Polymer membrane anion-selective electrodes

Development of Polymer Membrane Anion-Selective Electrodes Based on Molecular Recognition Principles... 

Several of the polymer membrane anion-selective electrodes described in the literature use quaternary ammonium salts as ion carriers (ionophores) (7). These electrodes respond according to the Hofmeister series (CIO4 > SCN > I > NO3 > Br - N3 > NC>2 > Cl > HCO3 acetate) (2, 5), which is the order of relative lipophilicity of the anions. Therefore, in strict terms, electrodes that respond according to this series could be considered "nonselective". 

In this paper, we report the development of ISEs that have been designed by using molecular recognition principles. Specific examples include the development of polymer membrane anion-selective electrodes based on hydrophobic vitamin B12 derivatives and a cobalt porphyrin. The selectivity patterns observed with these electrodes can be related to differences in the structure of the various ionophores, and to properties of the polymer film. 

In summary, it has been demonstrated that ISEs can be designed by employing molecular recognition principles. In particular, the feasibility of using hydrophobic vitamin B12 derivatives and electropolymerized porphyrin films in the development of polymer membrane anion-selective electrodes has been demonstrated. The studies indicated that the changes in the selectivity of these ISEs can be explained by the difference in structure of the ionophores. In addition, it was shown that by electropolymerization of a cobalt porphyrin, anion-selective electrodes can be prepared that have extended lifetimes compared with PVC-based ISEs, which use a similar compound as the ionophore. 

By running a potentiometric precipitation titration, we can determine both the compositions of the precipitate and its solubility product. Various cation- and anion-selective electrodes as well as metal (or metal amalgam) electrodes work as indicator electrodes. For example, Coetzee and Martin [23] determined the solubility products of metal fluorides in AN, using a fluoride ion-selective LaF3 single-crystal membrane electrode. Nakamura et al. [2] also determined the solubility product of sodium fluoride in AN and PC, using a fluoride ion-sensitive polymer membrane electrode, which was prepared by chemically bonding the phthalocyanin cobalt complex to polyacrylamide (PAA). The polymer membrane electrode was durable and responded in Nernstian ways to F and CN in solvents like AN and PC. 

A PhoE porin-lecithin membrane-BPG electrode was prepared as follows n-decane containing 0.5% egg lecithin and 0.25% cholesterol was brushed on the BPG electrode and dried in air. The resulting lecithin membrane-BPG electrode was inserted in 10 ml, 50 mM Tris-HCL buffer (pH 7.0), and coated again with the n-decane solution containing lecithin and cholesterol. After the lecithin membrane turned black, extracted PhoE porin was added to the lecithin membrane-BPG electrode and Tris-HCl buffer solution system. The anion-selective polymer membrane electrode was an Ag/AgCl electrode (0.422 cm2) coated with a PVC membrane containing 6% methyltridodecyl ammonium chloride and 30% nitrophenyloctyl ether. A PhoE porin-lecithin membrane-anion selective membrane electrode was prepared in the same way as the PhoE porin-lecithin membrane-BPG electrode described above. 

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. 

Traditionally, potentiometric sensors are distinguished by the membrane material. Glass electrodes are very well established especially in the detection of H+. However, fine-tuning of the potentiometric response of this type of membrane is chemically difficult. Solid-state membranes such as silver halides or metal sulphides are also well established for a number of cations and anions [25,26]. Their LOD is ideally a direct function of the solubility product of the materials [27], but it is often limited by dissolution of impurities [28-30]. Polymeric membrane-based ISEs are a group of the most versatile and widespread potentiometric sensors. Their versatility is based on the possibility of chemical tuning because the selectivity is based on the extraction of an ion into a polymer and its complexation with a receptor that can be chemically designed. Most research has been done on polymer-based ISEs and the remainder of this work will focus on this sensor type. 

