An indolium-based near-infrared fluorescent probe for non-invasive real-time monitoring of gastric pH in vitro and in vivo

Abnormal gastric acidity is closely linked to severe gastrointestinal diseases, making the real-time monitoring of gastric pH critical for investigating stomach-related physiological and pathological processes, diagnosing related diseases, and evaluating drug efficacy. In this study, we developed a near-infrared (NIR) fluorescent probe, named Hcy-pH, by conjugating a p-dimethylaminophenyl moiety with an indolium fluorophore via extended double bonds. The probe displayed significant NIR fluorescence at 820 nm in a PBS buffer, with a large Stokes shift of 240 nm. The fluorescence intensity of the probe decreased progressively as the pH decreased from 4.0 to 2.5, with a calculated pK_a of 2.98. Hcy-pH exhibited excellent biocompatibility and enabled the visualization of pH fluctuations in vitro by living HeLa cells. Moreover, the non-invasive monitoring of gastric pH in vivo was achieved in live mice, underscoring its great potential for studying stomach-related diseases and evaluating related pharmaceuticals.