Squeezing ionic liquids through nanopores

Conductivity of Ionic Liquids In the Bulk and during Infiltration in Nanopores

The conductivity of ionic liquids (ILs) in nanopores is essential when considering their application as materials for energy. However, no consensus has been reached about the influence of confinement on the mobility of the ions. A series of ILs bearing the same cation, 1-butyl-3-methylimidazolium ([BMIM]+), and six different anions ([Cl]-, [Br]-, [I]-, [BF4]-, [PF6]-, and [TFSI]-) with radii from 0.168 to 0.326 nm were investigated with respect to their self-assembly, the thermodynamics, and the ionic conductivity in the bulk, during flow and under confinement in cylindrical nanopores with sizes in the range from 400 to 25 nm. In the bulk, the [BMIM]+[X]- exhibits weak ordering as a result of cation-anion correlations (charge alteration peak), and nanophase separation of polar/apolar groups. Liquid-to-glass temperatures were found to differ by ∼50 K, their viscosities by a factor of ∼270, and their conductivities by a factor of 24 (all at a temperature of 303 K). Electrostatic interactions were largely responsible for variations in the glass temperature, the viscosity, and the conductivity. Confined ILs behave differently from the bulk. The majority of ILs in the bulk were prone to crystallization during heating but were unable to crystallize in the smaller pores. Changes in dc-conductivity were used as markers of the phase state. This allowed the construction of the effective phase diagrams under confinement. The ILs penetrate the pores with an effective viscosity of the order of their viscosity in their bulk state. However, within the pores the dc-conductivity was reduced relative to bulk, indicating the immobilization of ions at the pore walls. Hydrophobization of the pore walls by hexamethyldisilazane could partially restore the conductivity. ILs are model systems where the phase state and ion mobility can be controlled by confinement.

Label-free high-precise nanopore detection of endopeptidase activity of anthrax lethal factor regulated by diverse conditions

Endopeptidase activity of anthrax lethal factor (aLF) prevents the destroy of anthracis spore intracellularly by host macrophages, meanwhile disables the signaling pathways extracellularly that leads to host lethality. Hence, inhibitory of this activity is expected to be an alternative option to cure anthrax infection. Herein, we fabricated a nanopore platform via transmembrane pore construction in vitro, which allows precise mimics, monitoring of intercellular proteinic transport and enables the quantitative detection of aLF endopeptidase activity towards MAPKK signaling protein at single molecule level. Next, we inhibited the aLF activity via screening approaches of protein-metal ion acquisition and other condition controlment (proton/hydroxide strength, adapted temperature, ionizing irradiation), which were identified by nanopore electrokinetic study. Upon the results, we found that Ca2+, Mg2+, Mn2+, Ni2+ collaborating with Zn2+ promote aLF activity efficiently. In contrary, Cd2+, Co2+, Cu2+ have great inhibitory effect. Result further revealed that, the speed of aLF endopeptidase activity with different ions functions as the nanopore signal frequency in linear manner, which enables evident distinction of those divalent ions using this proteinase assay. We also found the higher strength of the proton or hydroxide, the higher the inhibitory to aLF activity. Besides, adapted temperature and γ-ray also play integral roles in inhibiting this activity. Our results lay experimental basis for accurate detection of aLF activity, meanwhile provide new direction to screening novel stimuli-responsive inhibitors specific to aLF.

Confinement Effects on the Magnetic Ionic Liquid 1-Ethyl-3-methylimidazolium Tetrachloroferrate(III)

Confinement effects for the magnetoresponsive ionic liquid 1-ethyl-3-methylimidazolium tetrachloroferrate(III), [C2mim]FeCl4, are explored from thermal, spectroscopic, and magnetic points of view. Placing the ionic liquid inside SBA-15 mesoporous silica produces a significant impact on the material’s response to temperature, pressure, and magnetic fields. Isobaric thermal experiments show melting point reductions that depend on the pore diameter of the mesopores. The confinement-induced reductions in phase transition temperature follow the Gibbs–Thomson equation if a 1.60 nm non-freezable interfacial layer is postulated to exist along the pore wall. Isothermal pressure-dependent infrared spectroscopy reveals a similar modification to phase transition pressures, with the confined ionic liquid requiring higher pressures to trigger phase transformation than the unconfined system. Confinement also impedes ion transport as activation energies are elevated when the ionic liquid is placed inside the mesopores. Finally, the antiferromagnetic ordering that characterizes unconfined [C2mim]FeCl4 is suppressed when the ionic liquid is confined in 5.39-nm pores. Thus, confinement provides another avenue for manipulating the magnetic properties of this compound.

