The addition of fluoride to drinking water or toothpaste is widespread and is mainly used for dental health and as a preventative measure against bone diseases. However, an excessively high concentration of fluoride can lead to health problems. Clear symptoms of this fluorosis, that occurs in developing countries in particular, are brownish-yellow stains on the teeth. Fluorinated drinking water is not being monitored everywhere so that children in particular can ingest too much fluoride. BAM Federal Institute for Materials Research and Testing has developed a very simple rapid colour test, which can help determine the fluoride concentration more easily in the future.
A change in the colour of a paper strip shows how much fluoride is dissolved in the water, similar to a litmus test to determine the pH value. The concentration is indicated by yellow-green dots, the darker the spot, the more fluoride is present. A dye developed by BAM is used in the procedure. "This BODIPY amidothiourea dye is applied as a small spot to a strip of nitrocellulose that responds to the fluoride ion" says Pichandi Ashokkumar, who, together with Knut Rurack, has developed the test.
The paper strip is then immersed in a glass of water for example. The analyst can read the concentration by the change in the perceived colour and the test strip is reusable because of the particular chemical dye chosen. As simple as it sounds, it was very difficult to implement because fluoride is bound to thiocarbamide by hydrogen bonds. Bonding triggers an electron transfer, which weakens the dye’s fluorescence so that the dye will appear darker. "The trick then was to develop a process that, in spite of the aqueous environment, still binds the fluoride and not the water's oxygen to the thiocarbamide" says project manager Knut Rurack. As the tests indicate, the scientists have succeeded. In order to evaluate the colour change better than with the human eye, the test strip’s colour may also be detected with the built-in camera of a mobile phone.
Fluorination of drinking water is common practice. In Europe this is practiced in the UK, Ireland and Switzerland and the U.S. also fluorinates the drinking water. In 2011, the U.S. Environmental Protection Agency (EPA) reduced the benchmark for fluoride in drinking water from 1 to 0.7 parts per million (ppm) and thus joined other countries. The detection limit of the new fluoride test is 0.2 ppm. The test was conducted using different water samples, including tap and sea water. "The test is not disturbed by other ions in tap water or salt water," says Rurack. However, too high a concentration of phosphate, as in some toothpastes, may cause problems. The scientists envisage a potentially large field of application for their testes especially in Asia and Africa.