Chemiluminescent oxidation of luminol

In aprotic media, the chemiluminescent oxidation of luminol requires only oxygen and a strong base2 4. 

As in the case of horseradish peroxidase, several synthetic metalloporphyrins in the presence of H2O2 have been found to be potent catalysts for the chemiluminescent oxidation of luminol or isoluminol. The microperoxidases, mainly MP8 and MPll, have been shown to act as functional peroxidase enzyme models. " However, they are readily inactivated within one min of catalytic turnover, and incorporation into a molecular sieve... 

Chemiluminescent reactions usually involve oxidation of an organic molecule. The chemiluminescent oxidation of luminol (19) is well known. 

The Emneus group in Denmark has reported the sensing of atrazine in surface water using microfluidic enzyme immunoassay and chemiluminescence as the transduction technique [19]. Silicon microchips were functionalized with proteins A and G via a hydrophilic polymer layer. An affinity capture competitive immunoassay was performed using a mixture of enzyme tracer [horseradish peroxidase (HRP)], atrazine sample, and antiatra-zine antibody which was subsequently injected on the microchannel, followed by addition of the substrate mixture [luminol/p-iodophenol (PIP)/H202]. Chemiluminescent oxidation of luminol/PIP/H202 in the presence of HRP enzyme was monitored via PMT. 

Titration Indicators. Concentrations of arsenic(III) as low as 2 x 10 M can be measured (272) by titration with iodine, using the chemiluminescent iodine oxidation of luminol to indicate the end point. Oxidation reactions have been titrated using siloxene the appearance of chemiluminescence indicates excess oxidant. Examples include titration of thallium (277) and lead (278) with dichromate and analysis of iron(II) by titration with cerium(IV) (279). 

The photosensitized results are from I.B.C. Matheson and J. Lee 118h It is seen that the quantum yields in photosensitized oxidation depend on the concentrations of luminol and base, and on temperature. At higher temperature (50°) and low luminol concentrations, the quantum yields reached those of hemin-catalyzed hydrogen peroxide oxidation of luminol in aqueous-alkaline solution. Primary products of the photosensitized oxidation are singlet oxygen (1Ag02) or a photoperoxide derived from methylene blue, but neither of these is directly responsible for the luminol chemiluminescence. 

Albrecht [10] was the first to report the CL of luminol (5-amino-2,3-dihydro-1,4-phthalazinedione), in 1928. The chemiluminescent reaction involves the oxidation of luminol (usually by H202) and often occurs in the presence of a catalyst (or co-oxidant) such as Fc(CN)6 3, Cu(II), or Co(II)) (Fig. 2). The light emission, which is blue in water and yellow-green in DMSO, is identical with the fluorescence of the 3-aminophthalate oxidation product [11],... 

The oxidation of luminol in basic solution is one of the best known and most efficient chemiluminescent reactions, having a quantum yield of CL of about 0.01 in water and 0.05 in DMSO. 

Much of the study of ECL reactions has centered on two areas electron transfer reactions between certain transition metal complexes, and radical ion-annihilation reactions between polyaromatic hydrocarbons. ECL also encompasses the electrochemical generation of conventional chemiluminescence (CL) reactions, such as the electrochemical oxidation of luminol. Cathodic luminescence from oxide-covered valve metal electrodes is also termed ECL in the literature, and has found applications in analytical chemistry. Hence this type of ECL will also be covered here. 

The enhanced chemiluminescence associated with the autoxidation of luminol (5-amino-2,3-dihydro-1,4-phthalazinedione) in the presence of trace amounts of iron(II) is being used extensively for selective determination of Fe(II) under natural conditions (149-152). The specificity of the reaction is that iron(II) induces chemiluminescence with 02, but not with H202, which was utilized as an oxidizing agent in the determination of other trace metals. The oxidation of luminol by 02 is often referred to as an iron(II)-catalyzed process but it is not a catalytic reaction in reality because iron(II) is not involved in a redox cycle, rather it is oxidized to iron(III). In other words, the lower oxidation state metal ion should be regarded as a co-substrate in this system. Nevertheless, the reaction deserves attention because it is one of the few cases where a metal ion significantly affects the autoxidation kinetics of a substrate without actually forming a complex with it. 

The source of chemiluminescence in the oxidation of luminol was explored by Merenyi and co-workers in detail (153). The oxidation of luminol yields aminophthalate as a final product and the reaction proceeds via a series of electron transfer steps. The primary oxidation product is the luminol radical which is transformed into either diazaquinone or the a-hydroxide-hydroperoxide intermediate (a-HHP). The latter oxidation step occurs between the deprotonated form of the luminol radical and O -. The chemiluminescence is due to the decomposition of the mono-anionic form of a-HHP into the final products ... 

Fig. 24 (a) Chemiluminescence reaction of luminol (63) to 3-aminophthalic acid (64). (b) Oxidation of diphenyloxalate (65) to produce the high-energy dioxetandione as a sensitizer for an organic dye fluorophore... 

The best known and most nsefnl of the chemiluminescent reactions involving electron transfer is the oxidation of luminol (3.100) or its derivatives in alkaline medium. The oxidant can be hydrogen peroxide, sodium ferricyanide or hypochlorite, usually with a catalyst that can be a transition metal ion, such as Cu " Co +, Fe + and Mtf+, or haem and haemproteins, e.g. peroxidases. The reaction mode is shown in Figure 3.22.4"... 

The chemiluminescence emission resulting from the oxidation of luminol (5-amino-2,3-dihydro-l,4-phthalazinedione) has been extensively studied since its discovery by Albrecht in 1928. Although luminol oxidation is one of the most commonly applied chemiluminescent reactions, to date no definitive mechanism is known . Efficient chemiluminescence emission is only observed when luminol (25) is oxidized under alkaline conditions. Depending on the medium, co-oxidants are required in addition to molecular oxygen for the observation of light emission, but under any condition, 3-aminophthalate (3-AP) and molecular nitrogen are the main reaction products (equation 10). 

Ferrous iron catalyses the oxidation of luminol (see below) by hydrogen peroxide. The intensity of the chemiluminescence which follows increases linearly with the concentration of Fe(II) as it rises from 10-10 M to 10-8 M. To measure a solution of unknown content in Fe(II), 2 ml of the solution was extracted and then introduced to 1 ml of water, followed by the addition of 2 ml of hydrogen peroxide and finally 1 ml of an alkali solution of luminol. The luminescence signal is emitted and integrated over a period of 10 seconds giving a value of 16.1 (arbitrary units). In a second attempt, 2 ml of the... 

Chemiluminescence of oxidized luminol has been the basis of several lumino-metric methods of estimation of TAC (Table 1). The mostcommon is to measure the induction time of the reaction. Often the chemiluminescence is first induced by an oxidant and then attenuated by addition of a sample, and the time to recover the initial fluorescence is measured. The enhanced chemiluminescent assay introduced a decade ago is based on the oxidation of luminol by perborate or by hydrogen peroxide in a reaction catalyzed by horseradish peroxidase. Enhancement (and stabilization) of luminescence is achieved by addition of p-iodophenol. The original procedure used a commercial reagent kit (ECL Anti-oxidant Detection Pack... 

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