Conical Nanopores Research Articles

Dynamic rotation featured translocations of human serum albumin with a conical glass nanopore

In this paper, translocation events of single ellipsoid human serum albumin (HSA) with a solid glass nanopore have been investigated in detail by common resistive pulses. In particular, the translocation events can be classified into three main groups according to the significant difference in current pulse amplitudes. By adjusting the electric field of the nanopore upon changing applied conditions in voltage, pH, and pore size, the observation would be originally associated with dynamic rotation rather than increased size exclusion of HSA. Finally, the rotation angles of translocated HSA are determined by both experimental and theoretical studies.

3D flow field measurements outside nanopores.

We demonstrate a non-stereoscopic, video-based particle tracking system with optical tweezers to study fluid flow in 3D in the vicinity of glass nanopores. In particular, we used the quadrant interpolation algorithm to extend our video-based particle tracking to displacements out of the trapping plane of the tweezers. This permitted the study of flow from nanopores oriented at an angle to the trapping plane, enabling the mounting of nanopores on a micromanipulator with which it was then possible to automate the mapping procedure. Mapping of the voltage driven flow in 3D volumes outside nanopores revealed polarity dependent flow fields. This is in agreement with the model of voltage driven flow in conical nanopores depending on the interaction of distinct flows within the nanopore and along the outer walls.

Nanoscale fluid pumping using a symmetric temperature gradient: a molecular dynamics study

ABSTRACT In this study, using the molecular dynamics simulation method, three systems for fluid pumping at the nanoscale have been proposed based on the thermo-osmotic mechanism. These pumps work by applying a symmetric temperature gradient along the wall of a nanopore, which is asymmetric in shape or material. The three systems are a composite nanotube, a conical nanotube, and a composite conical nanopore. The simulation results show that, in all of the proposed systems, the fluid can be pumped continuously by means of heat energy and without using any external force or moving component. The physical mechanisms of the flow in these pumps are clarified using the principles of the thermo-osmotic phenomenon. The simulations show the geometry of the pump and the fluid-solid interaction strength play an important role in determining the pumping strength in all systems. It is shown that a composite conical nanopump compared to other proposed systems has a better performance in fluid pumping.

Asymmetric Electrokinetic Energy Conversion in Slip Conical Nanopores.

Ion current rectification (ICR) phenomena in asymmetric nanofluidic structures, such as conical-shaped nanopores and funnel-shaped nanochannels, have been widely investigated in recent decades. To date, the effect of asymmetric nanofluidic structures on electrokinetic power generation driven by the streaming current/potential has not been explored. Accordingly, this study employed a numerical model based on the Poisson equation, Nernst–Planck equation, and Navier–Stokes equation to investigate the electrokinetic energy conversion (EKEC) in a conical nanopore while considering hydrodynamic slippage. The results indicated that the asymmetric characteristics of streaming current (short-circuit current), streaming potential (open-circuit voltage), maximum power generation, maximum conversion efficiency, and flow rate were observed in conical nanopores under the forward pressure bias (tip-to-base direction) and reverse pressure bias (base-to-tip direction) once the nonequilibrium ion concentration polarization (ICP) became considerable. The rectification behaviors in the streaming current, maximum power, and maximum conversion efficiency were all shown to be opposite to those of the well-known ICR in conical nanopores. In other words, the reverse pressure bias revealed a higher EKEC performance than the forward pressure bias. It was concluded that the asymmetric behavior in EKEC is attributed to the asymmetric electrical resistance resulting from asymmetric ion depletion and ion enrichment. Particularly, it was found that the decrease in electrical resistance (i.e., the change in electrical resistance dominated by the ion enrichment) observed in the reverse pressure bias enhanced the maximum power and maximum conversion efficiency. The asymmetric EKEC characteristics became more significant with increasing slip length, surface charge density, cone angle, and pressure bias, especially at lower salt concentrations. The present findings provide useful information for the future development of EKEC in engineered membranes with asymmetric nanopores.

