An optical ammonia sensor based on a spherical fiber tip coated with electrospun nanofibers

A fiber-optic ammonia (NH₃) sensor based on a spherical fiber tip integrated with electrospun nanofibrous layers is proposed. By engineering the fiber end-face into a spherical geometry and introducing a conductive indium tin oxide (ITO) coating, in situ electrospinning of bromocresol purple (BCP)-doped cellulose acetate nanofibers was achieved, forming a three-dimensional sensing architecture with high surface area. The spherical structure induces multiple internal reflections and leaky optical fields, significantly enhancing light-matter interaction within the sensing layer. The sensor exhibits a quadratic response to NH₃ in the range of 0.05-2.5 ppm, with a coefficient of determination (R²) of 0.991 based on the reflectance at 593 nm. The calculated limit of detection (LOD) reaches 0.0075 ppm (7.5 ppb), demonstrating sub-ppm sensitivity. The influence of environmental temperature and humidity was preliminarily investigated, and the sensor exhibits stable and reproducible responses over the ranges of 20-30 °C and 30-70% RH. This work demonstrates that the synergistic integration of spherical fiber geometry and electrospun nanofibrous sensing layers provides an effective strategy for enhancing light-matter interaction and gas adsorption, offering a promising approach for high-performance fiber-optic gas sensing.