Principles of Small-Molecule Transport through Synthetic Nanopores

Synthetic nanopores made from DNA replicate the key biological processes of transporting molecular cargo across lipid bilayers. Understanding transport across the confined lumen of the nanopores is of fundamental interest and of relevance to their rational design for biotechnological applications. Here we reveal the transport principles of organic molecules through DNA nanopores by synergistically combining experiments and computer simulations. Using a highly parallel nanostructured platform, we synchronously measure the kinetic flux across hundreds of individual pores to obtain rate constants. The single-channel transport kinetics are close to the theoretical maximum, while selectivity is determined by the interplay of cargo charge and size, the pores' sterics and electrostatics, and the composition of the surrounding lipid bilayer. The narrow distribution of transport rates implies a high structural homogeneity of DNA nanopores. The molecular passageway through the nanopore is elucidated via coarse-grained constant-velocity steered molecular dynamics simulations. The ensemble simulations pinpoint with high resolution and statistical validity the selectivity filter within the channel lumen and determine the energetic factors governing transport. Our findings on these synthetic pores' structure-function relationship will serve to guide their rational engineering to tailor transport selectivity for cell biological research, sensing, and drug delivery.

Conical Nanotubes Synthesized by Atomic Layer Deposition of Al2O3, TiO2, and SiO2 in Etched Ion-Track Nanochannels

Etched ion-track polycarbonate membranes with conical nanochannels of aspect ratios of ~3000 are coated with Al₂O₃, TiO₂, and SiO₂ thin films of thicknesses between 10 and 20 nm by atomic layer deposition (ALD). By combining ion-track technology and ALD, the fabrication of two kinds of functional structures with customized surfaces is presented: (i) arrays of free-standing conical nanotubes with controlled geometry and wall thickness, interesting for, e.g., drug delivery and surface wettability regulation, and (ii) single nanochannel membranes with inorganic surfaces and adjustable isoelectric points for nanofluidic applications.

From Water Solutions to Ionic Liquids with Solid State Nanopores as a Perspective to Study Transport and Translocation Phenomena

Solid state nanopores are single-molecular devices governed by nanoscale physics with a broad potential for technological applications. However, the control of translocation speed in these systems is still limited. Ionic liquids are molten salts which are commonly used as alternate solvents enabling the regulation of the chemical and physical interactions on solid-liquid interfaces. While their combination can be challenging to the understanding of nanoscopic processes, there has been limited attempts on bringing these two together. While summarizing the state of the art and open questions in these fields, several major advances are presented with a perspective on the next steps in the investigations of ionic-liquid filled nanopores, both from a theoretical and experimental standpoint. By analogy to aqueous solutions, it is argued that ionic liquids and nanopores can be combined to provide new nanofluidic functionalities, as well as to help resolve some of the pertinent problems in understanding transport phenomena in confined ionic liquids and providing better control of the speed of translocating analytes.

Ion Dynamics of Monomeric Ionic Liquids Polymerized In Situ within Silica Nanopores

Polymerized ionic liquids are a promising class of versatile solid-state electrolytes for applications ranging from electrochemical energy storage to flexible smart materials that remain limited by their relatively low ionic conductivities compared to conventional electrolytes. Here, we show that the in situ polymerization of the vinyl cationic monomer, 1-ethyl-3-vinylimidazolium with the bis(trifluoromethanesulfonyl)imide counteranion, under nanoconfinement within 7.5 ± 1.0 nm diameter nanopores results in a nearly 1000-fold enhancement in the ionic conductivity compared to the material polymerized in bulk. Using insights from broadband dielectric and Raman spectroscopic techniques, we attribute these results to the role of confinement on molecular conformations, ion coordination, and subsequently the ionic conductivity in the polymerized ionic liquid. These results contribute to the understanding of the dynamics of nanoconfined molecules and show that in situ polymerization under nanoscale geometric confinement is a promising path toward enhancing ion conductivity in polymer electrolytes.

