Luminol, chemiluminescent oxidation

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

The overall reaction scheme of the luminol chemiluminescence in an aqueous medium is shown in Figure 1. The luminol oxidation leads to the formation of an aminophthalate ion in an excited state, which then emits light on return to the ground state. The quantum yield of the reaction is low ( 0.01) compared with bioluminescence reactions and the emission spectrum shows a maximum1 at 425 nm. 

The luminol chemiluminescence reactions can be divided into two classes depending on the oxidation conditions 115> ... 

Table 5. Luminol chemiluminescence quantum yields in different solvents with some oxidative systems (after J. Lee and H. H. Seliger 115>)...

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. 

Figure 7.12. Role of protein biosynthesis in the ability of neutrophils to generate reactive oxidants. Neutrophils were incubated in the presence (O) and absence ( ) of 30 pg/ml cycloheximide at 37 °C. At time intervals, portions were removed and luminol chemiluminescence measured after stimulation with 1 pM fMet-Leu-Phe. Values presented are a percentage of the control value measured at time zero.

The effect of the hydroxyl radical (HO ) on luminol chemiluminescence has also been intensively studied Although detailed mechauisms for the reactiou of hydroxyl radicals with hydrazides remaiu uuknowu, two differeut processes are assumed to be involved oxidation of the hydrazide group and addition of hydroxyl radical to the aromatic ring (Scheme 19) ° . 

TABLE 3. Luminol chemiluminescence quantum yields in different solvents with some oxidative... 

In luminol chemiluminescence and that of other cyclic hydrazides the corresponding dicarboxylate ion is the emitter. The luminol is oxidized by Hj,02 and the oxidation is very sensitive to metal catalysts and pH of the solution. 

J. K. Robinson, M. J. Bollinger, and J. W. Birks, Luminol/H202 Chemiluminescence Detector for the Analysis of NO in Exhaled Breath, Anal. Chem. 1999, 71, 5131. Many substances can be analyzed by coupling their chemistry to luminol oxidation. See, for example, O. V. Zui and J. W. Birks, Trace Analysis of Phosphorus in Water by Sorption Preconcentration and Luminol Chemiluminescence, Anal. Chem. 2000, 72, 1699. 

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... 

Amino-2,3-dihydro-l,4-phthalazdione (3), known as luminol because oxidation of the substance is attended with a striking chemiluminescence, can be prepared in about 25 min. as follows. A mixture of 1 g. of 3-nitrophthalic acid and 2 ml. of an 8% aqueous solution of hydrazine in a 20 x 150-mm. test tube is heated over a free flame until the solid is dissolved, and 3 ml. of triethylene glycol is added. The solution... 

When luminol is oxidized and is observed in the dark, an eerie blue-green glow is produced through chemiluminescence. 

Of the artificial chemiluminescent compounds, luminol (5-amino-2,3-dihydro-l,4-phthalazinedione) is the most popular, and has been applied not only in analytical chemistry but also in other fields. The luminol chemiluminescence is based on the light emission from an excited 3-aminophthalate ion generated by oxidation with hydrogen peroxide or atmospheric oxygen in the presence of bases and catalysts. This peculiar chemiluminescence property and its industrial value have attracted continuous interest and prompted many chemists to investigate its reaction in detail... 

Since the emission spectra of enhanced and unenhanced luminol oxidations are similar, it seems clear that emission is from the excited-state aminophthalate derived from luminol itself and not from the enhancer. The enhancement is also specific for peroxidase i.e., it does not occur under conditions in which the heme would dissociate from the enzyme. Furthermore, luminol chemiluminescence triggered by heme-containing compounds, such as hemoglobin or cytochrome c, is actually reduced by enhancers such as p-iodophenol (T7). This latter phenomenon explains the potential usefulness of enhanced chemiluminescence, i.e., an amplified signal combined with a reduced background and reduced interference from the reagents as well as from endogenous heme compounds. 

The unit can also detect triacetone-triperoxide (TATP), which does not contain any nitro functionality. Flowever, when thermally decomposed TATP seems to release oxidants capable of inducing luminol chemiluminescence as shown in Figure 5. 

The alkyl nitrates also can be detected by chemiluminecent reactions similar to those for the PANs. The alkyl nitrates are converted to NO by hot catalysts, and the NO is detected by O3 chemiluminescence as described in Section l.E.l. The detector can be calibrated with an NO standard, which is more readily available, more reliable, and more easily transported than an alkyl nitrate standard. However, O3 chemiluminescence is less sensitive, by one or two orders of magnitude, than BCD. The alkyl nitrates can also be converted to NO2 by thermal decomposition with a postcolumn pyrolyzer consisting of a heated quartz tube. The NO2 can then be determined by luminol chemiluminescence. The sensitivity of this type of detector is comparable to that of the BCD. Both types of chemiluminescence detectors respond only to oxidized nitrogen species, therefore the halocarbon interference is eliminated. 

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