Michael V. Mirkin

Scanning electrochemical microscopy.

This review describes work done in scanning electrochemical microscopy (SECM) since 2000 with an emphasis on new applications and important trends, such as nanometer-sized tips. SECM has been adapted to investigate charge transport across liquid/liquid interfaces and to probe charge transport in thin films and membranes. It has been used in biological systems like single cells to study ion transport in channels, as well as cellular and enzyme activity. It is also a powerful and useful tool for the evaluation of the electrocatalytic activities of different materials for useful reactions, such as oxygen reduction and hydrogen oxidation. SECM has also been used as an electrochemical tool for studies of the local properties and reactivity of a wide variety of materials, including metals, insulators, and semiconductors. Finally, SECM has been combined with several other nonelectrochemical techniques, such as atomic force microscopy, to enhance and complement the information available from SECM alone.

Scanning electrochemical microscopy in the 21st century

The fundamentals of and recent advances in scanning electrochemical microscopy (SECM) are described. The focus is on applications of this method to studies of systems and processes of active current interest ranging from nanoelectrochemistry to electron transfer reactions and electrocatalysis to biological imaging.

Direct determination of diffusion coefficients by chronoamperometry at microdisk electrodes

The chronoamperometric response at a microdisk is used for the direct determination of the diffusion coefficient of an electroactive species. The method does not require knowledge of the bulk concentration and the number of electrons participating in the electrode reaction, and requires only a value for the disk radius. Subsequent determination of the number of electrons (n) for an electrode reaction or the concentration of electroactive species is also possible. This approach is demonstrated with the evaluation of the diffusion coefficient of Fe(CN)64− in KCl and that of borohydride ion in NaOH. In both cases, the values of n found remained constant over a wide time range and correspond to those expected for these processes.

Borohydride Oxidation at a Gold Electrode

The multistage process of borohydride oxidation in an 8 e - reaction to borate at a Au electrode has been studied by means of fast-scan cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM). The total irreversibility of this process observed previously is shown to be a result of the presence of very unstable intermediates. CV measurements showed that at least two stages of the process are quasi-reversible, and the presence of a coupled homogeneous chemical reaction was proved by SECM. The rate constant for this reaction as well as the electrochemical kinetic parameters for the first stage of oxidation are evaluated using digital simulation

Scanning electrochemical microscopy part 13. Evaluation of the tip shapes of nanometer size microelectrodes

Abstract The shape of steady-state current-distance (iT-d) curves obtained by scanning electrochemical microscopy (SECM) with a submicrometer conical-type tip is substantially different from that obtained with a microdisk tip. A model is proposed to describe the iT-d curves for tips shaped as cones or spherical segments. In each case a family of theoretical working curves, computed for different values of a single adjustable parameter (the height of a cone or a segment), was used to fit the experimental curves obtained with tips approaching a conductive or insulating substrate. A good fit of the experimental and theoretical curves can be obtained only for a very narrow range of the tip shape parameters, giving confidence in the reliability of the proposed method. The shape parameters for two substantially different microelectrodes are evaluated. A 420 nm radius tip was shown to be a fairly sharp cone with a height-to-radius ratio k of 0.8. An 80 nm radius tip can be represented as either a cone or spherical segment, because of the small k = 0.2. The radius values obtained from iT-d curves agree with those estimated from steady-state current values. The cyclic voltammograms (CVs) obtained at these microtip electrodes have a regular shape with very small capacitive and resistive background. New analytical approximations for iT-d curves are also proposed for SECM with a disk-shaped tip over conductive and insulating substrates.

Nanoelectrochemistry of mammalian cells.

There is a significant current interest in development of new techniques for direct characterization of the intracellular redox state and high-resolution imaging of living cells. We used nanometer-sized amperometric probes in combination with the scanning electrochemical microscope (SECM) to carry out spatially resolved electrochemical experiments in cultured human breast cells. With the tip radius ≈1,000 times smaller than that of a cell, an electrochemical probe can penetrate a cell and travel inside it without apparent damage to the membrane. The data demonstrate the possibility of measuring the rate of transmembrane charge transport and membrane potential and probing redox properties at the subcellular level. The same experimental setup was used for nanoscale electrochemical imaging of the cell surface.

Electroanalytical measurements using the scanning electrochemical microscope

This review describes the recent advances in the application of scanning electrochemical microscopy (SECM) to electroanalytical studies. SECM imaging of topography and chemical reactivity is also surveyed.

Adsorption/Desorption of Hydrogen on Pt Nanoelectrodes: Evidence of Surface Diffusion and Spillover

Nanoelectrochemical approaches were used to investigate adsorption/desorption of hydrogen on Pt electrodes. These processes, which have been extensively studied over the last century, remain of current interest because of their applications in energy storage systems. The effective surface area of a nanoelectrode was found to be much larger than its geometric surface area due to surface diffusion of adsorbed redox species at the Pt/glass interface. An additional peak of hydrogen desorption was observed and attributed to the spillover of hydrogen from the Pt surface into glass. The results were compared to those obtained for underpotential deposition of copper on Pt nanoelectrodes.

Electrochemistry of individual molecules in zeptoliter volumes.

Electrochemical experiments were carried out in a nanometer-sized cylindrical thin layer cell (TLC) formed by etching the surface of a disk-type platinum nanoelectrode (5- to 150-nm radius). Using high frequency ac voltage, the surface of such an electrode was etched to remove a very thin (> or = 1-nm-thick) layer of Pt. The resulting zeptoliter-scale cavity inside the glass sheath was filled with aqueous solution containing redox species, and the etched electrode was immersed in a dry (no external solution) pool of mercury to produce a TLC. Several approaches based on steady-state voltammetry and scanning electrochemical microscopy (SECM) were developed to independently evaluate the electrode radius and the etched volume. The number of redox molecules in the TLC could be varied between one and a few hundred by changing its volume and solution concentration. In this way, the transition between a random and deterministic number of trapped molecules was observed. High quality steady-state voltammograms of > or = 1 molecules were obtained for different neutral and charged redox species and different concentrations of supporting electrolyte. The analysis of such voltammograms yields information about mass transfer, adsorption, electron transfer kinetics, and double-layer effects on the nanoscale.

Nanoelectrodes for determination of reactive oxygen and nitrogen species inside murine macrophages.

Reactive oxygen and nitrogen species (ROS and RNS) produced by macrophages are essential for protecting a human body against bacteria and viruses. Micrometer-sized electrodes coated with Pt black have previously been used for selective and sensitive detection of ROS and RNS in biological systems. To determine ROS and RNS inside macrophages, one needs smaller (i.e., nanometer-sized) sensors. In this article, the methodologies have been extended to the fabrication and characterization of Pt/Pt black nanoelectrodes. Electrodes with the metal surface flush with glass insulator, most suitable for quantitative voltammetric experiments, were fabricated by electrodeposition of Pt black inside an etched nanocavity under the atomic force microscope control. Despite a nanometer-scale radius, the true surface area of Pt electrodes was sufficiently large to yield stable and reproducible responses to ROS and RNS in vitro. The prepared nanoprobes were used to penetrate cells and detect ROS and RNS inside macrophages. Weak and very short leaks of ROS/RNS from the vacuoles into the cytoplasm were detected, which a macrophage is equipped to clean within a couple of seconds, while higher intensity oxidative bursts due to the emptying of vacuoles outside persist on the time scale of tens of seconds.