Plasticized Polymeric Membranes Research Articles

Application of pfg NMR diffusometry to study diffusion and ionization phenomena in plasticized polymeric membranes doped with ionic liquids

Ionic liquids (ILs) are increasingly used as promising organic solvents for a variety of applications, including electrochemical processes, novel separation techniques and innovative methods of synthesis, due to their low volatility, excellent solvation properties, high ionic conductivity and electrochemical stability. Moreover, the tunable properties of ILs determined by their composition make them favorable candidates as dopants for composite and/or functional polymeric materials, e.g. in the membranes of chemical sensors. To effectively use ILs in polymeric materials and control the characteristics of the latter, one should develop a complete understanding of the properties of ILs in the polymeric phase, such as their diffusion and ionic behavior.In our previous work, we applied pulsed field gradient (PFG) NMR technique to verify the results of a chronopotentiometric method conventionally used to determine the diffusion coefficients and electrolytic properties of electrolytes in plasticized poly(vinyl chloride) (PVC) membranes of chemical sensors. The results obtained with these two independent methods were in good agreement, confirming that PEG NMR diffusometry is a viable technique for assessing the diffusion properties of species even in highly viscous polymeric media of low polarity.Here we report a comprehensive study of the transport and ionic behavior of IL 1-hexyl-3-methyl-1H-imidazolium bis[(trifluoromethyl)sulfonyl]amide ([C6Meim][NTf2]) in a poly(vinyl chloride) matrix, performed in situ by means of PFG NMR diffusometry. A number of sets of spectra for the 1H and 19F nuclei of the [C6Meim][NTf2] ionic liquid in PVC membranes doped with the plasticizer bis(2-ethylhexyl)sebacate were registered at different amplitudes of the magnetic field gradient. The mobility of both individual ions of the IL and the formed ion associates was investigated over a broad range of IL concentrations in the PVC matrix; in addition, the ionization degree and dissociation constant of [C6Meim][NTf2] were determined and their dependence on the IL content was traced.

Sensing and extraction of hazardous metals by di-phosphonates of heterocycles: a combined experimental and theoretical study.

In this study, pyridine and phenanthroline diphosphonate ligands were investigated for the first time from the context of solvent extraction and potentiometric sensing of Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Pb(II) cations. The extraction efficiency under the same conditions for phenanthroline-diphosphonates is considerably higher than that for pyridine ligands. At the same time, the pyridine-diphosphonates show pronounced selectivity towards lead in this metal series. The extraction systems with phenanthroline diphosphonates provided the most efficient extraction of Cd(II) and Pb(II) cations (D > 90). The newly developed pyridine and phenanthroline diphosphonate ligands have proven to be highly effective components in plasticized polymeric membranes. These ligands can be utilized to construct potentiometric ion sensors that exhibit a notable response specifically towards Pb(II) cations. Among the previously reported tetradentate ligands, the phenanthroline diphosphonate ligand, when incorporated into plasticized polymeric membranes, demonstrated the highest sensitivity towards d-metals and Pb(II). The structure of the single crystal complex of Pb(II) and Cd(II) with pyridine-diphosphonates was studied by X-ray diffraction analysis (XRD). The geometry of Cu(II), Zn(II), Cd(II) and Pb(II) complexes and the energy effect of the complex formation, including pseudo-oligomerization reactions, were determined by DFT calculations. The high sensing and extraction efficiency of diphosphonates with respect to Pb(II) is consistent with the minimum values of complex formation energies. The variation in sensory and extraction properties observed among the studied diphosphonate ligands is influenced by the ability to form polynuclear complexes with Pb(II) cations, whereas such properties are absent in the case of Cd(II) cations.

Substituted diamides of dipicolinic acid as extractants and ionophores for rare earth metals

The variety of newly synthesized diamides of dipicolinic acid (DPA) bearing methyl- and oxymethyl substituents in various positions of phenyl ring was investigated as potential ligands for liquid–liquid extraction of rare earth metal ions and as membrane active compounds in plasticized polymeric membranes of potentiometric sensors. It is found that scandium is extracted with 2,6-pyridinedicarboxylic acid diamides similarly to heavy lanthanides, while yttrium is extracted together with light group lanthanides. DPA shows high selectivity for the separation of cadmium/zinc, lead/nickel or copper/nickel pairs. The four out of six studied ligands demonstrate significant potentiometric sensitivity towards scandium, yttrium and lanthanides in 10−7–10−3 mol/L concentration range in nitric acid solutions. The trends in sensitivity values vary depending on a ligand structure and the highest values are observed for Sc3+ and heavy lanthanides. According to the evaluated selectivity Sm3+ is the most preferred ion, while La3+ is the most discriminated. The differences in sensor behavior in rare earth metal solutions assume that such devices can be applied in sensor arrays for potentiometric analysis of lanthanide mixtures.

