A fluorescent probe for alkaline phosphatase via excited state intramolecular proton transfer

Alkaline phosphatase (ALP) is an important biomarker for diagnostics, and excessive level of ALP is closely related to a variety of pathological processes; hence the development of convenient and sensitive methods for detecting ALP is of great significance for medical sciences and diagnostics. Herein, we report a fluorescent probe for detecting ALP via excited state intramolecular proton transfer (ESIPT). The probe was synthesized by coupling a phosphate group onto flavone-based fluorophore. For this probe, the phosphate group serves both as the blocking agent for ESIPT and the responsive moiety toward ALP. Due to the shielding of the hydroxyl group by the phosphate moiety, no ESIPT process occurs, the tautomer emission of the probe is therefore quenched; while in the presence of ALP, ALP cleaves phosphate group and generates hydroxyl group, thereby enabling the ESIPT and generating strong ESIPT-based emission. This probe exhibits high selectivity for ALP detection with a very low detection limit of 0.032 U/L. It is capable of detecting ALP in biological fluid like serum. Furthermore, it is of little cytotoxicity and can be easily internalized into cells for endogenous ALP imaging. Therefore, the probe herein displays strong potential for applications in biological diagnostics and pathological analysis.