Potentiometric response and mechanism of anionic recognition of heterocalixarene-based ion selective electrodes

The ion selective electrode (ISE)-based potentiometric approach is shown to be an effective means of characterizing the anion recognition sites in the molecular receptor calix[2]pyridino[2]pyrrole (CPP). In particular, potentiometric pH-measurements involving the use of experimental PVC-membranes based on CPP revealed the existence of both mono- and diprotonated forms of the receptor under readily accessible conditions. Based on these analyses, apparent surface protonation constants for this heterocalixarene were found to lie between 8.5-8.9 (pK_B1) and 3.3-3.8 (pK_B2). CPP was found to interact with targeted anionic analytes based on both coulombic and hydrogen bond interactions, as inferred from varying the kinds of ionic sites present within the membrane phase. Potentiometric selectivity studies revealed that CPP preferred “Y-shaped” anions (e.g. acetate, lactate, benzoate) over spherical anions (e.g. fluoride and chloride), fluoride over chloride within the set of spherical anions, and the ortho-isomer over the corresponding meta- and para-isomers in the case of hydroxybenzoate (salicylate and congeners). In the context of this study, the advantages of potentiometric determinations of acetylsalicylic acid using optimized PVC-membranes based on CPP relative to more conventional PVC-membrane ISEs based on traditional anion exchanger were also demonstrated.     

PVC matrix membrane electrodes based on cobalt and nickel phenanthroline complexes for selective determination of cobalt(II) and nickel(II) ions

Potentiometric sensors for determining cobalt and nickel ions are described. They are based on the use of cobalt and nickel tris(1,10-phenanthroline)-TPB as electroactive compounds dispersed in plasticized poly(vinyl chloride) matrix. The sensors exhibit fast, near-Nernstian responses for cobalt and nickel-phenanthroline cations over the pH range 3–11 with a slope of 30.3 ± 0.3 mV/concentration decade. In the presence of excess 1,10-phenanthroline reagent, cobalt(II) and nickel(II) ions at concentration levels as low as 4 × 10^–6 M are accurately determined. The results show an average metal ion recovery of 98.5% with a mean standard deviation of 0.5%. Cobalt in organometallic compounds and nickel in silicate rocks are determined by these sensors and results agreeing fairly well with atomic absorption spectrometry are obtained.     

Speciation of chromium using chronoamperometric biosensors based on screen-printed electrodes

Chronoamperometric assays based on tyrosinase and glucose oxidase (GOx) inactivation have been developed for the monitoring of Cr(III) and Cr(VI). Tyrosinase was immobilized by crosslinking on screen-printed carbon electrodes (SPCEs) containing tetrathiafulvalene (TTF) as electron transfer mediator. The tyrosinase/SPC_TTFE response to pyrocatechol is inhibited by Cr(III). This process, that is not affected by Cr(VI), allows the determination of Cr(III) with a capability of detection of 2.0 ± 0.2 μM and a reproducibility of 5.5%. GOx modified screen-printed carbon platinised electrodes (SPC_PtEs) were developed for the selective determination of Cr(VI) using ferricyanide as redox mediator. The biosensor was able to discriminate two different oxidation states of chromium being able to reject Cr(III) and to detect the toxic species Cr(VI). Chronoamperometric response of the biosensor towards glucose decreases with the presence of Cr(VI), with a capability of detection of 90.5 ± 7.6 nM and a reproducibility of 6.2%. A bipotentiostatic chronoamperometric biosensor was finally developed using a tyrosinase/SPC_TTFE and a GOx/SPC_PtE connected in array mode for the simultaneous determination of Cr(III) and Cr(VI) in spiked tap water and in waste water from a tannery factory samples.     

