Label-free IgG/anti-IgG biosensing based on long period fiber gratings: A comprehensive feasibility study

Long period fiber gratings (LPGs) have recently been proposed as label-free biosensors. A biochemical interaction occurring along the grating region can be evaluated as a refractive index (RI) change, which modifies the transmission spectrum of the fiber. This is an emergent, alternative choice with respect to other label-free optical systems, such as surface plasmon resonance, interferometric and in-fiber configurations, and resonating structures. In this work, various types of not-coated LPGs, in which the coupling occurs with increasing cladding mode orders, were manufactured for increasing the RI sensitivity of these sensors. After the functionalization of the fiber surface using Eudragit L100 copolymer, a label-free IgG/anti-IgG bioassay was realized for analyzing the antigen/antibody interaction following the same model assay. A comprehensive feasibility study was carried out among the different LPGs in order to assess and compare the biosensor performance, highlighting the advantages and the disadvantages of each type. Experimental results proved an improvement in the RI sensitivity and in the biosensor performance in the case of high-order cladding mode LPGs, with values of detection limit lower than 50 ng mL-1 (330 pM). The performance enhancement can be explained with the increase in the penetration depth of the evanescent field due to the increase of the cladding mode order. The sensor response was also studied using complex matrices made up of human serum.