Potentiometric anion selectivity of polymer-membrane electrodes based on cobalt, chromium, and aluminum salens

Metallo-salens of cobalt(II) (Co-Sal), chromium(III) (Cr-Sal), and aluminum(III) (Al-Sal) are used as the active ionophores within plasticized poly(vinyl chloride) membranes. It is shown that central metal-ion plays a critical role in directing the ionophore selectivity. Polymer-membrane electrodes based on Co-Sal, Cr-Sal, and Al-Sal are demonstrated to exhibit enhanced responses and selectivity toward nitrite/thiocyanate, thiocyanate, and fluoride anions, respectively. The improved anion selectivity of the three ionophore systems is shown to deviate significantly from the classical Hofmeister pattern that is based only on ion lipophilicity. For example, optimized membrane electrodes for nitrite ion based on Co-Sal exhibit logK(Nitrite,Anion)(pot) values of -5.22, -4.66, -4.48, -2.5 towards bromide, perchlorate, nitrate, and iodide anions, respectively. Optimized membrane electrodes based on Co-Sal and Cr-Sal show near-Nernstian responses towards nitrite (-57.9+/-0.9 mV/decade) and thiocyanate (-56.9+/-0.8 mV/decade), respectively, with fast response and recovery times. In contrast, Al-Sal based membrane electrodes respond to fluoride ion in a super-Nernstian (-70+/-3 mV/decade) and nearly an irreversible mode. The operative response mechanism of Co-Sal, Cr-Sal, and Al-Sal membrane electrodes is examined using the effect of added ionic sites on the potentiometric response characteristics. It is demonstrated that addition of lipophilic anionic sites to membrane electrodes based on the utilized metallo-salens enhances the selectivity towards the primary ion, while addition of cationic sites resulted in Hofmeister selectivity patterns suggesting that the operative response mechanism is of the charged carrier type. Electron spin resonance (ESR) data indicates that Co(II) metal-ion center of Co-Sal ionophore undergoes oxidation to Co(III). This process leads to formation of a charged anion-carrier that is consistent with the response behavior obtained for Co-Sal based membrane electrodes.     

Polymeric membrane electrodes with enhanced fluoride selectivity using Zr(IV)-porphyrins functioning as neutral carriers

Poly(vinyl chloride) polymeric membranes plasticized with o-NPOE (o-nitrophenyl octyl ether) or DOS (dibutyl sebacate) and containing Zr(IV)-octaethyl(OEP)- or Zr(IV)-tetraphenylporphyrins (TPP) along with lipophilic cationic additives (tridodecylmethylammonium chloride; TDMACl) are examined potentiometrically and optically with respect to their response toward fluoride. It is shown that these zirconium porphyrins can function as neutral anion carriers within the organic membranes of the electrodes. Spectrophotometric measurements of thin polymeric films indicate that the presence of lipophilic cationic sites in the form of TDMA⁺ and use of lower dielectric constant plasticizer (DOS) prevents formation of metalloporphyrin dimers in the organic polymer phase, which have been observed previously in polymeric membranes formulated with the same Zr(IV) porphyrins but with lipophilic anion site additives. By preventing dimer formation, rapid and Nernstian potentiometric response of the corresponding membrane electrodes toward fluoride ion is observed. Indeed, electrodes prepared with PVC/DOS membranes containing Zr(IV)-OEP and 15 mol% of TDMACl (relative to the ionophore) exhibit fast (t₉₅<15 s) and reversible response toward fluoride. The slope of calibration plots are near-Nernstian (−59.9 mV per decade). Such electrodes display the following selectivity pattern: ClO₄⁻>SCN⁻>F⁻>NO₃⁻>Br⁻>Cl⁻, which differs significantly from the classical Hofmeister series, with greatly enhanced potentiometric selectivity toward fluoride. The data presented herein, coupled with results from a previous study, confirm that Zr(IV) porphyrins can serve as either charged or neutral type anion carriers with respect to their enhanced interactions with fluoride when used as ionophores to prepare liquid-polymeric membrane electrodes, and that the nature of membrane additives and plasticizer dictates the response mechanism at play for given membrane formulations.     

