Chemiluminescence systems

Chemiluminescence reactions are currently exploited mainly either for analyte concentration measurements or for immunoanalysis and nucleic acid detection. In the latter case, a compound involved in the light emitting reaction is used as a label for immunoassays or for nucleic acid probes. In the former case, the analyte of interest directly participates in a chemiluminescence reaction or undergoes a chemical or an enzymatic transformation in such a way that one of the reaction products is a coreactant of a chemiluminescence reaction. In this respect, chemiluminescent systems that require H2O2 for the light emission are of particular interest in biochemical analysis. Hydrogen peroxide is in fact a product of several enzymatic reactions, which can be then coupled to a chemiluminescent detection. 

Chemically inert triplet quenchers e.g. trans-stilbene, anthracene, or pyrene, suppress the characteristic chemiluminescence of radical-ion recombination. When these quenchers are capable of fluorescence, as are anthracene and pyrene, the energy of the radical-ion recombination reaction is used for the excitation of the quencher fluorescence 15°). Trans-stilbene is a chemically inert 162> triplet quencher which is especially efficient where the energy of the first excited triplet state of a primary product is about 0.2 eV above that of trans-stilbene 163>. This condition is realized, for example, in the energy-deficient chemiluminescent system 10-methyl-phenothiazian radical cation and fluoranthene radical anion 164>. 

McCapra, F., in J. N. Bradley, R. D. Gillard, R. F. Hudson (eds.) Essays in chemistry, Vol. 3, p. 101. New York Academic Press 1972. This very condensed article also deals with other chemiluminescent systems. 

The leaving group of the oxalic ester has a strong effect on the efficiency of the peroxyoxalate chemiluminescent system. The electron-attracting power of the substituents on the phenyl rings of the substituted diphenyl oxalates is important to the overall efficiency of the chemiluminescent reactions. Steric effects... 

A wide variety of different classes of fluorescent molecules has been investigated in the peroxyoxalate chemiluminescent systems. Among those screened were fluorescent dyes such as rhodamines and fluoresceins, heterocyclic compounds such as benzoxazoles and benzothiazoles, and a number of polycyclic aromatic hydrocarbons such as anthracenes, tetracenes, and perylenes. The polycyclic aromatic hydrocarbons and some of their amino derivatives appear to be the best acceptors as they combine high fluorescence efficiency with high excitation efficiency in the chemiluminescent reaction [28],... 

Table 4 Drugs Analyzed with a Chemiluminescent System...

Some drugs will emit light under specific reaction conditions. The discovery of these direct chemiluminescent reactions has come about through trial and error. Table 3 lists some of these drugs and the method used for analysis. Other drugs that are not themselves chemiluminescent can be easily analyzed using a chemiluminescent system (Table 4). 

In the following sections the most important features of the organized media that are most frequently used in chemiluminescent reactions (micellar media and cyclodextrins) will be summarized as well as their influence on various chemiluminescent systems, including their corresponding applications in chemical analysis. 

From an analytical point of view, the use of anionic surfactants as enhancers of CL reactions is most limited. One of the most recent examples is the use of sodium dodecylbenzene sulfonate (SDBS) as a CL enhancer of the system Ru(hpy) - SO - KBr03 (bpy = 2,2 -bipyridyl) [60], The authors of this work propose the following mechanism for the chemiluminescent system ... 

Grayeski and Woolf have studied the influence of the presence of cyclodextrins in chemiluminescent systems [65, 66], They have found that aqueous solutions of a-, p-, and y-cyclodextrins enhance the CL intensity of the reaction of luci-... 

Clearly, a detailed knowledge of the electron-transfer process is required if more effective CIEEL-triggerable chemiluminescent systems are to be rationally designed, rather than engage in an empirical trial-and-error hunt. To understand the excited-state generation process, the nature of the CIEEL emitter and the chemiexcitation mechanism should be established. 

In this part of the chapter we will give a more detailed description of some highly efficient organic chemiluminescence systems, which occur with the involvement of peroxide intermediates. We have chosen to begin the subject with the well-known and widely applied luminol oxidation and will show that, even though this reaction has been exhaustively studied, several critical points in its mechanism remain unclear and are still the subject of... 

Steinfatt proposed an alternative mechanism for the formation of excited aminophth-alate, based on the concept of dioxirane-carbene mediated chemiexcitation, which is also attributed to other chemiluminescent systems ° °. After the attack of hydrogen peroxide on the diazaquinone 27 carbonyl carbon, a perhydrolysis step is postulated to result in the intramolecular dioxirane-carbene system (32) in the excited state ° ° . This species presumably rearranges to 3-aminophthalate dianion while still in the singlet-excited state (Scheme 23). Although this is a very interesting mechanistic proposal, it is based on experimental evidence obtained with indirect phthaloyl peroxide chemiluminescence and no further evidence corroborates this proposal. 

In this part of the chapter, we will focus essentially on mechanistic aspects of the peroxyoxalate reaction. For the discussion of the most important advances in mechanistic aspects of this chemiluminescent system, covering mainly literature reports published in the last two decades, we will divide the sequence operationally into three main parts (i) the kinetics of chemical reactions that take place before chemiexcitation, which ultimately produce the high-energy intermediate (HEI) (ii) the efforts to elucidate the structure of the proposed HEIs, either attempting to trap and synthesize them, or by indirect spectroscopic studies and lastly, (iii) the mechanism involved in chemiexcitation, whereby the interaction of the HEI with the activator leads to the formation of the electronically excited state of the latter, followed by fluorescence emission and decay to the ground state. 

Giri et al. (2) prepared mixed ammonium and phosphonium polymeric ionic liquids, (III), from chloromethylated polystyrene, which were useful as enhancers in chemiluminescent systems. 

As an example, a recent intercomparison (37) included three N02 measurement techniques aTDLAS-based system and two chemical-based systems— the photolysis-ozone chemiluminescence system diagramed in Figure 7 and an instrument based on N02 plus luminol chemiluminescence. Above 2 ppbv the three instruments gave similar results, but at sub-ppbv the results from the three techniques became dissimilar. Tests on the prepared mixtures showed that the luminol results were affected by expected interferences from 03 and PAN. No interferences were found in the TDLAS system, but near the detection limit the data analysis procedures calculated levels of N02 that were too high. The outcome of this intercomparison was close to the ideal the sensitivity, specificity, accuracy, and precision of each instrument were objectively analyzed previous data sets taken by different systems can now be reliably evaluated and each investigator was able to perceive areas in which the technique could be improved. 

Directly labeled chemiluminescent systems produce <1 photon/label and require complex chemical synthesis to produce each new labeled molecule (1,2). In contrast, chemiluminescent detection of enzyme labels combines the advantages of a high specific activity label with the convenience of relatively simple coupling chemistries, which use commercial reagents. A number of enzyme labels can be detected via chemiluminescent or bioluminescent reac-... 

The chemiluminescence spectrum is therefore the fluorescence spectrum of the dye. Many different colours of cold light can be obtained in chemiluminescent systems according to the nature of fluorescing species. 

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