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Ion current rectification (ICR), which is the departure of the experimentally measured current-voltage curves from ohmic behavior, has recently drawn intensive interests from both a fundamental point of view and microfluidic-based applications. In this case, the rectification factor (RF), which is defined as the absolute value of the quotient between the currents recorded for one voltage polarity and the currents recorded for the same absolute value of voltage at the opposite polarity, is one of the most important parameters to describe the rectification behavior. Herein, we interestingly observed that RF is strongly dependent on the bias voltage from both the experimental and theoretically simulated results, and this dependence exhibits nonlinear deviation with increasing the rectification degree. At the same surface charge density, the linear deviation increases with decreasing the electrolyte concentration. When the electrolyte concentration remains the same, the linear deviation increases with increasing the surface charge density. The simulated results demonstrate that the electro-osmotic flow is not the dominating factor to this nonlinear phenomenon. This nonlinear deviation is considered to possibly originate from the inhomogeneous conductivity and the nonlinear change of conductance with the bias potential. This study is not only useful to understanding the ICR behavior at a conical glass nanopipette, but also potentially offers a new parameter to describe the rectification degree with this nonlinear relationship. [ABSTRACT FROM AUTHOR]

Ion transport in a confined conical nanochannel with high solution concentrations was studied using molecular dynamics simulation. The simulation results indicated that the ion current rectification appeared at high solution concentrations, even without electrical double layer (EDL) overlapping, which was still influenced by both the solution concentration and surface charge properties as it would be in a low solution concentration. With solution concentrations increasing from 0.41 to 2.08 M, a maximum rectification ratio was obtained. This phenomenon was attributed to the competition between the axial binding energy gradient arising from the confined conical geometry intensifying the ion axial asymmetric concentration polarisation and the decreasing thickness of the EDL weakening the concentration polarisation. [ABSTRACT FROM PUBLISHER]

Glass conical nanopores functionalized with a macrocyclic dioxotetraamine derivative (C5) are shown to enable ultrasensitive determination of Hg(II). A complex is formed between Hg(II) and C5, and this converts the functionalized glass nanopore from a rectifying state to a non-rectifying state. The presence of Hg(II) in ~10 pM levels or higher can be detected by measuring the ion current rectification ratio in voltage range of ± 500 mV. This sensing strategy opens new possibilities for development of a simple, portable and convenient device for the quantitation of Hg(II) with very low limits of detection. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

A glass capillary-based nanopore (G-nanopore), due to its tapered tip, easy tunability in orifice size, and especially its flexible surface modifications that can be tailored to effectively capture and enhance the ionic current signal of single entities (single molecules, single cells, and single particles), offers a powerful and nanoconfined sensing platform for diverse biological measurements of single cells and single molecules. Compared with other artificial two-dimensional solid-state nanopores, its conical tip and high spatial and temporal resolution characteristics facilitate noninvasive single molecule and selected area (subcellular) single cell detections (e.g., DNA mutations, highly expressed proteins, and small molecule markers that reflect the change characteristics of the tumor), as a small G-nanopore (≤100 nm) does negligible damage to cell functions and cell membrane integrity when inserted through the cell membrane. In this brief review, we summarize the preparation of G-nanopores and discuss the advantages of them as solid-state sensing platforms for single molecule and single cell detection applications as well as for cancer diagnosis and treatment applications. We also describe the current bottlenecks that limit the widespread use of G-nanopores in clinical applications and provide an outlook on future developments. The brief review will provide the reader with a quick survey of this field and facilitate the rapid development of a G-nanopore sensing platform for future tumor diagnosis and personalized medicine based on single-molecule/single-cell bioassay.

In this article, we propose a nanopore-based approach to detect metal ions without any external functionalization. In detection of the biologically and environmentally relevant Cr 3+ ion as a prototypical example to prove our strategy, both selectivity and sensitivity were individually achieved. In contrast to mainstream research based on receptor-functionalized nanopores, we report a method for easy regeneration of the nanopore surface that allows elimination of the tedious functionalization steps. Besides, with the assistance of a strong chelator (EDTA), the asymmetric nanopore becomes highly resistant to the interference of the metal–ions matrix, and shows significant specificity towards Cr 3+ . The detection limit of this sensor was 16 nM (signal-to-noise ratio=3), which was comparable to reported values. By virtue of the reusability of the polymer surface, metal ion sensors based on asymmetric nanopores can be applied universally in combination with chelators sensitive to specific metal ions. [ABSTRACT FROM AUTHOR]

