From traditional medicine to advanced sensing: A natural Fraxin-based supramolecular probe for visual pH detection and in vivo gastrointestinal imaging.

Dynamic monitoring of intracellular pH homeostasis is critical for revealing pathological mechanisms and enabling early diagnosis of diseases. Accordingly, the development of fluorescent pH probes with high sensitivity, low-cost, and excellent biocompatibility is of considerable importance for biomedical diagnostics. In this study, a novel fluorescent probe based on a pH-induced reversible self-assembly mechanism was constructed using Fraxin, a natural small molecule extracted from the traditional Chinese medicine Cortex Fraxini. Experimental results show that Fraxin remains in a discrete state with quenched fluorescence in acidic microenvironments. As the pH increases to a weakly alkaline range, the molecules undergo self-assembly into nanosphere structures driven by π-π stacking, hydrogen bonding, and electrostatic interactions, triggering a pronounced "Off-On" fluorescence response. The probe exhibits an excellent linear correlation (R2 = 0.9985) within the pH range of 5.6-6.8 and demonstrates strong resistance to ambient light interference. In terms of practical applications, we developed a paper-based sensing platform for rapid screening of sulfur-fumigation in medicinal materials and established a smartphone-assisted point-of-care testing (POCT) system for digitalized pH monitoring. Regarding bioimaging, the probe demonstrates excellent cell permeability and specific mitochondrial targeting capability. In vivo studies further demonstrate its capability to monitor pH fluctuations in zebrafish and to track real-time changes in gastric acidity in mice following omeprazole treatment. Overall, this research not only elucidates the supramolecular assembly behavior of Fraxin but also provides new insights into the development of natural-sourced portable diagnostic tools and clinical gastrointestinal disease monitoring.