Ultramicroelectrodes types

The UMEs used in bioarrays can be divided into three types disk, ring, and strip electrodes. The theory of the disk, ring, and strip UMEs has been extensively studied [97-100], Due to the edge effect, the profile of the mass diffusion to the ultramicroelectrode surface is three dimensional, and can significantly enhance the mass transportation in comparison to the conventional large electrode with one-dimensional mass transportation. The steady-state measurement at a planar UME can be expressed as... 

Recently, arrayed electrodes, which consist of several ultramicroelectrodes of the same type or of different types are prepared and used in sophisticated ways [8h],... 

Solid-state electrochemistry — is traditionally seen as that branch of electrochemistry which concerns (a) the -> charge transport processes in -> solid electrolytes, and (b) the electrode processes in - insertion electrodes (see also -> insertion electrochemistry). More recently, also any other electrochemical reactions of solid compounds and materials are considered as part of solid state electrochemistry. Solid-state electrochemical systems are of great importance in many fields of science and technology including -> batteries, - fuel cells, - electrocatalysis, -> photoelectrochemistry, - sensors, and - corrosion. There are many different experimental approaches and types of applicable compounds. In general, solid-state electrochemical studies can be performed on thin solid films (- surface-modified electrodes), microparticles (-> voltammetry of immobilized microparticles), and even with millimeter-size bulk materials immobilized on electrode surfaces or investigated with use of ultramicroelectrodes. The actual measurements can be performed with liquid or solid electrolytes. 

Electrodes can be divided into four categories according to their characteristic dimensions 1) Macroelectrodes, such as parallel plates, 2) Microelectrodes, 3) Ultramicroelectrodes, and 4) Nanoelectrodes. The four types of electrodes are illustrated in Figure 11 together with their characteristic electrical properties important for their applicability resistance, R, double-layer capacitance, Cdi, and current, /. 

SECM involves the measurement of the current through an ultramicroelectrode (UME) (an electrode with a radius, a, of the order of a few nm to 25 (zm) when it is held or moved in a solution in the vicinity of a substrate. Substrates, which can be solid surfaces of different types (e.g., glass, metal, polymer, biological material) or liquids (e.g., mercury, immiscible oil), perturb the electrochemical response of the tip, and this perturbation provides information about the nature and properties of the substrate. The development of SECM depended on previous work on the use of ultramicroelectrodes in electrochemistry and the application of piezoelectric elements to position a tip, as in scanning tunneling microscopy (STM). Certain aspects of SECM behavior also have analogies in electrochemical thin-layer cells and arrays of interdigitated electrodes. 

While most of the SECM work has been carried out with amperometric tips for measuring feedback current or for use in the generation/collection mode, other types of tips such as potentiometric and enzymatic tips are also possible but are only briefly described in Section II.D. The readers who are interested in these tips are encouraged to refer to the appropriate chapters in this monograph. Finally, Section III deals with an approach to determining the shape of an ultramicroelectrode from its SECM response. 

The considerable complexity of SECM theory is due to the combination of a cylindrical diffusion to the ultramicroelectrode (UME) tip and a thin-layer-type diffusion space. The time-dependent diffusion problem for a simple quasireversible reaction in cylindrical coordinates is as follows (2,3) ... 

Consider an electrode covered with a film that has continuous pores or channels from the solution to the electrode (Figure 14.4.1, process 6). We can ask how the electrolysis of a species in solution at such an electrode differs from that at the bare (unfilmed) electrode. The answer depends upon the extent of coverage of the electrode by the film, the size and distribution of the pores, and the time scale of the experiment. The situation is complicated, because the pores can have different dimensions and degrees of tortuosity, and their distribution within the film may not be uniform. Thus, theoretical treatments of such films often use idealized models. The theory for electrodes of this type is closely related to that for ultramicroelectrode arrays (Section 5.9.3), which, however, often involve a better-defined geometry and uniform distribution of active sites (81, 82). 

The type of set up shown in Fig. 6 is only used for situations where low currents are measured (100 nA and smaller for typical electrolyte solutions), notably for experiments with microelectrodes (or ultramicroelectrodes), discussed in detail in Chapter 2.5. The requirement of a small current is both to make the ohmic term negligible and to ensure that there are minimal changes to the reference electrode composition that would, otherwise, lead to an unstable reference electrode potential. The ohmic term may, of course, also be minimized by making / goi as small as possible, for example, by working with added supporting electrolyte to increase... 

Electrochemical stripping analysis is carried out in a conventional ( beaker-type ) three-electrode cell (of 5-50-ml volume). The three electrodes, as well as the tube used for bubbling the deoxygenating gas, are supported in five holes in the cell cover. Various microceUs with 20 to 500 pi volumes can be used when the sample volume is limited. Particularly attractive are thin-layer cells in which the entire sample is confined within a thin layer (of less than 10-pm thickness) at the electrode surface [6]. Smaller sample volumes can be accommodated in connection with ultramicroelectrodes and advanced microfabrication processes. The latter result in planar strips that can be considered as disposable electrochemical cells onto which sample droplets are placed. 

In the last decade special attention has been paid to this type of electrode arrangement. Two eventualities appeared in electroanalytical practice. The first one is represented by assemblies of casually or regularly arranged micro- or ultramicroelectrodes. The second eventuality is realized by a set of microbands that have the length of macroscopic dimension (usually several mm of magnitude, cf. Fig. 10). These types are not microelectrodes in the proper sense of this term. They are fabricated usually by microlithographic techniques. 

Although there are many types of in vivo electrochemical sensors. All of these must meet certain criteria to provide interpretable results. The electrodes must exhibit a known selectivity for the ions or molecules of interest, and the response of the electrode should allow predictable calibration of the experiments. For situations where the electrode will be implanted for chronic use, sometimes for periods of time as long as a year, these requirements also exist, but require more stringent consideration of the stability of the electrode response. Nevertheless, these criteria have been met in many different types of applications. Thus, electrodes can be used as chemical sensors in a particular organ, or with ultramicroelectrodes, in a single cell. 

Figure 6.13.1 Different shapes of ultramicroelectrodes (UMEs). (A) A perfect microdisk electrode. (B) A conical disk-type planar UME due to sealing of the tnicrowire at an angle relative to the insulating shroud. (C) An irregular cylindrical protruding from the insulator. (D) A recessed microdisk electrode. (E) A recessed electrode in which the cavity in the insulator is large than the...

Studies using ultramicroelectrodes have been shown of great interest for electroanalysis [44], but initially the material for use on construction was known as fiber carbon. Such materials are prepared by carbonization on controlled high temperature of the polymer textiles or other means of obtaimnent is via catalytic chemical vapor deposition [16, 45], There are three types of carbon fiber with depending on the manufacturing process. Exist three types of... 

Scanning electrochemical microscopy (SECM) and related ultramicroelectrode (UME) methods have proven powerful for measuring the kinetics of homogeneous reactions coupled to heterogeneous electron transfer. For this type of investigation, the tip and substrate are both electrodes and one can usefully consider the tip/substrate electrode configuration as a variable gap ultrathin layer cell. In essence, the gap thickness determines the diffusional transit time of chemical species between the tip and substrate and hence the range of timescales that can be studied. 

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