Publications by authors named "Zuzanna Siwy"

85 Publications

Enhanced electro-osmosis in propylene carbonate salt solutions.

J Chem Phys 2021 Apr;154(13):134707

Department of Chemistry, University of California Irvine, Irvine, California 92697, USA.

Properties of solid-liquid interfaces and surface charge characteristics mediate ionic and molecular transport through porous systems, affecting many processes such as separations. Herein, we report experiments designed to probe the electrochemical properties of solid-liquid interfaces using a model system of a single polyethylene terephthalate (PET) pore in contact with aqueous and propylene carbonate solutions of LiClO. First, the existence and polarity of surface charges were inferred from current-voltage curves recorded when a pore was placed in contact with a LiClO concentration gradient. Second, the electro-osmotic transport of uncharged polystyrene particles through the PET pore provided information on the polarity and the magnitude of the pore walls' zeta potential. Our experiments show that the PET pores become effectively positively charged when in contact with LiClO solutions in propylene carbonate, even though in aqueous LiClO, the same pores are negatively charged. Additionally, the electro-osmotic velocity of the particles revealed a significantly higher magnitude of the positive zeta potential of the pores in propylene carbonate compared to the magnitude of the negative zeta potential in water. The presented methods of probing the properties of solid-liquid interfaces are expected to be applicable to a wide variety of solid and liquid systems.
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http://dx.doi.org/10.1063/5.0044402DOI Listing
April 2021

Tunable Nanopore Arrays as the Basis for Ionic Circuits.

ACS Appl Mater Interfaces 2020 Dec 7;12(50):56622-56631. Epub 2020 Dec 7.

Department of Physics and Astronomy, University of California, 210G Rowland Hall, Irvine, California 92697, United States.

There has been considerable interest in preparing ionic circuits capable of manipulating ionic and molecular transport in a solution. This direction of research is inspired by biological systems where multiple pores with different functionalities embedded in a cell membrane transmit external signals and underlie all physiological processes. In this manuscript, we describe the modeling of ion transport through small arrays of nanopores consisting of 3, 6, and 9 nanopores and an integrated gate electrode placed on the membrane surface next to one pore opening. We show that by tuning the gate voltage and strategically placing nanopores with nonlinear current-voltage characteristics, the local signal at the gate affects ionic transport through all nanopores in the array. Conditions were identified when the same gate voltage induced opposite rectification properties of neighboring nanopores. We also demonstrate that an ionic diode embedded in a nanopore array can modulate transport properties of neighboring pores even without a gate voltage. The results are explained by the role of concentration polarization and overlapping depletion zones on one side of the membrane. The modeling presented here is intended to become an inspiration to future experiments to create nanopore arrays that can transduce signals in space and time.
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http://dx.doi.org/10.1021/acsami.0c18574DOI Listing
December 2020

Modulation of Ionic Current Rectification in Ultrashort Conical Nanopores.

Anal Chem 2020 12 20;92(24):16188-16196. Epub 2020 Nov 20.

Key Laboratory of High Efficiency and Clean Mechanical Manufacture of Ministry of Education, National Demonstration Center for Experimental Mechanical Engineering Education, School of Mechanical Engineering, Shandong University, Jinan 250061, China.

Nanopores that exhibit ionic current rectification (ICR) behave like diodes such that they transport ions more efficiently in one direction than in the other. Conical nanopores have been shown to rectify ionic current, but only those with at least 500 nm in length exhibit significant ICR. Here, through the finite element method, we show how ICR of conical nanopores with lengths below 200 nm can be tuned by controlling individual charged surfaces, that is, the inner pore surface (surface) and exterior pore surfaces on the tip and base side (surface and surface). The charged surface and surface can induce obvious ICR individually, while the effects of the charged surface on ICR can be ignored. The fully charged surface alone could render the nanopore counterion-selective and induces significant ion concentration polarization in the tip region, which causes reverse ICR compared to nanopores with all surfaces charged. In addition, the direction and degree of rectification can be further tuned by the depth of the charged surface. When considering the exterior membrane surface only, the charged surface causes intrapore ionic enrichment and depletion under opposite biases, which results in significant ICR. Its effective region is within ∼40 nm beyond the tip orifice. We also found that individual charged parts of the pore system contributed to ICR in an additive way because of the additive effect on the ion concentration regulation along the pore axis. With various combinations of fully/partially charged surface and surface, diverse ICR ratios from ∼2 to ∼170 can be achieved. Our findings shed light on the mechanism of ICR in ultrashort conical nanopores and provide a useful guide to the design and modification of ultrashort conical nanopores in ionic circuits and nanofluidic sensors.
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http://dx.doi.org/10.1021/acs.analchem.0c03989DOI Listing
December 2020

