Electrokinetic Transport Phenomena in Nanofluidic Devices

Nanofluidic devices have wide potential applications in biology, chemistry and medicine, and have been proven to be very valuable in sensing biological particles (e.g., DNA and proteins) due to their efficiency, sensitivity and portability. Electrokinetic control of ion, fluid, and particle transport by using only electric field is the most popular method employed in nanofluidic devices. A comprehensive understanding of the electrokinetic ion, fluid, and particle transport in nanofluidics is essential for developing nanofluidic devices for the detection of single molecules, such as the next generation nanopore-based DNA sequencing technology. This research explored numerical simulation of electrokinetic ion and fluid transport in both solid-state and soft nanopores, and also explored the electric field induced translocation of nanoparticles through solid-state and soft nanopores using a continuum based model. In the first part of this dissertation, electrokinetic ion and fluid transport in two types of nanopores, charge-regulated solid-state and polyelectrolyte (PE)-modified soft nanopores, have been 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 Stokes and Brinkman equations for the flow fields. Concentration polarization phenomenon, ionic conductance, potential drop inside the nanopore, and flow field as functions of the solution properties including pH and ionic strength, charge properties of the nanopore, properties of the soft layer, and the electric field strength imposed were investigated. The results show that the electrokinetic ion and fluid transport in nanopore-based devices can be regulated by tuning pH and/or ionic strength and the properties of the polyelectrolyte layer grafted on the membrane wall. One could use the induced concentration polarization phenomenon to reduce the electric field inside the nanopore for slowing down nanoparticle translocation through the nanopore. One major challenge in the nanopore-based DNA sequencing technology is to slow down DNA translocation for improving the read-out accuracy. Therefore, the second part of this thesis focused on numerical investigations of nanoparticle translocation through a nanopore. Three types of nanoparticles, which include soft nanoparticle consisting of a rigid core covered by a soft layer, DNA, and charge-regulated soft nanoparticle such as protein, in both solid-state and soft nanopores were considered. Based on the results, regulating DNA translocation by using the soft nanopore was proposed to simultaneously enhance the nanopore capture rate and slow down DNA translocation inside the nanopore. Versatile manipulations of charge-regulated nanoparticles, including separation, focusing, trapping and pro-concentration by using soft nanopores can be achieved by adjusting pH, background salt concentration, and the properties of the soft layer grafted on the nanopore wall. Regulation of DNA translocation by using a solid-state nanopore with a floating electrode coated on the inner surface of the nanopore was also proposed and investigated using numerical simulation

Nanoscale Manipulation of Electrokinetic Transport for Iontronic Applications based on Surface Modification

