes utilized for osmotic energy generation are cation‐selective. Given that both anion‐ and cation‐selective membranes have the identical importance for energy conversion devices, it is of great significance to develop anion‐selective membranes. Herein, an anion‐selective membrane is synthesized by in situ growth of imine‐bridged covalent organic framework (COF) on ordered anodic aluminum oxide (AAO) at room temperature. The imine groups and residual amino groups of COF can combine with protons in neutral solution, enabling the COF positively charged and efficiently transport of anions. Particularly, due to the asymmetry in the charge and structure of COF/AAO, the as‐prepared membrane exhibits excellent ionic current rectification property, which can inhibit ion concentration polarization effectively and possess high ion selectivity and permeability. Using the present COF/AAO membrane, salinity gradient energy can be successfully harvested from solutions with high salt content, and the output power density reached 17.95W m−2 under a 500‐fold salinity gradient. The study provides a new avenue for construction and application of anion‐selective membranes in the smart ion transport and efficient energy conversion. [ABSTRACT FROM AUTHOR]

Ion current rectification (ICR) is the ratio of ion current by forward bias to backward bias and is a critical indicator of diode performance. In previous studies, there have been many attempts to improve the performance of this ICR, but there is the intrinsic problem for geometric changes that induce ionic rectification due to fabrication problems. Additionally, the high ICR could be achieved in the narrow salt concentration range only. Here, we propose a multi-layered bipolar ionic diode based on an asymmetric nanochannel network membrane (NCNM), which is realized by soft lithography and self-assembly of homogenous-sized nanoparticles. Owing to the freely changeable geometry based on soft lithography, the ICR performance can be explored according to the variation of microchannel shape. The presented diode with multi-layered configuration shows strong ICR performance, and in a broad range of salt concentrations (0.1 mM~100 mM), steady ICR performance. It is interesting to note that when each anion-selective (AS) and cation-selective (CS) NCNM volume was similar to each optimized volume in a single-layered device, the maximum ICR was obtained. Multi-physics simulation, which reveals greater ionic concentration at the bipolar diode junction under forward bias and less depletion under backward in comparison to the single-layer scenario, supports this tendency as well. Additionally, under different frequencies and salt concentrations, a large-area hysteresis loop emerges, which indicates fascinating potential for electroosmotic pumps, memristors, biosensors, etc. [ABSTRACT FROM AUTHOR]

Developing ion‐selective membranes with anti‐biofouling property and biocompatibility is highly crucial in harvesting osmotic energy in natural environments and for future biomimetic applications. However, the exploration of membranes with these properties in osmotic energy conversion remain largely unaddressed. Herein, a tough zwitterionic gradient double‐network hydrogel membrane (ZGDHM) with excellent biofouling resistance and cytocompatibility for sustainable osmotic energy harvesting is demonstrated. The ZGDHM, composed of negatively charged 2‐acrylamido‐2‐methylpropanesulfonic acid (AMPS) as the first scaffold network and zwitterionic sulfobetaine acrylamide (SBAA) as the second network, is prepared by a two‐step photopolymerization, thus creating continuous gradient double‐network nanoarchitecture and then remarkably enhanced mechanical properties. As verified by the experiments and simulations, the gradient nanoarchitecture endows the hydrogel membrane with apparent ionic diode effect and space‐charge‐governed transport property, thus facilitating directional ion transport. Consequently, the ZGDHM can achieve a power density of 5.44 W m−2 by mixing artificial seawater and river water, surpassing the commercial benchmark. Most importantly, the output power can be promoted to an unprecedented value of 49.6 W m−2 at the mixing of salt‐lake water and river water, nearly doubling up most of the existing nanofluidic membranes. This study paves a new avenue toward developing ultrahigh‐performance osmotic energy harvesters for biomimetic applications. [ABSTRACT FROM AUTHOR]

Biomimetic smart nanochannels can regulate ion transport behavior responsive to the external stimuli, having huge potential in nanofluidic devices, sensors and energy conversion. Field‐effect nanofluidic diodes or transistors based on electric‐responsive nanochannels are emerging owing to their advantages such as non‐invasiveness, in situ, real time, and high efficiency. However, simultaneously realizing the voltage‐control of the ion conductance and ion current rectification (ICR) properties is still a big challenge. Here, a field‐effect iontronic device is developed based on ionomer/anodic aluminum oxide/conducting polymer sandwich‐structured nanochannel to realize the multi‐control of ion transport behaviors including ion conductance, ICR magnitude, and ICR direction by modulating the surface charge, wettability, and morphology of the nanochannel. The electroactive conducting polymer carries tunable surface charges responsive to the electric stimuli, leading to the regulation of ICR values. The complex three‐segment structures lead to the reverse of ICR direction by reconfiguring the charge distribution along with the whole channel. The switching wettability between hydrophilic and hydrophobic results in the regulation of ion conductance. Furthermore, the field‐effect iontronic device functions in a wide salinity range especially in hypersaline environment, due to the salinity‐adaptive properties of the membrane. A new route is provided for designing more functional field‐effect nanofluidic devices. [ABSTRACT FROM AUTHOR]