A biomimetic ion-responsive single nanopore sensor was constructed by immobilizing zirconium(IV) ion on a film of poly(ethylene terephthalate) whose surface was modified with polyethyleneimine (PEI) to form nanopores. PEI is an excellent metal-chelating ligand that was initially modified on the surface of the membrane through amidation. The Zr-chelated nanopore responds to inorganic phosphate by a change in the ion current-voltage plot. This is due to a reversal of the charge from positive to negative after binding of the phosphate to the tip of the ion channel. Inorganic phosphate can be quantified by measurement of the extent of rectification. The limit of detection of the method is 118 nM, and response is linear in the 0 to 40 μM phosphate concentration range. The formation of the Zr(IV)/phosphate system on the surface was corroborated by X-ray photoelectron spectroscopy and current-voltage characterization. The single synthetic nanopore display excellent selectivity, high sensitivity and a wide response range for phosphate. The accuracy of the method and applicability of the nanopore-based detection scheme was proven by the successful determination of inorganic phosphate in (spiked) lake water and a cola drink. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Graphene, as an important ideal two-dimensional material, is driving more and more attention because of its unique mechanical strength and chemical stability. In this work, the rectification effect of nanopores in graphene/PET structure at different KCl concentration is investigated. Porous graphene membrane is obtained by using heavy ion irradiation technology. During investigating the ion current rectification effect, PET membrane with conical pores acts as porous graphene's substrate. The rectification coefficient of nanopores in graphene/PET is deduced from the current-voltage curves at KCl solution with different concentrations. The porous graphene/PET shows a relative higher rectification ratio and is more resistive to pH value while comparing the nanopores in PET. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Fluidic devices exhibiting ion current rectification (ICR), or ionic diodes, are of broad interest for applications including desalination, energy harvesting, and sensing, among others. For such applications a large conductance is desirable, which can be achieved by simultaneously using thin membranes and wide pores. In this paper we demonstrate ICR in micrometer sized conical channels in a thin silicon membrane with pore diameters comparable to the membrane thickness but both much larger than the electrolyte screening length. We show that for these pores the entrance resistance is key not only to Ohmic conductance around 0 V but also for understanding ICR, both of which we measure experimentally and capture within a single analytic theoretical framework. The only fit parameter in this theory is the membrane surface potential, for which we find that it is voltage dependent and its value is excessively large compared to the literature. From this we infer that surface charge outside the pore strongly contributes to the observed Ohmic conductance and rectification by a different extent. We experimentally verify this hypothesis in a small array of pores and find that ICR vanishes due to pore-pore interactions mediated through the membrane surface, while Ohmic conductance around 0 V remains unaffected. We find that the pore-pore interaction for ICR is set by a long-ranged decay of the concentration which explains the surprising finding that the ICR vanishes for even a sparsely populated array with a pore-pore spacing as large as 7 μm.

Abstract: In this work, we demonstrate a smart robust nanodevice that rectifies the ion current in response to sugar stimulation. The response is achieved by immobilizing 4-carboxyphenylboronic acid to the wall of a single glass conical nanopore. This smart robust nanodevice shows excellent sugar responsive capacity with the switch between the non-rectifying and rectifying states of the ion transport properties. The ion current rectification ratio increases linearly with increasing concentration of glucose and correlates to the concentration of the target by a simple linear regression equation, which makes quantitative nanosensing easily accessible for application in biomedical fields. [Copyright &y& Elsevier]

Abstract: Here, we demonstrate a novel sensing platform for the molecular recognition of monosaccharides (glucose and fructose) and glycoprotein (ovalbumin) using synthetic ion channels fabricated in polymeric membrane. To achieve this, receptor (3-aminophenylboronic acid) is covalently attached with carboxylic groups on the surface and inner walls of the channel. The detection principle rests on the reversible binding occurring between boronic acid and diols of saccharides or glycoprotein molecules, leading to a sensible change in the ionic flux across the nanochannel. In the presence of saccharides or glycoprotein at neutral pH, saccharide–boronic acid complex changes the channel surface from neutral to negative. The binding of ovalbumin onto the channel surface also leads to the reduction of the effective channel diameter, which in turn diminishes the channel conductance. Based on the experimental results, we believe that the presented sensing platform could also be applicable in the area of biomedical sciences for the recognition of cell-surface carbohydrate moieties. [Copyright &y& Elsevier]

