Biosensors are powerful allies for food safety, drug discovery, environmental monitoring, and clinical diagnosis. Optical biosensors are an active field of research worldwide, presenting rapid progress in recent years. In this perspective, optical biosensors based on refractometric sensing schemes have been developed with great successes in the last decades. Moreover, optical fibers (OF) based on evanescent wave sensing are an excellent platform to develop high-stability and high-sensitive optical biosensors. The quantitative and/or qualitative measurements result from the interaction of the analyte with the evanescent field of light at the fiber surface. It’s good biochemistry makes them appropriate for biochemical functionalization, creating very sensitive structures targeting viruses, drugs, and proteins. This work presents a method specially developed for the production of MIP, based on Stöber silica nanoparticles, immobilized in the surface of commercial SMF28 OF, serving has recognition elements for specific target molecules. The nanoparticles surface was functionalized by the introduction of an aminosilane (APTMS) followed by the covalent immobilization of the immunoglobulin G (IgG) from human serum. The antibody was activated by the EDC/NHS protocol to allow the interaction of the amine exposed groups, located on the surface of the silica nanoparticles, with the activated carboxyl acid groups of the IgG molecules. The resulting template was immobilized onto the surface of an OF. The sensing structure is based on long period fiber gratings (LPFGs), specially developed to allow the interaction of the electromagnetic wave with the target analytes through its evanescent field. The refractometric system comprises a Braggmetter unit (HBM, FiberSensing) working in a wavelength range from 1500 to 1600 nm, and a reference LPFG to correct possible false interactions. The resulting configuration was tested in the presence of anti-human IgG, recording the refractometric response of the modified LPFG in contact with different amounts of analyte.