Ionic amplifying circuits inspired by electronics and biology.

Nat Commun 2020 03 26;11(1):1568. Epub 2020 Mar 26.

Department of Physics and Astronomy, University of California, 4129 Frederick Reines Hall, Irvine, CA, 92697, USA.

Integrated circuits are present in all electronic devices, and enable signal amplification, modulation, and relay. Nature uses another type of circuits composed of channels in a cell membrane, which regulate and amplify transport of ions, not electrons and holes as is done in electronic systems. Here we show an abiotic ionic circuit that is inspired by concepts from electronics and biology. The circuit amplifies small ionic signals into ionic outputs, and its operation mimics the electronic Darlington amplifier composed of transistors. The individual transistors are pores equipped with three terminals including a gate that is able to enrich or deplete ions in the pore. The circuits we report function at gate voltages < 1 V, respond to sub-nA gate currents, and offer ion current amplification with a gain up to ~300. Ionic amplifiers are a logical step toward improving chemical and biochemical sensing, separations and amplification, among others.
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http://dx.doi.org/10.1038/s41467-020-15398-3DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7099069PMC
March 2020

Gating of Hydrophobic Nanopores with Large Anions.

ACS Nano 2020 Apr 19;14(4):4306-4315. Epub 2020 Mar 19.

Department of Chemistry, University of California, Irvine, California 92697, United States.

Understanding ion transport in nanoporous materials is critical to a wide variety of energy and environmental technologies, ranging from ion-selective membranes, drug delivery, and biosensing, to ion batteries and supercapacitors. While nanoscale transport is often described by continuum models that rely on a point charge description for ions and a homogeneous dielectric medium for the solvent, here, we show that transport of aqueous solutions at a hydrophobic interface can be highly dependent on the size and hydration strength of the solvated ions. Specifically, measurements of ion current through single silicon nitride nanopores that contain a hydrophobic-hydrophilic junction show that transport properties are dependent not only on applied voltage but also on the type of anion. We find that in Cl-containing solutions the nanopores only conducted ionic current above a negative voltage threshold. On the other hand, introduction of large polarizable anions, such as Br and I, facilitated the pore wetting, making the pore conductive at all examined voltages. Molecular dynamics simulations revealed that the large anions, Br and I, have a weaker solvation shell compared to that of Cl and consequently were prone to migrate from the aqueous solution to the hydrophobic surface, leading to the anion accumulation responsible for pore wetting. The results are essential for designing nanoporous systems that are selective to ions of the same charge, for realization of ion-induced wetting in hydrophobic pores, as well as for a fundamental understanding on the role of ion hydration shell on the properties of solid/liquid interfaces.
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http://dx.doi.org/10.1021/acsnano.9b09777DOI Listing
April 2020

Reading amino acids in a nanopore.

Nat Biotechnol 2020 02;38(2):159-160

Department of Physics and Astronomy, University of California, Irvine, Irvine, California, USA.

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http://dx.doi.org/10.1038/s41587-019-0401-yDOI Listing
February 2020

Charge Inversion and Calcium Gating in Mixtures of Ions in Nanopores.

J Am Chem Soc 2020 02 30;142(6):2925-2934. Epub 2020 Jan 30.

Department of Physics and Astronomy , University of California , Irvine , California 92697 , United States.

