Water-Soluble Photoluminescence <i>On–Off–On</i> Probe for Speedy and Selective Detection of Fluoride Ions

A CdTe QDs based new type of water-soluble switch <i>on–off–on</i> photoluminescence (PL) probe has been developed for specific detection of fluoride ions (F^–). Europium ions (Eu³⁺) stabilized by the carboxylic groups of 3-mercaptopropanoic acid (MPA) capped CdTe QDs result in quenching of PL (switch <i>off</i>) of QDs. PL is regained (switch <i>on</i>) with the addition of F^– due to the formation of EuF₃ as Eu³⁺ has higher bonding affinity with F^– compared to carboxylic acid groups. The quenching of the PL property of CdTe-MPA QDs toward Eu³⁺ and its regeneration in the presence of F^– is highly selective, sensitive (detection limit 5 ppm to 75 ppm), and prompt (less than 10 s). The technique has been successfully applied for the detection of HF vapors, which is actually an important issue from the industrial perspective. To the best of our knowledge, this is the first report of the use of one step synthesized water-soluble 3-MPA capped CdTe QDs, as a rapid, efficient, and most importantly selective photoluminescence <i>on–off–on</i> probe for fluoride ion detection