Prediction of Carbonate Selectivity of PVC-Plasticized Sensor Membranes with Newly Synthesized Ionophores through QSPR Modeling

Developing a potentiometric sensor with required target properties is a challenging task. This work explores the potential of quantitative structure-property relationship (QSPR) modeling in the prediction of potentiometric selectivity for plasticized polymeric membrane sensors based on newly synthesized ligands. As a case study, we have addressed sensors with selectivity towards carbonate—an important topic for environmental and biomedical studies. Using the logKsel(HCO3−/Cl−) selectivity data on 40 ionophores available in literature and their substructural molecular fragments as descriptors, we have constructed a QSPR model, which has demonstrated reasonable precision in predicting selectivities for newly synthesized ligands sharing similar molecular fragments with those employed for modeling.

Addressing the Detection of Ammonium Ion in Environmental Water Samples via Tandem Potentiometry-Ion Chromatography.

An analytical methodology for detecting ammonium ion (NH4+) in environmental water through potentiometry–ion chromatography (IC) in tandem is presented here. A multielectrode flow cell is implemented as a potentiometric detector after chromatographic separation of cations in the sample. The electrodes are fabricated via miniaturized all-solid-state configuration, using a nonactin-based plasticized polymeric membrane as the sensing element. The overall analytical setup is based on an injection valve, column, traditional conductometric detector, and new potentiometric detector (in that order), permitting the characterization of the analytical performance of the potentiometric detector while validating the results. The limit of detection was found to be ca. 3 × 10–7 M NH4+ concentration after linearization of the potentiometric response, and intra- and interelectrode variations of <10% were observed. Importantly, interference from other cations was suppressed in the tandem potentiometry–IC, and thus, the NH4+ content in fresh- and seawater samples from different locations was successfully analyzed. This analytical technology demonstrated a great potential for the reliable monitoring of NH4+ at micromolar levels, in contrast to the conductivity detector and previously reported NH4+ potentiometric sensors functioning in batch mode or even coupled with IC. Additionally, the suitability of the potentiometric cell for selective multi-ion analysis in the same sample, i.e., Na+, NH4+, and K+ in water, has been proven.

Transportation and Accumulation of Redox Active Species at the Buried Interfaces of Plasticized Membrane Electrodes.

The transportation and accumulation of redox active species at the buried interface between glassy carbon electrodes and plasticized polymeric membranes have been studied using synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS), near edge X-ray absorption fine structure (NEXAFS), in situ electrochemical Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy, cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS). Ferrocene tagged poly(vinyl chloride) [FcPVC], ferrocene (Fc), and its derivatives together with tetracyanoquinodimethane (TCNQ) doped plasticized polymeric membrane electrodes have been investigated, so as to extend the study of the mechanism of this reaction chemistry to different time scales (both small and large molecules with variable diffusion coefficients) using a range of complementary electrochemical and surface analysis techniques. This study also provides direct spectroscopic evidence for the transportation and electrochemical reactivity of redox active species, regardless of the size of the electrochemically reactive molecule, at the buried interface of the substrate electrode. With all redox dopants, when CA electrolysis was performed, redox active species were undetectable (<1 wt % of signature elements or below the detection limit of SR-XPS and NEXAFS) in the outermost surface layers of the membrane, while a high concentration of redox species was located at the electrode substrate as a consequence of the deposition of the reaction product (Fc(+)-anion complex) at the buried interface between the electrode and the membrane. This reaction chemistry for redox active species within plasticized polymeric membranes may be useful in the fashioning of multilayered polymeric devices (e.g., chemical sensors, organic electronic devices, protective laminates, etc.) based on an electrochemical tunable deposition of redox molecules at the buried substrate electrode beneath the membrane.

