Recognition and sensing of fluoride anion

Fluoride export (FEX) proteins from fungi, plants and animals are 'single barreled' channels containing one functional and one vestigial ion pore

The fluoride export protein (FEX) in yeast and other fungi provides tolerance to fluoride (F-), an environmentally ubiquitous anion. FEX efficiently eliminates intracellular fluoride that otherwise would accumulate at toxic concentrations. The FEX homolog in bacteria, Fluc, is a 'double-barreled' channel formed by dimerization of two identical or similar subunits. FEX in yeast and other eukaryotes is a monomer resulting from covalent fusion of the two subunits. As a result, both potential fluoride pores are created from different parts of the same protein. Here we identify FEX proteins from two multicellular eukaryotes, a plant Arabidopsis thaliana and an animal Amphimedon queenslandica, by demonstrating significant fluoride tolerance when these proteins are heterologously expressed in the yeast Saccharomyces cerevisiae. Residues important for eukaryotic FEX function were determined by phylogenetic sequence alignment and functional analysis using a yeast growth assay. Key residues of the fluoride channel are conserved in only one of the two potential fluoride-transporting pores. FEX activity is abolished upon mutation of residues in this conserved pore, suggesting that only one of the pores is functional. The same topology is conserved for the newly identified FEX proteins from plant and animal. These data suggest that FEX family of fluoride channels in eukaryotes are 'single-barreled' transporters containing one functional pore and a second non-functional vestigial remnant of a homologous gene fusion event.

Recent Advances in Macrocyclic Fluorescent Probes for Ion Sensing

Small-molecule fluorescent probes play a myriad of important roles in chemical sensing. Many such systems incorporating a receptor component designed to recognise and bind a specific analyte, and a reporter or transducer component which signals the binding event with a change in fluorescence output have been developed. Fluorescent probes use a variety of mechanisms to transmit the binding event to the reporter unit, including photoinduced electron transfer (PET), charge transfer (CT), Förster resonance energy transfer (FRET), excimer formation, and aggregation induced emission (AIE) or aggregation caused quenching (ACQ). These systems respond to a wide array of potential analytes including protons, metal cations, anions, carbohydrates, and other biomolecules. This review surveys important new fluorescence-based probes for these and other analytes that have been reported over the past five years, focusing on the most widely exploited macrocyclic recognition components, those based on cyclam, calixarenes, cyclodextrins and crown ethers; other macrocyclic and non-macrocyclic receptors are also discussed.

Insight into the deprotonation at the half-equivalence point of (thio)amido-benzimidazoles in the presence of anions

Three crystallographic structures highlight the acid–base half-equivalence point of hydrogen-bond donor (thio)amido-benzimidazoles induced by fluoride or benzoate salts with concomitant hydrogen-bonding and deprotonation as a merged synergic process.

Systematic size mediated trapping of anions of varied dimensionality within a dimeric capsular assembly of a flexible neutral bis-urea platform

The systematic and consistent size mediated recognition of spherical, planar and tetrahedral anions is observed within a neutral cation-sealed dimeric capsular assembly of a meta-difunctionalized organic bis-urea receptor in solid and solution states.

Rational Design of Azo-Azomethine Receptors for Sensing of Inorganic Fluoride: Construction of Molecular Logic Gates and DFT Study

New azo-azomethine receptors, HLn (n = 1–3), have been synthesised via condensation reaction of 5-(4-X-phenyl)-azo-salicylaldehyde (X = NO2, Cl and CH3) with (4-nitrobenzylidene)hydrazine. The receptor with a p-NO2 substituent on the aromatic ring of the azo moiety (HL1) has excellent sensitivity and selectivity towards basic anions with proper discrimination between F− and AcO− or H2PO4− in DMSO–water (4 : 1). A Job’s plot displays a 1 : 1 stoichiometry between HL1 and F− alone with a detection limit of 0.737 μM for fluoride ions. The solvatochromic behaviour of HL1 was probed by studying its UV-vis spectra in four pure organic solvents of different polarities and a meaningful correlation was observed. Furthermore, HL1 was used for detection of inorganic fluoride in toothpaste. The systematic density functional theory (DFT) and time dependent-DFT calculations have been carried out to investigate the mechanism of colourimetric sensing of fluoride ion by HL1 in the gas phase and in solution. Moreover, by using F− and H+ as chemical inputs, and the absorbance as output, a INHIBIT logic gate was constructed, which exhibits ‘Write–Read–Erase–Read’ ability without obvious degradation in its optical output.

