Researcher develops colour-changing fluoride sensor

A Florida State University (FSU) researcher has developed a molecular sensor that changes colour when a sample containing fluoride is added to it. The sensor could be used to help determine the level of fluoride in water, consumer products or the human body.

’The US Environmental Protection Agency recommends that one part per million (ppm) of fluoride ions is acceptable in drinking water, but above 2ppm is considered a serious health risk,’ said Sourav Saha, an assistant professor in FSU’s Department of Chemistry and Biochemistry.

’Because fluoride is carcinogenic, even at such small doses, a sensor is needed to detect fluoride selectively at very low concentrations and in the presence of other naturally occurring and biologically important ions.’

Working with a team of graduate students and postdoctoral researchers, Saha was able to develop just such a sensor. His research team discovered that a compound called naphthalene diimide (NDI) interacts with fluoride ions in a uniquely colourful way.

’NDIs are a family of neutral (albeit electron-deficient) aromatic compounds that are colourless until fluoride is added,’ Saha said. ’A small amount of fluoride will quickly turn the sample orange, while a larger amount will turn it pink. In this manner, it becomes very easy to determine not only the presence of fluoride in water, but at what levels.’

While other fluoride sensors exist, many of them cannot differentiate between fluoride and other anions − negatively charged ions − that might be present in the water.

’Although they can detect fluoride, they cannot accurately measure the levels of fluoride,’ Saha said. ’Naked-eye detection of fluoride at different concentration levels is an advantage of NDI-based sensors. Our sensor relies on an electron transfer from a fluoride ion to the NDI receptor for generating a visible response or signal, which in this case is colour change. The electron-transfer process can be reversed and the sensor can be regenerated and reused over and over again.’

The sensor Saha’s laboratory has developed can detect about one 10,000th of a milligram of fluoride in a litre of water, making it one of the most sensitive fluoride sensors known to date.