Polymeric Membrane Ion-selective Electrodes Research Articles

All-solid-state polymeric membrane ion-selective miniaturized electrodes based on a nanoporous gold film as solid contact.

A new type of all-solid-state polymeric membrane ion-selective electrodes (ISEs) is developed by using a nanoporous gold (NPG) film as solid contact. The NPG film is in situ formed on the surface of a gold wire electrode by the multicyclic electrochemical alloying/dealloying method. The characteristics of the NPG film, such as the large surface area, high double layer capacitance, and good conductivity, have been demonstrated by cyclic voltammetry and electrochemical impedance spectroscopy. The NPG film offers a well-defined interface between the electronic conductor and the ion-selective membrane. The NPG film-based all-solid-state K(+) ISE shows a stable Nernstian response within the concentration range from 10(-6) to 10(-2) M, and the detection limit is 4.0 × 10(-7) M. The proposed electrode exhibits an improved potential stability with a reduced water layer in comparison with the coated-wire K(+)-ISE, which is due to the bicontinuous electron- and ion-conducting properties of the ionophore-doped polymeric membrane/NPG film interlayer. Unlike the additionally coated intermediate layers as single-use solid contacts, the in situ formed NPG film as solid contact is reusable. This work provides a versatile method for fabricating the robust, reliable, and low-maintenance miniaturized ISEs.

ChemInform Abstract: Applications of Nanomaterials in Potentiometric Sensors

AbstractReview: 75 refs.

Nanomolar Detection of Alkyl Dimethyl Hydroxyethyl Ammonium Surfactant by a Lipophile-doped Solid Contact Electrode

Various methods for improving the lower detection limit of polymeric membrane ion-selective electrodes have been approached recently. The ion-selective electrode with solid-state membrane is described. Tetraphenyl borate as a lipophilic anion, is incorporated into the membrane of the electrode to improve its detection limit and selectivity. The detection limit was significantly lowered where nanomolar quantities were traced. Other favorable characteristics of the electrode are noticeably short response time of 5 seconds and good selectivity over the common interfering chemical species as well as its proven usefulness for determination of the surfactant in miscellaneous samples.

Construction and performance characteristics of polymeric membrane electrode and coated graphite electrode for the selective determination of Fe3 + ion

Novel Fe3+ ion-selective polymeric membrane electrodes (PMEs) were prepared using three different ionophores N-(4-(dimethylamino)benzylidene)thiazol-2-amine [L1], 5-((3-methylthiophene-2yl) methyleneamino)-1,3,4-thiadiazole-2-thiol [L2] and N-((3-methylthiophene-2yl)methylene)thiazol-2-amine [L3] and their potentiometric characteristics were discussed. Effect of various plasticizers and anion excluders was also studied in detail and improved performance was observed. The best performance was obtained for the membrane electrode having a composition of L2:PVC:o-NPOE:NaTPB as 3:38.5:56:2.5 (w/w; mg). A coated graphite electrode (CGE) was also prepared with the same composition and compared. CGE is found to perform better as it shows a wider working concentration range of 8.3×10−8–1.0×10−1molL−1, a lower detection limit of 2.3×10−8molL−1, and a near Nernstian slope of 19.5±0.4mVdecade−1 of activity with a response time of 10s. The CGE shows a shelf life of 6weeks and in view of high selectivity, it can be used to quantify Fe3+ ion in water, soil, vegetable and medicinal plants. It can also be used as an indicator electrode in potentiometric titration of EDTA with Fe3+ ion.

Pulsed Galvanostatic Control of a Polymeric Membrane Ion-Selective Electrode for Potentiometric Immunoassays

Pulsed galvanostatic control of ion fluxes across polymeric membrane ion-selective electrodes (ISEs) is an emerging field for potentiometric sensing. Herein we report a novel potentiometric enzyme immunoassay based on current-controlled release of an enzyme substrate, which eliminates the addition of marker ions in the sample solution. In this method, the carboxylated poly(vinyl chloride) matrix at the outer layer of the ISE membrane is employed to attach a primary antibody. A sandwich immunoassay with an alkaline phosphatase labeled antibody (ALP-Ab) as the reporter is used for the determination of human IgG (as a model protein). The large difference between the lipophilicity of the substrate ion and that of the product ion allows p-nitrophenyl phosphate to be used as the enzyme substrate for potentiometric immunosensors. After the immunoreactions, the captured ALP-Ab catalyzes the hydrolysis of the substrate ions released at the sample-membrane interface by using the pulsed galvanostatic technique. This process can be potentiometrically determined by measuring the open circuit potential of the ISE. Under optimal conditions, the potential response of the proposed immunosensor is proportional to the concentration of human IgG in the range of 50-1000 ng/mL with a detection limit of 30 ng/mL (3σ). Owing to simplicity and independence of sample volume and sample turbidity, the proposed potentiometric immunoassay offers a viable alternative to those based on optical absorbance.

