Polymer membranes, selective electrodes

Most polymer membrane ISEs are prepared by dissolving an ionophore in a polyvinylchloride (PVC) membrane. A large variety of plasticisers are used to increase the dielectric constant of the PVC and improve its hydrophilicity. Some membranes have complexes of the ions to be sensed to increase membrane conductivity, such as potassium tetraphenylborate in K -selective membranes. There is an extensive literature on the arcane arts of polymer membranes for electrodes with dissolved ionophores and a good review of this is given by Professor Ronald Armstrong in Section 3.7 of Gabor Harsanyi s book. Polymer films in sensor applications [14]. 

E. Bakker, Determination of Improved Selectivity Coefficients of Polymer Membrane Ion-Electrodes by Conditioning with a Discriminating Ion, J. Elec-trochem. Soc., 143(4) (1996) L83. 

Solutions of sodium dodecyl sulfate (SDS) and a variety of nonionic polymers listed in Table 13.4 were studied by Ghoreishi et al. (1999). The EMF method utilizing an SDS membrane selective electrode was applied to investigate the interaction between the polymers and SDS. 

PIMs share many characteristics with polymer ion-selective electrode (ISE) membranes which have been known for many years and are often referred to as plasticized membranes (Nghiem et al, 2006). PIMs are designed for fast transport of the target chemical species across the membrane. They often completely extract a particular species from a solution, as is common with separation techniques for hydrometallurgical or remedial environmental applications (Cox, 2004 Lo et al, 1983). These properties of PIMs are in sharp contrast with those of the plasticized membranes used in ISEs where the transmembrane transport is negligible. 

Table 10.1 summarizes the characteristics of common ISEs and a number of new sensors in this field. We have not included in this table the liquid or polymer membrane-based electrodes which are selective, but rather fragile (for more details on such membranes see References 58,59). ISEs of the first kind are not very numerous, e.g., F -ISE (monocrystalhne membrane based on LaFj), Ag" -ISE (silver salts), or Na" -ISE (Na alumino-silicate glass or polyciystalline NASICON [Na super ionic conductor] membranes). Most of the ISEs are of the second kind and are based on insoluble silver salts for example, halide ISEs (CE, Br, I"), Cd ", Pb ", Cu ", etc. Such ISEs use mixtures of insoluble salts based on silver sulfide or silver selenide. Recently, Vlasov etal. and Neshkova have proposed several glasses sensitive to transition metals. Typical ISE devices are shown in Figure 10.5. Thin-layer chemical sensors based on chalcogenide glasses have also been developed. ... 

An electrode in which an antibody or an antigen/hapten is incorporated in the sensing element is termed an immunoelectrode . The potential response of the immuno-electrode is based on an immunochemical reaction between the sensing element of the electrode and antibody or antigen/hapten in the sample solution. One example of such an electrode is the polymer membrane electrode shown in Fig. 7. The selective response of this electrode to specific immunoglobulins is based on the interaction between antibody in solution and an antigen-ionophore complex in the membrane ... 

A second surface modification has been reported by Yamamoto et al. These workers added stearic acid to their carbon paste mixture. This produced an electrode which was relatively insensitive to ascorbic acid and DOPAC relative to dopamine. It is theorized that this electrode works because of electrostatic repulsion of the anionic ascorbate and DOPAC by surface stearate groups. Ionic repulsion has also been employed by covering the surface of the working electrode with an anionic polymer membrane. Gerhardt et al. used Nafion, a hydrophobic sulfonated perfluoro-polymer, to make a dopamine selective electrode. This electrode exhibited selectivity coefficients as large as 250 1 for dopamine and norepinephrine over ascorbic acid, uric acid, and DOPAC. 

The membranes of the other ion-selective electrodes can be either homogeneous (a single crystal, a pressed polycrystalline pellet) or heterogeneous, where the crystalline substance is incorporated in the matrix of a suitable polymer (e.g. silicon rubber or Teflon). The equation controlling the potential is analogous to Eq. (6.3.9). 

E. Bakker, P. Buhlmann, and E. Pretsch, Polymer membrane ion-selective electrodes - what are the limits Electroanalysis 11, 915-933 (1999). 

T. Sokalski, A. Ceresa, T. Zwickl, and E. Pretsch, Large improvement of the lower detection limit of ion-selective polymer membrane electrodes. J. Am. Chem. Soc. 119, 11347-11348 (1997). 

E. Bakker, R.K. Meruva, E. Pretsch, and M.E. Meyerhoff, Selectivity of polymer membrane-based ion-selective electrodes — self-consistent model describing the potentiometric response in mixed ion solutions of different charge. Anal. Chem. 66, 3021—3030 (1994). 

E. Bakker, Determination of improved selectivity coefficients of polymer membrane ion-selective electrodes by conditioning with a discriminated ion. J. Electrochem. Soc. 43, L83—L85 (1996). 

The principle of pH electrode sensing mechanisms which are based on glass or polymer membranes is well investigated and understood. Common to all potentiometric ion selective sensors, a pH sensitive membrane is the key component for a sensing mechanism. When the pH sensitive membrane separates the internal standard solution with a constant pH from the test solution, the potential difference E across the membrane is determined by the Nemst equation ... 

Liquid membrane electrodes utilize porous polymer materials, such as PVC or other plastics. An organic liquid ion exchanger immiscible with water contacts and saturates the membrane from a reservoir around the outside of the tube containing the water solution of the analyte and the silver-silver chloride wire. See Figure 14.10. Important electrodes with this design are the calcium and nitrate ion-selective electrodes. 

Figure 5.7 shows a typical application of gas-diffusion membranes isolation of the circulating sample from a voltammetric or potentiometric electrode for the electrochemical determination of gaseous species. The ion-selective electrode depicted in this Figure includes a polymer membrane containing nonactin that is used for the potentiometric determination of ammonia produced in biocatalytic reactions. Interferences from alkali metal ions are overcome by covering the nonactin membrane with an outer hydro-... 

Recent research in the field of polymer membrane ion-selective electrodes [389-391], has revealed that their se-lectivities [392-396] and limits of detections [394-397] could be improved by several orders of magnitude. The review of Bakker and Pretsch [398] summarized recent progress in the development and application of potentiometric sensors with low detection limit in the range 10-8-10-11 M. 

Potentiometric Sensors Inthe field of ion-selective electrodes, considerable progress has been achieved in the last few years. By buffering the primary ions concentration on a low level in the internal solution, ionic fluxes in the membrane are affected [424-426]. Thus, primary ion leakage into sample solution is hindered, resulting in a tremendous shift of detection limits to lower values for Pb +-selective electrodes, the detection limit up to 10 M level has been achieved for internal solution electrodes [424, 427] and below 10 M for all-solid-state electrodes with conducting polymer solid... 

Shvedene, N.V., Chernyshov, D.V., Khrenova, M.G., Formanovsky, A.A., Baulin, V.E., and Pletnev, I.V., Ionic liquids plasticize and bring ion-sensing ability to polymer membranes of selective electrodes, Electroanal., 18,1416-1421, 2006. 

Redox potential pH Ionic activities Inert redox electrodes (Pt, Au, glassy carbon, etc.) pH-glass electrode pH-ISFET iridium oxide pH-sensor Electrodes of the first land and M" /M(Hg) electrodes) univalent cation-sensitive glass electrode (alkali metal ions, NHJ) solid membrane ion-selective electrodes (F, halide ions, heavy metal ions) polymer membrane electrodes (F, CN", alkali metal ions, alkaline earth metal ions)... 

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