Anion Selectivity Research Articles

Influence of electronic polarization in a chloride-pumping rhodopsin binding site.

Biological ion channels and membrane transport proteins are suggested to utilize anion-hydrophobic interactions to achieve selective anion transport. Here, we investigate a chloride-pumping rhodopsin (ClR) as an example of a Clˉ-selective protein known to contain a defined binding site composed predominantly of hydrophobic residues. Using molecular dynamics (MD) simulations, we explore Clˉ binding to this hydrophobic binding site and compare the dynamics arising when electronic polarization is either (i) neglected (CHARMM36 (c36) fixed-charge force field), (ii) included implicitly (via the prosECCo force field based on the electronic-continuum-correction method), or (iii) included explicitly (through the polarizable force field, AMOEBA). Free energy landscapes of a Clˉ moving out of the binding site and into bulk solution demonstrate stronger ion binding and exhibit a second metastable site with the inclusion of polarization effects. Simulations of a small protein fragment containing the binding site indicate longer binding durations and closer ion proximity with the inclusion of polarization. Finally, examination of larger protein fragment simulations consisting of the binding site and a neighboring loop reveal a mechanism that facilitates Clˉ rebinding events which are not observed with the non-polarizable force field. These results demonstrate the importance of modelling polarization (effects) which can influence the behavior of anions within real protein binding sites and suggest important mechanistic insights.

Stability Effects of Selective Anion Abstraction from Cesium Lead Halide Perovskite Nanocrystals

The dynamic surface chemistry and stabilization of cesium lead halide perovskite nanocrystals (NCs) remain principal challenges for the use of these materials in many applications. Here, trimethylsilyl trifluoromethanesulfonate (Me3SiOTf) was used as a Lewis acidic reagent for quantitative and irreversible silylation and removal of anionic ligands from colloidal oleate- and oleylamine-capped CsPbBr3 and CsPbCl3 NCs. 1H NMR and FTIR spectroscopies, combined with elemental analysis, show strong binding of a population of anionic ligands to the NC surfaces. Surface anion exchange for trifluoromethanesulfonate also results in an increase in photoluminescence quantum yields. For CsPbCl3 NCs, Me3SiOTf treatment abstracts chlorides and destabilizes the NC lattice. The thermodynamic differences between these reactions and their implications are discussed.

Separation of Hydrochloric Acid and Oxalic Acid from Rare Earth Oxalic Acid Precipitation Mother Liquor by Electrodialysis.

In this study, the hydrochloric acid from rare earth oxalic acid precipitation mother liquor was separated by electrodialysis (ED) with different anion exchange membranes, including selective anion exchange membrane (SAEM), polymer alloy anion exchange membrane (PAAEM), and homogenous anion exchange membrane (HAEM). In addition to actual wastewater, nine types of simulated solutions with different concentrations of hydrochloric acid and oxalic acid were used in the experiments. The results indicated that the hydrochloric acid could be separated effectively by electrodialysis with SAEM from simulated and real rare earth oxalic acid precipitation mother liquor under the operating voltage 15 V and ampere 2.2 A, in which the hydrochloric acid obtained in the concentrate chamber of ED is of higher purity (>91.5%) generally. It was found that the separation effect of the two acids was related to the concentrations and molar ratios of hydrochloric acid and oxalic acid contained in their mixtures. The SEM images and ESD-mapping analyses indicated that membrane fouling appeared on the surface of ACS and CSE at the diluted side of the ED membrane stack when electrodialysis was used to treat the real rare earth oxalic acid precipitation mother liquor. Fe, Yb, Al, and Dy were found in the CSE membrane section, and organic compounds containing carbon and sulfur were attached to the surface of the ACS. The results also indicated that the real rare earth precipitation mother liquor needed to be pretreated before the separation of hydrochloric acid and oxalic acid by electrodialysis.

Ultrasensitive electrochemical phosphate detection by pyridine–zinc(II) complex

Phosphorous is an important environmental health parameter as the availability of phosphorus within water systems plays an essential role in the prevalence of harmful algal blooms (cyanobacteria blooms). Currently, phosphates are detected using sensitive chromatographic and colorimetric techniques; however, major disadvantages stem from the lack of anion selectivity in samples with complex matrices, as well as the high cost of analysis. Electrochemical techniques utilizing self-assembled monolayers can provide a cheaper yet sensitive method of detection. This work explores the modification of a gold working electrode using pyridine–zinc(II) complexes. The implementation of self-assembled monolayers allows for an ultrasensitive and selective method of indirect detection of the H2PO4 − species, ranging in concentrations between 0.0 and 1.2 fmol/L phosphate. Electrochemical techniques such as cyclic voltammetry and square-wave voltammetry were explored for their phosphate-detection abilities, with detection limits of 4.0 × 10−16 and 9.0 × 10−17 mol/L H2PO4 −, respectively. X-ray photoelectron spectroscopy measurements were also taken to confirm the modification of the electrode. The selectivity of this sensor towards phosphate anions was successfully explored for this sensor in the presence of potential interfering agents (sulfate, chlorine, carbonate, fluoride, nitrite, and hypochlorite ions), and applicability of sensor was also explored through the detection of phosphate in a tap and lake water sample.

