Near-infrared fluorescent probe for sensing local microscopic extreme acidity and its application in mitochondria

• We developed a novel near-infrared (NIR) fluorescent probe Cy-BNN for sensing pH under extremely acidic conditions. • The pk a value of Cy-BNN as low as 3.32. • Cy-BNN could be used for monitoring pH changes in the mitochondria of living cells. In this paper, a novel near-infrared (NIR) fluorescent probe Cy-BNN for sensing local microscopic pH under extremely acidic conditions (pH < 4) was constructed and reported. For the preparation of Cy-BNN , a barbituric acid moiety was fused in the polymethine chain of the cyanine dye to introduce nitrogen atoms as protonation center. In different pH, the balance of quaternary ammonium salts and free amines of Cy-BNN was adjusted, and the pk a value of Cy-BNN was calculated based on the Sigmoidal equation. Surprisingly, the pk a value of Cy-BNN as low as 3.32, implying that Cy-BNN can be used for sensing extreme acidity. As expected, Cy-BNN exhibited highly selective response to H + over diverse analytes, and can be used to monitoring the intracellular pH fluctuation under extremely acidic conditions. Importantly, Cy-BNN can be used to specifically target mitochondria, providing potential possibilities for monitoring mitochondria–lysosome interactions in live cells and mitophagy process.