  In this contribution experimental evidence of plasmonic edge modes and
breathing acoustic modes in gold nanostars (AuNS) is reported. AuNS are
synthesized in a surfactant-free, one-step wet-chemistry method. Optical
extinction measurements of AuNS confirm the presence of localized surface
plasmon resonances (LSPRs), while electron energy-loss spectroscopy (EELS) in
the scanning transmission electron microscope (STEM) shows the spatial
distribution of LSPRs and reveals the presence of acoustic breathing modes.
Plasmonic hot-spots generated at the pinnacle of the sharp spikes, due to the
optically active edge dipolar mode, allow significant intensity enhancement of
local fields, hot-electron injection, and thus useful for size detection of
small protein molecules. The breathing modes observed away from the apices of
the nanostars are identified as stimulated dark modes - they have an acoustic
nature - and likely originate from the confinement of the surface plasmon by
the geometrical boundaries of a nanostructure. The presence of both types of
modes is verified by numerical simulations. Both these modes offer the
possibility to design nanoplasmonic antenna based on AuNS, which can provide
information both on mass and polarizability of biomolecules using a two-step
molecular detection process.