Gated, Selective Anion Exchange in Functionalized Self-Assembled Cage Complexes.

Appending functional groups to the exterior of Zn4 L4 self-assembled cages allows gated control of anion binding. While the unfunctionalized cages contain aryl groups in the ligand that can freely rotate, attaching inert functional groups creates a "doorstop", preventing rotation and slowing the guest exchange rate, even though the interiors of the host cavities are identically structured. The effects on anion exchange are subtle and depend on multiple factors, including anion size, the nature of the leaving anion, and the electron-withdrawing ability and steric bulk of the pendant groups. Multiple exchange mechanisms occur, and the nature of the external groups controls associative and dissociative exchange processes: these bulky groups affect both anion egress and ingress, introducing an extra layer of selectivity to the exchange. Small changes can have large effects: affinities for anions as similar as PF6 - and SbF6 - can vary by as much as 400-fold between identically sized cavities.

The impact of pathogenic and artificial mutations on Claudin-5 selectivity from molecular dynamics simulations

Tight-junctions (TJs) are multi-protein complexes between adjacent endothelial or epithelial cells. In the blood-brain-barrier (BBB), they seal the paracellular space and the Claudin-5 (Cldn5) protein forms their backbone. Despite the fundamental role in brain homeostasis, little is known on Cldn5-based TJ assemblies. Different structural models were suggested, with Cldn5 protomers generating paracellular pores that restrict the passage of ions and small molecules. Recently, the first Cldn5 pathogenic mutation, G60R, was identified and shown to induce Cl−-selective channels and Na+ barriers in BBB TJs, providing an excellent opportunity to validate the structural models. Here, we used molecular dynamics to study the permeation of ions and water through two distinct G60R-Cldn5 paracellular architectures. Only the so-called Pore I reproduces the functional modification observed in experiments, displaying a free energy (FE) minimum for Cl− and a barrier for Na+ consistent with anionic selectivity. We also studied the artificial Q57D and Q63D mutations in the constriction region, Q57 being conserved in Cldns except for cation permeable homologs. In both cases, we obtain FE profiles consistent with facilitated passage of cations. Our calculations provide the first in-silico description of a Cldn5 pathogenic mutation, further assessing the TJ Pore I model and yielding new insight on BBB’s paracellular selectivity.

Sensitive luminescent chemosensing of fluoride based on Eu-doped Zn-LMOF in aqueous media: structural and spectroscopic studies

Selective anion sensing/recognition by luminescent compounds able to operate in aqueous media is a central topic of supramolecular analytical chemistry that impacts biological and environmental chemistry.

Turn-on fluorescent capsule for selective fluoride detection and water purification.

It has been a long-standing challenge to develop organic molecular capsules for selective anion binding in water. Here, selective recognition of aqueous fluoride was achieved through triple protonation of a hemicryptophane (L), which is composed of a fluorescent cyclotriveratrylene (CTV) cap and tris(2-aminoethyl)amine (tren) as the anion binding site. Fluoride encapsulation by [3H-L]3+ was evidenced by 1H NMR, 19F NMR, LC-MS, and X-ray crystallography. In addition, [3H-L]3+ exhibited a 'turn-on' fluorescence signal (λ em = 324 nm) upon fluoride addition. An apparent association constant K A = (7.5 ± 0.4) × 104 M-1 and a detection limit of 570 nM fluoride were extracted from the fluorescence titration experiments in citrate buffer at pH 4.1. To the best of our knowledge, [3H-L]3+ is the first example of a metal-free molecular capsule that reports on fluoride binding in purely aqueous solutions with a fluorescence response. Finally, the protonated capsule was supported on silica gel, which enabled adsorptive removal of stoichiometric fluoride from water and highlights real-world applications of this organic host-guest chemistry.

Selective anion sensing in high salt water via a remote indicator displacement assay.

