A reflective optical fiber sensor for detecting acetic acid concentration using a zirconium-based metal-organic framework UiO-66

A reflective optical fiber gas sensor was developed for the accurate detection of acetic acid gas concentrations in real time. This sensor comprises a multimode quartz optical fiber and a sensing unit coated with a material that selectively responds to acetic acid. The sensing unit consists of a light channel coated with a Ag film, on which a composite film of TiO 2 /functionalized zirconium metal-organic framework (UiO-66–2CF 3 ) is deposited. Upon absorbing acetic acid, the refractive index of the TiO 2 /UiO-66–2CF 3 film increases, enhancing the light absorption at 326 nm while reducing the reflectance. This decrease in the transmitted light intensity enables the accurate measurement of the environmental acetic acid concentration by detecting the difference between the output and input optical intensities. Experiments were performed to investigate the dependence of the sensor performance on the TiO 2 content in TiO 2 /UiO-66–2CF 3 , coating thickness, light incidence angle in the optical channel, and ambient temperature and humidity. In the range of 10–40 °C and 40–80 % RH, the sensor with 45 wt% TiO 2 in the composite, a coating thickness of 55 μm, and an incident angle of 45° yielded an acetic acid gas response sensitivity of 0.0007 absorbance unit/ppm, a response time of 315 s, detection limit of 4.29 ppm, and maximum relative error below 5.0 %. The incorporation of the –CF 3 group into UiO-66 endowed the sensor with excellent humidity immunity, a high selectivity for acetic acid, and long-term stability.