Electrode materials sensing electrodes

Electrolyte material Sensing electrode Reference electrode... 

Biocatalytic membrane electrodes have an ISE or a gas sensing electrode in contact with a thin layer of biocatalytic material, which can be an immobilized enzyme, bacterial particles or a tissue slice, as shown in Fig. 3 The biocatalyst converts substrate (the analyte) into product, which is measured by the electrode. Electrodes of this type are often referred to as biosensors . 

CNTs have been one of the most actively studied electrode materials in the past few years due to their unique electronic and mechanical properties. From a chemistry point of view, CNTs are expected to exhibit inherent electrochemical properties similar to other carbon electrodes widely used in various electrochemical applications. Unlike other carbon-based nanomaterials such as C60 and C70 [31], CNTs show very different electrochemical properties. The subtle electronic properties suggest that carbon nanotubes will have the ability to mediate electron transfer reactions with electroactive species in solution when used as the electrode material. Up to now, carbon nanotube-based electrodes have been widely used in electrochemical sensing [32-35], CNT-modified electrodes show many advantages which are described in the following paragraphs. 

The schematic diagram of a gas-sensing electrode is illustrated in Figure 16.8, that comprises of essentially a reference electrode (E), a specific-ion electrode (B), and an internal electrolyte solution (F) contained in a cylindrical plastic tube (G). One end of the plastic tubing is provided with a thin, replaceable, gas-permeable membrane that separates the internal electrolyte solution from the external solution containing gaseous analyte. However, the exact composition and specifications of this gas-permeable membrane is usually described by its respective manufacturers. It is normally made up of a thin microporous film fabricated from a hydrophobic plastic material. 

Qiu, J.-D., et al., Controllable deposition of a platinum nanoparticle ensemble on a polyaniline/graphene hybrid as a novel electrode material for electrochemical sensing. Chemistry - A European Journal, 2012.18(25) p. 7950-7959. 

Strictly speaking, electrocatalysis applies to the dependence of the electrode reaction rate on the nature of the electrode material [152]. In the following, this term will be used in a broader sense and will be admitted to include the possibility that the catalyst be homogeneously dissolved in the electrolyte solution as well as the case where the catalyst is attached to the electrode surface. A short chapter on the electrocatalysis of inorganic chemicals by chemically modified electrodes can also be found in Vol. 10 of this Encyclopedia [9]. 

The reactant gas must diffuse through the electrode structure which contains air (02, N2) and any products of reaction (CO2, N02, NO, H2O vapor, etc.). Response characteristics are dependent on electrode material, Teflon content, electrode porosity, thickness and diffusion/reaction kinetics of the reactant gas on the catalytic surface. By optimizing catalytic activity for a given reaction and controlling the potentiostatic voltage on the sensing electrode, the concentration of reactant gas can be maintained at essentially zero at the electrode/electrolyte interface. All reactant species arriving at the electrode/electrolyte interface will be readily reacted. Under these conditions, the rate of diffusion is proportional to C, where... 

Another consideration is that the rds in a given reaction may change with pH, with temperature, and with catalyst. It would hardly make good sense to go through a detailed and costly investigation on just one electrode material at one set of conditions (see Table 7.15). 

The construction materials of each sensor part will influence its operating characteristics, as illustrated in the following examples. Choosing a Au rather than a Pt electrocatalyst for the sensing electrode allows for selective determination of in the... 

CO sensor allows detection of CO in the presence of hydrocarbons and other adsorbable contaminants. The membrane Is usually chosen for Its ability to protect the sensing electrode. However, If It has low permeability to air, the sensor will have a slower response time. The electrolyte and counter electrode have also been reported as Influencing selectivity and device performance In the determination of hydrazines (5) and NO2 (9), respectively. Finally, materials of construction are typically Teflon and high-density plastics like polypropylene because such materials must be compatible with reactive gases and corrosive electrolytes. 

The example considered is the redox polymer, [Os(bpy)2(PVP)ioCl]Cl, where PVP is poly(4-vinylpyridine) and 10 signifies the ratio of pyridine monomer units to metal centers. Figure 5.66 illustrates the structure of this metallopolymer. As discussed previously in Chapter 4, thin films of this material on electrode surfaces can be prepared by solvent evaporation or spin-coating. The voltammetric properties of the polymer-modified electrodes made by using this material are well-defined and are consistent with electrochemically reversible processes [90,91]. The redox properties of these polymers are based on the presence of the pendent redox-active groups, typically those associated with the Os(n/m) couple, since the polymer backbone is not redox-active. In sensing applications, the redox-active site, the osmium complex in this present example, acts as a mediator between a redox-active substrate in solution and the electrode. In this way, such redox-active layers can be used as electrocatalysts, thus giving them widespread use in biosensors. 

Conventional humidity sensors of the electric resistance variable type use hydrocarbon polyelectrolyte as a moisture sensing material. Therefore, the sensors usually have insufficient heat resistance, and cannot be used at temperatures of 60°C or more. Another problem is that they deteriorate when in contact with cigarette smoke and oil contained in the air [64,65]. When the fluorinated pitch-deposited coating was breathed upon, the electrical resistance quickly decreased, but electrical resistance quickly recovered when this action was stopped. Then, how to develop a humidity sensor excelling in humidity response sensitivity, heat resistance and durability was attempted [66]. Two kinds of comb-like electrodes with different electrode gaps were made, and a thin film was formed on the surfaces by vacuum deposition of fluorinated pitch. The obtained fluorinated pitch sensors were left at rest in a thermostatic chamber, and electrical resistance was determined under the following conditions. 

For the detection different electrode supports and a great variety of immobilization procedures have been used. Gold electrodes, screen-printed electrodes and carbon materials (such as graphite and glassy carbon) have been frequently used. The sensing phase is constructed onto the surface of the electrode by means... 

No single consumable electrode is ideal for all iontophoretic applications. Different materials meet different capacity needs, and because consumable electrodes consist of chemically reactive species, certain materials may be compatible with certain drugs or excipients but not all of them. The most popular electrodes are based on the silver/silver chloride redox couple. Silver and silver chloride have several advantageous characteristics They are biocompatible, perform well, and have an established history of use in medical applications including sensing electrodes. 

It is important to note that proteins tend to denature during such an adsorption process on noble metals or carbon electrodes. In addition, the stability of the adsorbed sensing layer is highly dependent on the pH value and ionic strength of the solution as well as the temperature, the electrode material, and other additional factors. For instance, as early as 1972 direct ET was observed on mercury electrodes employing cytochrome c as redox protein [108]. Reversible electrochemical behavior of cytochrome c was not observed because the protein denatured on the surface. 

Due to their ability to accommodate anionic guests in the interlayer region, such materials have been tested for different applications, from catalysis to sensing and as electrode materials for Ni batteries, as described in Chapter 10. Figure 6.5 shows the CV response of Co-Al LDH immersed into 0.10 M NaOH at two different potential... 

Porous materials continue to attract considerable attention because of their wide variety ot scientific and technological applications, such as catalysis, shape- and size-selective absorjition and adsorption, gas storage, and electrode materials. Roth research and applications of porous materials—via electroanalysis, electrosynthesis, sensing, fuel cells, capacitors, electro-optical devices, and other means—heavily rely on electrochemistry. 

As mentioned above (and shown in Figure 13.9c), when electrolyte chains are used the electrolyte in contact with the reference electrolyte can be different from that used for the sensing electrode, which provides flexibility in the design of the reference electrode material. 

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