Abstract: In this study, based on ion-current rectification in the conically shaped nanochannel embedded in polyethylene terephthalate (PET) membrane, we have selectively discriminated three small biomolecules. Because three positive biomolecules (Hoechst 33342, Propidium and Bupivacaine) have different hydrophobicities, their interactions with inside wall of the conical nanochannel are different and their binding affinities can be derived from Langmuir absorption model. Therefore, we can successfully discriminate these small biomolecules. The highest binding constant was obtained for the small molecule with highest hydrophobicity. Another interesting result is that the detection limit for the small molecule with the highest binding constant shifts to submicromole. [Copyright &y& Elsevier]

Abstract: In this study, a novel covalent modification method of the single glass conical nanopore channel with amphoteric 6-carboxymethyl-chitosan (CMC) was designed to obtain a smart device responsive to a broad range of pH stimuli. This response is highly sensitive, reversible and reproducible. The CMC modified channel possessing carboxyl and amino groups was able to regulate ion transport selectivity and ion current rectification properties which depend on surface charges at various pH values. Each modification step was characterized by simply measuring the current–voltage (I–V) curves of the nanopore channel. [ABSTRACT FROM AUTHOR]

At present, little attention has been paid to the ion current rectification (ICR) performance in the bilayer nanochannel with non-uniform charge distribution, which has the similar asymmetrical property with the conical nanochannel. In this work, we have established the ions transport model in the bilayer nanochannel with the coupled Poisson-Nernst-Planck (PNP) equations. Four configuration types bilayer nanochannel are compared on the ion rectification performance. The results show that asymmetrical geometry without the surface charge cannot induce ICR and the existence of surface charge is necessary. Besides, the config.3 has the best ICR performance and the physical mechanisms are discussed in detail. It is found that the ICR performance can be enhanced by regulating the surface charge density, geometry parameters and ion concentration. The ICR ratio increases as the absolute surface charge density increases and the effect is more significant to the change of left nanopore. When the total length of the bilayer nanochannel is fixed, the ICR ratio increases first and then decreases with the increasing length of the left nanopore. In addition, the forward current is more sensitive to the change of diameter of left nanopore and the reverse current is more sensitive to the change of diameter of right nanopore. When the bulk concentration varies, the ICR ratio decreases with the increasing concentration. The research results provide useful information for ions transport control and optimization of micro/nano fluidic devices. • Strong ion current rectification effect was observed in asymmetric charged bilayer nanochannels. • The factors which influence the ion current rectification performance were comprehensively investigated. • The optimal structure of nanochannel with appropriate small and large nanopore is demonstrated. • The research results provide useful information on ions transport control and optimization of micro/nano fluidic devices. [Display omitted]. [ABSTRACT FROM AUTHOR]

A higher rectification degree in ionic diodes is required to achieve better performance in applications. Nonetheless, the active geometrical change that is critical for inducing electrical potential asymmetry is difficult to realize in typical ionic diodes because of the intrinsic limitation of the fabrication method. Here, we propose a nanochannel-network-based bipolar diode with a high rectification degree of ∼1600─the highest value realized until now, to the best of our knowledge. Such a high rectification is obtained based on the synergetic effect of the bipolar surface charge and the optimization of the microchannel through experimental studies and multiphysics numerical simulations. It induces ion concentrations at the heterogeneous junction based on the accumulation effect under the forward potential bias. In particular, this proposed molecular concentration occurs in the ohmic region without vortex and instability that is inevitable at the conventional nano-electrokinetic concentration. Combining this accumulation with the horizontally aligned configuration of the nanochannel network membrane (NCNM), a highly sensitive and quantitative mercury ion (Hg 2+ ) sensor based on a fluorescent signal is fabricated that allows direct measurement using a general fluorescent microscope. The detection limit of Hg 2+ is 10 pM, which is ∼10 times lower than the best detection limit realized so far (∼100 pM) in fluorescent dye-based detection. This demonstrates the potential of asymmetric NCNM for high-performance ion transport in applications such as energy conversion, based on its design and material flexibility.