Synthesis and application of a new class of D–π–A type charge transfer probe containing imidazole – naphthalene units for detection of F− and CO2

A new class of D–π–A type charge transfer probe, 3 and 4, containing imidazole – naphthalene moieties as donor and acceptor, respectively, has been synthesized via a Suzuki coupling reaction.

A highly versatile fluorenone-based macrocycle for the sensitive detection of polycyclic aromatic hydrocarbons and fluoride anions

Reported herein is the high yielding synthesis of a new fluorenone-based triazolophane and its sensing capabilities for polycyclic aromatic hydrocarbons (PAHs) and fluoride anions.

A NIR fluorescent probe for the detection of fluoride ions and its application in in vivo bioimaging

A near-infrared fluorescent probe has been developed, which is available for visualizing exogenous fluoride ions in vitro and in vivo.

Transmembrane Fluoride Transport: Direct Measurement and Selectivity Studies

Fluoride has been overlooked as a target in the development of synthetic anion transporters despite natural fluoride transport channels being recently discovered. In this paper we report the direct measurement of fluoride transport across lipid bilayers facilitated by a series of strapped calix[4]pyrroles and show that these compounds facilitate transport via an electrogenic mechanism (determined using valinomycin and monensin coupled transport assays and an additional osmotic response assay). An HPTS transport assay was used to quantify this electrogenic process and assess the interference of naturally occurring fatty acids with the transport process and Cl^- over H⁺/OH^- transport selectivity.

Aza-Bambusurils En Route to Anion Transporters

Previous calculations of anion binding with various bambusuril analogs predicted that the replacement of oxygen by nitrogen atoms to produce semiaza-bambus[6]urils would award these new cavitands with multiple anion binding properties. This study validates the hypothesis by efficient synthesis, crystallography, thermogravimetric analysis and calorimetry. These unique host molecules are easily accessible from the corresponding semithio-bambusurils in a one-pot reaction, which converts a single anion receptor into a potential anion channel. Solid-state structures exhibit simultaneous accommodation of three anions, linearly positioned within the cavity along the main symmetry axis. The ability to hold anions at a short distance of about 4 Å is reminiscent of natural chloride channels in E. coli, which exhibit similar distances between their adjacent anion binding sites. The calculated transition-state energy for double-anion movement through the channel suggests that although these host-guest complexes are thermodynamically stable they enjoy high kinetic flexibility to render them efficient anion channels.

Reaction-based phosphorescent nanosensor for ratiometric and time-resolved luminescence imaging of fluoride in live cells

A two-channel phosphorescent nanosensor for fluoride with excellent selectivity and sensitivity has been designed and synthesized. By using the specific chemical affinity between silicon and fluoride, the nanosensor has been used for ratiometric and time-resolved luminescence detection of F(-) in aqueous media and live cells.

Fluoride-induced modulation of ionic transport in asymmetric nanopores functionalized with "caged" fluorescein moieties

We demonstrate experimentally and theoretically a nanofluidic fluoride sensing device based on a single conical pore functionalized with "caged" fluorescein moieties. The nanopore functionalization is based on an amine-terminated fluorescein whose phenolic hydroxyl groups are protected with tert-butyldiphenylsilyl (TBDPS) moieties. The protected fluorescein (Fcn-TBDPS-NH2) molecules are then immobilized on the nanopore surface via carbodiimide coupling chemistry. Exposure to fluoride ions removes the uncharged TBDPS moieties due to the fluoride-promoted cleavage of the silicon-oxygen bond, leading to the generation of negatively charged groups on the fluorescein moieties immobilized onto the pore surface. The asymmetrical distribution of these groups along the conical nanopore leads to the electrical rectification observed in the current-voltage (I-V) curve. On the contrary, other halides and anions are not able to induce any significant ionic rectification in the asymmetric pore. In each case, the success of the chemical functionalization and deprotection reactions is monitored through the changes observed in the I-V curves before and after the specified reaction step. The theoretical results based on the Nernst-Planck and Poisson equations further demonstrate the validity of an experimental approach to fluoride-induced modulation of nanopore current rectification behaviour.

