Development of LSPR based U-bent plastic optical fiber sensors

• U-bent probes fabrication by heat treatment of fibers placed in glass capillary.
• Probes with different bend diameters were fabricated for 250–1000 μm POF.
• Highest RI sensitivity for probes with bend diameter 2–3 times the fiber diameter.
• A lowest resolution of 0.2 mRIU was achieved with bare U-bent probe.
• RI sensitivity of LSPR POF probes are ∼1 order more sensitive than bare probes.

This study presents design and development of evanescent wave absorbance based U-bent plastic optical fiber (POF) probe with high sensitivity and its applications to refractive index (RI) sensing. The probes were fabricated by a controlled decladding procedure to remove the fluorinated polymer without damaging the poly methyl methacrylate (PMMA) core and a simple and scalable fabrication technique to obtain POF U-bent probes of desired geometry. U-bent probes of fiber diameter from 250 to 1000 μm were fabricated and optimum bend diameter for each fiber diameter was investigated. The sensitivity was found to be maximum when the bend diameter of the probe varies from 2 to 3 times the fiber diameter. Probes with 500 μm core and 1.25 mm bend diameter showed highest sensitivity (5.57ΔA560 nm/ΔRIU) in the visible region to RI changes from 1.33 to 1.47 with a resolution better than 1 milli RI units. Furthermore, U-bent probes were amine functionalized and coated with gold nanoparticles to obtain a localized surface plasmon resonance (LSPR) based RI sensor that has an 8-fold improvement in RI sensitivity, hence extending their applicability to plasmonic biosensing.