Estimation of the nano-pores diameter by conductometric measurements

s. Ionic transport properties and diameter of asymmetric conical nano-pores in polyethylene terephthalate membranes were calculated. These nano-pores were synthesized by chemical etching method in irradiated polyethylene terephthalate (PET) films. After etching, the transport characteristics weredocumented under stepping voltage with 2M ionic concentrations of Potassium halides i.e. KCl, KBr and KI. The symmetric bathing condition (same temperature and same pH value solution in both side of chamber) in electrochemical cell were accomplished. It is investigated that diameter of nano-pores is also evaluate by measuring the current rectification properties of track etched nano-pores.

Effect of Electrolyte Concentration and Pore Size on Ion Current Rectification Inversion.

A thorough understanding of nanoscale transport properties is vital for the development and optimization of nanopore sensors. The thickness of the electrical double layers (EDLs) at the internal walls of a nanopore, as well as the dimensions of the nanopore itself, plays a crucial role in determining transport properties. Herein, we demonstrate the effect of the electrolyte concentration, which is inversely proportional to the EDL thickness, and the effect of pore size, which controls the extent of the electrical double layer overlap, on the ion current rectification phenomenon observed for conical nanopores. Experimental and numerical results showed that as the electrolyte concentration is decreased, the rectification ratio reaches a maximum, then decreases, and eventually inverts below unity. We also show that as the pore size is decreased, the rectification maximum and the inversion take place at higher electrolyte concentrations. Numerical investigations revealed that both phenomena occur due to the shifting of ion enrichment distributions within the nanopore as the electrolyte concentration or the pore size is varied.

Ionic Current Rectification: A Result of the Series Connection of Nanochannels with Different Dukhin Numbers.

Originating from the ionic concentration polarization, ionic current rectification (ICR) is closely related to ion selectivity. Since it is the region with the greatest selectivity, the tip of the conical nanopore became the focus of research. However, even if the characteristic of the tip is fixed, the changes of pore length and cone angle still affect the magnitude of ICR─rectification factor (RF). This shows that only focusing on the selectivity of the tip is not comprehensive. Through the simulations based on the one-dimensional Poisson-Nernst-Planck model, it is found that the pore length and cone angle can influence the RF by changing the Dukhin number of pore base (Dubase). Here, Du is a parameter describing the ratio of excess ion concentration and bulk ion concentration. In addition, it is proved that the RF is determined by Dutip (Du of the pore tip) and Dubase together. On the basis of the results, we suggest that a uniformly charged conical nanopore can be equivalent to the series connection of many ultrashort nanochannels with different Du. The differences in Du between adjacent channels lead to unbalanced ion transport, ultimately leading to enrichment or depletion of ion concentration under different polarities. Besides, ICR in bipolar diodes also exhibits Du dependence. We anticipate that this work will provide help to understand the mechanism behind ICR.

Ion transport and current rectification in a charged conical nanopore filled with viscoelastic fluids

The ionic current rectification (ICR) is a non-linear current-voltage response upon switching the polarity of the potential across nanopore which is similar to the I–V response in the semiconductor diode. The ICR phenomenon finds several potential applications in micro/nano-fluidics (e.g., Bio-sensors and Lab-on-Chip applications). From a biological application viewpoint, most biological fluids (e.g., blood, saliva, mucus, etc.) exhibit non-Newtonian visco-elastic behavior; their rheological properties differ from Newtonian fluids. Therefore, the resultant flow-field should show an additional dependence on the rheological material properties of viscoelastic fluids such as fluid relaxation time (lambda ) and fluid extensibility (varepsilon ). Despite numerous potential applications, the comprehensive investigation of the viscoelastic behavior of the fluid on ionic concentration profile and ICR phenomena has not been attempted. ICR phenomena occur when the length scale and Debye layer thickness approaches to the same order. Therefore, this work extensively investigates the effect of visco-elasticity on the flow and ionic mass transfer along with the ICR phenomena in a single conical nanopore. The Poisson–Nernst–Planck (P–N–P) model coupled with momentum equations have been solved for a wide range of conditions such as, Deborah number, 1le De le 100, Debye length parameter, 1le kappa R_t le 50, fluid extensibility parameter, 0.05le varepsilon le 0.25, applied electric potential, -40le V le 40, and surface charge density sigma = -10 and -50. Limited results for Newtonian fluid (De = 0, and varepsilon = 0) have also been shown in order to demonstrate the effectiveness of non-Newtonian fluid behaviour over the Newtonian fluid behaviour. Four distinct novel characteristics of electro-osmotic flow (EOF) in a conical nanopore have been investigated here, namely (1) detailed structure of flow field and velocity distribution in viscoelastic fluids (2) influence of Deborah number and fluid extensibility parameter on ionic current rectification (ICR) (3) volumetric flow rate calculation as a function of Deborah number and fluid extensibility parameter (4) effect of viscoelastic parameters on concentration distribution of ions in the nanopore. At high applied voltage, both the extensibility parameter and Deborah number facilitate the ICR phenomena. In addition, the ICR phenomena are observed to be more pronounced at low values of kappa R_t than the high values of kappa R_t. This effect is due to the overlapping of the electric double layer at low values of kappa R_t.

