Oxidation-reduction sensors

Two methods are used to measure pH electrometric and chemical indicator (1 7). The most common is electrometric and uses the commercial pH meter with a glass electrode. This procedure is based on the measurement of the difference between the pH of an unknown or test solution and that of a standard solution. The instmment measures the emf developed between the glass electrode and a reference electrode of constant potential. The difference in emf when the electrodes are removed from the standard solution and placed in the test solution is converted to a difference in pH. Electrodes based on metal—metal oxides, eg, antimony—antimony oxide (see Antimony AND ANTIMONY ALLOYS Antimony COMPOUNDS), have also found use as pH sensors (8), especially for industrial appHcations where superior mechanical stabiUty is needed (see Sensors). However, because of the presence of the metallic element, these electrodes suffer from interferences by oxidation—reduction systems in the test solution. 

Photoinduced electron transfer (PET) has been widely used as the preferred tool in fluorescent sensor design for atomic and molecular species [52-57], PET sensors generally consist of a fluorophore and a receptor linked by a short spacer. The changes in the oxidation/reduction potential of the receptor upon guest binding can alter the PET process creating changes in fluorescence. 

High-temperature stabilized NO-, zirconia potentiometric sensors are also being utilized [187], The electrochemical reactions on zirconia devices take place at the triple-phase boundary, that is, the junction between the electrode, electrolyte, and gas [186], It has been reported that sensors composed of a W03 electrode, yttria-stabilized zirconia electrolyte, and Pt-loaded zeolite filters demonstrate high sensitivity toward NO,, and are free from interferences from CO, propane, and ammonia, and are subject to minimal interferences from humidity and oxygen, at levels typically present in combustion environments [188], In this sensor, a steady-state potential arises when the oxidation-reduction reaction [186,188]... 

The chemical stability and electrochemical reversibility of PVF films makes them potentially useful in a variety of applications. These include electrocatalysis of organic reductions [20] and oxidations [21], sensors [22], secondary batteries [23], electrochemical diodes [24] and non-aqueous reference electrodes [25]. These same characteristics also make PVF attractive as a model system for mechanistic studies. Classical electrochemical methods, such as voltammetry [26-28] chronoamperometry [26], chronopotentiometry [27], and electrochemical impedance [29], and in situ methods, such as spectroelectrochemistry [30], the SECM [26] and the EQCM [31-38] have been employed to this end. Of particular relevance here are the insights they have provided on anion exchange [31, 32], permselectivity [32, 33] and the kinetics of ion and solvent transfer [34-... 

Several commercial evidential breath alcohol measurement devices are available. The principle of measurement is either infrared absorption spectrometry (most common), dichromate-sulfuric acid oxidation-reduction (photometric), GC (flame ionization or thermal conductivity detection), electrochemical oxidation (fuel cell), or metal-oxide semiconductor sensors. A list has been published of DOT-approved breath alcohol devices.Some of these devices are approved for screening only. In this case, the second or confirmatory breath alcohol determination must be performed with an approved evidential breath alcohol analyzer. Breath alcohol devices may also be used for the medical evaluation of patients at the point of care (e.g., emergency department). A Fourier transform infrared point-of-care breath analyzer capable of measurement of... 

Petronilli, V., Costantini, P., Scorrano, L., Colonna, R., Passamonti, S., and Bemardi, P. (1994). The voltage sensor of the mitochondrial permeability transition pore is tuned by the oxidation-reduction state of vicinal thiols. Increase of the gating potential by oxidants and its reversal by reducing agents. J Biol Chem 269, 16638 2. 

For each parameter, the pH, DO (dissolved oxygen), ORP (oxidation-reduction potential), temperature, agitation speed, culture volume and pressure can be measured with sensors located in the fermenter. The output of the individual sensors is accepted by the computer for the on-line, continuous and real-time data analysis. Information stored in the computer control system then regulates the gas flow valves and the motors to the feed pumps. A model of a computer control system is shown in Fig. 11. The computer control systems, like the batch systems for mammalian cell culture, seem to level out at a maximum cell density of 10 cells/ml. It may be impossible for the batch culture method to solve the several limiting factors (Table 10) that set into high density culture where the levels are less than 10 cells/ml. 

Ihble 14-9 shows the enzyme sensors developed so far using biocatalytic analyte recycling for signal amplification. The Hrst of such sensors have been studied for measurement of pyridine nucleotides, glucose, and lactate [321, 324]. The NAD /NADH sensor involves the oxidation-reduction of the analyte by horseradish peroxidase and glucose dehydrogenase ... 

Qtdnhydrone Electrode. The quinhydrone electrode is an important hydrogen-ion electrode, and is perhaps typical of a whole class of such electrodes which function as pH sensors owing to a reversible organic oxidation-reduction pair involving protons. Quinhydrone (an equimolar compound of benzoquinone and hydroquinone) is only slightly soluble in water. The reversible oxidation-reduction couple... 

The resistance of a tin oxide gas sensor consists of bulk resistance, surface resistance and contact resistance. The reduction of contact resistance is useful for improving the properties of oxide semiconductor gas sensors. An ohmic contact between the electrode and sensing material can reduce the contact resistance. Zhou et al. compared conventional tin dioxide-gold electrode structures with devices in which an n+ layer was introduced between the sensor and electrode. The use of the metal-n+-n contact not only improved the sensitivity of the sensor to alcohol, but also the sensor selectivity to other gases did not change with the addition of an n+ layer. 

The unique structure and electronic properties of CNTs provide a tremendous potential for construction of CNTs and MOX hybrid materials in the field of gas-sensing applications. Advantages for mixing CNTs in metal oxides for gas sensors are the reduction of operating temperature and enhancement of sensitivity and selectivity due to the amplification effects of p-n heterojunctions with the gas reaction, formation of nanochannels for gas diffusion, high specific surface area, and increase of charge carrier on the surface. As a result of these advantages, the hybrid CNT/metal oxide gas sensor may be used instead of the popular commercial metal oxide gas sensors (such as TGS gas sensors) in the near future. 

Conjugated polymers, discovered in the late 1970s, have attracted a variety of attentions because of their unique properties, such as electrical conductivity and color versatility. The conjugated polymers with different colors can be used as ideal electrochromic materials, which have potential applications in sensors, mirrors, displayers, and textiles. Most of their reversible electrochromic behaviors are caused by the electro-induced oxidation-reduction, that is, the reversible change of a chemical species between two redox states under a certain voltage (Niklasson and Granqvist, 2007 Beaujuge et al., 2010). 

The same solid-gas interfaee can be the scene of multiple simultaneous and independent oxidation-reduction reactions. This has to be taken into consideration when modeling the electrical response of some electrochemical sensors, particularly those located in a gaseous atmosphere containing oxidant and reducing gases. 

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