  Plasmonic platforms are a promising solution for the next generation of
low-cost, integrated biomedical sensors. However, fabricating these
nanostructures often requires lengthy and challenging fabrication processes.
Here we exploit the peculiar features of Focused Ion Beam milling to obtain
single-step patterning of a plasmonic two-dimensional (2D) array of truncated
gold nano-pyramids (TNP), with gaps as small as 17 nm. We describe the
formation of plasmonic bandgaps in the arrangement of crossed tapered grooves
that separate the nano-pyramids, and we demonstrate refractive index sensing
via dark field imaging in patterned areas of 30 $\mu$m $\times$ 30 $\mu$m.
