Versatility of an intramolecularly hydrogen-bonded 4-(N,N-dimethylamino)benzoate group as a signaling subunit for anion recognition

The versatility of the 4-(N,N-dimethylamino)benzoate (DMAB) group embedded in host 1 as a signaling subunit for anion recognition was elucidated in terms of ¹H NMR, CD, and fluorescence studies. Host 1 showed 1:1 complexation with monovalent anions and stepwise 1:1 and 2:1 (host 1: anion) complexation with divalent phosphate anions. The binding constants between host 1 and anions were determined by means of ¹H NMR titrations in CD₃CN (HPO₄²⁻: log K_1:1=6.2, log K_2:1=4.9; H₂P₂O₇²⁻: log K_1:1=4.4, log K_2:1=1.8; AMP²⁻: log K_1:1>7, log K_2:1>5) and the affinity of host 1 toward divalent anions, HPO₄²⁻, H₂P₂O₇²⁻, and AMP²⁻, is stronger than that toward monovalent anions, NO₃⁻, BF₄⁻, ClO₄⁻, HSO₄⁻, and PF₆⁻. The CD exciton chirality studies of host 1 with divalent anions, HPO₄²⁻ and AMP²⁻, revealed that the two DMAB groups in the 2:1 complexes were arranged with negative chirality (counterclockwise). The dual fluorescence behavior of the DMAB group demonstrated not only the complexation stoichiometry but also the role(s) of the lipophilic countercation such as tetrabutylammonium and/or the hydrophilic residue in AMP during anion recognition.     

A ratiometric fluorescent probe for fluoride ion employing the excited-state intramolecular proton transfer

A ratiometric fluorescent probe 1 for fluoride ion was developed based on modulation of the excited-state intramolecular proton transfer (ESIPT) process of 2-(2′-hydroxyphenyl)benzimidazole (HPBI) through the hydroxyl group protection/deprotection reaction. The probe 1 was readily prepared by the reaction of HPBI with tert-butyldimethylsilyl chloride (TBS-Cl) and shows only fluorescence emission maximum at 360 nm. Upon treatment with fluoride in aqueous DMF solution, the TBS protective group of probe 1 was removed readily and ESIPT of the probe was switched on, which resulted in a decrease of the emission band at 360 nm and an increase of a new fluorescence peak around 454 nm. The fluorescent intensity ratio at 454 and 360 nm (I₄₅₄/I₃₆₀) increases linearly with fluoride ion concentration in the range 0.3-8.0 μmol L⁻¹ and the detection limit is 0.19 μmol L⁻¹. The proposed probe shows excellent selectivity toward fluoride ion over other common anions. The method has been successfully applied to the fluoride determination in toothpaste and tap water samples.     

Nickel pyrazolyl borate complexes: Synthesis, structure and analytical application in biological and environmental samples as anion selective sensors