Visible and FTIR Microscopic Observation of Bisthiourea Ionophore Aggregates in Ion‐Selective Electrode Membranes

Since conventional response models for ionophore‐based ISEs are based on the assumption of a homogeneous membrane phase, they cannot accurately predict the response of membranes containing self‐aggregating ionophores. However, meaningful conclusions about the relationship between ionophore structure and potentiometric responses can only be drawn if ionophore aggregation is properly recognized. This study demonstrates that dark field visible microscopy and FTIR microspectroscopy are valuable tools for the observation of such ionophore self‐aggregation and, thereby, the development of new ionophore‐based ISEs. Sulfate selective electrodes with solvent polymeric membranes containing bisthiourea ionophores that differ only by peripheral nonpolar substituents were shown to exhibit very different interferences from the sample pH. On one hand, optimized electrodes based on an ionophore with a phenyl substituent on each thiourea group ( 1 ) do not respond to pH at all and function well as sulfate‐selective electrodes. On the other hand, membranes containing a more lipophilic ionophore with two additional hexyl‐substituted adamantyl groups ( 2 ) exhibit severe pH interference at pH values as low as pH 5. The observation of membranes containing ionophore 2 with dark field visible microscopy and FTIR microspectroscopy shows supramolecular aggregation, and explains the startling difference between the potentiometric responses of the two types of electrodes.     

Highly selective PVC-membrane electrodes based on Co(II)-Salen for determination of nitrite ion.

A cobalt(II) derivative was used as a suitable ionophore for the preparation of a polymeric membrane nitrite-selective electrode. The electrode reveals a Nemstian behavior over a very wide NO2- ion concentration range (1.0 x 10(-6)-1.0 x 10(-1) M) and a very low detection limit (5.0 x 10(-7) M). The potentiometric response is independent of the pH of solution in the pH range 4.0-9.5. The electrode shows advantages such as low resistance, fast response and, most importantly, good selectivity relative to a wide variety of inorganic and organic anions. In fact, the selectivity behavior of the proposed NO2- ion-selective electrode shows great improvements compared to the previously reported electrodes for nitrite ion. The proposed electrodes could be used for at least 2 months without any significant changes in potentials. The electrode was successfully applied to the determination of nitrate ion concentrations in sausage and milk samples.     

New polymeric membrane cadmium(II)-selective electrodes using tripodal amine based ionophores

Fabrication of PVC membrane electrodes incorporating selective neutral carriers for Cd(2+) was reported. The ionophores were designed to have different topologies, donor atoms and lipophilicity by attaching tripodal amine (TPA) units to the lipophilic anthracene (ionophore I) and p-tert-butylcalix[4]arene (ionophores II, III and IV). The synthesized ionophores were incorporated to the plasticized PVC membranes to prepare Cd(II) ion selective electrodes (ISEs). The membrane electrodes were optimized by changing types and amounts of ionic sites and plasticizers. The selectivity of the membranes fabricated from the synthesized ionophores was evaluated, the relationship between structures of ionophores and membrane characteristics were explored. The ionophore IV which composed of two opposites TPA units on the calix[4]arene compartment showed the best selectivity toward Cd(2+). The best membrane electrode was fabricated from ionophore IV (10.2 mmol kg(-1)) with KTpClPB (50.1 mol% related to the ionophore) as an ion exchanger incorporated in the DOS plasticized PVC membrane (1:2; PVC:DOS). The Cd-ISE fabricated from ionophore IV exhibited good properties with a Nernstian response of 29.4±0.6 mV decade(-1) of activity for Cd(2+) ions and a working concentration range of 1.6×10(-6)-1.0×10(-2)M. The sensor has a fast response time of 10s and can be used for at least 1 week without any divergence in potential. The electrode can be used in the pH range of 6.0-9.0. The proposed electrodes using ionophores III and IV were employed as a probe for determining Cd(2+) from the oxidation of CdS QDs solution and the real treatment waste water sample with excellent results.     

