  Creating plasmonic nanoparticles on a tapered optical fiber tip enables a
remote SERS sensing probe, ideal for challenging sampling scenarios like
biological tissue, specific cells, on-site environmental monitoring, and deep
brain structures. However, nanoparticle patterns fabricated from current
bottom-up methods are mostly random, making geometry control difficult. Uneven
statistical distribution, clustering, and multilayer deposition introduce
uncertainty in correlating device performance with morphology. Here, we employ
a tunable solid-state dewetting method to create densely packed monolayer Au
nanoislands (NIs) with varied geometric parameters, directly contacting the
silica TF surface. These patterns exhibit analyzable nanoparticle sizes,
densities, and uniform distribution across the entire taper surface, enabling a
systematic investigation of particle size, density, and analyte effects on the
SERS performance of the through-fiber detection system. The study is focused on
the SERS response of a widely employed benchmark Rhodamine 6G molecule and
Serotonin, a neurotransmitter with high relevance for the neuroscience field.
The numerical simulations and limit of detection (LOD) experiments on R6G show
that the increase of the total near-field enhancement volume promotes the SERS
sensitivity of the probe. However, for serotonin we observed a different
behavior linked to its interaction with the nanoparticle's surface. The
obtained LOD is as low as 10-7 M, a value not achieved so far in a
through-fiber detection scheme. Therefore, we believe our work offers a
strategy to design nanoparticle-based remote SERS sensing probes and provide
new clues to discover and understand the intricate plasmonic-driven chemical
reactions.
