Sensing Fluoride Ion Research Articles

DNA-flavonoid pH sensitive host–guest mechanism to sense fluoride ions and construction of molecular logic gates

A fisetin derivative, 2-(3,4-diethoxyphenyl)-3-hydroxy-4H-benzo[h]chromen-4-one (DEPHBC), binds to the minor groove of DNA-duplex unlike its intercalating precursor. DEPHBC shows efficient excited state intramolecular proton transfer (ESIPT) on binding to the DNA-groove due to facilitation of intramolecular hydrogen bonding. The enhanced tautomer fluorescence of DEPHBC is used to sense fluoride ions (F−) very effectively at physiological pH. The sensibility is found to be significantly affected on minor alteration in the pH of the environment. Moreover, the results were used to fabricate DNA-based molecular logic devices using the threshold fluorescence yields for the ON and OFF signals. An IMPLICATION logic gate is constructed using acid and F− ions while the DNA and F− ion inputs form an INHIBIT logic gate.

Optical Anion Receptors with Urea/Thiourea Subunits on a TentaGel Support.

Two simple chemosensors with urea (L1) and thiourea (L2) groups were synthesized and studied by different spectroscopic techniques. Both receptors can sense acetate (Ac–), dihydrogen phosphate (H2PO4–), and fluoride (F–) anions, accompanied by changes in UV–vis and 1H NMR spectra, and an optical response is observed as a color change of the solutions due to deprotonation and hydrogen-bonding processes. Also, L1 and L2 were supported on TentaGel resins (R1 and R2), and their fluoride-sensing properties in DMSO and water solutions were studied. Interestingly, R2 can sense fluoride ions in sample solutions of 100% water.

Non-covalently functionalised rGO–fluorescein unit for selective detection of fluoride ions

The combination of reduced Graphene oxide and Fluorescein dye serves as a highly selective optical switch for sensing fluoride ion via colorimetric and fluorimetric modes. A turn off stage viz., quenching of optical signal of fluorescein was observed by the formation of a supramolecular assembly between reduced graphene oxide (host matrix) and the dye (indicator). This OFF stage was turned ON instantly with the advent of fluoride ion (guest), with high selectivity. The limit of detection values obtained are 4.2 ppm and 1.7 ppb respectively, via colourimetric and fluorimetric strategy. Solid state test strips of the sensor unit could detect fluoride in aqueous media at concentration down to 3 ppm with the help of UV source.

A study of silylated tris(styryl)benzenes as potential fluorescent sensors for aqueous fluoride

Different tris(styryl)benzenes bearing a tert-butyldimethylsilyl (TBDMS) group at the periphery have been efficiently prepared by a straightforward protocol. The potential of these compounds as fluorescent sensors for fluoride has been studied. In all cases, this anion caused significant quenching of the emission intensity upon cleavage of the silyl ether group and the formation of the corresponding phenolate anion. All compounds displayed good selectivity over a variety of other anions. They could sense fluoride ions in aqueous solutions with sensitivity in the lower μM range when dissolved in DMSO, demonstrating the validity of this approach.

Development of porphyrin-based chemosensor for highly selective sensing of fluoride ion in aqueous media

A porphyrin-based receptor to sense fluoride ion via hydrogen bonding has been prepared and investigated in aqueous media. The interactions between receptor and fluoride ion are confirmed by absorption and emission spectroscopy, and the influence of pH on the sensor performance has also been investigated. A dramatic color change from wine red to green, and photoswitching from the fluorescent on-state to the non-fluorescent off-state were observed after addition of fluoride ion to chemosensor POR. The detection limit for fluoride ion was calculated to be around 10 µM by both the colorimetric and fluorometric methods, which is under the tolerable amounts in drinking water by WHO guidelines. The Jobs plot showed a 1:2 reaction stoichiometry between POR and fluoride ion, which was further confirmed by mass analysis. The experimental results are fully supported by the DFT theoretical calculations. Furthermore, the changes in structural alignment of POR during fluoride detection were also proposed.