We experimentally characterize the ionic conduction of single and multipore nanoporous membranes in aprotic organic electrolytes. To this end, soft-etched (SE) membranes with pore diameters in the nanometer range and track-etched (TE) membranes with pore diameters in the tens of nanometers range are investigated. In aqueous conditions, the membrane ionic conduction rates follow the same trend of the bulk solution conductivities. However, the ionic transport through the narrow SE-nanopores dramatically decreases in aprotic electrolytes due to the formation of solvated metal cations and their adsorption on the pore surface. The current-voltage recordings of single conical nanopores in aprotic electrolyte solutions with different water mole fractions reveal that the solvated metal ion (M) species [M−(solvent) 4 ]+ formed in acetonitrile solvent are more tightly bounded to the pore walls compared with the cationic chelates obtained in propylene carbonate solvent. The basic findings reported here should be of interest for ionic/molecular nanofiltration processes in non-aqueous conditions as well as for moisture sensitive and energy storage nanofluidic devices. [Display omitted] • Soft-etch method is employed to fabricate subnanometer pores in heavy ion irradiated polyimide membranes. • Conical nanopores are prepared through asymmetric chemical track-etching technique. • Ion conduction characteristics of soft-etched and track-etched membranes are investigated in aqueous and aprotic electrolyte solutions. • Ionic transport through the soft-etched nanopores is significantly reduced in aprotic electrolytes due to the solvation of metal cations. • Track-etched nanopores in aqueous solutions conduct cations, while in aprotic electrolytes they become anion-selective. [ABSTRACT FROM AUTHOR]

Bioinspired nanochannels have emerged as a powerful tool for bioengineering and biomedical research due to their robust mechanical and controllable chemical properties. Inspired by inward-rectifier potassium (K + ) channels, herein, the charged peptide assembly has been introduced into a nano-confined space for the modulation of ion current rectification (ICR). Peptide-responsive reaction-triggered sequence changes can contribute to polarity conversion of the surface charge; therefore, ICR reversal (ICRR) is generated. Compared with other responsive elements, natural charged peptides show the merit of controllable charge polarity. By electrochemically monitoring the ICRR as an output signal, one can utilize the peptide assembly-mediated ICRR to construct an ionic sensory platform. In addition, a logic gate has been established to demonstrate the availability of an ionic sensory platform for inhibitor screening. As peptide nanoassemblies may also have various structures and functions due to their diverse properties, the ionic modulation system can provide alternatives for the assay of peptide-associated biotargets with biomedical applications.

• Single conical nanopores are fabricated in polymer membrane by track-etching technique. • Nanopore surface is modified by electrostatic immobilization of cationic polyelectrolytes. • Transport properties of positively charged conical nanopores are studied. • Concentration-dependent effect of various anions on nanopore transport is investigated. • The anions interaction with the positive charges leads to changes in the nanopore electrical readout. Here we present screening effect of various anions on nanopore fixed surface charges. The anions could not induce significant change in ion transport through negatively charged nanopore. For positively charged nanopore, the order of monovalent anions effect is: H 2 PO 4 − > HCO 3 – > NO 3 – > BrO 3 − ~ Cl−. The divalent HPO 4 2− exhibits significant changes in the rectified ion flux, while SO 4 2− could not induce any noticeable change even at higher concentrations. Our experimental results demonstrated the ability of various anions to interact with fixed surface charges and their effect on nanopore transport characteristics. [ABSTRACT FROM AUTHOR]

• Nanochannels (conical and cylindrical) are fabricated via track-etch technique in polymer membrane. • Immobilization of various cationic dendrimer generation on channels surface is achieved by carbodiimide coupling chemistry. • Effect of surface charge density on the ion transport through single conical nanochannels is evaluated through I−V measurements. • Dendrimer effect on mass transport is studied in multi-channel membranes through permeation experiments. The effect of polyamidoamine (PAMAM) dendrimers (generations G0–G3) on the ion transport properties of nanochannels (conical and cylindrical) is studied either by surface functionalization or by addition to the electrolyte solution. Surface functionalization with cationic dendrimers lead to inversion of the ion current rectification, indicating the anion selectivity of the modified nanochannels. This anion selectivity increases by immobilizing higher-generation dendrimers, as expected for an increase in the surface density of amino groups. However, compared to PAMAM G2, functionalization with PAMAM G3 results in higher cation and lower anion fluxes. Diffusion experiments of charged analytes across nanochannel membranes confirm the effects of the dendrimers. Our experimental and theoretical results show that the observed effects are independent of the nanochannel geometry, but primarily relate to the average charge density and distribution of fixed charges on the channel surface. Image, graphical abstract [ABSTRACT FROM AUTHOR]

