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The detection of fluoride ions in a competitive environment often poses several challenges. In this work, we have designed and synthesized a coumarin functionalized fluorescein dyad (R3) which represents an ideal through bond energy transfer (TBET) fluorophore with the coumarin unit as donor and fluorescein unit as acceptor. The bichromophoric dyad demonstrates the detection of fluoride ions in the parts per billion (ppb) concentration level (22.8 ppb) with high selectivity via a TBET emission signal at 548 nm with a diagnostic bright yellow colour fluorescence output. Based on UV-visible, fluorescence, 1H NMR and DFT studies, it is shown that the fluoride ion induces the opening of the spirolactam ring of the fluorescein moiety and provides a π-conjugation link between the donor and acceptor units enabling a TBET phenomenon with a larger pseudo-Stokes shift of 172 nm. To the best of our knowledge, this is the first report where the fluoride ion is detected via a TBET signal between the coumarin and fluorescein units in a bichromophoric dyad.

Detection of ions in chemical, biological and environmental samples has gathered tremendous momentum considering the beneficial as well as adverse effects of the ions. Generally, most of the ions are beneficial up to an optimum concentration, beyond which they are toxic to human health. However, most of the fluorescence-based ion sensors are only active in non-aqueous solution because of the low solubility of the sensor molecules in aqueous buffer medium. In the present work, we have demonstrated that encapsulation of an aqueous insoluble thiocarbonohydrazone-locked salicylidene-based macrocyclic ligand in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes allows the selective detection of Zn2+ in aqueous medium with approximately 3-fold enhanced efficiency compared to its efficiency in DMSO medium. We have further modulated the charge of the membrane surface by adding various concentrations of a negatively charged lipid, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG), and showed that negative surface charge further enhances the Zn2+ sensing efficiency up to approximately 6-fold. This strategy opens up a new avenue of utilizing organic sensors to detect vital ions in aqueous medium.

This work reports the aggregation induced emission (AIE) behaviour of a coumarin-salicylidene Schiff base probe 3 which shows bright yellow AIE fluorescence in DMSO–water medium (fw 80%) buffered by 10 mM HEPES at pH 7.4. The aggregated probe (AIEgen-3) selectively discriminates Cu2+ ions over a series of other interfering metal ions via a fluorescence “on–off” strategy through a CHEQ process. DLS measurements revealed an average particle size of 372 nm for AIEgen-3 at an 80% water fraction, which dramatically increases to 656 nm in the presence of Cu2+ ions, indicating the formation of more structured aggregates, which was further supported by TEM and ESI-MS experiments. A comprehensive analysis of the binding characteristics and interference studies of AIEgen-3 with various metal ions have been carried out by fluorescence and UV-visible experiments. AIEgen-3 shows a fluorescence detection limit (LOD) of 24 nM (∼1.5 ppb) for Cu2+ ions, which is considerably far below the values of detection limits recently reported for other Cu2+ ion sensors in aqueous medium. Further, the analytical applications of AIEgen-3 have been demonstrated by the estimation of Cu2+ ions in tap water and pharmaceutical samples both in solution and the solid phase, which shows the potentiality of the probe for on-site detection of Cu2+ ions in chemical, environmental and biological samples.

The detection of fluoride ions in a competitive environment often poses several challenges. In this work, we have designed and synthesized a coumarin functionalized fluorescein dyad (R3) which represents an ideal through bond energy transfer (TBET) fluorophore with the coumarin unit as donor and fluorescein unit as acceptor. The bichromophoric dyad demonstrates the detection of fluoride ions in the parts per billion (ppb) concentration level (22.8 ppb) with high selectivity

Two chromofluorescent thiocarbonohydrazone receptors (4 and 5) functionalized with coumarin derivatives have been synthesized and evaluated for selective detection of fluoride ions. Receptor 4 selectively recognizes fluoride ions via H-bond interaction and subsequent deprotonation to elicit a distinct visual colour change from colourless to pink with a significant 20-fold enhancement in its emission intensity to “turn on” a blue fluorescent. In contrast, receptor 5 exhibited a visible colour change from colourless to deep red upon interaction with fluoride ions over other anions. Detailed analysis of the binding characteristics of these receptors with fluoride ions revealed a 1 : 1 binding stoichiometry at lower concentrations while higher concentrations led to a 1 : 2 binding stoichiometry between the receptor and fluoride ions. These experimental results were further corroborated with quantum chemical calculations.

Herein, for the first time, we have reported a copper(II) bis(terpyridine) complex solution for instant ‘naked eye’ chromo-luminescent selective detection of fluoride ions in an acetonitrile medium at micromolar concentration. The copper complex [Cu(II) (L)2] (NO3)2 [where L = 4′-(4-N,N′-dimethylaminophenyl)-2,2′:6′,2′′-terpyridine] was characterized by mass spectroscopy and the terpyridine ligand by 1H NMR spectroscopy. The complex solution selectively discriminates F− ions from other anions such as AcO−, Br−, Cl−, CN−, H2PO4−, HSO4−, and I− in acetonitrile media via exceptional optical changes. The optical changes were evaluated by UV-visible and fluorescence techniques. Studies on the binding characteristics of the copper complex solution with fluoride ions revealed a displacement of copper ions from the complex solution as CuF2 resulting in the significant optical changes. Furthermore, displacement of Cu(II) from the complex was established by means of mass spectroscopy in the presence of 20 equivalents of fluoride ions. The limit of detection (LOD) was found to be 5.07 μM which is within the permissible range of fluoride ions in drinking water set by the World Health Organization (WHO).