  Single gold nanoparticles can act as nanoantennas for enhancing the
fluorescence of emitters in their near-fields. Here we present experimental and
theoretical studies of scanning antenna-based fluorescence microscopy as a
function of the diameter of the gold nanoparticle. We examine the interplay
between fluorescence enhancement and spatial resolution and discuss the
requirements for deciphering single molecules in a dense sample. Resolutions
better than 20 nm and fluorescence enhancement up to 30 times are demonstrated
experimentally. By accounting for the tip shaft and the sample interface in
finite-difference time-domain calculations, we explain why the measured
fluorescence enhancements are higher in the presence of an interface than the
values predicted for a homogeneous environment.
