Vacancy-modified Sc–N co-doped graphene coated on microfiber coupler for ultrasensitive NH₃ sensing

Ammonia (NH3) is a valuable industrial raw material, yet its toxicity necessitates highly efficient and accurate detection methods to protect human health and the environment. Optical fiber sensors offer advantages over conventional NH3 sensors, including high stability, precision, corrosion resistance, and strong anti-interference capability. Integrating optical fibers with sensitive materials has thus gained increasing attention in gas sensing. In this work, a highly sensitive micro-nano fiber coupler (MFC) was fabricated using a hydrogen – oxygen flame melting and tapering technique. Graphene, scandium-doped graphene, and scandium–nitrogen co-doped graphene were synthesized via solvothermal and liquid-phase dispersion methods. These materials were separately coated onto the MFC waist region by dip-coating to construct optical fiber NH3 sensors. The sensing units were placed in a vacuum gas chamber for ammonia concentration tests. A linear fit was applied to the interference wavelength shifts in the transmission spectra under different NH3 concentrations. The results confirm the linear response of graphene-based sensors to NH3. The measured sensitivities were 14.46 pm/ppm for graphene-modified MFC, 28.29 pm/ppm for Sc-doped graphene-modified MFC, and 52.14 pm/ppm for Sc–N co-doped graphene-modified MFC. The Sc–N co-doped graphene shows a significant enhancement in NH3 sensing performance. This study offers new insights and experimental support for developing high-performance optical fiber ammonia sensors.