Bifunctional polymeric membrane ion selective electrodes using phenylboronic acid as a precursor of anionic sites and fluoride as an effector: A potentiometric sensor for sodium ion and an impedimetric sensor for fluoride ion

A new concept in ion sensing mimicking allosteric membranes in biological systems was proposed and demonstrated using fluoride-effector Na+ selective electrodes (Na-ISEs). DOS-plasticized PVC membranes containing p-tert-butylcalix[4]arene tetraethyl ester (NaX) and 4-[(4-tert-butyl-2-methylphenoxy)methyl]phenyl boronic acid (50mol% compared to NaX) were found to give the best characteristic of Na-ISEs upon conditioning the membranes in 10−2M NaF: a response slope of 57.48mV per decade and detection limit of 7.42×10−7M. The selectivity coefficients (log KNa,jpot) for the optimal electrode were −2.29, −2.84, −3.16, −3.42, and −3.01 for K+, Li+, Cs+, Mg2+ and Ca2+, respectively. The optimal electrodes also gave good reversibility and could be used in a wide range of pH (pH 3.6–9.7). The fabricated Na-ISEs were found to have membrane resistances (Rm) varied with concentrations of fluoride ions used in membrane conditioning steps. Electrochemical impedance spectra showed a linear relationship between Rm and the logarithm of the concentration range of fluoride ions from 1×10−1 to 1×10−8M. Conditioning optimal membranes with various concentrations of Bu4NF gave a non-linear relationship. The proposed membrane electrode, thus, showed permselectivity toward Na+ using F− as an effector. Therefore, the proposed new concept led to the fabrication of a bifunctional sensor: a potentiometric sensor for sodium ion and an impedimetric sensor for fluoride ion.

Solvent polymeric membrane ion-selective electrodes under galvanostatic control: Powerful tool for analysis of extremely diluted samples

A recently introduced tuned galvanostatic mode of measurements with ion-selective electrodes (ISEs) is utilized to perform analytical measurements in the nanomolar concentration range with the same or better selectivity than within the traditional ISE working range. Experimental evidence is provided for Ohmic behavior of ISEs when polarized with relatively large currents. Complete unified protocol of the optimization of the compensating current densities for a given ISE in a given solution is presented, as well as the complete unified protocol of analysis of extremely diluted samples. It is also demonstrated that tuned galvanostatic polarization has beneficial effect on the selectivity of ISEs. The problems related to incomplete relaxation of ISEs when the current is shut off are discussed as well.

Potentiometric Determination of Dopamine Using a Solid-Contact Polymeric Membrane Ion-Selective Electrode

A simple solid-contact polymeric membrane ion-selective electrode for determination of dopamine was developed. The electrode is fabricated with polymeric membranes incorporating heptakis(2,3,6-tri-o-methyl)-beta-cyclodextrin as ionophore and sodium trakis[3,5-bis(trifluoromethyl)phenyl]borate as ionic additive. Using hydrophobic poly(3-octylthiophene) as solid-contact transducer, the sensor displays a stable potential response for detection of dopamine in 2 mM HAc-NaAc buffer solution (pH 4.4) within a wide concentration range of 3.0 x 10(-5) M-1.0 x 10(-3) M with a sub-Nernstian slope of 43.8 +/- 0.5 mV/dec. The detection limit calculated as the intersection of the two slope lines is 1.3 x 10(-5) M. The electrode exhibits good selectivity over inorganic ions, pharmaceutical excipients and ascorbic acid. In addition, the electrode shows a stable potential response over a wide pH range of 4.0 to 8.5, fast response time (less than 10 s) and excellent repeatability. The proposed electrode has been successfully applied for direct potentiometric determination of dopamine in pharmaceutical formulation without pretreatment procedures.