Electrokinetic behavior of conical nanopores functionalized with two polyelectrolyte layers: effect of pH gradient.

The behavior of ionic current rectification of a conical nanopore functionalized with two polyelectrolyte (PE) layers via layer-by-layer deposition subject to an extra applied pH gradient is investigated theoretically. The applied pH, the electric potential, the half-cone angle of the conical nanopore, and the fixed charge densities of the PE layers are examined in detail for their influence on the ionic current rectification (ICR) behavior of the nanopore. We found that this behavior depends highly on the direction of the pH gradient, which arises because the associated electroosmotic flow plays a significant role. The mechanisms of ionic transport in the present pH asymmetric system are discussed. The results gathered reveal that the ICR behavior of a nanopore can be tuned effectively by applying an extra pH gradient. We also examine the case where two PE layers are uniformly merged into one layer. In this case, both the fixed charge density and the concentration profile are quite different from those when two PE layers are present.

Origin of Ultrahigh Rectification in Polyelectrolyte Bilayers Modified Conical Nanopores.

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

Fluoride-Induced Negative Differential Resistance in Nanopores: Experimental and Theoretical Characterization.

We describe experimentally and theoretically the fluoride-induced negative differential resistance (NDR) phenomena observed in conical nanopores operating in aqueous electrolyte solutions. The threshold voltage switching occurs around 1 V and leads to sharp current drops in the nA range with a peak-to-valley ratio close to 10. The experimental characterization of the NDR effect with single pore and multipore samples concern different pore radii, charge concentrations, scan rates, salt concentrations, solvents, and cations. The experimental fact that the effective radius of the pore tip zone is of the same order of magnitude as the Debye length for the low salt concentrations used here is suggestive of a mixed pore surface and bulk conduction regime. Thus, we propose a two-region conductance model where the mobile cations in the vicinity of the negative pore charges are responsible for the surface conductance, while the bulk solution conductance is assumed for the pore center region.

Conical nanopores highlight the pro-aggregating effects of pyrimethanil fungicide on Aβ(1–42) peptides and dimeric splitting phenomena

The Aβ(1–42) aggregation is a key event in the physiopathology of Alzheimer's disease (AD). Exogenous factors such as environmental pollutants, and more particularly pesticides, can corrupt Aβ(1–42) assembly and could influence the occurrence and pathophysiology of AD. However, pesticide involvement in the early stages of Aβ(1–42) aggregation is still unknown. Here, we employed conical track-etched nanopore in order to analyse the Aβ(1–42) fibril formation in the presence of pyrimethanil, a widely used fungicide belonging to the anilinopyrimidine class. Our results evidenced a pro-aggregating effect of pyrimethanil on Aβ(1–42). Aβ(1–42) assemblies were successfully detected using conical nanopore coated with PEG. Using an analytical model, the large current blockades observed (>0.7) were assigned to species with size close to the sensing pore. The long dwell times (hundreds ms scale) were interpreted by the possible interactions amyloid/PEG using molecular dynamic simulation. Such interaction could leave until splitting phenomena of the dimer structure. Our work also evidences that the pyrimethanil induce an aggregation of Aβ(1–42) mechanism in two steps including the reorganization prior the elongation phase.

