Selectivity fluorescence

Kumar S, Singh AK, Krishnamoorthy G, Swaminathan R (2008) Thioflavin T displays enhanced fluorescence selectively inside anionic micelles and mammalian cells. J Fluoresc 18(6) 1199-1205... 

According to fluorescence selection rules, only molecules that have their absorption dipoles properly oriented with the electronic dipole of polarized light can be excited.(31) Consequently, the emission is also initially polarized. The polarization, P, is defined by the expression... 

Fig. 6.3. Cutoff fluorescence selection for screening. Instrumentation, labeling, and biological noise introduce spreading into a fluorescence measurement, such that the fluorescence probability distributions for wild-type and mutant cells overlap. The logarithm of single-cell fluorescence as measured by flow cytometry is generally well-approximated by a symmetrical Gaussian curve. A cutoff fluorescence value is selected for screening, with all cells above that value sorted out. The enrichment factor forthe mutants is the ratio of (dotted + striped areas)/(striped area), and the probability of retention of a given mutant clone at a single pass is the (striped + dotted area)/(all area under mutant curve).

The geometrical factor is the modulus of the normalized geometrical structure factor Sh for the ordered fluorescent-selected atoms ... 

In an extension of previous studies on functionalised caUxarenes with on-off fluorescence selectivity for Pb " and K, Chung and co-workers prepared 24. It was shown that fluorescence quenching occurred in the presence of Pb, but in the presence of excess K", strong fluorescence could be recovered. Using model... 

For coloured product family selection is made on nickel-chromium specimen of 30 pm, but for fluorescent product family selection is made on nickel-chromium specimen of 10 pm. Tests made on real defects confirm these results. 

Micellar structure has been a subject of much discussion [104]. Early proposals for spherical [159] and lamellar [160] micelles may both have merit. A schematic of a spherical micelle and a unilamellar vesicle is shown in Fig. Xni-11. In addition to the most common spherical micelles, scattering and microscopy experiments have shown the existence of rodlike [161, 162], disklike [163], threadlike [132] and even quadmple-helix [164] structures. Lattice models (see Fig. XIII-12) by Leermakers and Scheutjens have confirmed and characterized the properties of spherical and membrane like micelles [165]. Similar analyses exist for micelles formed by diblock copolymers in a selective solvent [166]. Other shapes proposed include ellipsoidal [167] and a sphere-to-cylinder transition [168]. Fluorescence depolarization and NMR studies both point to a rather fluid micellar core consistent with the disorder implied by Fig. Xm-12. 

Experimental access to the probabilities P(E ,E) for energy transfer in large molecules usually involves teclmiques providing just the first moment of this distribution, i.e. the average energy (AE) transferred in a collision. Such methods include UV absorption, infrared fluorescence and related spectroscopic teclmiques [11. 28. 71. 72, 73 and 74]. More advanced teclmiques, such as kinetically controlled selective ionization (KCSI [74]) have also provided infonnation on higher moments of P(E ,E), such as ((AE) ). 

Spectroscopists observed that molecules dissolved in rigid matrices gave both short-lived and long-lived emissions which were called fluorescence and phosphorescence, respectively. In 1944, Lewis and Kasha [25] proposed that molecular phosphorescence came from a triplet state and was long-lived because of the well known spin selection rule AS = 0, i.e. interactions with a light wave or with the surroundings do not readily change the spin of the electrons. 

CAHRS and CSHRS) [145, 146 and 147]. These 6WM spectroscopies depend on (Im for HRS) and obey the tlnee-photon selection rules. Their signals are always to the blue of the incident beam(s), thus avoiding fluorescence problems. The selection ndes allow one to probe, with optical frequencies, the usual IR spectrum (one photon), not the conventional Raman active vibrations (two photon), but also new vibrations that are synnnetry forbidden in both IR and conventional Raman methods. 

Perhaps the best known and most used optical spectroscopy which relies on the use of lasers is Raman spectroscopy. Because Raman spectroscopy is based on the inelastic scattering of photons, the signals are usually weak, and are often masked by fluorescence and/or Rayleigh scattering processes. The interest in usmg Raman for the vibrational characterization of surfaces arises from the fact that the teclmique can be used in situ under non-vacuum enviromnents, and also because it follows selection rules that complement those of IR spectroscopy. 

The pump-probe concept can be extended, of course, to other methods for detection. Zewail and co-workers [16,18, 19 and 2Q, 93] have used the probe pulse to drive population from a reactive state to a state that emits fluorescence [94, 95, 96, 97 and 98] or photodissociates, the latter situation allowing the use of mass spectrometry as a sensitive and selective detection method [99, 100]. 

Leone S R 1983 Infrared fluorescence a versatile probe of state-selected chemical dynamics Acc. Chem. Res. 16 88-95... 

The development of tunable, narrow-bandwidtli dye laser sources in tire early 1970s gave spectroscopists a new tool for selectively exciting small subsets of molecules witliin inhomogeneously broadened ensembles in tire solid state. The teclmique of fluorescence line-narrowing [1, 2 and 3] takes advantage of tire fact tliat relatively rigid chromophoric... 

Fries J R, Brand L, Eggeling C, Kdllner M and Seidel CAM 1998 Quantitative identification of different single molecules by selective time-resolved confocal fluorescence spectroscopy J. Phys. Chem. A 102 6602-13... 

Eggeling C, Fries J R, Brand L, Gunther R and Seidel CAM 1998 Monitoring conformational dynamics of a single molecule by selective fluorescence spectroscopy Proc. Natl Acad. Sc/. USA 95 1556-61... 

Sensitivity levels more typical of kinetic studies are of the order of lO molecules cm . A schematic diagram of an apparatus for kinetic LIF measurements is shown in figure C3.I.8. A limitation of this approach is that only relative concentrations are easily measured, in contrast to absorjDtion measurements, which yield absolute concentrations. Another important limitation is that not all molecules have measurable fluorescence, as radiationless transitions can be the dominant decay route for electronic excitation in polyatomic molecules. However, the latter situation can also be an advantage in complex molecules, such as proteins, where a lack of background fluorescence allow s the selective introduction of fluorescent chromophores as probes for kinetic studies. (Tryptophan is the only strongly fluorescent amino acid naturally present in proteins, for instance.)... 

An instrument for measuring fluorescence that uses filters to select the excitation and emission wavelengths. 

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