Voltammetric ion-channel sensing of ammonium ion using self-assembled monolayers modified with ionophoric receptors

Thiazole benzo-crown ether ethylamine-thioctic acid (TBCEAT) was evaluated as an ammonium (NH4+) ionophore for the preparation of ion-channel sensors (ICSs) based on self-assembled monolayers (SAMs) on a gold electrode. A SAM film based on TBCEAT on a gold electrode was assessed as a NH4+-selective sensor, particularly the voltammetric response. The working principle of this voltammetric sensor conceptually mimicked that of the ligand gated ion-channel proteins with regard to the chemically stimulated changes in the membrane permeability. The response to various analytes is based on the change in the electron transfer rates of a redox reaction of the reporter ion [Ru(NH3)6]3+/2+ before and after the binding of NH4+ to TBCEAT on the electrode surface, where electrostatic repulsion between the NH4+-TBCEAT complex and [Ru(NH3)6]3+/2+ induced a decrease in the rate constant. The selective ion-recognition properties of TBCEAT were characterized fully in the conventional ion-selective electrode (ISE) configuration using plasticized polymeric membranes. The mixed monolayer formed with alkylthiol and TBCEAT could bind ammonium ion selectively, and effectively control the flux of the reporter ion [Ru(NH3)6]3+/2+, resulting in ammonium selective ICS with comparable performance (in terms of selectivity and detection limits) to conventional ISE.

Use of a New Ziprasidone-Selective Electrode in Mixed Solvents and Its Application in the Analysis of Pharmaceuticals and Biological Fluids

The construction and characterization of a new ion-selective electrode for the determination of the antipsychotic ziprasidone in mixed solvents is presented. The electrode contains a plasticized polymeric membrane based on a ziprasidone-tetraphenylborate ion-exchanger. The influence of membrane composition on the electrode response towards ziprasidone in hydroalcoholic solutions was studied. The electrode displayed a stable response in a 2:3 (v/v) methanol/water medium from a ziprasidone concentration of 3 × 10−6 M with a fast response time of less than 20 s. The electrode also showed good selectivity towards ziprasidone over common inorganic and organic compounds and several species with pharmacological activity. The electrode was successfully applied to the determination of ziprasidone in pharmaceuticals and human urine and serum.

Benzodipyrrole derivates as new ionophores for anion-selective electrodes: Improving potentiometric selectivity towards divalent anions

Two open substituted benzodipyrroles were tested as hydrogen-bond-forming anion ionophores for the development of anion-selective electrodes. These compounds were incorporated in plasticized polymeric membranes with different plasticizers, using different membrane compositions to explore their response towards several anions. The electrodes constructed with membranes containing 2-nitrophenyl octyl ether and a 0.5 molar ratio ionic additive/ionophore showed pronounced anti-Hofmeister behaviour, providing a significantly enhanced response towards the divalent anions sulfate, sulfite, thiosulfate and oxalate. The selected electrodes were also evaluated in terms of detection limits and selectivity. 1H NMR experiments were carried out in an attempt to explain some aspects of the behaviour observed.

The Design and Development of Fluorescent Nano-Optodes for in Vivo Glucose Monitoring

The advent of fluorescent nanosensors has enabled intracellular monitoring of several physiological analytes, which was previously not possible with molecular dyes or other invasive techniques. We have extended the capability of these sensors to include the detection of small molecules with the development of glucose-sensitive nano-optodes. Herein, we discuss the design and development of glucose-sensitive nano-optodes, which have been proven functional both in vitro and in vivo. Throughout the design process, each of the sensor formulations was evaluated based on their response to changes in glucose levels. The percent change in signal, sensor reversibility, and the overall fluorescence intensity were the specific parameters used to assess each formulation. A hydrophobic boronic acid was selected that yielded a fully reversible fluorescence response to glucose in accordance with the sensor mechanism. The change in fluorescence signal in response to glucose was approximately 11%. The use of different additives or chromophores did not improve the response; however, modifications to the plasticized polymeric membrane extended sensor lifetime. Sensors were developed that yielded a dynamic response to glucose and through further modification of the components, sensor lifetime was improved. By following specific design criteria for the macrosensors, the sensors were miniaturized into nano-optodes that track changes in glucose levels in vivo.

