A novel hydrosoluble near-infrared fluorescence off-on probe has been developed for detecting carboxylesterase activity. The probe was designed by introducing (4-acetoxybenzyl)oxy as a quenching and recognizing moiety to the decomposed product of IR-783, which exhibits excellent near-infrared fluorescence features and good water solubility. The responding mechanism of novel probe 1 to carboxylesterase was investigated. It would lead to the cleavage of the carboxylic ester bond by carboxylesterase catalyze the spontaneous hydrolysis of the probe, resulting in the release of a near-infrared fluorophore. This behaviour leads to the development of a simple and sensitive fluorescent method for assaying carboxylesterase activity, with a detection limit of 3.4 × 10-3 U mL-1. Moreover, the probe displays excellent selectivity toward carboxylesterase over other substances. Notably, the imaging experimental results showed that the probe 1 is cell membrane permeable, and its applicability has been demonstrated for monitoring carboxylesterase activity in HeLa cells.
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