Wide-Acidity-Range pH Fluorescence Probes for Evaluation of Acidification in Mitochondria and Digestive Tract Mucosa

The cells control their pH change in a very accurate range. pH plays important roles in cell autophagy and apoptosis. Previous evidence implies that the internal milieu of a tumor is acidified. Although the acidification in cells is investigated, the biological effects from multiple stimulating factors under the complex intracellular environment have not been thoroughly elaborated yet. Currently, there are few pH probes that perform in a wide acidity range, and a probe that is capable of measuring a wide pH range needs to be developed. Herein, we report two new fluorescent probes (BHNBD and CM-BHNBD) for the detection of mitochondrial and intramucosal acidification. The two probes respond to pH via an H⁺-driven TICT (twist intramolecular charge transfer) mechanism, and they can linearly report pH within a wide pH range: 7.00-2.00 following ∼148-fold fluorescence increase. The two probes also possess excellent membrane permeability, good photostability, and negligible cytotoxicity. The probes are successfully applied for quantifying the acidification in HeLa cells under the simultaneous stimulation of nutrient deprivation and oxidative stress. Our results demonstrate that the mitochondrial pH is in a dynamic fluctuating state during the acidification process, which suggests a potential cross-talk effect between cell autophagy and apoptosis. We also use the probes for quantifying the intramucosal pH variation in stomach and esophagus via manipulating cellular proton pump. The development of our probes is potentially expected to be used to monitor the intracellular/intramucosal acidification for biomedical research.