Study on optical fiber ammonia sensor coated with cobalt-titanium co-doped graphene

Ammonia (NH3), as a toxic gas widely applied in industrial and agricultural fields, requires highly efficient and accurate detection techniques to mitigate environmental and health risks. Optical fiber sensing combined with sensitive materials is a hot spot in gas sensing. We investigated NH3 sensing properties of optical fiber sensors coated with cobalt (Co)-doped and titanium (Ti)-doped graphene aiming to improve its gas sensing performance. Co-doped graphene (CoG), Ti-doped graphene (IG-Ti), and Co-Ti co-doped graphene (CoG-Ti) were prepared using solvothermal, liquid phase dispersion, and combined methods, respectively. A microstructure optical fiber (MOF) with a Single Mode Fiber-Coreless Fiber-Seven Core Fiber-Coreless Fiber-Single Mode Fiber (SNSCNS) structure was fabricated and coated with intrinsic graphene (IG), CoG, IG-Ti, and CoG-Ti by the drop coating method. When the sensitive material is in contact with NH3, the refractive index changes and the optical fiber evanescent field changes, thus causing the interference spectrum to drift with wavelength. The spectra of the sensor uncoated sensitive materials without interference under NH3 concentration changes were used as the control group. It was found that the sensor coated with IG had a sensing response to NH3, and its sensitivity was 0.4 pm/ppm. The sensitivity of the sensor coated with CoG and IG-Ti was significantly improved compared with that coated with IG, reaching 2.02 and 1.68 pm/ppm, respectively, whereas the sensor coated with CoG-Ti reached the highest of 5.39 pm/ppm. We confirmed the feasibility of NH3 sensing by combining MOF with graphene-based sensitive materials. The doping of Co and Ti enhanced the NH3 sensing performance of graphene, and the co-doping of Co and Ti had the best effect, which provided a new idea and experimental support for the development of high-performance optical fiber gas sensors.