FRET on Surface of Silica Nanoparticle: Effect of Chromophore Concentration on Dynamics and Efficiency

Forster resonance energy transfer (FRET) has been studied between fluorescein (donor, D) and rhodamine B (acceptor, A) bound covalently to the surface of silica nanoparticle (SNP-dye). This is a part of an ongoing effort toward development of light harvesting nanoantennae. The role of the D:A ratio and the total chromophore content on the efficiency of FRET has been investigated. At low number density (ca. 75 +/- 15 dye molecules/particle) efficiency is dependent on the D:A ratio. It appears that the distribution of the dyes on the surface is inhomogeneous and that FRET occurs only between D and A molecules in very close proximity. The effect of self-quenching of the donor is not very significant for this number density. At higher number densities (ca. 700-1100 dye molecules/particle), a prominent rise in the acceptor emission is observed, perhaps indicating a more homogeneous distribution of the dye molecules, and D emission is quenched completely. However, D-D quenching is an issue here. So, it appears that the lower number density is a more favorable condition for FRET in these systems.