Water-soluble deep cavitands with cationic functions at the lower rim can selectively bind iodide anions in purely aqueous solution. By pairing this lower rim recognition with an indicator dye that is bound in the host cavity, optical sensing of anions is possible. The selectivity for iodide is high enough that micromolar concentrations of iodide can be detected in the presence of molar chloride. Iodide binding at the "remote" lower rim causes a conformational change in the host, displacing the bound dye from the cavity and effecting a fluorescence response. The sensing is sensitive, selective, and works in complex environments, so will be important for optical anion detection in biorelevant media.

Modulation of transmembrane anion transport of porphyrin boxes by dynamic window size engineering

The development of synthetic transmembrane anion transport systems is of considerable interest, not only for mimicking the functions of natural transmembrane proteins but also for practical applications. We have recently reported a porous organic cage, porphyrin box (PB(8)) having multiple windows surrounded by octyl chains as an iodide selective anion channel. Herein, we report the modulation of transmembrane transport of halides ([Formula: see text], [Formula: see text], and [Formula: see text] by dynamic window size engineering of the cage with different alkyl chain lengths (hexyl PB(6), octyl PB(8) and decyl PB(10)). ‘Apparent’ transport rates were measured by the HPTS fluorescence assay, which shows a gradual decrease in the transport rate upon increasing the length of alkyl chains of PB. We calculate the transport rate per PB in order to make a fair comparison as the ‘apparent’ transport rate is proportional to the number of PBs embedded in the lipid membrane. The transport rate per PB reveals that increasing the length of the alkyl chains of PBs results in a substantial fall in the iodide transport rate while only marginally decreasing the transport rates of bromide and chloride, thereby decreasing the selectivity of iodide transport.

Impact of Surface Roughness on Partition and Selectivity of Ionic Liquids Mixture in Porous Electrode.

Understanding the influence of surface roughness on the adsorption of ions from an ionic liquids (ILs) mixture is essential for designing supercapacitors. The classical density functional theory (DFT) is applied to investigate the adsorption behavior of ILs mixtures in rough nanopores. The model parameters for each ion are determined by fitting experimental data of pure IL density. The results show that the smaller anions are densely accumulated near the rough surface and are the dominant species at a high positive potential. The exclusion of larger anions is enhanced by roughness at almost all potentials. At negative potential, the surface roughness promotes the adsorption of cations, and the partition coefficient increases with roughness. The partition coefficient of smaller anions is virtually independent of roughness. At positive potential, the surface roughness only promotes the adsorption of smaller anions and raises the partition coefficient. The partition coefficient of smaller anions is far greater than one. The selectivity of smaller anions for rough surfaces is very high and increases with roughness. The surface charge of a more uneven surface is significantly higher (about 30%) at a high potential.

Nghiên cứu, chế tạo cặp màng lọc tích điện trái dấu từ graphen oxit hấp phụ ion Na+ và Cl-

In this research, we fabricate pairs of oppositely charged graphene oxide (GO) membranes (negatively charged n-GO and positively charged p-GO) with efficient cation or anion selectivity. The fabricated primary GO membrane carries a negative charge (n-GO). The positively charged p-GO membrane was prepared by ultrasonic stirring with polyethyleneimine. The electrical properties of the n-GO and p-GO films were examined based on a fluorescence electron microscope and zeta potentials in NaCl solution at different concentrations and times. The results show that the zeta potential of n-GO and p-GO both stabilize and maintain their charge properties in NaCl solution. The good ion selectivity of n-GO and p-GO was determined by X-ray spectroscopy (EDX) mapping.

Structural elucidation of N-amidothiourea derived from nicotinic acid hydrazide: A detailed study of its application as selective anion sensor

A novelN-amidothiourea receptor (1), synthesized by the reaction of nicotinic acid hydrazide with phenyl isothiocyanate, was evaluated for the detection of fluoride, acetate, and hydroxide ions selectively over other anions. Receptor1was characterized by various spectroscopic methods, and further structural elucidation of it was done by Single Crystal X-ray diffraction analysis. The efficacy of receptor1as a colorimetric sensor for fluoride ions was observed by an instant colour change from colourless to light yellow, which can be attributed to the H-bonding complex formation between receptor1and anion. The binding characteristics of the thiourea receptor with fluoride ion (F−) showed 1:1 binding stoichiometry at a lower concentration, whereas at a higher concentration, the binding stoichiometry increased to 1:2. The receptor, when treated with acetate and hydroxide ion, showed 1:2 and 1:1 binding stoichiometry, respectively. The binding affinity of receptor1was found to be following the Brønsted acidity order, i.e., OH− > AcO− > F−. Detailed quantum chemical calculations also supported the experimental findings.

