Long-period gratings in photonic crystal fiber as an optofluidic label-free biosensor.

Using long-period gratings (LPG) inscribed in photonic crystal fiber (PCF) and coupling this structure with an optically aligned flow cell, we have developed an optofluidic refractive index transduction platform for label-free biosensing. The LPG-PCF scheme possesses extremely high sensitivity to the change in refractive index induced by localized binding event in different solution media. A model immunoassay experiment was carried out inside the air channels of PCF by a series of surface modification steps in sequence that include adsorption of poly(allylamine hydrochloride) monolayer, immobilization of anti-rat bone sialoprotein monoclonal primary antibody, and binding interactions with non-specific goat anti-rabbit IgG (H+L) and specific secondary goat anti-mouse IgG (H+L) antibodies. These adsorption and binding events were monitored in situ using the LPG-PCF by measuring the shift of the core-to-cladding mode coupling resonance wavelength. Steady and significant resonance changes, about 0.75 nm per nanometer-thick adsorbed/bound bio-molecules, have been observed following the sequence of the surface events with monolayer sensitivity, suggesting the promising potential of LPG-PCF for biological sensing and evaluation.