A Schiff-based colorimetric fluorescent sensor with the potential for detection of fluoride ions

Excited State Intramolecular Proton Transfer (ESIPT)-Based Sensor for Ion Detection

C-2 and C-5 substituted imidazole skeleton was synthesized through a one-pot two-step strategy. Synthesized molecule emits the light on ESIPT (excited-state intramolecular proton transfer). This molecule was utilized for its proton donor ability, and we have observed that fluoride and cyanide ions can be detected selectively. Different cations and anions were selected to observe the response of the synthesized molecule. However, there were not any fluorometric and colorimetric response except for fluoride and cyanide ions. Detection limits of fluoride and cyanide ions were found to be 9.22 μM and 11.48 μM, respectively. ¹H-NMR spectra for the solution of the sensor and TBAF (tetrabuthylammoniumfluoride) were used for the identification of [L]^-[HF₂]^- species. 3 equiv. TBAF saturated the solution of the sensor in d₆-DMSO, and some of the proton resonances shifted to upfield due to the through-bond effect. The disappearance of NH proton with 0.5 equiv. TBAF or TBACN (tetrabuthylammoniumcyanide) showed that there was a proton abstraction by fluoride and cyanide ions, instead of the hydrogen bond. Solid-state application was utilized, and paper test strips were applied. Emission differences emerged when the sensor loaded strips were reacted with TBAF. Time resolved experiments revealed that solution of the sensor and TBAF in DMSO have multiexponential decay, and one of the lifetime was measured as 13.4 ns.

Highly Sensitive Ratiometric Fluorescent Paper Sensors for the Detection of Fluoride Ions

Two sensitive and ratiometric fluorescent probes (probe I and probe II) were developed for the detection of fluoride ions. Probe I can detect fluoride ions quantitatively within a range of 0-6 μM and a detection limit of 73 nM, while probe II has a range of 0-40 μM and a detection limit of 138 nM. The test strips from probe I are quickly able to recognize F- (5 min) inside of the F- safety level in drinking water (1.0 mg/L, ∼5 μM) under 254 nm ultraviolet light, and the test strips from probe II quickly recognize F- (12 min) in dangerously high F- levels in water (4.0 mg/L, ∼21 μM) under 254 nm ultraviolet light. This combination of fluorescent paper sensors from probe I and probe II can be used as a simple and convenient tool to determine whether water is safe to drink or dangerous.

Three-Fold C 3-Symmetric Off-On Fluorescent Chemo-Sensors for Fluoride

Three novel C 3-symmetric tris-salicylaldimine Schiff base based "off-on" fluorescence sensors have been designed and synthesized. The synthetic approach involves a simple imine bond (C = N) formation between 1,3,5-tris(4'-aminophenyl)benzene (TAPB) and 3,5-substituted salicylaldehydes. The presence of salicylaldehyde units on periphery of the tris-salicylaldimine Schiff bases masks the fluorescence of TAPB core. Interestingly, binding of fluoride ions to the salicylaldehyde units turns the fluorescence "on" in visible region. Fluoride ion detection limits for the present sensors have been calculated to be in the range of 0.17-3.0 ppm, which depends on the nature of the salicylaldehyde units.

A new dual fluorogenic and chromogenic "turn-on" chemosensor for Cu²⁺/F⁻ ions

Turn "off-on" chemosensor 2-(-2-((3',6'-bis(diethylamino)-3-oxospiro[isoindoline-1,9'-xanthen]-2-yl)imino)ethylidene)-N-phenylhydrazine-1-carbothioamide (RBS) was designed and synthesized. Using the naked eye, RBS showed favorable observation characteristics with both Cu(2+) and F(-) ions. The various modes of sensitivity shown by RBS toward the Cu(2+) and F(-) ions were investigated by spectral techniques, including UV-Vis, fluorescence and (1)H NMR spectroscopy. The Job's plot indicated the formation of 1:1 complex between RBS and Cu(2+)/F(-). The binding constant of the RBS-guest(-) complexes were found to be 1.3×10(4) and 6.2×10(3)M(-1) for the RBS-Cu(2+) and RBS-F(-), respectively.