Trace-Level Potentiometric Detection in the Presence of a High Electrolyte Background

Polymeric membrane ion-selective electrodes (ISEs) have become attractive tools for trace-level environmental and biological measurements. However, applications of such ISEs are often limited to measurements with low levels of electrolyte background. This paper describes an asymmetric membrane rotating ISE configuration for trace-level potentiometric detection with a high-interfering background. The membrane electrode is conditioned in a solution of interfering ions (e.g., Na(+)) so that no primary ions exist in the ISE membrane, thus avoiding the ion-exchange effect induced by high levels of interfering ones in the sample. When the electrode is in contact with the primary ions, the interfering ions in the membrane surface can be partially displaced by the primary ions due to the favorable ion-ligand interaction with the ionophore in the membrane, thus causing a steady-state potential response. By using the asymmetric membrane with an ion exchanger loaded on the membrane surface, the diffusion of the primary ions from the organic boundary layer into the bulk of the membrane can be effectively blocked; on the other hand, rotation of the membrane electrode dramatically reduces the diffusion layer thickness of the aqueous phase and significantly promotes the mass transfer of the primary ions to the sample-membrane interface. The induced accumulation of the primary ions in the membrane boundary layer largely enhances the nonequilibrium potential response. By using copper as a model, the new concept offers a subnanomolar detection limit for potentiometric measurements of heavy metals with a high electrolyte background of 0.5 M NaCl.

Potentiometric Sensor Based on Molecularly Imprinted Polymers for Rapid Determination of Clenbuterol in Pig Urine

A polymeric membrane ion-selective electrode for determination of clenbuterol is described. It is based on a molecularly imprinted polymer as an ionophore for molecular recognition, which can be synthesized by precipitation polymerization using clenbuterol hydrochloride as the template molecule. Under optimized conditions, the proposed membrane electrode exhibits Nernstian response to the protonated clenbuterol over the concentration range of 1.0 x 10(-7) M to 1.0 x 10(-4) M with a slope of 55.7 mV/dec., and a detection limit of 7.0 x 10(-8) M. The MIP-based sensor shows excellent selectivity, rapid response time and satisfactory long-term stability. The potentiometric sensor has been successfully applied to the determination of clenbuterol in pig urine samples with recoveries between 98% and 107% and an analysis time of less than 3 min.

Potentiometric Sensor Based on Molecularly Imprinted Polymer for Rapid Determination of Clenbuterol in Pig Urine

A polymeric membrane ion-selective electrode for the determination of clenbuterol is described.It is based on a molecularly imprinted polymer(MIP) as an ionophore for molecular recognition,which can be synthesized by precipitation polymerization using clenbuterol hydrochloride as a template molecule.Under the optimized conditions,the proposed membrane electrode exhibits Nernstian response to the protonated clenbuterol over the concentration range of 1.0×10-7-1.0×10-4 mol/L with a slope of 55.7 mV/decade,and the detection limit is 7.0×10-8 mol/L.The MIP-based sensor shows excellent selectivity,rapid response time and satisfactory long-term stability.The potentiometric sensor has been successfully applied to determine the clenbuterol in pig urine samples with recoveries between 98% and 107% and an analytical time of less than 3 min.

Potentiometric sensor for sensitive and selective detection of heparin

A polymeric membrane ion-selective electrode for determination of heparin is described in this paper. Protamine is incorporated into the organic membrane phase and functions as sensing element for selective recognition of heparin. The proposed membrane electrode exhibits high selectivity for heparin over lipophilic anions such as thiocyanide and salicylate. The potentiometric response to the concentration of heparin is linear in the range of 0.01-0.4 U/mL and a lower detection limit of 0.005 U/mL can be achieved.

Highly-sensitive ion selective electrode based on molecularly imprinted polymer particles for determination of tetracycline in aqueous samples

In this work, a highly-sensitive polymeric membrane ion selective electrode for determination of tetracycline was constructed by using molecularly imprinted polymer (MIP) particles as quasi-ionophore. The water-compatible MIP particles targeting tetracycline were synthesized with tetracycline as a template molecule, methacrylic acid as a functional monomer, ethylene glycol dimethacrylate as a cross-linker, 2,2′-azobisisobutyronitrile as an initiator and lanthanum ion as a mediator. Benefited from the distinctive performance of the quasi-ionophore and the optimized composition of the membrane and the inner filling solution, the lower detection limit of the electrode was decreased to about 1 × 10−8 mol/l. It exhibited a good electrode slope 59.8 mV/decade near the theoretical Nernstian one, with a wide linear working range from 2.0 × 10−8 to 1.0 × 10−3 mol/l. Due to the specific recognition of tetracycline by the MIP particles, the selectivity coefficients for routine interferences were less than 10−4. The fabricated electrode should be used in pH 2–4, response time of which was less than 200 s when the concentration of tetracycline was higher than 1.0 × 10−6 mol/l and no more than 30 min at the concentration of 1.0 × 10−8 mol/l. Finally, the proposed highly-sensitive ion selective electrode has been successfully applied to the determination of tetracycline in aqueous samples.

