Development of solution, film and membrane based fluorescent sensor for the detection of fluoride anions from water

Abstract

The synthesis and application of a neutral polymer, poly(1,4-bis-(8-imidazole-octyloxy)-benzene) (PPI), is performed by economical and simple reaction steps. The PPI polymer demonstrates exemplary activity to be used as a film on a TLC plate, or as a membrane by blending it with a desired polymer or in a solution phase to detect fluoride anions from contaminated water in the presence of competing anions at ppb levels easily and rapidly. This polymer PPI works on the simple displacement principle where fluorescence turn-on/turn-off are observed as signals. On selectively binding Cu²⁺ it displays extraordinary fluorescence quenching, resulting in >97% reduction in the fluorescence intensity. This effect could be visualized in solution phase, on a TLC plate and on a blended polymer membrane. Furthermore, the fluorescence of this PPI–Cu²⁺ assay showed 81% enhancement on selectively binding F⁻ anions in contaminated water in the presence of other competing anions with higher positive free energies of hydration. Polymeric systems with such robust fluorescence dequenching activity are novel, providing a unique platform for detection and possible removal of fluoride anions. To validate this potential, two experiments were performed: (a) preparation of a film on a TLC plate and (b) preparation of a membrane by mixing 1% PPI in polystyrene and casting as a membrane film of desired shape and thickness. Our results confirm that PPI–Cu²⁺ films and membranes described above have the highest specific activity to sense fluoride in a competitive environment, observed by the unique enhancements in fluorescence intensities at varying and extremely low quantities of 1 ppm, 10 ppm and 50 ppm of fluoride. The detection limit of fluoride in contaminated water for the TLC plate and membrane methods was very low and was in the range of 2.5–10.0 ppb. We have further used these methods for the detection of fluoride in natural ground water samples and ascertained the percentage of fluoride.