Proton-Triggered Fluorescence Switching in Self-Exfoliated Ionic Covalent Organic Nanosheets for Applications in Selective Detection of Anions.

The exfoliation of covalent organic frameworks into covalent organic nanosheets (CONs) not only helps to reduce fluorescence turn-off phenomena but also provides well-exposed active sites for fast response and recovery for various applications. The present work is an example of rational designing of a structure constructed by condensing triaminoguanidinium chloride (TGCl), an intrinsic ionic linker, with a fluorophore, 2, 5-dimethoxyterephthalaldehyde (DA), to produce highly fluorescent self-exfoliable ionic CONs (DATGCl-iCONs). These fluorescent iCONs are able to sense fluoride ions selectively down to the ppb level via the fluorescence turn-off mechanism. A closer look at the quenching mechanism via NMR, zeta potential measurement, lifetime measurement, and density functional theory calculations reveals unique proton-triggered fluorescence switching behavior of newly synthesized DATGCl-iCONs.

Dioxetane-based chemiluminescent probe for fluoride ion-sensing in aqueous solution and living imaging

Fluoride plays an important role in our daily life as it associates with a variety of biological and pathological processes. Hence, an analytic method that can detect fluoride ions under physiological conditions with high sensitivity and specificity, and fast response, is needed. Herein, we introduced a dioxetane-based chemiluminescent probe, CL-F, specifically designed for the detection of fluoride ions in physiological environment. The probe CL-F emitted strong green chemiluminescent light within 1 min when incubating with fluoride ions specifically. The linear range for sensing fluoride ions and limit of detection were determined to be 0–30 μM and 0.91 μM, respectively. The probe CL-F was then successfully applied in quantifying fluoride ions in toothpaste and in vivo imaging in living mice, demonstrating CL-F as a promising tool for sensing fluoride ion in vitro and in vivo.

Graphene-dye supramolecular assembly for parts per trillion level F− monitoring

An innovative and simple displacement approach based on Graphene and Rhodamine supramolecular assembly that could sense fluoride ion in aqueous solution is demonstrated. Graphene-Rhodamine unit engages in the formation of a supramolecular complex between Rhodamine (indicator) and Graphene (receptor), followed by the displacement of the indicator with the advent of F− ion. These changes alter optical characteristics of the supramolecular entity which can be quantified using spectroscopic techniques. The findings of UV–vis and Photoluminescence analysis together with the life-time measurement studies reveal that through the interactions of Graphene sheets with Rhodamine dye quenches the optical response of the dye which can be selectively restored by introducing traces of F−. The detection limit was found to be 11 ppb and 850 ppt by UV–vis and Photoluminescence studies respectively, falling admirably below the permissible F- level in drinking water i.e. 1.5 ppm. The proof of concept for this sensing strategy is demonstrated through paper-based testing strips that can function as a portable layman’s tool for sub ppb level detection of the test anion. Investigations on real water samples spiked with Fluoride ion assures that the proposed sensor moiety can swiftly monitor the test ion levels in complex samples and affirm the utility of the system for practical applications.

Coumarin Probe for Selective Detection of Fluoride Ions in Aqueous Solution and Its Bioimaging in Live Cells.

We have synthesized novel coumarin-based fluorescent chemosensors for detection of fluoride ions in aqueous solution. The detection mechanism relied on a fluoride-mediated desilylation triggering fluorogenic reaction and a strong interaction between fluoride and the silicon center. In this work, the hydroxyl-decorated coumarins containing oxysilyl moiety have been synthesized through the aldehyde-functionalized coumarins. The optical responses toward fluoride, as well as aqueous stability studies of both aldehyde and hydroxyl functionalized coumarins, have been investigated. Due to the highest fluorescence enhancement upon the addition of fluoride and good stability in aqueous solution, the hydroxyl-decorated coumarin connected with the bulky tert-butyldiphenyloxysilyl group (-OSitBuPh2) has been selected for further investigation of its potential as a fluoride sensor. This hydroxyl-decorated coumarin can selectively sense fluoride ions in aqueous media (contain 0.8% MeCN) with desirable response times (40 min). The limit of detection of this compound was determined as 0.043 ppm, satisfying the standard fluoride level (0.7 ppm) in drinking water recommended by U.S. Department of Health and Human Services. The application of this silyl-capped coumarin derivative for fluoride analysis in collected water samples displayed satisfactory analytical accuracy (<5% error). Finally, this compound was successfully employed in fluorescence bioimaging of fluoride ions in human liver cancer cells, indicating its excellent cell permeability, ability to retain inside the living cells, and good stability under physiological conditions.

