Novel Coumarin-Based Fluorescent Probes for Detecting Zn2+ in Living Cells

Zinc ion (Zn2+) is one of the most important transition-metal ions in the human body which is involved in many important life activities and many diseases can be displayed by its situation. Thus, monitoring of Zn2+ is very meaningful to diagnosis of diseases. Compared to the traditional detection methods, fluorescence probe is safer and more practical. Because coumarin derivatives possess several advantages in optics, we choose them as key structures to prepare new fluorescent probes. Based on the mechaism of photoinduced electron transfer (PET), the water-soluble fluorescent probes 1 and 2 that are coumarin-based derivatives were designed, which demonstrated sensitivity for Zn2+ and exhibited high selectivity to Zn2+ over other metal ions. The receptor unit serves as an electron donor in the absence of Zn2+, quenching the fluorophore excited state that makes the probe have no fluorescence. However, when Zn2+ binds to the receptor, photoinduced electron transfer is prevented and the quenching is blocked, resulting the fluorescence intensity significantly enhanced. There is a good correlation between fluorescence intensity and Zn2+ concentration. With the increase of Zn2+ concentration, fluorescence intensity becomes stronger. Furthermore, the two probes were successfully labelled on the MCF-7 cell and B. subtilis. The single crystals of the coumarin-based compounds 1 similar to 4 and zinc complex [Zn(2)] were also obtained. The X-ray crystal structure of the zinc complex [Zn(2)] reveals that the hydroxyl group and the 2,2-dipicolylaminomethyl group participate in coordination. Zn2+ is five-coordinated with three nitrogen atoms from the pyridine rings, the substituted amino group and two oxygen atoms from the 7-site hydroxyl group, also coordinated with water molecule forming a pentacoordinated bipyramid geometry.