Electrokinetic transport demonstrates a lot of new physical behaviors in nanofluidic systems due to the high surface-to-volume ratios and interactions between the fluid and walls of nanofluidic systems. The unique properties of electrokinetic transport in nanoscale offer possibilities for applications in many fields, such as biological computing, sensing, and drug delivery. Fundamental studies of electrokinetic transport phenomena in nanoscale have been investigated numerically and experimentally. However, the precise manipulation of electrokinetic transport in nanoscale is still a great challenge. Traditional nanofluidic devices are difficult to achieve practical applications due to their low accuracy and sensitivity. Surface modification is a promising method to develop nanofluidic devices with more functionalities. Up to date, the experimental study of electrokinetic transport in modified nanochannels is still limited. This thesis systematically studies the electrokinetic transport phenomena in surface-modified polydimethylsiloxane (PDMS) nanochannels, as well as applications of chip-scale nanofluidic devices with surface modifications. At the beginning of this thesis, electroosmotic flow (EOF) is measured in pristine PDMS single nanochannels by the current-slope method. This nanochannel is fabricated by using solvent-induced cracking method and used to form a nanofluidic chip. The effects of ion size, ion valence, and pH of electrolyte solutions on the velocity of EOF in the nanochannel are experimentally studied. These results will serve as control data for the following studies of electrokinetic transport phenomena in modified nanochannels. Then two fundamental research projects are conducted in nanochannels modified with DNA and charged polyelectrolytes to study the effects of surface modifications on electrokinetic transport phenomena. Electroosmotic flow is systematically investigated in DNA grafted hard PDMS (h-PDMS) channels with the channel size ranging from 50 nm to 2.5 μm by using the current-slope method. The effects of the DNA types, the incubation time, the pH value, the ionic concentration of electrolyte solutions, and the UV (ultraviolet) illumination on the EOF velocity are experimentally studied. The comparisons between the EOF in pristine nanochannels and DNA grafted nanochannels indicate that the surface modification of nanochannel can significantly affect the electrokinetic transport. Furthermore, the transport of fluid can be regulated by UV illumination in DNA grafted nanochannels. The size and surface charge of nanochannels after the layer-by-layer (LBL) deposition of polyelectrolytes are experimentally measured. The results reveal that the increment of the coated multilayer thickness will be limited in small nanochannels. A minimum size of nanochannel exists when the nanochannel is modified by using LBL deposition of polyelectrolytes. This minimum size depends on the salt additive in the polyelectrolyte solutions. In addition, the surface charge of the modified nanochannels is determined by the outmost coated layer. The EOF can be alternatively reversed in the modified nanochannels by repeatedly coating oppositely charged polyelectrolytes. Based on the results from the fundamental studies, a nanofluidic diode is developed by modifying the surface charge and size of a nanochannel with charged polyelectrolytes. The surface charge-governed electrokinetic transport of mobile ions results in diode-like behaviors of ionic current in the modified nanochannel. The working principle of the nanofluidic diode is explained and experimentally verified. The effects of the operation parameters, including ionic concentration, nanochannel length, and frequency, are systematically investigated. Two applications of the nanofluidic diode are presented in this thesis: improved resistive pulse sensing (RPS) system and iontronic circuits. A nanofluidic diode is fabricated and integrated into a RPS system serving as the sensing gate. A mathematic model for the modified RPS system is developed to evaluate the RPS signals. Nanoparticles with a diameter of 5 nm are also experimentally detected in the modified nanochannel-based RPS system. The experimental results are in good agreement with the numerical simulation results. By comparing the RPS signals in the modified nanochannel with that in the pristine nanochannel, it is found that RPS signals can be enhanced by approximately 50% when a nanofluidic diode is used as the sensing nanochannel. By integrating multiple nanofluidic diodes into a PDMS chip, iontronic circuits are developed. The performances of the iontronic circuits working as bipolar junction transistor and full-wave rectifier are examined and demonstrated. Signal manipulation and current rectification with high accuracy can be achieved by these iontronic circuits. This thesis develops simple methods to modulate electrokinetic transport in nanochannels by surface modifications. The fundamental research in this thesis expands the understanding of electrokinetic transport phenomena in nanochannels with various surface modifications. The iontronic devices fabricated by using modified nanochannels provide new possibilities in the development of nanofluidic systems with more functionalities, toward improved biological computing and sensing

Controlling ionic current through a nanopore by tuning pH: a local equilibrium Monte Carlo study

The purpose of this work is to create a model of a nanofluidic transistor which is able to mimic the effects of pH on nanopore conductance. The pH of the electrolyte is an experimentally controllable parameter through which the charge pattern can be tuned: pH affects the ratio of the protonated/deprotonated forms of the functional groups anchored to the surface of the nanopore (for example, amino and carboxyl groups). Thus, the behaviour of the bipolar transistor changes as it becomes ion selective in acidic/basic environments. We relate the surface charge to pH and perform particle simulations (Local Equilibrium Monte Carlo) with different nanopore geometries (cylindrical and double conical). The simulations form a self consistent system with the Nernst–Planck equation with which we compute ionic flux. We discuss the mechanism behind pH-control of ionic current: formation of depletion zones

Controlling ion transport through nanopores: modeling transistor behavior

We present a modeling study of a nanopore-based transistor computed by a mean-field continuum theory (Poisson-Nernst-Planck, PNP) and a hybrid method including particle simulation (Local Equilibrium Monte Carlo, LEMC) that is able to take ionic correlations into account including finite size of ions. The model is composed of three regions along the pore axis with the left and right regions determining the ionic species that is the main charge carrier, and the central region tuning the concentration of that species and, thus, the current flowing through the nanopore. We consider a model of small dimensions with the pore radius comparable to the Debye-screening length ($R_{\mathrm{pore}}/\lambda_{\mathrm{D}}\approx 1$), which, together with large surface charges provides a mechanism for creating depletion zones and, thus, controlling ionic current through the device. We report scaling behavior of the device as a function the $R_{\mathrm{pore}}/\lambda_{\mathrm{D}}$ parameter. Qualitative agreement between PNP and LEMC results indicates that mean-field electrostatic effects determine device behavior to the first order

