Voltage-Induced Modulation of Ionic Concentrations and Ion Current Rectification in Mesopores with Highly Charged Pore Walls

It is believed that ion current rectification (ICR), a property that assures preferential ionic transport in one direction, can only be observed in nanopores when the pore size is comparable to the thickness of the electric double layer (EDL). Rectifying nanopores became the basis of biological sensors and components of ionic circuits. Here we report that appreciable ICR can also occur in highly charged conical, polymer mesopores whose tip diameters are as large as 400 nm, thus over 100-fold larger than the EDL thickness. A rigorous model taking into account the surface equilibrium reaction of functional carboxyl groups on the pore wall and electroosmotic flow is employed to explain that unexpected phenomenon. Results show that the pore rectification results from the high density of surface charges as well as the presence of highly mobile hydroxide ions, whose concentration is enhanced for one voltage polarity. This work provides evidence that highly charged surfaces can extend the ICR of pores to the submicron scale, suggesting the potential use of highly charged large pores for energy and sensing applications. Our results also provide insight into how a mixture of ions with different mobilities can influence current–voltage curves and rectification

Importance of Ionic Polarization Effect on the Electrophoretic Behavior of Polyelectrolyte Nanoparticles in Aqueous Electrolyte Solutions

The electrophoresis of a polyelectrolyte, an entirely porous, charged nanoparticle, in various types of aqueous electrolyte solution is modeled taking account of the presence of multiple ionic species, and its applicability is verified by the experimental data of succinoglycan in the literature. We show that, in addition to the electroosmotic flow around a polyelectrolyte, two types of competing polarization effect are also significant: counterion polarization and co-ion polarization, both of them depend largely on the thickness of the double layer. The presence of these two polarization effects yields profound and interesting electrophoretic behaviors that are distinct to polyelectrolytes. The results gathered provide necessary theoretical background for the interpretation of various types of electrophoresis data in practice. Typical examples include that of a nanopore-based sensing device used, for instance, in DNA sequencing

Importance of Electroosmotic Flow and Multiple Ionic Species on the Electrophoresis of a Rigid Sphere in a Charge-Regulated Zwitterionic Cylindrical Pore

The influence of electroosmotic flow (EOF) on the electrophoretic behavior of a particle is investigated by considering a rigid sphere in a charge-regulated, zwitterionic cylindrical pore filled with an aqueous solution containing multiple ionic species. This extends conventional analyses to a more general and realistic case. Taking a pore with p<i>K</i>_a = 7 and p<i>K</i>_b = 2 (point of zero charge is pH = 2.5) filled with an aqueous NaCl solution as an example, several interesting results are observed. For instance, if pH < 5.5, the particle mobility is influenced mainly by boundary effect, and is influenced by both EOF and boundary effects if pH ≥ 5.5. If pH is sufficiently high, the particle behavior is dominated by EOF, which might alter the direction of electrophoresis. The ratio of (pore radius/particle radius) influences not only the boundary effect, but also the strength of EOF. If the boundary effect is insignificant, the mobility varies roughly linearly with log­(bulk salt concentration). These findings are of practical significance to both the interpretation of experimental data and the design of electrophoresis devices

Tunable Streaming Current in a pH-Regulated Nanochannel by a Field Effect Transistor

Many experimental results demonstrated that ion transport phenomena in nanofluidic devices are strongly dependent on the surface charge property of the nanochannel. In this study, active control of the surface charge property and the streaming current, generated by a pressure-driven flow, in a pH-regulated nanochannel using a field effect transistor (FET) are analyzed for the first time. Analytical expressions for the surface charge property and the streaming current/conductance have been derived taking into account multiple ionic species, surface chemistry reactions, and the Stern layer effect. The model is validated by the experimental data of the streaming conductance in the silica nanochannel available in the literature. Results show that the pH-dependent streaming conductance of the gated silica nanochannel is consistent with its modulated zeta potential; however, the salt concentration-dependent streaming conductance might be different from the zeta potential behavior, depending on the solution pH and the gate potential imposed. The performance of the field effect modulation of the zeta potential and the streaming conductance is significant for lower solution pH and salt concentration. The results gathered are informative for the design of the next-generation nanofluidics-based power generation apparatus

Importance of Boundary on the Electrophoresis of a Soft Cylindrical Particle

We modeled the electrophoresis of a soft cylindrical particle comprising a rigid core and a polyelectrolyte layer along the axis of a long, cylindrical pore, and the applicability of the model proposed is verified by the experimental data available in the literature. Previous analysis is extended to the case where the effects of double-layer polarization (DLP) and electroosmotic flow (EOF) can be significant. We show that the interaction between the particle’s double layer and the pore, the competition between the effective charge density and the local electric field strength, and the presence of EOF yield interesting and significant results. For example, if EOF is absent, the particle mobility as the bulk salt concentration varies depends highly on the amount of fixed charge of its polyelectrolyte layer: if that amount is small, the mobility decreases monotonically with increasing bulk salt concentration, and if that amount is large, then the mobility shows a local maximum. At a high bulk salt concentration, the longer the particle the larger is its mobility, that trend is reversed if it is low. That local minimum vanishes when the boundary effect is important. If the pore is positively charged, a positively charged particle can be driven to the direction opposite to that of the applied electric field. These provide necessary information for the design of electrophoresis devices

