Two sugar-rhodamine "turn-on" fluorescent probes for the selective detection of Fe³

Two new sugar-rhodamine fluorescent probes (RDG1 and RDG2) have been synthesized and characterized by ¹H NMR, ¹³C NMR and HRMS. Their UV-Vis, fluorescence spectra and fluorescence-response to Fe³⁺ are investigated and discussed. RDG1 had a very nice linear relationship between UV absorbance and Fe³⁺ concentration with the correlation coefficient as high as 0.997 and the detection limit is 3.46×10^-6M. Upon the addition of Fe³⁺, the spirolactam ring of RDG1 was opened and a 1:1 metal ligand complex was formed from Job's plot. The results showed that RDG1 can be used as an effective fluorescent probe for selective detection of Fe³⁺ in water. RDG2 was incorporated the well-known rhodamine group and a water-soluble d-glucose group within one molecule and can be used for detecting Fe³⁺ in natural water as a selective fluorescent sensor. The addition of Fe³⁺ into RDG2 resulted in a strongly enhanced fluorescence as well as color change of solution from colorless to pink. Job's plot of RDG2 indicated 1:1 stoichiometry of RDG2-Fe³⁺. RDG2 can serve as a probe for Fe³⁺ between pH=4.0 to 7.0 and it's detection limit is 2.09×10^-6M. The OFF-ON fluorescent mechanisms of RDG1-Fe³⁺ and RDG2-Fe³⁺ are proposed.