Molecular oxygen quenching

The basic structure of carotenoids is a conjugated polyprenoid chain. Carotenoids exert a positive impact on human welfare. They show marked singlet molecular oxygen quenching capacity [11] and they protect against photosensitized peroxidation of lipids [12],... 

M. Montenegro, M. Nazareno, E. Durantini and C. Borsarelli, Singlet molecular oxygen quenching ability of carotenoids in a reverse-micelle membrane mimetic system. Photochem. Photobiol. 75 (2002) 353-361. 

Typical singlet lifetimes are measured in nanoseconds while triplet lifetimes of organic molecules in rigid solutions are usually measured in milliseconds or even seconds. In liquid media where drfifiision is rapid the triplet states are usually quenched, often by tire nearly iibiqitoiis molecular oxygen. Because of that, phosphorescence is seldom observed in liquid solutions. In the spectroscopy of molecules the tenn fluorescence is now usually used to refer to emission from an excited singlet state and phosphorescence to emission from a triplet state, regardless of the actual lifetimes. 

The dye is excited by light suppHed through the optical fiber (see Fiber optics), and its fluorescence monitored, also via the optical fiber. Because molecular oxygen, O2, quenches the fluorescence of the dyes employed, the iatensity of the fluorescence is related to the concentration of O2 at the surface of the optical fiber. Any glucose present ia the test solution reduces the local O2 concentration because of the immobilized enzyme resulting ia an iacrease ia fluorescence iatensity. This biosensor has a detection limit for glucose of approximately 100 ]lM , response times are on the order of a miaute. 

In turn, 1O2 is a very electrophilic excited state species of molecular oxygen that interacts efficiently with electron-rich molecules, such as aminoadd residues of proteins like histidine, metionine, tryptophan, tyrosine, etc., by both physical and chemical quenching processes, eqns. 9 and 10 (Davies, 2003 Bisby et al., 1999). 

Fig. 2. a) Transient absorption spectra of RF (35 pM) in N2-saturated MeOH-Water (1 1) solution observed after 60 ps of the laser pulse as a function of the concentration of GA. b) Effect of dissolved molecular oxygen ( 2) on the decay of the 3RF at 700 nm. Inset Stern-Volmer plot for the quenching of 3RF by 3O2. 

Later on, such S-layer-based sensing layers were also used in the development of optical biosensors (optodes), where the electrochemical transduction principle was replaced by an optical one [97] (Fig. 10c). In this approach an oxygen-sensitive fluorescent dye (ruthenium(II) complex) was immobilized on the S-layer in close proximity to the glucose oxidase-sensing layer [97]. The fluorescence of the Ru(II) complex is dynamically quenched by molecular oxygen. Thus, a decrease in the local oxygen pressure as a result of... 

In the presence of oxygen, the situation is more complicated, as more active species can be formed, which can contribute to NO decomposition or formation in the plasma. It was found [46] that the excited states of N2 do not contribute significantly to NO evolution, but mainly influence the dissociation or quenching of 02. As the concentration of 02 increases, their contribution to 02 dissociation also increases. Only nitrogen atoms in the ground state (N(2D)) or excited state (N(4S)) may contribute directly to NO formation by reactions with molecular oxygen ... 

Excited electronic states of molecular oxygen are essentially unreactive, and only oxygen atoms in the ground state (0(3P)) or the first electronic excited state (O( D)) are involved in reactions with NO. The excited O atoms are mostly quenched by the background N2 and 02 molecules, therefore, only ground-state O atoms contribute to the evolution of NO in the plasma. 

In this group, there are collisional interactions, which are responsible for quenching of excited states by molecular oxygen, paramagnetic species, heavy atoms, etc. [1, 2, 13-15]. Probability of such quenching can be calculated as ... 

Quenching of a photoluminescent dye (D) by molecular oxygen (O2) is a non-chemical collisional process which can be presented as follows ... 

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