Electroactive compounds

A liquid chromatographic experiment resulted in the same retention time for the electroactive compounds A and B. Which electrochemical detection scheme would offer a selective detection of the two coeluting analytes Explain your selection. (E = +0.43 V Eg = +0.77 V.)... 

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. 

Voltammetry has been adapted to HPLC (when the mobile phase is conducting), and CE as a detection technique for electroactive compounds. In this usage, the voltammetric cell has been miniaturised (to about 1 p.L) in order not to dilute the analytes after separation. This method of amperometric detection in the pulsed mode is very sensitive. However, this device makes it possible to detect few analytically important molecules besides phenols, aromatic amines and thiols. 

Audebert, P. and Sanches, C. (1994) Modified electrodes from hydrophobic alkoxide silica gels — insertion of electroactive compounds as glucose oxidase. Journal of Sol-Gel Science and Technology, 2, 809-812. 

Amperometric sensors for redox-inactive cations and electroactive compounds... 

AMPEROMETRIC SENSORS FOR REDOX-INACTIVE CATIONS AND ELECTROACTIVE COMPOUNDS... 

Whereas detection of electroinactive ions was principally worked out at the end of last century, the use of transition metal hexacyanoferrates as sensors for various electroactive compounds still attracts particular interest of scientists. Although the cross-selectivity of such compounds must be low, a number of them have been successfully used for analysis of real objects. 

Originally, a Clark oxygen electrode was used to measure a reduction in current due to the consumption of oxygen. Anodic detection of the hydrogen peroxide by oxidation at a platinum or carbon electrode was then introduced but, owing to the high electrode potential required, suffered from interference from other electroactive compounds in the sample. 

Amperometric detection is based on transfer of an electron to or from an electroactive compound at a solid electrode under the influence of externally applied DC voltage. Amperometric detection is limited to a certain type of compounds and is quite difficult to mount on a CE unit. ... 

Electrochromic materials are electroactive compounds whose visible spectra depend on the oxidation state. Possible applications are smart windows, displays, mirrors, and so on. Among the most important performance aspects in electrochromic materials, the reversibility and lifetime of the material to repeated cycles, the time of response (usually in order of seconds), the colors of the oxidized/reduced forms and the change in absorbance upon redox switching (contrast) are of interest. 

Unfortunately in quite a few cases voltammetric data become inaccessible because the electroactive compound is adsorbed on the electrode. By variation of the electrode material, the concentration of the different solutes, the solvent and the scanning time adsorption phenomena may occasionally be overcome. 

Nearly all the nonfluorescent emission bands reported in these studies have been found at longer wavelengths than the expected fluorescence. They have been variously ascribed to eximers6,15 or to phosphorescence1315 of the electroactive substance. Some apparently do not arise from the electroactive compound itself.16 On oxidation of the pyrene anion radical with Wurster s blue perchlorate, an emission band was obtained which corresponded closely to the known pyrene eximer emission.15 Several spectra obtained from polycyclic aromatic hydrocarbons by electrochemical treatment at constant applied voltage have... 

Cuest-Induced Changes in Membrane Permeability. Calixarene derivatives are also used for sensing systems other than ISEs or optodes. Recently, a systematic investigation on the control of membrane permeability by use of oriented monolayers composed of calixarene esters was carried out. The hosts used were short alkyl chain esters of calix[6]arene [28 (R = Bu )] and calix[4]arene [26 (R = Bu ), 30 both cone conformers]. The permeabilities through the intermo-lecular voids of these monolayers were evaluated by cyclic voltammetry, as described earlier for oriented membranes of nucleobase derivatives. Cationic, anionic, and neutral electroactive compounds were used as the permeability markers. The voltammetric measurements were carried out either for a monolayer... 

