We introduce an innovative approach to enhance refractive index sensing utilizing Long Period Optical Fiber Gratings (LPGs) processed through microsphere-assisted superlens laser nanostructuring technology. This method involves the self-assembly of a silica microsphere monolayer on the outer surface of LPGs, followed by pulsed laser irradiation to create nanoholes (with a diameter of 300–500 nm) on the surface, for forming the nanohole-structured LPGs (NS-LPGs). The sensing capabilities of NS-LPGs, featuring two different nanohole densities (5 % and 7.9 %), were experimentally evaluated in sucrose and glycerin solutions. The results revealed a notable improvement in sensitivity, with increases of 16.08 % and 19.57 %, respectively, compared to conventional LPGs. This suggests that higher nanohole density contributes to greater enhancement in refractive index sensing. Furthermore, the permanent nanohole structures showed improved durability and lifespan in various environments compared to surface-coating-based LPGs. Further enhancements can be achieved by refining the nanostructuring density and controlling the dimensions of the nanoholes.