Full-range optical pH sensor based on imaging techniques

A new colour-based disposable sensor array for a full pH range (0–14) is described. The pH sensing elements are a set of different pH indicators immobilized in plasticized polymeric membranes working by ion-exchange or co-extraction. The colour changes of the 11 elements of the optical array are obtained from a commercial scanner using the hue or H component of the hue, saturation, value ( HSV) colour space, which provides a robust and precise parameter, as the analytical parameter. Three different approaches for pH prediction from the hue H of the array of sensing elements previously equilibrated with an unknown solution were studied: Linear model, Sigmoid competition model and Sigmoid surface model providing mean square errors (MSE) of 0.1115, 0.0751 and 0.2663, respectively, in the full-range studied (0–14). The performance of the optical disposable sensor was tested for pH measurement, validating the results against a potentiometric reference procedure. The proposed method is quick, inexpensive, selective and sensitive and produces results similar to other more complex optical approaches for broad pH sensing.

Cation transport across plasticized polymeric membranes containing N,N,N′,N′-tetraoctyl-3-oxapentanediamide(TODGA) as the carrier

Facilitated transport of trivalent actinides (Am, Cm), lanthanides (La, Eu, and Lu) and fission product (Sr) was investigated from 1 M HNO 3 into 0.5 M α-hydroxy isobutyric acid (α-HIBA) through polymer inclusion membrane (PIM) consisting of cellulose triacetate (CTA) as a polymer matrix, 2-nitrophenyl n-octyl ether (NPOE) as a plasticizer and N,N,N′,N′-tetraoctyl-3-oxapentanediamide (TODGA) as a carrier. The transport of Cm(III) was higher as compared to Am(III) which was ascribed to the higher ionic potential of the former. Effect of ionic potential was also demonstrated for lanthanides where the transport of Lu(III) was more as compared to La(III). The experimental results indicated that the transport of metal ions was controlled by the diffusion of the metal–ligand complex in the membrane phase.

C3-Functionalized Cyclotriveratrylene Derivative Bearing Quinolinyl Group as a Fluorescent Probe for Cu2+

A new C(3)-functionalized cyclotriveratrylene (CTV) bearing three fluorogenic quinolinyl groups was synthesized and studied as a fluorescent probe. The CTV derivative exhibited a selective response of fluorescence enhancement toward Cu(2+). The fluorescence intensity in CH(3)CN was increased until the [Cu(2+)]/[CTV derivative] mole ratio was 3. Moreover, C(3)-functionalized CTV(2-quinolinemethyl)(3) was applied to a visible fluorescent probe by using a plasticized polymeric membrane. The emission profile was observed by a fluorescence microscope after immersing the membrane in a 1 x 10(-5) M aqueous solution of Cu(2+).

Development of a One-Shot Optical Citrate Sensor Based on a Guanidinium Synthetic Receptor

A new optical absorption-based one-shot sensor for the determination of citrate is described. The citrate-sensitive element is a plasticized polymeric membrane containing a synthetic guanidinium receptor and a pH-sensitive lipophilic dye. The rigid amide-based receptor with C3 symmetry controls the anion selectivity pattern of the optical element. The optical selectivity coefficients obtained for citrate over a variety of common carboxylate anions meet the requirements for the determination of citrate in different types of samples. The one-shot sensor responds rapidly – in 40 s – and reversibly to citrate over a dynamic range of 0.779 to 25.0 mM with a sensor-to-sensor reproducibility of 5.7%, as log acit, in the medium level of the range. The performance of the optical one-shot sensor has been tested in the analysis of citrate in soft drinks and pharmaceuticals, validating the results against a reference procedure. The proposed method is quick, inexpensive, selective and sensitive, and uses conventional instrumentation only.