A [{hydrotris(3-phenyl-5-methyl-1-pyrazolyl)borate}(3-phenyl-5-methyl-pyrazole) nickel chloride] [Tp^Ph,MeNi(Cl)Pz^Ph,MeH] (I) has been synthesized and explored as ionophores for the preparation of a poly (vinyl chloride) (PVC) membrane sensor for azide and thiocyanate anions. The compounds [Tp^Ph,MeNi(N₃)Pz^Ph,MeH] (II) and [Tp^Ph,MeNi(SCN)Pz^Ph,MeH] (III) were characterized by their crystal structures and proved to be bonded as monodentate through nitrogen atom of azide and thiocyanate anion. Potentiometric investigations also indicate high affinity of this receptor for thiocyanate and azide ions. PVC based membranes of I using as hexadecyltrimethylammonium bromide (HTAB) cation discriminator and o-nitrophenyloctyl ether (o-NPOE), dibutylphthalate (DBP), acetophenone (AP) and tributylphosphate (TBP) as plasticizing solvent mediators were prepared and investigated as SCN⁻ and N₃⁻ selective sensors. The best performance was shown by the membrane of thiocyanate with composition (w/w) of (I) (7%):PVC (31%):DBP (60%):HTAB (2%). This sensor works well over a wide concentration range 5.3 × 10⁻⁷ to 1.0 × 10⁻² M with Nernstian compliance (59.2 mV decade⁻¹ of activity) within pH range 2.5-9.0 with a response time of 11 s and showed good selectivity for thiocyanate ion over a number of anions. The sensor exhibits adequate life (3 months) and could be used successfully for the determination of thiocyanate content in human urine, saliva and river water samples. While the membrane of [Tp^Ph,MeNi(Cl)Pz^Ph,MeH] ionophore with composition (I) (6%):HTAB (4%):PVC (31%):TBP (59%) showed highest sensitivity and widest linear range for azide ion. These sensors exhibit the maximum working concentration range of 8.1 × 10⁻⁶ to 1.0 × 10⁻² M with Nernstian slope of 59.3 mV decade⁻¹ of activity. It can be applied for the monitoring of the azide ions concentration in aqueous black tea and orange juice samples.     

Biindolyl-based molecular clefts that bind anions by hydrogen-bonding interactions

Molecular clefts were synthesized from 2,2′-biindolyl scaffold that contains good hydrogen bond donors of two indole NHs. The molecular clefts were systematically modified in two different manners to increase binding affinities toward chloride. The association constant dramatically increased when additional hydrogen-bonding sites of two benzamide units were incorporated to the biindolyl scaffold. For example, the association constants of 1a and 1b are 5.1 × 10³ and 1.4 × 10⁴ M⁻¹ in CH₃CN at 22 ± 1 °C, while reference molecule 10 having only two indole NHs showed the association constant of 340 M⁻¹ under the same conditions. When the biindolyl backbone was structurally preorganized, the binding affinities toward anions were further increased with additional stabilization energy (−ΔΔG) of 2.0 ± 0.2 kcal/mol).     

Potentiometric evaluation of calix[4]arene anion receptors in membrane electrodes: phosphate detection

Ion-selective membrane electrodes doped with the urea- or thiourea-functionalised calix[4]arenes, 5,11,17,23-tetra-tert-butyl-25,27-bis[[4-N′-(phenylureido)butyl]oxy]-26,28-dipropoxy calix[4]arene (I) and 5,11,17,23-tetra-tert-butyl-25,27-bis[[4-(N′-phenylthioureido)-butyl]oxy]-26,28-dipropoxy calix[4]arene (II), were evaluated for anion sensing. Potentiometric results show that these calixarene ionophore-based membrane electrodes exhibit a good sensitivity to aqueous solutions of the monohydrogen orthophosphate species HPO₄²⁻ in the concentration range 5.0 × 10⁻⁵ to 1.0 × 10⁻¹ M, with near-Nernstian response slopes of −33.0 and −28.0 mV dec⁻¹ for ionophores I and II, respectively. Selectivity coefficient values for monohydrogen orthophosphate over a range of common anions were determined by the fixed interference and matched potential methods and indicated that these membrane electrodes exhibit a good selectivity for HPO₄²⁻ with respect to the other anions, including sulfate and nitrate.     

Allosteric anion recognition by metal complexation of tris(bipyridine-imidazolium) ligand

A new tripodal imidazolium ligand 2 comprising three 2,2′-bipyridine-imidazolium subunits connected through a mesityl spacer has been synthesized and 1:1 complexes of 2 with Fe(II) and Ru(II) ions have been prepared. ¹H NMR spectroscopy including NOE analysis and molecular modeling study established that the complexes exist as a pseudocryptand type twisted structure in solution. The Fe(II) and Ru(II) complexes show strong 1:1 binding of Cl⁻, Br⁻, and I⁻ anions in MeCN-d₃ with a large enhancement of the guest selectivity (Cl⁻ > Br⁻ > I⁻) upon metal complexation.     