Calcium ions play important roles in many physiological processes, yet their concentration is much lower than the concentrations of potassium and sodium ions. The selectivity of calcium channels is often probed in mixtures of calcium and a monovalent salt, e.g., KCl or NaCl, prepared such that the concentration of cations is kept constant with the mole fraction of calcium varying from 0 and 1. In biological channels, even sub-mM concentration of calcium can modulate the channels' transport characteristics; this effect is often explained via the existence of high affinity Ca binding sites on the channel walls. Inspired by properties of biological calcium-selective channels, we prepared a set of nanopores with tunable opening diameters that exhibited a similar response to the presence of calcium ions as biochannels. Nanopores in 15 nm thick silicon nitride films were drilled using focused ion beam and e-beam in a transmission electron microscope and subsequently rendered negatively charged through silanization. We found that nanopores with diameters smaller than 20 nm were blocked by calcium ions such that the ion currents in mixtures of KCl and CaCl and in CaCl were even ten times smaller than the ion currents in KCl solution. The ion current blockage was explained by the effect of local charge inversion where accumulated calcium ions switch the effective surface charge from negative to positive. The modulation of surface charge with calcium leads to concentration and voltage dependent local charge density and ion current. The combined experimental and modeling results provide a link between calcium ion-induced changes in surface charge properties and resulting ionic transport.
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http://dx.doi.org/10.1021/jacs.9b11537DOI Listing
February 2020

Electrodiffusioosmosis-Induced Negative Differential Resistance in pH-Regulated Mesopores Containing Purely Monovalent Solutions.

ACS Appl Mater Interfaces 2020 Jan 31;12(2):3198-3204. Epub 2019 Dec 31.

Department of Chemical Engineering , National Taiwan University of Science and Technology , Taipei 10607 , Taiwan.

Negative differential resistance (NDR) refers to a unique electrical property where current decreases with increasing voltage. Herein, we report experimental evidence showing that the NDR effect can be observed in mesopores that feature charged pore walls and are subjected to a KCl concentration gradient. NDR in our system originates from the solution and ion flows driven by the synergistic effects of electroosmosis [electroosmotic flow (EOF)] and diffusioosmosis, the so-called electrodiffusioosmosis. Experiments reveal that in addition to the ion current rectification, the mesopores considered here exhibit the NDR phenomenon that is dependent on the magnitude and direction of the salinity gradient and on pH. The NDR behavior can be observed only at conditions at which the EOF and diffusioosmosis occur in the opposite directions: diffusioosmosis fills the tip opening with a high concentration solution, while EOF brings a low concentration solution to the pore. All experimental findings are supported by our numerical model, which takes into account the interfacial site reactions of acidic and basic functional groups on the entire pore membrane surfaces. Our results provide an important insight into how liquid pH, salinity gradients, interfacial site reactions, and pore geometries can influence the current-voltage characteristics of mesopores, enriching transport modes that can be induced by voltage.
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http://dx.doi.org/10.1021/acsami.9b18524DOI Listing
January 2020

Tunable Current Rectification and Selectivity Demonstrated in Nanofluidic Diodes through Kinetic Functionalization.

J Phys Chem Lett 2020 Jan 13;11(1):60-66. Epub 2019 Dec 13.

Department of Chemical Engineering , National Taiwan University , Taipei 10617 , Taiwan.

The possibility of tuning the current rectification and selectivity in nanofluidic diodes is demonstrated both experimentally and theoretically through dynamically functionalizing a conical nanopore with poly-l-lysine. We identified an optimum functionalization time equivalent to optimum modification depth that assures the highest rectification degrees. Results showed that the functionalization time-dependent rectification behavior of nanofluidic diodes is dominated by the properties of current at positive voltages that in our electrode configuration indicate the "on" state of the diode and accumulation of ions in the nanopore. The functionalization time also tunes the ion selectivity of the diode. If the functionalization time is sufficiently short, an unusual depletion of counterions near the bipolar interface results in a cation-selective nanopore. However, a further increase in the duration of functionalization renders a nanopore that is an anion-selective nanopore. The dynamic functionalization presented in this Letter enables tuning ion selectivity of nanopores.
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http://dx.doi.org/10.1021/acs.jpclett.9b03344DOI Listing
January 2020

Preface.

Anal Chim Acta 2019 Dec 12;1086:14-15. Epub 2019 Sep 12.