Shale gas transport through the inorganic cylindrical and conical nanopores: A density gradient driven molecular dynamics

Pores heterogeneity is an essential part of porous media, however, the effect of the changeable pore is usually overlooked in studies on the shale gas transport in inorganic nanopores. Research currently focuses on inorganic nanoslits that fail to simulate the realistic methane transport through cylindrical and conical inorganic nanopores because variable apertures are not captured in the simulation. However, the impact of such varied apex angles and diameters on methane transport capacity in nanopores has not been studied yet in inorganic shale nanopores. To compensate for the defect, density gradient-driven molecular dynamics (DGD-MD) are used to study the methane transport behavior through the cylindrical and conical inorganic nanopores. The clay material (Ca-montmorillonite) is used to establish the conical nanopores with different apex angles and the cylindrical inorganic shale nanopores with different diameters. Main results show that the permeability of methane increases with enlarged nanopores diameter in cylindrical inorganic nanopores and can be described by the Hagen-Poiseuille (HP) equation with the effective viscosity. However, the flux of methane through cylindrical nanopores is greater than that obtained by using the HP equation under high pressure drop owing to the intensive surface diffusion of gas and positive slippage. Methane is most readily absorbed in the conical nanopores with an apex angle of 9.6°. The flow behavior of methane molecules gradually changes from negative slippage to positive slippage through the conical nanopores and this will result in the overestimation flux obtained by the HP equation. The flow capacity of confined methane predicted by the modified HP equation combining with apex angle is in good agreement with the simulation results. This study provides a molecular-level understanding of the key relationship between variable apex angles and permeability through the cylindrical and conical nanopores, and it can apply to the flow behavior of gas at the nanoscale in biological, chemical, medical, and physical fields.

Detection of Amyloid-β Fibrils Using Track-Etched Nanopores: Effect of Geometry and Crowding.

Several neurodegenerative diseases have been linked to proteins or peptides that are prone to aggregate in different brain regions. Aggregation of amyloid-β (Aβ) peptides is recognized as the main cause of Alzheimer's disease (AD) progression, leading to the formation of toxic Aβ oligomers and amyloid fibrils. The molecular mechanism of Aβ aggregation is complex and still not fully understood. Nanopore technology provides a new way to obtain kinetic and morphological aspects of Aβ aggregation at a single-molecule scale without labeling by detecting the electrochemical signal of the peptides when they pass through the hole. Here, we investigate the influence of nanoscale geometry (conical and bullet-like shape) of a track-etched nanopore pore and the effect of molecular crowding (polyethylene glycol-functionalized pores) on Aβ fibril sensing and analysis. Various Aβ fibril samples that differed by their length were produced by sonication of fibrils obtained in the presence of epigallocatechin gallate. The conical nanopore functionalized with polyethylene glycol (PEG) 5 kDa is suitable for discrimination of the fibril size from relative current blockade. The bullet-like-shaped nanopore enhances the amplitude of the current and increases the dwell time, allowing us to well discern the fibrils. Finally, the nanopore crowded with PEG 20 kDa enhances the relative current blockade and increases the dwell time; however, the discrimination is not improved compared to the "bullet-shaped" nanopore.

Probing Multidimensional Structural Information of Single Molecules Transporting through a Sub-10 nm Conical Plasmonic Nanopore by SERS.

Probing the orientation and oxygenation state of single molecules (SMs) is of great importance for understanding the advanced structure of individual molecules. Here, we manipulate molecules transporting through the hot spot of a sub-10 nm conical gold nanopore and acquire the multidimensional structural information of the SMs by surface enhanced Raman scattering (SERS) detection. The sub-10 nm size and conical shape of the plasmonic nanopore guarantee its high detection sensitivity. SERS spectra show a high correlation with the orientations of small-sized single rhodamine 6G (R6G) during transport. Meanwhile, SERS spectra of a single hemoglobin (Hb) reveal both the vertical/parallel orientations of the porphyrin ring and oxygenated/deoxygenated states of Hb. The present study provides a new strategy for bridging the primary sequence and the advanced structure of SMs.

