Bimodal chemiluminescence

The present study demonstrates the pH-dependent bimodal chemiluminescence of 6,8-diaryl-imidazopyrazinone la induced by superoxide. Despite its lower sensitivity to superoxide than CLA la showed enough sensitivity to superoxide in comparison with luminol and appears to be capable of superoxide detection. What the advantage of la over CLA is that it exhibits the pH-dependent bimodal chemiluminescence, and, therefore, the direct estimation of the local pH in a cell or a droplet is feasible based on the luminescence ratio, Iank /(la nii+Iaiiiai)- This property of la allows us to apply la for simultaneous detection of the two concurrent events, superoxide evolution and pH alteration, in biological systems. 

Saito R, Inoue C, Katoh A. Well-divided aixl pH-dqjendent bimodal chemiluminescence of 2-methyl-6-phenyl-8-(4-substituted phenyl)imidazo[l,2-a]pyrazin-3(7//)-ones induced by superoxide anion. Heterocycles 2004 63 759-64. 

Among the first cluster reactions studied were reactions of readily formed dimers. These are usually dimers of open-shell molecules, such as NO2. Wren and Menzinger studied the chemiluminescence resulting from the reaction of Ba( S) with NOj monomers and dimers. They concentrated on the effect of the collision energy on the reaction s cross section. The monomers in this case react with appreciable cross sections even at low collision energy, and their excitation function is bimodal. The cross section for the clusters, in contrast, rises monotonically from a threshold near zero collision energy. 

Camacho and Karlsson [35] investigated the thermal and thermo-oxidative stability of recycled PP, HDPE and a 20 80 PP/HDPE blend. These samples were subject to extrusion cycles. The oxidation induction time of all three samples decreased with the number of extrusion cycles, as did the temperature of oxidation. Chemiluminescence runs showed two peaks one sharp peak (corresponding to PP) and the other bimodal in shape (corresponding to PE). 

Figure 2.8 shows the chemiluminescence curves of PP, HDPE and their blend after the first extrusion step. The PP exhibits a sharp peak at the maximum on chemiluminescence intensity, whereas the HDPE curve shows a broad bimodal behaviour [36]. In the chemiluminescence curves of the blend, all these features were observed. This may be a strong indication of the existence of a two-phase system in the molten state, which was also pointed out by Braun and co-workers [37], although this was based on peroxide treatment of PP/PE blend melts. It appears that PP oxidises first and the oxidation sites created during this process accelerate, to some extent, the oxidation of PE phase. The overlap between the PP and PE traces in the blend can be interpreted as the interface of these two phases where the PE starts oxidising. In addition, the shape of the curves confirms that the oxidation mechanisms of these resins are different, and that this difference remains during the oxidation of the hlend in the molten state. 

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