In Situ Ni(II) Complexation Induced Deprotonation of Bis-Thiourea-Based Tweezers in DMSO-Water Medium: An Approach toward Recognition of Fluoride Ions in Water with Organic Probe Molecules.

Recognition of fluoride in water through the fluoride-induced Brönsted acid-base deprotonation reaction of an organic probe molecule is still a challenging task owing to the lower basicity of fluoride ions and the instability of the conjugate base of the probe molecules in aqueous medium. Herein, we report a complementary strategy in which the conjugate base of the studied bis-thiourea molecule in dimethyl sulfoxide (DMSO) medium is simultaneously stabilized through chelation of the Ni(II) ion, which eventually facilitates the recognition of the fluoride ion in water samples. The recognition methodology is validated colorimetrically and electrochemically, and finally, the applicability of the approach is explored with water samples collected from fluoride-affected areas. The limit of detection value for the fluoride ion in water medium was found to be 0.2 and 0.3 ppm with UV-visible spectroscopy and differential pulse voltammetry measurements, respectively. The methodology is also demonstrated on a paper strip for the detection of the fluoride ion with the naked eye and a smartphone-based RGB sensor. The scheme has been shown to be effective in enhancing the aqueous fluoride recognition ability of the organic probe molecules with acidic hydrogen prone to deprotonation by the fluoride ion.