Micelles fluorescence quenching

Fluorescence quenching studies in micellar systems provide quantitative information not only on the aggregation number but also on counterion binding and on the effect of additives on the micellization process. The solubilizing process (partition coefficients between the aqueous phase and the micellar pseudo-phase, entry and exit rates of solutes) can also be characterized by fluorescence quenching. 

M. R. Eftink and C. A. Ghiron, Fluorescence quenching of indole and model micelle systems, J. Phys. Chem. 80, 486 193 (1976). 

E. Blatt and W. H. Sawyer, Depth-dependent fluorescent quenching in micelles and membranes, Biochim. Biophys. Acta 822, 43-62 (1985). 

Let us recall the micellar aqueous system, as this procedure is actually the basic one. The chemistry is based on fatty acids, that build micelles in higher pH ranges and vesicles at pH c. 8.0-8.5 (Hargreaves and Deamer, 1978a). The interest in fatty acids lies also in the fact that they are considered possible candidates for the first prebiotic membranes, as will be seen later on. The experimental apparatus is particularly simple, also a reminder of a possible prebiotic situation the water-insoluble ethyl caprylate is overlaid on an aqueous alkaline solution, so that at the macroscopic interphase there is an hydrolysis reaction that produces caprylate ions. The reaction is very slow, as shown in Figure 7.15, but eventually the critical micelle concentration (cmc) is reached in solution, and thus the first caprylate micelles are formed. Aqueous micelles can actually be seen as lipophylic spherical surfaces, to which the lipophylic ethyl caprylate (EC) avidly binds. The efficient molecular dispersion of EC on the micellar surface speeds up its hydrolysis, (a kind of physical micellar catalysis) and caprylate ions are rapidly formed. This results in the formation of more micelles. However, more micelles determine more binding of the water-insoluble EC, with the formation of more and more micelles a typical autocatalytic behavior. The increase in micelle population was directly monitored by fluorescence quenching techniques, as already used in the case of the... 

AOT-isooctane-H20 reversed micelles 50-A-diameter CdS particles generated in situ in reversed micelles from CdCl2 or Cd(N03)2 by H2S Reversed-micelle-entrapped CdS was fluorescence quenched by methylviologen band-gap excitation in the presence of Rh as catalyst and PhSH as sacrificial electron donor resulted in water photoreduction 611... 

A0T-heptane-H20 reversed micelles Small (10 A diameter) CdS particles prepared in situ in reversed micelles from Cd(C104)2 and Na2S Agglomeration number grew discontinuously CdS was fluorescence quenched by MV2+ 612... 

Fluorescence quenching in micelles. Consider an aqueous solution with a high concentration of micelles (Box 26-1) and relatively low concentrations of the fluorescent molecule pyrene and a quencher (cetylpyridinium chloride, designated Q), both of which dissolve in the micelles. 

The probe molecule pyrene (-10"6 M) was used in time-resolved fluorescence quenching experiments using a single photon counting apparatus, cetylpiridinium chloride (CpyC, 10"3 M) being introduced as a quencher of the pyrene fluorescence[ll-13]. All the experiments were performed at 303K. From these fluorescence studies the micelle aggregation number (N) and the pyrene fluorescence lifetime (x) were obtained [14]. 

In fact, it is fairly easy to correct the fluorescence quenching constants by considering that both of the reagents are restricted only to the portion of the solution actually occupied by the surfactant micelle. When this is done the values listed as K are obtained. 

Almgren, M., J. Alsins, and P. Bahadur. 1991. Fluorescence quenching and excimer formation to probe the micellization of a poly(ethylene oxide)-poly(propylene oxde)-poly(ethylene oxide) block copolymer, as modulated by potassium uoride in aqueous solillHngmuir7 446—450. 

Fig. 2 Fluorescence quenching of ZnTMePyP4+ by MV2+wp in reversed micelles of BF1DC ( s =0.1 mol dnT3) at several w0 values 4, 12, 22.

Many surfactants, e.g. benzenesulphonates, contain aryl groups and it is found that they will form charge-transfer complexes with 1,2,4,5-tetracyano-benzene which can be detected by ultraviolet absorption and fluorescence spectroscopy (Masuhara et al., 1979) a similar result was obtained with an amphiphatic system. Fluorescence quenching in such micelles has been studied, an example being the quenching of the fluorescence of benzyl anthroate by triethylamine in Triton X (Costa and Macanita, 1978). 

The ratio of the slope above to the slope below the cmc provided an estimate of P [7]. The binding constant was also used to estimate the concentration of alcohol in the bulk phase. Time resolved fluorescence quenching of dimethylnaphthalene by cetylpyridinium bromide solubilized in the micelles was used to obtain estimates of the mean aggregation number of the surfactant in the mixed micelles. 

The rate at which the microemulsion droplets exchange the contents of their aqueous cores is another important transport variable. Exchange kinetics have been measured using fluorescence techniques. Dynamic fluorescence quenching has also been used to study micelle size. f The technique is, however, more useful as a tool to explore the exchange kinetics between microemulsion droplets. The systems studied were AOT or didodecyldimethyl ammonium bromide and water in propane up to 100°C and... 

H -tetramethylbenzidine in anionic-cationic mixed micelles has been studied in detail by ESR . The photochemistry of the semi-oxidised forms of eosin Y and rose bengal have been investigated in colloidal solutions. Relevant to the fluorescence of proteins is a study of fluorescence quenching of indolic compounds by amino-acids in SOS, CTAB, and CTAC micelles O Rate constants for proton transfer of several hydroxyaromatic compounds have been measured in a variety of surfactant solutions. Photoprotolytic dissociation does not require exit of the reactant molecules from the micelles. Micellar solutions can be used to improve the fluorescence determination of 2-naphthol by inhibiting proton transfer or proton inducing reactions z2. jpe decay of the radical pair composed of diphenylphosphonyl and 2,4,6-trimethyl benzoyl radicals in SDS is affected by magnetic... 

The signal can be enhanced by so-called amplifiers, probably by an interaction of the relatively long-lived AMP D anion with hydrophobic microdomains on the enhancer molecules. The hydrophobic domains exclude water with its protons (which could quench chemiluminescence) from the direct environment of the anion emitter. Just the addition of 0.1% BSA may amplify the signal several-fold. This may show possible problems for quantification due to unknown effects of sample components with hydrophobic domains. Amplification with an independent system can be achieved by adding commercially available fluorescent micelles (Schaap et al., 1989), such as those prepared from CTAB (Section 3.1.2.5) and 5-A-tetradecanoyl-aminofluorescein, which sequester the more hydrophobic AMP D. Chemiluminescence transfers enough energy to the co-micellized fluorescer to provide for a = 100-fold increase in photon output. 

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