A2B corroles: Fluorescence signaling systems for sensing fluoride ions

Four free base corroles, 1–4, A2B, (where A = nitrophenyl, and B = pentafluorophenyl, 2, 6-difluoro, 3, 4, 5-trifluoro and 4-carboxymethylphenyl group) have been synthesized, characterized and demonstrated as excellent chemosensor for the detection of fluoride ions selectively in toluene solution. The reported corroles shows highest quantum yield in free base form of porphyrinoid systems so far. All these corrole, 1–4, have the excellent ability to sense fluoride ion. Cumulative effect of static and dynamic factors is responsible for the quenching of fluorescence which indicates the detection of fluoride ion in solution.

Novel luminescent lanthanide(iii) hybrid materials: fluorescence sensing of fluoride ions and N,N-dimethylformamide.

Two novel luminescent organic-inorganic hybrid materials, Ln(L-SBA15)3phen (Ln = Eu, Tb), were designed and synthesized. The organic ligand phenylurea was modified by the silane coupling agent 3-(triethoxysilyl)propyl isocyanate (TESPIC) to form a precursor (L), which was covalently bonded onto a mesoporous SBA-15 backbone to form mesoporous L-SBA15 through co-hydrolysis and co-condensation reactions. 1,10-Phenanthroline monohydrate (phen) was selected as the second ligand to improve the luminescence of the final products, and the two mesoporous hybrid materials Ln(L-SBA15)3phen were obtained after the coordination reaction between the organic ligands (phen and L-SBA15) and Ln(iii) ions. Both the mesoporous hybrid materials were examined by Fourier transform infrared spectroscopy, transmission electron microscopy, small-angle X-ray diffraction, N2 adsorption-desorption curves, and photoluminescence spectroscopy. The results show that both the hybrid materials showed highly ordered mesoporous structures, high surface areas, and excellent photophysical properties (long luminescence lifetimes and high quantum efficiencies). Furthermore, the fluorescence sensing properties of the materials were investigated systematically, and the hybrid materials were revealed to be promising examples of dual functional materials with good ability to sense fluoride ions and a small organic molecule N,N-dimethylformamide.

Colorimetric and NIR fluorescence receptors for F− ion detection in aqueous condition and its Live cell imaging

Abstract In this paper, a new kind of colorimetric receptors R1 and R2 for fluoride ions have been synthesized and characterized by various spectral techniques. Single crystal X-ray diffraction study confirms, the receptor R2 possesses the orthorhombic centrosymmetric lattice with the donor atoms of ONS. In naked eye, absorption and fluorescence studies were evidently predicting the potential sensing capability of the receptors R1 and R2 that detect fluoride ions by competing with various ions. The detection limit was found to be more or less same as low as 10 −7 M for R1 and R2 with F − . We proposed the 1:1 binding formation of R1 and R2 by means of F − ion was further supported by NMR titration and Job’s plot analysis. Besides, the density functional theory (DFT) calculations have been theoretically sustained with the sensing mechanism. Moreover, both the two receptors had the possibility of potentially applied in an analytical application for detecting the trace amounts of fluoride ion in the real water sample analysis was demonstrated. Live cell imaging experiments with the receptors displayed that these receptors extensively applicable to detect fluoride ions in HeLa cells under physiological conditions. Finally, the use of test strips containing R1 and R2 that sense fluoride ions in solution medium and supported on solid medium (silica) was exhibited.

