Synergistic enhancement of ammonia sensing using U-shaped tapered no-core fiber and functional group-modulated MXene

The development of high-sensitivity sensing technologies for ammonia (NH₃), a common and toxic industrial gas, is crucial for public safety and environmental protection. Two-dimensional materials MXene have shown great potential in gas sensing due to their excellent electronic and optical properties, yet their integration with fiber-optic platforms and the underlying sensing mechanisms require further exploration. In this study, we propose and fabricate an NH₃ sensor based on a Ti₃C₂T_x MXene-functionalized U-shaped Tapered No-core fiber (UTNCF). This structure enhances the light-matter interaction with the MXene material through a strong evanescent field effect. Concurrently, we employed density functional theory (DFT) to systematically simulate the interaction between NH₃ molecules and Ti₃C₂T_x with varying ratios of functional groups, calculating the band structure and charge transfer. Furthermore, the BoltzTrap module was utilized to analyze the changes in Ti₃C₂T_x ’s optical refractive index upon NH₃ adsorption, thereby revealing the intrinsic physical mechanism of the sensor. Experimental results demonstrate that the developed sensor exhibits good performance over an NH₃ concentration range of 0–320 ppm. The sensor's transmission spectrum shows a significant redshift with increasing NH₃ concentration, achieving a high sensitivity of 1.8 pm/ppm with good linearity. Theoretical calculations confirm that the primary sensing mechanism is the charge transfer from NH₃ molecules to the Ti₃C₂T_x surface. This process alters the material's carrier concentration, leading to a measurable change in its complex refractive index, which is in high agreement with our experimental observations. This work successfully combines theoretical calculations with experimental validation to not only develop a high-sensitivity fiber-optic NH₃ sensor but also to elucidate its sensing mechanism from a first-principle's perspective. It provides an effective strategy for designing novel, high-performance Ti₃C₂T_x-based optical gas sensors with significant application prospects in industrial safety and environmental monitoring.