Long period gratings working in transition mode as a valuable technological platform for biosensing

this work, we present preliminary results concerning the development of a platform for biochemical sensing based on overlayered Long Period Gratings (LPGs) working in transition mode. The surrounding medium refractive index (SRI) sensitivity of the bare device was greatly enhanced by means of the modal transition phenomenon which is enabled in LPGs with a nanoscale polymer overlay of higher refractive index than the cladding. This strategy was pursued to develop a sensitive tool for biochemical detection in aqueous solutions. Dip-coating was used to deposit thin layers of Polystyrene (PS) with different thicknesses on the same LPG to test the performances of the device in different working points of its modified sensitivity characteristic. The PS overlay surface was modified through adsorption of biotinylated bovin serum albumin (BBSA). Adsorption dynamic of BBSA was observed by on-line monitoring the shift of the LPGs attenuation bands. In the same way also a subsequent steptavidin (SA) binding dynamic on the byotinilated sites of the adsorbed proteins was recorded. Several washing cycles between the steps showed a rather stable immobilization of the protein monolayers. Experimental results show that overlayered LPGs are among the most sensitive refractive index transducers to be employed in biochemical detection and that wide margins of further optimization exist