Electrode Materials and Types

After the immunological event has taken part, it has to be transduced. As was commented in Section 9.1, a sensor consists of two parts recognition and transduction, usually very selective and sensitive. Both are commonly combined in a single device (except in the case of the so-called magnetoimmunosensors or automatized devices) with complicated architectures. Surface is carefully engineered to get better performance and the best analytical features. Therefore, the primary transducer (e.g., a SPE) is converted into a different one that could be called the secondary transducer. Connection with the primary transducer is often achieved with the help of conductive nanomaterials (e.g., CNTs, molecular wires, etc.) or polymers (e.g., polyaniline). In this section that is devoted to the important transduction of the immunological events, the main electrode materials and types, the approaches for the detection, the use of labels and label-free formats, as well as the main techniques are summarized. 

The important trend of assay parallelization (similarly to the ELISAs (enzyme linked immunosorbent assays) performed in microtiter plates) has increased the use of miniaturized electrodes and, film electrodes, especially those fabricated by screen-printed technology are widespreading. By their importance, these will be discussed in a separate section. 

The use of films as electrodes makes possible numerous experiments that would be difficult or impractical to implement with conventional bulk electrodes. Conductive films employed as electrodes are usually classified as thin (thickness in the nanometer range) or thick (in the micrometer scale). There are numerous film fabrication methods available, depending on the material, and are included in thin-film and thick-film technologies. The most common materials for thin-film electrodes are gold and platinum metals and are deposited as a continuous film by sputtering or vacuum evaporation, commonly on an insulating substrate such as quartz, fused silica, glass, or polymeric materials. In many cases, adhesion is improved by a thin layer of an intermediate material as titanium or chromium. 

Multielectrode configuration allows parallelization of the assays. In this way, eight-electrode arrays are available, in a way that material employed in conventional ELISA microtiter plates (8x 12 wells) such as eight-channel micropipettes could be used. This format was employed for detection of antimicrobial sulfonamides in honey [44] or clen-buterol in livestock urine [55] with 8 or 16-electrode arrays, respectively. Apart from these arrays, dual-electrode formats in which both working electrodes share the reference and auxiliary electrodes can be employed for single [45] or multianalyte determinations [143], similarly to what happens with a four-channel SPCE (screen-printed carbon electrode) design for simultaneous bianalyte determination [70] (see also Section 9.7.1). 

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Amperometry is the most widely reported EC detection mode for CE microchips, which primarily relies on oxidation or reduction of elect-rochemically active species by applying a constant potential to a working electrode. The current is then monitored as a function of time. Since it is based on the redox reaction that occurs at the electrode surface, electrodes can be miniaturised without loss in sensitivity. The relevance of this simple technique is reported in several reviews [48,74], In this section, a general overview of the combination of this detection technique to CE microchips together with special sections for different amperometric techniques and electrode materials and types are considered. 

Very little work (relative to research of electrode materials and electrolytes) is directed toward characterizing and developing new separators. Similarly, not much attention has been given to separators in publications reviewing batteries.A number of reviews on the on cell fabrication, their performance, and application in real life have appeared in recent years, but none have discussed separators in detail. Recently a few reviews have been published in both English and Japanese which discuss different types of separators for various batteries. A detailed review of lead-acid and lithium-ion (li-ion) battery separators was published by Boehnstedt and Spot-nitz, respectively, in the Handbook of Battery Materials. Earlier Kinoshita et al. had done a survey of different types of membranes/separators used in different electrochemical systems, including batteries."... 

In certain cases and together with the electrode reaction, in particular that of oxygen reduction at metal surfaces, a non-electrochemical regeneration mechanism operates which is heterogeneous in nature and involves the adsorbed product [165] obviously, it is very dependent on the electrode material and the available surface states. The reaction scheme is thus of the type... 

As pointed out above, the cathodic limit of an SSE can often be extended toward such negative potentials that the problem of identifying the electroactive species never becomes acute. However, three types of possible complication deserve mentioning, one mainly due to the nature of the electrode material and the others to the nature of the SSE. 

Several are the parameters that affect the areh-diseharge nanotubes production such as gas type, pressure and flow rate, eleetrie field strength, electrode materials and dimensions, in addition to imquantified variables sueh as... 

The emichment of tritium is usuaUy determined by the tritium separation (fractionation) factor during electrolysis, and by measuring the initial and final amounts of water. However, many workers have reported that the value of the separation factor of tritium depends on the electrode material, the type of electrolytic emiclunent ceU, the current density, the mode by which water is fed into the electrolytic cell, and the temperature of the electrolytic cell. In 1991, a rehable method was proposed for estimating tritium concentrations in water, based on a rehable correlation between the water electrolytic emiclunent of deuterium and tritium. The constancy of the ratio, k, during the electrolysis, k = a(fi — — 1), was... 

Electrosorption technique, which may use the electrical potential as the 3" driving force to the traditional adsorption and ion exchange mechanism, has reversible characteristics of purifying waste solution by adsorption and concentrating contaminants by desorption. Carbon materials satisfy the basic requirements for an efficient electrode material, and have good radiation and chemical-stability. Especially activated carbon fiber (ACF), which can be easily made into a variety of types (textures or sheet), has a high specific surfece area and electrical conductivity. 

Initial experiments were performed to verify and demonstrate the feasibility of the electrolytic reduction method with uranium, followed by experiments with a mixture of uranium and plutonium. Experiments were conducted batchwise in a small electrolytic cell. Basic parameters, such as concentration of solutes and type of holding agents (in the aqueous phase) for removal of any nitrite which would reoxidize the reduced heavy metal, electrode material and geometry, off-gas composition and type of diaphragm, were also determined. These data were valuable in the conceptual design of the first continuously operating column for the electrolytic reduction process. 

Vacuum arcs. This type of low-pressure arc, operating with cathode spots, is special because the gas-phase working fluid is provided by erosion and evaporation of the electrode material. This type of arc is of importance in high-current electrical equipment, high-current vacuum circuit breakers, and switches. 

In all primary lithium cells, the negative electrode is made of metallic lithium. Thus, different types of lithium cells differ in the positive electrode material and in the type of electrolyte. A variety of oxidant materials was offered as the active material of the positive electrode. These included different oxides, sulfides, selenides, oxysulfides, oxychlorides, and some other substances perfluorinated carbon and sulfur. However, only a small number of electrochemical systems in the cells actually reached the industrial production stage. The electrochemical systems of the cells produced industrially are given in Table 11.1. This Table also presents the values of open circuit voltage (OCV) of these cells and the theoretical values of their energy density. 

Although many models for the double layer have been published in the literature, there is no general model that can be used in all experimental situations. This is because the double-layer structure and its capacity depend on several parameters such as electrode material (metals, types of carbon, semiconductors, material porosity, the presence of layers of either oxides or polymeric films or other solid materials at the surface), type of solvent, type of supporting eleetrolyte, extent of specific adsorption of ions and molecules, and temperature. 

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