Combining SECM with other techniques

9 PROBING SINGLE ENZYME MOLECULES USING SECM 

However, each probe must be fabricated individually for this purpose [76,77]. Recent instrumental developments in the field of SPM are aimed at combined techniques, which provide simultaneous information on the surface topography and on the surface physical and chemical properties. 

AFM measurements made during an SECM experiment allow for an independent determination of the zero distance (rf = 0, if the tip is not recessed) and aid in the positioning of the tip. Moreover, this combination of techniques opens the way to experiments in which changes in topography are smdied by AFM as a reactant is generated electrochemically at the tip. 

The existing SECM theory requires some modifications, as the mass transfer now is no longer purely diffusional. 

An alternative approach is the use of a tuning fork attached to an insulated SECM tip (7, 8). The SECM tip can be prepared by the procedures described in Section 12.3.1.1. A tuning fork-based SPM combined with electrochemical techniques can then be used for both topographic imaging and SECM measurements. A key issue in the application of AEM/SECM is the ability to obtain good quantitative approach curves for both insulators and conductors that agree with the theoretical treatment. 

There have been several reports of the use of conunercial NSOM with a W-tip coated with varnish for SECM and photoelectrochemical experiments (11). However, no SECM approach curves for these tips have been shown. Most recently, Lee and cowoikers (10,12) have demonstrated the combined tuning fork-based NSOM/SECM technique to simultaneously obtain topographic, electrochemical, and optical images of an interdigitated array electrode. Although the resolution is still on the order of pm, with the construction of a smaller tip an improved spatial resolution should be possible, leading to NSOM combined with SECM and simultaneous topography. 

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COMBINING SECM WITH OTHER TECHNIQUES Photodiode... 

Ring UME tips (Figure 12.6d) have received attention only recently because of the increased interest in the combination of SECM with other techniques such as AFM (6) and optical miCToscopy (10). Fabrication of ring UMEs involves several steps ... 

Although SECM is a mature technique, there is still room for further developments in different directions, such as new SECM probes and new fabrication methods, new SECM operation modes, combination of SECM with other techniques, and the application of SECM to novel interfaces and systems under extreme conditions. These are discussed in the following sections. 

The quartz crystal nanobalance (QCN) can be combined with practically any electrochemical methods, such as cyclic voltammetry, chronoamperometry, chronocoulometry, potentiostatic, galvanostatic, rotating disc electrode [11], or potentiometric measurements. The EQCN can be further combined with other techniques, e.g., with UV-Vis spectroscopy [12], probe beam deflection (PBD) [13], radiotracer [14], atomic force microscopy (AEM) [15], and scanning electrochemical microscopy (SECM) [16]. The concept and the instrumentation of... 

In Chapter 17, the combination of SECM with other techniqnes has been presented. Hybrid systems, like SECM-AFM, are useful for obtaining more information abont processes at an interface. In addition to these techniques, SECM can be combined with other methodologies that allow further sample analysis and afford the extraction of sample structural information, for instance, by coupling SECM with mass spectrometry (MS). Several approaches for scanning mass spectrometry have been reported for the spatial detection of different species at solid-liqnid, liquid-liquid, or air-solid interface. Both chemical " and electrochemical reactions have been characterized by these means. These approaches are based on the use of concentric capillaries that allow the flux of carrier gas or redox components to the substrate (i.e., through the outer capillary) and the extraction... 

A highly significant recent development is the integration of atomic force microscopy with SECM (AFM-SECM) [220]. Further details are described in Chapter 3.2 of this volume. An etched Pt wire was flattened and then insulated with electrophoretic paint. This probe acts as the cantilever in AFM and as the SECM tip providing dual force-sensing and electrochemical capabilities. Submicron resolution for SECM and for topography by AEM was achieved for test samples of track-etched polycarbonate membranes and ionic crystal surfaces. As the resolution of the technique improves and it is combined with other scanning probes, it seems likely that SECM can be applied in an ever-wider spectrum of scientific fields. 

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