Organotin compounds: an ionophore system for fluoride ion recognition

Ion-selective properties were established for membrane electrodes prepared by using organotin compounds of type (L^CNRSnF₂)_n, (R = n-Bu (I), = Ph (II)) and (L^CNSnF₃)_n (III) (L^CN = C₆H₄(CH₂NMe₂)-2). Electrodes formulated with the optimized membranes containing the organotin compounds I-III as ionophores and sodium tetraphenylborate (10-30%) exhibited high selectivity for fluoride over other anions. An electrode prepared with ionophore II using dibutyl phthalate as the plasticizer and 15% sodium tetraphenylborate (NaTPB) as anion additive, possesses the best potentiometric response characteristics. It shows a detection limit of 7.9 × 10⁻⁷ M with a slope of 62.7 mV decade⁻¹ of activity in buffer solutions of pH 5.5. The interference from other anions is suppressed under this optimized measurement conditions. An entirely non-Hofmeister selectivity sequence (F⁻ > CH₃COO⁻ > Cl⁻ > I⁻ ∼ Br⁻ >ClO₄⁻ > NO₂⁻ > NO₃⁻ > SCN⁻) with remarkable preference towards fluoride is obtained. The influence on the electrode performances by anion additive was studied, and the possible response mechanism was investigated by UV-vis spectra. The electrode has been used for direct determination of fluoride in drinking mineral water with satisfactory results.     

A new heteroditopic receptor and sensor highly selective for bromide in the presence of a bound cation

The heteroditopic receptor 2 containing a crown ether and amidoferrocence groups was synthesized and the binding abilities with various anions are reported in the presence and absence of metal cations. In the presence of Na⁺, 2 showed positive co-operative binding towards Br⁻ with the binding affinity K_ass = 16,096 M⁻¹. Therefore, receptor 2 showed a switched-on binding for Br⁻ in the presence of Na⁺ and a switched-off binding in the absence of Na⁺. Compound 2 was also found to sense Cl⁻ and Br⁻ electrochemically.     

Influence of microwave heating on fluoride, chloride, nitrate and sulfate concentrations in water

This paper describes a study about the influence of microwave radiation using closed vessels on fluoride, chloride, nitrate and sulfate concentrations in aqueous media. The experiments were processed by heating water using PFA vessels and a microwave cavity oven, determining the anions by ion chromatography. The influence of the exposure time, the atmospheric composition, the kind of heating (water bath or microwave radiation) and the possible formation of hydrogen peroxide were investigated. The limits of quantification for fluoride, chloride, nitrate and sulfate were respectively of 0.17, 0.15, 0.55 and 0.57 μg L⁻¹, and precision, expressed as RSD, was <4% for all considered anions. The hydrogen peroxide was quantified by spectrophotometry, and the limit of quantification and precision were 24 μg L⁻¹ and <5% (n = 10), respectively. The results demonstrate a significant increase in the anion concentration levels (between 63 and 89%) when microwave heating was used in comparison with heating by water bath. In addition, these changes observed can be mainly attributed to the species transfers, either between gaseous (atmospheric gases) and liquid (water) phases for nitrate, or between vessels walls and water for fluoride, chloride and sulfate. Additionally, hydrogen peroxide concentration higher than 45 μg L⁻¹ was determined when water was exposed to microwave radiation.     

Carbamate triserine lactone receptors for anion recognition

We have taken advantage of the ability of the cyclic triester of l-serine to organize ligand units appended to the three α-amine functionalities, as in enterobactin, a siderophore produced by enteric bacteria that binds ferric ion exceptionally well (K_f = 10⁴⁹). As an extension of the preorganization concept, this work describes the preparation and characterization of carbamate triserine lactone receptors 1-6 and their ability to act as molecular receptors for anions. These receptors bind halides (F⁻ ? Cl⁻ > Br⁻ > I⁻) with binding constants K in the range 21-7350 L mol⁻¹.     

