Macromolecular detection of streptavidin with gold-coated tilted FBG refractometers

Surface Plasmon resonance (SPR) biosensors are optical sensors exploiting tiny surrounding refractive index changes to measure interactions between host and guest molecules. Biosensors prepared on optical fibers constitute a miniaturized counterpart to the traditional SPR prism-based sensor, so-called Kretschmann prism. They possess all the advantages inherent to the use of optical fibers and intrinsically allow in situ and real time measurements in very small volumes. The optical fiber sensors used for this study are tilted fiber Bragg gratings covered by a nano-scale-gold coating allowing the SPR generation along the outer surface of the sensors. Using orthogonally polarized lightwaves matching the TFBG eigenmodes, we are able to retrieve the surrounding refractive index with a sensitivity of 10-5 refractive index unit. This confirms that gold-coated TFBGs can be deployed for chemical species profiling in diluted solutions. In this paper, we assess the protein detection and quantification capabilities of SPR-TFBGs using a well-acknowledged label-free mechanism. A self-assembled monolayer of Biotin is formed on the gold surface of the SPR-TFBGs biosensors to quantify Streptavidin diluted in phosphate buffered saline. We demonstrate the capability of this new technology to accurately detect molecules concentrations as lows as 10-10 g/ml.