Ion‐Selective Electrodes for Thiocyanate Based on the Dinuclear Zinc(II) Complex of a Bis‐N,O‐bidentate Schiff Base

An ion selective electrode based on the dinuclear complex formed by two zinc(II) ions and two molecules of the bis‐N,O‐bidentate Schiff base 2,2′‐[methylenebis(4,1‐phenylenenitrilomethylidyne)]bisphenol exhibits thiocyanate selectivity with a good discrimination of nitrite, nitrate, and azide. The selectivities of electrode membranes with various compositions indicate that this potentiometric selectivity is based on the formation of a 1 : 1 complex between the thiocyanate anion and the dinuclear ionophore. The 2 : 1 ratio of thiocyanate ions and the dinuclear ionophore that results from higher ratios of cationic sites and ionophore worsens the selectivity, suggesting that binding of a thiocyanate to both zinc(II) centers of the dinuclear ionophore is not favorable. Interestingly, the selectivity patterns of these electrodes differ radically from that of a highly sulfate selective electrode based on a compound reported previously to be the analogous mononuclear 1 : 1 complex of zinc(II) and the same Schiff base. It is suggested that the previously reported 1 : 1 complex with zinc(II) may indeed have been a polymer of the same elemental composition.     

Coated wire ion-selective electrode for palladium(II) and its application to catalyst analysis

The construction, performance and application of polymeric membrane (PME) and coated wire (CWE) palladium(II)-selective electrodes based on the ion pair between tetra bromopalladate(II) and hexadecylpyridinium cation (HDP⁺) in a poly(vinyl chloride) matrix, plasticized with o-NPOE are described. The influence of membrane composition, bromide ion concentration and pH on the potentiometric responses of electrodes were investigated. Nernstian responses were obtained for the two type of electrodes with low limits of detection (1.0×10⁻⁶ M for PME and 5.0×10⁻⁸ M for CWE). The potentiometric responses are independent of the pH of test solution in the range 3–8. The response time of electrodes are fast (30 s for PME and 10 s for CWE), and they can be used for at least 3 months without any considerable divergence in potentials. The proposed electrodes revealed good selectivity for palladium(II) respect to different cations and anions. They were used to the direct potentiometric determination of palladium(II) in silicon-alumina catalysts.     

New polyacrylate-based lead(II) ion-selective electrodes

A novel polyacrylate-based matrix for potentiometric ion-selective electrodes has been developed. Isododecyl acrylate, acrylonitrile and hexanedioldiacrylate co-monomers along with the thermo-initiator 2,2-dimethoxy-2-phenylacetophenone were used as polymeric matrix components. A lead(II)-selective electrode (Pb-ISE) was constructed using the above matrix. The electrode showed comparable analytical performance in the micromolar range to Pb-ISEs with conventional poly(vinyl chloride)-based membranes containing neutral ionophore and with solid-state membranes containing a mixture of lead sulphide and silver sulphide. Electrochemical impedance spectroscopy studies revealed much lower ion mobility in the polyacrylate membrane than in plasticized poly(vinyl chloride) membranes. This result additionally indicates the possibility of obtaining a lower detection limit for ISEs using the new acrylate matrix.     

Highly sensitive and selective solid-contact calcium sensor based on Schiff base of benzil with 3-aminosalycilic acid covalently attached to polyacrylic acid amide for health care

