Diffusional Motion of a Particle Translocating through a Nanopore

The influence of diffusional motion on the capture and release of individual nanoparticles as they are driven through a conical-shaped glass nanopore membrane (GNM) by pressure-induced flow is reported. In these experiments, one to several hundred particles are driven through the orifice of the nanopore. Following the initial translocation, the pressure is reversed and the particles are driven through the GNM orifice in the reverse direction. The resistive-pulse technique is used to monitor the temporal sequence of particle capture and release translocations. The size of the particles (120–160 nm) and the direction of translocation can be determined from the pulse amplitude and shape. The stochastic influence of diffusion on particle trajectories has been investigated, including instantaneous transfer rate, release probability, and cumulative release success rate. We demonstrate that the sequence of particle translocations in the capture step (<i>a</i>, <i>b</i>, <i>c</i>... where the letters represent different particles) is largely preserved and can be read out by resistive-pulse signature during the release translocations (...<i>c</i>, <i>b</i>, <i>a</i>). The observed stochastic events are in good agreement with a convective diffusion model of particle trajectory within the confined geometry of the nanopore. The pressure-reversal technique opens new avenues for chemical analysis of particles using resistive-pulse methods

Electrochemical Generation of a Hydrogen Bubble at a Recessed Platinum Nanopore Electrode

We report the electrochemical generation of a <i>single</i> hydrogen bubble within the cavity of a recessed Pt nanopore electrode. The recessed Pt electrode is a conical pore in glass that contains a micrometer-scale Pt disk (1–10 μm radius) at the nanopore base and a nanometer-scale orifice (10–100 nm radius) that restricts diffusion of electroactive molecules and dissolved gas between the nanopore cavity and bulk solution. The formation of a H₂ bubble at the Pt disk electrode in voltammetric experiments results from the reduction of H⁺ in a 0.25 M H₂SO₄ solution; the liquid-to-gas phase transformation is indicated in the voltammetric response by a precipitous decrease in the cathodic current due to rapid bubble nucleation and growth within the nanopore cavity. Finite element simulations of the concentration distribution of dissolved H₂ within the nanopore cavity, as a function of the H⁺ reduction current, indicate that H₂ bubble nucleation at the recessed Pt electrode surface occurs at a critical supersaturation concentration of ∼0.22 M, in agreement with the value previously obtained at (nonrecessed) Pt disk electrodes (∼0.25 M). Because the nanopore orifice limits the diffusion of H₂ out of the nanopore cavity, an anodic peak corresponding to the oxidation of gaseous and dissolved H₂ trapped in the recessed cavity is readily observed on the reverse voltammetric scan. Integration of the charge associated with the H₂ oxidation peak is found to approach that of the H⁺ reduction peak at high scan rates, confirming the assignment of the anodic peak to H₂ oxidation. Preliminary results for the electrochemical generation of O₂ bubbles from water oxidation at a recessed nanopore electrode are consistent with the electrogeneration of H₂ bubbles

Single-Molecule Electrical Currents Associated with Valinomycin Transport of K⁺

A quantitative description of ionophore-mediated ion transport is important in understanding ionophore activity in biological systems and developing ionophore applications. Herein, we describe the direct measurement of the electrical current resulting from K+ transport mediated by individual valinomycin (val) ionophores. Step fluctuations in current measured across a 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) bilayer suspended over a ∼400 nm radius glass nanopore result from dynamic partitioning of val between the bilayer and torus region, effectively increasing or decreasing the total number of val present in the membrane. In our studies, approximately 30 val are present in the membrane on average with a val entering or leaving the bilayer approximately every 50 s, allowing measurement of changes in electrical current associated with individual val. The single-molecule val(K+) transport current at 0.1 V applied potential is (1.3 ± 0.6) × 10–15 A, consistent with estimates of the transport kinetics based on large val ensembles. This methodology for analyzing single ionophore transport is general and can be applied to other carrier-type ionophores

