Nanotransferred plasmonic metasurface fiber sensor for in situ sulfide monitoring in marine environments

Dissolved sulfide plays an important role in marine sulfur cycling and serves as an indicator of marine environmental quality. However, conventional sulfide analysis relies on sample collection and laboratory testing, limiting rapid on-site analysis and dynamic monitoring. Here, we present a fiber-end plasmonic sensing strategy for in situ sulfide monitoring in marine environments. Ordered gold plasmonic nanoarrays were fabricated by thermal nanoimprint lithography and electrodeposition, followed by transfer onto multimode fiber end facets to form a compact plasmonic sensing interface. A chitosan-Cu²⁺ (CS-Cu) functional layer was integrated onto the metasurface as a sulfide-responsive coating. Sulfide exposure produced a blue shift of the resonance wavelength, accompanied by chemical and structural changes in planar CS-Cu model films, consistent with restructuring of the coordination network and changes in the interfacial optical environment. The sensor exhibited a refractometric sensitivity of 271.7 nm RIU⁻¹ and a logarithmic response to Na₂S-derived dissolved sulfide concentrations ranging from 0.05 to 10 mmol L⁻¹, with a statistically estimated detection -limit of 0.08 mmol L⁻¹. Stable optical baselines were maintained during 7-day immersion in representative high-ionic-strength solutions. This work demonstrates a compact nanotransferred fiber-optic plasmonic platform for quantitative sulfide sensing in marine-relevant aqueous media.