Solid-state potentiometric calcium sensors based on newly synthesized Schiff’s base of 3-aminosalycilic acid with benzil [2-hydroxy-3-(2-oxo-1,2-diphenylethylidene)amino) benzoic acid] ionophore I and with isatin [2-hydroxy-3-(2-oxoindolin-3-ylidene amino)benzoic acid] ionophore II ionophores and their covalently attached to polyacrylamide ionophores III and IV, respectively, were developed. The all-solid-state sensors were constructed by the application of a thin film of polymeric membrane cocktail onto gold electrodes that were pre-coated with the conducting polymer poly (3,4-ethylenedioxy-thiophen) as an ion and electron transducer. More than 40 sensors with membranes containing plasticized PVC or poly(butyl methacrylate-co-dodecyl methacrylate as a plasticizer-free membrane matrix were investigated. The constructed sensors contained various amounts of the different ionophores with and without anionic lipophilic additive. The sensor containing 10% of ionophore III and 3% tetra (p-chlorophenyl) borate in acrylate copolymer exhibited a stable potentiometric response over a wide pH range of 4–9. It possessed a linear concentration range of 6 10^?10 to 1 10^?2 mol L^?1 with a Nernstian slope of 28.5 mV/decade and a limit of detection (LOD) of 2 10^?10 mol L^?1. It exhibited a good selectivity for calcium to other cations. The selectivity coefficients towards different mono-, di- and trivalent cations were determined with the fixed interference method (FIM) and separate solution method (SSM). The sensor’s life time is more than 3 months, without significant deterioration in the slope. The proposed sensors were utilized for the determination of calcium concentration in serum. The results were compared with those obtained from routine clinical laboratory electrolyte analyser. The results reveal that the all-solid-state calcium sensor is promising for the point of care testing.     

Polymethacrylate polymers with appended aluminum(III)-tetraphenylporphyrins: Synthesis, characterization and evaluation as macromolecular ionophores for electrochemical and optical fluoride sensors

The synthesis and characterization of a novel polymethacylate polymer with covalently linked Al(III)-tetraphenylporphyrin (Al(III)-TPP) groups is reported. The new polymer is examined as a potential macromolecular ionophore for the preparation of polymeric membrane-based potentiometric and optical fluoride selective sensors. To prepare the polymer, an Al(III) porphyrin monomer modified with a methacrylate functionality is synthesized, allowing insertion into a polymethacrylate block copolymer (methyl methacrylate and decyl methacrylate) backbone. The resulting polymer can then be incorporated, along with appropriate additives, into conventional plasticized poly(vinyl chloride) films for testing electrochemical and optical fluoride response properties. The covalent attachment of the Al(III)-TPP ionophore to the copolymer matrix provides potentiometric sensors that exhibit significant selectivity for fluoride ion with extended lifetimes (compared to ion-selective membrane electrodes formulated with conventional free Al(III)-TPP structure). However, quite surprisingly, the attachment of the ionophore to the polymer does not eliminate the interaction of Al(III)-TPP structures to form dimeric species within the membrane phase in the presence of fluoride ion. Such interactions are confirmed by UV/visible spectroscopy of the blended polymeric films. Use of the new polymer-Al(III)-TPP conjugates to prepare optical fluoride sensors by co-incorporating a lipophilic pH indicator (4′,5′-dibromofluorescein octadecyl ester; ETH7075) is also examined and the resulting optical sensing films are shown to exhibit excellent selectivity for fluoride, with the potential for prolonged operational lifetime.     

Zirconium(IV)-porphyrins as novel ionophores for fluoride-selective polymeric membrane electrodes

The feasibility of using Zr(IV)-porphyrins as novel ionophores for preparing anion-selective polymeric membrane electrodes is examined. Electrodes constructed using o-nitrophenyl octyl ether plasticized poly(vinyl chloride) membranes containing Zr(IV)-octaethylporphyrin (OEP) dichloride (Zr(IV)[OEP]Cl₂) or Zr(IV)-tetraphenylporphyrin (TPP) dichloride (Zr(IV)[TPP]Cl₂) were found to exhibit enhanced potentiometric selectivity toward fluoride compared to electrodes based on a typical anion-exchanger (e.g. tridodecylmethylammonium chloride). At pH 5.5, the electrodes displayed the following selectivity sequences: ClO₄⁻ > SCN⁻ > I⁻ > F⁻ > NO₃⁻ > Br⁻ > NO₂⁻ > Cl⁻ and F⁻ > ClO₄⁻ > SCN⁻ > I⁻ > NO₂⁻ > NO₃⁻ > Br⁻ > Cl⁻ for membranes doped with Zr(IV)[OEP]Cl₂) and Zr(IV)[TPP]Cl₂, respectively. Both ionophores are shown to operate via a charged carrier mechanism, with 10 mol% of lipophilic tetraphenylborate derivative in the membrane phase required to achieve optimal selectivity. Electrodes prepared with both metalloporphyrin species display super-Nernstian response toward fluoride with slopes typically greater than −100 mV per decade. It is shown, via UV-VIS spectroscopy of the membrane phase, that this behavior occurs due to spontaneous formation of hydroxide ion bridged porphyrin dimers in the membrane in the presence of the lipophilic anionic additive. The dimers are easily converted to monomeric species upon increasing the concentration of fluoride in the sample solution. Decreasing the pH of sample buffer background solution (from pH 5.5 to pH 3) decreases the lower detection limit (DL) of the electrode response toward fluoride (by two-order of magnitude) and improves the electrodes’ selectivity.     