Cation and anion exchange on clay modified electrodes

Carbon paste electrodes modified with clay minerals were used for the study of ion-exchange properties of these clay minerals. Cation exchange of Cu(II) was studied by the dependence on the pH, ionic strength and the form of the minerals. The highest anionic exchange of Hg(II) acetates and chlorides was found on montmorillonites. Anionic exchange of Au(III) chloro complexes was found on montmorillonite, but their sorption is strongly influenced by other anions such as chlorides and thiocyanates. The results found are important with regard to the study of the anion-exchange properties of clay minerals. Received: 2 December 1999 / Accepted: 2 March 2000     

Determination of trace cobalt(II) by adsorptive stripping voltammetry on disposable microfabricated electrochemical cells with integrated planar metal-film electrodes

This work reports the determination of trace Co(II) by adsorptive stripping voltammetry on disposable three-electrode cells with on-chip metal-film electrodes. The heart of the sensors was a bismuth-film electrode (BiFE) with Ag and Pt planar strips serving as the reference and counter electrodes, respectively. Metals were deposited on a silicon chip by sputtering while the areas of the electrodes were patterned via a metal mask. Co(II) was determined by square wave adsorptive stripping voltammetry (SWAdSV) after complexation with dimethylglyoxime (DMG). The experimental variables (the DMG concentration, the preconcentration potential, the accumulation time and the SW parameters), as well as potential interferences, were investigated. Using the selected conditions, the 3σ limit of detection was 0.09 μg l⁻¹ of Co(II) (for 90 s of preconcentration) and the relative standard deviation for Co(II) was 3.8% at the 2 μg l⁻¹ level (n = 8). The method was applied to the determination of Co(II) in a certified river water sample. These mercury-free electrochemical devices present increased scope for field analysis and μ-TAS applications.     

硫脲类阴离子受体的设计合成与阴离子识别

硫脲类化合物具有生物活性,是优良的氢键供体,与阴离子特别是含氧阴离子形成氢键配合物。本文设计合成了系列二苯基硫脲衍生物,通过取代基效应调控硫脲衍生物分子内电转移过程,阴离子与硫脲基团结合后,增强了分子内供体的给电子能力,进一步促进电荷转移过程,据此识别不同的阴离子。由于主体分子与阴离子间形成氢键的能力及阴离子碱性的差异,可达到选择性结合的目的。     

Tunability of anion binding strength based on acyl-thiourea receptors containing isatin group

Some acyl-thiourea derivatives containing isatin group were synthesized and their interactions with anions were investigated using UV–vis spectroscopy and ¹H NMR titrations in DMSO and DMSO-d₆, respectively. These compounds have a same molecular framework, functionalising with different groups lead to different anion binding strength of these receptors. Receptor 1 showed a higher binding affinity for AcO⁻ than for F⁻, due to the cooperative multiple hydrogen bond interactions of AcO⁻ with the acyl-thiourea group and N–H group in the indole unit of receptor 1. Displacing the N–H proton in the indole unit with –CH₃ group, receptor 2 showed no obviously discriminative responses for F⁻, AcO⁻ and H₂PO₄⁻ due to lack of such additional binding. In the case of receptor 3, which was functionalised with strong electron-withdrawing group, it showed selectively chromogenic response for F⁻ based on double deprotonation of the receptor in DMSO, whereas AcO⁻ and H₂PO₄⁻ induced single deprotonation only.     

The ion adsorption effect on selectivity of liquid state, O,O'-didecylodithiophosphate chelate based ion-selective electrodes

Ion-selective electrodes with liquid membranes including O,O′-didecylo-dithiophosphate complexes of Tl(I), Pb(II), Cd(II) and Ni(II) are characterised and results of the study on their selectivity are reported. A short review of problems related to determination and interpretation of selectivity coefficients of ion-selective electrodes is presented with particular emphasis on the drawbacks of the hitherto used methods. A new method is proposed, which in the experimental part is close to that of mixed solutions recommended by IUPAC but can be applied also when the latter is of no use. The method proposed for determination of selectivity coefficients simultaneously allows concluding about the mechanism of potential generation. A few examples of relations between selectivity coefficients of the electrodes and concentrations of disturbing ions in solutions, are given. An interpretation of the above relations as results of the processes of ion adsorption at the interface of the electrode membrane and water solution is proposed. The results obtained have confirmed the hypothesis given by Pungor, according to which the main role in the mechanism of generation of ion-selective electrodes potential is played by the processes of ion chemisorption at the interface of the membrane and water solution.     

