Finding Fluoride Via Electron Transfer

With the help of an unusual anion-π orbital interaction, scientists have created a chemical sensor that changes color in the presence of fluoride (J. Am. Chem. Soc., DOI: 10.1021/ja107382x). The sensor molecule could be used to detect fluoride in drinking water and consumer products, as well as in bone and muscle tissue for the early detection of fluoride-related diseases. The new sensor, developed by Florida State University’s Sourav Saha and Samit Guha, makes use of a π-electron-deficient naphthalene diimide (NDI) receptor. Strong interactions between fluoride’s lone pair of electrons and NDI’s π*-orbitals lead to an unprecedented F^– to NDI electron-transfer event, which produces an orange NDI radical anion. Additional fluoride reduces the sensor to a pink NDI dianion. The sensor is specific for fluoride and remains colorless in the presence of other anions, such as chloride or iodide. “The selectivity and reusability of NDI-based sensors distinguish them from existing fluoride sensors,” Saha notes. By tethering two NDI moieties together in overlapping positions with intramolecularly hydrogen-bonded folded polyamide linkers, Saha and Guha were able to push the sensor’s sensitivity into the nanomolar range.