Long period grating coated with graphene oxide as platform for optical fiber biosensors

In this work we report about a novel label-free biosensing configuration consisting of a long period grating (LPG) inscribed into a double cladding fiber (DCF) and coated with a thin layer of graphene oxide (GO). The DCF had a W-type refractive index (RI) profile and permitted the tuning of the device working point to the mode transition region through etching of the outer cladding, resulting in high RI sensitivity and good visibility of the LPG resonance. The GO overlay provided carboxylic functional groups on the sensor surface for the immobilization of a capture antibody. The sensing characteristics of the biosensor were evaluated through the detection of C-reactive protein in serum, where a limit of detection lower than 1 ng/mL was achieved.