Adding fluoride to water has been common practice in a number of countries including the U.S. In low concentrations (below 1.5 mg/L), fluoride can help prevent tooth decay and strengthen bones; however, at higher levels, it can have the opposite effect, causing serious dental and bone disease, especially in children and developing fetuses. To determine whether drinking water is safe, fluoride must be detected in water at a parts-per-million (ppm) level. Measuring fluoride at such low concentrations with sufficient accuracy is expensive and requires a well-equipped chemical lab.

The prototype device used with SION-105 to detect fluoride anions in drinking water. (Credit: Marie-Thé and Etienne Roux)

A device was developed that can accurately measure fluoride concentrations using only a few drops of water even with low-level contamination, resulting in a simple change in color brightness. The device, named SION-105, is portable, considerably less expensive than current methods, and can be used on-site by virtually anyone.

A diagram of the SION-105 prototype device. A vial containing a red luminescent suspension of SION-105 crystals is placed inside the device. When two drops of water are added to the vial, changes in the brightness of SION-105 are measured by the photodiode, and instantly converted into a quantitative reading of the fluoride concentration in the sample. The inset shows how the luminescence is quenched at the molecular level. (Credit: Mish Ebrahim)

The key to the device design is a novel material that was synthesized (SION-105). The material belongs to the family of metal-organic frameworks (MOFs) — compounds made up of a metal ion (or a cluster of metal ions) connected to organic ligands, thus forming one-, two-, or three-dimensional structures. Because of their structural versatility, MOFs can be used in an ever-growing list of applications such as separating petrochemicals, detoxing water, and removing hydrogen or gold from water.

SION-105 is luminescent but darkens when it encounters fluoride ions. Adding a few droplets of water and monitoring the color change of the MOF indicates whether it is safe to drink the water. The researchers used the device to determine the fluoride content in different groundwater samples from three geographic areas. The data corresponded very well when compared to measurements made using ion chromatography, a standard method for measuring fluoride concentration in water.

For more information, contact Nikolaos Papageorgiou at This email address is being protected from spambots. You need JavaScript enabled to view it.; +41 21 69 32105.

Tech Briefs Magazine

This article first appeared in the September, 2019 issue of Tech Briefs Magazine.

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