Fluorescence prompt

Detection of the PSP toxins has proven to be one of the largest hurdles in the development of analytical methods. The traditional means, and still in wide use today, is determination of mouse death times for a 1 mL injection of the test solution. There are a variety of drawbacks to utilization of this technique in routine analytical methods, that have prompted the search for replacements. In 1975 Bates and Rapoport (3) reported the development of a fluorescence technique that has proven to be highly selective for the PSP toxins, and very sensitive for many of them. This detection technique has formed the basis for analytical methods involving TLC (77), electrophoresis (72), column chromatography (7J), autoanalyzers (7 ), and HPLC (5,6,7). 

The first observations of P-type delayed fluorescence arose from the photoluminescence of organic vapors.<15) It was reported that phenanthrene, anthracene, perylene, and pyrene vapors all exhibited two-component emission spectra. One of these was found to have a short lifetime characteristic of prompt fluorescence while the other was much longer lived. For phenanthrene it was observed that the ratio of the intensity of the longer lived emission to that of the total emission increased with increasing phenanthrene vapor... 

Plasma excitation would occur on a much longer timescale, while fluorescence from prompt excitation of the 2p level in H could be readily quenched through the ionization of the 2p state with the second laser pulse. 

The technique of fluorescence up-conversion (see Chapter 11), allowing observations at the time-scale of picoseconds and femtoseconds, prompted a number of fundamental investigations on solvation dynamics that turned out to be quite complex (Barbara and Jarzeba, 1990) (see Box 7.1). 

The applications of visible fluorescent immunosensors described are all susceptible to large interferences from biomolecules such as bilirubin and porphyrins. A more comprehensive review of immunosensors has been published by Robinson.(10S) The spectral interferences associated with visible fluorophores have prompted the design of an NIR optical immunosensor techniques with high sensitivity and low interference. 

In the past ten years, numerous applications of fluorescence methods for monitoring homogeneous and heterogeneous immunoassays have been reported. Advances in the design of fluorescent labels have prompted the development of various fluorescent immunoassay schemes such as the substrate-labeled fluorescent immunoassay and the fluorescence excitation transfer immunoassay. As sophisticated fluorescence instrumentation for lifetime measurement became available, the phase-resolved and time-resolved fluorescent immunoassays have also developed. With the current emphasis on satellite and physician s office testing, future innovations in fluorescence immunoassay development will be expected to center on the simplification of assay protocol and the development of solid-state miniaturized fluorescence readers for on-site testing. 

For several compounds probabilities of excited singlet state formation as a result of TTA processes have been determined in our previous paper ( ) with the help of a method based on measurement of the ratio of the integrated intensities of ADF and prompt fluorescence (PF) under the conditions of powerful photoexcitation... 

Application of continuous anti-Stokes excitation makes it possible to record the emission from the S2 state when the PF S2 > Sq is absent ( ). Prompt S2 > Sq fluorescence has been measured at direct Sq > 2 excitation (krypton laser ... 

A rapid reaction kinetic technique (time scale = 10-1000 ps) that typically uses a Van de Graff accelerator or a microwave linear electron accelerator to promptly generate a pulse of electrons at sufficient power levels for excitation and ionization of target substances by electron impact. The technique is the direct radiation chemical analog of flash photolysis and the ensuing kinetic measurements are accomplished optically by IR/visible/UV adsorption spectroscopy or by fluorescence spectroscopy. 

It is interesting to note that excimer bands of phenanthrene67 and of anthracene,124 which have defied detection in the prompt fluorescence spectra even at low temperatures, have been observed in the delayed emission spectra of these compounds at — 75°K. Presumably at the low temperatures necessary to observe these bands the high solvent viscosity completely suppresses photoassociation at the reduced concentration available, i.e., WM] 1 /r , whereas the reduced triplet-triplet annihilation rate constant mstationary concentration of the triplet state. 

Chandross, Longworth, and Visco126 have reported the observation of long-wave structureless emission bands in the vicinity of the electrodes during the ac electrolysis of anthracene, phenanthrene, perylene, and 3,4-benzpyrene in polar solvents such as acetonitrile and dimethylformamide the similarity between the perylene band and the crystal fluorescence spectrum prompted the assignment of these bands to excimer fluorescence originating in the process... 

Fig. 9. Microscope luminescence imaging of a model system consisting of oxirane acrylic beads (diameter < 250 pm) containing [Eu(HL)]+ or fluorescein. The left panel shows the prompt fluorescence image and the right panel the TR luminescence image. The figures represent the average luminescence intensities measured for each bead. Reprinted with permission from...

The processes III and IV termed as E-type and P-type delayed emissions have emission spectra identical with that of the normal fluorescence but with longer radiative lifetime. The long life is due to the involvement of the triplet state as an intermediate. Hence the short-lived direct fluorescence emission from the Sx state is referred to as prompt fluorescence. E-type delayed fluorescence was called a-phosphorescence by Lewis in his early works. 

The long-lived delayed emission as phosphorescence has spectral characteristics very different from fluorescence. But there are delayed emissions whose spectra coincide exactly with the prompt fluorescence from the lowest singlet state, the only difference being in their lifetimes. These processes are known as delayed fluorescence. Two most important types of delayed fluorescence are (A) E-type delayed fluorescence and (B) P-type delayed fluorescence. 

If rate constants are competitive, we expect the emission of prompt fluorescence, phosphorescence and delayed fluorescence of energy quanta /7v/, h jp and //vn) respectively. Although //v/ is equal to ftvED, the lifetime of delayed fluorescence will match the lifetime of triplet decay. The rate constant ArEn for E-type delayed fluorescence is temperature dependent and can be expressed as... 

Since j> is independent of I , the ratio of the intensity of P-type delayed emission to that of prompt fluorescence should show linear dependence on the intensity of absorption. The square law dependence indicates the necessity of two photons for the act of delayed emission and is hence known as biphotonic process. It has been observed in fluid solutions of many compounds and also in the vapour state. 

Besides long lived phosphorescence emission, which has spectral characteristics different from fluorescence, there are few other delayed emission processes which have same spectral characteristics as prompt fluorescence. 

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