Graphene Oxide Membrane/Conical Nanoporous Polyimide Composites for Regulating Ion Transport

The ionic rectification effect, an important phenomenon existing in the solid-state nanopore system, has attracted much attention due to its ability to significantly regulate ion transport in the nanopores. However, current research mainly focuses on the single nanopore systems with low ionic currents, and thus further practical applications are limited. In this work, the conical nanoporous polyimide (CNPI) membrane was prepared by ion irradiation and asymmetric chemical etching to increase the ionic current, and the graphene oxide membrane (GOM) was spin-coated on it to improve the corresponding ionic flux and rectification effect after that. It was found that the ionic rectification ratio (0.05 M KCl at 3 V) increased from 17 (CNPI membrane) to 63 (GOM/CNPI composite) with the ionic current in microamperes. The enhanced rectification effect was ascribed to the increased geometry asymmetry and charge asymmetry, which further increased the difference of ionic conductance at opposite bias. Moreover, the corresponding COMSOL simulation revealed that the enhanced ion depletion and enrichment appeared in the nanochannel, especially at the tip side of conical nanopores after the GOM was spin-coated on the CNPI membrane. Finally, it was also confirmed that the GOM/CNPI composite can exhibit a good rectification effect and chemical durability in a wide acidic condition (pH 1 to 6), which extends the practical applications of the conical nanopore in a nanofluidic system.

Ionic conduction through single-pore and multipore polymer membranes in aprotic organic electrolytes

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.

Influence of finite ion size and dielectric decrement on the ion current rectification in a single conical nanopore

The ion current rectification (ICR) arising due to the transport of ionized liquids within a geometrically asymmetric nanopore is of great significance for the development of smart nanogadgets with unique working capabilities. Though the theoretical framework for the ICR is well developed, the influence of the finite size of ions on the ICR phenomena had not been addressed before. The ion steric repulsion due to finite ion size and dielectric decrement of the medium creates a counterion saturation. In this study, a modified electrokinetic model is adopted to describe the ICR phenomena of a single conical nanopore by considering the hydrated ions as finite-sized dielectric charged spheres. The Nernst–Planck equations for ion transport are modified to incorporate the short-range ion steric interactions modeled by the Boublik–Mansoori–Carnahan–Starling–Leland equation as well as Born force and dielectrophoretic force acting on the hydrated ions engender by the ion–solvent interactions. The counterion saturation attenuates the shielding effect of the surface charge of the nanopore leading to a larger ζ-potential and hence, a larger volume flux and reduced conduction. We find that the ion steric interactions and the dielectric decrement significantly influence the ICR phenomena as well as the ion selectivity of a conical nanopore, especially for moderate to high range of surface charge density, bulk concentration, and applied bias. We find that ICR varies linearly with temperature; however, the variation is found to be marginal. Our results show that the volume flux and the rectification factor of the conical nanopore can be suitably tuned by adding salt of larger counterion size or multivalent ions.

Negative differential resistance and threshold-switching in conical nanopores with KF solutions

Negative differential resistance (NDR) phenomena are under-explored in nanostructures operating in the liquid state. We characterize experimentally the NDR and threshold switching phenomena observed when conical nanopores are immersed in two identical KF solutions at low concentration. Sharp current drops in the nA range are obtained for applied voltages exceeding thresholds close to 1 V and a wide frequency window, which suggests that the threshold switching can be used to amplify small electrical perturbations because a small change in voltage typically results in a large change in current. While we have not given a detailed physical mechanism here, a phenomenological model is also included.

Higher Ion Selectivity with Lower Energy Usage Promoted by Electro-osmotic Flow in the Transport through Conical Nanopores

The tradeoff between selectivity and throughput presents fundamental challenges to improve desalination and charge storage, salinity gradient-based energy harvesting, memory device/circuit developm...