TiO2-Coated Fiber-Optic Sensor for Monitoring Ambient pH Fabricated Using the Sol–Gel Method

Environmental pollution, particularly water contamination, requires the continuous development of robust, effective monitoring technologies. While chemical optical sensors offer significant advantages for environmental monitoring, many traditional devices rely on complex combinations of chemical dyes with specific acid dissociation constants. To overcome these limitations, this study presents a dye-free approach centered on a fiber-optic pH sensor based on a TiO2 thin film. The protective coating of multimode optical fibers was successfully removed using a controlled hydrofluoric acid (HF) treatment, enabling deposition of TiO2 films via a low-cost sol–gel dip-coating method. Structural characterizations through X-ray diffraction (XRD) and Raman spectroscopy confirmed the preferential growth and high purity of the anatase phase. Furthermore, energy-dispersive X-ray spectroscopy (EDS) confirmed the presence and relatively uniform distribution of Ti on the fiber surface. The optical performance of the sensor was evaluated using a 940 nm light source over a broad pH range (4–14), where the device exhibited distinguishable stepped optical-power responses at four pH conditions spanning pH 4–14. A preliminary analysis yielded an apparent average slope of 0.40 µW/pH. These results support the feasibility of using sol–gel derived TiO2 coatings in the preliminary development of dye-free optical devices for aqueous pH monitoring.