Quantum dots excitation enhancement

Furthermore, the dense, saturation packing of R6G or other dye molecules on colloidal gold/silver surface does not limit Raman intensities in the way that fluorescence intensities are lowered by excited state dipole-dipole interactions. In comparison with flnorescent dyes and quantum dots, the enhanced Raman probes have an intrinsic amplification mechanism, with improved encoding and mnltiplexing capabilities. If multiple (4 or even more) sizes of gold/silver nanoparticles in the 50-300 nm diameter range with a narrow size distribution can be prepared, and then loaded with 20 different dye molecules with unique SERS fingerprints, this would provide 80 probes for each metal nanoparticle. 

Again, the question arises why do we see different degree of photoluminescence enhancements from the same particle as we vary the excitation wavelength Unlike the previous experiments with variety of organic fluorc hores shown in Figure 4.10, here we only use one kind of fluorc hore CdSe quantum dots... 

Figure 4.13 Enhancement of quantum dots photoluminescence near a nancqiarticle as a function of excitation wavelength for several single metal nani iarticles. Black dots are the ratio of the photoliuninescence near a nanc article to photoluminescence ir from a nanoparticle. The trace in each spectra is the LSPR...

In summary, the photoluminescence of CdSe quantum dots can be strongly enhanced by nearby metal nanoparticles, where most of the enhancement results from excitation effects. We observed that the shape of the PLE spectra of the quantum dots near a metal nanoparticle is significantly altered for both gold and Ag nanoparticles, and shows a new PLE peak coincident with the LSPR peak of the metal nanoparticle. Although the absolute enhancement factor varies from one metal nanoparticle to another, the wavelength dep>endence of the total enhancement factor still mirrors the line shape of the metal nanoparticle s scattering spectrum. There may be a small offset in the maximum excitation enhancement from the nanoparticle s scattering peak (as was described for the total fluorescence in Section 4.3 above), but at present our experiments have not had sufficient spectral resolution to identify any such shift. 

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