A new rhodamine-based fluorescent chemosensor (1) has been designed and synthesized by linking rhodamine 6G hydrazide with N-methylisatin via an imine linkage. The receptor can selectively recognize and sense Pb, Hg and Cd by showing different fluorescence characteristics. In ethanol/HEPES buffer medium, the addition of Cd caused a yellowish-green fluorescence, while the presence of Pb or Hg gave rise to an orange fluorescence. Additionally, the sensor shows an irreversible fluorescence response to Pb and reversible fluorescence responses to Hg and Cd.

A novel fluorescent probe (NT) was developed by merging 2-hydrazinylbenzothiazole with 2-hydroxy-1-naphthaldehyde for the detection of Cd and Cu. The probe alone is almost nonfluorescent due to the isomerization of C=N in the excited state. The addition of Cd can cause an immediate strong green fluorescence owing to the suppression of C=N isomerization by Cd-coordination. Furthermore, NT gives a delayed turn-on fluorescence response to Cu although it is a vigorous fluorescence quencher, which was thanks to the inhibition of the electron transfer between excited fluorophore and paramagnetic Cu by sulfur donor. Based on fluorescence spectra and ESI-MS analysis, the binding modes between NT and Cd/Cu were proposed.

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.