A cerium(III) selective polyvinyl chloride membrane sensor based on a Schiff base complex of N,N'-bis[2-(salicylideneamino)ethyl]ethane-1,2-diamine

A polyvinyl chloride (PVC) based membrane sensor for cerium ions was prepared by employing N,N′-bis[2-(salicylideneamino)ethyl]ethane-1,2-diamine as an ionophore, oleic acid (OA) as anion excluder and o-nitrophenyloctyl ether (o-NPOE) as plasticizer. The plasticized membrane sensor exhibits a Nernstian response for Ce(III) ions over a wide concentration range (1.41 × 10⁻⁷ to 1.0 × 10⁻² M) with a limit of detection as low as 8.91 × 10⁻⁸ M. It has a fast response time (<10 s) and can be used for 4 months. The sensor revealed a very good selectivity with respect to common alkali, alkaline earth and heavy metal ions. The response of the proposed sensor is independent of pH between 3.0 and 8.0. It was used as an indicator electrode in potentiometric titration of fluoride, carbonate and oxalate anions and determination of cerium in simulated mixtures.     

The use of a polymer inclusion membrane for separation and preconcentration of orthophosphate in flow analysis

A highly sensitive flow analysis system has been developed for the trace determination of reactive phosphate in natural waters, which uses a polymer inclusion membrane (PIM) with Aliquat 336 as the carrier for on-line analyte separation and preconcentration. The system operates under flow injection (FI) and continuous flow (CF) conditions. Under optimal FI conditions the system is characterised by a linear concentration range between 0.5 and 1000 μg L⁻¹ P, a sampling rate of 10 h⁻¹, a limit of detection of 0.5 μg L⁻¹ P and RSDs of 3.2% (n = 10, 100 μg L⁻¹) and 7.7% (n = 10, 10 μg L⁻¹). Under CF conditions with 10 min stop-flow time and sample solution flow rate of 1.32 mL min⁻¹ the flow system offers a limit of detection of 0.04 μg L⁻¹ P, a sampling rate of 5 h⁻¹ and an RSD of 3.4% (n = 5, 2.0 μg L⁻¹). Interference studies revealed that anions commonly found in natural waters did not interfere when in excess of at least one order of magnitude. The flow system, operating under CF conditions, was successfully applied to the analysis of natural water samples containing concentrations of phosphate in the low μg L⁻¹ P range, using the multipoint standard addition method.     

Electrosorption-enhanced solid-phase microextraction of trace anions using a platinum plate coated with single-walled carbon nanotubes

A platinum plate coated with single-walled carbon nanotubes (SWCNTs@Pt) was prepared by means of electrophoretic deposition. Using the SWCNTs@Pt plate, an electrosorption-enhanced solid-phase microextraction (EE-SPME) technique was proposed for the extraction of trace anions in water, described as follows: a positive potential was applied to the SWCNTs@Pt plate to extract F⁻, Cl⁻, Br⁻, NO₃⁻ and SO₄²⁻ from water using electrosorption, and then a negative potential was applied to the plate placed in ultra-pure water for the desorption of the absorbed anions, and finally the desorbed anions were analyzed using ion chromatography (IC). The EE-SPME parameters, including extraction potential and time as well as desorption potential and time, were investigated. An analytical method based on the above procedures, i.e., EE-SPME-IC, was established and used for the analysis of trace anions in water. The results showed that the application of potential on the SWCNTs@Pt plate significantly enhanced the ion extraction efficiency, and an enrichment factor of 15-38 was achieved. The SWCNTs@Pt plate could be used more than 50 times without significant decay. The linear range, the limit of detection (S/N = 3), the limit of quantification (S/N = 10) and repeatability (n = 7) of our EE-SPME-IC method were 1.0-150.0 μg/L, 0.06-0.26 μg/L, 0.19-0.85 μg/L and 2.1-8.0%, respectively. The proposed method was successfully applied for the analysis of trace anions in deionized water, and acceptable recoveries between 65.3 and 121.1% were obtained for the spiked deionized water samples.     