In this paper, an ultrasensitive nanochannel sensor has been proposed for label-free Ochratoxin A (OTA) assay in combination with graphene oxide (GO) and catalyzed hairpin assembly (CHA). The high-performance sensor is segmented into two parts. One is composed of graphene oxide (GO) and DNA probes. In the presence of target OTA, OTA works as a catalyst to trigger the self-assembly pathway of the two probes and initiate the cycling of CHA circuits, which results in numerous double-stranded DNAs (dsDNA) in solution. The excess ssDNA probes are removed by GO. The other part is composed of biomimetic nanochannel coated with polyethyleneimine (PEI) and Zr4+, which can quantify the concentration of OTA by detecting the dsDNA in solution. The nanofluidic device has a detection limit of as low as 6.2 pM with an excellent selectivity. The nanochannel based assay was used to analyse food samples (red wine) with satisfied results. Thus, the proposed analytical method will provide a new approach the detection of OTA and can be applied for quality control to ensure food safety. Image 1 • A label-free and novel nanochannel sensor for OTA detection is proposed. • The sensor is constructed in combination with GO and nanochannel. • The approach evaded direct immobilization of DNA probes onto nanochannel surface. • CHA is introduced in nanochannel platform to amplify the detection signal. [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 × 10 6   C / m 3 ), 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. [ABSTRACT FROM AUTHOR]

The nanopore-based technique is a useful tool for single-molecule sensing and characterization. In this work, we have developed a new DNA-functionalized gold-modified nanopore, and analytes can induce the conformational switch of i-motif DNA formed on the inner surface of the nanopore. i-Motif DNA structure can be formed in the presence of silver ions (Ag + ), which will result in the change in surface charge and structure of the nanopore tip and ion current rectification (ICR) ratio. The i-motif DNA structure on nanopore surface will be destroyed after the addition of glutathione (GSH) due to the strong interaction of Ag-S bond, which results in the recovery of surface charge, steric hindrance, and ICR ratio. This analyte-triggered conformational switch of i-motif DNA can help us deeply understand the DNA technology inside single nanopore and will benefit the possible applications in an ultrasensitive detection and biological/chemical analysis.

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., (© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Mesoscale ionic diodes, which can rectify ionic current at conditions at which their pore size is larger than 100 nm and thus over 100 times larger than the Debye length, have been recently discovered with potential applications in ionic circuits as well as osmotic power generation. Compared with the conventional nanoscale ionic diodes, the mesoscale ionic diodes can offer much higher conductance, ionic current resolution, and power generated. However, the thermal response, which has been proven playing a crucial role in nanofluidic devices, of the mesoscale ionic diode remains significantly unexplored. Here, we report the thermal dependence of the mesoscale ionic diode comprising a conical pore with a tip opening diameter of ∼400 nm. To capture its underlying physics more accurately, our model takes into account the practical equilibrium chemistry reaction of functional carboxyl groups on the pore surface. Modeling results predict that in the mesoscale ionic diode prepared currents increase but the performance decreases with the increase of temperature, which is consistent with our experimental data and indicates that the ion transport properties apparently depend on the presence of highly mobile hydroxide ions. The results gathered can provide important guidance for the design of new mesoscale ionic diodes, enriching their applications in thermoelectric power and thermoresponsive chemical sensors.

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.

In vivo electrochemical sensing based on implantable microelectrodes is a strong driving force of analytical neurochemistry in brain. The complex and dynamic neurochemical network sets stringent standards of in vivo electrochemical sensors including high spatiotemporal resolution, selectivity, sensitivity, and minimized disturbance on brain function. Although advanced materials and novel technologies have promoted the development of in vivo electrochemical sensors drastically, gaps with the goals still exist. This Review mainly focuses on recent attempts on the key issues of in vivo electrochemical sensors including selectivity, tissue response and sensing reliability, and compatibility with electrophysiological techniques. In vivo electrochemical methods with bare carbon fiber electrodes, of which the selectivity is achieved either with electrochemical techniques such as fast-scan cyclic voltammetry and differential pulse voltammetry or based on the physiological nature will not be reviewed. Following the elaboration of each issue involved in in vivo electrochemical sensors, possible solutions supported by the latest methodological progress will be discussed, aiming to provide inspiring and practical instructions for future research.

