Oxygen quenching effect

The first co-reactant discovered was oxalate in 1977. The introduction of the co-reactant in ECL exhibits distinct advantage in comparison with the annihilation reaction (1) it can overcome the limited potential window of solvent and the poor stability of radical anions or cations (2) the coreactant ECL can be beneficial for some fluorescent compounds that have only a electrochemical reduction or oxidation (3) the use of co-reactant can produce more intense ECL emission when the annihilation reaction between oxidized and reduced species is not efficient (4) it can eliminate the oxygen quenching effect frequently encountered in ion annihilation reaction and facilitate the ECL in the air. All commercially available ECL analytical instruments are based on this pathway. According to the generated intermediates and the polarity of the applied potential, the corresponding coreactant ECL can be classified as oxidative-reduction ECL and reductive-oxidation ECL, respectively. 

In order to determine the multiplicity of the reactive species, the photodimerization was carried out in the presence of the triplet quenchers oxygen and ferrocene. The results of these experiments are shown in Table 10.4.<41) It is obvious that the presence of oxygen exerts a large quenching effect on the production of the tram dimer and a smaller but significant effect on the formation of the cis dimer (the formation of tram dimer is decreased by oxygen by a factor of 25, while the cis dimer is decreased by a factor of 1.2). As with oxygen, the production of the tram dimer was quenched in the... 

Calibration curves are linear over several orders of magnitude but eventually show curvature and even reversal due to quenching effects. These are caused by partial or total absorption of the emitted radiation by unexcited analyte molecules, dissolved oxygen and other species, particularly if they are paramagnetic. Unlike absorptiometry, sensitivity can be improved by increasing the intensity of the exciting radiation, I0. 

Size, shape, and density The shielding effects of dendritic shells can likewise be caused by steric factors. Thus, the access of foreign molecules to the central functional unit can be hindered or prevented according to size and density of the dendritic shell. Sometimes, even a certain size selectivity is observed. These effects are especially interesting for electrochemically, catalytically active, redox-and photo-active functional units, since interactions with foreign molecules, such as oxygen quenching of the luminescence (photo-active units) or the access of substrates (catalytically active units) can be influenced.14 11 17,221... 

The carrier used for this purpose consisted of a 0.1 M phosphate buffer of pH 7. The appearance of the sensing microzone is shown in Fig. 5.5.B. The oxygen optrode used was based on a 10-pm silicone rubber film containing dissolved decacyclene as indicator (S) that was fixed on a 110-pm thick polyester support (PS). A 9-pm black PTFE membrane (I) was used for optical insulation. The dye fluorescence was found to be markedly dependent on the concentration of oxygen, which exerted a quenching effect on it. The enzyme (glutamate oxidase) was immobilized on a 150-pm thick immunoaffmity membrane (E). The sensor was prepared similarly as reported by Trettnak et al. [7]. 

The apphcation of an optical transducer based on the fluorescence quenching effect of oxygen was described by Preininger et al. [3]. Another interesting technique is represented by the use of the luminous bacterium Photobacterium phos-phoreum [34]. This device is based on the correlation of the intensity of luminescence to the cellular assimilation of organic compounds from the wastewater. 

Ruthenium(II) bipyridyl and Cr(III) aquo complexes luminesce strongly when photostimulated. The emission of light can be quenched effectively by such species as oxygen, paraquat, Fe(II) aquo complexes, Ru(II) complexes and Cr(NCS)i (Sutin [15]). Pfeil [16] found that the quenching rate coefficients are typically a third to a half of the value which might be predicted from the Smoluchowski theory [3]. 

Ring substitution can have a very different effect on the phosphorescent properties depending on its size, polarity and ring position. Oxygen quenching studies are consistent with the presence of at least two types of inclusion sites. 

Scurlock R, Rougee M, Bensasson RV. 1990. Redox properties of phenols, their relationships to singlet oxygen quenching and to their inhibitory effects on benzo(a)-pyrene-induced neoplasia. Free Radic Res Commun 8 251-258. 

Table 2 Solvent Effect on Singlet-Oxygen Quenching Rate Constants...

De Silva et al. [28] prepared a naphthalene derivative (3) with logic functions (Scheme 1). Here, the bromonaphthalene unit exhibits phosphorescence in the presence of both the calcium ion and (3-CD [28], However, without them, oxygen quenches the phosphorescence of 2-bromonaphthalene phosphor because the protection effect of (3-CD is absent and photoinduced electron transfer from the tetracarboxylate receptor to the 2-bromonaphthalene phosphor occurs. Thus, phosphorescence output occurs only when the calcium ion and (3-CD inputs are active. The operation of these two inputs with a phosphorescence output corresponds to the AND logic function. The input to the NOT gate is oxygen. In the presence of oxygen without either calcium or (3-CD, the AND gate is disabled. 

In the following we present an application of laser induced fluorescence to a study of the chemistry of sulfur in rich hydrogen/oxygen/nitrogen (H2/O2/N2) flames and demonstrate a simple rationale for taking quench effects into account. Fluorescence measurements for S2, SH, S02, SO, and OH along with measurements of flame temperature and H-atom (in sulfur free flames) have been employed to develop a kinetic model for the highly coupled flame chemistry of sulfur. The kinetic aspects of the study already have been presented in considerable detail (6). 

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