A coumarin-based colorimetric receptor coupled with a smartphone for the determination of fluoride concentration in drinking water

The excess residues of fluoride ions (F⁻) in drinking water cause serious human health problems, making their detection and measurement highly essential. In this work, a coumarin-based receptor (4-methyl-1-(1-(6-nitro-2-oxo-2H-chromen-3-yl)ethylidene)thiosemicarbazide, 3-acetyl coumarin hydrazone derivative) was synthesized via a microwave-assisted protocol. The colorimetric receptor was found to be highly selective and sensitive and showed visible changes towards F⁻ detection. The ¹H NMR and HRMS results demonstrated that N-H in the receptor could bind with F⁻via intermolecular hydrogen bonds (N-H⋯F⁻), which caused a colour shift (from pale yellow to pink) and spectral change (a new absorption peak at 546 nm). The colorimetric responses of the receptor towards F⁻ were quantified by UV-vis spectroscopy, and simultaneously, a smartphone installed with the freely accessible Color Grab app was used to extract the RGB values of the captured images. The obtained calibration curves from both the UV-vis spectrophotometry and smartphone colorimetry showed good linear responses to F⁻ concentration, and the detection limit of smartphone colorimetry was 0.088 mg L⁻¹. The agreement of results obtained from the developed smartphone-based colorimetric method with that obtained from the traditional UV-vis technique and an ion-selective electrode validated the possibility of smartphones as a novel analytical tool for the low cost, versatile, and handy analysis of F⁻ in real water samples (such as groundwater, lake, rain, snow, commercial mineral water and tap water).