Excess fluoride in drinking water can have severe implications for human health, with effects ranging from dental and skeletal fluorosis, to the potentially fatal osteosarcoma (cancer of the bones). 

Pradyut Ghosh and colleague, from the Indian Association for the Cultivation of Science in Kolkata, have developed a tripodal receptor based on a 1,3,5-methyl substituted benzene ring with nitro substituted aryl terminals, which can effectively encapsulate fluoride anions in solution. This bowl-shaped molecule is electron deficient, and upon exposure to electron rich fluoride anions, forms a dimeric capsule which sequesters the potentially dangerous fluoride anions away from the bulk of the solvent medium. 

The recognition and sensing of fluoride in aqueous media has been a problematic area of research, due to thermodynamic constraints related to the high hydration energy of fluoride. In a recent feature article published in ChemComm, Cametti and Rissanen postulated that when designing fluoride ion receptors and sensors, a partially hydrated fluoride should be the target species instead of a naked fluoride anion.¹ Ghosh has expanded on this work by targeting a hydrated fluoride cluster ([F₂(H₂0)₆]^2-), which forms when the fluoride ion is dissolved in aqueous media. It is this cluster that acts as a template for the assembly of the dimeric capsule. This method of assembly is specific to the fluoride anion, as other anions (such as chloride, nitrate and acetate) cause the receptor to dimerise via a zipper-like mechanism and form a non-capsular aggregate. 

This technology could also be used to isolate reactive intermediates, which could help towards the understanding of reaction mechanisms, although the potential application of the method for the removal of fluoride anions from drinking water shows great promise. 

Katie Dryden-Holt