Sensitivity enhancement of gold nanospheres assisted CO2 laser tapered optical fiber for refractive index sensor

•Clean and symmetric tapered single mode and multimode fibers have been successfully prepared using CO2 laser.•The controllable CO2 laser tapering process could be done in just 5 s.•Different CO2 laser powers influenced the final waist taper diameter.•Smaller diameter of tapers promoted more evanescence waves which enhanced the refractive index sensing sensitivity.•Au nanospheres immobilization on tapered multimode fiber could improve the sensitivity up to 122% due to localized surface plasmon resonance. Tapered optical fibers (TOFs) have been intensively used as sensors due to their high sensitivity, simple structure, and excellent coupling efficiency. In this paper, we reported the successful rapid tapering of single mode and multimode fibers using CO2 laser beam at working wavelength of 10.6 μm. Clean and symmetric tapers could be produced in just 5 s using this method. Arduino-based microcontroller was included in the tapering set-up to precisely control the fiber pulling speed and distance. Smaller waist diameters that were observed in multimode than in single mode fibers generated more evanescent waves on the cladding of tapered multimode fibers (TMMFs). This implied that TMMFs performed better refractive index (RI) sensitivity, within the range of 1.3325–1.4266, than tapered single mode fibers (TSMFs). Furthermore, localized surface plasmon resonance (LSPR) effect was coupled with evanescent wave through AuNSs immobilization on the TOF surface. The AuNSs having average particle diameter of 12.70 nm was synthesized by citrate reduction route. The formation of AuNSs was confirmed by (ultraviolet–visible) UV–vis and high-resolution transmission electron microscopy (HRTEM) data. Self-assembly monolayer approach was conducted to immobilize AuNSs on the 3-mercaptopropyl trimethoxysilane (MPTMS)-functionalized surface of the TOFs. The AuNSs immobilization on TMMFs could improve the sensitivity up to 122 %. Besides sensitive to RI changes, the sensors were also excellent in terms of repeatability and reversibility performances. Finally, this paper described the amplitude modality-based RI sensing mechanism due to evanescent wave and LSPR effects.