Biochemical sensing with Surface Plasmon-assisted optical fibers

Surface Plasmon resonance (SPR) optical fibre biosensors appear as a cost-effective and relatively straightforward technique to yield in situ (or even possibly in vivo) macromolecular detection. The biosensor configuration reported in this work uses nanometric-scale gold-coated tilted fibre Bragg gratings (TFBGs) interrogated by light polarized radially to the optical fibre outer surface. Recently, we have reported unprecedented surrounding refractive index (SRI) sensitivity for these SPR sensors, namely 10^-5 refractive index unit. We have also shown that neighbouring cladding mode resonances present a differential sensitivity to slight SRI changes such as those induced by biomolecules attachment. In this work, this peculiar behaviour is exploited to assess the protein detection and quantification capabilities of these SPR-TFBGs. For this, self-assembled monolayers of biotin were formed on the gold surface of the sensors to quantify streptavidin molecules diluted in phosphate buffered saline. We demonstrate that a single SPR-TFBG can be used not only to monitor the SAM formation in situ but also to accurately quantify molecules concentrations down to 2 pM.