Metallic nanoparticles quenching

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. 

Recently, there has been a flurry of experimental work investigating the phenomenon of MEF with a range of metal nanoparticle shapes and sizes. In this Section, we provide a brief overview of some recent experiments that have shown quenching and enhancement effects, and demonstrate a correlation between observed effects on fluorescence and the morphology of the nanoparticles employed. 

Many experiments have been carried out where the distance between the fluorophore and the metal nanoparticle surface is varied yet only quenching is observed. In these cases the nanoparticles are usually small spherical metal nanoparticles. The following examples demonstrate distance-dependent quenching in a couple of nanoparticle-fluorophore systems. 

One may ask why these experiments only showed quenching and not enhancement. The first thing to take note is the fact that the metal nanoparticles here are spherical, and therefore the SPR does not produce a very large enhancement of the local field. Also, the nanoparticles are small, which, as will be explained in Section 11.3,... 

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. 

The radiative and nonradiative decay rates depend also on a possible aggregation state of the dye molecules. The lifetime of aggregates can be longer than that of single molecules on the other hand, the fluorescence may be almost entirely quenched. Extremely strong effects on the decay rates must also be expected if dye molecules are bound to metal surfaces, especially to metallic nanoparticles [182,309, 337]. 

The fluorescence lifetimes of typical fluorophores used in cell imaging are of the order of a few ns. However, the lifetime of autofiuorescence components and of the quenched donor fraetion in FRET experiments can be as short as 100 ps. In cells, lifetimes of dye aggregates as short as 50 ps have been found [261]. The lifetime of fluorophores eonneeted to metallic nanoparticles [182, 183, 309, 337] can be 100 ps and shorter. 

Multicomponent composites built by LbL assembly may consist not only of nanocrystals but also of nanowires, biomolecules, dyes, and functional polymers. In addition, metal nanoparticles may be employed with potential applications in the fields of drug delivery and biodetection, energy harvesting, optical signal processing, and emission enhancement or quenching [70, 93]. 

In Chapter 10, Samnel Hemandez-Rivera and cowoikers show that Raman detection of trace amoimts of explosives and other hazardous materials on surfaces can be improved by ten to one thousand times with the addition of colloidal metallic nanoparticles to contaminated areas. Banahalli Ratna and colleagues demonstrate that orgarrized spatial distribution of fluorescent reporter molecules on a virus capsid eliminates the commonly encountered problem of fluorescence quenching. Using such viral nanoparticles, they show in Chapter 11 that enhanced fluorescence for the detection of protein toxins is possible. 

In two more recent papers on single molecule fluorescence near thin metallic layers, Enderlein reminds us that the quenching by metal nanoparticles shortens the lifetime of the excited state and by doing this, increases the number of excitation cycles that the molecule can survive before it is photobleached. This is an extremely important point, especially when dealing with single molecules, as fluorescence quantum yields are not nearly as important as the number of photons that are emitted before photobleaching occurs. 

Part of the energy transferred from the molecule to the excitation of plasmon is actually dissipated within the metal nanoparticle. This is an additional channel of de-excitation of the molecule that is not leading to photon emission, and it is not included in Prad- The energy flux method calculates the decay rate associated to this quenching by the net flux of S (equal to the absorbed power Pahs ) through a closed surface that contains the metal nanoparticle but not the molecule (see Fig. 5.4). The non-radiative decay -/nr is then calculated as for... 

By this approach, in Ref. [57] it was found that even for a very small nanoparticle, where excitations are of molecular character, quenching efficiency is as high as when plasmon resonances are present. We report in Fig. 5.12 the non-radiative decay rate of a dye molecule (PDl) as a function of its distance from a metal cluster composed of 20 Au atoms. For such a small cluster, ab initio results predict a molecular-like excitation spectrum, while applying a continuum model for the molecule is the same as assuming that the Au surface plasmon excitations of larger metal nanoparticle are conserved also for this size. 

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