Selective fluorescent probes based on CN isomerization and intramolecular charge transfer (ICT) for zinc ions in aqueous solution

As the second most abundant transition-metal ion in the human body, Zn2+ plays crucial roles in many important biological processes; while in the environment, an excessive concentration of Zn2+ may reduce the soil microbial activity resulting in phytotoxic effects. Therefore, developing effective and sensitive detection method for Zn2+ has become crucially important and necessary both in life and environment science. Two new fluorescence probes, 2-((2-hydroxynaphthalen-1-yl)methyleneamino)-3-(1H-imidazol-5-yl) propanoic acid (2) and 2-hydroxy-2-((2-hydroxynaphthalen-1-yl) methyleneamino) acetic acid (3), were easily prepared by a one step reaction between 2-hydroxy-1-naphthaldehyde with histidine and serine, respectively, in ethanol. The optical properties of them were investigated by fluorescence spectra, which displayed specific and sensitive recognition to Zn2+ and especially avoided the interference of Cd2+ when they were tested against a range of physiological and environmentally relevant metal ions in aqueous solution. The responsive mechanism of the two probes to Zn2+ were involved both the CN isomerization and ICT, which were clarified by NBO charge analysis and the HOMO-LUMO energy gap calculation by using B3LYP/6-31G density functional theory.