  Metallic nanoparticles were shown to affect F\"orster energy transfer between
fluorophore pairs. However, to date, the net plasmonic effect on FRET is still
under dispute, with experiments showing efficiency enhancement and reduction.
This controversy is due to the challenges involved in the precise positioning
of FRET pairs in the near field of a metallic nanostructure, as well as in the
accurate characterization of the plasmonic impact on the FRET mechanism. Here,
we use the DNA origami technique to place a FRET pair 10 nm away from the
surface of gold nanoparticles with sizes ranging from 5 to 20 nm. In this
configuration, the fluorophores experience only moderate plasmonic quenching.
We use the acceptor bleaching approach to extract the FRET rate constant and
efficiency on immobilized single FRET pairs based solely on the donor lifetime.
This technique does not require a posteriori correction factors neither a
priori knowledge of the acceptor quantum yield, and importantly, it is
performed in a single spectral channel. Our results allow us to conclude that,
despite the plasmon-assisted Purcell enhancement experienced by donor and
acceptor partners, the gold nanoparticles in our samples have a negligible
effect on the FRET rate, which in turns yields a reduction of the transfer
efficiency.