Ion current rectification (ICR) is one of the electrodynamic properties in asymmetric nanochannels, which has been utilized to develop biosensors, heavy metal ion detection, and ion diodes. Previous studies on ICR mainly focused on long nanopores (length >500 nm), and relatively few studies on ion rectification performance in short nanopores. Here, we use a finite element numerical solution to simulate the effect of coupling asymmetric temperature and surface charge distribution on the ion rectification performance of ultrashort conical nanopores (length = 100 nm). The results show that the ion rectification performance is significantly improved when the inner and outer surfaces of the nanopore are charged non-uniformly, which is more than three times that when the inner surface of the nanopore is charged non-uniformly or the outer surface is charged non-uniformly. In addition, the positive temperature difference enhances the ion enrichment and dissipation and improves the ion rectification performance of nanopores. On the contrary, negative temperature difference inhibits ion enrichment and dissipation in pores, which is not conducive to improving ion rectification performance. Abnormal behavior is observed for nanopores non-uniform charged only on the outer wall surface, where the negative temperature gradient enhances the accumulation and dissipation of ions within the pores, thereby increasing the rectification ratio. In addition, in order to more clearly understand the ion transport behavior in the ultrashort nanopore under asymmetric temperature conditions, we further investigated the influence of the tip radius of the nanopore, the solution volume concentration, and the charge type of each surface charge distribution on the ion current rectification performance. The results reveal the ion transport mechanism of ultrashort conical nanopores and provide practical guidance for designing and optimizing nanofluidic devices. [Display omitted] • Ion current rectification (ICR) performance in ultrashort conical nanopores was investigated. • ICR performance can be improved by configurating the surface charge density. • Temperature distribution has an important impact on ICR. [ABSTRACT FROM AUTHOR]

Due to its advantages of label-free and highly sensitive, the resistive pulse sensing with a nanopore has recently become even more potent for the discrimination of analytes in single molecule level. Generally, a transient interruption of ion current originated from the captured molecule passing through a nanopore will provide the rich information on the structure, charge and translocation dynamics of the analytes. Therefore, nanopore sensors have been widely used in the fields of DNA sequencing, protein recognition, and the portable detection of varied macromolecules and particles. However, the conventional nanopore devices are still lack of sufficient selectivity and sensitivity to distinguish more metabolic molecules involving ATP, glucose, amino acids and small molecular drugs because it is hard to receive a large number of identifiable signals with the fabricated pores comparable in size to small molecules for nanopore sensing. For all this, a series of innovative strategies developed in the past decades have been summarized in this review, including host-guest recognition, engineering alteration of protein channel, the introduction of nucleic acid aptamers and various delivery carriers integrating signal amplification sections based on the biological and solid nanopore platforms, to achieve the high resolution for the small molecules sensing in micro-nano environment. These works have greatly enhanced the powerful sensing capabilities and extended the potential application of nanopore sensors. [Display omitted] • A topical review covering advances of small molecule detection in nanopore sensing. • Detection of small molecules are critical for bioanalysis. • New ways enhance nanopore sensing capabilities for small molecule detection. • Recent advances offer more opportunities for nanopore sensing. [ABSTRACT FROM AUTHOR]

• Impact of dielectric polarization of a membrane on electroosmosis. • Ion transport incorporating steric and electrostriction due to ion hydration. • Impact of induced surface charge on conductivity and ion current rectification. • Variation of permittivity and viscosity of the electrolyte on ionic concentration. • Numerical computation of the modified electrokientic transport. The impact of solid polarization of a dielectric membrane on ion transport in a conical nanopore drilled through a membrane is studied by considering ions as finite-sized dielectric charged spheres. The excess polarization of the hydrated ions creates a dielectric decrement. A mean-field-based approach is adopted by modifying the Nernst-Planck equation to incorporate the ion steric interactions, the Born force, and the dielectrophoretic force arising due to the spatial variation of electrolyte permittivity. The viscosity of the medium is considered to vary with the local ionic concentration through the Bachelor-Green equation, which leads to a reduction in the mobility of ions near a charged surface. Governing equations are solved numerically through the control volume method along with a pressure-correction-based iterative algorithm. The solid polarization of the membrane, which is often neglected in existing studies, is found to have a strong impact on the volume flux, conductance, selectivity, and ion current rectification in the conical nanopore. The ion saturation in the vicinity of the charged surface, as governed by the present modified model, creates a pronounced alteration in the electrokinetics compared to that predicted by the standard Poisson-Nernst-Planck (PNP-model) based on the point charge consideration of ions. This discrepancy due to ion steric repulsion and ion-solvent interactions is pronounced at a higher ionic concentration as well as a highly polarizable surface. A close agreement between the predictions of the present modified model and the existing experimental data, compared with that of the standard PNP model, is encouraging. The present modified model provides an accurate analysis of the electroosmotic flow and ion transport in a dielectric charged conical nanopore beyond the low charge density and dilute solution limits. [ABSTRACT FROM AUTHOR]

Nanofluidic ionic diodes have attracted much attention due to their unique functions as unidirectional ion transportation ability and promising applications from molecular sensing, and energy harvesting to emerging neuromorphic devices. However, it remains a challenge to fabricate diode-like nanofluidic systems with ultrathin film thickness <100 nm. Herein the formation of ultrathin ionic diodes from hybrid nanoassemblies of nanoporous (NP) SiO 2 nanofilms and polyelectrolyte layer-by-layer (LbL) multilayers is described. Ultrathin ionic diodes are prepared by integrating polyelectrolyte multilayers onto photo-oxidized NP SiO 2 nanofilms obtained from silsesquioxane-containing block copolymer thin films as a template. The obtained ultrathin ionic diodes exhibit ion current rectification (ICR) properties with high ICR factor = ≈20 under low ionic strength and asymmetric pH conditions. It is concluded that this ICR behavior arises from effective ion accumulation and depletion at the interface of NP SiO 2 nanofilms and LbL multilayers attributed to high ion selectivity by combining the experimental data and theoretical calculations using finite element methods. These results demonstrate that the hybrid nano assemblies of NP SiO 2 nanofilms and polyelectrolyte LbL multilayers have potential applications for (bio)sensing materials and integrated ionic circuits for seamless connection of human-machine interfaces., (© 2024 The Author(s). Small published by Wiley‐VCH GmbH.)

