Showing papers by "Jyh-Ping Hsu published in 2012"

Field effect control of surface charge property and electroosmotic flow in nanofluidics

Abstract: The surface charge property of nanofluidic devices plays an essential role in electrokinetic transport of ions, fluids, and particles in them. The nanofluidic field effect transistor (FET), referring to a nanochannel embedded with an electrically controllable gate electrode, provides a simple way to rapidly regulate its surface charge property, which in turn controls the electrokinetic transport phenomena within the nanochannel. In this study, approximate analytical expressions are derived for the first time to estimate the surface-charge property and electroosmotic flow (EOF) in charge-regulated nanochannels tuned by the nanofluidic FET and are validated by comparing their predictions to the existing experimental data available from the literature. The control of the surface charge property as well as the EOF by the nanofluidic FET depends on the pH and ionic concentration of the aqueous solution.

Ion Concentration Polarization in Polyelectrolyte-Modified Nanopores

Abstract: Nanopores functionalized with synthetic or biological polyelectrolyte (PE) brushes have significant potentials to rectify ionic current and probe single biomacromolecules. In this work, electric-field-induced ion transport and the resulting conductance in a PE-modified nanopore are theoretically studied using a continuum-based model, composed of coupled Poisson–Nernst–Planck (PNP) equations for the ionic mass transport, and Stokes and Brinkman equations for the hydrodynamic fields in the exterior and interior of the PE layer, respectively. Because of the competition between the transport of counterions and co-ions in the nanopore, two distinct types of ion concentration polarization (CP) occur at either opening of the PE-modified nanopore. These distinct CP behaviors, which significantly affect the nanopore conductance, can be easily manipulated by adjusting the bulk salt concentration and the imposed potential bias. The induced CP in the PE-modified nanopore is more appreciable than that in the correspon...

Regulating DNA translocation through functionalized soft nanopores

Abstract: 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

Abstract: 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.

Controlling pH-regulated bionanoparticles translocation through nanopores with polyelectrolyte brushes.

Abstract: 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 ...

DNA Electrokinetic Translocation through a Nanopore: Local Permittivity Environment Effect

Abstract: 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,...

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

Abstract: 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 SiO2 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 sig...

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

Abstract: 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.

Electrokinetics of pH-regulated zwitterionic polyelectrolyte nanoparticles.

Abstract: The electrokinetic behavior of pH-regulated, zwitterionic polyelectrolyte (PE) nanoparticles (NPs) in a general electrolyte solution containing multiple ionic species is investigated for the first time. The NPs considered are capable of simulating entities such as proteins, biomolecules, and synthetic polymers. The applicability of the model proposed is verified by the experimental data of succinoglycan nanoparticles available in the literature. We show that, in addition to their effective charge density, counterion condensation, double-layer polarization, and electro-osmotic flow of unbalanced counterions inside the double layer all significantly affect the electrophoretic behaviors of NPs. Our model successfully predicts many interesting electrophoretic behaviors, which qualitatively agree with experimental observations available in the literature. In contrast, because the effects of double-layer polarization and charge regulation are neglected, the existing theoretical models fail to explain the experimental results. The results gathered provide necessary information for the interpretation of relevant electrophoresis data in practice, and for nanofluidic applications such as biomimetic ion channels and nanopore-based sensing of single biomolecules.

Influence of the shape of a polyelectrolyte on its electrophoretic behavior

Abstract: The deformation of a non-rigid particle, such as proteins, DNA, and microorganisms, during its electrophoresis can be significant in practice. This effect is simulated by considering the electrophoresis of an ellipsoidal polyelectrolyte (PE) of varying aspect ratios at a fixed volume. We show that both the double-layer polarization and the counterion condensation can be influenced significantly by the shape of an ellipsoidal PE, making its behaviors different both quantitatively and qualitatively from those of a spherical PE. In addition, the electroosmotic retardation flow inside an ellipsoidal PE also plays an important role as its shape varies. The electrophoretic mobility of a PE might have a local maximum as the amount of its fixed charge varies, which is consistent with the experimental observations in the literature. The level of the amount of the fixed charge at which the local maximum in the electrophoretic mobility occurs depends upon the PE shape, which has not been reported in previous theoretical studies. The results gathered in this study provide valuable information for both the interpretation of experimental observation and the design of relevant electrophoresis devices.

