A novel fluorescent probe based on dansyl-appended rhodamine B was developed. The probe can selectively recognize and sense Hg 2+ and Cu 2+ from other common metal ions by showing unique fluorescence and absorption characteristics. In MeCN/HEPES buffer solution, the probe gives a ratiometric fluorescent response to Hg 2+ , which was ascribed to the fluorescence resonance energy transfer from dansyl moiety to the ring-opened rhodamine B moiety, while the presence of Cu 2+ causes fluorescence quenching. Beside the fluorescence change, the presence of Cu 2+ and Hg 2+ can induce intensive absorption at about 555 nm, which resulted in a color change from colorless to pink.

The fluorescent turn-on detection of metal ions is highly desirable for public health and environmental security. Herein, we report a rationally designed fluorescent probe (1) for the detection of Cu2+ synthesized by integrating 2-hydrazinylbenzothiazole with 3-acetyl-7-hydroxycoumarin. The probe alone is non-fluorescent due to the isomerization of C=N in the excited state. The addition of Cu2+ can cause a delayed fluorescence enhancement. A comparative study of 1 and its analogues indicated that the turn-on fluorescence response was thanks to the sulfur atom coordinating to Cu2+. The response delay of 1 in sensing Cu2+ was ascribed to the gradual transition from N-coordination to S-coordination (N and S in thiazole moiety). The proposed new function of S-donor would provide a new approach for the turn-on fluorescence sensation of Cu2+.