Role of Surface Chemistry in Nanoscale Electrokinetic Transport

This dissertation work presents the efforts to study the electrofluidics phenomena, with a focus on surface charge properties in nanoscale systems with the potential applications in imaging, energy conversion, ultrafiltration, DNA analysis/sequencing, DNA and protein transport, drug delivery, biological/chemical agent detection and micro/nano chip sensors. Since the ion or molecular or particle transport and also liquid confinement in nano-structures are strongly dominated by the surface charge properties, in regards of the fundamental understanding of electrofluidics at nanoscale, we have used surface charge chemistry properties based on 2-pK charging mechanism. Using this mechanism, we theoretically and analytically showed the surface charge properties of silica nanoparticles as a function of their size, pH level and salt ionic strength of aqueous solution. For a fixed particle size, the magnitude of the surface charge typically increases with an increase in pH or background salt concentration. Furthermore, we investigated the surface charge properties of a charged dielectric nanoparticle and flat wall in electrostatic interactions. According to the theoretical results strong interactions cause a non-uniform surface charge density on the nanoparticle and the plate as a result of the enhancement of proton concentration in the gap between the particle and the plate. This effect increases with decreased separation distance (Kh). We moreover investigated the ion confinement inside the nanospaces and using a continuum model, we showed the proton enhancement in extended nanochannels. The proton enrichment at the center of the nanochannel is significant when the bulk pH is medium high and the salt concentration is relatively low. The results gathered are informative for the development of biomimetic nanofluidic apparatuses and the interpretation of relevant experimental data. Later, we have developed an analytical model for electroosmotic ion transport inside pH-regulated nanoslits and compared the results with the numerical study. We showed the influences of background salt concentration, pH level and the length of nanoslit on EOF velocity. The predictions show that the EOF velocity increases first and then decrease with background salt concentration increasing and the EOF velocity increases with pH level of aqueous solution

Angstrofluidics:walking to the limit

Angstrom-scale fluidic channels are ubiquitous in nature, and play an important role in regulating cellular traffic, signaling, and responding to stimuli. Synthetic channels are now a reality with the emergence of several cutting-edge bottom-up and top-down fabrication methods. In particular, the use of atomically thin two dimensional (2D) materials and nanotubes as components to build fluidic conduits has pushed the limits of fabrication to the Angstrom-scale. Here, we provide an overview of the recent developments in the fabrication methods for nano- and angstrofluidic channels while categorizing them on the basis of dimensionality (0D pores, 1D tubes, 2D slits), along with the latest advances in measurement techniques. We discuss the ionic transport governed by various stimuli in these channels and draw comparison of ionic mobility, streaming and osmotic power, with varying pore sizes across all the dimensionalities. Towards the end of the review, we highlight the unique future opportunities in the development of smart ionic devices.Comment: Keywords: Angstrofluidics, nanofluidics, confinement, ion transport, 2D materials, molecular transport 6 figures, review articl

