Li-Hsien Yeh

Ion Transport in a pH-Regulated Nanopore

Fundamental understanding of ion transport phenomena in nanopores is crucial for designing the next-generation nanofluidic devices. Due to surface reactions of dissociable functional groups on the nanopore wall, the surface charge density highly depends upon the proton concentration on the nanopore wall, which in turn affects the electrokinetic transport of ions, fluid, and particles within the nanopore. Electrokinetic ion transport in a pH-regulated nanopore, taking into account both multiple ionic species and charge regulation on the nanopore wall, is theoretically investigated for the first time. The model is verified by the experimental data of nanopore conductance available in the literature. The results demonstrate that the spatial distribution of the surface charge density at the nanopore wall and the resulting ion transport phenomena, such as ion concentration polarization (ICP), ion selectivity, and conductance, are significantly affected by the background solution properties, such as the pH and salt concentration.

Regulating DNA translocation through functionalized soft nanopores

Nanopores have emerged as promising next-generation devices for DNA sequencing technology. The two major challenges in such devices are: (i) find an efficient way to raise the DNA capture rate prior to funnelling a nanopore, and (ii) reduce the translocation velocity inside it so that single base resolution can be attained efficiently. To achieve these, a novel soft nanopore comprising a solid-state nanopore and a functionalized soft layer is proposed to regulate the DNA electrokinetic translocation. We show that, in addition to the presence of an electroosmotic flow (EOF), which reduces the DNA translocation velocity, counterion concentration polarization (CP) occurs near the entrance of the nanopore. The latter establishes an enrichment of the counterion concentration field, thereby electrostatically enhancing the capture rate. The dependence of the ionic current on the bulk salt concentration, the soft layer properties, and the length of the nanopore are investigated. We show that if the salt concentration is low, the ionic current depends largely upon the length of the nanopore, and the density of the fixed charge of the soft layer, but not upon its degree of softness. On the other hand, if it is high, ionic current blockade always occurs, regardless of the levels of the other parameters. The proposed soft nanopore is capable of enhancing the performance of DNA translocation while maintaining its basic signature of the ionic current at high salt concentration. The results gathered provide the necessary information for designing devices used in DNA sequencing.

Electrokinetic ion and fluid transport in nanopores functionalized by polyelectrolyte brushes

Chemically functionalized nanopores in solid-state membranes have recently emerged as versatile tools for regulating ion transport and sensing single biomolecules. This study theoretically investigated the importance of the bulk salt concentration, the geometries of the nanopore, and both the thickness and the grafting density of the polyelectrolyte (PE) brushes on the electrokinetic ion and fluid transport in two types of PE brush functionalized nanopore: PE brushes are end-grafted to the entire membrane surface (system I), and to its inner surface only (nanopore wall) (system II). Due to a more significant ion concentration polarization (CP), the enhanced local electric field inside the nanopore, the conductance, and the electroosmotic flow (EOF) velocity in system II are remarkably smaller than those in system I. In addition to a significantly enhanced EOF inside the nanopore, the direction of the flow field near both nanopore openings in system I is opposite to that of EOF inside the nanopore. This feature can be applied to regulate the electrokinetic translocation of biomolecules through a nanopore in the nanopore-based DNA sequencing platform.

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.

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.

Regulating Current Rectification and Nanoparticle Transport Through a Salt Gradient in Bipolar Nanopores

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.

Importance of temperature effect on the electrophoretic behavior of charge-regulated particles.

The Joule heating effect is inevitable in electrophoresis operations. To assess its influence on the performance of electrophoresis, we consider the case of a charge-regulated particle in a solution containing multiple ionic species at temperatures ranging from 298 to 308 K. Using an aqueous SiO(2) dispersion as an example, we show that an increase in the temperature leads to a decrease in both the dielectric constant and the viscosity of the liquid phase, and an increase in both the diffusivity of ions and the particle surface potential. For a particle having a constant surface potential, its electrophoretic mobility is most influenced by the variation in the liquid viscosity as the temperature varies, but for a charged-regulated particle both the liquid viscosity and the surface potential can play an important role. Depending upon the level of pH, the degree of increase in the mobility can be on the order of 40% for a 5 K increase in the temperature. The presence of double-layer polarization, which is significant when the surface potential is sufficiently high, has the effect of inhibiting that increase in the mobility. This implies that the influence of the temperature on the mobility of the particle is most significant when the pH is close to the point of zero charge.

Influence of boundary on the effect of double-layer polarization and the electrophoretic behavior of soft biocolloids

The electrophoresis of a soft particle comprising a rigid core and a charged porous membrane layer in a narrow space is modeled. This simulates, for example, the capillary electrophoresis of biocolloids such as cells and microorganisms, and biosensor types of device. We show that, in addition to the boundary effect, the effects of double-layer polarization (DLP) and the electroosmotic retardation flow can be significant, yielding interesting electrophoretic behaviors. For example, if the friction coefficient of the membrane layer and/or the boundary is large, then the DLP effect can be offset by the electroosmotic retardation flow, making the particle mobility to decrease with increasing double layer thickness, which is qualitatively consistent with many experimental observations in the literature, but has not been explained clearly in previous analyses. In addition, depending upon the thickness of double layer, the friction of the membrane layer of a particle can either retard or accelerate its movement, an interesting result which has not been reported previously. This work is the first attempt to show solid evidence for the influence of a boundary on the effect of DLP and the electrophoretic behavior of soft particles. The model proposed is verified by the experimental data in the literature. The results of numerical simulation provide valuable information for the design of bio-analytical apparatus such as nanopore-based sensing applications and for the interpretation of relevant experimental data.

Electrophoresis of a Charge-Regulated Soft Sphere in a Charged Cylindrical Pore

The boundary effect on the electrophoresis of a soft spherical particle, which comprises a rigid core and a porous layer, along the axis of a cylindrical pore is analyzed under the conditions of low surface potential and weak applied electric field. The porous layer of the particle is of charge-regulated nature where the distribution of fixed charge depends on the degree of dissociation/association reaction of the ionizable function groups contained in the porous layer. The pore might be charged, implying that the effect of electroosmotic flow can play a role. The influences of the nature of the porous layer of a particle, the thickness of the double layer, the pH of the bulk liquid, and the relative size of a pore on the electrophoretic behavior of the particle are investigated. Several unexpected and interesting results are obtained. For instance, if the pH value of the bulk liquid or the thickness of the membrane layer takes a medium large value, the electrophoretic mobility of the particle has a local maximum as the thickness of the double layer varies.

Field effect regulation of Donnan potential and electrokinetic flow in a functionalized soft nanochannel

Controlling charge properties and electrokinetic flow (EKF) in a nanochannel is essential for the development of next generation nanofluidics based biosensing devices. In this study, active control of the Donnan potential and EKF in a functionalized soft nanochannel, comprising a solid-state nanochannel functionalized with a polyelectrolyte (PE) brush layer, using a field effect transistor (FET) is theoretically investigated for the first time. Both the magnitude and sign of the Donnan potential in the functionalized soft nanochannel can be effectively regulated by the gate potential imposed on the FET and the salt concentration. In addition, the salt concentration dependence of the EKF velocity profiles inside and outside the PE brush layer of the functionalized soft nanochannel are distinctly different.