Nanocapillary confinement of imidazolium based ionic liquids

Room temperature ionic liquids are salts which are molten at or around room temperature without any added solvent or solution. In bulk they exhibit glass like dependence of conductivity with temperature as well as coupling of structural and transport properties. Interfaces of ionic liquids have been found to induce structural changes with evidence of long range structural ordering on solid-liquid interfaces spanning length scales of 10-100 nm. Our aim is to characterize the influence of confinement on the structural properties of ionic liquids. We present the first conductivity measurements on ionic liquids of the imidazolium type in single conical glass nanopores with confinements as low as tens of nanometers. We probe glassy dynamics of ionic liquids in a large range of temperatures (-20 to 70 °C) and nanopore opening sizes (20-600 nm) in silica glass nanocapillaries. Our results indicate no long range freezing effects due to confinement in nanopores with diameters as low as 20 nm. The studied ionic liquids are found to behave as glass like liquids across the whole accessible confinement size and temperature range.

Surface coatings for solid-state nanopores

Since their introduction in 2001, solid-state nanopores have been increasingly exploited for the detection and characterization of biomolecules ranging from single DNA strands to protein complexes. A major factor that enables the application of nanopores to the analysis and characterization of a broad range of macromolecules is the preparation of coatings on the pore wall to either prevent non-specific adhesion of molecules or to facilitate specific interactions of molecules of interest within the pore. Surface coatings can therefore be useful to minimize clogging of nanopores or to increase the residence time of target analytes in the pore. This review article describes various coatings and their utility for changing pore diameters, increasing the stability of nanopores, reducing non-specific interactions, manipulating surface charges, enabling interactions with specific target molecules, and reducing the noise of current recordings through nanopores. We compare the coating methods with respect to the ease of preparing the coating, the stability of the coating and the requirement for specialized equipment to prepare the coating.

A Direct Sensor to Measure Minute Liquid Flow Rates

Nanofluidics finds its root in the study of fluids and flows at the nanoscale. Flow rate is a quantity that is both central when dealing with flows and notoriously difficult to measure experimentally at the scale of an individual nanopore or nanochannel. We show in this letter that minute flow rate can be directly measured accumulating liquid over time within the compliant membrane of a commercial piezoresistive pressure sensor. Our flow rate sensor is versatile and can be operated independently of the nature of the liquid, flow profile, and type of nanochannel. We demonstrate this method by measuring the pressure-driven flow of silicon oil in a single nanochannel of average radius 200 nm. This approach gives reliable measurement of the flow rate up to 1 pL/min. Unlike other nanoscale flow measurements methods based, for instance, on particle tracking, our sensor delivers a direct voltage output suitable for nanoflow control applications.

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

The ionic conductivity of silica ionogel based solid electrolyte on meso and nanoscales is measured.

Achieving enhanced ionic mobility in nanoporous silica by controlled surface interactions

Upon chemical modification of the silica surface the ionic mobility is increased by one order of magnitude inside the nano-pores.

Ionic liquid prolongs DNA translocation through graphene nanopores

Ionic liquid molecules interact strongly with DNA and effectively reduce its translocation speed via graphene nanopore.

Interface and core relaxation dynamics of IL molecules in nanopores of ordered mesoporous MCM-41: a dielectric spectroscopy study

The dynamics of ionic liquid molecules confined in ordered porous MCM-41 has been investigated by dielectric measurements and two types of confined states of IL were found; pore wall surface adsorbed IL and core of pore filled IL.

Rectification of nanopores in aprotic solvents--transport properties of nanopores with surface dipoles

Nanopores have become a model system to understand transport properties at the nanoscale. We report experiments and modeling of ionic current in aprotic solvents with different dipole moments through conically shaped nanopores in a polycarbonate film and through glass nanopipettes. We focus on solutions of the salt LiClO4, which is of great importance in modeling lithium based batteries. Results presented suggest ion current rectification observed results from two effects: (i) adsorption of Li(+) ions to the pore walls, and (ii) a finite dipole moment rendered by adsorbed solvent molecules. Properties of surfaces in various solvents were probed by means of scanning ion conductance microscopy, which confirmed existence of an effectively positive surface potential in aprotic solvents with high dipole moments.