Ion current rectification (ICR) is the ratio of ion current by forward bias to backward bias and is a critical indicator of diode performance. In previous studies, there have been many attempts to improve the performance of this ICR, but there is the intrinsic problem for geometric changes that induce ionic rectification due to fabrication problems. Additionally, the high ICR could be achieved in the narrow salt concentration range only. Here, we propose a multi-layered bipolar ionic diode based on an asymmetric nanochannel network membrane (NCNM), which is realized by soft lithography and self-assembly of homogenous-sized nanoparticles. Owing to the freely changeable geometry based on soft lithography, the ICR performance can be explored according to the variation of microchannel shape. The presented diode with multi-layered configuration shows strong ICR performance, and in a broad range of salt concentrations (0.1 mM~100 mM), steady ICR performance. It is interesting to note that when each anion-selective (AS) and cation-selective (CS) NCNM volume was similar to each optimized volume in a single-layered device, the maximum ICR was obtained. Multi-physics simulation, which reveals greater ionic concentration at the bipolar diode junction under forward bias and less depletion under backward in comparison to the single-layer scenario, supports this tendency as well. Additionally, under different frequencies and salt concentrations, a large-area hysteresis loop emerges, which indicates fascinating potential for electroosmotic pumps, memristors, biosensors, etc.

MicroRNAs, as oncogenes or tumor suppressors, enable to up or down-regulate gene expression during tumorigenesis. The detection of miRNAs with high sensitivity is crucial for the early diagnosis of cancer. Inspired by biological ion channels, artificial nanochannels are considered as an excellent biosensing platform with relatively high sensitivity and stability. The current nanochannel biosensors are mainly based on homogeneous membranes, and their monotonous structure and functionality limit its further development. Therefore, it is necessary to develop a heterostructured nanochannel with high ionic current rectification to achieve highly sensitive miRNA detection. In this work, an asymmetric heterostructured nanochannel constructed from dendrimer-gold nanoparticles network and anodic aluminum oxide are designed through an interfacial super-assembly method, which can regulate ion transport and achieve sensitive detection of target miRNA. The symmetry breaking is demonstrated to endow the heterostructured nanochannels with an outstanding ionic current rectification performance. Arising from the change of surface charges in the nanochannels triggered by DNA cascade signal amplification in solution, the proposed heterogeneous nanochannels exhibits excellent DNA-regulated ionic current response. Relying on the nucleic acid's hybridization and configuration transformation, the target miRNA-122 associated with liver cancer can be indirectly quantified with a detection limit of 1 fM and a wide dynamic range from 1 fM to 10 pM. The correlation fitting coefficient R2 of the calibration curve can reach to 0.996. The experimental results show that the method has a good recovery rate (98%–105 %) in synthetic samples. This study reveals how the surface charge density of nanochannels regulate the ionic current response in the heterostructured nanochannels. The designed heterogeneous nanochannels not only possess high ionic current rectification property, but also enable to induce superior transport performance by the variation of surface chemistry. The proposed biosensor is promising for applications in early diagnosis of cancers, life science research, and single-entity electrochemical detection. [Display omitted] • The symmetry breaking endows the nanochannels with an outstanding ionic current rectification property. • An enlarged ion current signal of target miRNA can be achieved by DNA cascade amplification. • The designed biosensor can detect miRNA-122 as low as 1 fM. • The designed biosensors can be extended to the detection of charged molecules. [ABSTRACT FROM AUTHOR]

The present study gives an insight into ion flow dynamics of aqueous electrolytes via functionalized multiple nanochannels in polyethylene terephthalate (PET) membrane. The selectivity of certain ions (anions/cations) of various electrolyte combinations KCl- KClO 4 (S1), KBr-KCl (S2), KBr-KClO 4 (S3) over their counter-ions by the nanochannels (generated via swift heavy ion irradiation followed by chemical etching) has been studied. Tuning of the ion transport and ion current rectification has been done using different functionalized nanochannels (with charged carboxyl and protonated amino groups). The impact of ion selectivity of the nanochannels is reflected in the short circuit current and ion current saturation of different systems. The collection of counter ions outside the channels along with the in-built potential due to the flow of ions inside the channels play a crucial role in the ion transport behaviour of the collective system. The cation-selective channels (with charged carboxyl groups) show low rectification ratio compared to the anion-selective channels. The functionalized nanochannels with two -OH polar charge groups (generated through the amino-silane based reagent (3-aminopropyl) trimethoxysilane (APTMS)) exhibit higher current saturation values (S1 AP ∼31 μA, S2 AP ∼30 μA and S3 AP ∼ 30.5 μA) and higher short circuit current values (S1 AP ∼32.3μA, S2 AP ∼ 29.6μA and S3 AP ∼31.3μA) due to the overlapping of electric double layer (EDL). Systematic study of tuning of ion transport through nanochannels helps to understand the nanofluidic flow and has imminent application in the fabrication of nanoelectronic and bioinspired nanodevices. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Electrokinetic behavior of cylindrical nanopores subject to an extra pH-gradient is modeled. • ICR performance depends highly on the level of applied potential bias, and is optimum at a medium strong pH gradient. • Influence of PE layer thickness on ICR performance is important only if pH gradient is medium strong. • The optimum ICR performance occurs at a medium thick PE layer (~3 nm) • The underlying mechanisms of the present novel system are investigated in detail for the first time. Considering versatile potential applications of bioinspired membranes, we simulate the electrokinetic behavior of a cylindrical nanopore, surface modified by a polyelectrolyte (PE) layer. Taking account of the effect of electroosmotic flow and an additionally applied pH gradient, the influences of the strength of the pH gradient, the PE layer thickness, the length of the nanopore and its radius on its conductance and ion current rectification (ICR) performance are assessed. We show that if pH U (the pH at the higher pH end of the nanopore) is fixed at 11 and pH L (the pH at the lower pH end of the nanopore) varies from 3 to 11, the rectification factor R f has a local maximum occurring in 6 < pH L <8; the greater the magnitude of the applied potential bias | V | the smaller the pH L at which the local maximum occurs. The influence of the PE layer thickness on the nanopore rectification performance is important only if 5 < pH L <8, and the optimum performance is reached at a medium thick PE layer (ca. 3 nm). Possible mechanisms associated with the ion transport phenomenon under consideration are proposed and discussed in detail. [ABSTRACT FROM AUTHOR]

