Ultrasmall gold nanoparticle-coated tapered optical fibers for advanced biochemical sensing

Surface-enhanced Raman spectroscopy (SERS) is a powerful label-free sensing technique with applications in biomedical diagnostics, chemical identification, food safety, and environmental monitoring. Integrating SERS-active substrates with endoscopic probes enables molecular sensing in previously inaccessible environments. In this context, tapered optical fibers (TFs) represent an ideal substrate due to their unique modal properties and large active area. Here, we introduce a high-throughput, room-temperature method for fabricating SERS-active TFs by leveraging physical vapor deposition to nucleate gold nanoparticles. The non-planar TF surface drives nanoparticle self-organization, transitioning from a rough interconnected gold film to uniformly distributed spherical particles with sub-5 nm interparticle gaps. These sensors demonstrate remarkable sensitivity, detecting serotonin at its physiological resting concentration (10⁻⁹ M) even in a through-fiber measurement scheme. Additionally, SERS spectra obtained at varying concentrations reveal sensitivity to different stereoisomeric forms of serotonin, opening new avenues for biochemical sensing..