Oct 252007
A clever little experiment appeared in this week’s JACS preprints, from an Italian group at the University of Bologna. Their efforts concern fluorescent nanoparticles, specifically ones in which the fluorophores have been encapsulated in a silica substance. The advantage of this sort of construct is twofold: it concentrates a large number of fluorescent molecules in the same spot, and also it sequesters them from ions in the solution that might quench their fluorescence. The latter effect could be maximized if you synthesized a nanoparticle that had a fluorophore-doped center and a fluorophore-free shell. Rampazzo et al. have accomplished just this, and demonstrated that the system can be tuned to produce some interesting effects.

Using a pyrene derivative that was only weakly fluorescent in oxygenated water, Rampazzo et al. constructed a reaction to create a doped nanoparticle. In an initial case, they added the dye to about 0.1%, and then grew nanoparticles up to a size of about 90 nanometers. The dye was almost completely incorporated into the particles, and the quantum yield correspondingly increased in an almost linear fashion. Similarly, the results of light-scattering experiments indicated that the particles were growing with a rate equal to that of the increase in quantum yield. The interesting feature here is that although these rates are equal, the fluorescence plateaus significantly before the apparent particle radius reaches its maximum. This result indicates that the fluorescent dye is incorporated relatively early but that the particle continues to grow after this supply is exhausted, thus creating precisely the kind of shell we wanted (A, B, C below):

This success seems to be based entirely on a fortuitous choice of dye concentration. What if that concentration is changed? Rampazzo et al. changed the concentration by an order of magnitude and produced another interesting effect. It turns out that when there are a number of these dye molecules close to one another, they form excimers with an emission maximum at a wavelength of light ~100 nm longer than the monomer. When the dye constitutes 1% of the reaction mixture, there is an initial decrease in emission from the monomer that occurs with roughly the same rate as an increase in excimer emission and in particle radius. Later the monomer emissions recover, and all three processes plateau at approximately the same time. This suggests the formation of a heterogeneous particle as shown in D, E, F above.

This time the dye is not sequestered entirely from the solution, but local concentration at the core of the particle is so high that an unusual fluorescent property is observed. It’s a conceptually simple little experiment, but it has an interesting result, and one that will have to be considered in future efforts to construct nanoparticles of this kind.

At the same time, if extreme local concentrations of some molecule have fortuitous or useful properties, this potential problem for nanoparticle construction might become an advantage. Consider if you have some molecule of interest that forms an excimer or exciplex with a known dye. Taking a sample containing traces of this molecule, one could use the encapsulation technique to construct a highly sensitive fluorescent detector.

 Posted by at 8:59 PM

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