AmmPatch: A Visual, Flexible, and Reusable Film Sensor for Ammonia Gas

Reliable detection of ammonia (NH ₃ ) is essential for environmental monitoring, industrial safety, and public health protection. Here, we report a flexible, paper‐based colorimetric NH ₃ sensor patch fabricated by deep‐coating bromocresol green (BCG)–polyvinylpyrrolidone (PVP) composite onto a porous Whatman filter paper substrate, named as “AmmPatch” . The sensing mechanism is governed by a reversible protonation–deprotonation equilibrium of BCG, producing a distinct yellow‐to‐blue color transition upon exposure to NH ₃ . Quantitative optical response analysis demonstrated a rapid response (<30 ms) , excellent linearity over the 10–100 ppm concentration range, and a limit of detection near 5 ppm, suitable for safe exposure monitoring. The sensor maintained consistent performance across varying temperature (20°C–36°C) and humidity (27%–92%) conditions and exhibited negligible signal drift during 50 days of continuous testing , confirming exceptional stability and reproducibility. Ultraviolet‐visible spectroscopy (UV–Vis) and Fourier transform infrared spectroscopy (FTIR) spectroscopy confirmed that NH ₃ induces reversible proton abstraction from the sulfonephthalein groups of BCG, leading to optical switching between protonated‐deprotonated forms. The flexible, adhesive, and low‐cost design of the patch enables on‐site and real‐time monitoring when integrated with digital color analysis or smart device interfaces. This work establishes a scalable, visually intuitive, and environmentally robust colorimetric sensing platform for practical NH ₃ detection in industrial, and occupational safety applications.