Light emission kinetics

Luminescence reaction (Viviani et al., 2002a) The luciferin-luciferase luminescence reaction was carried out in 0.1 M Tris-HCl, pH 8.0, containing 2mM ATP and 4mM Mg2+. Mixing luciferase with luciferin and ATP resulted in an emission of light with rapid onset and a kinetically complex decay. Further additions of fresh luciferase, after the luminescence has decayed to about 10% of its maximum value, resulted in additional luminescence responses similar to the initial one (Fig. 1.15). According to the authors, the repetitive light emission occurred in consequence of the inhibition of luciferase by a reaction product, as seen in the case of the firefly system (McElroy et al., 1953). The luminescence spectrum showed a peak at 487nm (Fig. 1.16). 

Chemiluminescence is light emission from the relaxation of electrons populating excited states in an elementary step of a chemical reaction. Since, the process of population of excited states is related kinetically to the kinetics of the given chemical reaction, the emission of chemiluminescence over time should thus be related to the rate of the chemical reaction. 

Equation (13) appears to be a good approximation for describing isothermal chemiluminescence kinetics for homogeneous systems where oxidation takes place uniformly. However, as has been shown by several authors [53-58], the different sections of a polymer sample may oxidize with its autonomous kinetics determined by different rates of primary initiation. A chemiluminescence imaging technique revealed that the light emission may be spread from some sites of the polymer film and the isothermal chemiluminescence vs. time runs are then modified, particularly in the stage of an advanced oxidation reaction [59]. 

With the advent of picosecond-pulse radiolysis and laser technologies, it has been possible to study geminate-ion recombination (Jonah et al, 1979 Sauer and Jonah, 1980 Tagawa et al 1982a, b) and subsequently electron-ion recombination (Katsumura et al, 1982 Tagawa et al, 1983 Jonah, 1983) in hydrocarbon liquids. Using cyclohexane solutions of 9,10-diphenylanthracene (DPA) and p-terphenyl (PT), Jonah et al. (1979) observed light emission from the first excited state of the solutes, interpreted in terms of solute cation-anion recombination. In the early work of Sauer and Jonah (1980), the kinetics of solute excited state formation was studied in cyclohexane solutions of DPA and PT, and some inconsistency with respect to the solution of the diffusion equation was noted.1... 

Fluorescent chemical sensors occupy nowadays a prominent place among the optical devices due to its superb sensitivity (just a single photon sometimes suffices for quantifying luminescence compared to detecting the intensity difference between two beams of light in absorption techniques), combined with the required selectivity that photo- or chemi-luminescence impart to the electronic excitation. This is due to the fact that the excitation and emission wavelengths can be selected from those of the absorption and luminescence bands of the luminophore molecule in addition, the emission kinetics and anisotropy features of the latter add specificity to luminescent measurements8 10. 

Seitz, Suydam, and Hercules 186> recently developed on the basis of luminol chemiluminescence a method for chromium-III ion determination which has a detection limit of about 0.025 ppb. The method is specific for free chromium-III ions as chromium-VI compounds have no catalytic effect and other metal ions can be converted to a non-catalytic form by complexing with EDTA, since the chromium-III complex of EDTA, which is in any case not catalytically active, is formed kinetically slowly 186>. To detect extremely small light emissions, and hence very small metal concentrations, a flow system was used which allows the reactants to be mixed directly in front of a multiplier. (For a detailed description of the apparatus, see 186>). 

The main features of the chemiluminescence mechanism are exemplarily illustrated in Scheme 11 for the reaction of bis(2,4,6-trichlorophenyl)oxalate (TCPO) with hydrogen peroxide in the presence of imidazole (IMI-H) as base catalyst and the chemiluminescent activators (ACT) anthracene, 9,10-diphenylanthracene, 2,5-diphenyloxazole, perylene and rubrene. In this mechanism, the replacement of the phenolic substituents in TCPO by IMI-H constitutes the slow step, whereas the nucleophilic attack of hydrogen peroxide on the intermediary l,l -oxalyl diimidazole (ODI) is fast. This rate difference is manifested by a two-exponential behavior of the chemiluminescence kinetics. The observed dependence of the chemiexcitation yield on the electrochemical characteristics of the activator has been rationalized in terms of the intermolecular CIEEL mechanism (Scheme 12), in which the free-energy balance for the electron back-transfer (BET) determines whether the singlet-excited activator, the species responsible for the light emission, is formed ... 

FIGURE 3. Typical emission (A) and absorbance (B) profiles observed in kinetic experiments with the peroxyoxalate system of oxalic esters reacting with base. A Light emission in the presence of ACT. B Absorption change due to phenol release in the absence of ACT... 

The latter reaction is widely used as a proving ground for new theories. Particles B (called also scavengers) are unsaturable energy sinks and thus after rapid light emission can absorb it anew. In this case the reaction scheme is A + B —> B (concentration of B s remains constant raB) which could be described by the following kinetic equation... 

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