Selective colorimetric sensing of fluoride ion via H-bonding in 80% aqueous solution by transition metal chelates

A series of four coordinated transition metal chelates of composition [M(R1)2], where M=Cu(II), Co(II), Ni(II) and Zn(II) and R1=quinone-imidazole ensemble, have been prepared and characterized by using standard analytical and spectral techniques. These metal chelates selectively (over many anions) and sensitively (detection limit down to nM) sense fluoride ion colorimetrically with an instantaneous striking colour change from yellow to red. They do so in H2O:DMF (80:20% v/v) medium, while free R1 imparts similar colour change only in DMF. The mechanism of sensing involves formation of H-bond between imidazole NH group and fluoride ion. The chelates binds strongly (Ka 107M−1) with fluoride ion to form a 1:2 (receptor:fluoride) complex. The coordination of transition metals to R1 not only enhances the H-bond donor ability of the NH group but also accommodates 80% of water in the sensing medium, which is a novel finding. Electrochemical and theoretical calculations have also been used to substantiate the results obtained in the spectral studies. The spectral data were applied to construct logic gate at molecular level.

Colorimetric and ratiometric fluorescent sensor for F− based on benzimidazole-naphthalene conjugate: Reversible and reusable study & design of logic gate function

A novel benzimidazole-naphthalene conjugate molecule has been successfully designed and applied as a colorimetric and ratiometric fluorescent sensor for direct visual detection of F− with high selectivity over other anions. Addition of F− to the sensor resulted in an obvious color change from colorless to pale green under ambient light and blue to green under 365 nm UV light, respectively. 1H NMR and DFT calculations studies confirm that the recognition towards F− is due to the formation of an NH⋯F hydrogen bond and subsequent deprotonation. In addition, the sensor can be reversible and reusable for sensing fluoride ion in the presence of H+, thus it can be developed as an INHIBIT type logic gate at molecular level. Moreover, the sensor with high sensitivity and fast response has also been successfully applied to the detection of F− from inorganic origins and in the solid state.

Design and application of tri-benzimidazolyl star-shape molecules as fluorescent chemosensors for the fast-response detection of fluoride ion

Three tri-benzimidazolyl star-shape molecules have been successfully designed and applied as new fluorescent chemosensors for the detection of fluoride ion for the first time. They are easily obtained by the simple condensation of o-phenylenediamines and trimesic acid in the presence of polyphosphoric acid as catalyst. Three chemosensors show good selectivity, high sensitivity, and fast response toward F−. The addition of fluoride ion to the chemosensors causes a dramatically observable colour change from blue to light cyan not only in solution, but also in solid state under UV light. 1H NMR and DFT calculations studies confirm that the F− ion facilitates its recognition due to the formation of NH⋯F hydrogen bond and subsequent deprotonation as designed. In addition, chemosensors 3 can be reversible and reusable for sensing fluoride ion, resulting that they can be developed as an IMPLICATION type logic gate at molecular level. Moreover, these chemosensors have also been successfully applied to the detection of fluoride ion from inorganic origin and commercial toothpaste sample.

Fluorescent carbon nanodots for optical detection of fluoride ion in aqueous media

In this work, a nanoprobe for sensing fluoride ion in aqueous solution has been introduced. The operation of this system is based on the quenching of the fluorescence signal of carbon nanodots (CDs) with a simple organic receptor. The linear trend in fluorescence signal with respect to fluoride concentration proved that Stern-Volmer equation was adapted successfully to the system with a linear range of 0–26.7mM and LOD value of 1.1×10−4M. This system showed to be nonresponsive toward weak basic anions except for fluoride. Using toothpaste as a real sample, the level of fluoride was successfully measured by this method.

