Superoxide anion radical luminol oxidation

It is generally agreed that the CL obtained with luminol (124) is based on the series of transformations shown in Scheme 3, where the analyte (oxidant) as such or in combination with a catalyst produces a free radical (125), which in turn captures a superoxide anion to yield an endoperoxide (126), which on elimination of N2 produces an excited intermediate (127), which finally settles down to the 3-aminophthalate ion (128) on emission of a photon. A linear correlation may be established between the intensity of the CL emission and the concentration of the analyte334. 

Other cationic surfactants such as TTAB, DTAB, DODAB, STAC, CEDAB, and DDDAB have been used in CL reactions with less frequency. Thus, tetradecyltrimethylammonium bromide [TTAB] has been used to increase the sensitivity of the method to determine Fe(II) and total Fe based on the catalytic action of Fe(II) in the oxidation of luminol with hydrogen peroxide in an alkaline medium [47], While other surfactants such as HTAB, hexadecylpiridinium bromide (HPB), Brij-35, and SDS do not enhance the CL intensity, TTAB shows a maximum enhancement at a concentration of 2.7 X 10 2 M (Fig. 11). At the same time it was found that the catalytic effect of Fe(II) is extremely efficient in the presence of citric acid. With regard to the mechanism of the reaction, it is thought that Fe(II) forms an anionic complex with citric acid, being later concentrated on the surface of the TTAB cationic micelle. The complex reacts with the hydrogen peroxide to form hydroxy radical or superoxide ion on the... 

The reaction between hydroxyl radical and luminol results in the quantitative formation of hydroxylated products, which are known to be radical scavengers (Scheme 19) °. The reduction of luminol radical anion by these species decreases the emission intensity, unless HO is generated rapidly in high concentrations . The emission observed under these conditions may be attributed to one-electron oxidation of luminol by hydroxyl radicals, yielding luminol radical anion, or by hydroxyl-mediated generation of potent one-electron oxidants from its reaction with bases. Moreover, the addition of HO to luminol or its monoanion results in an adduct able to react very rapidly with O2, yielding, in addition to hydroxylated luminol, superoxide (Scheme 19) °. 

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