Abstract
We demonstrate strong spectral dependence of the efficiency of fluorescence quenching in molecular systems composed of organic dyes and gold nanoparticles. In order to probe the coupling with metallic nanoparticles we use dyes with varied spectral overlap between the plasmon resonance and their absorption. Hybrid molecular structures were obtained via conjugation of metallic nanoparticles with the dyes using biotin-streptavidin linkage. For dyes featuring absorption above the plasmon excitation in gold nanoparticles, laser excitation induces minute changes in the fluorescence intensity and its lifetime for both conjugated and non-conjugated mixtures, which are the reference. In contrast, when the absorption of the dye overlaps with the plasmon resonance, the effect is quite dramatic, reaching 85% and 95% fluorescence quenching for non-conjugated and conjugated mixtures, respectively. The degree of fluorescence quenching strongly depends upon the concentration of metallic nanoparticles. Importantly, the origin of the fluorescence quenching is different in the case of the conjugated mixture, as evidenced by time-resolved fluorescence. For conjugated mixtures of dyes resonant with plasmon, excitation features two-exponential decay. This is in contrast to the single exponential decay measured for the off-resonant configuration. The results provide valuable insight into spectral dependence of the fluorescence quenching in molecular assemblies involving organic dyes and metallic nanoparticles.
Highlights
Plasmon excitation, the result of collective electron oscillation in metallic nanoparticles, has been used for more than a decade to control the optical properties of fluorophores
The distance between the fluorophore and the metallic nanoparticle determines whether the fluorescence is enhanced due to local electromagnetic field created by the metallic nanoparticle or, alternatively, if the energy is efficiently transferred from the fluorophore to the metallic nanoparticle, which would result in fluorescence quenching [4,5]
We studied the fluorescence properties of Atto dyes coupled to metallic nanoparticles
Summary
The result of collective electron oscillation in metallic nanoparticles, has been used for more than a decade to control the optical properties of fluorophores. The ability to control and very often dramatically modify absorption or fluorescence of a molecule by placing a metallic nanoparticle in its vicinity owes to strong localization of the electromagnetic field by the latter. The relation between the energy of plasmon excitation and the optical properties of a fluorophore implies whether the emission or the absorption of the fluorophore is enhanced. The relative orientation of the fluorophore and the metallic nanoparticle to the laser excitation could contribute to the net effect measured for an assembly comprising fluorophores and metallic nanoparticles. This parameter has proven to be the hardest to control. It has been found that there exists an optimal distance, typically in the range of 10 to 20 nm, for which the enhancement of the fluorescence intensity is the strongest
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