17 research outputs found

    Resistive Pulse Delivery of Single Nanoparticles to Electrochemical Interfaces

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    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><sup>3</sup>, in excellent agreement with an analytical expression for convective fluid flow from a pipet orifice

    Ion Transport within High Electric Fields in Nanogap Electrochemical Cells

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    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<sup>+/2+</sup>) 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

    Critical Nuclei Size, Rate, and Activation Energy of H<sub>2</sub> Gas Nucleation

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    Electrochemical measurements of the nucleation rate of individual H<sub>2</sub> bubbles at the surface of Pt nanoelectrodes (radius = 7–41 nm) are used to determine the critical size and geometry of H<sub>2</sub> nuclei leading to stable bubbles. Precise knowledge of the H<sub>2</sub> concentration at the electrode surface, <i>C</i><sub>H<sub>2</sub></sub><sup>surf</sup>, is obtained by controlled current reduction of H<sup>+</sup> in a H<sub>2</sub>SO<sub>4</sub> solution. Induction times of single-bubble nucleation events are measured by stepping the current, to control <i>C</i><sub>H<sub>2</sub></sub><sup>surf</sup>, while monitoring the voltage. We find that gas nucleation follows a first-order rate process; a bubble spontaneously nucleates after a stochastic time delay, as indicated by a sudden voltage spike that results from impeded transport of H<sup>+</sup> to the electrode. Hundreds of individual induction times, at different applied currents and using different Pt nanoelectrodes, are used to characterize the kinetics of phase nucleation. The rate of bubble nucleation increases by four orders of magnitude (0.3–2000 s<sup>–1</sup>) over a very small relative change in <i>C</i><sub>H<sub>2</sub></sub><sup>surf</sup> (0.21–0.26 M, corresponding to a ∼0.025 V increase in driving force). Classical nucleation theory yields thermodynamic radii of curvature for critical nuclei of 4.4 to 5.3 nm, corresponding to internal pressures of 330 to 270 atm, and activation energies for nuclei formation of 14 to 26 <i>kT</i>, respectively. The dependence of nucleation rate on H<sub>2</sub> concentration indicates that nucleation occurs by a heterogeneous mechanism, where the nuclei have a contact angle of ∼150° with the electrode surface and contain between 35 and 55 H<sub>2</sub> molecules

    Redox Cycling in Nanogap Electrochemical Cells. The Role of Electrostatics in Determining the Cell Response

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    Ion transport near interfaces is a fundamental phenomenon of importance in electrochemical, biological, and colloidal systems. In particular, electric double layers in highly confined spaces have implications for ion transport in nanoporous energy storage materials. By exploiting redox cycling amplification in lithographically fabricated thin-layer electrochemical cells comprising two platinum electrodes separated by a distance of 150–450 nm, we observed current enhancement during cyclic voltammetry of the hexaamine­ruthenium­(III) chloride redox couple (Ru­(NH<sub>3</sub>)<sub>6</sub><sup>3/2+</sup>) at low supporting electrolyte concentrations, resulting from ion enrichment of Ru­(NH<sub>3</sub>)<sub>6</sub><sup>3/2+</sup> in the electrical double layers and an enhanced ion migration contribution to mass transport. The steady-state redox cycling was shown to decrease to predominately diffusion controlled level with increasing supporting electrolyte concentration. Through independent biasing of the potential on the individual Pt electrodes, the voltammetric transport limited current can be controlled without changing the electrochemical nature at the system. Using finite-element simulations based on numerical solutions to the Poisson and Nernst–Planck equations with Butler–Volmer type boundary conditions, we are able to semiquantitatively predict the voltammetric behavior of the nanogap cell that results from coupling of surface electrostatics and ion transport

    Multipass Resistive-Pulse Observations of the Rotational Tumbling of Individual Nanorods

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    Rotational tumbling of nanorods as they translocate through a glass nanopipet modulates the flux of charge carrying ions, generating a resistive pulse with multiple peaks. The measured times between maxima and minima in the resistive pulse correspond to an average rotation of approximately 90° and can be used to compute the rotational diffusion coefficient, <i>D</i><sub>r</sub>. Analytical expressions for the rotational diffusion coefficient (<i>D</i><sub>r</sub>) in terms of the nanorod length (<i>L</i>) allow the calculation of the rod length. We report experiments in which an individual Au nanorod (nominal length of 77–122 nm) is driven repeatedly through the nanopipet orifice by voltage switching at up to 30 Hz, allowing rapid measurement of <i>D</i><sub>r</sub> and <i>L</i> of individual nanorods with ∼15% error. Measured values of <i>D</i><sub>r</sub> between 2000 and 4000 rad<sup>2</sup> s<sup>–1</sup> for Au nanorods of 77–122 nm length are in good agreement with theoretical predictions

    Characterization of Solute Distribution Following Iontophoresis from a Micropipet

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    Iontophoresis uses a current to eject solution from the tip of a barrel formed from a pulled glass capillary and has been employed as a method of drug delivery for neurochemical investigations. Much attention has been devoted to resolving perhaps the greatest limitation of iontophoresis, the inability to determine the concentration of substances delivered by ejections. To further address this issue, we evaluate the properties of typical ejections such as barrel solution velocity and its relation to the ejection current using an amperometric and liquid chromatographic approach. These properties were used to predict the concentration distribution of ejected solute that was then confirmed by fluorescence microscopy. Additionally, incorporation of oppositely charged fluorophores into the barrel investigated the role of migration on the mass transport of an ejected species. Results indicate that location relative to the barrel tip is the primary influence on the distribution of ejected species. At short distances (<100 μm), advection from electroosmotic transport of the barrel solution may significantly contribute to the distribution, but this effect can be minimized through the use of low to moderate ejection currents. However, as the distance from the source increases (>100 μm), even solute ejected using high currents exhibits diffusion-limited behavior. Lastly a time-dependent theoretical model was constructed and is used with experimental fluorescent profiles to demonstrate how iontophoresis can generate near-uniform concentration distributions near the ejection source

