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A novel ratiometric fluorescent probe derived from 2-(2′-hydroxy-3′-methoxyphenyl) benzothiazole (HMBT) for human carboxylesterase 1 (hCES1) has been synthesized and well characterized. The probe was designed by modulating the excited state intramolecular proton transfer (ESIPT) emission of HMBT via introducing of an octanoyl group. Under physiological conditions, the probe displayed satisfying stability with very low background fluorescence signal at 489 nm, but it could be selectively hydrolyzed by hCES1 to release its metabolite HMBT which brings remarkable changes in fluorescence spectrum. The newly designed probe exhibited excellent selectivity towards hCES1 over other enzymes, while the interference of endogenous chemicals was negligible. The probe can be used for accurately measuring the actual activities of hCES1 in complex biological samples. The potential biological applications of the probe including inhibitor screening, quantification of hCES1 in serum, as well as fluorescence imaging in vivo, have been demonstrated. Image 1 • An ESIPT-based fluorescent probe with large Stokes shift has been developed for detection of human carboxylesterase 1. • The probe can be selectively hydrolyzed by hCES1 which brings remarkable changes in fluorescence spectrum. • The probe can be used for accurately measuring the actual activities of hCES1 in complex biological samples. • The potential biological applications of the probe included inhibitor screening, as well as fluorescence imaging in vivo. [ABSTRACT FROM AUTHOR]