University of Louisville, USA. Electronic address:

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http://dx.doi.org/10.1016/j.aca.2019.09.027DOI Listing
December 2019

Modulation of Charge Density and Charge Polarity of Nanopore Wall by Salt Gradient and Voltage.

ACS Nano 2019 09 31;13(9):9868-9879. Epub 2019 Jul 31.

Department of Chemical Engineering , National Taiwan University , Taipei 10617 , Taiwan.

Surface charge plays a very important role in biological processes including ionic and molecular transport across a cell membrane. Placement of charges and charge patterns on walls of polymer and solid-state nanopores allowed preparation of ion-selective systems as well as ionic diodes and transistors to be applied in building biological sensors and ionic circuits. In this article, we show that the surface charge of a 10 nm diameter silicon nitride nanopore placed in contact with a salt gradient is not a constant value, but rather it depends on applied voltage and magnitude of the salt gradient. We found that even when a nanopore was in contact with solutions of pH equivalent to the isoelectric point of the pore surface, the pore walls became charged with voltage-dependent charge density. Implications of the charge gating for detection of proteins passing through a nanopore were considered, as well. Experiments performed with single 30 nm long silicon nitride nanopores were described by continuum modeling, which took into account the surface reactions on the nanopore walls and local modulation of the solution pH in the pore and at the pore entrances. The results revealed that manipulation of surface charge can occur without changing pH of the background electrolyte, which is especially important for applications where maintaining pH at a constant and physiological level is necessary. The system presented also offers a possibility to modulate polarity and magnitude of surface charges in a two-electrode setup, which previously was accomplished in more complex multielectrode systems.
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http://dx.doi.org/10.1021/acsnano.9b01357DOI Listing
September 2019

Electrokinetic Phenomena in Organic Solvents.

J Phys Chem B 2019 Jul 2;123(28):6123-6131. Epub 2019 Jul 2.

Solid/liquid interfaces play a key role in separation processes, energy storage devices, and transport in nanoscale systems. Nanopores and mesopores with well-defined geometry and chemical characteristics have been a valuable tool to unravel electrochemical properties of interfaces, but the majority of studies have been focused on aqueous solutions. Here, we present experiments and numerical modeling aimed at characterizing effective surface charge of polymer pores in mixtures of water and alcohols as well as in propylene carbonate and acetone. The charge properties of pore walls are probed through analysis of current-voltage curves recorded in the presence of salt concentration gradients. The presence and direction of electro-osmotic flow lead to asymmetric current-voltage curves, with rectification characteristics determined by the polarity of surface charge. The results suggest that the effective surface charge of the pore walls depends not only on the type of solvent but also on the concentration of the electrolyte and voltage. We identified conditions at which polymer pores that are negatively charged in aqueous solutions become positively charged in propylene carbonate and acetone. The findings are of importance for nonaqueous separations, fundamental knowledge on solid/liquid interfaces in organic media, and preparation of porous devices with tunable surface charge characteristics.
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http://dx.doi.org/10.1021/acs.jpcb.9b04969DOI Listing
July 2019

A nanofluidic ion regulation membrane with aligned cellulose nanofibers.

Sci Adv 2019 Feb 22;5(2):eaau4238. Epub 2019 Feb 22.

Department of Materials Science and Engineering, University of Maryland College Park, College Park, MD 20742, USA.

The advancement of nanofluidic applications will require the identification of materials with high-conductivity nanoscale channels that can be readily obtained at massive scale. Inspired by the transpiration in mesostructured trees, we report a nanofluidic membrane consisting of densely packed cellulose nanofibers directly derived from wood. Numerous nanochannels are produced among an expansive array of one-dimensional cellulose nanofibers. The abundant functional groups of cellulose enable facile tuning of the surface charge density via chemical modification. The nanofiber-nanofiber spacing can also be tuned from ~2 to ~20 nm by structural engineering. The surface-charge-governed ionic transport region shows a high ionic conductivity plateau of ~2 mS cm (up to 10 mM). The nanofluidic membrane also exhibits excellent mechanical flexibility, demonstrating stable performance even when the membrane is folded 150°. Combining the inherent advantages of cellulose, this novel class of membrane offers an environmentally responsible strategy for flexible and printable nanofluidic applications.
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http://dx.doi.org/10.1126/sciadv.aau4238DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6386557PMC
February 2019

Biomimetic potassium-selective nanopores.

