Highly sensitive detection of molecular interactions with plasmonic optical fiber grating sensors

• High resolution optical fiber biosensing configuration suitable for remote operation in very small volumes (of the order of microliter) of analytes.
• In situ and real time monitoring of self-assembled monolayer (SAM) formation on the sensor surface, allowing to confirm the correct operation of this process.
• Accurate quantification of streptavidin molecules concentrations down to 2 pM, lowest limit of detection (LOD) reported for optical fiber biosensing configurations so far.
• Immunosensing experiment with human transferrins, confirming the high sensitivity of these biosensors and demonstrating their reversibility and robustness.

Surface Plasmon resonance (SPR) optical fiber biosensors constitute a miniaturized counterpart to the bulky prism configuration and offer remote operation in very small volumes of analyte. They are a cost-effective and relatively straightforward technique to yield in situ (or even possibly in vivo) molecular detection. The biosensor configuration reported in this work uses nanometric-scale gold-coated tilted fiber Bragg gratings (TFBGs) interrogated by light polarized radially to the optical fiber outer surface, so as to maximize the optical coupling with the SPR. These gratings were recently associated to aptamers to assess their label-free biorecognition capability in buffer and serum solutions. In this work, using the well-acknowledged biotin–streptavidin pair as a benchmark, we go forward in the demonstration of their unique sensitivity. In addition to the monitoring of the self-assembled monolayer (SAM) in real time, we report an unprecedented limit of detection (LOD) as low as 2 pM. Finally, an immunosensing experiment is realized with human transferrin (dissociation constant Kd∼10−8 M−1). It allows to assess both the reversibility and the robustness of the SPR-TFBG biosensors and to confirm their high sensitivity.