Electrochemical microelectrode ultramicroelectrodes

Bard AJ, Crayston JA, Kittlesen GP, Shea TV, Wrighton MS (1986) Digital simulation of the measured electrochemical response of reversible redox couples at microelectrode arrays consequences arising from closely spaced ultramicroelectrodes. Anal Chem 58 2321-2331... 

Since micro-gravimetry with the EQCM lacks specificity only the difference of cation and anion fluxes can be obtained by microgravimetry and therefore an independent measurement of specific ions is needed. Scanning electrochemical microscopy (SECM) coupled with a quartz crystal microbalance with independent potential control of the tip and substrate has been recently done by Cliffel and Bard [28]. In this experiment generation at the substrate (EQCM crystaj) working electrode and collection at the tip of an ultramicroelectrode (UNE) that was approached perpendicular to the EQCM crystal was employed with measurement of A/. Hillier and Ward [8] had previously used a scanning microelectrode to map the mass sensitivity across the surface of the QCM crystal. Reflection of longitudinal waves at the UME tip limits these experiments due to oscillations. 

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. A microelectrode with a small diameter (e.g. 10-20 pm, such an electrode is sometimes also called ultramicroelectrode (UME) [112-116]) is exposed to an electrolyte solution containing an electrochemically active substance. The electrode potential is adjusted to a value sufficiently negative to drive the electrochemical reaction O -h riQ R under diffusion control. Diffusion of reactive species to the electrode surface is hemispherical instead of planar, as in the case of large electrodes. The current I flowing across the solid/electrolyte solution interface of the microelectrode tip quickly reaches a steady state value /xa = nFDcr with n as the number of electrons transferred in the electrochemical reaction step, F the Faraday constant, D the diffusion coefficient of the reacting species, c its concentration and r the tip radius. The experimental setup is pictured schematically in Fig. 7.10. 

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. 

Two different microelectrode arrays are presented the first array consists of 100 parallel connected ultramicroelectrodes. Counter as well as reference electrodes are integrated on the same chip. Although still exhibiting microelectrode features, this array yields analytical currents very similar to those of macroelectrodes. The second array consists of 400 individually addressable microelectrodes. It enables redundant as well as multianalyte measurements. Here, we demonstrate the electrochemical imaging of two-dimensional distributions of ammonium chloride as well as urea. 

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. 

A theoretical study of the current-time relationship aimed at the explanation of anomalous response in differential pulse polarography was reported by Lovric and Zelic. The effect was explained by the adsorption of reactant at the electrode surface (Lovric Zelic, 2008). The situation connected with the formation of metal preconcentration at the electrode surface, followed by electrodissolution was modelled by Cutters and Compton. The theory to explore the electrochemical signals in such a case at a microelectrode or ultramicroelectrode arrays was derived (Cutress Compton, 2009). 

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