Electrochemical heparin sensing at liquid/liquid interfaces and polymeric membranes

The monitoring of heparin and its derivatives in blood samples is important for the safe usage of these anticoagulants and antithrombotics in many medical procedures. Such an analytical task is, however, highly challenging due to their low therapeutic levels in the complex blood matrix, and it still relies on classical, indirect, clot-based assays. Here we review recent progress in the direct electrochemical sensing of heparin and its analogs at liquid/liquid interfaces and polymeric membranes. This progress has been made by utilizing the principle of electrochemical ion transfer at the interface between two immiscible electrolyte solutions (ITIES) to voltammetrically drive the interfacial transfer of polyanionic heparin and monitoring the resulting ionic current as a direct measure of heparin concentration. The sensitivity, selectivity, and reproducibility of the ion-transfer voltammetry of heparin are dramatically enhanced compared to those of traditional potentiometry. This voltammetric principle was successfully applied for the detection of heparin in undiluted blood samples, and was used to develop highly sensitive ion-selective electrodes based on thin polymeric membranes that are intended for analytical applications beyond heparin detection. The mechanism of heparin recognition and transfer at liquid/liquid interfaces was assessed quantitatively via sophisticated micropipet techniques, which aided the development of a powerful ionophore that can extract large heparin molecules into nonpolar organic media. Moreover, the reversible potentiometric detection of a lethal heparin-like contaminant in commercial heparin preparations was achieved through the use of a PVC membrane doped with methyltridodecylammonium chloride, which enables charge density dependent polyanion selectivity.     

Polymeric membrane electrodes with improved fluoride selectivity and lifetime based on Zr(IV)- and Al(III)-tetraphenylporphyrin derivatives

Novel aluminum(III)- and zirconium(IV)-tetraphenylporhyrin (TPP) derivatives are examined as fluoride-selective ionophores for preparing polymer membrane-based ion-selective electrodes (ISEs). The influence of t-butyl- or dichloro-phenyl ring substituents as well as the nature of the metal ion center (Al(III) versus Zr(IV)) on the anion complexation constants of TPP derivative ionophores are reported. The anion binding stability constants of the ionophores are characterized by the so-called “sandwich membrane” method. All of the metalloporphyrins examined form their strongest anion complexes with fluoride. The influence of plasticizer as well as the type of lipophilic ionic site additive and their amounts in the sensing membrane are discussed. It is shown that membrane electrodes formulated with the metalloporphyrin derivatives and appropriate anionic or cationic additives exhibit enhanced potentiometric response toward fluoride over all other anions tested. Since selectivity toward fluoride is enhanced in the presence of both anionic and cationic additives, the metalloporphyrins can function as either charged or neutral carriers within the organic membrane phase. In contrast to previously reported fluoride-selective polymeric membrane electrodes based on metalloporphyrins, nernstian or near-nernstian (−51.2 to −60.1 mV decade⁻¹) as well as rapid (t < 80 s) and fully reversible potentiometric fluoride responses are observed. Moreover, use of aluminum(III)-t-butyltetraphenylporphyrin as the ionophore provides fluoride sensors with prolonged (7 months) functional lifetime.     

Nitrogenous synergists induced potentiometric response to metal ions with polymeric liquid membranes containing thenoyltrifluoroacetone as an ionophore.

