Ti3C2 MXene-TiO2 hybrid-modified U-bend fiberoptic sensor for improved refractive index sensitivity and ammonia detection

This study introduces the U-shaped evanescent optical fibers (UOFEs), coated with TiO2 and Ti3C2 MXene composites, for refractive index (RI) measurement and ammonia sensing applications. The composites of Ti3C2 MXene multilayers and TiO2 were prepared using a novel preparation technique based on the liquid-phase deposition of (NH4)2TiF6. This is a safe alternative of conventional synthesis techniques in which hazardous HF acid is used. The UOFEs coated with Ti3C2 MXene/TiO2 hybrid films were 12-fold more sensitive to RI with a maximum RI response value of 943–1056%/RIU than bare fibers. In addition, UOFEs exhibited a linear response to ammonia concentrations between 0.5 and 20 ppm at 25 °C. The limit of detection (LOD) of UOFEs for detecting ammonia in water was 1.02 ppm in the detection range of 0–7.5 ppm. The response of UOFEs could be restored by immersing them in water for 5 min. Furthermore, UOFEs demonstrated selective detection of ammonia over aqueous solutions with a concentration of 300 ppm of other routinely used solvents, such as ethanol, methanol, hexane, acetonitrile, acetone, and chloroform. These results demonstrate good reversibility and reproducibility of UOFEs, indicating that UOFE-based sensor for detection of ammonia can be developed with potential biomedical applications.