Conical Nanopores Research Articles

Performance of Single Nanopore and Multi-Pore Membranes for Blue Energy.

The salinity gradient power extracted from the mixing of electrolyte solutions at different concentrations through selective nanoporous membranes is a promising route to renewable energy. However, several challenges need to be addressed to make this technology profitable, one of the most relevant being the increase of the extractable power per membrane area. Here, the performance of asymmetric conical and bullet-shaped nanopores in a 50 nm thick membrane are studied via electrohydrodynamic simulations, varying the pore radius, curvature, and surface charge. The output power reaches ~60 pW per pore for positively charged membranes (surface charge σw=160 mC/m2) and ~30 pW for negatively charges ones, σw=-160 mC/m2 and it is robust to minor variations of nanopore shape and radius. A theoretical argument that takes into account the interaction among neighbour pores allows to extrapolate the single-pore performance to multi-pore membranes showing that power densities from tens to hundreds of W/m2 can be reached by proper tuning of the nanopore number density and the boundary layer thickness. Our model for scaling single-pore performance to multi-pore membrane can be applied also to experimental data providing a simple tool to effectively compare different nanopore membranes in blue energy applications.

Nanofluidic ion-exchange membranes: Can their conductance compete with polymeric ion-exchange membranes?

Nanofluidic membranes (NFMs) are gaining prominence as alternative ion-exchange membranes, because of their distinct selectivity mechanism, which does not rely on functional groups on a polymeric backbone but rather on charged nanopores that allow straight ion-conductive pathways for efficient ion transport. We measured the conductivity of commercial anodized aluminum oxide membranes with different pore sizes under different current densities and electrolyte concentrations. We also simulated a nanopore channel with charged walls between two electrolyte reservoirs. Our findings indicate that electrolyte concentration is the main parameter that determines NFM conductivity, with a linear dependence at least up to 1 M. Our study shows that the optimal pore length is between 0.5 and 5 μm considering the trade-off between selectivity and conductance. On the other hand, the conductance is not sensitive to the pore diameter. Conical nanopores are a way to increase conductance, but according to our results, this increase comes at the expense of selectivity. Our findings suggest that NFMs can outperform polymeric ion-exchange membranes in certain electrochemical applications, such as reverse electrodialysis, but not in applications that use low electrolyte concentrations on both sides of the membrane.

Numerical Modelling of Nanopores

Nanopores are used for resistive pulse sensing of single analytes and as nanopipette probes for electrochemical scanned probe microscopy. The current-voltage responses of nanopores display a rich range of behaviors, which arise due to interactions between the electric fields (applied potential and surface charges), ion concentrations, and fluid flows.1 To understand these behaviors and interpret experimental results, numerical simulation of the coupled physics are typically performed.2 Yet despite simulations of conical nanopores being commonplace, they remain challenging. As the electric double layer requires resolution of concentrations and electric fields at sub-nm length scales, while concentration enhancements and depletions occur 10s of μm inside the pore simulations must resolve differences over length scales that vary by ~5 orders of magnitude. While various strategies have been employed to make these simulations more manageable including simulating a small portion of the pore, ignoring surface charge beyond a certain distance [others]. However, no consensus exists as to best practices or the impact of these simplifications.In this work we assess the impact of commonly made simplifications and describe best practices for efficient modelling of conical nanopores to ensure accurate results are readily obtained. Suggestions include effective choices of mesh, initial conditions, and the handling of semi-infinite boundaries.(1) Lan, W.-J.; Holden, D. A.; White, H. S. Pressure-Dependent Ion Current Rectification in Conical-Shaped Glass Nanopores. J. Am. Chem. Soc. 2011, 133 (34), 13300–13303. https://doi.org/10.1021/ja205773a.(2) White, H. S.; Bund, A. Ion Current Rectification at Nanopores in Glass Membranes. Langmuir 2008, 24 (5), 2212–2218. https://doi.org/10.1021/la702955k.

Ion current rectification coupled with asymmetric temperature and surface charge in ultrashort conical nanopores

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.