Sci Adv 2019 02 8;5(2):eaav2568. Epub 2019 Feb 8.

Department of Physics and Astronomy, University of California, Irvine, Irvine, CA 92697, USA.

Reproducing the exquisite ion selectivity displayed by biological ion channels in artificial nanopore systems has proven to be one of the most challenging tasks undertaken by the nanopore community, yet a successful achievement of this goal offers immense technological potential. Here, we show a strategy to design solid-state nanopores that selectively transport potassium ions and show negligible conductance for sodium ions. The nanopores contain walls decorated with 4'-aminobenzo-18-crown-6 ether and single-stranded DNA (ssDNA) molecules located at one pore entrance. The ionic selectivity stems from facilitated transport of potassium ions in the pore region containing crown ether, while the highly charged ssDNA plays the role of a cation filter. Achieving potassium selectivity in solid-state nanopores opens new avenues toward advanced separation processes, more efficient biosensing technologies, and novel biomimetic nanopore systems.
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http://dx.doi.org/10.1126/sciadv.aav2568DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6368432PMC
February 2019

Rectification of Concentration Polarization in Mesopores Leads To High Conductance Ionic Diodes and High Performance Osmotic Power.

J Am Chem Soc 2019 02 13;141(8):3691-3698. Epub 2019 Feb 13.

Department of Chemical Engineering , National Taiwan University of Science and Technology , Taipei 10607 , Taiwan.

Nanopores exhibit a set of interesting transport properties that stem from interactions of the passing ions and molecules with the pore walls. Nanopores are used, for example, as ionic diodes and transistors, biosensors, and osmotic power generators. Using nanopores is however disadvantaged by their high resistance, small switching currents in nA range, low power generated, and signals that can be difficult to distinguish from the background. Here, we present a mesopore with ionic conductance reaching μS that rectifies ion current in salt concentrations as high as 1 M. The mesopore is conically shaped, and its region close to the narrow opening is filled with high molecular weight poly-l-lysine. To elucidate the underlying mechanism of ion current rectification (ICR), a continuum model based on a set of Poisson-Nernst-Planck and Stokes-Brinkman equations was adopted. The results revealed that embedding the polyelectrolyte in a conical pore leads to rectification of the effect of concentration polarization (CP) that is induced by the polyelectrolyte, and observed as voltage polarity-dependent modulations of ionic concentrations in the pore, and consequently ICR. Our work reveals the link between ICR and CP, significantly extending the knowledge of how charged polyelectrolytes modulate ion transport on nano- and mesoscales. The osmotic power application is also demonstrated with the developed polyelectrolyte-filled mesopores, which enable a power of up to ∼120 pW from one pore, which is much higher than the reported values using single nanoscale pores.
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http://dx.doi.org/10.1021/jacs.8b13497DOI Listing
February 2019

Abnormal Ionic-Current Rectification Caused by Reversed Electroosmotic Flow under Viscosity Gradients across Thin Nanopores.

Anal Chem 2019 01 18;91(1):996-1004. Epub 2018 Dec 18.

Department of Physics , Northeastern University , Boston , Massachusetts 02115 , United States.

Single nanopores have attracted much scientific interest because of their versatile applications. The majority of experiments have been performed with nanopores being in contact with the same electrolyte on both sides of the membrane, although solution gradients across semipermeable membranes are omnipresent in natural systems. In this manuscript, we studied ionic and fluidic movement through thin nanopores under viscosity gradients both experimentally and using simulations. Ionic-current rectification was observed under these conditions because solutions with different conductivities filled across the pore under different biases caused by electroosmotic flow. We found that a pore filled with high-viscosity solutions exhibited a current increase with applied voltage in a steeper slope beyond a threshold voltage, which abnormally reduced the current-rectification ratio. Through simulations, we found that reversed electroosmotic flow, which filled the pore with aqueous solutions of lower viscosities, was responsible for this behavior. The reversed electroosmotic flow could be explained by slower depletion of co-ions than of counterions along the pore. By increasing the surface charge density of pore surfaces, current-rectification ratio could reach the value of the viscosity gradient across thin nanopores. Our findings shed light on fundamental aspects to be considered when performing experiments with viscosity gradients across nanopores and nanofluidic channels.
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http://dx.doi.org/10.1021/acs.analchem.8b04225DOI Listing
January 2019

Probing ion current in solid-electrolytes at the meso- and nanoscale.

