Internal self-quenching

Bimolecular reactions such as quenching, either by molecules of the same kind, self-quenching, or by added substances, impurity quenching, inhibit emission because frequency of bimolecular collisions in gases as well as in solution, k 1010. v can compete with fluorescence emission. Solvent quenching may involve other physical parameters as well such as solute-solvent interactions. Since the solvent acts as the medium in which the solute molecules are bathed, solvent quenching may be classified under unimolecular processes and a clear distinction between it and internal conversion St - S0 is difficult. 

One facile method for investigation of cellular interactions of liposomes, and more precisely, their ability to accumulate within cells, is the fluorescent microscopy. To meet this objective, liposomes are loaded with fluorescent marker (calcein or other suitable dye) at high concentration, whereby its fluorescence is self-quenched. Upon cellular fusion and internalization the dye is released from the carrier, diluted in the environment so the selfquenching effect is lost and the increased fluorescence is detected by fluorescent microscopy. In the case of effective cellular internalization of pH-sensitive liposomes, that is, without endosomal sequestration, calcein would have been diluted several-hundred-fold and the cells will display uniform cytosolic fluorescence. If the liposomes have been taken up by cells by endocytosis, punctuate fluorescence will be restricted to the secondary lysosomal and endocytic vacuoles. In contrast, adsorbed liposomes should... 

Internal conversion Self quenching External quenching... 

Fig. 2. Liposome structure (A) Biorecognition elements can be covalently conjugated to or inserted into lipid bilayers through hydrophobic interactions (not to scale.) (B) The large internal volume of unilamellar vesicles can encapsulate hundreds of thousands of hydrophilic signaling molecules and provide for their stability. (C) Surfactant introduction can provide for instantaneous signal enhancement through release of encapsulants. Fluorophores encapsulated within liposomes at high concentrations undergo self-quenching, which is overcome upon release into the surrounding medium.

Scheme 7.2 Fluorescence (F) and phosphorescence (P). Normal fluorescence (F) arises from Sj (independently from which singlet has been initially reached, and thus its shape is not affected by the energy of the wavelength used, provided that this has been absorbed. This is the Vavilov rule, rationalized by Kasha as due to the fact that internal conversion among states of the same multiplicity is always the fastest process, see Chap. 3). Delayed fluorescence may involve triple-triplet annihilation, in which self-quenching of the triplet leads back to the singlet Sj, with the usual fluorescence spectrum but the triplet lifetime...

Superregenerative technique Superregenerative oscillators (SRO) have been widely used as oscillator-detectors in the study of NQR because of their high sensitivity at high RF levels, reliability of operation and simplicity of construction. The phenomenon of NQR was discovered by Demelt and Kruger using a SRO spectrometer. SRO is actually an oscillator-detector, with oscillation periodically quenched either with the help of an external quench oscillator (quencher mode), or with the help of internal circuits (self-quenched mode). A high sensitivity of the SRO... 

Self-quenching appears to compete favorably with Reaction (35) at high concentrations of coumarin and is probably the principal mode of destruction of since the quantum yield of dimerization is low even in ethanol. The results can be accounted for if it is assumed that the relative rates of self-quenching and dimerization are solvent-dependent (Hanunond et al., 1964b). It appears possible, therefore, that an excimer is involved whose fate, collapse to dimer, or internal conversion and dissociation, is determined by the solvent. 

However, it was also shown that the CL-time profile was sensitive to the purity of the resin and the amine, DDS, which was found to contain a quenching agent, so the quantum yield of emission was reduced in the fluid state (Equation (3.28)). It is therefore concluded that simple CL experiments are not genemlly applicable as a method for monitoring the cure of epoxy resins. In addition to the self-probe fluorescence of TGDDM, the CL measurements rely on absolute emission intensities, which, in the absence of an internal standard for calibration, is unreliable. 

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