Highly selective thiocyanate membrane electrode based on butane-2,3-dione bis(salicylhydrazonato)zinc(II) complex

A highly selective poly(vinyl chloride) (PVC) membrane electrode based on butane -2,3-dione bis(salicylhydrazonato) zinc(II) [Zn (BDSH)] complex as carrier for thiocyanate-selective electrode is reported. The influence of membrane composition, pH and possible interfering anions were investigated on the response properties of the electrode. The sensor responds to thiocyanate in linear range from 1.0 × 10⁻⁶ to 1.0 × 10⁻¹ M with a slope −56.5 ± 1.1 mV decade⁻¹, over a wide pH range of 3.5-8.5. The limit of detection of the electrode was 7.0 × 10⁻⁷ M SCN⁻. Selectivity coefficients determined with fixed interference method (FIM) indicate a good discriminating ability towards SCN⁻ ion in comparison to other anions. The proposed sensor has a fast response time of about 5-15 s and can be used for at least 3 months without any considerable divergence in potential. It was applied as indicator electrode in titration of thiocyanate with Ag⁺ and in potentiometric determination of thiocyanate in saliva and urine samples.     

Organic tin compounds combined with anionic additives-an ionophore system leading to a phosphate ion-selective electrode?

The potentiometric response characteristics of electrodes based on organic trialkyl/aryl-tin compounds combined with various amounts of anionic additive (NaTFPB) were investigated in 0.1 M bis-Tris-H₂SO₄ buffer solution at pH 7.0. The best result for phosphate sensing was obtained for the electrode membrane containing tributyltin chloride and 25 mol% NaTFPB, where the electrode exhibits high selectivity towards phosphate anions with a slope of −60 mV per decade. It was demonstrated that the interference from more lipophilic anions is drastically suppressed (, i=H₂PO₄⁻: salicylate, 0.5; SCN⁻, −0.8; ClO₄⁻, −2.3) under this optimized measurement conditions.     

Synthesis, optical and electrochemical properties of new receptors and sensors containing anthraquinone and benzimidazole units

Novel anion receptors and sensors, HBIMANQ and BIMANQ fabricated from the imidazolole unit and anthraquinone moieties were synthesized. ¹H NMR spectroscopy and UV-vis titrations in DMSO-d₆ and DMSO, respectively, showed that both receptors underwent deprotonation at the NH- moiety of the amide-anthraquinone unit in the presence of basic anions such as F⁻ and AcO⁻. These phenomena gave a dramatic color change due to charge transfer transition corresponding to the shift of λ_max from 371 nm to 489 nm. Redox chemistry of HBIMANQ and BIMANQ in the presence of anions (F⁻, Cl⁻, AcO⁻, BzO⁻, and H₂PO₄⁻) using cyclic voltammetry showed the different CV responses upon addition of various anions. In the case of HBIMANQ with various anions, the CV changes are dependent on the basic strength of anions in order of F⁻>AcO⁻, BzO⁻>H₂PO₄⁻>Cl⁻, Br⁻. Interestingly, the CV responses of BIMANQ with H₂PO₄⁻ exhibited the most significant changes. BIMANQ, thus, has an excellent electrochemical selectivity toward H₂PO₄⁻.     

