Polymer membrane anion-selective

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. 

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. 

In the case of co-extraction, a selective anion-carrier (ionophore) extracts the analyte anion into the lipophilic sensor membrane. In order to maintain electroneutrality, a proton is co-extracted into the membrane where it protonates a pH indicator dye contained in the polymer membrane. Due to protonation, the dye undergoes a change in either absorption or fluorescence. (Figure 6 and Tables 13 and 14). 

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. 

As with the majority of ISEs, all of the aforementioned receptors are immobilised within close proximity to the transducer element. However, conducting polymers (electroactive conjugated polymers) are now emerging rapidly as one of the most promising classes of transducer for use within chemical sensors. Here, the receptor can be doped within the polymer matrix, i.e. within the transducer element itself. This will facilitate the production of reliable, cost-effective, miniaturised anion-selective sensors, as it will be possible to move away from plasticiser-based membranes, but allow for ion recognition sites in conjunction with all-solid-state ion-to-electron transducers. 

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. 

Compound 48b was shown to display enantiose-lectivity in the extraction of chiral potassium salts from water into the organic phase.227 The supramolecular polymer possesses a homochiral helical architecture onto which one of the anionic enantiomers preferentially binds. Intriguingly, for some of the anions the octamer and polymer show opposite selectivity, illustrating the difference in supramolecular chirality of the two. Furthermore, the polymer is capable of inducing a Cotton effect in the achiral potassium N- (2,4-d initrophenyl)glycinate. The use of these polymers as artificial ion channels is currently under investigation, as the apolar side-chains would allow incorporation into a membrane.224,228 Also,... 

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... 

A large number of optodes developed for the selective detection of inorganic anions and cations, so-called ion-selective optodes (see Table 13.2), consist of polymer membranes that contain transducers. The latter are mostly physically admixed, but in some cases they are covalently bound to the polymer matrix. Most of these optodes [7, 8] are based on poly(vinyl chloride), plasticized with DOS, BBPA, DOP, o-NPOE or other plasticizers (see Chart 13.3). Typically, membranes are composed of 33 wt.% PVC, 66 wt% plasticizer, and 1 wt% ionophore (analyte-complexing agent) and lipophilic salt (ion-exchanger). Other polymers occasionally employed in hydrophobic optodes include polysiloxanes and poly(vi-... 

Metalloporphyrins, one carbon larger but structurally analogous to the corrin ring of vitamin B12, exhibited unique anion ionophore properties when incorporated into a polymer membrane. Mn(III), Co(III), Ru(II), and Sn(IV) porphyrin-based ion-selective sensors exhibit high selectivity for thiocyanate, nitrite/thiocyanate, thiocyanate and siicylate, respectively. doo-io7,i 10-114 nature of the... 

Very large areas of membrane are used in an electrodialysis cell and hence they should be as cheap as possible. Thus while the perfluoropolymers have excellent properties, they are too expensive and the membranes used are copolymers of styrene and divinylbenzene. The cation-selective materials are activated by sulphonation while the anion-selective membranes are substituted with quaternary ammonium centres. To give the membranes the required mechanical and dimensional stability, the copolymerization reaction is initiated around a reinforcing mesh or within a porous polyethylene or thermoplastic sheet the first method is generally preferred since it gives membranes of lowest resistance. The ratio of styrene to divinylbenzene determines the extent of crosslinking and hence, after activation, the amount of water absorbed into the polymer. The water creates channels across the membrane through which the ions can diffuse and the quantity of water controls the maximum size of ion which can be transported. 

Solvated rubbery 1,4 cis-polyisoprene polymer dissolved in hydrocarbon cement solutions are emulsified into oil-in-water emulsions with the aid of anionic surfactants and are converted to artificial latices by partitioning the emulsion through a membrane of selective permeability to the hydrocarbon solvent relative to both water and the polymer. This allows for the selective removal of hydrocarbon solvent from the emulsion, thus producing an artificial latex. The difficult problems associated with solution foaming often associated with removing hydrocarbon solvent from aqueous anionic emulsions by conventional means are thus avoided. Membrane vapour... 

A variety of components are either freely dissolved in this hydrophobic matrix or covalently anchored onto the polymeric backbone of the membrane. These membrane components mediate the selective extraction of many analytes and also make sure that the ISE membrane exhibits ion-exchanger properties. Thus far, liq-uid/polymer membrane ISEs for more than five-dozen analytes have been described [15, 27, 28]. They are routinely used in clinical analysis for the direct potentiometric detection of many anions and cations, and their application is steadily broadening with the advent of more selective membrane materials, advances in miniaturization, and the availability of more rugged sensors. Two main classes of liquid membrane ISEs can be distinguished one that contains an ion-exchanger without molecular receptor properties, and the other that is based on highly selective ionophores. While modem chemical research is mainly directed to the improvement of the second class, many commercial IS Es are still based on the first. 

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. 

The basic concept to use block co-polymer for the application to the DMFC is that ordered hydrophilic/hydrophobic phase separations offer a route for the selective transport of proton ions with reduced methanol crossover in the hydrophilic domains, because block co-polymers can be selectively sulfonated using post-sulfonation methods, and the block co-polymers can be verified over a wide range of structures during anionic polymerization. For example, methanol transport behaviors of a triblock co-polymer ionomer, sulfonated poly(styrene-isobutylene-styrene) (S-SIBS), were compared with Nafion to determine whether the sulfonated block co-polymer could serve as a viable alternative membrane for application to the DMFC [62]. The S-SIBS membranes showed approximately 5-10 times more methanol selectivity than that of Nafionll , although the S-SIBS membranes exhibited low conductivity compared with Nafion 117. 

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