Chemiluminescent compounds types

ImmunO lSS iy. Chemiluminescence compounds (eg, acridinium esters and sulfonamides, isoluminol), luciferases (eg, firefly, marine bacterial, Benilla and Varela luciferase), photoproteins (eg, aequorin, Benilld), and components of bioluminescence reactions have been tested as replacements for radioactive labels in both competitive and sandwich-type immunoassays. Acridinium ester labels are used extensively in routine clinical immunoassay analysis designed to detect a wide range of hormones, cancer markers, specific antibodies, specific proteins, and therapeutic dmgs. An acridinium ester label produces a flash of light when it reacts with an alkaline solution of hydrogen peroxide. The detection limit for the label is 0.5 amol. 

Titrimetric luminescence methods record changes in the indicator emission of radiation during titration. This change is noted visually or by instruments normally used in luminescence analysis. Most luminescence indicators are complex organic compounds which are classified into fluorescent and chemiluminescent, compounds according to the type of emission of radiation. As in titrimetry with adsorption of colored indicators, luminescence titration makes use of acid-base, precipitation, redox, and complexation reactions. Unlike color reactions, luminescence indicators enable the determination of ions in turbid or colored media and permit the detection limit to be lowered by a factor of nearly one thousand. In comparison with direct luminescence determination, the luminescence titrimetric method is more precise. 

Depending on the label, lAs are classified in different groups. Radioisotopes are used in RIAs, enzymes in enzyme-linked immunosorbent assays (ELISAs) or EIAs, fluorophores in FIAs or PFIAs and chemiluminescent compounds in CLIAs. Additional types of lA exist, but are not very common in pesticide analysis. A more detailed description of these lAs can be found in Gosling. 

Nowadays all over the world considerable attention is focused on development of chemical sensors for the detection of various organic compounds in solutions and gas phase. One of the possible sensor types for organic compounds in solutions detection is optochemotronic sensor - device of liquid-phase optoelectronics that utilize effect of electrogenerated chemiluminescence. In order to enhance selectivity and broaden the range of detected substances the modification of working electrode of optochemotronic cell with organic films is used. Composition and deposition technique of modifying films considerably influence on electrochemical and physical processes in the sensor. 

The fact that the kinetic chain length of dimedone autoxidation is very low appears to indicate structural effects in autoxidation reactions. These may account for some of the discrepancies found in autoxidation chemiluminescence studies of different types of compounds. 

In the preceding paragraph peroxides were described as key intermediates in autoxidation chemiluminescence. In most cases hydroperoxides were involved. The majority are well-defined compounds (e.g. cumene hydroperoxide), but autoxidation reactions are rather complex and peroxides are only one, though very important type of compound involved. 

In DMS0/tert.Bu0K/02 quantum yields found were 3 x I0 4 for 50 and 5 X 104 for 49. These values are not regarded as maximum values, as a strong influence of the reaction conditions was noted 104). Whereas, in luminol-type compounds, any substitution in the cyclic hydrazide ring system renders the compound non-chemiluminescent 105>, this is not the case with these open-chain hydrazides the methylated compounds 49, 50 (R=N(CH3)NH2) and 50 (R=N(CH3)NH(CH3)) were also found to be chemiluminescent, though less so than the unsubstituted hydrazide. E. H. White and coworkers 104> therefore suggest a mechanism via acyl anions for the non-cyclic hydrazides (see Section VI. C.). 

Similarly the N-methylacridone derivative 59 yielded chemiluminescence which was about a third that of 58. In this case the donor part of the molecule, i.e. the phthalic hydrazide, cannot produce a fluorescent product at all, but on its oxidation excited phthalate dianions very probably are produced 124>. From a series of other compounds of this type (60, 61, 62)... 

Indole chemiluminescence (J> p. 112, I80,i8ia)j can a]so be regarded as a special type of Schiff s base chemiluminescence, for the indole compounds very probably react via a hydroperoxide derived from the respective indolenine form. 

In recent years different types of surfactants have been used, in concentrations above their cmc, forming normal micelles, to improve different aspects of chemiluminescent reactions. Though the choice of the best surfactant depends on the characteristics of the chemiluminescent reaction, the surfactants most used have been the cationic (fundamentally quaternary ammonium salts) and to a lesser degree the anionic, the nonionic, and the zwiterionic compounds. 

The dismutation (disproportioning) of two free radicals is accompanied by release of a portion of reaction energy as a light quantum. As the quantum yield of such a process is extremely low, the detection of this type of chemiluminescence is technically complicated. Several compounds like lucigenin and luminol have a high quantum yield after reaction with peroxide radicals. Therefore, they are widely used for the detection of these radicals, particularly in the examination of phagocyting cells. 

Fluorescence detection is usually more sensitive than absorption detection, but the number of naturally fluorescent compounds is limited. Pre- or postcolumn derivatization can also be applied for this type of detection. Chemiluminescence detection is the most sensitive method for some fluorescent compounds. 

In this part of the chapter, we will briefly outline the main types of CL reactions which can be functionally classified by the nature of the excitation process that leads to the formation of the electronically excited state of the light-emitting species. Direct chemiluminescence is the term employed for a reaction in which the excited product is formed directly from the unimolecular reaction of a high-energy intermediate that has been formed in prior reaction steps. The simplest example of this type of CL is the unimolecular decomposition of 1,2-dioxetanes, which are isolated HEI. Thermal decomposition of 1,2-dioxetanes leads mainly to the formation of triplet-excited carbonyl compounds. Although singlet-excited carbonyl compounds are produced in much lower yields, their fluorescence emission constitutes the direct chemiluminescence emission observed in these transformations under normal conditions in aerated solutions ... 

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