Faraday Discuss 2018 10;210(0):55-67

Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA.

We present experimental approaches to probe the ionic conductivity of solid electrolytes at the meso- and nanoscales. Silica ionogel based electrolytes have emerged as an important class of solid electrolytes because they maintain both fluidic and high-conductivity states at the nanoscale, but at the macroscale they are basically solid. Single mesopores in polymer films are shown to serve as templates for cast ionogels. The ionic conductivity of the ionogels was probed by two experimental approaches. In the first approach, the single-pore/ionogel membranes were placed between two chambers of a conductivity cell, in a set-up similar to that used for investigating liquid electrolytes. The second approach involved depositing contacts directly onto the membrane and measuring conductivity without the bulk solution present. Ionic conductivity determined by the two methods was in excellent agreement with macroscopic measurements, which suggested that the electrochemical properties of ionogel based electrolytes are preserved at the mesoscale, and ionogels can be useful in designing meso-scaled energy-storage devices.
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http://dx.doi.org/10.1039/c8fd00071aDOI Listing
October 2018

Advances in Multi-Scale Pores and Channels Systems.

Small 2018 05;14(18):e1800908

Lehrstuhl für Technische Chemie II, Universität Duisburg-Essen, 45141, Essen, Germany.

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http://dx.doi.org/10.1002/smll.201800908DOI Listing
May 2018

The Design and Characterization of Multifunctional Aptamer Nanopore Sensors.

ACS Nano 2018 05 8;12(5):4844-4852. Epub 2018 May 8.

Department of Chemistry , Loughborough University , Loughborough LE11 3TU , United Kingdom.

Aptamer-modified nanomaterials provide a simple, yet powerful sensing platform when combined with resistive pulse sensing technologies. Aptamers adopt a more stable tertiary structure in the presence of a target analyte, which results in a change in charge density and velocity of the carrier particle. In practice the tertiary structure is specific for each aptamer and target, and the strength of the signal varies with different applications and experimental conditions. Resistive pulse sensors (RPS) have single particle resolution, allowing for the detailed characterization of the sample. Measuring the velocity of aptamer-modified nanomaterials as they traverse the RPS provides information on their charge state and densities. To help understand how the aptamer structure and charge density effects the sensitivity of aptamer-RPS assays, here we study two metal binding aptamers. This creates a sensor for mercury and lead ions that is capable of being run in a range of electrolyte concentrations, equivalent to river to seawater conditions. The observed results are in excellent agreement with our proposed model. Building on this we combine two aptamers together in an attempt to form a dual sensing strand of DNA for the simultaneous detection of two metal ions. We show experimental and theoretical responses for the aptamer which creates layers of differing charge densities around the nanomaterial. The density and diameter of these zones effects both the viability and sensitivity of the assay. While this approach allows the interrogation of the DNA structure, the data also highlight the limitations and considerations for future assays.
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http://dx.doi.org/10.1021/acsnano.8b01583DOI Listing
May 2018

Information Dynamics of a Nonlinear Stochastic Nanopore System.

Entropy (Basel) 2018 Mar 23;20(4). Epub 2018 Mar 23.

Department of Chemistry, University of California-Irvine, Irvine, CA 92697-2025, USA.