Anodic oxidation of organometallic sandwich complexes using [Al(OC(CF3)3)4] or [AsF6] as the supporting electrolyte anion

Anodic voltammetry and electrolysis of the metallocenes ferrocene, ruthenocene, and nickelocene have been studied in dichloromethane containing two different fluorine-containing anions in the supporting electrolyte. The perfluoroalkoxyaluminate anion [Al(OC(CF₃)₃)₄]⁻ has very low nucleophilicity, as shown by its inertness towards the strong electrophile [RuCp₂]⁺ and by computation of its electrostatic potential in comparison to other frequently used electrolyte anions. The low ion-pairing ability of this anion was shown by the large spread in E_1/2 potentials (ΔE_1/2 = 769 mV) for the two one-electron oxidations of bis(fulvalene)dinickel. The hexafluoroarsenate anion [AsF₆]⁻, on the other hand, reacts rapidly with the ruthenocenium ion and is much more strongly ion-pairing towards oxidized bis(fulvalene)dinickel (ΔE_1/2 = 492 mV). In terms of applications of these two anions to the anodic oxidation of organometallic sandwich complexes, the behavior of [Al(OC(CF₃)₃)₄]⁻ is similar to that of other weakly-coordinating anions such as [B(C₆F₅)₄]⁻, whereas that of [AsF₆]⁻ is similar to the more traditional electrolyte anions such as [PF₆]⁻ and [BF₄]⁻. Additionally, the synthesis and crystal structure of [Cp₂Fe][Al(OC(CF₃)₃)₄] are reported.     

Oxidation of substituted pyridines by dimethyldioxirane: kinetics and solvent effects

The oxidation of a series of substituted pyridines by dimethyldioxirane (1) produced the expected N-oxides in quantitative yields. The second order rate constants (k₂) for the oxidation of a series of substituted pyridines (2a-g) by dimethyldioxirane were determined in dried acetone at 23 °C. An excellent correlation with Hammett sigma values was found (ρ = −2.91, r = 0.995). Kinetic studies for the oxidation of 4-trifluoromethylpyridine by 1 were carried out in the following dried solvent systems: acetone (k₂ = 0.017 M⁻¹ s⁻¹), carbon tetrachloride/acetone (7:3; k₂ = 0.014 M⁻¹ s⁻¹), acetonitrile/acetone (7:3; k₂ = 0.047 M⁻¹ s⁻¹), and methanol/acetone (7:3; k₂ = 0.68 M⁻¹ s⁻¹). Kinetic studies of the oxidation of pyridine by 1 versus mole fraction of water in acetone [k₂ = 0.78 M⁻¹ s⁻¹ (χ = 0) to k₂ = 11.1 M⁻¹ s⁻¹ (χ = 0.52)] were carried out. The results showed the reaction to be very sensitive to protic, polar solvents.     

A novel histidine assay using tetraphenylporphyrin manganese (III) chloride as a molecular recognition probe by resonance light scattering technique

A novel histidine-selective method has been developed for the determination of histidine in aqueous solutions by resonance light scattering (RLS) technique. At pH 8.0, the weak RLS intensity of tetraphenylporphyrin manganese (III) chloride [MnTPPCl] was greatly enhanced by the addition of histidine with the maximum peak located at 483 nm. Under the optimum conditions, it was found that the enhanced RLS intensity was in proportion to the concentration of histidine in the range 7.8 × 10⁻⁷-2.4 × 10⁻⁵ mol l⁻¹. Low detection limit of 9.2 × 10^-8 mol l⁻¹ has been achieved. The histidine concentrations in synthetic samples and real samples were determined with satisfactory results. The sensitivity and selectivity of this method are high enough to permit the determination of trace amounts of histidine without any significant interference from high levels of other components such as common anions and especially, other amino acids.     

Fluorescent sensing of pyrophosphate and ATP in 100% aqueous solution using a fluorescein derivative and Mn

A new fluorescein derivative has been synthesized for the detection of pyrophosphate (PPi) and ATP in 100% aqueous solution. Chemosensor 1 in the presence of Mn²⁺ (2.5 equiv) displayed selective fluorescent enhancements with PPi and ATP at pH 7.4. among the anions examined. The association constant of 1 in the presence of Mn²⁺ with PPi and ATP was calculated as 4.2 × 10⁴ and 3.5 × 10⁴ M⁻¹.