Ion conduction property of track- etched conical nanopores modified by atom layer deposition method under applied pressure

In this work, single conical nanopores in Kapton films were fabricated using the etch-track and atomic layer deposition (ALD) methods. Nanopores with different tip sizes (34 nm, 123 nm, 194 nm, respectively) were used to investigate the effect of pressure on the ionic conduction rectification (ICR) of the nanopores. Results showed that the effect of pressure on the ICR of the nanopores is related to the tip size of the nanopore and the rectification coefficient of the nanopores in the electrolyte solution. When pressure was applied across the nanopore, a volumetric flow Q was engendered through the conical-shaped nanopore. Only when the tip size of the nanopore reached a certain size could the pressure-driven liquid flow eliminate the hydraulic resistance of the pore, changing the ion distribution in the nanopore and further altering the ICR of the nanopore. For nanopores with a tip size of 34 nm, the effect of pressure on ICR was almost negligible. Only when the tip size of the nanopore was greater than a certain extent did the applied pressure significantly affect the ICR. The influence of pressure on ICR was also related to the strength of the rectification coefficient of the nanopore in the electrolyte. Nanopores exhibiting smaller ion rectification coefficients were more susceptible to the effects of pressure on their ICR.

Controlling Electroosmosis in Nanopores Without Altering the Nanopore Sensing Region.

Nanopores are powerful tools for single-molecule sensing of biomolecules and nanoparticles. The signal coming from the molecule to be analyzed strongly depends on its interaction with the narrower sectionof the nanopore (constriction) that may be tailored to increase sensing accuracy. Modifications of nanopore constriction have also been commonly used to induce electroosmosis, that favors the capture of molecules in the nanopore under a voltage bias and independently of their charge. However, engineering nanopores for increasing both electroosmosis and sensing accuracy is challenging. Here it is shown that large electroosmotic flows can be achieved without altering the nanopore constriction. Using continuum electrohydrodynamic simulations, it is found that an external charged ring generates strong electroosmosis in cylindrical nanopores. Similarly, for conical nanopores it is shown that moving charges away from the cone tip still results in an electroosmotic flow (EOF), whose intensity reduces increasing the diameter of the nanopore sectionwhere charges are placed. This paradigm is applied to engineered biological nanopores showing, via atomistic simulations and experiments, that mutations outside the constriction induce a relatively intense electroosmosis. This strategy provides much more flexibility in nanopore design since electroosmosis can be controlled independently from the constriction, which can be optimized to improve sensingaccuracy.

Dynamic Response of Ionic Current in Conical Nanopores.

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.

Memristive switching of nanofluidic diodes by ionic concentration gradients

We demonstrate that nanofluidic diodes exhibit a memristive behavior that can be controlled not only by the amplitude and frequency of the driving oscillatory signal but also by changing the relative orientations of the externally imposed ionic concentration difference with respect to the pore charge density gradient. Because ionic concentration differences, memristive properties, and nanofluidic pores are of broad interest to electrochemical technology and membrane bioelectrochemistry, this study can be a useful contribution to the field. In our case, the memristor is a polymer membrane with current rectifying conical nanopores. The electrical interaction between the mobile ions in the aqueous solution and the pore surface charges leads to rich physico-chemical phenomena, including memory effects and the neuromorphic-like potentiation of the membrane conductance following voltage pulses (spikes). The multipore membrane can be used in the design of electrochemical circuits for signal conversion and information processing in iontronic hybrid devices.

Nanopore single molecule ATP detection based on the dual effects of specific capturing and signal amplification

Adenosine triphosphate (ATP) plays a crucial role in energy metabolism. Some serious diseases such as Alzheimer’s disease are related to abnormal ATP concentrations. Fast, sensitive, and specific ATP detection is important and challenging in disease diagnosis and biomedical research. In present study, conical nanopore was designed and employed to detect ATP at single molecule level. For overcoming the inherent defects such as non-selectivity and low sensitivity of nanopore in sensing small target molecule, two collaborative improvements of specific identification and signal amplification were proposed. We first used split aptamer capture probes (H1 and S1) to anchor ATP molecule, and then used auxiliary probes (H2 and H3) to form a long-nicked duplex ATP-DNA concatemer by hybridization chain reaction (HCR). Referring ATP-DNA concatemer rather than native ATP to nanopore sensor, characteristic current signals with high signal-to-noise ratio were obtained. By such indirect way, specific and sensitive ATP detection was realized with a linear range from 20 pM to 5 nM and a detection limit of 3.9 pM (S/N = 3). Additionally, the sensor exhibited many merits such as simplify, anti-interference, and especially allowing ATP detection in real sample. So, we believe that the nanopore sensor has good application prospects in screening and management of ATP-related diseases.

Negative Differential Resistance in Conical Nanopore Iontronic Memristors.

