Reflector-Less Shallow-Tapered Optical Fiber Biosensors for Rapid Detection of Cancer Biomarkers

Reflector-less biosensors based on optical fibers enable the detection of the refractive index (RI) variations, in reflection mode, without the need for any reflective element (such as a grating or an interferometer). The sensing system works by interrogating the wavelength shift occurring in the Rayleigh scattering signal at each location of a thinned fiber through an optical backscatter reflectometer (OBR); a high-scattering fiber is required in order to compensate for the propagation losses. The approaches reported so far make use of etched fibers, which are however fragile and require tens of minutes of fabrication, and U-bent fibers which are incompatible with in-situ devices. In this article, we present the working principle and main results of reflector-less biosensors fabricated through shallow tapering; this concept allows the fastest fabrication (∼20 seconds through a CO₂ automated laser splicer) reported so far, and allows maintaining a compact and robust form factor as the fiber maintains a diameter >28  μm (>22% of the initial thickness). The fabrication process allows achieving a peak sensitivity up to 1.33 nm/RIU (1.05 nm/RIU average over 5 tapers), using MgO-nanoparticle doped fibers. The tapered biosensors have been functionalized through the silanization method, in order to detect CD44 protein (from 100 fM to 10 nM), a breast cancer biomarker. The results show a successful detection with a low limit of detection (16.4 pM) and high specificity over two controls.