Diffusiophoresis of a polyelectrolyte in a salt concentration gradient

Abstract: The diffusiophoresis of a polyelectrolyte subject to an applied salt concentration gradient is modeled theoretically. The entirely porous type of particle is capable of simulating entities such as DNA, protein, and synthetic polymeric particles. The dependence of the diffusiophoretic behavior of the polyelectrolyte on its physical properties, and the types of ionic species and their bulk concentrations are discussed in detail. We show that in addition to the effects coming from the polarization of double layer and the difference in the ionic diffusivities, the polarization of the condensed counterions inside the polyelectrolyte might also be significant. The last effect, which has not been reported previously, reduces both the electric force and the hydrodynamic force acting on the polyelectrolyte. Both the direction and the magnitude of the diffusiophoretic velocity of the polyelectrolyte are found to highly depend upon its physical properties. These results provide valuable references for applications such as DNA sequencing and catalytic nano- or micromotors.

Electrophoresis of a Particle at an Arbitrary Surface Potential and Double Layer Thickness: Importance of Nonuniformly Charged Conditions

Abstract: Recent advances in material science and technology yield not only various kinds of nano- and sub-micro-scaled particles but also particles of various charged conditions such as Janus particles. The characterization of these particles can be challenging because conventional electrophoresis theory is usually based on drastic assumptions that are unable to realistically describe the actual situation. In this study, the influence of the nonuniform charged conditions on the surface of a particle at an arbitrary level of surface potential and double layer thickness on its electrophoretic behavior is investigated for the first time in the literature taking account of the effect of double-layer polarization. Several important results are observed. For instance, for the same averaged surface potential, the mobility of a nonuniformly charged particle is generally smaller than that of a uniformly charged particle, and the difference between the two depends upon the thickness of double layer. This implies that using the conventional electrophoresis theory may result in appreciable deviation, which can be on the order of ca. 20%. In addition, the nonuniform surface charge can yield double vortex in the vicinity of a particle by breaking the symmetric of the flow field, which has potential applications in mixing and/or regulating the medium confined in a submicrometer-sized space, where conventional mixing devices are inapplicable.

Importance of boundary on the electrophoresis of a soft cylindrical particle.

Abstract: 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.

Importance of electroosmotic flow and multiple ionic species on the electrophoresis of a rigid sphere in a charge-regulated zwitterionic cylindrical pore.

Abstract: 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 pKa = 7 and pKb = 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 o...

Importance of Boundary Effect on the Diffusiophoretic Behavior of a Charged Particle in an Electrolyte Medium

Abstract: The presence of a boundary on the diffusiophoretic behavior of a particle is analyzed by considering the diffusiophoresis of a charged, finite cylindrical particle along the axis of an uncharged cylindrical pore filled with an electrolyte solution. Compared with the other similar geometries considered in the literature, the boundary effect is the most significant in the present geometry, thereby highlighting its importance. The influence of chemiphoresis arising from two types of double-layer polarization (DLP) and that of the electrophoresis effect coming from the difference in the ionic diffusivities on the diffusiophoretic behavior of a particle are discussed. We show that this behavior can be influenced both quantitatively and qualitatively by the boundary. This is because all of the relevant factors, including the DLP effect, the electrophoresis effect, the electric repulsive force between the particle and the co-ions outside its double layer, and the hydrodynamic drag all depend highly on the degree...

Importance of Multiple Ionic Species on the Diffusiophoresis of a Rigid, Charged-Regulated, Zwitterionic Sphere

Abstract: The importance of the presence of multiple ionic species in the liquid phase on the diffusiophoresis of a charged-regulated, zwitterionic, rigid sphere is investigated. This analysis extends previo...