Ionic current rectification (ICR) of charged conical nanopores has various applications in fields including nanofluidics, biosensing, and energy conversion, whose function is closely related to the dynamic response of nanopores. The occurrence of ICR originates from the ion enrichment and depletion in conical pores, whose formation is found to be affected by the scanning rate of voltages. Here, through time-dependent simulations, we investigate the variation of ion current under electric fields and the dynamic formation of ion enrichment and depletion, which can reflect the response time of conical nanopores. The response time of nanopores when ion enrichment forms, i.e., at the "on" state is significantly longer than that with the formation of ion depletion, i.e., at the "off" state. Our simulation results reveal the regulation of response time by different nanopore parameters including the surface charge density, pore length, tip, and base radius, as well as the applied conditions such as the voltage and bulk concentration. The response time of nanopores is closely related to the surface charge density, pore length, voltage, and bulk concentration. Our uncovered dynamic response mechanism of the ionic current can guide the design of nanofluidic devices with conical nanopores, including memristors, ionic switches, and rectifiers.

The confinement of ionic liquid inside the nanochannels and its transportation through it under applied potential is being reported. The nanochannels in SiON/Si structure have been created by swift heavy ion bombardment of Au26+ ionś beam at a fluence of 108 ions/cm2 having 350 MeV energy. After chemical etching, these channels (~ 50 to 80 nm diameter) have been filled by ionic liquid (IL) to see their behaviour under applied electrical bias. Due to the high self-diffusion coefficient and low viscosity (compared to other ionic liquids), EMImTFSI has been chosen for the said purpose. The effect of temperature (35–50 °C) and concentration of ionic liquid (undiluted and 50%, 60% and 75% diluted) on IL-SiON/Si structures has been studied. Dissociation of the ionic liquid by the applied voltage and charge extraction/injection through ion channels results in diode-like current–voltage behaviour. Ion current rectification presented in this paper depends on ionś concentration and the temperature of the ionic liquid. It also depends on the applied voltage at a given temperature. It has been found that the ion current rectification ratio decreases with dilution of the ionic liquid. For the system with 75% diluted IL, SiON/Si system has the same behaviour as the one without IL. Since the temperature affects the mobility of ions hence, it also affects the current–voltage behaviour of the system. [ABSTRACT FROM AUTHOR]

Summary of main observation and conclusion: The outstanding combination of selectivity and permeability observed in ultra‐thin nanoporous membranes attracts broad interest from both fundamental research and practical application aspects. It is reported that the asymmetrical ion transport can be realized through the modification of polyelectrolytes on the outer membranes surface. However, the related mechanisms are still unclear which hinders their applications. Herein, we systematically investigate the origin of the ion current rectification in heterogeneously charged ultra‐thin nanopores. The calculated results based on the continuity model suggest that, the ion transport across the ultra‐thin membrane is dominated by the charge on the outer membrane surface rather than the pore wall. The overlap of electric double layer outside the pore entrance is essential for the ion asymmetric transport. And it accordingly results in the fact that the ion concentration enrichment and depletion effect occurring outside the nanopore orifices governs the high and low ionic conductance states under the applied bias. These features can be regulated by the membrane surface state, which is truly attractive because the surfaces of the outer membrane can be easily modified by experimental approaches. These findings provide the renew inspiration for the design of high‐performance 2D nanofluidic devices. [ABSTRACT FROM AUTHOR]

Over the decades, nanochannels have been widely used for single molecule detection, smart sensors, and energy transfer and storage based on its unique ion transport properties. Although various ion transport phenomena of nanochannels have been reported, the discovery of new ion transport phenomena is still of great significance for understanding material transport of nanochannels and development of nanodevices with unique working capabilities. This article reports a novel nanochannel ion transport phenomenon - ion current rectification in combination with ion current saturation (ICR-S), which arised from a mesoporous titania conical microplug generated in situ in the glass micropipette tip cavity by space confinement evaporation. The ion current of forward voltage is greater than that of reverse voltage, and the saturation currents appear in both the forward and reverse voltages, the ratio of forward and reverse saturation current can reaches to 10. In addition, the influence of pH, ionic strength, and micropipette angle on ICR-S is also investigated. Image 1 • Ion current rectification with current saturation, ICR-S is found for the first time. • ICR-S occurs at mesoporous titania conical microplug (MTCP). • MTCP is generated in situ in glass micropipette tip by space confinement evaporation. • Forward voltage of ion current is greater than reverse voltage in absolute value. • Saturation currents appear in both of forward and reverse voltages with ratio of 10. [ABSTRACT FROM AUTHOR]

We report a nanofluidic device for the label‐free detection of phosphoprotein (PPn) analytes. To achieve this goal, a metal ion chelator, namely 4‐[bis(2‐pyridylmethyl)aminomethyl]aniline (DPA−NH2) compound was synthesized. Single asymmetric nanofluidic channels were fabricated in polyethylene terephthalate (PET) membranes. The chelator (DPA−NH2) molecules are subsequently immobilized on the nanochannel surface, followed by the zinc ion complexation to afford DPA−Zn2+ chelates, which act as ligand moieties for the specific binding of phosphoproteins. The success of the chemical reaction and biomolecular recognition process that occur in a confined geometry can be monitored from the changes in electrical readout of the nanochannel. The nanofluidic sensor has the ability to sensitively and specifically detect lower concentrations (≥1 nM) of phosphoprotein (albumin and α‐casein) in the surrounding environment as evidenced from the significant decrease in ion current flowing through the nanochannels. However, dephosphoproteins such as lysozyme and dephospho‐α‐casein even at higher concentration (>1 μM) could not induce any significant change in the transmembrane ion flux. This observation indicated the sensitivity and specificity of the proposed nanofluidic sensor towards PPn proteins, and has potential for use in differentiating between phosphoproteins and dephosphoproteins. [ABSTRACT FROM AUTHOR]