Emerging ion transport dynamics with memory effects at nanoscale solution-substrate interfaces offers a unique opportunity to overcome the bottlenecks in traditional computational architectures, trade-offs in selectivity and throughput in separation, and electrochemical energy conversions. Negative differential resistance (NDR), a decrease in conductance with increasing potential, constitutes a new function from the perspective of time-dependent instead of steady-state nanoscale electrokinetic ion transport but remains unexplored in ionotronics to develop higher-order complexity and advanced capabilities. Herein, NDR is introduced in hysteretic and rectified ion transport through single conical nanopipettes (NPs) as ionic memristors. Deterministic and chaotic behaviors are controlled via an electric field as the sole stimulus. The NDR arises fundamentally from the availability and redistribution of the ionic charges during the hysteretic and rectified transport at asymmetric nanointerfaces. The elucidated mechanism is generalizable, and the drastically simplified operations enable tunable state-switching dynamics with higher-order complexity besides the first-order synaptic functions in multiple excitatory and inhibitory states.

Memristive arrangements of nanofluidic pores.

We demonstrate that nanofluidic diodes in multipore membranes show a memristive behavior that can be controlled not only by the amplitude and frequency of the external signal but also by series and parallel arrangements of the membranes. Each memristor consists of a polymeric membrane with conical nanopores that allow current rectification due to the electrical interaction between the ionic solution and the pore surface charges. This surface charge-regulated ionic transport shows a rich nonlinear physics, including memory and inductive effects, which are characterized here by the current-voltage curves and electrical impedance spectroscopy. Also, neuromorphiclike potentiation of the membrane conductance following voltage pulses (spikes) is observed. The multipore membrane with nanofluidic diodes shows physical concepts that should have application for information processing and signal conversion in iontronics hybrid devices.

Modeling of memory effects in nanofluidic diodes

Nanofluidic diodes and ionic solutions find application in electrochemical circuits for information processing, controlled release, and signal conversion in hybrid devices. Here, we describe a physical model that accounts for the memory effects observed in conical nanopores in terms of the driving signal and ionic solution characteristics. The concepts invoked describe the device operation on the basis of the electrical interaction between the pore surface charges and the nanoconfined ionic solution. The physical insights provided can explain the experimental dependence of the nanofluidic tunability on the amplitude and frequency of the driving signal, the ionic concentration, and the solution pH. The model should also be useful for the design of electrochemical circuits based on ionic conduction in asymmetric memristors.

Improved theoretical prediction of nanoparticle sizes with the resistive-pulse technique

With the resistive-pulse technique (RPT), nanopores serve as the nanofluidic sensors of various analytes for their many physical and chemical properties. Here, we focus on the size measurement and its theoretical prediction for sub-200 nm nanoparticles with RPT. Through systematical investigation of the current blockade of nanoparticles across cylindrical nanopores with simulations, Maxwell's method considering the shape coefficient and access resistances agrees well with simulation results. However, the widely used integration method of the resistance has distinct deviations in various cases. With the introduction of a correction factor β to the integration method, our revised equations can provide good predictions for simulation results. β shows a strong dependence on the diameter ratio (d/D) of the nanoparticle and nanopore. Following the same strategy, modified equations are provided for the accurate size prediction for nanoparticles across conical nanopores, where the integration method is the default convenient way. The correction factor β′ relates to β in cylindrical nanopores. β′ exhibits independence on the pore geometry parameters and diameters of nanoparticles, but dependence on the surface charge density of conical nanopores. Our improved equations can provide theoretical predictions for the accurate size detection of 100–200 nm diameter nanoparticles across cylindrical and conical nanopores.

Low-cost and convenient fabrication of polymer micro/nanopores with the needle punching process and their applications in nanofluidic sensing.

Solid-state micro/nanopores play an important role in the sensing field because of their high stability and controllable size. Aiming at problems of complex processes and high costs in pore manufacturing, we propose a convenient and low-cost micro/nanopore fabrication technique based on the needle punching method. The thin film is pierced by controlling the feed of a microscale tungsten needle, and the size variations of the micropore are monitored by the current feedback system. Based on the positive correlation between the micropore size and the current threshold, the size-controllable preparation of micropores is achieved. The preparation of nanopores is realized by the combination of needle punching and chemical etching. First, a conical defect is prepared on the film with the tungsten needle. Then, nanopores are obtained by unilateral chemical etching of the film. Using the prepared conical micropores, resistive-pulse detection of nanoparticles is performed. Significant ionic current rectification is also obtained with our conical nanopores. It is proved that the properties of micro/nanopores prepared by our method are comparable to those prepared by the track-etching method. The simple and controllable fabrication process proposed here will advance the development of low-cost micro/nanopore sensors.