The effects of nitrogenous synergists on the potentiometric responses to divalent transition metal ions were investigated concerning polymeric liquid membranes containing thenoyltrifluoroacetone (Htta) as an ionophore. The tested synergists were pyridine (py) and 4,4'-dioctyl-2,2'-bipyridyl (C8bpy). The potentiometric responses to metal ions, such as Cd2+, Co2+, Ni2+ and Zn2+, were induced by adding the synergists into the liquid membrane systems. The coexistence of Htta and a synergist was necessary for generating the membrane potential. The tta- anion adsorbed at the liquid membrane/solution interface and the complex formation between the synergist and a given metal ion appeared to participate in preferential uptake of metal ions.     

Graphene-based Disposable Electrochemical Sensor for Chlorhexidine Determination

In this work, simple, reliable, eco-friendly, and quantitative electrochemical sensors were developed to detect chlorhexidine Digluconate (CHX) in a variety of dosage forms, including mouthwashes and intimate douches, as well as chlorhexidine in spiked human saliva. Without any sample pre-treatment or extraction processes, CHX was measured in colored aqueous formulations. Based on carbon screen-printed electrodes, two potentiometric sensors (sensors I and II), utilizing graphene nanocomposites (Gr-NC), were designed (SPEs). An ionophore, 2-hydroxypropyl-β-cyclodextrin, was doped into the Poly Vinyl Chloride (PVC) polymeric membrane to improve sensor selectivity.     

Potentiometric and theoretical studies of the carbonate sensors based on 3-bromo-4-hexyl-5-nitrotrifluoroacetophenone

Novel carbonate ionophore, trifluoroacetophenone derivative (TFA) substituted by two acceptor substituents in the phenyl ring (3-bromo-4-hexyl-5-nitrotrifluoroacetophenone), was synthesized. Solvent polymeric membrane sensors based on this ionophore exhibited heightened selectivity to carbonate ions in the presence of the most important interfering anions. A wide range of potentiometric properties were studied and compared with those of sensors based on mono-substituted ionophores. Special attention was paid to pH dependence of sensor responses and to elaboration of appropriate conditions for carbonate analysis. A segmented-sandwich membrane method was applied for determination of the stoichiometry of ionophore-carbonate complexes, which was determined to be 1:3, and apparent complex formation constants which were 14.4 and 13.6 for DOS- and NPOE-plasticized membranes, respectively. Theoretical studies on TFA derivatives by semi-empirical (AM1 and PM3) and ab initio(6-31+G*) methods were performed, considering different types of possible ionophore-ion interactions. The formation of hydrogen bonds between carbonate and hydrated TFA was proved to be much more favourable in terms of energy compared to tetrahedral nucleophilic adducts that earlier were postulated to being formed in the membrane phase. The final conclusion on the mechanism of carbonate sensing by TFA-based solvent polymeric membrane sensors was made on the basis of computational data and detailed analysis of the literature.     

以单环有机锡配合物为载体的水杨酸根阴离子选择电极的研究

Two new cyclic pentacoordinative organotin(Ⅳ) complexes, 2,4-dihydroxyacetophenone tribenzyltin(HAPTBT) and 2,4-dihydroxyacetophenone triphenyltin(HAPTPT) were synthesized and used as anion ionophore for PVC membrane electrode. The new electrodes exhibit specificity selectivity for salicylate. The electrode based on HAPTBT as a neutral carrier displays a highly potentiometric response to salicylate and an anti-Hofmeister selectivity sequence in the following order: Sal^- ＞ SCN^- ＞ I^- ＞ NO₂^- ＞ Br^- ＞ Cl^- ＞ ClO₄^- ＞ NO₃^- ＞ SO₄^2-. The electrode has the advantages of fast response, stability and reproducibility, simplicity. The response mechanism is discussed in view of UV spectroscopy technique. The results show that there are close relationship between the potentiometric response characteristics and structure of organotin(Ⅳ) complexes. The electrode was applied to medicine analysis with satisfactory results.     

