Immobilized liquid membrane electrodes

The designation membrane is used here in a very general sense, as many materials are used. Commercially available electrodes include liquid membrane units, solid-state electrodes, glass membrane electrodes, and plastic membrane electrodes. General classes of ion specific electrodes in addition to those cited are immobilized-liquid membrane electrodes, mixed-crystal membrane electrodes, enzyme electrodes, and antibiotic electrodes (Rechnitz, 1973). Certain of these membrane electrodes are now discussed in some detail. A generalized membrane electrode is illustrated in Figure 6.1a. 

As an alternative to the use of a porous disk as a rigid supporting medium, it is possible to immobilize liquid exchangers in tough I VC membranes. In this type of electrode, the liquid ion exchanger and PVC are dissolved in a solvent such as tetrahydrofuran. The soivcnl is evaporated to leave behind a flexible membrane lhal can be cut. shaped, and bonded to the end of a glass or plastic tube. Membranes formed in this way behave in much the same way is those in which the ion exchanger is encased as a ljt uid in the pores of a disk. Most liquid-membrane electrodes arc of this newer type. 

Guilbault and Montalvo were the first, in 1969, to detail a potentiometric enzyme electrode. They described a urea biosensor based on urease immobilized at an ammonium-selective liquid membrane electrode. Since then, over hundreds of different applications have appeared in the literature, due to the significant development of ion-selective electrodes (ISEs) observed during the last 30 years. The electrodes used to assemble a potentiometric biosensor include glass electrodes for the measurement of pH or monovalent ions, ISEs sensitive to anions or cations, gas electrodes such as the CO2 or the NH3 probes, and metal electrodes able to detect redox species some of these electrodes useful in the construction of potentiometric enzyme electrodes are listed in Table 1. 

The separation between the internal and external solutions of the specific electrode is achieved using a porous and hydrophobic disc (of 3 mm diameter). This disc is saturated with an organic solvent that is immiscible with water on either side and contains also an ion-carrier called an ionophore (Figure 19.4). Hence, the membrane behaves as a lipophile immobilized liquid. The counter ion is a molecule... 

For many years, a typical polymer membrane of an ion-selective electrode has been comprised of around 60-70% (w/w) of plasticizer, 30% (w/w) of PVC and 1-5% (w/w) ionophore with a small addition of lipophilic salt (ion exchanger), depending on the type of ionophore used [14,15]. Earlier, a liquid membrane was immobilized... 

The utility of liquid membranes as ion-selective electrodes lies in the mobility of their exchange sites. These are of molecular size, however, so that it is possible to immobilize an exchanger liquid in a bulk matrix. The constraint on the matrix is that it be permeable to microscopic charge carriers. Collodion is one such bulk material. 

Electrocatalytic enzyme mediation has been demonstrated using quinones, viol-ogens, 2,2-azinobis(3-ethylbenzothiazohne-6-sulfonate) (ABTS), and complexes of iron, ruthenium, cobalt, osmium, and many other compounds [22-24]. Much early work concerned the GOx anode, intended for a glucose sensor. In 1974, Schlapfer et al. tested 11 different mediators for a GOx electrode with a semipermeable membrane [25]. Ten years later, Cass et al. reported membrane-bound electrodes that operated in whole blood [26]. In 1986, Bourdillon et al. presented an analysis arguing that immobilized enzyme electrodes have higher efficiency than those with free enzyme in solution [27]. These examples demonstrate several possible enzyme/ mediator configurations. Both enzyme and mediator can exist as firee species in the liquid electrolyte, or one or both can be immobilized on the electrode surface. As an alternative to immobilization, enzyme and mediator can also be confined near the electrode by a semipermeable membrane. 

One example of a liquid-based ion-selective electrode is that for Ca +, which uses a porous plastic membrane saturated with di-(n-decyl) phosphate (Figure 11.13). As shown in Figure 11.14, the membrane is placed at the end of a nonconducting cylindrical tube and is in contact with two reservoirs. The outer reservoir contains di-(n-decyl) phosphate in di- -octylphenylphosphonate, which soaks into the porous membrane. The inner reservoir contains a standard aqueous solution of Ca + and a Ag/AgCl reference electrode. Calcium ion-selective electrodes are also available in which the di-(n-decyl) phosphate is immobilized in a polyvinyl chloride... 

A thin film of a membrane-forming material is deposited on a metal wire (silver, platinum, nickel, etc.) or on carbon, the membrane being, for example, a liquid exchanger immobilized in PVC (Fig. 13.12). This electrode is thus similar to an ion-exchange selective electrode, but without solution and without internal reference. The detailed mechanism of its operation is not clear but it is certain, and logical, that the interface... 

Polymer membranes have also been used as a "sandwich". In this configuration, the liquid film is supported between two polymer membranes. Ward (18) used two silicone rubber membranes to contain a solution of ferrous ions in formamide. Ward noted that Bernard convection cells could be maintained if the complex were formed at the upper surface. Ward (19) used this same system and membrane configuration to study electrically-induced, facilitated gas transport. The silicone rubber membranes provided the mechanical support so the electrodes could be placed next to each liquid surface. Otto and Quinn (20) immobilized the liquid film in a horizontal layer between two polymer films. The polymer was described as an experimental silicone copolymer having high CO2 permeability as well as excellent mechanical properties. They were studying CO2 facilitated transport in bicarbonate solutions with and without carbonic anhydrase. 

In commercial electrodes, the liquid ion-exchanger is in a form in which the chelating agent is immobilized in a hydrophobic polymer membrane like poly(vinylchloride) (Pig-ure 2.4.4). Electrodes based on this design (called polymer or plastic membrane ISEs) are more rugged and generally offer superior performance. 

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