Distinguishing free and total calcium with a single pulsed galvanostatic ion-selective electrode

A pulsed galvanostatic technique is presented to distinguish free and total levels of calcium with a single membrane electrode by varying the magnitude of the applied current. Pulsed chronopotentiometry creates the possibility of accurate control of ion fluxes across ion-selective plasticized polymeric membranes without ion-exchanger properties (note, however, that a low concentration of ion-exchanger impurities is always present in such membranes). During a constant current pulse ions are forced to extract from the aqueous sample into the contacting membrane phase. Each current pulse is followed by a constant potential pulse to remove the extracted ions from the membrane. The induced concentration gradients are reproducible from pulse to pulse. At relatively small applied currents and in the presence of labile complexes in the sample, the sensor responds to the ion activity, in analogy to a conventional ISE. If a larger current is applied, the flux of complexed ions dominates the sensor response because of its increased magnitude, and the observed potential is now a function of the total ionic concentration. This approach allows one to detect, with the same sensor, the levels of free and total ionic concentration by varying the magnitude of the applied current. The technique utilized here gives much more stable signals than with earlier work demonstrating the principle with zero-current potentiometry. This concept is illustrated with calcium selective membranes based on the ionophore ETH 5234.

Lanthanum(IlI) PVC membrane electrodes based on 1,3,5-trithiacyclohexane.

Novel plasticized polymeric membrane (PPME) and membrane-coated graphite (MCGE) electrodes based on 1,3,5-trithiacyclohexane for highly selective determination of La3+ ion have been developed. The electrodes exhibit Nernstian responses over very wide concentration ranges (8.0 x 10(-6)-5.0 x 10(-2) M for PPME and 4.0 x 10(-8)-1.0 x 10(-2) M for MCGE). The limit of detections were 5.0 x 10(-6) and 2.0 x 10(-8) M for PPME and MCGE, respectively. The electrodes possess a fast response time of approximately 10 s and can be used for at least 6 months without observing any deviation. The proposed electrodes revealed excellent selectivities for La3+ over a wide variety of alkali, alkaline earth, transition, and heavy metal ions and could be used in a pH range of 5.0-8.0. The practical utility of the electrodes has been demonstrated by their use as indicator electrodes in the potentiometric titration of La3+ ions with EDTA and in determination of F- in some mouthwash preparations.

Optical films for protamine detection with lipophilic dichlorofluorescein derivatives

Lipophilic 2′,7′-dichlorofluorescein esters have been synthesized and incorporated into thin plasticized polymeric membrane films as chromoionophores for the optical sensing and detection of polycationic protamine. The membrane response is based on the extraction of protamine into the film due to the interaction between the anionic fluorescein and protamine polycation which results in a cation exchange between the protamine and proton and thus, induces an absorbance spectra change of the polymeric film. The response speed is controlled by protamine diffusion through the stagnant diffusion layer adjacent to the film surface as well as within the bulk of the polymer film. When limited exposure time (3 min) and non-stirring detection modes are used in a buffer solution of pH 7.4, absorbance (at 536 nm) of a film (thickness: 2 μm) changes as a function of the protamine concentration in the range of 2 to 60 μg m −1 (0.44–13.3 μM). The sensing film showed good selectivity over most of the common small cations, and, therefore, it can be used in the determination of protamine in diluted serum. No response was observed when a protamine complexing reagent such as heparin is present, thus it can be used as an indicator for the protamine-heparin titration. The operating principle and applications of the present sensing membrane are discussed.

Polymeric membrane salicylate-sensitive electrodes based on organotin(IV) carboxylates

Selectivity properties were established for membrane electrodes prepared by incorporating tribenzyltin carboxylates in plasticized polymeric membranes. These electrodes display high selectivity for salicylate with respect to many common anions. An electrode prepared with tribenzyltin octoate, using o-nitrophenyl octyl ether (o-NPOE) as the plasticizer, possesses the best potentiometric response characteristics, including a fast response time. It shows a linear response towards salicylate ions over the concentration range 0.1–5 × 10–6 mol l–1 with a slope of –57.5 mV decade–1 in buffer solutions of pH 5.5. The behaviour of the electrode is considerably influenced by the plasticizer employed and the optimum response appears to result when o-NPOE is present. The electrode was applied to the determination of salicylate in pharmaceutical and urine samples.

Characteristics of Semiconductor and Ion-Sensitive Layers That Affect Response of Ion-Sensing Devices Based on n-SnO2/Ion-Exchange Membrane Junction

Abstract The carrier density and thickness of both the semiconductor and ion-sensing membrane were found to significantly affect the sensitivity of ion-sensing devices with a junction of n-type SnO2 and cation-selective plasticized polymeric membranes. When an anion-selective membrane was employed instead, the resulting device responded in a different way from the corresponding cation-sensing device.