Development of Fluorescence Probes for Transition Metal Cations, based upon Fluorescence Resonance Energy Transfer

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A novel fluorescent probes for metal cations, i. e. DBD-ED-CNEDTA, ABD-ED-CNEDTA, DBD-Apy-CNEDTA and ABD-Apy-CNEDTA, which have long Stokes shift, were synthesized from the reaction of N-(3-carboxy-2-naphthyl)-ethylenediamine-N, N', N'-triacetic acid (CNEDTA) with a benzofurazan (DBD-ED, ABD-ED, DBD-Apy or ABD-Apy). The long Stokes shifts are due to the fluorescence resonance energy transfer (FRET) phenomenon between a donor (CNEDTA) and the acceptors (benzofurazans) fluorophores. When the fluorescent probes were excited at 240 nm, 340 nm, and 440 nm, only emission maximum was observed at 560 nm. However, the fluorescence (FL) at 480 nm, based upon the CNEDTA moiety, was not detected with excitation at 340 nm. Among the metal probes synthesized, DBD-ED-CNEDTA was suitable for the determination of transition metals in terms of FL property of the resulting chelates. The FL intensity of DBD-ED-CNEDTA was dependent upon the acidity of the medium and highest at pH4. 1. DBD-ED-CNEDTA reacted with metal cations, i. e., Zn, Cd, Al, Y, and La, in aqueous medium to form the chelates. The spectral change of FL excitation and emission was small before and after the addition of the metal cations. However, the FL intensity was dependent upon the concentrations of the metals. In the case of Zn^<2+>, the molar ratio bound with DBD-ED-CNEDTA was calculated as one-to-one. The FL intensities after chelate formation of Zn/DBD-ED-CNEDTA(1:1) were enhanced by 3.8-fold (excitation at 340 nm, emission at 240 nm, emission at 560 nm), respectively. The FL probe was applied to the determination of Zn in food supplement.