Metal-enhanced fluorescence distance dependence

Ray, K., Badugu, R. and Lakowicz, J. R. (2006). Distance-dependent metal-enhanced fluorescence from Langmuir-Blodgett monolayers of alkyl-NBD derivatives on silver island films. Langmuir 22 8374-8378. 

Figure 10.9 Sample architecture for the distance dependence of metal-enhanced Superoxide generation (Top), and graphical representation of the interpretation of metal-enhanced superoxide generation with an enhanced and distance dependent excitation rate (Bottom). F - Fluorophore, MEF - Metal-Enhanced Fluorescence, MEP — Metal-enhanced Phosphorescence, SiFs — Silver Island Films. EF- Enhancement factor = I Silver /I Glass. Adopted from ref [31 ].

Both Geddes and the Lakowicz group s have investigated the metal-enhanced fluorescence of fluorophores on silver island films (SIFs) [11,26,27] and variously aggregated silver nanoparticles in solution [28,29]. One example of enhancement on SIFs is discussed below [26]. In this work the distance-dependent MEF of a monolayer of sulforfiodamine B (SRB) on SIFs was studied. A SRB monolayer was electrostatically incorporated into the Langmuir-Blodgett (LB) layers of octadecylamine (ODA) deposited... 

Figure 7.1 (A) Schematic representation of the Metal-Enhanced Fluorescence phenomena (B) FDTD calculations for two silver nanoparticle arrays (d = 100 nm). (C) Wavelength-dependent calculated Ej maximum intensity for silver nanoparticle arrays (d = 100 nm). Geometries and incident field polarization [p-polarized) and propagation direction are shown in the insets. The gap between the nanopaiticles was assumed to be 2 nm in the calculations. (D) Calculated field enhancement as a function of distance for a single silver nanoparticle (d = 100 nm).The inset shows these results as an FDTD E image above the nanoparticle.

Very recently Bawendi and co-workers [4] reported about fivefold increase in the observed fluorescence intensity of single CdSe/ZnS nanocrystalls (NCs) and striking reduction in their fluorescence blinking behavior due to interactions with a rough metal film. The distance-dependent enhancement and quenching of NC fluorescence has been observed by us using the layer-by-layer polyelectrolyte deposition technique to insert well-defined spacer between gold colloidal films and NCs [5] with maximum enhancement for the 9-layer spacer ( 11 nm in thickness). 

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. 

One common misunderstanding about the underlying physics of the plasmon-enhanced fluorescence is made clear by the distance dependence data. Researchers often cite Fbrster transfer from the molecular chromophore to the electrcxi plasma in the metal as a quenching mechanism for the excited state. If this... 

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. 

In 1988, the distance dependence of surface-enhanced fluorescence was studied for Langmuir-Blodgett monolayers deposited on silver island films. This study was inspired in part by two earlier reports that examined the distance dependence of SERS of LB films on metal surfaces. Varying numbers of spacer layers of arachidic acid were employed in order to probe the competition between EM enhancement and radiationless energy transfer for a phthalocyanine monolayer. In direct contact with the metal surface, a broadened, enhanced, and red-shifted fluorescence spectrum was observed. These spectral changes can be attributed to a drastic decrease in the fluorescence lifetime of the molecule when it is placed in contact with the metal surface. However, an enhanced version of the unperturbed spectrum was observed when intervening spacer layers were introduced. It was found at enhancements on the order of about 400 could be realized when S monolayers were placed between the Ag island film and the phthalocyanine monolayer. 

N. Akbay, J.R. Lakowicz, K. Ray, Distance-dependent metal-enhanced intrinsic fluorescence of proteins using polyelectrolyte layer-by-layer assembly and aluminum nanoparticles. J. Phys. Chem. C 116, 10766-10773 (2012)... 

Although it has been difficult to separate the effects of excitation and emission enhancement, both of these effects should be extremely sensitive functions of the shape of the metal particle, the orientation of the fluorophore, and the distance between the fluorophore and the metal, because the local-field effects depend strongly on these parameters. Many groups have studied variations in fluorescence intensity as a function of the distance between a layer of fluorophores and a number of nanostructured metal surfaces, adsorbed colloidal particles or suspended colloidal particles. Single-molecule experiments have even provided strong evidence for the existence of a local maximum in the fluorescence intensity versus distance curve. ... 

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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