Effect of plasmonic nanostructures and nanofilms on fluorescence resonance energy transfer

We investigate the effect of silver nanostructures and nanofilms on fluorescence resonance energy transfer (FRET). Thin layers of poly (vinyl alcohol) of 10 or 20 nm with homogenously distributed donor (Cy3) and acceptor (Cy5) molecules were coated on silver islands and continuous silver films. Several geometrical configurations were studied with multiple donor-acceptor (D-A) samples with various acceptor concentrations. The acceptor concentrations were adjusted to yield FRET efficiencies from about 10 to 92% on bare glass surfaces. Time-resolved fluorescence spectroscopy was used to determine the energy transfer efficiency and the relative transfer rate. It was found that the relative transfer rate increased 2-fold in the presence of continuous silver films and about 25-fold in presence of silver island films. The largest enhancements in the energy transfer efficiency, up to 90%, were observed for low acceptor concentrations (large donor-acceptor separations). To a first approximation, observed changes in FRET efficiencies were explained by an increase of the quantum yield of the donor molecules in the presence of silver islands.