Synthesis,characterization, and spectroscopic studies of tetradentate Schiff base chromium(III) complexes

Eight chromium(III) complexes of tetradentate Schiff bases have been prepared in situ by condensing of a substituted salicylaldehyde compound with ethylenediamine. These were characterized by elemental analysis, m.p., IR, molar conductivity, magnetic moment measurements, and electronic spectra. The free ligands were also characterized by ¹H and ¹³C NMR spectra. The ¹³C NMR spectra are discussed in terms of possible substituent effects. The IR and electronic spectra of the free ligand and the complexes are compared and discussed. The electrospray ionization (ESI) mass spectra of four free ligands and their complexes were measured. The deconvolution of the visible spectra of the complexes, C_2v symmetry, in DMSO yields three peaks at ca. 15 600–17 600, 18 400–20 400 and 20 000–23 100, and are assigned to the three d–d transitions, ⁴B_1g → ⁴E_g(⁴T_2g); ⁴B_1g → ⁴B_2g(⁴T_2g); ⁴B_1g → ⁴E_g(⁴T_1g), respectively. The complexes showed magnetic moment in the range of 3.5–4.2 BM which corresponds to three unpaired electrons.     

A cobalt(II)-selective PVC membrane based on a Schiff base complex of N,N′-bis(salicylidene)-3,4-diaminotoluene

A new PVC membrane electrode for Co²⁺ based on N,N′-bis(salicylidene)-3,4-diaminotoluene, an excellent neutral carrier, has been fabricated using sodium tetraphenylborate (NaTPB) as an anionic excluder and dioctylphthalte (DOP) as a solvent mediator. The electrode exhibits a linear potential response in the concentration range of 7.9 × 10⁻⁸ to 1.0 × 10⁻¹ M with a slope of 30 ± 0.2 mV per decade. The detection limit of the proposed sensor is 5.0 × 10⁻⁸ M and it can be used over a period of 5 months. The proposed sensor revealed good selectivity over a wide variety of other cations including alkali, alkaline earth, heavy and transition metals and could be used in the pH range of 2.0-9.0. This electrode was successfully applied for the determination of Co²⁺in real samples and as an indicator electrode in potentiometric titration of cobalt ions.     

Potentiometric detection of polyions based on functionalized magnetic nanoparticles

A novel potentiometric detection strategy based on functionalized magnetic nanoparticles has been developed for rapid and sensitive sensing of polyions. Highly dispersed magnetic nanoparticles coated with ion exchanger and plasticizer could promote an in situ cooperative ion-pairing interaction between the ion exchanger and the polyion analyte in sample solution by dramatically reducing the mass-transfer distance. With applying a magnetic field, the nanoparticles can be attached to the surface of ion exchanger free polymeric membrane. The observed potential signals are related to the polyion concentrations. The proposed polymeric membrane electrode exhibits a linear relationship between the greatest potential response slope (dE/dt) and the logarithm of protamine concentration in the range of 0.05−5 μg/mL with a lower detection limit of 0.033 μg/mL.     

Electrocatalytic oxidation of hydrazine at cobalt hexacyanoferrate- modified glassy carbon, Pt and Au electrodes

The electrocatalytic oxidation of hydrazine has been studied on glassy carbon, Pt and Au electrodes modified by cobalt hexacyanoferrate (CoHCF) using cyclic voltammetry and rotating disc techniques. It has been shown that the oxidation of hydrazine to nitrogen occurs at the potential coinciding with that of Co(II) to Co(III) transformation in a CoHCF film, where no oxidation signal is observed at a bare glassy carbon electrode. A Tafel plot, derived from RDE voltammograms, exhibits a slope of 150 mV, indicating a one-electron charge transfer process to be the rate-limiting step. The electrocatalytic efficiency of the modified electrode towards hydrazine oxidation depends on solution pH, and the optimum range was found to be located between pH 5 and pH 7. The kinetic behaviour and location of the electrocatalytic process were examined using the W.J. Albery diagnosis table, and it was concluded that the reaction has either a “surface” or a “layer” reaction mechanism. Pt- and Au-CoHCF-modified electrodes show no significant electrocatalytic activity towards hydrazine oxidation. Received: 25 April 1997 / Accepted: 12 August 1997     