    Collision and Oxidation of Silver Nanoparticles on a Gold Nanoband Electrode

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    We report the use of gold nanoband electrodes ranging from 60 to 180 nm in width to study collision and oxidation of single Ag nanoparticles (NPs). The use of nanoscale electrodes has enabled the observation of unique single-NP collision responses indicating a strong electrode size effect when the critical dimension of the electrode (the bandwidth) is reduced to that of NPs. In addition to multipeak events, NP collision on a nanoband electrode displays reduced collision frequency, significantly higher probability of single-peak events, and fewer subpeaks. More importantly, the average charge transferred in a single-peak event is about 50% less than that of the first subpeak of a multipeak event. The reduced charge of single-peak collisions and the more frequent appearance on nanoelectrodes are strong evidence that NPs start to behave differently at the electrode/solution interface when the size of the electrode is reduced to be comparable to that of the NPs. The reduced charge is likely due to a weaker particle–electrode interaction when the particle collides on the edge of the nanoband electrode. Random walk numerical simulation was used to further understand the electrode size effect in single-particle collision and oxidation. The simulated results are in good agreement with the experiments. A detailed analysis of the collision signal reveals that a Ag NP is more likely to diffuse away after making its initial contact with a nanoband electrode, due to the electrode’s smaller critical dimension and a possible strong edge effect from the negatively charged silicon nitride/oxide. This study offers a deeper insight into the dynamic collision behavior of metal NPs on the electrode surface

    Electrochemical Generation of Individual O<sub>2</sub> Nanobubbles via H<sub>2</sub>O<sub>2</sub> Oxidation

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    Herein, we use Pt nanodisk electrodes (apparent radii from 4 to 80 nm) to investigate the nucleation of individual O<sub>2</sub> nanobubbles generated by electrooxidation of hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>). A single bubble reproducibly nucleates when the dissolved O<sub>2</sub> concentration reaches ∼0.17 M at the Pt electrode surface. This nucleation concentration is ∼130 times higher than the equilibrium saturation concentration of O<sub>2</sub> and is independent of electrode size. Moreover, in acidic H<sub>2</sub>O<sub>2</sub> solutions (1 M HClO<sub>4</sub>), in addition to producing an O<sub>2</sub> nanobubble through H<sub>2</sub>O<sub>2</sub> oxidation at positive potentials, individual H<sub>2</sub> nanobubbles can also be generated at negative potentials. Alternating generation of single O<sub>2</sub> and H<sub>2</sub> bubbles within the same experiment allows direct comparison of the critical concentrations for nucleation of each nanobubble without knowing the precise size/geometry of the electrode or the exact viscosity/temperature of the solution

    High-Speed Multipass Coulter Counter with Ultrahigh Resolution

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    Coulter counters measure the size of particles in solution by passing them through an orifice and measuring a resistive pulse, <i>i</i>.<i>e</i>., a drop in the ionic current flowing between two electrodes placed on either side of the orifice. The magnitude of the pulse gives information on the size of the particle; however, resolution is limited by variability in the path of the translocation, due to the Brownian motion of the particle. We present a simple yet powerful modified Coulter counter that uses programmable data acquisition hardware to switch the voltage after sensing the resistive pulse of a nanoparticle passing through the orifice of a nanopipet. Switching the voltage reverses the direction of the driving force on the particle and, when this detect–switch cycle is repeated, allows us to pass an individual nanoparticle through the orifice thousands of times. By measuring individual particles more than 100 times per second we rapidly determine the distribution of the resistive pulses for each particle, which allows us to accurately determine the mean pulse amplitude and deliver considerably improved size resolution over a conventional Coulter counter. We show that single polystyrene nanoparticles can be shuttled back and forth and monitored for minutes, leading to a precisely determined mean blocking current equating to sub-angstrom size resolution

    The Dynamic Steady State of an Electrochemically Generated Nanobubble

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    This article describes the dynamic steady state of individual H<sub>2</sub> nanobubbles generated by H<sup>+</sup> reduction at inlaid and recessed Pt nanodisk electrodes. Electrochemical measurements coupled with finite element simulations allow analysis of the nanobubble geometry at dynamic equilibrium. We demonstrate that a bubble is sustainable at Pt nanodisks due to the balance of nanobubble shrinkage due to H<sub>2</sub> dissolution and growth due to H<sub>2</sub> electrogeneration. Specifically, simulations are used to predict stable geometries of the H<sub>2</sub>/Pt/solution three-phase interface and the width of exposed Pt at the disk circumference required to sustain the nanobubble via steady-state H<sub>2</sub> electrogeneration. Experimentally measured currents, <i>i</i><sup>ss</sup>, corresponding to the electrogeneration of H<sub>2</sub>, at or near the three-phase interface, needed to sustain the nanobubble are between 0.2 and 2.4 nA for Pt nanodisk electrodes with radii between 2.5 and 40 nm. However, simple theoretical analysis shows that the diffusion-limited currents required to sustain such a single nanobubble at an inlaid Pt nanodisk are 1–2 orders larger than the observed values. Finite element simulation of the dynamic steady state of a nanobubble at an inlaid disk also demonstrates that the expected steady-state currents are much larger than the experimental currents. Better agreement between the simulated and experimental values of <i>i</i><sup>ss</sup> is obtained by considering recession of the Pt disk nanoelectrode below the plane of the insulating surface, which reduces the outward flux of H<sub>2</sub> from the nanobubble and results in smaller values of <i>i</i><sup>ss</sup>
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