Surface enhanced fluorescence, SEF

Le Ru, E. C., and Etchegoin, P. G. (2005). Surface-Enhanced Raman Scattering (SERS) and Surface-Enhanced Fluorescence (SEF) in the context of modified spontaneous emission. arXiv physics/0509154vl pp.1-14. 

Progress in technology of nanosized materials has renewed attention to surface-enhanced optical phenomena. The nanoscale metals, which have important applications in snrface-enhanced Raman scattering (SERS) [1], surface-enhanced fluorescence (SEF) [2, 3] and optoelectronic nanodevices are of particular interest. 

Due to the surface sensitivity surface enhanced fluorescence has become particularly popular in the characterisation of thin molecular films, such as Langmuir-Blodgett films and self-assembled biomembranes. Two surface enhanced spectroscopic techniques (surface enhanced IR absorption, SEIRA, and surface enhanced fluorescence, SEF) were recently applied to the study of biomembrane systems by the group of Reiner Salzer [323]. With SEIRA, specific fingerprints of biomolecules could be obtained with a tenfold IR intensity enhancement With SEF signal enhancement factors greater than 100 were obtained. The enhancement factor was very dependent on the properties and structure of the metal clusters used. With the two techniques biomembranes formed from vesicles with embedded nicotinic acetylchoHne receptors were spectroscopically characterized. 

The surface-enhanced fluorescence, SEF, and surface-enhanced Raman scattering, SERS, phenomena are inextricably tied to one another due to their conunon electromagnetic enhancement origin, and thus there is a very strong overlap in their lit ature. In fact, there is often a direct competition between fluorescence and Raman scattering that, as we shall see, with strategic experimental design, can be exploited. There are, however, a few very important points on which these two effects differ, the most important of which is their distance dependence. 

Finally, it is evident that SERS should be considered from a wider perspective and comparing the performance of SERS with other techniques can always be useful. For example, fluorescence microscopy and spectroscopy are extremely sensitive and widely used techniques in many areas of the life sciences. Further development and expanding of other surface-enhanced spectroscopic techniques such as surface-enhanced infrared absorption (SEIRA), surface-enhanced fluorescence (SEF) and tip-enhanced Raman scattering (TERS) are also highly desirable. It is highly appreciable so that these techniques will help to refine information obtained by SERS in many bioanalytical, biomolecular and medical studies. 

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. 

Thus, the importance of the Langmuir-Blodgett technique in the study and application of surface-enhanced fluorescence has been demonstrated through a variety of examples. The LB technique provides a unique method for generating organized and well-defined molecular structures at precise distances from metal surfaces with known adsorbate concentrations, and so provides an excellent tool for manipulating important factors such as metal-adsorbate separation distance, and surface coverage for SEF measurements. 

For some time now, we have been working with many different perylene derivatives in the study of surface-enhanced spectroscopies such as SERS, SERRS, SEF, and SEIRA (surface-enhanced infiared absorption). These molecules show a very strong tendency towards aggregation at high concentrations, and often display excimer fluorescence in concentrated Langmuir-Blodgett monolayers. The surface-enhanced... 

A large portion of the work that has been done in our group on the surface-enhancement of fluorescence from LB monolayers has been in conjunction with surface-enhanced Raman studies. In theses studies, SEF has been helpful in providing information about the aggregation, and orientation of dye molecules, as well as information about the thermal stability of the monolayers used. A separate study reported the effects of different variables pertaining to the fabrication of LB films. In particular. 

The electromagnetic field enhancement provided by nanostructure plasmonics is the key factor to manipulate the quantum efficiency. However, as it is illustrated in the unified theory of enhancement, since both the radiative and non-radiative rates of the molecular systems are affected by proximity of the nanostructure, the tuning has to be done on a case by case basis. In addition, there are factors due to molecule-metal interactions and molecular orientation at the surface causing effects that are much more molecule dependent and as are much more difficult to predict. Given the fact that fluorescence cross sections are the one of the highest in optical spectroscopy the analytical horizon of SEF or MEF is enormous, in particular in the expanding field of nano-bio science. 

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