Emission enhancement fluorescence quenching

Keywords Rare earth complexes Langmuir-Blodgett (LB) films Fluorescence emission Fluorescence enhancement Fluorescence quenching. 

In this chapter, a brief theoretical overview is provided that discusses, among other things, EM enhancement of emission, enhanced absorption, quenching to metal surfaces, the distance, coverage, and temperature dependence of SEF, and the effects of quantum efficiencies on enhancement. Also discussed, is the preparation and characteristics of several different nanoparticle metal substrates that have been employed in the collection of SEF, and the surface-enhanced fluorescence of Langmuir-Blodgett (LB) monolayers. Finally, a summary of these concepts is presented, and the future of SEF is discussed. 

The extent of fluorescence quenching often depends on the sorbent medium and is generally more severe for silica gel than for chemically bonded sorbents [183]. In many cases the emission signal can be enhanced by application of a viscous liquid to the layer before scanning the plate. Common fluorescence enhancing... 

At wavelengths shorter than 1520 A photolysis of N20 is accompanied by fluorescence152 from the B2IIr state of NO (fi emission). Added or product NO quenches this P emission and enhances fluorescence of the y bands from NO(<42 +). The fluorescent states of NO must152 arise in secondary reactions and these are believed151,152 to be, for P emission,... 

In calixarene-based compound M-8 (Figure 10.28), bearing four anthracene moieties on the lower rim, some changes in fluorescence intensity were observed on binding of alkali metal ions but no excimer emission was detected. Quenching of the fluorescence by Na+ may arise from interaction of four anthracene residues brought in closer proximity to one another enhancement of fluorescence by K+ is difficult to explain. 

FLUORESCENCE MEASUREMENTS OF LIGAND BINDING. In principle, ligand binding may either enhance or quench the intrinsic or extrinsic fluorescence of its macromolecular receptor or it may change the polarization of the fluorescence emission (see below). 

The fluorescence emission maximum, quantum yield, and lifetime of a fluorophore are very sensitive to its immediate environment. A blue shift in the emission maximum and an increase in the fluorescence quantum yield or lifetime is generally observed when a fluorophore is transferred form a polar solvent to a nonpolar one or when it binds to a hydro-phobic protein site. Furthermore, fluorescence quenching or enhancement may result from interactions of the fluorophore with various structural elements in its vicinity. 

Metal nanostructures can act as small antennas that aid in the reception and broadcasting (absorption and emission) of light from nearby fluorophores. Whether fluorescence enhancement or quenching is observed in a given system is determined by the relative extent of excitation enhancement (increased light absorption), emission enhancement (increased radiative decay), and quenching (increased non-... 

At very short metal nanoparticle-fluorophore distances ( 1 to 3 nm), a large decrease in fluorescence, known as quenching, is expected [8,19,20]. At greater distances however, the fluorescence can undergo enhancement or continue to experience a degree of quenching. The examples outlined below will illustrate that whether enhancement or quenching is observed depends on nanoparticle size and shape, the distance between the fluorophore and the metal nanoparticle surface, and on the overlap between the SPR and the excitation and/or emission transitions in the fluorophore. 

It is possible that surface enhancement effects, similar to the observations made earlier in metal-fluorophore systems [11, 83-85] may occur. Metal surfaces are known to have effects on fluorophores such as increasing or decreasing rates of radiative decay or resonance energy transfer. A similar effect may take place in ZnO nanomaterial platforms. However, decay lengths of fluorescence enhancement observed in the semiconducting ZnO NRs are not commensurate with the length scale seen on metals such as Au or Ag. For effective metal enhanced fluorescence, fluorophores should be placed approximately between 5-20 nm away from the metal surface. However, fluorescence enhancement effect on ZnO NRs is observed even when fluorophores are located well beyond 20 nm away from the NR surface. At the same time, no quenching effec en when they are placed directly onto ZnO NR surfaces. In addition, there overlap between the absorption and emission... 

---