Towards sensitive label-free immunosensing by means of turn-around point long period fiber gratings

•A new label-free optical biosensor based on a turn-around point long period grating.•Long-term measurements by using of a thermo-stabilized low-volume flow cell.•Limit of detection of 70 µg L−1 (460 pM) in human serum for an IgG/anti-IgG bioassay.•Improvement of the TAP LPG sensor performance by using a high order cladding mode.

Long period fiber gratings have been effectively used in the field of biochemical sensing since a few years. Compared to other well-known label-free optical approaches, long period gratings (LPGs) take advantage of the typical peculiarity of optical fibers. Coupling the propagating core mode with a high-order cladding mode near its turn-around point (TAP) was the strategy adopted to achieve good performances without additional coatings, except for the sensing and selective biolayer deposited on the fiber. Both the modeling and manufacturing of TAP LPGs were discussed. After the functionalization of the fiber surface with the deposition of a Eudragit L100 copolymer layer followed by immunoglobulin G (IgG) covalent immobilization, an IgG/anti-IgG bioassay was implemented along the grating region and the kinetics of antibody/antigen interaction was analyzed. A quantitative comparison between a TAP LPG and a non-TAP LPG was carried out to highlight the improvement of the proposed immunosensor. The real effectiveness and feasibility of an LPG-based biosensor were demonstrated by using a complex matrix consisting of human serum, which also confirmed the specificity of the assay, and a limit of detection of 70 μg L−1 (460 pM) was achieved.