Biological ion channels are known as membrane proteins which can turn on and off under environmental stimulus to regulate ion transport and energy conversion. Rapid progress made in biological ion channels provides inspiration for developing artificial nanochannels to mimic the structures and functions of ion transport systems and energy conversion in biological ion channels. Due to the advantages of abundant pore channels, metal–organic frameworks (MOFs) have become competitive materials to control the nanofluidic transport. Herein, a facile in situ synthesis method is developed to prepare hybrid nanochannels constructed by 2D MOFs and porous anodic aluminum (PAA). The introduction of asymmetries in the chemical composition and surface charge properties gives the hybrid outstanding ion current rectification properties and excellent ion selectivity. A power density of 1.6 W m−2 is achieved by integrating it into a salinity‐gradient‐driven device. With advantages of facile fabrication method and high ion selectivity, the prepared 2D MOFs/PAA hybrid membrane offers a promising candidate for power conversion and water desalination. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Nanopore electrochemical sensors have been developed to detect various emerging pollutants at the single-molecule level with high sensitivity and specificity due to their high resolution in past three decades, which is of great significance for emerging hazardous pollutants control. Electrochemical detection methods based on nanopore technology are primarily divided into two categories: resistive-pulse and rectification sensing. The main detection mechanism of the resistive-pulse method is based on the volume exclusion effect generated when the analyte enters through the nanopore, and it involves three primary detection strategies. The first approach involves nanopore technology for direct pollutant detection, where pollutant structure and concentration information is assessed based on nanopore-generated ionic current's blockade strength, duration, and frequency. The second strategy relies on weak intermolecular interactions when direct detection of pollutants is challenging. Molecular probes interact with target molecules to enhance detection sensitivity and achieve specific recognition. The third strategy is based on the biological/chemical reaction of pollutant molecules, through indirect detection method to achieve specific detection of target pollutant molecules. Additionally, relies on changes in surface charge polarity or nanopore spatial structure induced by the analyte molecules to achieve the detection of target molecules within the nanopore. This work demonstrates the research progress achieved in the application of nanopore electrochemical sensing technology to high-sensitivity detection of hazardous pollutants, utilizing the unique confined spatial characteristics of nanopores, combined with biorecognition techniques and principles of electrochemical reactions. [ABSTRACT FROM AUTHOR]

Ion current rectification (ICR) is an electrodynamic phenomenon in electrolyte solution which is defined as the asymmetric potential-dependent ion flux through a confined environment, giving rise to asymmetric electrical current-voltage characteristics induced by the influence of an asymmetric electrical double layer structure. Since the discovery of the ICR phenomenon, the observation and application of ICR at nanoscale and microscale have been widely investigated experimentally and theoretically. Here, the recent progress of ICR from nanoscale to microscale is systematically reviewed. Nano/micropore structures of different materials, shapes and pore sizes are first discussed. Then, the factors influencing ICRs by thermodynamically or kinetically regulating the electrical double layer structure are introduced. Moreover, theoretical models are presented to explain the mechanism of ICRs. Based on the understanding of this phenomenon, the applications, especially in biosensors, are discussed. Finally, future developments of this area are briefly presented. This review covers the representative related literature published since 2010 and is intended to give a systematic introduction to this area. [ABSTRACT FROM AUTHOR]

Bioinspired artificial asymmetric nanochannels for more precise controls of ion transport and ion pump at the nanometer scale have been attracting some attention for extensive application in biosensors, ions separation technologies, nanofluidic diodes and energy device. Herein, we construct the first asymmetric heterostructured SiO 2 /Al 2 O 3 nanofluidic diodes based on the introduction of physical structures and chemical surface activity. Due to asymmetric chemically deposited coatings and surface charges distribution, the SiO 2 /Al 2 O 3 nanofluidic diodes are capable of regulating ion transport selectivity and possessing ionic current rectification properties. These asymmetric heterostructured SiO 2 /Al 2 O 3 nanofluidic diodes are further introduced spiropyran into nanochannels to create a lightgating device, which were triggered by ultraviolet light to be "ON" state from the neutral spiropyran blocked "OFF" state. This asymmetric heterostructured SiO 2 /Al 2 O 3 nanofluidic diode epitomizes high sensitivity, fine selectivity, superior stability and highly reversibility. The combination with photoswitchable compounds spiropyrans shows great promise for easy application in the optical device such as highly sensitive light gating devices. [ABSTRACT FROM AUTHOR]

