Lissamine-gold nanoparticle

Figure 4 Lissamine - gold nanoparticle composite system. Lissamine molecules with dipole moment are attached onto gold nanoparticles of radius r via a thioether linker. The distance between the chromophoric part and the nanoparticle surface is d = Inm.

Figure 9 Fluorescence decay signal from Lissamine - gold nanoparticle composites after removal of the ultrafast gold nanoparticle spike and the long lived signal from unbound Lissamine molecules. The fastest and the intermediate decay curves are from composites comprising nanoparticles with r = 30 nm and r = 1 nm radius. The slowest decay is that of free Lissamine molecules. Straight lines are single exponential fits.

A recent experimental test of the theory was performed using time-resolved fluorescence. Lissamine fluorophores were attached to gold nanoparticles via thio ether groups. Both the radiative and non-radiative decay chaimels were studied. The sizes of the Au particles were varied in several steps between a = 1 and a = 30 nm, whereas the distance between the fluorphore and the nanoparticle was kept constant at about d=r-a = 1 nm. The optical excitation source was a 120 fs laser pulse at a wavelength of 400 nm (3.10 eV), well away from the dipolar plasmon resonance at 520 run (2.4 eV) for a gold sphere in water. It was believed that the transition dipole of the fluorophore was parallel to the surface of the particle. 

Figure 10 shows the absorption spectrum of the gold particles in solution (dashed curve) and the fluorescence spectrum of a solution of lissamine fluorophores both before and after they are attached to the gold particles. As expected, the gold particles in solution show an absorption peak centered at the dipolar plasmon frequency. It is seen that the fluorescence of the lissamine is strongly quenched by presence of the gold nanoparticles. It was estimated that the quenching efficiency exceeded 99%, when the residual fluorescence of unbound fluorophores was taken into account. 

Figure 6 Optical density of a 0.029 nM gold nanoparticle solution (solid line) and fluorescence spectra of a 0.18 pM Lissamine solution (open circles), a solution of 0.18 pM Lissamine and 0.029 nM passivated gold nanoparticles (dash-dotted curve) and a solution of 0.18 pM Lissamine bound to gold nanoparticles with a concentration of 0.029 nM (dotted curve).

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. 

Figure 8 Time resolved fluorescence signal from a 0.37 /tM Lissamine solution (upper curve) and a mixed solution of 0.37 fiM Lissamine and of0.2S nM gold nanoparticles of r = IS nm radius. The ultrafast spike at t K 0 stems from the gold particles, while the long lived component is from unbound Lissamine molecules. The intermediate decay signal is from Lissamine - nanoparticle composites.

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