Ultra-microelectrodes

Cyclic voltammetric and related techniques are particularly valuable for determining Ea values in cases where one member of the redox couple is unstable. At a microelectrode in electrolytes of low resistivity, cyclic voltammograms can be recorded at scan rates up to about 100 V s-1 and at low temperatures. This allows the detection of reversibility when the unstable partner has a life-time of the order of a millisecond or so. Ultra-microelectrode techniques promise to lower this limit even further.1-3... 

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],... 

Dimensionless) time parameter or characteristic time scale Any dimensionless combination of time (s) and physical parameters, e.g., Dt/r20, kit, k r /D (for ultra microelectrodes), RT/Fv for cyclic voltammetry, or1 for AC methods [i-ii]. 

High-speed cyclic voltammetry, using ultra-microelectrodes and performed on [Ni(dmit)2] complexes, showed that the formation of [Ni(dmit)2] species follows an EC mechanism before the crystallisation of the compound occurs on the electrode... 

FIG. 10 Experimental approach curve obtained with a 4 /rm diameter spherical self-assembled ultra-microelectrode variation of the normalized current with the dimensionless tip-substrate separation d/a (d is the tip-substrate separation, a is the electrode effective radius). (----) Theoretical approach curve for a spherical electrode calculated as described in the text. (----) Theoretical approach curve for a... 

Human blood specimens Pb Centrifuge filter mix filtrate with Hg++/HCl [N/MT] Perform analyses using the Osteryoung square wave stripping voltammetry mode on electrochemical analyzer with glass or PE shrouded carbon disk ultra-microelectrodes [SWSV] [N/MT-SWSV] Feldman et al. 1994... 

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

Microelectrodes, also called ultra-microelectrodes, have some very special diffusion properties. The electrode can be prepared by melting an ultra-thin metal wire into a glass rod. The small radius of the wire with only a few micrometers leads to a situation in which the radial resistance of the electrolyte in front of the electrode snbstitntes the diffusion limitation by the diffusion layer. A microelectrode and the current distribution in front of the microelectrode are shown in Figure 5.14. 

Array electrodes Replacement of a single electrode (with dimensions in the micrometer or centimeter range) by an array of (ultra)microelectrodes [66]. 

The specific features of voltammetry at microelectrodes (absence of interferences arising from charging currents, uncompensated resistance and instrumental imperfections, high signal/noise ratio) were emphasized in chapter 2 (section 2.3) of this volume. Various construction modes and application of (ultra)-microelectrodes in chemical and biochemical practice will be treated in chapter 7 of this volume. In this subsection the exploitation of microelectrodes (under steady-state conditions) to investigation of electrode mechanisms and homogeneous reaction kinetics is discussed. 

Cyclic voltammetry is probably the electrochemical technique that is simulated most often, aiming at the analysis of electrode processes with respect to mechanism, kinetics, and thermodynamics of the reaction steps as well as transport properties of the molecules involved. The simulation of processes at (ultra)microelectrodes is also popular and highly important for the analysis of scanning electrochemical microscopy experiments [10]. 

In short, SECM is a scanning probe technique similar to STM or atomic force microscopy (AFM). A tip current arises due to an electrochemical reaction (faradaic process) at an ultra-microelectrode (UME) tip (see Chapter 6). The tip generally consists of a Pt wire of diameter between 1 and 25 pm that is sealed in a glass capillary and polished to get a flat electrode surface. A typical voltammogram recorded on a UME is shown in Figure 9.28b. It is a sigmoidal, steady-state current-potential curve without any hysteresis. 

Rush, S., Lepeschkin, E., and Brooks, H. O., 1968, Electrical and thermal properties of double-barreled ultra microelectrodes, Trans. IEEE, BME 15 80-93. 

Until now most efforts in the development of miniaturized analyzers are based on the use of photometric and electrochemical detection. During the workshop many examples will be given. It can be easily shown that miniaturizing the devices with respect to diameters of channels, injected volumes, etc. asks for a more than proportional reduction in the detector volume [6]. In this respect the important development of (ultra-) microelectrodes has to be mentioned. Because electrochemical techniques, but also optical ones like surface plasmon resonance, are essentially based on surface phenomena they are critically dependent on the condition of the sensing surfaces. As these surfaces can often easily be affected in an irreversible way, extreme care in handling is required to get reproducible results. The search for other techniques related to bulk properties of the samples should therefore be continued. These may comprise optical as well as magnetic or enthalpimetric principles. 

Hirata, Y., S. Yabuki, and F. Mizutani, Application of integrated SECM ultra-microelectrode and AFM force probe to biosensor surfaces, Bioelectrochemistry, Vol. 63, 2004 pp. 217-224. 

Pocaznoi, D., Erable, B., Delia, M. Bergel, A. Ultra microelectrodes increase the current density provided by electroactive biofilms by improving their electron transport ability. Energy Environ. Sci. 5 1 (2012), pp. 5287-5296. 

LAUNAY - Could you tell us more about these ultra microelectrodes and why do they allow such very high scan rates ... 

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