Abstract Bioinspired nanofluidic systems have shown a promising application in ion transport tuning and energy conversion. Here, we have numerically investigated the ionic current rectification behavior and power generation via bioinspired fractal nanochannels. Fractal nanochannels can present a strong ionic current rectification effect due to the asymmetric distribution of the electrical double layer overlapping induced by the fractal geometry. At a concentration of 200 mM, the ionic current rectification factor in the trifurcated nanochannel is almost improved by 100%, compared to that in the bifurcated nanochannel. Moreover, ionic current can rectify at much higher salt concentrations in trifurcated nanochannel. Fractal nanochannels can considerably improve the power density and energy conversion efficiency for enhanced osmotic current, diffusive voltage and ion selectivity originating from considerably improved electrical double layer overlapping degree. At a concentration ratio of 1000-fold, compared to the straight nanochannel, the power densities of the bifurcated and trifurcated nanochannels are, respectively, improved by 153% and 357% while the energy conversion efficiency are augmented by 147% and 336%. Furthermore, the salt dependent ion transportation and power generation via the furcated nanochannels are also discussed. This paper may contribute to designing appropriate artificial nanofluidic systems for efficient ion transport tuning and energy conversion. Highlights • ICR and power generation via bioinspired fractal nanochannels are investigated. • ICR factor in the trifurcated nanochannel is almost improved by 100%. • Power density can be significantly improved via the furcated nanochannels. • Cation diffusion coefficient presents opposite impact on ICR and power generation. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract Extending previous electrokinetic analyses based on a Newtonian fluid to power-law fluids, we investigate the behaviors of the ion current rectification (ICR) and the ion selectivity S of a conical nanopore having a pH-regulated surface. The bulk salt concentration C bulk , the solution pH, and the power-law index n are examined in detail for their influences on these behaviors. We show that the ICR ratio for the case where pH is lower than the isoelectric point (IEP) of the nanopore surface is different both quantitatively and qualitatively from that for the case where pH > IEP. The relative magnitude of the ICR ratio as n varies depends largely on the level of C bulk. In contrast, S (pH < IEP) is qualitatively similar to that for S (pH > IEP), where | S | decreases with increasing C bulk and/or decreasing n. In addition, S is very sensitive to n , for example, a decrease of n from 1.0 (Newtonian fluid) to 0.9 (pseudoplastic fluid) can yield a 245% increase in S at C bulk = 100 mM. Implying that the performance of ion separation can be improved by tuning the fluid viscosity. Mechanisms are proposed for explaining the observed behaviors in the ICR ratio. [ABSTRACT FROM AUTHOR]

Abstract The unique properties of nanopores depend largely on the functionalization of the pore surface, which has always been a great challenge. In this study, a facile method for functionalizing the surface of a glass nanopipette to establish thiol-based linkage without involving Au film coating was successfully developed by utilizing the thiol–maleimide click reaction which works rapidly and with high specificity. Cysteine was employed as a model molecule to functionalize the nanopipette, resulting in pH-modulated ion-current rectification behavior which showed a good reproducible response to pH. Due to the widespread existence of thiol groups in nature, this nanopipette functionalization method has the potential and versatility to be used in a wide range of applications, including mimicking biological ion channels and fabricating sensing devices. Graphical abstract Unlabelled Image Highlights • A glass nanopipette was functionalised via a thiol–maleimide click reaction without Au film coating. • pH-modulated ion-current-rectification behavior of a cysteine-functionalized glass nanopipette was achieved. • The nanopipette ion-current rectification direction was modulated by pH. [ABSTRACT FROM AUTHOR]

In this study, a polyethyleneimine (PEI)/Zr4+-functionalized nanofluidic sensing platform based on nonlinear hybridization chain reaction (NHCR) was developed for PNK activity assay. With the existence of PNK, the hairpin H PNK was cleaved by λ exonuclease, liberating the initiator T-DNA. Then T-DNA triggered the nonlinear HCR in solution and the reaction products were absorbed onto the nanopore, which changed the surface charge of nanofluidic device and could be detected by current-voltage characteristic curves. Compared to traditional linear HCR, the nonlinear HCR exhibits a higher sensitivity and order of growth kinetics, making it a powerful signal amplifier in bioanalysis. Due to the powerful amplification efficiency of nonlinear HCR, high sensitivity of the nanopore and specific recognition site of PNK/λ-Exo, an ultrasensitive and selective PNK sensing approach had been developed and applied to precisely quantitate the PNK activity with a LOD of 0.0001 U/mL. Moreover, utilizing this nanofluidic system as a foundation, we constructed a logic circuit that utilized PNK, adenosine diphosphate (ADP), and (NH 4) 2 SO 4 as input elements. ADP and (NH 4) 2 SO 4 had a crucial function in facilitating the PNK to regulate the DNA logic gate. By modifying the target and inhibitors, the nanofluidic device could detect a variety of stimuli and execute more advanced logical operations. [Display omitted] [ABSTRACT FROM AUTHOR]

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. [Display omitted] • The effect of membrane thermal conductivity (MTC) on the ICR in conical nanochannel is investigated. • The MTC noticeably affects the temperature and ion concentration distribution. • The ICR under different MTC is obtained. • The ICR decreases as the MTC increases under positive temperature difference. [ABSTRACT FROM AUTHOR]

Single-cell analysis (SCA) could provide in-depth understanding of cell biology yet is of great challenge. Many attempts have been made to develop novel techniques for SCA. Nanopipette-based ion current rectification (ICR) electroanalysis represents an emerging methodology to combat in this arena. In this review, the working rationale of ICR and its key regulation factors are discussed, followed by the sensing strategies established for SCA using illustrative examples. Future perspectives in this field are also proposed. • Ion current rectification (ICR)-nanopipette technique for single-cell analysis (SCA) is reviewed. • The fundamentals, operation rationales, and the state of the art are discussed. • Trends and prospects of this field are presented. [ABSTRACT FROM AUTHOR]

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.

This study shows that it is possible to customize the ion-current-rectification (ICR)-based pH response in glass nanopipettes by adjusting the ratio of 3-aminopropyldimethylethoxysilane (APTES) and (3-triethoxysilylpropyl) succinic acid anhydride (TESP-SA) used in silanization. The silanization time and concentration were optimized by investigating the rectification ratio of the nanopipettes. For each APTES/TESP-SA silane ratio, analysis of variance (ANOVA) tests indicated that the rectification ratio of the nanopipette had good linearity in the pH range 3–8. Moreover, nanopipettes silanized with equal concentrations of amino and carboxylic groups gave the highest pH response of 1.06 × 10 −4  pH  −1 . This method for customizing the ICR-based pH response in nanopipettes has the potential and versatility to be used in applications such as mimicking biological ion channels and developing nanosized pH sensing devices. [ABSTRACT FROM AUTHOR]

