Time Resolved Spectroscopy of Fluorophores Bound to Metal Nanoparticles

TIME RESOLVED SPECTROSCOPY OF FLUOROPHORES BOUND TO METAL NANOPARTICLES 

The preceding chapter showed that many different processes have to be considered if one would like to fully understand the interactions between a fluorophore and a nanostructured metallic template. Depending on the distance regime, classical image theory, electrodynamic theory, nonlocal effects or even wave functions of conduction band electrons leaking out of the metal surface have to be considered. Furthermore, each of the theories gives different results for fluorophores oriented perpendicular or tangential to the metallic surface. Different situations are also expected when either the absorption spectrum or the emission spectrum of the fluorophore overlaps with the plasmon 

The composite sample under investigation is shown in figure 4. The Rhodamine-type dye Lissamine with molecular dipole moment Hm, has been attached to the gdd nanoparticles via a thioether group. The radii of the particles have been 1, 10, 15, and 30 nanometers with a size distribution of less than 15% each. 

The fluorescence spectrum of the composite system, i.e. a solution of 0.029 nM unpassivated gold nanoparticles (r = 30 nm) and 0.18 fiM Lissamine dye molecules, is shown by the dotted curve in figure 6. In fact, it is hardly distinguishable from the abscissa. Compared to the fluorescence of the solution with passivated particles, only -5 % of fluorescence intensity is left over. From the following discussion it will become clear that most of this residual fluorescence is due to fluorescence fi-om unbound Lissamine molecules inevitably present due to the thermodynamic equilibrium between bound and unbound molecules. Only time resolved measurements are able to distinguish between fluorescence of bound and unbound molecules. Below it will become clear that the nanoparticles quench the fluorescence of the bound molecules by more than 99 %. 

Lissamine molecules in the vicinity of gold nanoparticles because the [H oportionality of Einstein coefficients of absorption and emissiai must hold as long as absorption and emission occur without a Stokes shift. We find that the absorption cross section is changed by 30% for nanoparticles of 30 nm radius. This effect is ready included in the fluorescence quenching efficiency above and will be included in a similar way in the analysis of the following time resolved measurements. 

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