Quantitive binding constants of H(+)-selective chromoionophores and anion ionophores in solvent polymeric sensing membranes

The binding properties of neutral or charged chromoionophores and anion ionophores in solvent polymeric membranes were characterized in situ by the so-called sandwich membrane method. Acidity constants (pK(a)) of eight chromoionophores (ETH 5294, ETH 2439, ETH 5350, ETH 5418, ETH 5315, ETH 7061, ETH 7075, ETH 2412) were measured in bis(2-ethylhexyl)sebacate (DOS) and o-nitrophenyloctylether (NPOE) plasticized poly(vinyl chloride) (PVC) membranes commonly used in optical and potentiometric ion sensors. The pK(a) values of all chromoionophores in DOS membranes are by 2-3 orders of magnitude smaller than in NPOE membranes. The weak alkali metal ion binding properties with neutral H(+)-chromoionophore and anion binding with electrically charged chromoionophores were also studied quantitatively. The complex formation constants of the commercially available Co(III)cobyrinate nitrite ionophore and the organomercury chloride ionophore, ETH 9009, were also measured. The very low stability constant observed for ETH 9009 (logbeta(2)=3.60+/-0.03 in PVC-DOS and 3.61+/-0.01 in PVC-NPOE) was explained by the decomposition of the ionophore in contact with chloride samples. On the other hand, the electrically charged nitrite ionophore showed strong complexation with nitrite ions, with logbeta=10.58 and 10.59 in DOS and NPOE membranes, respectively. In contrast to cation ionophores, the stability constant of the NO(2)(-) ionophore does not change with different plasticizers.     

Aluminum(III) Porphyrins as Ionophores for Fluoride Selective Polymeric Membrane Electrodes

Aluminum(III) porphyrins are examined as potential fluoride selective ionophores in polymeric membrane type ion‐selective electrodes. Membranes formulated with Al(III) tetraphenyl (TPP) or octaethyl (OEP) porphyrins are shown to exhibit enhanced potentiometric selectivity for fluoride over more lipophilic anions, including perchlorate and thiocyanate. However, such membrane electrodes display undesirable super‐Nernstian behavior, with concomitant slow response and recovery times. By employing a sterically hindered Al(III) picket fence porphyrin (PFP) complex as the membrane active species, fully reversible and Nernstian response toward fluoride is achieved. This finding suggests that the super‐Nernstian behavior observed with the nonpicket fence metalloporphyrins is due to the formation of aggregate porphyrin species (likely dimers) within the membrane phase. The steric hindrance of the PFP ligand structure eliminates such chemistry, thus leading to theoretical response slopes toward fluoride. Addition of lipophilic anionic sites into the organic membranes enhances response and selectivity, indicating that the Al(III) porphyrin ionophores function as charged carrier type ionophores. Optimized membranes formulated with Al(III)‐PFP in an o‐nitrophenyloctyl ether plasticized PVC film exhibit fast response to fluoride down to 40 μM, with very high selectivity over SCN^?, ClO₄^?, Cl^?, Br^? and NO₃^? (k^pot<10^?3 for all anions tested). With further refinements in the membrane chemistry, it is anticipated that Al(III) porphyrin‐based membrane electrodes can exhibit potentiometric fluoride response and selectivity that approaches that of the classical solid‐state LaF₃ crystal‐based fluoride sensor.     

Coated-wire electrodes containing polymer immobilized ionophores blended with poly(vinyl chloride)

Polymers containing covalently attached 18-crown-6 or 2.2.2 cryptand units were incorporated into plasticized PVC membranes and the composite membranes were examined as potassium ion sensor elements. Ionophores were linked to carboxy-PVC and to poly(acrylic acid) via amide linkages to an alkyl spacer unit. Coated-wire electrodes (CWEs) from the immobilized ionophores gave acceptable responses, but conventional ion-selective membrane electrodes (ISEs) prepared by solvent casting were inactive. Dip-cast membranes did give active ISEs. Potassium electrode performance was independent of the loading of the ionophore within the acrylate support polymer, but depended upon the spacer length. Ion selectivity varied with the ionophore loading within the support polymer. Selectivity is a composite of the ionophore selectivity and ion-exchange interactions with the acrylate backbone, giving selectivities akin to carboxylate substituted crown ethers, notably enhanced monovalent/divalent ion discrimination relative to the ionophore in solution. Polymer immobilization extended the lifetime of active electrodes.