In this study, control of ion transport is demonstrated by surface modification of bipolar polyelectrolyte (PE) brushes on a nanopore wall. Three types of nanopore are considered: (i) Only the inner wall of a nanopore is modified and its outer walls are charge-free (type I). (ii) Only the outer walls of a nanopore are modified (type II) and its inner wall is charge-free. (iii) Both the inner and the outer walls of a nanopore are modified (type III). Results reveal that for the levels of salt concentration considered all these three types of nanopore show apparent current rectification. The rectification ability depends highly on both the structure and the space charge densities of the bipolar PE brush layers. For example, if the charge densities of this layer are low, the rectification ability of the types of nanopore ranks as type III > type II > type I; otherwise, it ranks as type III > type I > type II if they are sufficiently high. In addition, if the charge densities of those layers are symmetric, the rectification ability increases with increasing charge density for all three types of nanopore. For nanopores having asymmetric charge densities in their PE brush layers, similar charge density-dependent rectification behavior occurs only to type II nanopore. It is interesting to observe in the other two types of nanopore, the increase in the charge densities results in a decrease in the rectification effect. This is unexpected because previous studies suggested that the nanopore rectification depends positively on its charge density. [ABSTRACT FROM AUTHOR]

• Polymer conical nanopores are prepared using the track-etching technique. • The nanopore acts as an electrochemical valve with on/off conductance states. • Zero-average fluctuating potentials are applied as driving forces. • Cation pumping against an external concentration difference is demonstrated. • A load capacitor characterizes the pumping for a range of experimental data. We study the cation transport against an external concentration gradient (cation pumping) that occurs in conical nanopores when zero-average oscillatory and white noise potentials are externally applied. This pumping, based on the electrically asymmetric nanostructure, is characterized here by a load capacitor arrangement. In the case of white noise signals, the conical nanopore acts as an electrical valve that allows extraction of order from chaos. No molecular carriers, specific ion pumps, and competitive ion-binding phenomena are required. The nanopore conductance on / off states mimic those of the voltage-gated ion channels in the cell membrane. These channels allow modulating membrane potentials and ionic concentration gradients along oscillatory pulses in circadian rhythms and the cell cycle. We show that the combination of asymmetric nanostructures with load capacitors can be useful for the understanding of nanofluidic processes based on bioelectrochemical gradients. [ABSTRACT FROM AUTHOR]

• The effects of channel shape and surface charge on the ion current rectification in asymmetric nanochannels were comprehensively investigated. • Simple increase or decrease of the spatial size will not beneficial for improving the ICR performance. • The specified asymmetric surface charge on the outer wall surface can strongly enhance the ICR performance. • The research results provide essential insights on the ion transport and useful guidelines for the design and performance optimization of biological sensors and nanofluidic devices. Ion current rectification (ICR) in nanochannels has attracted increasing attention for its great potential in the applications of ionic circuits and biological sensors. Herein, the influence of nanochannel shape and surface charge on the ion transport and ICR performance of asymmetric nanochannels was numerically investigated. Firstly, three asymmetric nanochannels with different shapes (bullet, conical, and trumpet) were constructed to investigate the spatial size effect on the ICR behavior. And then selected the best-performing one to further analyze the effect of the surface charge on the ICR. In the investigation of spatial size effect, it is found that the increases and the decreases of spatial size will not be beneficial for improving the ICR performance and the conical nanochannel exhibits the best performance. In the investigation of surface charge effect, it is found that specified asymmetric surface charge on the outer wall surface enhances the corresponding ICR performance, revealing the cooperative role with inner wall surface charge. Besides, it is found that the outer wall surface charge is dominant when the nanochannel length is small and the inner wall surface charge is dominant when the nanochannel length is large. Moreover, the finite length enhancement effect for ICR performance is illustrated by tuning the range of outer wall surface charged zone, indicating the charged zone near the entrance and exit plays the dominant role on ion transport. The obtained results provide essential insights on the ion transport and useful guidelines for the design and performance optimization of biological sensors and nanofluidic devices. [Display omitted] [ABSTRACT FROM AUTHOR]

• The construction of solid nanochannel-based biosensor with submicron pore size. • Polydopamine-Au multilayer assembly in submicrochannel heteromembrane. • Adjustable size (∼10 and 89 nm) and density of Au nanoparticles in AAO. • Ion current rectification induced by pH in submicrochannel heteromembrane. • Non-enzymatic glucose response at ultra-low concentration (1 pM) The construction of solid nanochannel-based biosensor with submicron pore size has great significance to the analysis of biomolecule with high sensitivity and low detection limit. In this paper, a submicrochannel heterogeneous membrane with asymmetric geometry is prepared as the base by combining thermo-crosslinked hydrophobic conducting polymer and porous anodic aluminium oxide channel (AAO) with submicron pore size (269 nm). After the polydopamine-Au multilayer assembly, a large number of Au nanoparticles with adjustable size (∼10 and 89 nm) and density are in situ generated and fixed on the inner wall of AAO. Then 4-mercapto phenylboric acid with high density is grafted through Au-S bond to construct the submicrochannel heterogeneous membrane with the characteristic of ion current rectification in response to pH and saccharide. Benefit from the blocking effect of hydrophobic conducting polymer and the multilayer assembly of polydopamine-Au, the current of background response by the membrane device is effectively reduced, the ion current rectification strength can be effectively regulated in the range of pH 3–9, and the response to ultra-low glucose concentration without enzyme is achieved in the linear range of 1 pM-0.1 μM. A universal substrate platform is provided for the construction of solid nanochannel-based biosensor with submicron pore size. [ABSTRACT FROM AUTHOR]

