Method for highly selective, ultrasensitive fluorimetric detection of Cu and Al by Schiff bases containing o-phenylenediamine and o-aminophenol.

Schiff base probes (1 and 2) made from o-phenylenediamine and o-aminophenol were appeared as highly selective fluorimetric chemosensor of Cu and Al ions respectively. Strong fluorescence emission of probe 1 at 415 nm (excitation at 350 nm) was instantly turned off on addition of Cu. Very weak fluorescence of probe 2 at 506 nm (excitation at 400 nm) was immediately turned on specifically by Al. Job's plot and ESI-MS results suggested 1:1 molar stoichiometric ratio of metal ion and probe in their respective complexes. Probe 1 and 2 had demonstrated very low detection limit (9.9 and 2.5 nM respectively). Binding of Cu with probe 1 was found chemically reversible on addition of EDTA, while complexation between Al and probe 2 was not reversible. On the basis of density functional theory (DFT) and spectroscopic results, probable mode of sensing of the metal ions by the probes were proposed. Quenching of the fluorescence of probe 1 by Cu was attributed to the extensive transfer of charge from the probe molecule to paramagnetic copper ion. Whereas, in the Al-complex of probe 2, photo-induced electron transfer (PET) process from the imine nitrogen to salicylaldehyde moiety was restricted and thereby the weak emission intensity of probe 2 was enhanced significantly. Effective pH range of sensing the metal ions by probe 1 and 2 were 4 to 8 and 6 to 10 respectively. Probe 1 was also applied in the design of a logic gate for Cu detection. Moreover, probe 1 and 2 was also used in water sample analysis for quantitative estimation of Cu and Al respectively.