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.

Small molecule fluoride toxicity agonists

Fluoride is a ubiquitous anion that inhibits a wide variety of metabolic processes. Here, we report the identification of a series of compounds that enhance fluoride toxicity in Escherichia coli and Streptococcus mutans. These molecules were isolated by using a high-throughput screen (HTS) for compounds that increase intracellular fluoride levels as determined via a fluoride riboswitch reporter fusion construct. A series of derivatives were synthesized to examine structure-activity relationships, leading to the identification of compounds with improved activity. Thus, we demonstrate that small molecule fluoride toxicity agonists can be identified by HTS from existing chemical libraries by exploiting a natural fluoride riboswitch. In addition, our findings suggest that some molecules might be further optimized to function as binary antibacterial agents when combined with fluoride.

Anion-Exchange Properties of Trifluoroacetate and Triflate Salts of N-Alkylammonium Resorcinarenes

The synthesis of N-benzyl- and N-cyclohexylammonium resorcinarene trifluoroacetate (TFA) and triflate (OTf) salt receptors was investigated. Solid-state analysis by single-crystal X-ray diffraction revealed that the N-alkylammonium resorcinarene salts (NARSs) with different upper substituents had different cavity sizes and different affinities for anions. Anion-exchange experiments by mixing equimolar amounts of N-benzylammonium resorcinarene trifluoroacetate and N-cyclohexylammonium resorcinarene triflate, as well as N-benzylammonium resorcinarene triflate and N-cyclohexylammonium resorcinarene trifluoroacetate showed that the NARS with flexible benzyl groups preferred the larger OTf anion, whereas the rigid cyclohexyl groups preferred the smaller TFA anions. The anion-exchange processes were confirmed in the solid state by single-crystal and powder X-ray diffraction experiments and in the gas phase by electrospray ionization mass spectrometry.

Highly selective naked-eye anion sensors based on thioureido or amido calix[4]arenes

Calix[4]arene-thiourea and -tetraamide naked-eye receptors do not show any tendency to self-aggregation and are highly sensitive towards small monoanions; association constants in DMSO for halogenides (chloride to iodide) and HSO4 – are <200 m –1. Basic anions deprotonate both receptors leading to a high and selective optical readout. Binding constants for carboxylates, fluoride, and dihydrogen phosphate are three orders of magnitude higher (~105 m –1) in case of the tetrathiourea receptor.

Recognition of halides and Y-shaped oxoanions by carbonylchromium-based urea-like molecules: A theoretical analysis of hydrogen bonding modes

One of the major challenges in anion recognition is to design hosts that can be used to distinguish between anions of different shapes. Urea-based molecules are widely used in anion recognition because the pair of -NH groups acts as an electron acceptor. Although these hosts can bind to both spherical anions (halides) and Y-shaped anions (oxoanions), experimental evidence to date does not provide a clear picture of what differences in the nature of the hydrogen bonding interactions could be used to distinguish between anions of different shapes. Here, we use several computational topology analyses to study the non-covalent interactions between Cr(CO)3-based organometallic urea-like hosts and halides and Y-shaped oxoanions. Our results suggest that the F(-) and AcO(-) anions are recognized experimentally due to a combination of strong interaction and large infrared (IR) shifts upon complexation, verifying the remarkable IR-reporting ability of the Cr(CO)3 moiety and its potential applications in anion recognition. The lone pairs of the oxygen atom in Y-shaped oxoanions directly interact with the -NH groups of the hosts, while all the shell electrons of the halides participate as a group in the interaction; however, the relative contributions of electrostatic and charge-transfer interactions are quite similar for the two types of anions. This insight into the nature of the anion-host interactions can be used to provide guidance for the design of hosts that differentiate between anions.

