New Fiber-Optic Sensor Detects Ammonia at Parts-Per-Billion Levels

Optical sensors have carved out a niche as effective tools to be used in emissions monitoring, food quality control, and environmental sensing. In a new advancement in this space, a team of researchers from Zhejiang Gongshang University have developed a fiber-optic ammonia sensor capable of detecting concentrations as low as 7.5 parts per billion (ppb), which is a sensitivity level that could benefit industrial emissions monitoring, food quality control, and environmental sensing applications.¹ The findings were published in the journal Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy.¹

What makes this newly developed sensor unique compared to other sensors?

Most fiber-optic sensors are designed similarly, often with a flat-tipped design.² What makes the sensor built in this study different is the spherical geometry at the end-face of the optical fiber.¹ The researchers also deposited a conductive indium tin oxide (ITO) coating to enable in situ electrospinning.¹ That process deposited nanofibers of cellulose acetate doped with the pH-sensitive dye bromocresol purple (BCP) directly onto the tip, forming a three-dimensional (3D) sensing architecture with elevated surface area.¹

The spherical geometry is not for aesthetic purposes. This new design is critical to how the sensor performs. Unlike a flat fiber end-face, the curved surface induces multiple internal reflections and generates leaky optical fields that extend further into the surrounding sensing layer, increasing the degree to which light interacts with adsorbed gas molecules.¹ Combined with the nanofibrous layer's high surface area, the architecture enables the sensor to capture and respond to low-concentration ammonia more effectively than conventional flat-tip designs.¹

How did the sensor perform in laboratory testing?

The researchers discovered during laboratory testing that their sensor demonstrated a quadratic response across the 0.05 to 2.5 parts-per-million (ppm) range at a wavelength of 593 nm, with a coefficient of determination of 0.991.¹ The calculated detection limit reached 7.5 ppb, which is well below occupational exposure thresholds and within the range required for sensitive environmental monitoring.¹ The sensor also showed stable, reproducible results across temperatures of 20–30 °C and relative humidity levels of 30–70%, which are conditions representative of many real-world deployment environments.¹

The authors noted that the performance gains result from the combined effect of the optical geometry and the electrospun sensing layer, rather than either element alone.¹ By demonstrating that fiber-tip geometry can be used as an active design parameter, the study offers a template for improving sensitivity in fiber-optic gas sensors more broadly. Although fiber-optic sensors have been routine used in telecommunications, this study underscores how these types of sensors are beginning to be used in other areas such as environmental monitoring.^1,2

References

1. Chen, Y.; Zang, X.; Zhu, X. An Optical Ammonia Sensor Based on a Spherical Fiber Tip Coated with Electrospun Nanofibers. Spectrosc. Acta Part A: Mol. Biomol. Spectrosc. 2026, 361, 128028. DOI: 10.1016/j.saa.2026.128028
2. Elsherif, M.; Salih, A. E.; Munoz, M. G.; et al. Optical Fiber Sensors: Working Principle, Applications, and Limitations. Adv. Photo. Res. 2022, 3 (11), 2100371. DOI: 10.1002/adpr.202100371