Metal-enhanced fluorescence nanoparticle interactions

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. 

This handbook presents a comprehensive overview on the physics of the plasmon-emitter interaction, ranging from electromagnetism to quantum mechanics, from metal-enhanced fluorescence to surface-enhanced Raman scattering, and from optical microscopy to the synthesis of metal nanoparticles, filling the gap in the literature of this emerging field. It is useful for graduate students as well as researchers from various fields who want to enter the field of molecular plasmonics. The text allows experimentalists to have a solid theoretical reference at a different level of accuracy and theoreticians to find new stimuli for novel computational methods and emerging applications. 

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. 

Understanding the field enhancement of radiative rates is insufficient to predict how molecular photophysical properties such as enhancement of fluorescence quantum yield will be affected by interactions of the molecule with plasmons. A more detailed model of the photophysics that accounts for non-radiative rates is necessary to deduce effects on photoluminescence (PL) yields. Such a model must include decay pathways present in the absence of metal nanoparticles as well as additional pathtvays such as charge transfer quenching that are associated with the introduction of the metal particles. Schematically, we depict the simplest conceivable model in Figure 19. IB. Note that both the contributions of radiative rate enhancement and the excited state quenching by proximity to the metal surface will depend on distance of the chromophore from the metal assembly. In most circumstances, one expects the optimal distance of the chromophores from the surface to be dictated by the competition between quenching when it is too close and reduction of enhancement when it is too far. The amount of PL will be increased both due to absorption enhancement and emissive rate enhancement. Hence, it is possible to increase PL substantially even for molecules with 100 % fluorescence yield in the absence of metal nanoparticles. 

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