Resistive Pulse Delivery of Single Nanoparticles to Electrochemical Interfaces

An experimental system for controlling and interrogating the collisions of <i>individual</i> nanoparticles at electrode/electrolyte interfaces is described. A nanopipet positioned over a 400 nm radius Pt ultramicroelectrode is used to deliver individual nanoparticles, via pressure-driven solution flow, to the underlying electrode, where the nanoparticles undergo collisions and are detected electrochemically. High-velocity collisions result in elastic collisions of negatively charged polystyrene nanospheres at the Pt/water interface, while low-velocity collisions result in nanoparticle adsorption (“sticky” collisions). The ability to position the nanopipet with respect to the underlying ultramicroelectrode also allows the time between particle release from the nanopipet and electrode collision to be investigated as a function of nanopipet–electrode separation, <i>d</i>. The time between release and collision of the nanoparticle is found to be proportional to <i>d</i>³, in excellent agreement with an analytical expression for convective fluid flow from a pipet orifice

Tunable Negative Differential Electrolyte Resistance in a Conical Nanopore in Glass

Liquid-phase negative differential resistance (NDR) is observed in the <i>i–V</i> behavior of a conical nanopore (∼300 nm orifice radius) in a glass membrane that separates an external <i>low-conductivity</i> 5 mM KCl solution of dimethylsulfoxide (DMSO)/water (v/v 3:1) from an internal <i>high-conductivity</i> 5 mM KCl aqueous solution. NDR appears in the <i>i–V</i> curve of the negatively charged nanopore as the voltage-dependent electro-osmotic force opposes an externally applied pressure force, continuously moving the location of the interfacial zone between the two miscible solutions to a position just inside the nanopore orifice. An ∼80% decrease in the ionic current occurs over less that a ∼10 mV increase in applied voltage. The NDR turn-on voltage was found to be tunable over a ∼1 V window by adjusting the applied external pressure from 0 to 50 mmHg. Finite-element simulations based on solution of Navier–Stokes, Poisson, and convective Nernst–Planck equations for mixed solvent electrolytes within a negatively charged nanopore yield predictions of the NDR behavior that are in qualitative agreement with the experimental observations. Applications in chemical sensing of a tunable, solution-based electrical switch based on the NDR effect are discussed

Resistive-Pulse Detection of Multilamellar Liposomes

The resistive-pulse method was used to monitor the pressure-driven translocation of multilamellar liposomes with radii between 190 and 450 nm through a single conical nanopore embedded in a glass membrane. Liposomes (0% and 5% 1,2-dioleoyl-<i>sn</i>-glycero-3-phospho-l-serine (sodium salt) in 1,2-dilauroyl-<i>sn</i>-glycero-3-phosphocholine or 0%, 5%, and 9% 1,2-dipalmitoyl-<i>sn</i>-glycero-3-phospho­(1′-<i>rac</i>-glycerol) (sodium salt) in 1,2-dipalmitoyl-<i>sn</i>-glycero-3-phosphocholine) were prepared by extrusion through a polycarbonate membrane. Liposome translocation through a glass nanopore was studied as a function of nanopore size and the temperature relative to the lipid bilayer transition temperature, <i>T</i>_c. All translocation events through pores larger than the liposome, regardless of temperature, show translocation times between 30 and 300 μs and current pulse heights between 0.2% and 15% from the open pore baseline. However, liposomes at temperatures below the <i>T</i>_c were captured at the pore orifice when translocation was attempted through pores of smaller dimensions, but squeezed through the same pores when the temperature was raised above <i>T</i>_c. The results provide insights into the deformation and translocation of individual liposomes through a porous material