Analytical study on cofactor biorecognition by immobilized alkaline apophosphatase

Alkaline apophosphatase (apoALP) was examined in flow bioanalytical systems as an immobilized bioligand for reversible recognition of zinc ions. For such investigations plastic open-tubular bioreactor coupled with spectrophotometric detector and potentiometric biosensor based on plasticized polymeric membrane ion-selective electrode were applied. Both biodevices contain covalently immobilized monomolecular layer of alkaline phosphatase (ALP) and operate according to the indirect biosensing scheme based on cofactor biorecognition by in situ generated apoenzymatic bioreceptor. An implementation of immobilized ALP into flow system enables automation of the developed bioanalytical procedure allowing multiple and reproducible apoenzyme formation/regeneration, analyte biorecognition as well as assays for recovered enzyme activity. Both developed bioanalytical systems offer relatively fast zinc biodetermination in the ppm range of concentrations. Additionally, effects from potential chelators and metal ions involving in holo–apo and apo–holo enzyme conversions were investigated. It was found, that the selectivity of the reported biosensing scheme is defined by alternative mechanisms for apoenzyme and holoenzyme formation. A model, clarifying such conversions, has been proposed and confirmed by simple analytical experiments performed under flow conditions.

Electrochemistry and ion-sensing properties of calix[4]arene derivatives

The cyclic voltammetric properties of several substituted calix[4]arenes were examined in acetonitrile and dichloromethane. The compounds that contained one phenolic group in the macrocyclic cavity were able to be electrochemically oxidised at positive potentials. In acetonitrile, cyclic voltammetry experiments indicated that the phenolic compounds were oxidised in a two-electron (one-proton) process over all measured scan rates (up to 50 V s −1), while in dichloromethane, the oxidation process occurred by one-electron at scan rates ≥5 V s −1, to most likely form the radical cations. In both solvents, longer timescale (minutes to hours) controlled potential coulometry experiments indicated that the oxidation process occurred by two-electrons per molecule, to form reactive diamagnetic cations that could not be reduced back to the starting materials under electrolysis conditions. The ion-sensing properties of the compounds were investigated in polymer membrane ion-selective electrodes and it was found that they responded reversibly in a Nernstian fashion to Groups 1 and 2 metals and had the highest selectivity to the cesium cation.

Lead-selective poly(vinyl choride) membrane electrodes based on heterocyclic thiocarboxylic acid

Heterocyclic thiocarboxylic acids have been designed to prepare polymeric membrane ion-selective electrode (ISE) for Pb2+. Construction, response characteristic and application of the lead ISEs are investigated. Better results have been obtained with membranes containing ligands L1∼L3 with bis(2-ethylhexyl) sebacate (DOS) as a plasticizer. Ionophores L1∼L3 are [(4,6-dimethyl-2-pyrimidinyl) thio] acetic acid (L1), (1,3,4-thiadiazole-2,5-diyldithio) diacetic acid (L2) and (1,3,4-thiadiazole-2,5-diyldithio) dipropionic acid (L3). The optimum electrodes have the composition of L1 (1.6): PVC (32.7): DOS (65.3): KTp ClPB (0.4) (w/w), L2 (1.0): PVC (32.8): DOS (66.0): KTpClPB (0.2) (w/w), and L3 (1.0): PVC (32.7): DOS (65.4): KTpClPB (0.9) (w/w). The optimized membrane electrodes work well over a wide range of concentrations (1.0 × 10−5 ∼1.0 × 10−2 M, 1.0 × 10−6 ∼1.0 × 10−2 M, and 1.0 × 10−6 ∼1.0 × 10−2 M) with the response slope of 27.4, 30.1 and 29.2 mV/decade, respectively. Potentiometric selectivities of the ISEs based on L1 ∼ L3 for Pb2+ over other interfering ions are determined with the fix interference method. The electrodes display good selectivity over a number of alkali, alkaline earth, transition and heavy metal ions. The lifetime of the electrodes is about 2 months and their response time is 20 s. Applications of these electrodes for the determination of lead in real samples and as indicator electrodes for potentiometric titration of Na2SO4 using Pb2+ solution are reported.

