Selective fluoride sensing using organic-inorganic hybrid nanomaterials containing anthraquinone

Anthraquinone-based fluorescent receptor 1 was immobilized on mesoporous silica (AFMS) or on silica particles (AFSP) via a sol-gel reaction. The sensing abilities of AFMS and AFSP were studied by addition of the anions F^-, Cl^-, Br^-, I^- and HSO₄^- to water suspensions of the assayed solid. The addition of fluoride ions to a suspension of AFMS resulted in a large decrease in the fluorescence intensity of the anthraquinone of AFMS. Thus, fluoride ions bind to two urea N-H protons of receptor 1 in AFMS. In contrast, the addition of Cl^-, Br^-, I^- or HSO₄^- did not reduce the fluorescence of AFMS. In the case of AFSP, the sensitivity for fluoride ions was 10 times lower than that of AFMS due to the immobilization of smaller amounts of receptor 1 on the silica particles. A linear response of AFMS upon the addition of fluoride ions was observed between 0.50 μM and 10.0 μM, with a detection limit of ∼0.50 μM. These results suggest that mesoporous silica with its large surface area is useful as a supporting material. Furthermore, receptor 1 was also immobilized on a glass slide surface by a sol-gel reaction. The fluorescence of 1 immobilized on a glass slide was quenched when dipped into F^- solution. On the other hand, no fluorescence change was observed in Cl^- solution. These results imply that 1 immobilized on a glass slide is applicable as a portable fluorescent sensor for detection of fluoride ions in the biological and environmental fields.