Patrick R. Unwin

Chemical Imaging of Surfaces with the Scanning Electrochemical Microscope

Scanning electrochemical microscopy is a scanning probe technique that is based on faradaic current changes as a small electrode is moved across the surface of a sample. The images obtained depend on the sample topography and surface reactivity. The response of the scanning electrochemical microscope is sensitive to the presence of conducting and electroactive species, which makes it useful for imaging heterogeneous surfaces. The principles and instrumentation used to obtain images and surface reaction-kinetic information are discussed, and examples of applications to the study of electrodes, minerals, and biological samples are given.

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.

Electrochemistry at carbon nanotubes: perspective and issues

Electrochemistry at carbon nanotubes (CNTs) is a large and growing field, but one in which there is still uncertainty about the fundamental activity of CNTs as electrode materials. On the one hand, there are many reports which focus on the favourable electrochemical properties of CNT electrodes, such as enhanced detection sensitivity, electrocatalytic effects and reduced fouling. On the other hand, other studies suggest that CNTs may be no more electroactive than graphitic powder. Furthermore, it has been proposed that the catalytic nanoparticles from which CNTs are formed may dominate the electrochemical characteristics in some instances. A considerable body of the literature presumes that the CNT sidewall is inert and that edge-plane-graphite-like open ends and defect sites are responsible for the electron transfer activity observed. In contrast, studies of well characterised single-walled nanotube (SWNT) electrodes, either as individual tubes or as two-dimensional networks, suggest sidewall activity. This review highlights how the various discrepancies in CNT electrochemistry may have arisen, by taking a historical view of the field and identifying crucial issues that still need to be solved. When assessing the behaviour of CNT electrodes, it is vitally important that careful consideration is given to the type of CNT used (SWNT or multi-walled), the quality of the material (presence of impurities), the effect of chemical processing steps in the fabrication of electrodes and the experimental arrangements adopted. Understanding these key features is an essential requirement to develop a fundamental understanding of CNT electrochemistry, to allow a wide range of electroanalytical applications, and to move the field forward rationally. As part of this process, high resolution electrochemical and electrical imaging techniques are expected to play a significant role in the future, as well as theoretical developments which examine the fundamentals of electron transfer at different types of CNTs and their characteristic surface sites.

Electrochemistry of Nanoparticles

Metal nanoparticles (NPs) find widespread application as a result of their unique physical and chemical properties. NPs have generated considerable interest in catalysis and electrocatalysis, where they provide a high surface area to mass ratio and can be tailored to promote particular reaction pathways. The activity of NPs can be analyzed especially well using electrochemistry, which probes interfacial chemistry directly. In this Review, we discuss key issues related to the electrochemistry of NPs. We highlight model studies that demonstrate exceptional control over the NP shape and size, or mass-transport conditions, which can provide key insights into the behavior of ensembles of NPs. Particular focus is on the challenge of ultimately measuring reactions at individual NPs, and relating the response to their structure, which is leading to imaginative experiments that have an impact on electrochemistry in general as well as broader surface and colloid science.

Multifunctional Nanoprobes for Nanoscale Chemical Imaging and Localized Chemical Delivery at Surfaces and Interfaces

Double take: Double-barrel carbon nanoprobes with integrated distance control for simultaneous nanoscale electrochemical and ion conductance microscopy can be fabricated with a wide range of probe sizes in less than two minutes. The nanoprobes allow simultaneous noncontact topographical (left image) and electrochemical imaging (right) of living neurons, as well as localized K+ delivery and simultaneous neurotransmitter detection.

A New View of Electrochemistry at Highly Oriented Pyrolytic Graphite

Major new insights on electrochemical processes at graphite electrodes are reported, following extensive investigations of two of the most studied redox couples, Fe(CN)(6)(4-/3-) and Ru(NH(3))(6)(3+/2+). Experiments have been carried out on five different grades of highly oriented pyrolytic graphite (HOPG) that vary in step-edge height and surface coverage. Significantly, the same electrochemical characteristic is observed on all surfaces, independent of surface quality: initial cyclic voltammetry (CV) is close to reversible on freshly cleaved surfaces (>400 measurements for Fe(CN)(6)(4-/3-) and >100 for Ru(NH(3))(6)(3+/2+)), in marked contrast to previous studies that have found very slow electron transfer (ET) kinetics, with an interpretation that ET only occurs at step edges. Significantly, high spatial resolution electrochemical imaging with scanning electrochemical cell microscopy, on the highest quality mechanically cleaved HOPG, demonstrates definitively that the pristine basal surface supports fast ET, and that ET is not confined to step edges. However, the history of the HOPG surface strongly influences the electrochemical behavior. Thus, Fe(CN)(6)(4-/3-) shows markedly diminished ET kinetics with either extended exposure of the HOPG surface to the ambient environment or repeated CV measurements. In situ atomic force microscopy (AFM) reveals that the deterioration in apparent ET kinetics is coupled with the deposition of material on the HOPG electrode, while conducting-AFM highlights that, after cleaving, the local surface conductivity of HOPG deteriorates significantly with time. These observations and new insights are not only important for graphite, but have significant implications for electrochemistry at related carbon materials such as graphene and carbon nanotubes.

