A protein-based biointerfacing route toward label-free immunoassays with long period gratings in transition mode

We present a fast and effective method for anchoring bioreceptors to optical waveguides exhibiting a poorly reactive polymer interface and that have to be minimally perturbed with respect to their design. The study originated from the need to biofunctionalize a fiber optic Long Period Grating (LPG) that is tuned in a highly sensitive working point, the so-called transition mode, through the deposition of a high refractive index overlay. In particular, a thin film of atactic polystyrene (PS) was dip-coated onto the LPG with a thickness suitable to optimize the LPG sensitivity to refractive index changes of the surrounding medium. Bovine serum albumin was selected as sacrificial layer for its well-known adhesion capabilities to PS surfaces, then glutaraldehyde was used to conjugate IgGs, serving as prototypical bioreceptor, on the device surface. The effectiveness of the immobilization method was assessed by studying the interaction between the immobilized IgG with a suitable anti-IgG. In a preliminary study performed by means of ELISA and surface plasmon resonance, optimal conditions for the biomolecular testing with the LPG were assessed. Four distinct interactions were thus monitored in real time following the shift of the LPG attenuation band. These experiments suggest a novel and interesting biofunctionalization approach of unreactive polymers with applications in immunosensing and basic life science research.