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/m 2 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.

Herein, we report the ultrasensitive DNA detection through designing an elegant nanopore biosensor as the first case to realize the reversal of current rectification direction for sensing. Attributed to the unique asymmetric structure, the glass conical nanopore exhibits the sensitive response to the surface charge, which can be facilely monitored by ion current rectification curves. In our design, an enzymatic cleavage reaction was employed to alter the surface charge of the nanopore for DNA sensing. The measured ion current rectification was strongly responsive to DNA concentrations, even reaching to the reversed status from the negative ratio (-6.5) to the positive ratio (+16.1). The detectable concentration for DNA was as low as 0.1 fM. This is an ultrasensitive and label-free DNA sensing approach, based on the rectification direction-reversed amplification in a single glass conical nanopore., (© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A CO-regulated ion nanochannel is demonstrated, which is inspired by the living activity of CO-induced smooth muscle vasodilation. The mechanism relies on the fact that CO has a higher affinity with ferroporphyrin compared to carboxyl groups on the surface of the nanochannels. The cooperation effect of the released carboxyl groups and the conical asymmetric shape leads the ion transportation to be diode-like., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Nanopores fabricated from glass microcapillaries are used in applications ranging from scanning ion conductance microscopy to single-molecule detection. Still, evaluating the nanocapillary tip by a noninvasive means remains challenging. For instance, electron microscopy characterization techniques can charge, heat, and contaminate the glass surface and typically require conductive coatings that influence the final tip geometry. Per contra, electrical characterization by the means of ion current through the capillary lumen provides only indirect geometrical details of the tips. Here, we show that helium scanning transmission ion microscopy provides a nondestructive and precise determination of glass nanocapillary tip geometries. This enables the reproducible fabrication of axially asymmetric blunt, bullet, and hourglass-shaped tips with opening diameters from 20 to 400 nm by laser-assisted pulling. Accordingly, this allows for an evaluation of how tip shape, pore diameter, and opening angle impact ionic current rectification behavior and the translocation of single molecules. Our analysis shows that current drops and translocation dwell times are dominated by the pore diameter and opening angles regardless of nanocapillary tip shape.

pH-activated gates intelligently govern the ion transport behaviors of a wide range of bioinspired ion channels, but the mechanisms between the gate locations and the functionalities of the ion channels remain poorly understood. Here, we construct an artificial gate-location-tunable single-nanochannel system to systematically investigate the impact of the gate location on the ion transport property of the biomimetic ion channel. The gate-location-controllable single nanochannels are prepared by asymmetrically grafting pH-responsive polymer gates on one side of single nanochannels with gradual shape transformation. Experimental ion current measurements show that the gating abilities and rectification effects of the pH-gated nanochannels can be gradually altered by precisely locating the artificial pH gates on the different sites of the channels. The experimental gate-location-dependent gating and rectification of ion current in the bioinspired ion channel system is further well confirmed by theoretical simulation. This work, as an example, provides a new avenue to optimize the smart ion transport features of diverse artificial nanogate devices via precisely locating the gates on the appropriate sites of the artificial nanochannels.

Tuning of ion and nanoparticle transport is validated through applying a salt gradient in two types of nanopores: the inner wall of a nanopore has bipolar charges and its outer wall neutral (type I), and both the inner and outer walls of a nanopore have bipolar charges (type II). The ion current rectification (ICR) behavior of these nanopores can be regulated by an applied salt gradient: if it is small, the degree of ICR in type II nanopore is more significant than that in type I nanopore; a reversed trend is observed at a sufficiently large salt gradient. If the applied salt gradient and electric field have the same direction, type I nanopore exhibits two significant features that are not observed in type II nanopore: (i) a cation-rich concentration polarization field and an enhanced funneling electric field are present near the cathode side of the nanopore, and (ii) the magnitude of the axial electric field inside the nanopore is reduced. These features imply that applying a salt gradient to type I nanopore is capable of simultaneously enhancing the nanoparticle capture into the nanopore and reducing its translocation velocity inside, so that high sensing performance and resolution can be achieved., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)