Nanopores have become a subject of interest in the scientific community due to their potential uses in nanometer-scale laboratory and research applications, including infectious disease diagnostics and DNA sequencing. Additionally, they display behavioral similarity to molecular and cellular scale physiological processes. Recent advances in information theory have made it possible to probe the information dynamics of nonlinear stochastic dynamical systems, such as autonomously fluctuating nanopore systems, which has enhanced our understanding of the physical systems they model. We present the results of local (LER) and specific entropy rate (SER) computations from a simulation study of an autonomously fluctuating nanopore system. We learn that both metrics show increases that correspond to fluctuations in the nanopore current, indicating fundamental changes in information generation surrounding these fluctuations.
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http://dx.doi.org/10.3390/e20040221DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7512734PMC
March 2018

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

J Phys Chem Lett 2018 Jan 9;9(2):393-398. Epub 2018 Jan 9.

Department of Physics and Astronomy, University of California , Irvine, California 92697, United States.

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.
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http://dx.doi.org/10.1021/acs.jpclett.7b03099DOI Listing
January 2018

Ion transport in gel and gel-liquid systems for LiClO-doped PMMA at the meso- and nanoscales.

Nanoscale 2017 Nov;9(42):16232-16243

Department of Physics and Astronomy, University of California, Irvine, California 92697, USA.

Solid and gel electrolytes offer significant advantages for cycle stability and longevity in energy storage technologies. These advantages come with trade-offs such as reduced conductivity and ion mobility, which can impact power density in storage devices even at the nanoscale. Here we propose experiments aimed at exploring the ion transport properties of a hybrid electrolyte system of liquid and gel electrolytes with meso and nanoscale components. We focus on single pore systems featuring LiClO-propylene carbonate and LiClO-PMMA gel, which are model electrolytes for energy storage devices. We identified conditions at which the systems considered featured rectifying current-voltage curves, indicating a preferential direction of ion transport. The presented ion current rectification suggests different mechanisms arising from the unique hybrid system: (i) PMMA structure imposing selectivity in fully immersed systems and (ii) ionic selectivity linked to ion sourcing from media of different ionic mobility. These mechanisms were observed to interplay with ion transport properties linked to nanopore structure i.e. cylindrical and conical.
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http://dx.doi.org/10.1039/c7nr06719dDOI Listing
November 2017

Probing charges on solid-liquid interfaces with the resistive-pulse technique.

Nanoscale 2017 Sep;9(36):13527-13537

Department of Physics and Astronomy, University of California, Irvine, CA 92697, USA.

Our manuscript addresses the issue of probing an effective surface charge that any surface can acquire at the solid/liquid interface. Even if a particle is predicted to be neutral based on its chemical structure, the particle can carry finite surface charges when placed in a solution. We present tools to probe the presence of surface charge densities of meso-particles, characterized with zeta potentials below 10 mV. The tools are based on the resistive-pulse technique, which uses single pores to probe properties of individual objects including molecules, particles, and cells. The presented experiments were performed with particles 280 and 400 nm in diameter and single pores with opening diameter tuned between ∼ 200 nm and one micron. Surface charge properties were probed in two modes: (i) the passage of the particles through pores of diameters larger than the particles, as well as (ii) an approach curve of a particle to a pore that is smaller than the particle diameter. The curve in the latter mode has a biphasic character starting with a low-amplitude current decrease, followed by a current enhancement reaching an amplitude of ∼10% of the baseline current. The current increase was long-lasting and stable, and shown to strongly depend on the particle surface charge density. The results are explained via voltage-modulation of ionic concentrations in the pore.
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http://dx.doi.org/10.1039/c7nr03998kDOI Listing
September 2017

A hybrid resistive pulse-optical detection platform for microfluidic experiments.

Sci Rep 2017 08 31;7(1):10173. Epub 2017 Aug 31.

Department of Physics and Astronomy, University of California, Irvine, Irvine, CA, 92617, USA.

Resistive-pulse sensing is a label-free method for characterizing individual particles as they pass through ion-conducting channels or pores. During a resistive pulse experiment, the ionic current through a conducting channel is monitored as particles suspended in the solution translocate through the channel. The amplitude of the current decrease during a translocation, or 'pulse', depends not only on the ratio of the particle and channel sizes, but also on the particle position, which is difficult to resolve with the resistive pulse signal alone. We present experiments of simultaneous electrical and optical detection of particles passing through microfluidic channels to resolve the positional dependencies of the resistive pulses. Particles were tracked simultaneously in the two signals to create a mapping of the particle position to resistive pulse amplitude at the same instant in time. The hybrid approach will improve the accuracy of object characterization and will pave the way for observing dynamic changes of the objects such as deformation or change in orientation. This combined approach of optical detection and resistive pulse sensing will join with other attempts at hybridizing high-throughput detection techniques such as imaging flow cytometry.
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http://dx.doi.org/10.1038/s41598-017-10000-1DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5579027PMC
August 2017

Improving on aquaporins.

