Selective membranes using ionophores

This experiment describes the preparation and evaluation of two liquid-membrane Na+ ion-selective electrodes, using either the sodium salt of monensin or a hemisodium ionophore as ion exchangers incorporated into a PVG matrix. Electrodes prepared using monensin performed poorly, but those prepared using hemisodium showed a linear response over a range of 0.1 M to 3 X 10 M Na+ with slopes close to the theoretical value. 

In this review, recent development of active transport of ions accross the liquid membranes using the synthetic ionophores such as crown ethers and other acyclic ligands, which selectively complex with cations based on the ion-dipole interaction, was surveyed,... 

Especially sensitive and selective potassium and some other ion-selective electrodes employ special complexing agents in their membranes, termed ionophores (discussed in detail on page 445). These substances, which often have cyclic structures, bind alkali metal ions and some other cations in complexes with widely varying stability constants. The membrane of an ion-selective electrode contains the salt of the determined cation with a hydrophobic anion (usually tetraphenylborate) and excess ionophore, so that the cation is mostly bound in the complex in the membrane. It can readily be demonstrated that the membrane potential obeys Eq. (6.3.3). In the presence of interferents, the selectivity coefficient is given approximately by the ratio of the stability constants of the complexes of the two ions with the ionophore. For the determination of potassium ions in the presence of interfering sodium ions, where the ionophore is the cyclic depsipeptide, valinomycin, the selectivity coefficient is Na+ 10"4, so that this electrode can be used to determine potassium ions in the presence of a 104-fold excess of sodium ions. 

FIGURE 4.1 Structures of common ionophores used in ion-selective membranes. 

Among all the polymers used in preparing ion-selective membranes, poly(vinylchloride) (PVC) is the most widely used matrix due to its simplicity of membrane preparation [32, 70], In order to ensure the mobility of the trapped ionophore, a large amount of plasticizer (approximately 66%) is used to modify the PVC membrane matrix (approximately 33%). Such a membrane is quite similar to the liquid phase, because diffusion coefficients for dissolved low molecular weight ionophores are high, on the order of 10 7-10 8cm2/s [59],... 

At present the plastic membrane with ionophore ETH 2137 is used. The introduction of ionophore ETH 2137 generated a stable selectivity towards sodium ion allowing for numerical correction of sodium influence using the values of independently measured sodium. Thus a complete procedure covering therapeutic concentration range of lithium (which should be <1.2mmol/L) is available. 

Solid-contact pH sensors can be constructed by using polypyrrole [45,59] or polyaniline [92,96] as ion-to-electron transducer in combination with pH-selective membranes based on plasticized PVC [45,59,92,96]. The dynamic pH range of the sensors depend on the pH ionophore used in the plasticized PVC membranes, as follows tri-n-dodecylamine (pH 2-12) [45], tris(2-phenylethyl)amine (pH 4.5-12.6) [59], tris(3-phenylpropyl)amine (pH 4.6-13.2) [59], tribenzylamine (pH 2.5-11.2) [92,96], dibenzylnaphtalenemethylamine (pH 0.65-10.0) [96], dibenzylpyrenemethyl-amine (pH 0.50-10.2) [96]. Suggested applications include pH measurements in body fluids such as serum [45,96], whole blood [92], and cow milk [59]. 

The membrane used to activate this potassium-selective IWAO [134] consists of a potassium bulk optode based on 0.5 wt % chromoionophore ETH 5294, 1.0 wt% ionophore valinomycin, 0.5 wt% ionic additive potassium tetrakis(4-chlorophenyl)borate (KtpClPB), 31.0 wt % polymer PVC, 67.5 wt % organic solvent and plasticizer bis(2-ethylhexyl)sebacate (DOS) [142], This commercially available optode not only acts as an example of the development of an enhanced ion-selective IWAO, but also serves to validate the previously remarked features, because results can be compared with the ones obtained with membranes of the same composition and thickness in a con-... 

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. 

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]. 

Permselectivity is the ability of a membrane to allow a high flux of one chemical species (typically the analyte) while reducing or eliminating the flux of other species (chemical or sensor interferences). An extreme example of permselectivity is the liquid polymer membranes used in ion selective electrodes. The ionophores contained in these membranes bind selectively to a specific ion. However, this binding is often so strong that the diffusion constant for these ions through the membrane is vanishingly small [15]. 

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