In the context of the voltammetry of microparticles methodology, the H-point standard addition method has been adapted for determining organic dyes [241] as well as lead and tin in ceramics [242]. Let us consider a mixture of material containing unknown amounts of two electroactive compounds, A and B, and a reference compound, R. It is assumed that weighted amounts of both materials are accurately powdered and thoroughly mixed so that the mass ratio between the A,B-containing material and the reference compound, m/mR, is known. 

If all A, B, and R compounds are electroactive in a suitable electrolyte, voltam-mograms of the mixture must provide peaks corresponding to their respective redox processes. In solid-state voltammetry, in which separate microparticles of each one of the electroactive compounds are mechanically transferred to the surface of an inert electrode, independent electrochemical processes must occur. Accordingly, peak currents, ip j) j =A,B,R), recorded for A-, B-, and R-centered voltammetric processes, can generally be taken as proportional to the amount of each one of the compounds deposited on the electrode [131, 243-245]. If separate peaks are recorded for A, B, and R, the respective peak currents must satisfy the relationship... 

Figure 7.1 Representative examples of electroactive compounds with pendant thiol groups able to self-assemble into monolayers on the surface of gold electrodes.

Figure 7.4 Electroactive compounds incorporating 4,4 -bipyridinium dications and either one (6) or two (7) thiol groups.

Numerous postcolumn enzymatic reactors have been designed for LCEC. Enzymes can be used to produce an electroactive compound from the analyte of interest or, alternatively, to generate an electroactive species that is proportional to analyte concentration. An example of the latter is the detection of acetylcholine [46]. In this case, acetylcholinesterase is used to convert acetylcholine to choline. The resulting choline is reacted with choline oxidase to produce hydrogen peroxide. The amount of hydrogen peroxide produced is directly proportional to the initial concentration of acetylcholine. Detection limits are in the 100 femto-mole range. 

One of the advantages of CE, which is also a disadvantage, is the very small volume needed for analysis. Typical injection volumes are 1-10 nL. This ability to inject small volumes has been shown to be useful in the analysis of single cells and microdialysis samples. Although the concentration-based detection limits of LCEC and CEEC are similar, detection limits based on actual mass analyzed are much lower for CEEC. Typical mass limits of detection for an electroactive compound are in the attomole range. 

In Fig. 12.12, current-potential curves are shown of increasing S02 concentration, recorded in a pH=10 buffer at a carbon fibre modified with [Fe(II)TSPc]4+ It can be seen that in alkaline solution only one wave is obtained at 0.75 V vs. AglAgCl, which is identical to the one obtained when started from sodium sulphite. Therefore, it can be proposed that SO-,2 is the main (Table 12.1) electroactive compound in solution. As expected, the slope between peak current and S02 or Na2S03 concentrations corresponds to exchange of two electrons (Fig. 12.10, curve 7), and a detection limit of 7.3+0.1 xlO - molI 1 was obtained. 

A typical approach is to utilise a substrate which when hydrolysed by the enzyme gives rise to a product which can be easily detected elect-rochemically. Thiocholine can be easily detected using screen-printed carbon electrodes doped with cobalt phthalocyanine (CoPC) [18,19], which acts as an electrocatalyst for the oxidation of thiocholine at a lowered working potential of approximately +100 mV (vs. Ag/AgCl) [18,19], thereby minimising interference from other electroactive compounds ... 

OPH-based biosensors have been fully discussed in previous reviews [2,165]. AChE-based biosensors are based on the principle that OP pesticides have an inhibitory effect on the activity of AChE that may be permanent or partially reversible. The extent of the inhibition is directly related to the concentration of the pesticide and therefore enzyme activity may be used as a measure of the inhibition [166]. The amperometric measurement of AChE activity can be based on the measurement of any of the following three mechanisms [167] (1) production of hydrogen peroxide from choline, (2) oxygen consumption during the enzyme reaction or (3) production of electroactive compounds directly from the oxidation of acetylthiocholine chloride such as thiocholine. The measurement of hydrogen peroxide and oxygen consumption has been described in more details in other reviews [167]. 

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