Generalized selectivity description for polymeric ion-selective electrodes based on the phase boundary potential model

A generalized description of the response behavior of potentiometric polymer membrane ion-selective electrodes is presented on the basis of ion-exchange equilibrium considerations at the sample–membrane interface. This paper includes and extends on previously reported theoretical advances in a more compact yet more comprehensive form. Specifically, the phase boundary potential model is used to derive the origin of the Nernstian response behavior in a single expression, which is valid for a membrane containing any charge type and complex stoichiometry of ionophore and ion-exchanger. This forms the basis for a generalized expression of the selectivity coefficient, which may be used for the selectivity optimization of ion-selective membranes containing electrically charged and neutral ionophores of any desired stoichiometry. It is shown to reduce to expressions published previously for specialized cases, and may be effectively applied to problems relevant in modern potentiometry. The treatment is extended to mixed ion solutions, offering a comprehensive yet formally compact derivation of the response behavior of ion-selective electrodes to a mixture of ions of any desired charge. It is compared to predictions by the less accurate Nicolsky–Eisenman equation. The influence of ion fluxes or any form of electrochemical excitation is not considered here, but may be readily incorporated if an ion-exchange equilibrium at the interface may be assumed in these cases.

Current-Driven Ion Fluxes of Polymeric Membrane Ion-Selective Electrode for Potentiometric Biosensing

A polymeric membrane ion-selective electrode (ISE) for rapid potentiometric biosensing has been developed. Butyrylcholinesterase, as a target enzyme, is immobilized on the surface of the polymeric membrane. A macro-command-controlled procedure for switching between the potentiostatic and galvanostatic steps has been designed to provide a current-driven release of the butyrylcholine substrate for in situ biosensing of the enzyme and its inhibitors. This system has the flexibility of trapping substrate ions in the ISE inner filling solution and provides rapid, continuous, and reproducible measurements of enzymes and other bioanalytes involved in enzymatic systems.

A highly selective polymeric membrane barium(II) electrode based on a macrocyclic tetrabasic ligand as neutral carrier

A novel barium(II) ion-selective polymeric membrane electrode based on a macrocyclic tetrabasic ligand that derived from the reaction of 4,6-diacetylresorcinol and ethylenediamine in the molar ratio 1:1 as neutral carrier was described. The electrode, with optimum membrane composition, exhibited an excellent near-Nernstian response for Ba(2+) ion ranging from 3.6 x 10(-6) to 1.0 x 10(-1) mol/L with a detection limit of 1.9 x 10(-6) mol/L and a slope of 29.7 +/- 0.2 mV/dec in pH 4.0 nitrate buffer solution at 25 degrees C. It had a relatively fast response time (10 s), good selectivity towards Ba(2+) ion, and could be used over a wide pH range of 2.5-7.5. Then, the response mechanism was discussed in view of UV-visible spectroscopy and the A.C. impedance technique. Finally, the proposed electrode was successfully applied to the detection of Ba(2+) ions in real samples as well as in sulfate(II) ions in water.

Potentiometric sensor based on molecularly imprinted polymer for determination of melamine in milk

A polymeric membrane ion-selective electrode for determination of melamine is described in this paper. It is based on a molecularly imprinted polymer (MIP) for selective recognition, which can be synthesized by using melamine as a template molecule, methacrylic acid as a functional monomer and ethylene glycol dimethacrylate as a cross-linking agent. The membrane electrode shows near-Nernstian response (54 mV/decade) to the protonated melamine over the concentration range of 5.0 × 10 −6 to 1.0 × 10 −2 mol L −1. The electrode exhibits a short response time of ∼16 s and can be stable for more than 2 months. Combined with flow analysis system, the potentiometric sensor has been successfully applied to the determination of melamine in milk samples. Interference from high concentrations of ions co-existing in milk samples such as K + and Na + can be effectively eliminated by on-line introduction of anion- and cation-exchanger tandem columns placed upstream, while melamine existing as neutral molecules in milk of pH 6.7 can flow through the ion-exchanger columns and be measured downstream by the proposed electrode in an acetate buffer solution of pH 3.7.