  Plasmonic optical nanoantennas offer compelling solutions for enhancing
light-matter interactions at the nanoscale. However, until now, their focus has
been mainly limited to the visible and near-infrared regions, overlooking the
immense potential of the ultraviolet (UV) range, where molecules exhibit their
strongest absorption. Here, we present the realization of UV resonant nanogap
antennas constructed from paired rhodium nanocubes. Rhodium emerges as a robust
alternative to aluminum, offering enhanced stability in wet environments and
ensuring reliable performance in the UV range. Our results showcase the
nanoantenna ability to enhance the UV autofluorescence of label-free
streptavidin and hemoglobin proteins. We achieve significant enhancements of
the autofluorescence brightness per protein by up to 120-fold, and reach
zeptoliter detection volumes enabling UV autofluorescence correlation
spectroscopy (UV-FCS) at high concentrations of several tens of micromolar. We
investigate the modulation of fluorescence photokinetic rates and report
excellent agreement between experimental results and numerical simulations.
This work expands the applicability of plasmonic nanoantennas into the deep UV
range, unlocking the investigation of label-free proteins at physiological
concentrations.
