Scanning Electrochemical Microscopy

Abstract

AbstractScanning electrochemical microscopy (SECM) is one of a number of scanned probe microscopy (SPM) techniques invented following the demonstration of the scanning tunneling microscope. The use of an electrochemical process for image formation defines SECM. In most applications of the method, an ultramicroelectrode (UME) is used as the probe and the probe signal is the Faradic current arising from the electrolysis of solution species. In other cases, the use of an ion‐selective electrode (ISE) as the probe provides a probe signal proportional to the logarithm of the activity of an ion in solution (eg., pH). In SECM, the primary interaction between probe tip and sample is mediated by diffusion of solution species between the sample and the tip of the probe, which distinguishes SECM from other SPM methods that may use an electrochemically active probe. An electrochemically active probe permits a versatile range of experiments, an essential aspect of which is chemical sensitivity or control of chemical processes occurring at a substrate surface.Forming an image in an SPM technique requires that the probe signal be perturbed in a reproducible fashion by some aspect of the imaged substrate. There are two principal image‐forming modes in SECM: feedback and generation/collection (GC). The feedback mode uses the Faradaic current that flows from electrolysis of an intentionally‐added or naturally present mediator species at an UME probe.Imaging in the feedback mode provides topographic images of blocking or conducting surfaces. Images of many types of surfaces have been obtained in the feedback mode; examples include images of electrodes, polymer films, and immiscible liquid interfaces.It is possible to manipulate the SECM imaging conditions to produce images that representing chemical and electrochemical activity.The generation/collection (GC) mode uses the probe to detect changes in the concentration of a chemical species at the surface of the imaged material. Ideally, the probe acts as a passive sensor to produce concentration maps of a particular chemical species near the substrate surface. GC mode imaging is described further by the type of sensing probe used. In amperometric GC imaging, the probe is an UME, which detects species by electrolysis. First reported as a method to map electrochemically active areas on electrodes, amperometric GC is used to make high‐resolution chemical concentration maps of corroding metal surfaces, biological materials, and polymeric materials. In addition, measurements of ion fluxes through porous materials, such as skin and dental material, are useful applications. In potentiometric GC imaging, the probe is an ISE, which has the advantage of increased sensitivity to nonelectroactive ions and improved selectivity for imaging a desired ion concentration.