Nanoscale electrohydrodynamic ion transport: Influences of channel geometry and polarization-induced surface charges.

Electrohydrodynamic ion transport has been studied in nanotubes, nanoslits, and nanopores to mimic the advanced functionalities of biological ion channels. However, probing how the intricate interplay between the electrical and mechanical interactions affects ion conduction in asymmetric nanoconduits presents further obstacles. Here, ion transport across a conical nanopore embedded in a polarizable membrane under an electric field and pressure is analyzed by numerically solving a continuum model based on the Poisson, Nernst-Planck, and Navier-Stokes equations. We report an anomalous ionic current depletion, of up to 75%, and an unexpected rise in current rectification when pressure is exerted along the external electric field. Membrane polarization is revealed as the prerequisite to obtain this previously undetected electrohydrodynamic coupling. The electric field induces large surface charges at the pore tip due to its conical shape, creating nonuniform electrical double layers (EDL) with a massive accumulation of electrolyte ions near the orifice. Once applied, the pressure distorts the quasiequilibrium distribution of the EDL ions to influence the nanopore conductivity. Our fundamental approach to inspect the effect of pressure on the channel EDL (and thus ionic conductance) in contrast to its effect on the current arising from the hydrodynamic streaming of ions further explains the pressure-sensitive ion transport in different nanochannels and physical regimes manifested in past experiments, including the hitherto inexplicit mechanism behind the mechanically activated ion transport in carbon nanotubes. This enhances our broad understanding of nanoscale electrohydrodynamic ion transport, yielding a platform to build nanofluidic devices and ionic circuits with more robust and tunable responses to electrical and mechanical stimuli.

Distinct DNA conformations during forward and backward translocations through a conical nanopore.

DNA conformations, which encompass the three-dimensional structures of the DNA strand, play a crucial role in genome regulation. During DNA translocation in a nanopore, various conformations occur due to interactions among force fields, the fluidic environment, and polymer features. The most common conformation is folding, where DNA moves through the nanopore in a two-strand or multi-strand manner, influencing the current signature. Factors such as hydrodynamic drag, ionic environments, and DNA length significantly affect these conformations. Notably, conical nanopores, with their asymmetrical geometry, impose unique constraints on DNA translocation. Our findings reveal that during forward translocation, from the narrow (cis) end to the wide (trans) end, DNA experiences less resistance, resulting in shorter translocation times and higher blockade currents. Conversely, backward translocation, from the wide (trans) end to the narrow (cis) end, leads to longer translocation times and more complex conformations due to increased hydrodynamic drag and geometric constraints. This study employs molecular ping-pong methods to confine DNA, further highlighting the intricate dynamics of DNA folding within nanopores. These insights enhance the understanding of DNA behavior in confined environments, contributing to advancements in nanopore-based sensing and sequencing technologies, with implications for genome regulation and biomedical applications.

A Comparable Study of Single Stranded DNA Sensing Using Track‐Etched Nanopore Sensors

AbstractDetection of single‐stranded DNA (ss‐DNA) has great importance not only for decoding genetic information but understanding its role in DNA replication, recombination, and repair. Recently, its detection in body fluids and the development of its library brought new perspectives to diagnose cancer. Track‐etched nanopores are synthetic pores that can be used in several applications to sense and distinguish molecules. Biomolecules such as DNA have special importance due to their usage and applications. Even if most of the studies focus on double‐stranded DNA (ds‐DNA), nanopore sensing and discriminating single‐stranded DNA (ss‐DNA) is still of great interest. However, sensing ss‐DNA is relatively more challenging than ds‐DNA since it is more prone to ionic interactions which changes its translocation dynamics. In this study, single‐tracked poly(ethylene terephthalate) (PET) membranes were used to generate conical nanopores. Due to its chemical stability, ease of fabrication at various geometries, and tunable pore size with respect to other polymer nanopores, PET nanopores were preferred. By chemical etching, single nanopores were fabricated, surface modified, and used for ss‐ and ds‐DNA sensing. We presented our results in a comparative way relative to ds‐DNA. We observed shorter current‐pulse amplitude (for ss‐DNA) with similar duration with respect to ds‐DNA. Such behavior may indicate the conformational changes of single and double‐strand forms during translocation. We also report the effect of electrolyte concentration on the frequency of ss‐DNA translocation. Our results demonstrated the feasibility and performance of track‐etched PET nanopore for ss‐DNA sensing. It provided insight into the electrophoretic transport dynamics of ss‐DNA in solution such as linear current‐pulse frequency against concentration and applied potential. Additionally, the current‐pulse amplitude of ss‐DNA was relatively smaller than ds‐DNA which enabled to differentiate the DNA strands. Moreover, the results showed the capability and potential use of the track‐etched PET nanopore and the limits as a sensor. Finally, the results on electrolyte concentration and its effect on the translocation frequency provided additional insight into the possible non‐uniform DNA‐ion interactions.

