Optical Fiber HMS Ammonia Sensor with Cellular-like N-CQDs/Mg_<i>x</i>Zn_1–<i>x</i>O at Room Temperature

This paper presents an optical fiber hollow malposition structure (HMS) elaborated with cellular-like N-CQDs/Mg_<i>x</i>Zn_1-<i>x</i>O for detecting ammonia (NH₃) at room temperature. The gas detection was realized by combining the HMS to excite the surface evanescent field on the outer surface of the hollow core fiber (HCF) and the surface-coated cellular-like N-CQDs/Mg_<i>x</i>Zn_1-<i>x</i>O material for improving sensitivity. Experimental results demonstrate that the sensor coated with N-CQDs/Mg_0.2Zn_0.8O exhibits the highest sensitivity, reaching 8.9 pm/ppm, which is 11.6 times higher than that with Mg_0.2Zn_0.8O. Additionally, the NH₃ sensor can determine a minimum concentration of 1 ppm and has response and recovery times of 6.2 and 7.1 s, respectively. Density functional theory (DFT) calculations confirm that N-CQDs reduces the bandgap of the composite material and increase the carrier concentration on its surface, thereby enhancing gas adsorption capacity. The intrinsic characteristics of the proposed sensor including small size, high sensitivity, and good stability reveal its promising prospects in detecting low concentrations of NH₃.