Electrochemical scanning microscope microelectrode

A different development but with similar importance for electrochemistry was the development of the electrochemical scanning microscope. In this case an ultra-microelectrode... 

Figure 3.5 Scanning electron microscope images of the surfaces of (a) a bare carbon fiber microelectrode and (b) a multiwalled carbon nanotube -[C4CjIm][PF ]-modified carbon fiber microelectrode. (Reprinted from Liu, Y., Zou, X., and Dong, S., Electrochem. Commun., 8,1429-1434,2006. Copyright 2006 Elsevier. With permission.)...

A third screening method for arrays of electrocatalysts was recently introduced by Hillier and coworkers [15, 29, 30]. Using a scanning electrochemical microscope (SECM), a microelectrode tip is moved over an electrocatalyst array. The resulting electrochemical feedback currents are measured and used to generate an activity map of the electrocatalyst library. This method does not require individual electronic addressability for each electrocatalyst... 

The main purpose of this contribution, however, is to review recent advances in solid state ionics achieved by means of microelectrodes, i.e. electrodes whose size is in the micrometer range (typically 1-250 pm). In liquid electrolytes (ultra)-microelectrodes are rather common and applied for several reasons they exhibit a very fast response in voltametric studies, facilitate the investigation of fast charge transfer reactions and strongly reduce the importance of ohmic drops in the electrolyte, thus allowing e.g. measurements in low-conductive electrolytes [33, 34], Microelectrodes are also employed to localize reactions on electrodes and to scan electrochemical properties of electrode surfaces (scanning electrochemical microscope [35, 36]) further developments refer to arrays of microelectrodes, e.g. for (partly spatially resolved) electroanalysis [37-39], applications in bioelectrochemistry and medicine [40, 41] or spatially resolved pH measurements [42], Reviews on these and other applications of microelectrodes are, for example, given in Ref. [33, 34, 43-47],... 

Isik, S. Etienne, M. Oni, J. Blochl, A. Reiter, S. Schuhmann, W., Dual microelectrodes for distance control and detection of nitric oxide from endothelial cells by means of scanning electrochemical microscope, Anal. Chem. 2004, 76, 6389-6394... 

An alternative electrochemical approach to the measurement of fast interfacial kinetics exploits the use of the scanning electrochemical microscope (SECM). A schematic of this device is shown in Fig. 14 the principle of the method rests on the perturbation of the intrinsic diffusive flux to the microelectrode, described by Eq. (34) above. A number of reviews of the technique exist [109,110]. In the case of the L-L interface, the microelectrode probe is moved toward the interface once the probe-interface separation falls within the diffusion layer, a perturbation of the current-distance response is seen, which can be used to determine the rate of interfacial processes, generally by numerical solution of the mass-transport equations with appropriate interfacial boundary conditions. The method has been... 

Other dual electrode systems that operate at steady state and show similar shielding and collection effects include microelectrode arrays and the scanning electrochemical microscope (SECM). With microelectrode arrays (Section 5.9.3), one monitors diffusion between two neighboring electrodes. In a similar way, one can use the SECM (Section 16.4) to study diffusion between an ultramicroelectrode tip and substrate electrode. In both of these systems, convective effects are absent and the time for interelectrode transit is governed by the distance between the electrodes. 

Fundamentals. Based on the functional principles of the scanning electrochemical microscope, other scanning probe methods used to determine localized surface properties of the electrode under investigation or of the solution phase adjacent to this surface have been developed utilizing suitable microelectrodes. A pH-sensitive microelectrode based on a glass capillary filled with a pH-constant buffer solution and containing an internal reference electrode that has a tip filled with a proton-selective ionophor cocktail is scanned across the surface. The potential of the internal reference electrode with respect to an external reference electrode is directly correlated to the local pH value. A schematic cross section of this microelectrode is shown in Fig. 7.18. 

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. 

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