Anion coordinated capsules and pseudocapsules of tripodal amide, urea and thiourea scaffolds

This review provides a detailed and comparative account of the solid- and solution-states anion (halides and oxyanions) binding affinities of hydrogen bonding tripodal scaffolds.

A novel displacement-type colorimetric chemosensor for the detection of Cu2+and GSH in aqueous solution

A new selective and sensitive chemosensor1for the sequential detection of Cu²⁺and GSHvianaked-eye has been developed.

C 3v-symmetric anion receptors with guanidine recognition motifs for ratiometric sensing of fluoride

Two new tripodal receptors 3 and 4 derived from a trindane framework having guanidine groups acting as hydrogen bond acceptors are synthesized and characterized for the selective recognition of anions.

Chromogenic and fluorogenic multianalyte detection with a tuned receptor: refining selectivity for toxic anions and nerve agents

In the present study, a chemical probe was finely tuned for the highly selective and sensitive chromogenic and fluorogenic detection of toxic anions and a nerve agent.

Metal organic frameworks mimicking natural enzymes: a structural and functional analogy

In this review, we have portrayed the structure, synthesis and applications of a variety of biomimetic MOFs from an unprecedented angle.

In vivo detection of fluoride at trace levels and its removal from raw water at neutral pH utilizing a cyanobacterium pigment as a luminescent probe

A selective method has been developed for trace level (0.01 mg L⁻¹) fluoride detection in HEPES buffer in the presence of interfering ions (such as Cl⁻, I⁻, Br⁻, SO₄²⁻, ClO₄⁻, and CH₃COO⁻) using a cyanobacterium as a luminescent probe.

A switchable electrochemical redox ratiometric substrate based on ferrocene for highly selective and sensitive fluoride detection

A ferrocene based electrochemical redox substrate for highly selective and sensitive fluoride detection in non-transparent solutions.

A turn-on fluorescent sensor for relay recognition of two ions: from a F−-selective sensor to highly Zn2+-selective sensor by tuning electronic effects

A simple ion sensor bearing quinoline and an amide group was designed and synthesized, which showed both colorimetric detection for F⁻ and a fluorescence turn-on response for Zn²⁺. Moreover, sensor L2 can distinguish F⁻ and Zn²⁺via different sensing mechanisms.

An AIE based tetraphenylethylene derivative for highly selective and light-up sensing of fluoride ions in aqueous solution and in living cells

MOTIPS-TPE displayed highly selectivity for F⁻ in PBS and was employed as an AIE light-up probe for F⁻ in living HeLa cells.

Fluorometric selective detection of fluoride ions in aqueous media using Ag doped CdS/ZnS core/shell nanoparticles

In this study, a fluorometric method based on Ag–CdS/Ag–ZnS core/shell nanoparticles is developed for fluoride ion detection.

An efficient ICT based fluorescent turn-on dyad for selective detection of fluoride and carbon dioxide

A new intramolecular charge transfer based fluorescent turn-on ratiometric probe exhibiting high sensitivity for F⁻ and CO₂.

A colorimetric and ratiometric fluorescence sensor for sensitive detection of fluoride ions in water and toothpaste

The colorimetric and ratiometric sensor shows significant spectral response towards fluoride ions in water with a detection limit as low as 2.7 ppb, enabling direct observation with the bare eye as well as quantitative determination of F⁻ in toothpaste.

Experimental investigation of anion–π interactions – applications and biochemical relevance

Anion–π interactions, intuitively repulsive forces, turned from controversial to a well-established non-covalent interaction over the past quarter of a century.

Coumarin functionalized thiocarbonohydrazones as a new class of chromofluorescent receptors for selective detection of fluoride ion

Thiocarbonohydrazone receptors selectively recognize fluoride ionsviaH-bond interactions and subsequent deprotonation to elicit a distinct visual and fluorescence colour change.