Semicarbazone and thiosemicarbazone appended 4-diethylamino-2‑hydroxybenzyl compounds as highly selective bifunctional chemosensors: An experimental and computational approach

Three bifunctional chemosensors, 2-(4-(diethylamino)-2-hydroxybenzylidene)hydrazinecarboxamide (S1), 2-(4-(diethylamino)-2-hydroxybenzylidene)hydrazinecarbothioamide (S2) and 2-(4-(diethylamino)-2-hydroxybenzylidene)-N-methylhydrazinecarbothioamide (S3) have been designed and synthesized for recognition of inorganic cations and anions in solution at room temperature. The structure of the receptor S2 was confirmed by single crystal X-ray diffraction technique, which crystallizes in the monoclinic P21/n space group. The selectivity and sensitivity of cations (Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+ and Pb2+) and anions (F−, Cl−, Br−, I−, HSO4− and H2PO4−) were investigated using colorimetric, UV–vis absorption, emission and 1H NMR studies. The receptors (S1–S3) with ONO/ONS donors allow bare-face recognition of Co2+, Ni2+ and Cu2+ cations, and F− anion in a mixture of solvent, CH3CN:CH3OH (8:2, v/v). The receptor S3 possesses high sensitivity and selectivity in comparison with the receptors S1 and S2. The binding constant and limit of detection of the receptors with sensed cations and anion showed high sensitivity and selectivity towards Co2+ cation with order of magnitude 6, greater than the other sensed cations. In addition, the receptors S3 also shows 6 order of magnitude for Cu2+ cation and F− anion. The optimized geometry determined by DFT studies shows that the receptors in its conjugated state provide binding sites for cations and F− anion. The HOMO-LUMO energy gap between the receptors and the Co2+ cation was found to be the least among the studied receptors-cations binding interaction, possibly due to charge transfer effect.

Extractive desulfurization of liquid hydrocarbon fuel: Task-specific ionic liquid development and experimental study

Extractive desulfurization has been used as an alternative method to remove sulfur compounds from liquid hydrocarbon fuels because of its simplicity and easy operating conditions. Ionic liquids (ILs) were applied as extractant solvent for extractive desulfurization, due to their unique properties. But the selection of proper cations and anions are still the most challenging parts of all researches. In the present study, a new methodology was developed for screening of cation and anion molecules to obtain maximum sulfur removal efficiency. At first, the best structural descriptors were selected using Genetic Algorithm. The different kinds of molecular descriptors for a diverse library of cation/anion have been evaluated, including 114 cations and 48 anions. Then, the most proper cations and anions have been selected using Principal Component Analysis (PCA) after evaluating the most accurate structural descriptors for various cations and anions. It was concluded that ionic liquids (ILs) comprise imidazolium and pyridinium cations with hexyl chain, combined with the [N(CN)2] and [Ac] anions, are promising ILs as extraction solvents for extractive desulfurization. To evaluate the obtained results, one of the selected ILs, 1-Hexyl-pyridinium dicyanamide ([Hpy][N(CN)2]), has been synthesized for the first time. The as-synthesized IL was characterized through NMR and FTIR analysis. After that, by investigating results, it is observed that the sulfur removal of [Hpy][N(CN)2] for DBT and BT is nearly 62% and 67%, respectively. Therefore, [Hpy][N(CN)2] is a proper solvent for extraction of DBT or BT from n-heptane.

Toward explicit anion transport nanochannels for osmotic power energy using positive charged MXene membrane via amination strategy

The reverse electrodialysis (RED) is a potential technique to harvest osmotic energy caused by different salinities. Owing to the negative surface-charged controlling effects of nanofluidic channel in 2D membrane, the power density has been achieved with high energy conversion efficiency for cations (Na+ with the counterpart of Cl−). However, anion-selective nanofluidic channel is rare but still required for promising application of RED due to its high higher diffusion rate resulting in desirable osmotic power energy. In this work, we constructed a two-dimensional layered MXene membrane with positive charge via amination strategy of 3-aminopropyltriethoxysilane (APTES) toward anion selectivity for efficient osmotic power harvesting. The power density of the resultant membrane can achieve a maximum of 10.98 W m−2 at a natural salinity gradient, higher than the commercial benchmark of 5 W m−2. Under regulating environmental conditions including operation temperatures, pH and the existence of pollutants (heavy metal ions and organic pollution) of the simulated electrolyte solution, a higher power density of 20.66 W m−2 can be reached through Cl− transmembrane transport. During 10-h testing, the attenuation of output power density was just 3%. The applicable stability still maintains under the contaminative environment of heavy metal ions or organic pollution. These results will bring an innovative design of anion-selective membrane for osmotic energy conversion and widen the practical application in ion selectivity.

