Electron-Transfer Gated Ion Transport in Carbon Nanopipets

Coating the inner wall of a quartz nanopipet with a thin layer of carbon yields a nanopore with tunable surface charge and chemical state for resistive-pulse and rectification sensing. Herein we report the experimental study and modeling of the electron-transfer gated ion transport processes in carbon nanopipets. The potential of the unbiased carbon layer can be tuned by adding very low (sub-nM) concentrations of redox species to the solution via bipolar electrochemistry. The potential of the carbon layer determines the electrical double-layer structure that, in turn, affects the ionic transport processes. The ion current rectification decreased when redox species with a relatively positive formal potential (e.g., Fe(CN)₆^3/4–) were added to the solution and increased upon adding redox species with a negative formal potential (e.g., Ru­(NH₃)₆^3/2+). Additionally, the ion current displays high sensitivity to redox species, suggesting the possibility of trace-level analysis

Toward More Reliable Measurements of Electron-Transfer Kinetics at Nanoelectrodes: Next Approximation

Steady-state voltammetry at nanoelectrodes and scanning electrochemical microscopy (SECM) have recently been used to measure kinetics of several rapid heterogeneous electron transfer (ET) reactions. One problem with those experiments was that the dependence of the shape of the steady-state voltammogram on kinetic parameters becomes weak when the reaction rate approaches the diffusion limit. The possibility to fit the same experimental voltammogram using different combinations of the standard rate constant, transfer coefficient, and standard potential results in significant uncertainties in extracted parameter values. In this article, the reliability of the kinetic analysis was improved by obtaining steady-state voltammograms with both oxidized and reduced forms of redox species initially present in solution. Additional improvements were attained by characterizing the nanoelectrode geometry with the atomic force microscope and using water with a very low level of organic contaminants (TOC ≤ 1 ppb). This approach was used to re-evaluate the ET rate constants measured for several electroactive species, including ferrocene, ferrocenemethanol, 7,7,8,8-tetracyanoquinodimethane (TCNQ), and ferrocyanide at Pt electrodes. The obtained standard rate constants are higher than the values measured earlier at Pt and Au nanoelectrodes but comparable to those obtained in recent nanogap/SECM experiments

Scanning Electrochemical Microscopy of Single Spherical Nanoparticles: Theory and Particle Size Evaluation

Experiments at individual metal nanoparticles (NPs) can provide important information about their electrochemical and catalytic properties. The scanning electrochemical microscope (SECM) equipped with a nanometer-sized tip was recently used to image single 10 or 20 nm gold particles and quantitatively investigate electrochemical reactions occurring at their surfaces. In this Article, the theory is developed for SECM current vs distance curves obtained with a disk-shaped tip approaching a comparably sized, surface-bound conductive or insulating spherical NP. The possibility of evaluating the size of a surface-bound particle by fitting the experimental current–distance curve to the theory is shown for NPs and tips of different radii. The effects of the NP being partially buried into an insulating layer and the imperfect positioning of the tip with respect to the NP center are considered. The collection efficiency is calculated for redox species generated at the nanoparticle surface and collected at the tip

Atomic Force Microscopy of Electrochemical Nanoelectrodes

Nanometer-sized electrodes have recently been used to investigate important chemical and biological systems on the nanoscale. Although nanoelectrodes offer a number of advantages over macroscopic electrochemical probes, visualization of their surfaces remains challenging. Thus, the interpretation of the electrochemical response relies on assumptions about the electrode shape and size prior to the experiment and the changes induced by surface reactions (e.g., electrodeposition). In this paper, we present first AFM images of nanoelectrodes, which provide detailed and unambiguous information about the electrode geometry. The effects of polishing and cleaning nanoelectrodes are investigated, and AFM results are compared to those obtained by voltammetry and SEM. <i>In situ</i> AFM is potentially useful for monitoring surface reactions at nanoelectrodes

Electrochemical Evaluation of the Number of Au Atoms in Polymeric Gold Thiolates by Single Particle Collisions

Polymeric gold thiolates, [Au­(I)­SR]_<i>n</i>, are common synthetic intermediate precursors of gold nanoclusters and larger nanoparticles. The size and dispersity of the precursors strongly influence the properties of the synthesis products. Evaluating the size of the precursors is not straightforward because they are irregularly shaped (nonspherical) and hard to isolate from solution. Herein, we propose an effective method for determining the number of Au atoms in polymeric thiolate particles from current transients resulting from single precursor collisions, where individual [Au­(I)­SR]_<i>n</i> species are electrochemically reduced at the collector ultramicroelectrode. The developed approach can lead to a better control over the mean size and dispersity of colloidal metal nanoclusters and nanoparticles

Diffuse Layer Effect on Electron-Transfer Kinetics Measured by Scanning Electrochemical Microscopy (SECM)

