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.