Shape, size and composition dependence of efficiency and dynamics of Forster resonance energy transfer in dye-silica nanoconjugates

The role of relative concentrations of energy donors (fluorescein, D), acceptors (rhodamine, A) and silica on Forster resonance energy transfer (FRET) efficiency and dynamics in dye silica conjugates has been studied, as a part of our initial attempts to ascertain the potential of dye-silica nanoconjugates as light harvesting nanoantennae. Two types of dye-silica nanoconjugates, prepared by the co-condensation method, have been examined. The first is based on silica nanoshells (SNSdye) while the second is based on silica nanoparticles (SNP-dye). Both these nanostructures have a diameter of approximately 25 nm. Efficient energy transfer (91% and 97%, respectively) has been observed in both, for total fluorophore concentration upto 5-6 mmol, irrespective of the D : A ratio. The lower efficiency at dye concentrations greater than these has been rationalized by the competitive self-quenching of D. A risetime of approximately 500 fs is observed in the A emission in SNS-dye, but there is no such feature in SNP-dye. The shape and size dependence of the FRET efficiency and dynamics has been rationalized as follows: the initial step of dye rich core formation in nanoparticles results in high proximity of dye molecules to each other, leading to highly efficient FRET than in nanoshells. In larger SNP-dye nanoconjugates of 65 nm in diameter, the FRET efficiency decreases to 85%, while a risetime in D emission emerges. This provides support to the proposed correlation between efficiency and packing. Hence, it is inferred that total fluorophore concentration, rather than D : A ratios, governs the FRET dynamics and efficiency in these systems.