Science 2017 08;357(6353):753

Lawrence Livermore National Laboratory, Livermore, CA 94550, USA.

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http://dx.doi.org/10.1126/science.aao2440DOI Listing
August 2017

Viscosity and Conductivity Tunable Diode-like Behavior for Meso- and Micropores.

J Phys Chem Lett 2017 Aug 4;8(16):3846-3852. Epub 2017 Aug 4.

Department of Physics and Astronomy, ‡Department of Chemistry, §Department of Biomedical Engineering, University of California , Irvine, California 92697, United States.

Rectifying pores, which transport ions mainly in one direction blocking the ionic flow in the other, were shown to be important in the preparation of chemical sensors, components of ionic circuits, and mimics of biological channels. Ionic rectification has been shown with various engineered systems, but pores with similar opening diameters often rectify to a various uncontrolled extent. In this Letter we present a system of single meso-pores, whose current-voltage curves and rectification can be tuned with great precision via viscosity and conductivity gradients of solutions placed on both sides of the membrane. The mechanism of rectification is based on electroosmotically induced flow, which fills the entire volume of the pore with a single solution from either side of the membrane. The highly predictable rectifying system can find various applications, including measuring viscosity of unknown media and tuning electrokinetic passage of particles.
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http://dx.doi.org/10.1021/acs.jpclett.7b01804DOI Listing
August 2017

Nanopores and Nanochannels: From Gene Sequencing to Genome Mapping.

ACS Nano 2016 11 10;10(11):9768-9771. Epub 2016 Nov 10.

Department of Physics and Astronomy, University of California, Irvine , 210G Rowland Hall, Irvine, California 92697, United States.

DNA strands can be analyzed at the single-molecule level by isolating them inside nanoscale holes. The strategy is used for the label-free and portable sequencing with nanopores. Nanochannels can also be applied to map genomes with high resolution, as shown by Jeffet et al. in this issue of ACS Nano. Here, we compare the two strategies in terms of biophysical similarities and differences and describe that both are complementary and can improve the DNA analysis for genomic research and diagnostics.
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http://dx.doi.org/10.1021/acsnano.6b07041DOI Listing
November 2016

Polarization of Gold in Nanopores Leads to Ion Current Rectification.

J Phys Chem Lett 2016 Oct 6;7(20):4152-4158. Epub 2016 Oct 6.

Department of Chemistry, University of California , Irvine, California 92697, United States.

Biomimetic nanopores with rectifying properties are relevant components of ionic switches, ionic circuits, and biological sensors. Rectification indicates that currents for voltages of one polarity are higher than currents for voltages of the opposite polarity. Ion current rectification requires the presence of surface charges on the pore walls, achieved either by the attachment of charged groups or in multielectrode systems by applying voltage to integrated gate electrodes. Here we present a simpler concept for introducing surface charges via polarization of a thin layer of Au present at one entrance of a silicon nitride nanopore. In an electric field applied by two electrodes placed in bulk solution on both sides of the membrane, the Au layer polarizes such that excess positive charge locally concentrates at one end and negative charge concentrates at the other end. Consequently, a junction is formed between zones with enhanced anion and cation concentrations in the solution adjacent to the Au layer. This bipolar double layer together with enhanced cation concentration in a negatively charged silicon nitride nanopore leads to voltage-controlled surface-charge patterns and ion current rectification. The experimental findings are supported by numerical modeling that confirm modulation of ionic concentrations by the Au layer and ion current rectification even in low-aspect ratio nanopores. Our findings enable a new strategy for creating ionic circuits with diodes and transistors.
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http://dx.doi.org/10.1021/acs.jpclett.6b01971DOI Listing
October 2016