Realization of enhanced evanescent field long period fiber grating near turn around point for label-free immunosensing

A Long period Fiber Grating (LPFG) with maximum enhancement of evanescent field has been designed and fabricated, along with theoretical modeling, by working near the turn-around point (TAP) of lowest order symmetric cladding mode (LP_0,2 cladding mode). The LPFG was fabricated using point by point inscription technique and it was characterized in term of surrounding refractive index (SRI) within the range of 1.333 to 1.3335 using a thermostated flow-cell. This closed cell, made of poly(methyl methacrylate) (PMMA), was designed for better handling of the sensor, because during the fabrication process the diameter of the LPFG was reduced up to ~ 20 µm by chemical etching, for the maximum enhancement of evanescent field. The sensitivity of dual peak resonance of the LP_0,2 cladding mode near TAP was measured and resulted to be ~ 8751 nm/SRIU with a resolution of the order of 10^-5 RIU. The sensor was further used for the label-free immunosensing application by the implementation of Immunoglobulin G (IgG)/ anti- Immunoglobulin G (anti-IgG) bioassay on the grating region inside the thermostated closed flow cell and for real effectiveness along with the confirmation of the specificity, we perform the negative control test of the sensor using a complex matrix consisting of human serum.