Identification of single nucleotides in MoS2 nanopores

The size of the sensing region in solid-state nanopores is determined by the size of the pore and the thickness of the pore membrane, so ultrathin membranes such as graphene and single-layer molybdenum disulphide could potentially offer the necessary spatial resolution for nanopore DNA sequencing. However, the fast translocation speeds (3,000-50,000 nt ms(-1)) of DNA molecules moving across such membranes limit their usability. Here, we show that a viscosity gradient system based on room-temperature ionic liquids can be used to control the dynamics of DNA translocation through MoS2 nanopores. The approach can be used to statistically detect all four types of nucleotide, which are identified according to current signatures recorded during their transient residence in the narrow orifice of the atomically thin MoS2 nanopore. Our technique, which exploits the high viscosity of room-temperature ionic liquids, provides optimal single nucleotide translocation speeds for DNA sequencing, while maintaining a signal-to-noise ratio higher than 10.

DNA nanopore translocation in glutamate solutions

Nanopore experiments have traditionally been carried out with chloride-based solutions. Here we introduce silver/silver-glutamate-based electrochemistry as an alternative, and study the viscosity, conductivity, and nanopore translocation characteristics of potassium-, sodium-, and lithium-glutamate solutions. We show that it has a linear response at typical voltages and can be used to detect DNA translocations through a nanopore. The glutamate anion also acts as a redox-capable thickening agent, with high-viscosity solutions capable of slowing down the DNA translocation process by up to 11 times, with a corresponding 7 time reduction in signal. These results demonstrate that glutamate can replace chloride as the primary anion in nanopore resistive pulse sensing.

Accurate characterization of single track-etched, conical nanopores

Deviation from cone geometry significantly influences the ion current rectification through track-etched nanopores with tip radii smaller than 10 nm.

The effect of electronic energy loss on irradiation-induced grain growth in nanocrystalline oxides

The unraveling disorder-driven grain growth mechanism may be utilized to control grain sizes and tailor the functionality of nanocrystalline materials.

Rectification of ion current in nanopipettes by external substrates

We describe ion distribution and the current-voltage (i-V) response of nanopipettes at different probe-to-substrate distances (Dps) as simulated by finite-element methods. Results suggest electrostatic interactions between a charged substrate and the nanopipette dominate electrophoretic ion transport through the nanopipette when Dps is within 1 order of magnitude of the Debye length (∼10 nm for a 1 mM solution as employed in the simulation). Ion current rectification (ICR) and permselectivity associated with a neutral or charged nanopipette can be reversibly enhanced or reduced dependent on Dps, charge polarity, and charge density (σ) of the substrate. Regulation of nanopipette current is a consequence of the enrichment or depletion of ions within the nanopipette interior, which influences conductivity of the nanopipette. When the external substrate is less negatively charged than the nanopipette, the substrate first reduces, and then enhances the ICR as Dps decreases. Surprisingly, both experimental and simulated data show that a neutral substrate was also able to reduce and reverse the ICR of a slightly negatively charged nanopipette. Simulated results ascribe such effects to the elimination of ion depletion within the nanopipette at positive potentials.

Review article: Fabrication of nanofluidic devices

Thanks to its unique features at the nanoscale, nanofluidics, the study and application of fluid flow in nanochannels/nanopores with at least one characteristic size smaller than 100 nm, has enabled the occurrence of many interesting transport phenomena and has shown great potential in both bio- and energy-related fields. The unprecedented growth of this research field is apparently attributed to the rapid development of micro/nanofabrication techniques. In this review, we summarize recent activities and achievements of nanofabrication for nanofluidic devices, especially those reported in the past four years. Three major nanofabrication strategies, including nanolithography, microelectromechanical system based techniques, and methods using various nanomaterials, are introduced with specific fabrication approaches. Other unconventional fabrication attempts which utilize special polymer properties, various microfabrication failure mechanisms, and macro/microscale machining techniques are also presented. Based on these fabrication techniques, an inclusive guideline for materials and processes selection in the preparation of nanofluidic devices is provided. Finally, technical challenges along with possible opportunities in the present nanofabrication for nanofluidic study are discussed.

