Colorimetric ‘naked eye’ sensor for fluoride ion based on isatin hydrazones via hydrogen bond formation: Design, synthesis and characterization ascertained by Nuclear Magnetic Resonance, Ultraviolet–Visible, Computational and Electrochemical studies

Abstract

A new colorimetric sensor ‘R-IND’ based on isatin hydrazones have been designed and synthesized by simple Schiff base reaction. The anion sensing properties of R-IND were examined, characterized by common spectroscopic techniques such as Fourier Transform-Infrared (FT-IR), Ultraviolet–Visible (UV–Visible), Proton and Carbon Nuclear Magnetic Resonance (1H &13C NMR), Electrospray Ionization Mass Spectroscopy (ESI-MS) and electrochemical studies. The sensor R-IND bearing ortho hydroxy group and amine group acting as binding sites. The sensor R-IND exhibited sensitive and selective sensing towards F− ion over the other competitive anions like Cl−, I−, Br−, H2PO4−, AcO−, CN–, NO3–, ClO4− and HSO4− in 100% Aetonitrile and 20% aqua Acetonitrile media. The bathochromic shift of 222 nm and 228 nm was observed in UV–Vis spectra with a color change from pale yellow to purple which is due to the complex formation between R-IND and F− ion through hydrogen bond formation. This was further accounted by 1H NMR titration. The Job’s plot reveals 1:2 stoichiometry between the host-guest complex. Binding and association constants were found to be 1.72 × 109 M−1, 2.82 × 109 M−1 and 6.0 × 105 M−1, 2.0 × 106 M−1 in 100% ACN and 20% aqua ACN respectively. The detection limit of sensor R-IND was turned out to be 0.45 and 0.41 ppm for F− ion in organic and aqua organic medium which is lesser than World Health Organization (WHO) permission level (1 ppm). The characteristic potential difference of 1.069 V has been observed for R-IND+F− complex in the electrochemical study. Finally, economically viable paper-based colorimetric test strips of R-IND were developed to detect the F− ions in 100% aqueous solution.