Fluoride-philic reduced graphene oxide–fluorophore anion sensors

Abstract

The detection and quantification of the fluoride ion, one of the most significant anions, have attracted much research interest because of its striking role in oral/bone health and clinical treatment of osteoporosis. A set of F⁻ ion sensors have been reported here, which operate through the fluorescence turn-on response of reduced graphene oxide (rGO)–fluorophore noncovalent conjugate. The developed sensing systems perform well at neutral pH in aqueous solutions, and the response towards F⁻ ions was initiated within seconds. The high specificity for these optical sensors towards F⁻ ions, among a set of significant competing anions, was notable. For the three fluorescent organic dyes selected for study, tetraphenylporphyrin (D_TPP), curcumin (D_CURN), and coumarin (D_CMN), the low-level detection (LOD) ability increased with an increase in π-interactions between rGO and the fluorophore. A fall in LOD to the attomolar level could be achieved for the rGO-D_TPP system. The turn-on fluorescence strategy was extended further to develop solid-state sensor strips for F⁻ ion detection at the attomolar level. The fluoride-philic nature of rGO–fluorophore systems was traced by systematic investigations using FT-IR, XRD, and XPS techniques, which revealed that the interaction between the most electronegative F⁻ ion and the rGO in the sensor unit leads to the formation of the stable compound graphite fluoride, and this conversion in turn switches on the quenched fluorescence of the fluorophore.