Fluorescence nitroxide concentrations

TMDBIO for the alkyl radicals formed by initiator (hydroperoxide) thermolysis. The further quantitation of this reaction is difficult without assuming a kinetic model or obtaining further analytical information such as the actual concentration of nitroxide that has reacted during the retardation reaction. In the special case of the pro-fluorescent nitroxide, this can be achieved by fluorescence analysis of the formed alkoxyamine R2NOP. 

It may be seen from the clear aromatic vibronic progression that the fluorescence spectrum is that of phenanthrene, the chromophore present in TMDBIO. This fluorescence, before trapping of the alkyl radicals, was suppressed by the spin-orbit coupling of the nitroxide group located on the iso-indoline framework as seen from its structure (Introduction). The dependence of the fluorescence on the nitroxide concentration may be seen in Figure 6. 

When the nitroxide is present, the alkyl peroxy radicals POO and the resultant hydroperoxide POOH are still formed, but the concentration is reduced due to the competition between oxygen and nitroxide for the alkyl radicals P. The rates of these reactions are given in Figure 2 for homogeneous kinetics. The extent of this competitive reaction may be estimated by the decrease in the slope of the CL - time curves in the retardation period as shown in Figure 3 and also by the formation of the alkoxy amine R2NOP. The concentration of the alkoxy amine R2NOP may be measured by fluorescence (see next section). 

As shown in Fig. 1.16a, upon addition of CATl the fluorescence of P6 is efficiently quenched. The ( )-6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid (trolox) is added and allowed to equilibrate for 35 min, which reverses the quenching due to the scavenging of nitroxide radicals via hydrogen transfer from trolox to CATl. The fluorescence recovery depends on the concentration of trolox (Fig. 1.16b). The trolox can be detected in the range of 10 100 pM. The same probe can also be used to detect the capabilities of a variety of antioxidants. Sensing for ascorbic acid can be accomplished with high sensitivity and selectivity because of its excellent antioxidant capabilities. The ascorbic acid concentration can be determined in the 50 nM to 200 pM range (Fig. 1.17a). Control experiments were also done with a nonspecific quencher. A , A -dimethyl-d, 4 -bipyridinium (MV +) for ascorbic acid. As shown in Fig. 1.17b, upon addition of MV +, the fluorescence of P6 is... 

Beside the estimation of medium microviscosity, quartz plates modified by BFLl were used for the quantitative analysis of ascorbate. For this purpose, a number of solutions of ascorbic acid were prepared with different concentrations, and the kinetics of change in steady-state fluorescence was recorded. There are two parallel processes that influence fluorescence (a) trans-cis photoisomerization and (b) reduction of the nitroxide moiety in traws-BFLl. After a correction taking into account the influence of trans-cis photoisomerization (curve b ), the pseudo-first-order kinetics of BFL reduction appears as curve c. The correction was done by performing a parallel measurement under identical conditions but without adding ascorbate. 

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