Approach to modulate the sensing range of molecular transducers

Transduction occurs upon binding of an analyte with a transducer. Specific binding involves the design and synthesis of receptors that is non trivial. Indeed, the task of designing a receptor for the smallest anion fluoride is very challenging. Herein, we take advantage of the basicity of fluoride ions that has affinity toward acidic proton. A commercially available 6-bromoisatin can sense fluoride ion in the concentration range of 0.5–3.9ppm. Although the sensor is reaction based, the response is rapid. To further increase the linear range, dibromoisoindigo was synthesized. The linear range of this transducer was found to be between 0.5ppm and 10.4ppm. The transducer can be regenerated by adding proton source such as trifluoroacetic acid. Thus, transducers with variable sensing range have been designed and synthesized using acid base interaction.

Diketopyrrolopyrrole-based fluorescent conjugated polymer for application of sensing fluoride ion and bioimaging

Diketopyrrolopyrrole (DPP)-based polymer (P1) was designed as the conjugated amplifying polymer for sensing fluoride ion and bioimaging. The delocalized electronic structure of this polymer was composed by copolymerization of DPP and fluorene monomers in the main chain. Some unalkylation imide moieties in DPP monomer (1) offered emission traps for the excited state (exciton) transport. Fluoride ion was induced as quencher to interrupt electronic energy transfer (EET) by strong deprotonated interaction. The obvious quenching of fluorescence and red-to-purple color change shows P1 can act as the great promise sensor for the detection of fluoride ion. Furthermore, P1 was fabricated to polymeric nanoparticles for bioimaging. This report provides a feasible way to construct the useful fluorescent sensing and bioimaging materials.

Colorimetric detection of fluoride ion by 5-arylidenebarbituric acids: dual interaction mode for fluoride ion with single receptor

Two 5-arylidenebarbituric acid derivatives (IH and IM) have been synthesized by the Knoevenagel condensation of barbituric acid with 4-N,N-dimethylamino benzaldehyde and studied for anion sensing activities. Both receptors sense fluoride ion with high selectivity and sensitivity and the sensing action has been demonstrated by naked eye detection, UV-visible absorption, and fluorescence spectral changes in the presence of F(-). Indeed, the F(-) sensing mechanism for receptor IH depends on F(-) ion concentration. While at higher concentrations F(-) forms strong hydrogen bonding interaction with the N-H proton of the receptor, at lower concentrations sensing is influenced by the deprotonation of the methylene proton, followed by the chemical reaction, which is also confirmed by the (1)H-NMR technique. On the other hand, when replacing the N-H proton with a methyl group, IM does not show any concentration dependent behaviour with F(-). The F(-) concentration dependent sensing is attributed to the changes in the receptor-anion interaction equilibrium, where at higher F(-) concentrations, F(-) interacts with the receptor through hydrogen bonding and at lower concentrations it induces a chemical reaction.

Tuning of the H-bonding ability of imidazole N–H towards the colorimetric sensing of fluoride and cyanide ions as their sodium salts in water

Imidazole functionalized receptors, 2-R-1H-naphtho[2,3-d]imidazole-4,9-dione (2a–h), containing naphthoquinone as a chromogenic signalling unit have been synthesized from the reaction of 2,3-diaminonaphthoquinone and different aldehydes. These receptors showed a color change upon addition of fluoride and cyanide ions in DMSO with a bathochromic shift of the characteristic intramolecular charge transfer (ICT) transition band. No color change was observed upon addition of other anions such as Cl−, Br−, I−, NO3−, AcO− and H2PO4−. 1H NMR and electrochemical studies revealed that these receptors sense fluoride and cyanide ion via the formation of H-bond with the imidazole N–H moiety. Electronic and spectrofluorimetric studies indicated that the binding constants of these receptors with F− and CN− ions were in the order of ∼106. The results of the spectral studies indicated that, by changing the R group in the receptor, the acidity of the imidazole N–H can be varied from δH 13.70 (for isopropyl) to 14.94 ppm (for thiophene). Theoretical calculations based on Density Functional Theory showed that the HOMO–LUMO energy gap for the ICT transition corroborate the results of the spectral studies. Receptor 2f (R = thiophene) was also able to detect fluoride and cyanide ions as their sodium salts in aqueous solution with a visual color change.