Anti-Hofmeister Anion Selectivity via a Mechanical Bond Effect in Neutral Halogen-Bonding [2]Rotaxanes.

Exceptionally strong halogen bonding (XB) donor-chloride interactions are exploited for the chloride anion template synthesis of neutral XB [2]rotaxane host systems which contain perfluoroaryl-functionalised axle components, including a remarkably potent novel 4,6-dinitro-1,3-bis-iodotriazole motif. Halide anion recognition properties in aqueous-organic media, determined via extensive 1 H NMR halide anion titration experiments, reveal the rotaxane host systems exhibit dramatically enhanced affinities for hydrophilic Cl- and Br- , but conversely diminished affinities for hydrophobic I- , relative to their non-interlocked axle counterparts. Crucially, this mechanical bond effect induces a binding selectivity which directly opposes Hofmeister bias. Free-energy analysis of this mechanical bond enhancement demonstrates anion recognition by neutral XB interlocked host systems as a rare and general strategy to engineer anti-Hofmeister bias anion selectivity in synthetic receptor design.

Anti‐Hofmeister Anion Selectivity via a Mechanical Bond Effect in Neutral Halogen‐Bonding [2]Rotaxanes

AbstractExceptionally strong halogen bonding (XB) donor‐chloride interactions are exploited for the chloride anion template synthesis of neutral XB [2]rotaxane host systems which contain perfluoroaryl‐functionalised axle components, including a remarkably potent novel 4,6‐dinitro‐1,3‐bis‐iodotriazole motif. Halide anion recognition properties in aqueous‐organic media, determined via extensive 1H NMR halide anion titration experiments, reveal the rotaxane host systems exhibit dramatically enhanced affinities for hydrophilic Cl− and Br−, but conversely diminished affinities for hydrophobic I−, relative to their non‐interlocked axle counterparts. Crucially, this mechanical bond effect induces a binding selectivity which directly opposes Hofmeister bias. Free‐energy analysis of this mechanical bond enhancement demonstrates anion recognition by neutral XB interlocked host systems as a rare and general strategy to engineer anti‐Hofmeister bias anion selectivity in synthetic receptor design.

Regulation of Structure and Anion-Exchange Performance of Layered Double Hydroxide: Function of the Metal Cation Composition of a Brucite-like Layer.

As anion-exchange materials, layered double hydroxides (LDHs) have attracted increasing attention in the fields of selective adsorption and separation, controlled drug release, and environmental remediation. The metal cation composition of the laminate is the essential factor that determines the anion-exchange performance of LDHs. Herein, we review the regulating effects of the metal cation composition on the anion-exchange properties and LDH structure. Specifically, the internal factors affecting the anion-exchange performance of LDHs were analyzed and summarized. These include the intercalation driving force, interlayer domain environment, and LDH morphology, which significantly affect the anion selectivity, anion-exchange capacity, and anion arrangement. By changing the species, valence state, size, and mole ratio of the metal cations, the structural characteristics, charge density, and interlayer spacing of LDHs can be adjusted, which affect the anion-exchange performance of LDHs. The present challenges and future prospects of LDHs are also discussed. To the best of our knowledge, this is the first review to summarize the essential relationship between the metal ion composition and anion-exchange performance of laminates, providing important insights for regulating the anion-exchange performance of LDHs.

Narrowing the pore size distribution of polyamide nanofiltration membranes via dragging piperazines to enhance ion selectivity

Traditional polyamide nanofiltration membranes often can't obtain ideal ion selectivity due to their wide pore size distribution. In this work, a β-cyclodextrin modified sodium hyaluronate (HA-CD) was introduced to achieve the dragging effect of piperazine by non-specific hydrogen bond interaction between HA-CD and substrate and piperazine, thus realizing the purpose of sustained release of piperazine. The MD simulation results demonstrated that the diffusion rate of piperazine was reduced by approximately two orders of magnitude due to the interaction effect. In addition to making the polyamide layer thinner and smoother, HA-CD also narrows the pore size distribution. The salt filtration test showed that the prepared membrane has excellent anion selectivity (αmax:114) and permeability (10.66 L m−2 h−1 bar−1). Long-term filtration and nano-scratch test results demonstrated that the prepared membranes have good operational stability (the permeability only decreased about 30%) and mechanical strength (the interface bonding strength increased about 270%). By exploring the effect of the interaction between HA-CD and substrate and piperazine on the interfacial polymerization process, which proved the feasibility of realizing high permeability selectivity and stability of the membrane by constructing the functional bridge between substrates and aqueous monomers.