Enhanced Forster resonance energy transfer on single metal particle. 2. Dependence on donor-acceptor separation distance, particle size, and distance from metal surface

We studied the effect of metal particles on Forster resonance energy transfer (FRET) between nearby donor-acceptor pairs. The studies included the effect of donor-acceptor distance, silver particle size, and distance from the metal surface. The metal particles were synthesized with average diameters of 15, 40, and 80 nm, respectively. A Cy5-labeled oligonucleotide was chemically bound to a single silver particle with a distance of 2 or 10 nm from the surface of metal core. A Cy5.5-labeled complementary oligonucleotide was bound to the particle-conjugated oligonucleotide by hybridization. The spacer length between donor-acceptor was adjusted by the number of base pairs. FRET between the donor-acceptor pair was investigated by dual-channel single-molecule fluorescence detection. Both the emission intensities and lifetimes indicated that FRET was enhanced efficiently by the metal particles. The results showed an increase of apparent energy transfer distance with the size of silver particle and distance from the metal core. Simulations by finite-difference time-domain (FDTD) calculations were used to compare with the experimental results. The local fields at the location of the donor-acceptor pair appeared to correlate with the FRET efficiency. These results will aid in the design of metal particles for using FRET to determine biomolecule proximity at distances beyond the usual Forster distance.