Recent theoretical and experimental studies revealed strong effects of the electrical double layer (EDL) on mass transfer at nanometer-sized electrodes and in electrochemical nanogaps. Although the EDL effect is much stronger in weakly supported media, it can significantly influence the kinetics of electron-transfer processes involving multicharged ionic redox species, even at high concentrations of supporting electrolyte. We measured the kinetics of Fe­(CN)₆^4– oxidation in 1 M KCl solution at the Pt nanoelectrode used as a tip in the scanning electrochemical microscope. The apparent standard rate constant values extracted from tip voltammograms without double-layer correction increased markedly with the decreasing separation distance between the tip and substrate electrodes. The same steady-state voltammograms were fitted to the theory including the EDL effect and yielded the rate constant essentially independent of the separation distance

Scanning Electrochemical Microscopy Study of Electron-Transfer Kinetics and Catalysis at Nanoporous Electrodes

The complicated electrochemical properties of nanoporous electrodes arising from their geometry remain poorly understood because their complex structure defies easy interpretation of experimental results. The large surface area of a porous electrode results in a higher double-layer charging current and faster apparent heterogeneous rate constants, which are difficult to measure by voltammetry and other transient electrochemical techniques. In this article, we used a scanning electrochemical microscope equipped with a nanometer- or micrometer-sized tip to measure the rates of the same electron-transfer process at the flat and nanoporous Au electrodes. The origins and magnitude of the rate constant enhancement at the nanoporous surface (after the roughness factor correction) are discussed. Using the substrate generation/tip collection mode of the scanning electrochemical microscope operation, the higher catalytic activity of nanoporous Au for the oxygen reduction reaction was found from both substrate and tip voltammograms that can also be used for analyzing the reaction products

Surface Patterning Using Diazonium Ink Filled Nanopipette

Molecular grafting of diazonium is a widely employed surface modification technique. Local electrografting of this species is a promising approach to surface doping and related properties tailoring. The instability of diazonium cation complicates this process, so that this species was generated in situ in many reported studies. In this Article, we report the egress transfer of aryl diazonium cation across the liquid/liquid interface supported at the nanopipette tip that can be used for controlled delivery this species to the external aqueous phase for local substrate patterning. An aryl diazonium salt was prepared with weakly coordinating and lipophilic tetrakis­(pentafluorophenyl)­borate anion stable as a solid and soluble in low polarity media. The chemically stable solution of this salt in 1,2-dichloroethane can be used as “diazonium ink”. The ink-filled nanopipette was employed as a tip in the scanning electrochemical microscope (SECM) for surface patterning with the spatial resolution controlled by the pipette orifice radius and a few nanometers film thickness. The submicrometer-size grafted spots produced on the HOPG surface were located and imaged with the atomic force microscope (AFM)

Voltage-Driven Molecular Catalysis: A Promising Approach to Electrosynthesis

The combination of electrocatalysis and molecular catalysis is an increasingly popular approach to designing catalysts for electrosynthetic processes. We recently found that the electrostatic potential drop across the double layer contributes to the driving force for electron transfer between a dissolved reactant and a molecular catalyst immobilized directly on the electrode surface. The applied electrode potential can increase the oxidizing (or reducing) ability of a surface-bound molecular catalyst, thus making it suitable for charge-transfer processes, which it normally would not be able to catalyze. In this article, we report the initial application of voltage-driven molecular catalysis to electroorganic synthesis. The metal-free, purely organic molecular catalyst (TEMPO) attached to a carbon electrode showed potential-dependent activity for the oxidation of toluene, which does not occur if TEMPO is used as a homogeneous catalyst. Surface-attached TEMPO also shows significant catalytic activity toward benzyl alcohol oxidation even at pH 7. The products of toluene and benzyl alcohol oxidations were identified by nuclear magnetic resonance, Fourier transform infrared, and ultraviolet–visible spectroscopy to evaluate the reaction yield and selectivity. The effect of the applied electrode potential on these catalytic processes was elucidated by density functional theory calculations

Collisions of Ir Oxide Nanoparticles with Carbon Nanopipettes: Experiments with One Nanoparticle

Investigating the collisions of individual metal nanoparticles (NPs) with electrodes can provide new insights into their electrocatalytic behavior, mass transport, and interactions with surfaces. Here we report a new experimental setup for studying NP collisions based on the use of carbon nanopipettes to enable monitoring multiple collision events involving the same NP captured inside the pipet cavity. A patch clamp amplifier capable of measuring pA-range currents on the microsecond time scale with a very low noise and stable background was used to record the collision transients. The analysis of current transients produced by oxidation of hydrogen peroxide at one IrO_<i>x</i> NP provided information about the origins of deactivation of catalytic NPs and the effects of various experimental conditions on the collision dynamics. High-resolution TEM of carbon pipettes was used to attain better understanding of the NP capture and collisions