Anion Complexation Studies of 3‑Nitrophenyl-Substituted Tripodal Thiourea Receptor: A Naked-Eye Detection of Sulfate via Fluoride Displacement Assay

A thiourea-based
tripodal receptor L substituted with
3-nitrophenyl groups has been synthesized, and the binding affinity
for a variety of anions has been studied by 1H NMR titrations
and nuclear Overhauser enhancement spectroscopy experiments in dimethyl
sulfoxide-d6. As investigated by 1H NMR titrations, the receptor binds an anion in a 1:1 binding
mode, showing the highest binding and strong selectivity for sulfate
anion. A competitive colorimetric assay in the presence of fluoride
suggests that the sulfate is capable of displacing the bound fluoride,
showing a sharp visible color change. The strong affinity of L for sulfate was further supported by UV–vis titrations
and density functional theory (DFT) calculations. Time-dependent DFT
calculations indicate that the fluoride complex possesses a different
optical absorption spectrum (due to charge transfer between the fluoride
and the surrounding ligand) than the sulfate complex, reflecting the
observed colorimetric change in these two complexes. The receptor
was further tested for its biocompatibility on primary human foreskin
fibroblasts and HeLa cells, exhibiting an excellent cell viability
up to 100 μM concentration.