Amino-naphthoquinone and its metal chelates for selective sensing of fluoride ions

The fluoride ion sensing ability of 2-(2-(dimethylamino)ethylamino)-3-chloronaphthalene-1,4-dione (R1) and its metal chelates viz. [Cu(R1)Cl2], [Co(R1)Cl2].3H2O, [Zn(R1)Cl2] and [Ni(R1)Cl2] was studied through visual detection experiment, UV-Vis, fluorescence and 1H NMR titrations. The receptors were highly selective and sensitive for distinguishing fluoride ions from other common anions through a conspicuous change of UV-Vis and fluorescence spectra. The sensing mechanism, as confirmed by 1H NMR titration, involves formation of H-bond between the N-H moiety and F-ions. The coordination of metal ions was found to increase the H-bond donor ability of the receptor unit (NH). The binding constant for the [Zn(R1)Cl2] + F− complex was found to be 6.2 × 104 M−1 which is 15 times higher than that of R1 + F− complex. The sensing mechanism was further supported by electro chemical studies and DFT calculations using Gaussian 03 program. The detection limits of the probes in the determination of F− ions were found to 0.05-0.4 μM, which are lower than the World Health Organization (WHO) permissible level (1.5 mg L−1).The test strips prepared using [Zn(R1)Cl2] was found to impart striking colour change instantaneously with F-ions in water.