17 research outputs found
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)<sub>6</sub><sup>3/4–</sup>) were added to the solution
and increased upon adding redox species with a negative formal potential
(e.g., RuÂ(NH<sub>3</sub>)<sub>6</sub><sup>3/2+</sup>). 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,
[AuÂ(I)ÂSR]<sub><i>n</i></sub>, 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 [AuÂ(I)ÂSR]<sub><i>n</i></sub> 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 FeÂ(CN)<sub>6</sub><sup>4–</sup> 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 tetrakisÂ(pentafluorophenyl)Âborate 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<sub><i>x</i></sub> 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