| Summary: | In this work, a highly selective fluorescent chemosensor <i>N</i>-(2-(2-<i>butyl</i>-1,3-dioxo-2,3-dihydro-1<i>H</i>-benzo[<i>de</i>]isoquinolin-6-yl)hydrazine-1-carbonothioyl)benzamide (<b>L</b>) was prepared and characterized. An assay to detect the presence of cobalt(II) ions was developed by utilizing turn-on fluorescence enhancement with visual colorimetric response. Upon treatment with Co<sup>2+</sup>, a remarkable fluorescence enhancement located at 450 nm was visible to naked eyes accompanied with a distinct color change (from pink to colorless) in a CH<sub>3</sub>CN/HEPES (4/1, <i>v</i>/<i>v</i>, pH = 7.4) solution due to the formation of a 1:1 complex at room temperature. In addition, the linear concentration range for Co<sup>2+</sup> was 0−25 µM with the limit of detection down to 0.26 µM. Thus, a highly sensitive fluorescent method based on chelation-assisted fluorescence enhancement was developed for the trace-level detection of Co<sup>2+</sup>. The sensor was found to be highly selective toward Co<sup>2+</sup> ions with a large number of coexisting ions. Furthermore, the <b>L</b> probe can serve as a fluorescent sensor for Co<sup>2+</sup> detecting in biological environments, demonstrating its low toxic properties to organisms and good cell permeability in live cell imaging.
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