High-temperature gating of solid-state nanopores with thermo-responsive macromolecular nanoactuators in ionic liquids

Stimuli-responsive nanofluidic systems in room temperature ionic liquids (RTILs). The nanofluidic device can withstand high temperatures up to 200 °C, in which conventional water-based smart materials and nanodevices are invalid. The smart nanopores can be "irreversibly" turned off above the transition temperature of ca. 120-150 °C, actuated by the conformational change of the chemically-modified polymer brushes.

Single-nanopore investigations with ion conductance microscopy

A three-electrode scanning ion conductance microscope (SICM) was used to investigate the local current-voltage properties of a single nanopore. In this experimental configuration, the response measured is a function of changes in the resistances involved in the pathways of ion migration. Single-nanopore membranes utilized in this study were prepared with an epoxy painting procedure to isolate a single nanopore from a track-etch multipore membrane. Current-voltage responses measured with the SICM probe in the vicinity of a single nanopore were investigated in detail and agreed well with equivalent circuit models proposed in this study. With this modified SICM, the current-voltage responses characterized for the case of a single cylindrical pore and a single conical pore exhibit distinct conductance properties that originate from the geometry of nanopores.

Rectification of nanopores at surfaces

At the nanoscale, methods to measure surface charge can prove challenging. Herein we describe a general method to report surface charge through the measurement of ion current rectification of a nanopipette brought in close proximity to a charged substrate. This method is able to discriminate between charged cationic and anionic substrates when the nanopipette is brought within distances from ten to hundreds of nanometers from the surface. Further studies of the pH dependence on the observed rectification support a surface-induced mechanism and demonstrate the ability to further discriminate between cationic and nominally uncharged surfaces. This method could find application in measurement and mapping of heterogeneous surface charges and is particularly attractive for future biological measurements, where noninvasive, noncontact probing of surface charge will prove valuable.

Structural modification of nanocrystalline ceria by ion beams

Exceptional size-dependent electronic-ionic conductivity of nanostructured ceria can significantly alter materials properties in chemical, physical, electronic and optical applications. Using energetic ions, we have demonstrated effective modification of interface volume and grain size in nanocrystalline ceria from a few nm up to ∼25 nm, which is the critical region for controlling size-dependent material property. The grain size increases and follows an exponential law as a function of ion fluence that increases with temperature, while the cubic phase is stable under the irradiation. The unique self-healing response of radiation damage at grain boundaries is utilized to control the grain size at the nanoscale. Structural modification by energetic ions is proposed to achieve desirable electronic-ionic conductivity.

Pink noise of ionic conductance through single artificial nanopores revisited

We report voltage-clamp measurements through single conical nanopore obtained by chemical etching of a single ion track in polyimide film. Special attention is paid to the pink noise of the ionic current (i.e., 1/f noise) measured with different filling liquids. The relative pink-noise amplitude is almost independent of concentration and pH for KCl solutions, but varies strongly using ionic liquids. In particular, we show that depending on the ionic liquid, the transport of charge carriers is strongly facilitated (low noise and higher conductivity than in the bulk) or jammed. These results show that the origin of the pink noise can be ascribed neither to fluctuations of the pore geometry nor to the pore wall charges, but rather to a cooperative effect on ions motion in confined geometry.

Current oscillations generated by precipitate formation in the mixing zone between two solutions inside a nanopore

Unlike biological protein pores in lipid membranes, nanopores fabricated in synthetic materials can withstand a wide range of environmental conditions including the presence of organic solvents. This capability expands the potential of synthetic nanopores to monitor chemical reactions occurring at the interface between solutions of organic and aqueous character. In this work, nanopores fabricated in borosilicate glass or silicon nitride connected a predominantly organic solvent to an aqueous solvent, thereby generating a mixing zone between these solutions inside the pore. This configuration was exploited to precipitate small organic molecules with low aqueous solubility inside the nanopores, and concomitantly, to monitor this precipitation by the decrease of the ionic conductance through the nanopores over time. Hence, this method provides a means to induce and investigate the formation of nanoprecipitates or nanoparticles. Interestingly, precipitates with a slight electric charge were cleared from the pore, causing the conductance of the pore to return to its original value. This process repeated, resulting in stable oscillations of the ionic current. Although such oscillations might be useful in fluidic logic circuits, few conditions capable of generating oscillations in ionic currents have been reported. The frequency and amplitude of oscillations could be tuned by changing the concentration of the precipitating molecule, the pH of the electrolyte, and the applied potential bias. In addition to generating oscillations, nanopores that separate two different solutions may be useful for monitoring and mediating chemical reactions in the mixing zone between two solutions.

