Toward Plasmonic Neural Probes: SERS Detection of Neurotransmitters through Gold Nanoislands-decorated Tapered Optical fibers with sub-10 nm Gaps.

Integration of plasmonic nanostructures with fiber optics-based neural probes enables label-free detection of molecular fingerprints via surface-enhanced Raman spectroscopy (SERS), and it represents a fascinating technological horizon to investigate brain function. However, developing neuro-plasmonic probes that can interface with deep brain regions with minimal invasiveness while providing the sensitivity to detect bio-molecular signatures in a physiological environment is challenging. In particular, because the same waveguide must be employed for both delivering excitation light and collecting the resulting scattered photons. Here, we present a SERS-active neural probe based on a tapered optical fiber (TF) decorated with gold nanoislands (NIs) that can detect neurotransmitters down to the micro-molar range. To do this, we developed a novel, non-planar repeated dewetting technique to fabricate gold NIs with sub-10 nm gaps, uniformly distributed on the wide (∼mm 2 ), highly curved surface of TF. We experimentally and numerically show that the amplified broadband near-field enhancement of the high-density NIs layer allows achieving a limit of detection in aqueous solution of 10 -7  M for rhodamine 6G and 10 -5  M for serotonin and dopamine through SERS at near-infrared wavelengths. We envision our technology as a first step towards the unexplored frontier of in-vivo label-free plasmonic neural interface. This article is protected by copyright. All rights reserved.