All-plastic-fiber D-shaped functionalized biosensor through ROI-camera analysis: towards IoT-based DIY saliva analysis

Existing optical biosensor systems are still limited by bulky instrumentation, constrained portability, restricting their clinical and point-of-care implementation. This study proposes a smartphone-based plastic optical fiber (POF) biosensing platform for biomarker detection. The platform employs a manually polished, antibody-functionalized D-shaped POF to enhance evanescent-field interaction with the surrounding medium. This approach combines non-plasmonic fiber architecture with direct smartphone-camera intensity interrogation for target protein detection, without the metal coatings, external lenses, or optical filters used in previously reported smartphone-integrated plasmonic fiber biosensors. The proposed all-fiber system uses fiber to couple light from the smartphone's flashlight and camera for signal detection. To facilitate the measurements, a custom Android application is built with Camera2 API. The application allows controlling image acquisition, selecting the region of interest (ROI), and visualizing response curves. The sensors were functionalized with interleukin-8 (IL-8) antibody and used in IL-8 protein detection in artificial and healthy human saliva. In RI sensing, the D-shaped sensor depicts a sensitivity of 361%/RIU and R² = 0.9835. In biomarker detection, the proposed biosensor showed a concentration-dependent response with R² = 0.9996 for the four-point calibration and an estimated LOD of 162 aM. Preliminary measurements using IL-8-spiked healthy human saliva also demonstrated an estimated LOD of 219 aM. Repeatability studies show similar signal changes across several biosensors towards IL-8 protein. Sensors functionalized with no antibodies produce negligent signals when incubated with the target protein.