Metallic nanoparticles single molecule fluorescence

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. 

In addition to the benefits of MEF from metal nanostructures deposited onto solid supports that are very useful in surface bioassays, MEF can also be observed from individual nanostructures in bioassays carried out in solution. In this regard, fluorophores and metal nanostructures can be assembled in core-shell architecture and can be used as fluorescent nanoparticles as indicators in biological plications such as imaging of cellular activity or single-molecule sensing. 

Shape The radiative emission from molecules confined within metallic nanocavities and on the surface of nanoparticles is of great relevance to biotechnology. In 1986, it has been suggested that fluorescence enhancement and reduced observation volumes could be obtained from small metal apertures (85). Nanocavities of different shapes could induce different surface plasmon (SP) fields. More recently, some studies has been done for different shapes, such as circular (86-90), elliptical (91), coaxial (92), or rectangular (93, 94) metallic nanocavity(95). In 2003, single-molecule detection from a nanocavity was demonstrated (86). However, it might be difficult to position the biospecies in the nanocavities. 

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]. 

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