Direct Nucleation of Hierarchical Nanostructures on Plasmonic Fiber Optics Enables Enhanced SERS Performance

An innovative fabrication method is presented to achieve bottom-up in situ surface-overstructured Au nanoislands (NIs) with tunable grades of surface coverage, elongation, and branching, directly on micro-optical fibers for sensing applications. These all-in-gold hierarchical nanostructures consist of NIs coated with surface protrusions of various morphologies. They are created in solution using a selective seeded growth approach, whereby additional gold growth is achieved over Au NIs formerly developed on the fiber facet by a solid-state dewetting approach. The morphology of nanosized surface-NI overstructuring can be adjusted from multi-dot-decorated Au NIs to multi-arm-decorated Au NIs. This engineering of optical fibers allows for improved remote surface-enhanced Raman spectroscopy (SERS) molecular detection. By combining solid-state dewetting and wet-chemical approaches, stable in-contact deposition of surface-overstructured NIs with the optical fiber solid substrate is achieved, alongside precise control over branching morphology and anisotropy extent. The fiber-optic probes engineered by surface-overstructured NIs exhibit outstanding sensing performance in an instant and through-fiber detection scheme, achieving a remarkable detection limit at 10^-7 M for the R6G aqueous solution. These engineered probes demonstrate an improved detection limit by one order of magnitude and enhanced peak prominence compared to devices solely decorated with pristine NIs.