Fluoride can help to prevent tooth decay. In some regions, it is added to the drinking water for this reason. However, fluoride can also enter the water supply by other means, e.g., by leaching from minerals or contamination with industrial waste. Its overconsumption can cause health issues, particularly in children. Simple fluoride sensors could, thus, be useful, e.g., to check the safety of drinking water in remote regions.

Kyriakos C. Stylianou, Ecole Polytechnique Fédérale de Lausanne (EPFL), Sion, Switzerland, and colleagues have developed a lanthanide-based luminescent metal–organic framework (MOF) for the detection of fluoride. The team designed the MOF, named SION-105, with a boron receptor in the active site that attracts fluoride electrostatically. The active site is surrounded by bulky substituents, which prevent larger ions from interacting with the boron center and ensures the selective detection of F^–. The metal used in the framework is Eu^III, which causes a red luminescence. This luminescence is quenched when the MOF interacts with F^–.

The MOF was prepared by combining a tris(p-carboxylic acid)tridurylborane ligand (H₃tctb) and Eu(NO₃)₃·6H₂O in a mixture of dimethylformamide (DMF) and water at 120 °C. The resulting crystals can be ground into powder, suspended in tetrahydrofuran (THF), and combined with a water sample. The luminescence quenching can be observed using a portable, miniaturized fluorimeter device developed by the researchers. This allows a fast and user-friendly measurement of fluoride levels. Due to the noncovalent interaction with the fluoride ions, the MOF can be recycled and reused.

• Selective, Fast-Response, and Regenerable Metal–Organic Framework for Sampling Excess Fluoride Levels in Drinking Water,
  Fatmah Mish Ebrahim, Tu N. Nguyen, Serhii Shyshkanov, Andrzej Gładysiak, Patrick Favre, Anna Zacharia, Grigorios Itskos, Paul J. Dyson, Kyriakos C. Stylianou,
  J. Am. Chem. Soc. 2019.
  https://doi.org/10.1021/jacs.8b11907