Localized high resolution electrochemistry and multifunctional imaging : scanning electrochemical cell microscopy

We describe highly localized electrochemical measurements and imaging using a simple, mobile theta pipet cell. Each channel (diameter <500 nm) of a tapered theta pipet is filled with electrolyte solution and a Ag/AgCl electrode, between which a bias is applied, resulting in a conductance current across a thin meniscus of solution at the end of the pipet, which is typically deployed in air or a controlled gaseous environment. When the position of the pipet normal to a surface of interest is oscillated, an oscillating component in the conductance current is generated when the meniscus at the end of the probe comes into contact with the surface and undergoes periodic (reversible) deformation, so as to modulate the solution resistance. This oscillating current component can be used to maintain gentle contact of the solution from the pipet cell with the surface and as a set point for high resolution topographical imaging with the pipet. Simultaneously, the mean conductance current that flows between the pipet channels can be measured and is sensitive to the local nature of the interface, informing one, for example, on wettability and ion flow into or out of the surface investigated. Furthermore, conductor or semiconductor surfaces can be connected as a working electrode, with one of the electrodes in the pipet serving as a quasi-reference electrode. This pipet cell then constitutes part of a dynamic electrochemical cell, with which direct voltammetric-amperometric imaging can be carried out simultaneously with conductance and topographical imaging. This provides multifunctional electrochemical maps of surfaces and interfaces at high spatial resolution. The prospects for the use of this new methodology widely are highlighted through exemplar studies and a brief discussion of future applications.

Landing and Catalytic Characterization of Individual Nanoparticles on Electrode Surfaces

We demonstrate a novel and versatile pipet-based approach to study the landing of individual nanoparticles (NPs) on various electrode materials without any need for encapsulation or fabrication of complex substrate electrode structures, providing great flexibility with respect to electrode materials. Because of the small electrode area defined by the pipet dimensions, the background current is low, allowing for the detection of minute current signals with good time resolution. This approach was used to characterize the potential-dependent activity of Au NPs and to measure the catalytic activity of a single NP on a TEM grid, combining electrochemical and physical characterization at the single NP level for the first time. Such measurements open up the possibility of studying the relation between the size, structure and activity of catalyst particles unambiguously.

Structural Correlations in Heterogeneous Electron Transfer at Monolayer and Multilayer Graphene Electrodes

As a new form of carbon, graphene is attracting intense interest as an electrode material with widespread applications. In the present study, the heterogeneous electron transfer (ET) activity of graphene is investigated using scanning electrochemical cell microscopy (SECCM), which allows electrochemical currents to be mapped at high spatial resolution across a surface for correlation with the corresponding structure and properties of the graphene surface. We establish that the rate of heterogeneous ET at graphene increases systematically with the number of graphene layers, and show that the stacking in multilayers also has a subtle influence on ET kinetics.

Topographical and electrochemical nanoscale imaging of living cells using voltage-switching mode scanning electrochemical microscopy

We describe voltage-switching mode scanning electrochemical microscopy (VSM-SECM), in which a single SECM tip electrode was used to acquire high-quality topographical and electrochemical images of living cells simultaneously. This was achieved by switching the applied voltage so as to change the faradaic current from a hindered diffusion feedback signal (for distance control and topographical imaging) to the electrochemical flux measurement of interest. This imaging method is robust, and a single nanoscale SECM electrode, which is simple to produce, is used for both topography and activity measurements. In order to minimize the delay at voltage switching, we used pyrolytic carbon nanoelectrodes with 6.5–100 nm radii that rapidly reached a steady-state current, typically in less than 20 ms for the largest electrodes and faster for smaller electrodes. In addition, these carbon nanoelectrodes are suitable for convoluted cell topography imaging because the RG value (ratio of overall probe diameter to active electrode diameter) is typically in the range of 1.5–3.0. We first evaluated the resolution of constant-current mode topography imaging using carbon nanoelectrodes. Next, we performed VSM-SECM measurements to visualize membrane proteins on A431 cells and to detect neurotransmitters from a PC12 cells. We also combined VSM-SECM with surface confocal microscopy to allow simultaneous fluorescence and topographical imaging. VSM-SECM opens up new opportunities in nanoscale chemical mapping at interfaces, and should find wide application in the physical and biological sciences.