Instrumentation SECM cells

A CHI Model 900 scanning electrochemical microscope (CH Instruments, Austin, TX) can be used to control UME tip potentials, obtain approach curves and monitor the tip to underlying substrate distance. Voltanunetric and SECM experiments can be performed either with the SECM head in a glove bag under positive pressure or the SECM cell covered with parafilm and in the presence of an argon blanket. Where SECM characterization is not possible, any potentiostat can be used to deposit and characterize the Hg/Pt UME. 

To facilitate and maintain Upid organization, specialized Teflon SECM cells and unusual UME arrangements have to be used. In bilayer lipid membrane (BLM) systems, for example. Teflon cells adapted to the SECM instrument usually have to be made to uniformly distribute the pressure above and below the BLM (Figure 12.33). Also, to visualize the formation of the BLM, the use of an angled microscope combined to a television facilitates the approach of the UME to the BLM (159). 

SECM instrumentation (see footnote 1). Nevertheless, the use of a shear force system adds another level of complexity to the experiment because the mechanical properties of the UME, the sample, and the entire setup become important and have to be optimized. Shear force systems have been used to investigate electrode arrays [22,114,116] and detect metabolic activity of living cells [115] or following cell-cell communication processes [112] by the group of Schuhmann. 

Finally, SECM offers a new application area for miniaturized electrochemical and biochemical sensors. They can be used in connection with a positioning system to solve, for instance, problems of cell biology, material science, and interfacial geochemistry. Since SECM instruments are now available from different commercial sources, a much broader application in the electrochemical sensor community is expected within the next years. 

An understanding of the operation of the SECM and an appreciation of the quantitative aspects of measurements with this instrument depends upon an understanding of electrochemistry at small electrodes. The behavior of ultramicroelectrodes in bulk solution (far from a substrate) has been the subject of a number of reviews (17-21). A simplified experimental setup for an electrochemical experiment is shown in Figure 1. The solution contains a species, O, at a concentration, c, and usually contains supporting electrolyte to decrease the solution resistance and insure that transport of O to the electrode occurs predominantly by diffusion. The electrochemical cell also contains an auxiliary electrode that completes the circuit via the power supply. As the power supply voltage is increased, a reduction reaction, O + ne — R, occurs at the tip, resulting in a current flow. An oxidation reaction will occur at the auxiliary electrode, but this reaction is usually not of interest in SECM, since this electrode is placed sufficiently far from the UME... 

The model 900 SECM (CH Instruments) is the first commercial instrument specifically designed for SECM. Although STM instruments equipped with electrochemical accessories can be adapted for some SECM experiments, they cannot replace a general purpose SECM instrument. This section will describe the features of the model 900. The model 900 includes the cell and probe positioner illustrated in Figure 5 as well as a computer, motor controller, and bipotentiostat (35). 

With scanning electrochemical microscopy (SECM) becoming a widely used and mature technique, with a wide range of applications, many in fields outside of fundamental or applied electrochemistry, relatively few researchers construct their own instruments. Thus, we will not deal with homebuilt instruments in this chapter. Those interested in constructing an SECM are referred to a chapter that goes into details about the basics of this in the first edition of this book. A number of companies have commercialized SECM instrumentation (Table 2.1). Thus in this chapter we will discuss, as an example, the operation of a commercial SECM, the CH Instruments model series 900, which is the most frequently used instrument. Information about the other instruments can be obtained from the manufacturers or their websites. Some practical aspects of cell construction and using a commercial SECM are also addressed. 

An illustration showing translators, tip, cell, and cell mount for the commercially available CH Instruments model 920C SECM is shown in Fignre 2.2 as an example. 

The shear force constant distance mode SECM was later mounted onto an inverted optical microscope, in a so-called Bio-SECM configuration, in order to study individual living cells. The Bio-SECM instrument was notably used to detect nitric oxide released from single cells [78]. In parallel, the shear force setup was modified by replacing the optical detection system, used to monitor the tip vibrating motion, by a piezoelectric element [79]. Electrical detection of the tip vibration was shown to be much easier and more convenient than optical detection, partly because the delicate laser alignment on the tip was made unnecessary. Further developments to shear force SECM have seen the implementation of high-resolution constant distance mode AC-SECM, which was used for the visualization of corrosion pits on stainless steel samples [80,81]. 

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