Electrochemical Nucleation of Stable N₂ Nanobubbles at Pt Nanoelectrodes

Exploring the nucleation of gas bubbles at interfaces is of fundamental interest. Herein, we report the nucleation of individual N₂ nanobubbles at Pt nanodisk electrodes (6–90 nm) via the irreversible electrooxidation of hydrazine (N₂H₄ → N₂ + 4H⁺ + 4e^–). The nucleation and growth of a stable N₂ nanobubble at the Pt electrode is indicated by a sudden drop in voltammetric current, a consequence of restricted mass transport of N₂H₄ to the electrode surface following the liquid-to-gas phase transition. The critical surface concentration of dissolved N₂ required for nanobubble nucleation, <i>C</i>_{N₂,critical}^s, obtained from the faradaic current at the moment just prior to bubble formation, is measured to be ∼0.11 M and is independent of the electrode radius and the bulk N₂H₄ concentration. Our results suggest that the size of stable gas bubble nuclei depends only on the local concentration of N₂ near the electrode surface, consistent with previously reported studies of the electrogeneration of H₂ nanobubbles. <i>C</i>_{N₂,critical}^s is ∼160 times larger than the N₂ saturation concentration at room temperature and atmospheric pressure. The residual current for N₂H₄ oxidation after formation of a stable N₂ nanobubble at the electrode surface is proportional to the N₂H₄ concentration as well as the nanoelectrode radius, indicating that the dynamic equilibrium required for the existence of a stable N₂ nanobubble is determined by N₂H₄ electrooxidation at the three phase contact line

Ion Transport within High Electric Fields in Nanogap Electrochemical Cells

Ion transport near an electrically charged electrolyte/electrode interface is a fundamental electrochemical phenomenon that is important in many electrochemical energy systems. We investigated this phenomenon using lithographically fabricated thin-layer electrochemical cells comprising two Pt planar electrodes separated by an electrolyte of nanometer thickness (50–200 nm). By exploiting redox cycling amplification, we observed the influence of the electric double layer on transport of a charged redox couple within the confined electrolyte. Nonclassical steady-state peak shaped voltammograms for redox cycling of the ferrocenylmethyltrimethylammonium redox couple (FcTMA^+/2+) at low concentrations of supporting electrolyte (≤10 mM) results from electrostatic interactions between the redox ions and the charged Pt electrodes. This behavior contrasts to sigmoidal voltammograms with a diffusion-limited plateau observed in the same electrochemical cells in the presence of sufficient electrolyte to screen the electrode surface charge (200 mM). Moreover, steady-state redox cycling was depressed significantly within the confined electrolyte as the supporting electrolyte concentration was decreased or as the cell thickness was reduced. The experimental results are in excellent agreement with predictions from finite-element simulations coupling the governing equations for ion transport, electric fields, and the redox reactions. Double layer effects on ion transport are generally anticipated in highly confined electrolyte and may have implications for ion transport in thin layer and nanoporous energy storage materials

Sizing Individual Au Nanoparticles in Solution with Sub-Nanometer Resolution

Resistive-pulse sensing has generated considerable interest as a technique for characterizing nanoparticle suspensions. The size, charge, and shape of individual particles can be estimated from features of the resistive pulse, but the technique suffers from an inherent variability due to the stochastic nature of particles translocating through a small orifice or channel. Here, we report a method, and associated automated instrumentation, that allows repeated pressure-driven translocation of individual particles back and forth across the orifice of a conical nanopore, greatly reducing uncertainty in particle size that results from streamline path distributions, particle diffusion, particle asphericity, and electronic noise. We demonstrate ∼0.3 nm resolution in measuring the size of nominally 30 and 60 nm radius Au nanoparticles of spherical geometry; Au nanoparticles in solution that differ by ∼1 nm in radius are readily distinguished. The repetitive translocation method also allows differentiating particles based on surface charge density, and provides insights into factors that determine the distribution of measured particle sizes

Sizing Individual Au Nanoparticles in Solution with Sub-Nanometer Resolution

Resistive-pulse sensing has generated considerable interest as a technique for characterizing nanoparticle suspensions. The size, charge, and shape of individual particles can be estimated from features of the resistive pulse, but the technique suffers from an inherent variability due to the stochastic nature of particles translocating through a small orifice or channel. Here, we report a method, and associated automated instrumentation, that allows repeated pressure-driven translocation of individual particles back and forth across the orifice of a conical nanopore, greatly reducing uncertainty in particle size that results from streamline path distributions, particle diffusion, particle asphericity, and electronic noise. We demonstrate ∼0.3 nm resolution in measuring the size of nominally 30 and 60 nm radius Au nanoparticles of spherical geometry; Au nanoparticles in solution that differ by ∼1 nm in radius are readily distinguished. The repetitive translocation method also allows differentiating particles based on surface charge density, and provides insights into factors that determine the distribution of measured particle sizes