In situ Synthesis of Au-ZnO on Composite Fibre for Wearable and High-Sensitivity Ammonia Sensing

Gas sensors play a vital role in human health monitoring, particularly for detecting disease-related biomarkers in exhaled breath. However, conventional metal-oxide semiconductor gas sensors face limitations in wearable respiratory monitoring applications due to their inflexibility and high heat loss. Here, we fabricated an Au-ZnO fibre-based gas sensor for the detection of ammonia (NH₃), an indicator of kidney dysfunction. The device is constructed from a composite flexible fibre comprising a liquid-metal core and a silica cladding, onto which an Au-ZnO gas-sensing layer is synthesised in situ. Liquid metal directly conducts thermal energy to the insulating cladding through Joule heating, providing thermal energy to the gas-sensing layer. This design enables low heat loss and low-current operation while generating sufficient thermal energy to enhance the gas-sensing response. The resulting Au-ZnO fibre sensor exhibits a stable high response to NH₃ (response of 2.2 to 2 ppm NH3) even under bending (bending radius of 15 mm). The response time and recovery time are 153 s and 156 s at a working temperature of 280 oC. This demonstrates that the Au-ZnO fibre sensor has good flexibility and suitability for wearable respiratory monitoring. To showcase its practical utility, a real-time monitoring sensor system was developed by designing a signal detection circuit, and the concentration of NH₃ was monitored in real time through different coloured light-emitting diodes. This work presents a practical, home-based approach for respiratory monitoring of NH₃ and offers a simple, effective solution to distinguish between healthy individuals and patients with different stages of kidney disease.