High sensitivity transition-tuned long period grating for label-free immunosensing

Abstract

We present real-time monitoring of multiple biomolecular interactions by means of high refractive index (HRI) coated long period grating (LPG) working in transition region. The coating was deposited by dip-coating as a thin layer of few hundreds nanometers of atactic polystyrene (PS). The HRI coating was used to enhance the LPG sensitivity to refractive index changes of the surrounding medium and its thickness was determined in order to tune the coated device at the desired working point. To allow immobilization of specific biomolecules, the PS overlay surface was functionalized through bovine serum albumin (BSA) and glutaraldehyde. In particular, BSA was directly adsorbed on PS through strong hydrophobic interactions, then glutaraldehyde was linked atop exploiting the lysines side chains. This intermediate layer allowed to covalently immobilize human IgG (antibody) on the device surface. To prove the immobilization method, the interaction between IgG and its proper anti-human IgG (antigen) was studied. The four binding interactions were on-line monitored following the shift of the LPG attenuation band. The experiment also suggests a novel and interesting biofunctionalization approach of unreactive synthetic polymers with applications in immunosensing.