Conducting polymers have already been well documented in conjunction with the classical ionophore-based solvent polymeric ion-selective membrane as an ion-to-electron transducer. This approach has been applied to both macro- and microelectrodes. However, with careful control of the optimisation process (i.e. ionic/electronic transport properties of the polymer), the doping of the polymer matrix with anion-recognition sites will ultimately allow selective anion recognition and ion-to-electron transduction to occur within the same molecule. This is obviously ideal and would allow for the production of durable microsensors, as conducting polymer-based electrodes, and due to the nature of their manufacture these are suited to miniaturisation. There are various examples of anion-selective sensors formed using this technique reported in the literature, some of which are listed below. 

Ion-selective electrodes (ISEs) are relatively simple membrane-based po-tentiometric devices which are capable of accurately measuring the activity of ions in solution. Selectivity of these transducers for one ion over another is determined by the nature and composition of the membrane materials used to fabricate the electrode. While many scientists are quite familiar with the glass membrane pH electrode first described by Cremer (CIO), most are for less aware of the other types of ISEs which may be prepared with crystalline, liquid, and polymer membranes and which allow for the selective measurement of a wide variety of cations and anions (e.g., Na" ", K" ", Ca ", Ag" ", Cl, Br , F , and organic ions). Moreover, in recent years, the range of measurable species has been further extended to include dissolved gases and... 

The first electrode of this type was based on the Ca-dodecylphos-phate/dioctylphenyl phosphonate system [71]. A mixture of 5% PVC in cyclohexanone and 0.1 M calcium dodecylphosphate in dioctylphenyl phosphonate was dried on the end of a platinum wire. This electrode exhibits greater selectivity for Ca-" over other divalent cations, as compared to traditional i.s.e.s, with the exception of Pb-" and. Its response relies upon the complexation of aqueous Ca by dodecylphosphate dispersed in the organic (membrane) phase. Anion-selective CWEs can be prepared in a similar manner, e.g., by the incorporation of methyltricaprylammonium salts into a polymer membrane placed on a copper wire [72]. Other mediators, including particularly neutral carriers, show promise for utilization in CWE construction. In some cases, polymethylmethacrylate or epoxy resin could be substituted for PVC with retention of response. 

Ion-selective electrodes (ISEs) constitute an example of potentiometric sensors that offer several advantages over other analytical techniques for the analysis of environmentally important ions. Specifically, the sensing platform of a membrane-based ISE consists of an ion carrier (ionophore) entrapped within a liquid polymeric membrane. The membrane does offer some interaction with numerous species, but the main interaction governing the selectivity of the sensor is between the analyte/interferences and the ionophore. Once an ionophore that offers the preferred selectivity has been developed and the polymer components that are ionophore-compatible have been optimized, the production of a functional ISE is rather facile and rapid. Presently, ISEs have been reported for several species including metal ions, anions, surfactants, and gases (5). 

Model HA301), a function generator (Hokuto Denko, Model HB104) and an X-Y recorder (Riken Denshi, F35). The measurement cell was of all-glass construction, approximately 10 ml in volume, incorporating a conventional three-electrode system. An anion-selective polymer membrane electrode was also coated with a PhoE porin-lecithin membrane. Potentiometric measurements were made with an electrometer (Hokuto Denko, Model HE-IOIA) in conjunction with a recorder (Riken Denshi, Model SP-J3C). 

An ion-selective field effect transistor device has been invented to serve as an anionic or cationic surfactant-selective electrode. In this case, a plasticized PVC membrane is made incorporating a surfactant salt. This membrane is cast over the gates area of a field effect transistor, the whole (except the sensing surface) is encapsulated in epoxy resin, and a suitable measuring circuit is connected. Application is similar to the polymer membrane electrodes described above (124-126). 

Another aspect of tin as a constituent of electrode material is shown by tin(IV)TPP complexes incorporated into PVC membrane electrodes. These increase the selectivity to salicylate over anions such as Cl-, Br- I-, I()4, Cl()4, citrate, lactate and acetate. The specificity is attributed to the oxophilic character of the Sn ion in TPP at the axial coordination sites. Indeed, carboxyl groups incorporated into the membrane polymer compete for these binding sites. The complete complex structure is important. Substitution of TPP with octaethylporphirine results in loss of salicylate selectivity231. Preparation and analytical evaluation of a lead-selective membrane electrode, containing lead diethyldithiocarbamate chelate, has also been described232. 

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