Hydrogel-integrated plasmonic nanostructures on optical fiber facet for remote and real-time pH sensing

Abstract

In this work, we use ultraviolet nanoimprint lithography (UV-NIL) to transfer metallic nanostructures from a polymer mold to the facet of the optical fiber with 200 μm core diameter. Once a polymer mold carrying nanopillar array is fabricated by thermal embossing, a thin layer of gold is deposited on it by thermal evaporation. Then the metallic nanostructure is transferred onto fiber facet by the cross-linked UV-cured resist. The transferred metallic nanostructures feature closely spaced double layers of disks and holes. Strong coupling between the metal disk and hole generates resonantly enhanced local electrical field under incident excitation light, as revealed by peaks and dips in the reflection spectra. A layer of hydrogel is coated and cross-linked on fiber facet as a pH-sensing element. Hydrogel shrinks in acid and swells in basic solutions by containing different amount of water and thus has a different refractive index, which can be detected from the resonant reflection peaks/dips of plasmonic fiber probe. Our hydrogel fiber probe shows obvious spectrum response to solutions with pH values ranging from 1 to 8. Under cycling test, the sensor remains stable for three cycles when switching between acid and basic solutions.