Electroosmotic flow can generate ion current rectification in nano- and micropores

This paper introduces a strategy for generating ion current rectification through nano- and micropores. This method generates ion current rectification by electroosmotic-driven flow of liquids of varying viscosity (and hence varying conductance) into or out of the narrowest constriction of a pore. The magnitude of current rectification was described by a rectification factor, R(f), which is defined by the ratio of the current measured at a positive voltage divided by the current measured at a negative voltage. This method achieved rectification factors in the range of 5-15 using pores with diameters ranging from 10 nm to 2.2 microm. These R(f) values are similar to the rectification factors reported in other nanopore-based methods that did not employ segmented surface charges. Interestingly, this work showed that in cylindrical nanopores with diameters of 10 nm and a length of at least 275 nm, electroosmotic flow was present and could generate ion current rectification. Unlike previous methods for generating ion current rectification that require nanopores with diameters comparable to the Debye length, this work demonstrated ion current rectification in micropores with diameters 500 times larger than the Debye length. Thus this method extends the concept of fluidic diodes to the micropore range. Several experiments designed to alter or remove electroosmotic flow through the pore demonstrated that electroosmotic flow was required for the mode of ion current rectification reported here. Consequently, the magnitude of current rectification could be used to indicate the presence of electroosmotic flow and the breakdown of electroosmotic flow with decreasing ionic strength and hence increasing electric double layer overlap inside nanopores.

Pressure-driven ionic transport through nanochannels with inhomogenous charge distributions

The effect of spatially inhomogeneous fixed charge distributions on the pressure-driven transport of ions through cylindrical nanopores have been investigated theoretically by means of an approximate version of the Poisson-Nernst-Planck model that can be used with confidence for moderately charged nanopores with radius smaller than the Debye screening length of the system. Salt rejection rate has been computed as a function of the applied pressure difference for various one-dimensional (1D) unipolar charge distributions and has been compared with that obtained for a homogeneously charged nanochannel with an identical average volume charge density. The ion rejection capabilities of charged nanopores can be optimized by an appropriate distribution of the fixed charge concentration. When ions are forced to enter the nanopores by the end with the lowest fixed charged concentration, the salt rejection rate exhibits a nonmonotonous variation with the applied pressure. This phenomenon has been attributed to the influence of the inhomogeneous charge distribution on the electric field that arises spontaneously so as to maintain the electroneutrality within the nanopore.

DNA strands from denatured duplexes are translocated through engineered protein nanopores at alkaline pH

Nanopores are under development for the detection of a variety of analytes and the investigation of chemical reactions at the single molecule level. In particular, the analysis of nucleic acid molecules is under intense investigation, including the development of systems for rapid, low-cost DNA sequencing. Here, we show that DNA can be translocated through an engineered alphaHL protein pore at pH 11.7, a value at which dsDNA is denatured. Therefore, the alphaHL pore is sufficiently stable to entertain the possibility of direct nanopore sequencing of genomic dsDNA samples, which are more readily obtained and handled than ssDNA.

Molecular simulation of ionic liquids: current status and future opportunities

Ionic liquids are salts that are liquid near ambient conditions. Interest in these unusual compounds has exploded in the last decade, both at the academic and commercial level. Molecular simulations based on classical potentials have played an important role in helping researchers understand how condensed phase properties of these materials are linked to chemical structure and composition. Simulations have also predicted many properties and unexpected phenomena that have subsequently been confirmed experimentally. The beneficial impact molecular simulations have had on this field is due in large part to excellent timing. Just when computing power and simulation methods matured to the point where complex fluids could be studied in great detail, a new class of materials virtually unknown to experimentalists came on the scene and demanded attention. This topical review explores some of the history of ionic liquid molecular simulations, and then gives examples of the recent use of molecular dynamics and Monte Carlo simulation in understanding the structure of ionic liquids, the sorption of small molecules in ionic liquids, the nature of ionic liquids in the vapor phase and the dynamics of ionic liquids. This review concludes with a discussion of some of the outstanding problems facing the ionic liquid modeling community and how condensed phase molecular simulation experts not presently working on ionic liquids might help advance the field.