"Turn-on" benzophenone based fluorescence and colorimetric sensor for the selective detection of Fe2+ in aqueous media: Validation of sensing mechanism by spectroscopic and computational studies.

Abstract A diaminobenzophenone Schiff base derived probe 1, was synthesized and structure elucidation was carried out by spectroscopic studies viz., FT-IR, UV-vis, 1H and 13C NMR and mass spectrometry. The sensing phenomenon with different metal ions (Cr3+, Mn2+, Fe2+, Fe3+,Co2+, Ni2+, Cu2+, Zn2+ ,Cd2+) was investigated by employing absorption and fluorescence titrations which demonstrated that the probe 1 exhibited selective fluorescent sensing behavior towards Fe2+ ion among various other metal ions. The selectivity of the probe towards Fe2+ was also examined by colorimetric assay which revealed a change in the color from light yellow to brown upon addition of Fe2+ ion. A remarkable increase in the fluorescence intensity of the probe 1 was observed towards Fe2+ ion, which was found to be associated with the inhibition of photoinduced electron-transfer (PET) and C=N isomerization processes, respectively. The chemosensor exhibited an association constant value of 6.173×107 M-2 as determined by using non-linear least square fit data. The binding stoichiometry was calculated by Job’s plot and the sensing phenomenon of Fe2+ towards the probe was further supported by Density Functional theory (DFT) calculations and 1H NMR studies. The detection limit of the probe 1 was found to be 0.0363 µM which is below the permissible limits according to the WHO guideline (5 μM) for Fe2+ ions in the drinking water. Furthermore, the practical application of the probe 1 was studied by analyzing the content of Fe2+ in different water samples.