[Display omitted] • The asymmetric branch-type alumina nanochannel (BAN) membrane is engineered. • The BAN shows rectification inversion and amplified ion transport in a pH gradient. • Inversion of ion current rectification boosts osmotic power. • A jump of output power from 2.1 W/m2 to an ultrahigh 5.9 W/m2 is achieved. • Our rigorous simulations support all experimental findings. Biological channel membranes regulate direction and degree of ion transport in response to external stimuli, yet it is a challenge to develop artificial nanofluidics with such functions without any chemical modification of the channel walls. Herein, we report the asymmetric branch-type alumina nanochannel (BAN) membrane, that comprises large-pore stem channels and small-pore branched channels and exhibits reversed and significantly enhanced ionic rectification when subjected to a pH gradient. The anomalous transport properties are supported by our rigorous model that takes into account the equilibrium surface reactions of alumina hydroxyl groups on the nanochannel walls with surface protons. The simulation results indicated that the pH gradient-induced bipolar surface charge distributions on the BAN walls lead to the reversal and enhancement of ion transport properties. This work highlights the link between surface chemistry, concentration polarization and ion current rectification, significantly extending the knowledge of how asymmetric pH environments influence transport properties of asymmetric nanofluidics. The application of remarkably amplified osmotic power is also demonstrated, highlighting that the BAN membrane achieves a jump of power density output from ∼2.1 to ∼5.9 W/m2 in the presence of a pH gradient, thus exceeding previously reported values. [ABSTRACT FROM AUTHOR]

Since the outbreak of COVID-19 in the world, it has spread rapidly all over the world. Rapid and effective detection methods have been a focus of research. The SARS-CoV-2 N protein (NP) detection methods currently in use focus on specific recognition of antibodies, but the reagents are expensive and difficult to be produced. Here, aptamer-functionalized nanopipettes utilize the unique ion current rectification (ICR) of nanopipette to achieve rapid and highly sensitive detection of trace NP, and can significantly reduce the cost of NP detection. In the presence of NP, the surface charge at the tip of the nanopipette changes, which affects ion transport and changes the degree of rectification. Quantitative detection of NP is achieved through quantitative analysis. Relying on the high sensitivity of nanopipettes to charge fluctuations, this sensor platform achieves excellent sensing performance. The sensor platform exhibited a dynamic working range from 102-106 pg/mL with a detection limit of 73.204 pg/mL, which showed great potential as a tool for rapidly detecting SARS-CoV-2. As parallel and serial testing are widely used in the clinic to avoid missed diagnosis or misdiagnosis, we hope this platform can play a role in controlling the spread and prevention of COVID-19. [Display omitted] • The aptamer-functionalized nanopipettes were successfully constructed. • The N protein sensor platform was constructed by using the unique ion current rectification of nanopipettes. • The constructed sensor achieved a good detection effect on N protein in serum samples. [ABSTRACT FROM AUTHOR]

[Display omitted] • The MOF-based subnanochannel IDM is engineered for osmotic energy conversion. • The IDM shows rectified, directional, and amplified Li+ ion transport in methanol. • Angstrom-scale pores of ZIF-8/PSS render IDM high cation selectivity in methanol. • A record power density of 23.4 W/m2 is achieved in 2 M/0M LiCl-methanol system. Harvesting Gibbs energy from a salinity gradient of waste organic solutions can not only alleviate the environmental pollution but also provide new energy resources to mitigate the energy shortage. However, the membranes with high ion selectivity, excellent ionic flux, and high-performance power output in organic solutions remain considerably unexplored. Here, we report on the metal–organic framework (MOF) based subnanochannel ionic diode membrane (IDM), comprising a large-area and continuous zeolitic imidazolate framework-8/polystyrene sulfonate membrane and a highly ordered alumina nanochannel membrane, for highly efficient osmotic energy conversion in organic solution. We show that a distinctly asymmetric design of pore sizes, charges, and wettabilities and the numerous highly negatively charged and angstrom-scale window-cavity pore structures in the MOF layer endow the subnanochannel IDM with directional and amplified Li+ ion transport and enhanced selectivity in methanol. Thus, an unprecedented power density of ∼23.4 W/m2 is achieved when a 2 M LiCl-methanol solution is mixed with the pure methanol, 5-times more than the existing value. This work provides significant insights into the use of rectified ion channel-mimetic membranes for achieving ultrafast Li+ ion transport at the confinement and high-performance energy harvesting from organic solutions. [ABSTRACT FROM AUTHOR]

Electric eels can convert ionic concentration gradients into a high-efficiency power via a large number of sub-2 nm transmembrane ion channels, which can exhibit high ion selectivity and strong diode-like ion rectification property. Inspired by this, herein, we report on a sub-2 nm scale ionic diode membrane, composed of an ultrathin (∼110 nm) β-ketoenamine-linked two-dimensional covalent-organic framework (COF) membrane and a highly ordered alumina nanochannel membrane (ANM), for highly efficient osmotic energy harvesting. As verified by our experimental and simulation results, the heterostructured membrane with the features of asymmetric charges and pore sizes in two aligned COF (1.1 nm) and ANM (100 nm) channels is capable of highly rectifying ion transport even in high 0.5 M salt solution. Benefiting from the presence of abundant sub-2 nm COF-based ion channels and the strong ionic diode effect, an unprecedented power density of up to 27.8 W/m2 is achieved by mixing the artificial salt-lake water and river water (500-fold NaCl gradient). This study will open new avenues of using the rectified ion channel-mimetic nanofluidic membrane as a new platform towards the exploration and development of an ultrahigh osmotic power generator. [Display omitted] • The sub-2 nm-scale COF-based ionic diode membrane has been developed for efficient osmotic energy conversion. • The ion channel-mimetic nanofluidic membrane can rectify ion transport even in seawater simulated solution. • A record power density of 27.8 W/m2 is achieved by mixing artificial salt-lake and river water. • Simulations attribute the breakthrough power output to the numerous sub-2 nm 1D COF channels and strong ionic diode effect. [ABSTRACT FROM AUTHOR]