Evaluation of GaN and In0.2Ga0.8N Semiconductors as Potentiometric Anion Selective Electrodes

The response of potentiometric anion selective electrodes consisting of undoped GaN or In_0.2Ga_0.8N films grown on Al₂O₃ (sapphire) was measured in electrolyte solutions of F^?, NO₃^?, Cl^?, SCN^?, ClO₄^? or Br^? anions at concentrations ranging from 10^?6 to 10^?1 M. The slope of the linear regions varied between ?32.8 and ?51.9 mV/decade for the GaN electrode and between ?31.0 and ?72.0 mV/decade for the In_0.2Ga_0.8N electrode. The drift of the GaN electrode reached 1.57 mV/day in KNO₃ solutions, whereas the drift of the In_0.2Ga_0.8N electrode could not be evaluated due to large drops in the slope of its linear range over time. Both electrodes were sensitive to pH variations over the pH range from 12.8 to 1.3. The GaN electrode surface could be electrochemically etched under anodic polarization; however, both GaN and In_0.2Ga_0.8N electrodes remained chemically stable and mechanically intact under open circuit conditions even after prolonged use.     

Design and synthesis of novel tweezer anion receptors based on deoxycholic acid

A novel type of molecular tweezer receptors based on deoxycholic acid has been designed and synthesized and their binding properties were examined by UV-vis spectral titration. These molecular tweezers showed a high selectivity toward F- over Cl-,Br-, I-, AcO-, H2PO4-.     

Vertically aligned carbon nanotube based electrodes: Fabrication, characterisation and prospects

We present a methodology to fabricate carbon nanotube based electrodes using plasma enhanced chemical vapour deposition. The metal catalyst nanoparticles used to promote nanotube growth are removed using a water plasma treatment in combination with an acid attack. The final integrated microelectrode-based devices present excellent electrocatalytic properties that make them suitable for electrochemical applications. The presented methodology enables the construction of highly regular and dense vertically aligned carbon nanotube (VACNT) forests that can be confined within the patterned bounds of a desired surface. These VACNT electrodes display very low capacitive currents and are amenable to further chemical modifications.     

Effect of cyclodextrins on the electrochemical behaviour of ascorbic acid on platinum electrodes in acidic and neutral solutions

The effect of β-CD and α-CD on the electrochemical behaviour of H₂A and HA⁻ on platinum is studied. The adsorption of β-CD on this electrode is demonstrated and proved to be dependent on the base electrolyte composition. The maximum adsorption coverage was reached in phosphate solution at pH 6.95. The homogeneous H₂A---β-CD complex formation produced a decrease in the oxidation current and a positive shift in the oxidation peak potential. These effects are predominant in acid solutions. In neutral solutions the opposite behaviour is observed, i.e. an increase in the oxidation current at lower oxidation potentials. The presence of a parallel oxidation route for the vitamin involving strongly adsorbed CO residues is considered, and evidence for a decrease in CO_ad in the presence of β-CD was given by variations in hydrogen adsorption charges. This fact, more important in neutral solutions, must be responsible for the catalytic effect observed. α-CD was not adsorbed, neither did it modify the electro-oxidation behaviour of H₂A and HA⁻.     

Urea and thiourea based anion receptors in solution and on polymer supports

Herein we report the development of a new series of surface bound anion sensors exploiting the urea or thiourea motif capable of binding anions through hydrogen bonding interactions. The use of high resolution magic angle spinning ¹H NMR allows the direct comparison of the anion binding properties of these receptors in solution versus those tethered to polymer resins. Some intramolecular hydrogen bonding and solvent effects were observed at the solution:surface interface however in general the anion binding properties of the polymer bound urea and thiourea receptors were maintained.     

Recent developments in the field of screen-printed electrodes and their related applications

The development of analytical methods that respond to the growing need to perform rapid ‘in situ’ analyses shows disposable screen-printed electrodes (SPEs) as an alternative to the traditional electrodes. This review presents recent developments in the electrochemical application of disposable screen-printed sensors, according to the types of materials used to modify the working electrode. Therefore, unmodified SPE, film-modified SPE, enzyme-modified SPE and antigen/antibody-modified SPE are described. Applications are included where available.     

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.