Selective fluoride sensing by a novel series of lanthanide-based one-dimensional coordination polymers through intramolecular proton transfer

Abstract

A novel series of one-dimensional coordination polymers (CPs) is achieved via a facile one-pot synthesis strategy employing the nitrate salts of trivalent lanthanides, a pentadentate chelating ligand, and triphenylphosphine oxide at a controlled stoichiometry under ambient conditions. All the CPs are characterized comprehensively using spectroscopic, X-ray crystallographic and magnetometric studies. The CPs are found to be thermally stable up to a significantly high temperature and resistant to water for an indefinite time. They are photoactive and exhibit selective fluoride ion (F⁻) sensing with excellent efficiency both colorimetrically and fluorimetrically in the solid-state as well as in solution. The presence of F⁻ concomitantly sensitizes the photoluminescence enhancement and visual decolourization of the CPs in solution owing to the ground-state intra-molecular proton transfer. The photophysical response of the CPs to F⁻ in solution was found to be instantaneous (<30 s). The sensitivity of detection is observed to be significantly high over a wide range of F⁻ concentrations, covering the beneficial and detrimental domains of F⁻ concentrations in drinking water. The limit of detection (LoD) under ambient conditions was found to be in the micromolar (μM) range—the best being 0.22 μM found using UV-vis spectrometry and 7.5 μM using fluorimetry. In comparison, the USEPA standard cut-off for the upper limit of F⁻ concentration in drinking water is 211 μM, and the LoD of measuring F⁻ concentration using the USEPA standard method using a fluoride-selective electrode is 26.3 μM. The CPs display markedly high selectivity toward F⁻ with negligible-to-no interference from the commonly abundant ions (Cl⁻, Br⁻, I⁻, CH₃CO₂⁻, CO₃²⁻, SO₄²⁻, HPO₄²⁻, NH₄⁺, Na⁺, K⁺, Mg²⁺, and Ca²⁺) in terms of UV-vis spectral change. Moreover, they also exhibit solid-state IR-spectrometric sensitivity towards F⁻ under ambient conditions.