Highly Reversible “Off‐On‐Off” Dual‐channel Fluorescence Probe Based on Amino Pyrazole and Phenothiazine for Sensing Extremely Alkaline Solution

Abstract Acid‐base balance is related to many cell functions, and its disorders can lead to many diseases. Real‐time detection of intracellular pH fluctuation is very important to prevent many diseases and environmental water samples. Here, we report a dual‐channel fluorescence probe PTZ‐aminopyrazole based on low‐cost phenothiazine and 3‐aminopyrazole, which can detect pH fluctuation highly sensitively through protonation under acidic condition and deprotonation under alkaline condition. The probe exhibits a fluorescence intensity and UV absorbance characteristics in the extreme acidity range of 2.6‐5.5 and especially in the extreme alkaline range of 11.1‐13.8, respectively. In addition, PTZ‐ aminopyrazole has high pH‐selective reaction to metal ions, good reversibility and excellent photostability. Furthermore, PTZ‐aminopyrazole can be fixed on a test paper, which shows a rapid and reversible colorimetric response to HCl/NaOH vapor with naked‐eye.