Local probes—scanning electrochemical microscopy

A particularly interesting approach for probing electrode reactions at the micro-level has arisen from the combination of microelectrodes and the accurate control instrumentation associated with scanning probe microscopies. 

The usual objective of scanning probe microscopy techniques [41] is to provide images of a solid surface—normally topographic information— with up to atomic resolution. However, they can also be used to probe local solution composition and electrode reactions, as will be described. 

An important extension of these techniques is when the substrate is made into an electrode in a small electrochemical cell, so that the change in surface morphology of the substrate can be monitored with time. In this way, for example, direct visualisation of electrodeposition at the sub-microscopic level can be seen in real time, both nucleation and growth phases, e.g., the electrodeposition of copper in media with and without organic additives [42]. 

For the investigation of electrode reaction parameters and chemistry at these dimensions, another approach is necessary, in order to make the system species-selective through monitoring the electrochemistry. This involves making tip and substrate into independent electrodes the tip is thus a microelectrode. The microelectrode tip is scanned over the surface this is known as scanning electrochemical microscopy (SECM) [43] and due to its local probe nature can be used to probe microvolumes. 

The response of SECM is distance-dependent as shown in Fig. 16.8. At large distances from the substrate the current measured is that of the microelectrode tip. The tip is a disc microelectrode, so that far from a substrate in the steady state the diffusion-limited tip current, /T., will be measured. For a simple n-electron electrode reaction this is 

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Scanning electrochemical microscopy can also be applied to study localized biological activity, as desired, for example, for in-situ characterization of biosensors (59,60). In this mode, the tip is used to probe the biological generation or consumption of electroactive species, for example, the product of an enzymatic surface reaction. The utility of potentiometric (pH-selective) tips has also been... 

Scanning Electrochemical Microscopy as a Local Probe of Chemical Processes at Liquid Interfaces... 

As described in the introduction, submicrometer disk electrodes are extremely useful to probe local chemical events at the surface of a variety of substrates. However, when an electrode is placed close to a surface, the diffusion layer may extend from the microelectrode to the surface. Under these conditions, the equations developed for semi-infinite linear diffusion are no longer appropriate because the boundary conditions are no longer correct [97]. If the substrate is an insulator, the measured current will be lower than under conditions of semi-infinite linear diffusion, because the microelectrode and substrate both block free diffusion to the electrode. This phenomena is referred to as shielding. On the other hand, if the substrate is a conductor, the current will be enhanced if the couple examined is chemically stable. For example, a species that is reduced at the microelectrode can be oxidized at the conductor and then return to the microelectrode, a process referred to as feedback. This will occur even if the conductor is not electrically connected to a potentiostat, because the potential of the conductor will be the same as that of the solution. Both shielding and feedback are sensitive to the diameter of the insulating material surrounding the microelectrode surface, because this will affect the size and shape of the diffusion layer. When these concepts are taken into account, the use of scanning electrochemical microscopy can provide quantitative results. For example, with the use of a 30-nm conical electrode, diffusion coefficients have been measured inside a polymer film that is itself only 200 nm thick [98]. 

Barker, A.L., Slevin, C.J., Unwin, P.R. and Zhang, J. (2001) Scanning electrochemical microscopy as a local probe of chemical processes at liquid interfaces. Chapter 12 in A.G. Volkov (Ed.) Liquid Interfaces in Chemical, Biological and Pharmaceutical Applications. Dekker, New York. See also Chapter 6. 

Other local probe techniques to be discussed, of an electrochemical nature, which rely on much of the same instrumental technology, are scanning electrochemical microscopy (SECM) and scanning ion conductance microscopy (SICM). 

JV Macpherson, MA Beeston, PR Unwin, NP Hughes, D Littlewood. Scanning electrochemical microscopy as a probe of local fluid-flow through porous solids—application to the measurement of convective rates through a single dentinal tubule. J Chem Soc Faraday Trans 91 1407-1410, 1995. 

SECM (Scanning electrochemical microscopy) is a technique to characterize the local electrochemical nature of various materials by scanning a probe microelectrode [1,2]. The spatial resolution of SECM is inferior to the conventional scanning probe microscopes such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) as the fabrication of the probe, microelectrode, with nanometer sizes is quite difficult and the faradaic current of the microprobe is very small (often picoamps or less). However, SECM has unique characteristics that cannot be expected for STM and AFM SECM can image localized chemical reactions and it also can induce localized chemical reactions in a controlled manner. 

Scanning electrochemical microscopy (SECM, see Chapter 12) is another microelectrode technique that has been used at the cellular surface. Briefly, with SECM a microelectrode (UME, see Chapter 6) functions as a scanning probe that detects local electrochanical activity. When the UME is rastered over a sample, electrochemical data is recorded at multiple positions and an image is constructed based on the local electrochemical properties of the area of interest SECM has been thoroughly reviewed (19,20,52, Chapter 12 of this... 

Scanning electrochemical microscopy (SECM), a member of the scanning probe microscopies (SPM), uses an ultramicroelectrode as the probe to characterize the localized electrochemical properties of the surfaces (1-3). Although the lateral resolution of SECM is inferior to that of STM or AFM because it is difficult to fabricate a small probe microelectrode with atom-size radius, SECM has abilities of detecting chemical reactions. In addition, it can induce chemical reactions in an extremely small volume. Using the unique properties of SECM, a single molecule (4) or a radical with a short lifetime (5) was recently detected. 

Scanning electrochemical microscopy (SECM) is a scanning probe technique that provides a local chemical interrogation of an interface. In a typical experimental setup (Figure 8.1), the probe, also named the tip, is an ultramicroelectrode (UME), a microelectrode of radius <25 pm, which is rastered with micropositioners in the vicinity of an interface in order to detect an elec-trochemically active species. Contrary to other scanning probe microscopies. 

Other applications snch as the measuranent of local concentrations in environmental analy-sis44,45 using potentiometric probes as detectors in various types of scanning electrochemical microscopies inclnding probes with dnal functionality are emerging, but are stiU largely limited to micrometer-size electrodes due to inherent difficulties of using nanoelectrodes. 

Gonsalves M, Barker AL, Macpherson JV, Unwin PR, O Hare D, Winlove CP (2000) Scanning electrochemical microscopy as a local probe of oxygen permeability in cartilage. Biophys J 78 1578-1588... 

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