Field Effect Control of Electrokinetic Transport Phenomena in Nanofluidics

Naturally nanofluidics has applications demanding the samples to be handled in exceedingly small quantities due to the small size of the fluidic channels in nanofluidic devices, such as characterization of single biomolecules. Fluids confined in channels of nanometer characteristic dimensions exhibit physical behaviors not observed in large conduits. Charge properties of the nanochannel wall in contact with an aqueous solution play essential roles in the involved electrokinetic transport phenomena occurring in nanofluidic devices. In addition to tuning the charge properties of the nanofluidic channel wall by adjusting the solution properties such as pH and background salt concentration, field effect transistor (FET) with a gate electrode embedded beneath the nanochannel wall has been demonstrated to rapidly tune the surface charge condition and accordingly the electrokinetic transport phenomena in nanofluidic devices. The first part of the dissertation develops a mathematical model for the charge properties of the nanofluidic channel and the electroosmotic flow (EOF) in a nanoslit gated by a FET. In contrast to the previous studies, surface chemistry is considered for the first time. The obtained results demonstrated that the surface charge property as well as the direction and magnitude of the EOF can be actively tuned by the FET. The performance of FET control is more sensitive when the pH and/or the bulk electrolyte concentration is relatively low. Since the nanofluidics-based biosensing is based on discriminating the ionic current or conductance signal, active control of the surface charge property and accordingly the ionic current/conductance in nanofluidics is investigated in the second part of the dissertation. An analytical model for the surface charge property and the ionic conductance in a FET-gated silica nanochannel is developed considering practical effects of surface chemistry reactions, multiple ionic species, the Stern layer, and the EOF. The results show that the performance of the FET control on ionic conductance is more significant when the background salt concentration and pH are low. Experimental studies demonstrated that the streaming current in the nanochannel provides a simple and effective scenario for converting hydrodynamics to electrical power. The third part of the dissertation investigated streaming current in a pH-regulated nanochannel gated by FET. Analytical models for the streaming current/conductance with and without considering the electroviscous effects have been derived. The models take into account multiple ionic species, surface chemistry reactions, and the Stern layer effect. Results show the performance of the field effect modulation of the streaming conductance is significant for lower solution pH and salt concentration. The last part of the dissertation extends the previous studies by considering the overlapping of the EDLs inside the nanochannel. The model takes into account the surface chemistry, Stern layer, multiple ionic species and the EDL overlapping effect. The model is validated by the existing experimental data of the ionic conductance in the silica nanochannel with significant EDL overlapping effect. Results from the model with and without considering EDL overlapping are compared

Mass Transport through Conical Nanopipettes and its Applications in Energy Conversion and Crystallization

With the advancement of current nanotechnology and deeper understanding of mass transport through single solid state nanopores, more applications of nanopores are emerging have been inspired and brought up by biological nanopore. Steady state response has been widely studied and explored. However, dynamic ionic response has seldom been explored. Our group has studied ionic transport kinetics and reported unique time dependent ionic transport behavior through bench-top fabricated single glass nanopores. Compared to other solid state nanopore compartments, single nanopipettes have found applications in scanning ion conductance microscopy, controlled small volume delivery and biological imaging, due to their ease to fabricate and special geometry for precise tip spatial control. Other than generally considered radius and half cone angle, long shank geometry in nanopipettes is another parameter to affect ionic transport behaviors compared to other nanopores with shorter shank length. In this dissertation, the first research topic is dynamic ionic transport behaviors through single quartz nanopipettes from fundamental perspective. An important non-zero cross point separating normal and negative hysteresis current-potential (I-V) loops will be introduced and discussed by electroanalytical analysis. Strong time dependent I-V hysteresis at low frequency and interesting negative resistance behavior reveals the impacts of finite variation in nanogeometry specifically channel length effect. Next, dynamic ion transport through single nanopipettes is studied under a series of concentration gradient introduced. Ion transport dynamics through asymmetric nanogeometry contributed by migration and diffusion is deconvoluted and its implication in salinity gradient energy conversion is explained. In the third project, a new method to crystallize matter based on dynamic control of mass transport through single nanopipette is demonstrated using protein insulin

ELECTROKINETIC TRANSPORT IN NANOCHANNELS GRAFTED WITH BACKBONE CHARGED POLYELECTROLYTE BRUSHES

In this thesis, we study the electrokinetic transport in nanochannels functionalized with pH-responsive backbone charged polyelectrolyte (PE) brushes modeled using thermodynamically self-consistent augmented strong stretching theory. We investigate the electroosmotic (EOS) transport, induced by the application of external electric field, and the diffusioosmotic (DOS) transport due to applied salt concentration gradient induced electroosmotic transport in brush functionalized and brushless nanochannels with equal surface charge density. We find massive enhancement in the electrokinetic transport in PE brush functionalized nanochannels when compared to brushless nanochannels which can be ascribed to the brush induced localization of the EDL and hence the net EOS body force away from the flow retarding walls. Further, we establish that both EOS and DOS transport in nanochannels grafted with backbone charged PE brushes is larger in magnitude when compared to that in nanochannels grafted with end charged PE brushes