Plasmon-activated intermolecular nonradiative energy transfer in spherical nanoreactors

Abstract

The effect of silver nanoparticles upon intermolecular radiationless energy transfer between the dye molecules confined in the nanopores of silica was studied both experimentally and by theoretical modeling. Silver nanoparticles were synthesized directly inside the nanopores of silica from AgNO3 using aqueous solution of NaBH4 as a reducing agent. Silica samples with or without nanoparticles were soaked with ethanol solutions of dyes, Acridine Orange (AO) as a donor of electronic excitation energy, and Nile Blue as the acceptor. Studies of fluorescence spectra of samples at different acceptor concentration have shown that addition of nanoparticles decreases the relative quantum yield of fluorescence of donor accompanied by the increase of that of acceptor. This was interpreted as a plasmonic contribution to the rate of nonradiative energy transfer. A theoretical model was proposed, which describes the quenching kinetics of the excited donor molecules in the presence of accepting molecules and metal nanoparticles in a polarized spherical nanopore, as well as the changes in spectral bands of the system. Besides, simulation of the interaction of a point dipole source field with plasmonic silver nanoparticles in a silica cavity was carried out using finite-difference time-domain (FDTD) method. The results of field calculation with the two models are in a good qualitative agreement between each other and predict the regions of local electric field enhancement in the gap between silver nanoparticles and walls of pores that is supposed to account for the experimentally observed increased efficiency of the nonradiative intermolecular singlet–singlet energy transfer in porous silica matrix with silver nanoparticles. The results obtained can be of interest for FRET SNOM.