Fluorophore-metallic surface interactions

Gersten, J. (2005] Theory of fluorophore-metallic surface interaction, in Topics in Fiuorescence Spectroscopy, Volume 8 Radiative Decay Engineering (ed. by Geddes, C. D., and Lakowicz, J. R., Springer), New-York. 

Figure 1.1 A schematic diagram of the unified plasmon/fluorophore description. Fluorophores induce surface plasmons in metals and energy is effectively transferred in a non-radiative fashion. This interaction of excited states with surface plasmons leads to a wealth of new fluorescence, chemiluminescence and phosphorescence phenomena and technologies we describe as a Unified Description.

Several other studies (150-153) reported that metal surfaces were able to either enhance or suppress the radiative decay rates of fluorophores. Furthermore, an increase in the extent of resonance energy transfer was also observed. These effects might be due to the interactions of excited-state fluorophores with SPs, which in turn produce constructive effects on the fluorophore. The effects of metallic surfaces include fluorophore quenching at short distances ( 0-5 nm), spatial variation of the incident light field (-0-15 nm), and changes in the radiative decay rates (-0-20 nm) (64). The term of metal-enhanced fluorescence could be referred to the appplication of fluorophore and metal interactions in biomedical diagnosis (64). 

The RDE concept of placing the fluorophore in the presence of a metal surface (either metal nanoparticles or a thin, roughened metal layer) now takes advantage of the fact that the interaction can be described by the introduction of a new decay path PLQY and lifetime are then given by (7) and (8). As F increases, increases... 

The preceding chapter showed that many different processes have to be considered if one would like to fully understand the interactions between a fluorophore and a nanostructured metallic template. Depending on the distance regime, classical image theory, electrodynamic theory, nonlocal effects or even wave functions of conduction band electrons leaking out of the metal surface have to be considered. Furthermore, each of the theories gives different results for fluorophores oriented perpendicular or tangential to the metallic surface. Different situations are also expected when either the absorption spectrum or the emission spectrum of the fluorophore overlaps with the plasmon... 

In about 2000, my laboratory started to study the interactions of fluorophores with metallic nanoparticles, both solution-based and surface-immobilized. Our findings agreed with other workers whom had observed increases in fluorescence emission coupled with a decrease in the fluorophores radiative lifetime. Subsequently, we applied classical far-field fluorescence descriptions to these experimental observations, which ultimately suggested a modification in the fluorophores s intrinsic radiative decay rate, a rate thought to be mostly unchanged and only weakly dependent on external environmental factors. This simple description, coupled with what seemed like a limitless amount of applications led to a paper published by our laboratory in 2001 entitled Metal-Enhanced Fluorescence , or MEF, a term now widely used today almost a decade later. 

As briefly mentioned in the Introduction the MEF phenomenon is a result of the interactions between the excited states of the fluorophores and the induced surface plasmons of metal nanopaiticles or roughened surfaces. Figure 1.2A depicts our laboratory s interpretation of the processes involved in the interactions of fluorophores with metal nanoparticles in close proximity to one another. There are two main processes thought responsible in MEF 1) non-radiative coupling from the excited state of the fluorescent species to surface plasmons of the metallic... 

There are essentially two models that describe the interaction between an excited fluorophore and the SPR of the metal to account for quenching and enhancement of the fluorescence. They both depend on coupling of the fluorophore excited state to the SPR and this is dependent of the spectral overlap of the emission of the fluorophore and the SPR, and the distance between the fluorophore and the metal nanoparticle surface. 

The photochemical properties of various nanoassemblies discussed in this chapter highlight the ways in which the metal and semiconductor nanopartides interact with light. Furthermore, one can fine tune these responses by subjecting nano-stmctures to an externally applied electrochemical bias. The ability to functionalize these nanopartides with photoactive molecules has opened new avenues to utilize these nanoassemblies in light energy conversion and catalytic applications. By suitably modulating the fluorescence of the surface bound fluorophore these... 

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