Electrophoresis of Deformable Polyelectrolytes in a Nanofluidic Channel

The influence of the shape of a polyelectrolyte (PE) on its electrophoretic behavior in a nanofluidic channel is investigated by considering the translocation of a deformable ellipsoidal PE along the axis of a cylindrical nanochannel. A continuum model comprising a Poisson equation for the electric potential, Nernst–Planck equations for the ionic concentrations, and modified Stokes equations for the flow field is adopted. The effects of the PE shape, boundary, bulk ionic concentration, counterion condensation, electroosmotic retardation flow, and electroosmotic flow (EOF) on the PE mobility are discussed. Several interesting behaviors are observed. For example, if the nanochannel is uncharged and the double layer is thick, then the PE mobility increases (decreases) with increasing double-layer thickness for a smaller (larger) boundary, which has not been reported previously. If the nanochannel is negatively charged and the double layer is thick, then a negatively charged PE moves in the direction of the applied electric field. The results gathered provide necessary information for both the interpretation of experimental data and the design of nanochannel-based sensing devices

pH-Regulated Ionic Conductance in a Nanochannel with Overlapped Electric Double Layers

Accurately and rapidly analyzing the ionic current/conductance in a nanochannel, especially under the condition of overlapped electric double layers (EDLs), is of fundamental significance for the design and development of novel nanofluidic devices. To achieve this, an analytical model for the surface charge properties and ionic current/conductance in a pH-regulated nanochannel is developed for the first time. The developed model takes into account the effects of the EDL overlap, electroosmotic flow, Stern layer, multiple ionic species, and the site dissociation/association reactions on the channel walls. In addition to good agreement with the existing experimental data of nanochannel conductance available from the literature, our analytical model is also validated by the full model with the Poisson–Nernst–Planck and Navier–Stokes equations. The EDL overlap effect is significant at small nanochannel height, low salt concentration, and medium low pH. Neglecting the EDL overlap effect could result in a wrong estimation in the zeta potential and conductance of the nanochannel in a single measurement

DNA Electrokinetic Translocation through a Nanopore: Local Permittivity Environment Effect

The effect of the local liquid permittivity surrounding the DNA nanoparticle, referred to as the local permittivity environment (LPE) effect, on its electrokinetic translocation through a nanopore is investigated for the first time using a continuum-based model, composed of the coupled Poisson–Nernst–Planck (PNP) equations for the ionic mass transport and the Stokes and Brinkman equations for the hydrodynamic fields in the region outside of the DNA and within the ion-penetrable layer of the DNA nanoparticle, respectively. The nanoparticle translocation velocity and the resulting current deviation are systematically investigated for both uniform and spatially varying permittivities surrounding the DNA nanoparticle under various conditions. The LPE effect in general reduces the particle translocation velocity. The LPE effect on the current deviation is insignificant when the imposed electric field is relatively high. However, when the electric field and the bulk electrolyte concentration are relatively low, both current blockade and enhancement are predicted with the LPE effect incorporated, while only current blockade is predicted with the assumption of constant liquid permittivity. It is thereby shown that regardless of the electric field imposed the predictions on ionic current with considering the LPE effect are in good qualitative agreement with the experimental observations obtained in the literature

Controlling pH-Regulated Bionanoparticles Translocation through Nanopores with Polyelectrolyte Brushes

A novel polyelectrolyte (PE)-modified nanopore, comprising a solid-state nanopore functionalized by a nonregulated PE brush layer connecting two large reservoirs, is proposed to regulate the electrokinetic translocation of a soft nanoparticle (NP), comprising a rigid core covered by a pH-regulated, zwitterionic, soft layer, through it. The type of NP considered mimics bionanoparticles such as proteins and biomolecules. We find that a significant enrichment of H⁺ occurs near the inlet of a charged solid-state nanopore, appreciably reducing the charge density of the NP as it approaches there, thereby lowering the NP translocation velocity and making it harder to thread the nanopore. This difficulty can be resolved by the proposed PE-modified nanopore, which raises effectively both the capture rate and the capture velocity of the soft NP and simultaneously reduces its translocation velocity through the nanopore so that both the sensing efficiency and the resolution are enhanced. The results gathered provide a conceptual framework for the interpretation of relevant experimental data and for the design of nanopore-based devices used in single biomolecules sensing and DNA sequencing

Highly Charged Particles Cause a Larger Current Blockage in Micropores Compared to Neutral Particles

Single pores in the resistive-pulse technique are used as an analytics tool to detect, size, and characterize physical as well as chemical properties of individual objects such as molecules and particles. Each object passing through a pore causes a transient change of the transmembrane current called a resistive pulse. In high salt concentrations when the pore diameter is significantly larger than the screening Debye length, it is assumed that the particle size and surface charge can be determined independently from the same experiment. In this article we challenge this assumption and show that highly charged hard spheres can cause a significant increase of the resistive-pulse amplitude compared to neutral particles of a similar diameter. As a result, resistive pulses overestimate the size of charged particles by even 20%. The observation is explained by the effect of concentration polarization created across particles in a pore, revealed by numerical modeling of ionic concentrations, ion current, and local electric fields. It is notable that in resistive-pulse experiments with cylindrical pores, concentration polarization was previously shown to influence ionic concentrations only at pore entrances; consequently, additional and transient modulation of resistive pulses was observed when a particle entered or left the pore. Here we postulate that concentration polarization can occur across transported particles at any particle position along the pore axis and affect the magnitude of the entire resistive pulse. Consequently, the recorded resistive pulses of highly charged particles reflect not only the particles’ volume but also the size of the depletion zone created in front of the moving particle. Moreover, the modeling identified that the effective surface charge density of particles depended not only on the density of functional groups on the particle but also on the capacitance of the Stern layer. The findings are of crucial importance for sizing particles and characterizing their surface charge properties