The translocation of a polymer through a nanopore with sandglass‐like geometry

AbstractIn recent years, non‐uniformly geometrical nanopores, such as conical nanopores, have gained significant attention in nanopore sensing technology due to their advantages in analyte manipulation and clear output signals. This paper focuses on the polymer translocation through a sandglass‐like nanopore, characterized by a double‐conical geometry with a tip at the middle. We systematically investigate the effects of pore geometry on the translocation dynamics through computational simulations and theoretical analyses. The polymer translocation process is divided into three stages: approaching, threading, escaping, corresponding to the movement from the entry to the tip, through the tip, and from the tip to the exit, respectively. Our findings reveal that the duration of the approaching stage highly depends on the polymer conformations, while the durations of the threading and escaping stages are primarily influenced by the driving force associated with the pore geometry and the accompanying free energy change. Additionally, we report a relationship between the threading speed of the polymer at the tip and the combination of the driving force there and the free energy change during the threading stage.

Rectification behavior of a conical nanopore subject to extra simultaneously applied concentration and thermal gradients

The transport of ions in biological cells can be efficiently regulated by a uniformly distributed salt concentration and temperature in their ion channels. Attempting to understand the interactions between these two factors, we consider the ionic transport of a conical nanopore functionalized with bipolar polyelectrolyte (PE) layers subject to applied electric field, concentration gradient (∇C), and thermal gradient (∇T). The electrokinetic behavior of the system considered is primarily governed by ∇C which contributes to diffusion flow. In contrast, the contribution of ∇T is only auxiliary. Through varying the directions of ∇C and ∇T (four possible combinations), we show that the accumulation and depletion of ions in the nanopore are mainly influenced by the shielding effect of the charged region of PE layers. If a small negative voltage bias is applied, an anion-selective nanopore switches to a cation-selective one, and this phenomenon is pronounced when a higher concentration is placed on the nanopore tip side. However, that phenomenon is temperature independent. When ∇C is applied, the ionic current rectification factor Rf shows a maximum value as a higher concentration is placed on the nanopore tip side, while it monotonically decreases as a higher concentration is placed on the other side. As for applying ∇T, the rectification capability of the nanopore is only slightly affected. The ion selectivity S under opposite polarities of voltage bias shows unique phenomena: If V=+1V, S mainly depends on temperature, which is almost uniform in the nanopore. In contrast, S is dominated by concentration when V=−1V, which arises from the enhancement or offset of diffusion and migration fluxes.

Numerical study of rectified electroosmotic flow in nanofluidics: Influence of surface charge and geometrical asymmetry

The transport of ions in nanofluidic systems, specifically the rectified ion transport or the ionic diode phenomenon occurring in the presence of asymmetrical geometry and/or charge distribution, has drawn considerable attention due to its relevance in energy conversion and biosensing applications. However, previous numerical research has frequently overlooked the concurrent liquid flow within these systems, even though multiple experimental studies have highlighted intriguing flow patterns in ionic diode configurations. In the present study, we employ comprehensive numerical simulations to probe the influence of geometrical or charge asymmetry in a nanofluidic system on electroosmotic flow and ion transport. These simulations employ the Poisson–Nernst–Planck equation in conjunction with the Navier–Stokes equation. Our findings reveal that even when the current rectification trend is consistent between conical and straight nanopores, charge asymmetry and geometric asymmetry can generate significant variations in the rectification effects of electroosmotic flow. Furthermore, our research indicates that the direction of ion rectification and flow rectification can be independently manipulated by utilizing charge asymmetry in conjunction with geometric asymmetry, thereby facilitating advanced control of ions and flows within nanofluidic systems. Collectively, our findings contribute to a more profound understanding of the mechanisms underlying osmotic flow rectification and propose a novel approach for developing efficient ion and flow rectification systems.