Fluorescence thermal quenching

In general, fluorescence intensity is more sensitive to temperature than lifetime, thus fluorescence thermal quenching followed by fluorescence intensity will allow probing the local dynamics of the fluorophore. Also, it will be possible to understand the nature of the fluorophore interaction with its binding site. 

Figure 4.44. Fluorescence thermal quenching of calcofluor free in solution (a), bound to sialylated ai - acid glycoprotein (b) and bound to asialylated ai - acid glycoprotein (c). The concentration of the fluorophore (5 pM) is equal to that of the protein. A.ex = 330 nm and Xem 435 nm. Source Albani, J. R. 2003, Carbohydrate Research 338, 1097-1101.

In summary, the use of fluorescence lifetime monitoring for temperature sensing at high temperatures is based on the phenomenon of thermal quenching of fluorescence, while this phenomenon is j u st the very obstacle that blocks the extending of the measurement further into higher temperatures. Therefore, fluorescence thermometry is intrinsically more effective for measurement within moderate temperature regions, due to this fundamental nature of the fluorescence emission itself. 

Many other fluorophores are temperature-sensitive only when they are bound to macromolecules. Figure 10.16 shows the effect of temperature on the fluorescence intensity of native and guanidine unfolded AEDANS-RNase. Increasing the temperature from 10 to 30°C induces a decrease in fluorescence intensity for both protein states. The intensity decrease in native protein is more affected by temperature than the guanidine-unfolded protein. This thermal quenching is the consequence of rapid movements of the protein structure around the fluorescent probe. These movements occur during the lifetime of the excited state, and their rate is temperature-dependent. 

The activated internal conversion, which is a hallmark of the photoexcited DNA bases, also occurs in a number of aromatic ethynes and nitriles, including dipheny-lacetylene (DPA) and 4-(dimethylamino)benzonitrile (DMABN). Thus, as in the case of the nucleobases, DPA exhibits an abrupt break-off (loss) of fluorescence in supersonic free jet [35], Figure 15-16, and the strong thermal quenching of... 

Thermal Fluorescence intensity quenching of TNS bound tightly to LCA shows two slopes equal to -2.5% per °C and -1.6% per °C. The breaking temperature is equal to 20°C. One can notice that there is no big difference between the two slopes (Fig. 

Figure 4.43. Thermal fluorescence intensity quenching of free FITC (a), of FITC bound to LCA (b), in presence of LCA-LTF complex (c) and in presence of LCA-STF complex (d). Source Albani, J. R. 1998. Biochim. Biophys. Acta. 1425, 405-410.

The fluorescent probe by itself shows no thermal quenching (Fig. 4.43a). Therefore thermal quenching is not related to environment polarity alone. The presence of a structured matrix around the fluorophore induces the sensitivity of the fluorophore to temperature. The origin of this thermal quenching is the fast motions of the microenvironment surrounding the fluorophore. 

In the fluorescence intensity quenching (thermal and with iodide), it is the fluorescein environment consisting of amino acids (thermal quenching) and of amino acids and solvent dipoles that is relaxing around the excited fluorescein. In the fluorescence anisotropy experiments, on the other hand, the displacement of the emission dipole moment of the fluorescein is monitored. In the first approach, it is the environment that is either fluid or rigid. In the second approach, the restricted reorientational motion of the fluorophore is followed. 

Improvements BaMgAlioOi7 Eu " is not only a blue-emitting phosphor for three-band fluorescent lamps but also an important phosphor for plasma display. However, BaMgAlioOn u " blue-emitting phosphor has severe thermal quenching, and many efforts have been made to reduce its luminescence degradation and quenching. 

Increasing a solution temperature increases the chance for molecules to interact and thus fluorophore coUisional quenching is expected to be greater at higher temperatures. However, not all fluorophores are deactivated in the same manner. Thus thermal quenching of DOM fluorescence is a physical phenomenon that can be used to characterize DOM in natural waters (Baker, 2(X)5 Seredynska-Sobecka et al., 2007) as much as it is a consideration for the environmental conditions DOM encounters (Spencer et al., 2007b). 

Thermal quenching appears to increase the deactivation of the excited state by internal conversion (Senesi, 1990). Indeed this effect was observed from 10°C to 45°C by Baker (2005) on examining several riverine and wastewater DOM samples and humic standards. Interestingly, protein fluorescence appeared most susceptible to thermal quenching in several instances as compared to fulvic material fluorescence. As thermal quenching is sensitive to the fluorophores exposure to the energy supplied by increasingtemperature, these results imply that some species may be more easily perturbed than others and can provide information on DOM sources (Baker, 2005). 

Shim and coworkers [129] synthesized poly(2-fluoro-l,4-phenylene vinylene) 75 by the thermal conversion method. This polymer exhibits almost the same absorbance spectra as PPV 1 (Amax 410 nm), but the fluorescence band (Amax = 560 nm) is red-shifted by ca. 20 nm. The LUMO level was shifted down by ca. 0.15eV, facilitating electron injection but, in contrast to the above polymer 74, no fluorescence quenching was observed. Consequently, the PLED devices fabricated as ITO/75/A1 have about ten times higher EL efficiency than those fabricated with PPV 1 under identical conditions. 

Although, in principle, it is possible for some fraction of the events to follow the Odd surface beyond this second intersection and to thus lead to JOdd product molecules that might fluoresce, quenching is known to be rapid in most polyatomic molecules as a result, reactions which are chemiluminescent are rare. An appropriate introduction to the use of OCD s, CCD s, and SCD s as well as the radiationless processes that can occur in thermal and photochemical reactions is given in the text Energetic Principles of Chemical Reactions, J. Simons, Jones and Bartlett, Boston (1983). 

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