Visualization of Gaseous Ammonia Using an Ionic Liquid‐Based pH‐Responsive Dye

Ammonia (NH 3 ) is an important resource that is used as a raw material in the production of fertilizers and fuels. It is generated from animal excrement and plant decomposition. Because NH 3 is highly toxic, a high‐sensitivity and responsivity method for its visual detection is required. Fluorescent dyes that respond to NH 3 have been developed for visual detection. However, ultraviolet irradiation is required to confirm changes in their fluorescence colors. Herein, the visualization of gaseous NH 3 under natural light using an ionic liquid–based pH‐responsive dye is reported. The dye is synthesized using trihexyl(tetradecyl)phosphonium (P 66,614 ) and bromocresol purple (BCP). In addition, a [P 66614 ] 2 [BCP] thin film that exhibits a visible response to gaseous NH 3 is synthesized, manifested as a color change from yellow to dark blue. Additionally, it exhibits sensitivity with a limit of detection of 66 ppm, repeatability for >100 exposure cycles, and a rapid response to gaseous NH 3 . To demonstrate practical applicability, gaseous NH 3 emitted from food is detected using the thin film. The thin film responds to trace amounts of NH 3 released from pork, which is visible as a color change from yellow to blue. Thus, [P 66614 ] 2 [BCP] has good application potential as a gaseous NH 3 sensor.