Electrochemical response

Most of the usual conducting polymers have a cross-linked stmcture (Fig. 3), but again they can be electrochemically oxidized and reduced. The electrochemical responses must follow electrochemical models and... 

If such a model is to be self-consistent, it has to include, and to be able to simulate, all the electrochemical responses treated by the previous models as well as all the so-called anomalous effects. 180-183 Any variable acting on those anomalous effects has to be described by the model without need of further development. 

The electrochemical responses were quantified through both compaction and relaxation coefficients that represent the energy required to compact or relax 1 mol of polymeric segments. A series of experiments were performed in each solvent by potential steps from different cathodic potentials (-1600, -1800, -2000, -2200, and -2400 mV vs. SCE) with the same anodic potential, changing this potential for every series (-250,... 

Electrochemical reaction orders in electrode polymerization, 317 Electrochemical relaxation, as a function of cathode potential, 388 Electrochemical responses during polymer formation, 400... 

Polymer formation icont.) diffusion components and, 421 diffusion control of oxidation in, 389 electrochemical responses, 400 influence of concentration, 397 and kinetic equations, 381 nucleation and, 379 oxidized area, 387... 

The electrochemical behavior of heterometallic clusters has been reviewed clsewbcre."" The interest in examining clusters stems from their potential to act as "electron sinks " in principle, an aggregate of several metal atoms may be capable of multiple redox state changes. The incorporation of heterometals provides the opportunity to tune the electrochemical response, effects which should be maximized in very mixed"-metal clusters. Few very mixed -metal clusters have been subjected to detailed electrochemical studies the majority of reports deal with cyclic voltammetry only. Table XII contains a summary of electrochemical investigations of "very mixed"-metal clusters. 

In addition to chromatography based on adsorption, ion pair chromatography (IP-HPLC) and capillary electrophoresis (CE) or capillary zone electrophoresis (CZE) are new methods that became popular and are sufficiently accurate for these types of investigations. Other methods involving electrochemical responses include differential pulse polarography, adsorptive and derived voltammetry, and more recently, electrochemical sensors. 

While electroanalytical techniques are inherently quite sensitive, the resolution of a mixture of electroactive compounds is very difficult. Practical considerations limit the usable potential window to no more than 3 V and typically around 1.5 V. This is because at more extreme potentials the medium or the electrode itself begin to oxidize or reduce. In addition, the electrochemical response of compounds as a function of applied potential is fairly broad so that at least a 200-400 mV difference in half-wave potentials is required for adequate resolution. This typically limits electrochemical resolution of mixtures to no more than three or four electroactive compounds. 

The electrochemical responses of the 100 electrodes in the array were measured simultaneously using a potentiostat, two 64-channel current followers, and two data... 

The large size of redox enzymes means that diffusion to an electrode surface will be prohibitively slow, and, for enzyme in solution, an electrochemical response is usually only observed if small, soluble electron transfer mediator molecules are added. In this chapter, discussion is limited to examples in which the enzyme of interest is attached to the electrode surface. Electrochemical experiments on enzymes can be very simple, involving direct adsorption of the protein onto a carbon or modified metal surface from dilute solution. Protein film voltammetry, a method in which a film of enzyme in direct... 

If the analyte is electrochemically inactive, it is possible to add a background of an electrochemically active material to the eluent post-column. If dispersion is kept a minimum, the analyte elutes as a well defined band with a lower electrochemical response than the background, forming a vacancy peak. 

J. Hayon, D. Ozer, J. Rishpon, and A. Bettelheim, Spectroscopic and electrochemical response to nitrogen monoxide of a cationic iron porphyrin immobilized in nafion-coated electrodes or membranes. J. Chem. Soc.-Chem. Commun. 619-620 (1994). 

The electrochemical response of analytes at the CNT-modified electrodes is influenced by the surfactants which are used as dispersants. CNT-modified electrodes using cationic surfactant CTAB as a dispersant showed an improved catalytic effect for negatively charged small molecular analytes, such as potassium ferricyanide and ascorbic acid, whereas anionic surfactants such as SDS showed a better catalytic activity for a positively charged analyte such as dopamine. This effect, which is ascribed mainly to the electrostatic interactions, is also observed for the electrochemical response of a negatively charged macromolecule such as DNA on the CNT (surfactant)-modified electrodes (see Fig. 15.12). An oxidation peak current near +1.0 V was observed only at the CNT/CTAB-modified electrode in the DNA solution (curve (ii) in Fig. 15.12a). The differential pulse voltammetry of DNA at the CNT/CTAB-modified electrode also showed a sharp peak current, which is due to the oxidation of the adenine residue in DNA (curve (ii) in Fig. 15.12b). The different effects of surfactants for CNTs to promote the electron transfer of DNA are in agreement with the electrostatic interactions... 

Some efforts have been taken to obtain the electrochemical response of Hb at solid electrode surfaces. Fan s electrochemical researches revealed that the electron-transfer reactivity of Hb could be greatly enhanced, simply by treating it with an organic solvent, dimethyl sulfoxide (DMSO) [115], Hb can also achieve its direct electron transfer in /V,/V-dimcthy I form am idc (DMF) film, as Xu [116] reported. These, therefore, suggested that there are many different factors that regulate electron-transfer reactivity of proteins. It also pointed out the complicated and precise regulation mechanisms of proteins in vivo. 

XOD is one of the most complex flavoproteins and is composed of two identical and catalytically independent subunits each subunit contains one molybdenium center, two iron sulfur centers, and flavine adenine dinucleotide. The enzyme activity is due to a complicated interaction of FAD, molybdenium, iron, and labile sulfur moieties at or near the active site [260], It can be used to detect xanthine and hypoxanthine by immobilizing xanthine oxidase on a glassy carbon paste electrode [261], The elements are based on the chronoamperometric monitoring of the current that occurs due to the oxidation of the hydrogen peroxide which liberates during the enzymatic reaction. The biosensor showed linear dependence in the concentration range between 5.0 X 10 7 and 4.0 X 10-5M for xanthine and 2.0 X 10 5 and 8.0 X 10 5M for hypoxanthine, respectively. The detection limit values were estimated as 1.0 X 10 7 M for xanthine and 5.3 X 10-6M for hypoxanthine, respectively. Li used DNA to embed xanthine oxidase and obtained the electrochemical response of FAD and molybdenum center of xanthine oxidase [262], Moreover, the enzyme keeps its native catalytic activity to hypoxanthine in the DNA film. So the biosensor for hypoxanthine can be based on... 

Y. Sato and F. Mizutani, Electrochemical responses of cytochrome c on gold electrodes modified with nucleic acid base derivatives - electrochemical and quartz crystal microbalance studies. Electrochim. Acta 45, 2869-2875 (2000). 

Building on the success of 37 in recognition of barium ions, this unit was incorporated into monomer units derived from pyrrole and EDOT (42, 43 and 44).113 Electrodes modified by the corresponding electrodeposited polymers showed an electrochemical response in the presence of Ba2+ which was similar to 37, albeit of a lesser magnitude. 

Bard AJ, Crayston JA, Kittlesen GP, Shea TV, Wrighton MS (1986) Digital simulation of the measured electrochemical response of reversible redox couples at microelectrode arrays consequences arising from closely spaced ultramicroelectrodes. Anal Chem 58 2321-2331... 

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