Sensing electrode surface

Table II is a list of gases which could potentially interfere with an N0 analysis along with the concentrations of these gases which produce a signal equivalent to 1 ppm N0. Only H2S had an effect on sensor cell performance and was found to decrease the response level by 0.2 a/ppm. H2S, SO2 and NO2 were effectively filtered from the gas stream by use of triethanolamine (TEA) as shown in Table II. To prevent TEA vapors from reaching (and thus poisoning) the sensing electrode surface, a short column of a cation exchange bead was placed after the filter.

Potcntiomctric Biosensors Potentiometric electrodes for the analysis of molecules of biochemical importance can be constructed in a fashion similar to that used for gas-sensing electrodes. The most common class of potentiometric biosensors are the so-called enzyme electrodes, in which an enzyme is trapped or immobilized at the surface of an ion-selective electrode. Reaction of the analyte with the enzyme produces a product whose concentration is monitored by the ion-selective electrode. Potentiometric biosensors have also been designed around other biologically active species, including antibodies, bacterial particles, tissue, and hormone receptors. 

Chemically modified electrodes (CMEs) represent a modem approach to electrode systems. These rely on the placement of a reagent onto the surface, to impart the behavior of that reagent to the modified surface. Such deliberate alteration of electrode surfaces can thus meet the needs of many electroanalytical problems, and may form the basis for new analytical applications and different sensing devices. 

If substrate diffusion becomes rate determining, only a small fraction of the film at the film/solution interface will be used. On the other hand, if charge diffusion becomes rate determining, the catalytic reaction can take place only in a film fraction close to the electrode surface. Each of these effects will render parts of the film superfluous, and it is obvious that there is no sense in designing very thick redox films, rather there is an optimal layer thickness to be expected depending on the individual system. 

Gas-sensing electrodes consist of an ion-selective electrode in contact with a thin layer of aqueous electrolyte that is confined to the electrode surface by an outer membrane as shown schematically for a COj electrode in Fig. 2. The outer membrane,... 

Conventional ion-selective electrodes have been used as detectors for immunoassays. Antibody binding measurements can be made with hapten-selective electrodes such as the trimethylphenylammonium ion electrode Enzyme immunoassays in which the enzyme label catalyzes the production of a product that is detected by an ion-selective or gas-sensing electrode take advantage of the amplification effect of enzyme catalysis in order to reach lower detection limits. Systems for hepatitis B surface antigen and estradiol use horseradish peroxidase as the enzyme label and... 

Thiols are easily oxidized to disulfides in solution, but this reaction occurs only very slowly at most electrode surfaces. However, use can be made of the unique reaction between thiols and mercury to detect these compounds at very favorable potentials. The thiol and mercury form a stable complex which is easily oxidized, in a formal sense it is mercury and not the thiol which is actually oxidized in these reactions. For the LCEC determination of thiols a Au/Hg amalgam electrode is used Using a series dual-electrode both thiols and disulfides can be determined in a single chromatographic experiment... 

Special electrochemical sensors that operate on the principle of the voltammetric cell have been developed. The area of chemically modified solid electrodes (CMSEs) is a rapidly growing field, giving rise to the development of new electroanalytical methods with increased selectivity and sensitivity for the determination of a wide variety of analytes [490]. CMSEs are typically used to preconcentrate the electroactive target analyte(s) from the solution. The use of polymer coatings showing electrocatalytic activity to modify electrode surfaces constitutes an interesting approach to fabricate sensing surfaces useful for analytical purposes [491]. 

Wall-mounted probes, which measure field rather than electrostatic potential, may seem a better choice than suspended probes, but they suffer from the serious problem that charged fines often adhere tenaciously to the sensing electrodes and other surfaces of probes. These charged particles will shield the sensing electrode from the field within the bed and interfere with the measurements. Probably for this reason, very few attempts to use wall-mounted probes are found in the literature. 

The molecular models are in a sense a subset of the geometrical ones - we assume that we know which molecules are present and we assume that we know their geometries (indeed sometimes we assume more than that, such as the usual assumption that thiol end groups lose their protons when forming their asymmetric bond with gold). In this we also necessarily assume that there are no other species, either on the electrode surface or in the surrounding media, that influence the current flow through the system. 

Bearing in mind that diffusing ions move randomly in all directions, it is reasonable to say that the diffusing ions sense selfsimilar scaling property of the electrode surface irrespective of whether the fractal surface has self-similar scaling property or self-affine scaling property. Therefore, it is experimentally justified that the fractal dimension of the self-affine fractal surface determined by using the diffusion-limited electrochemical technique represents the apparent self-similar fractal dimension.43... 

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 most convenient and reliable electrical biasing method for use with a hydrated SPE cell has been shown to be a three electrode potentiostatic circuit which maintains the sensing electrode at a predetermined potential vs. a stable reference (1 >3.>j0e The most reversible reference is a Pt/Hp, H+, static or dynamic. In practical instruments, however, good accuracy and convenience are achieved using a large surface area platinoid metal black/air (Op). All work reported in this study utilized the air reference which has a potential of approximately +1.05 V vs. a standard hydrogen electrode (SHE). For convenience, all potentials reported are vs. the SHE ... 

Electrochemical Reaction/Transport. Electrochemical reactions occur at the electrode/electrolyte interface when gas is brought to the electrode surface using a small pump. Gas diffuses through the electrode structure to the electrode/electrolyte interface, where it is electrochemically reacted. Some parasitic chemical reactions can also occur on the electrocatalytic surface between the reactant gas and air. To achieve maximum response and reproducibility, the chemical combination must be minimized and controlled by proper selection of catalyst sensor potential and cell configuration. For CO, water is required to complete the anodic reaction at the sensing electrode according to the following reaction ... 

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... 

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