Coated-wire ISE

The polymer film membrane is being used to coat metal wire electrodes to make miniature ISEs for in vivo analysis. These coated wire ISEs require no internal reference solution and have been made with electrode tip diameters of about 0.1 xm. Electrodes have been made small enough to measure ions inside a single cell, as seen in Fig. 15.11. 

With regard to simple miniaturization, coated-wire ISEs are an attractive approach (39- 0). This involves coating the corresponding conventional polymer membrane onto a platinum or other wire. Numerous coated-wire electrodes selective to different ions have been prepared (i41-j 2), despite the lingering controversy over the performance of these devices in terms of... 

In attempts already made in the early 1970s, the ion-selective polymeric membrane was directly applied to a metal wire. From a theoretical point of view, this is an unsatisfactory situation because in all but a few exceptions these membranes contain neither a redox-active cation of the metal of which the wire is made nor an electron acceptor/donor pair that determines the redox potential. As a result, the phase boundary potential at the membrane/metal interface of most coated-wire ISEs is poorly defined. It is still not well understood why in real life freshly prepared coated-wire electrodes sometimes perform surprisingly well under circumstances when they can be recalibrated relatively frequently. However, formation of a water layer at the metal-membrane interface leads eventually to memory effects and, after delamination of the sensitive membrane, to catastrophic failure. 

The coated-wire ISEs represented the first major approach to the fabrication of SC ISEs based on polymeric manbranes. These electrodes involved coating the reference element directly with the ISM without having a thermodynamically defined interface, which obviously impeded their long-term potential stability. The failure of this straightforward approach in a number of practical... 

The first and very simple solid contact polymeric sensors were proposed in the early 1970s by Cattrall and Freiser and comprised of a metal wire coated with an ion-selective polymeric membrane [94], These coated wire electrodes (CWEs) had similar sensitivity and selectivity and even somewhat better DLs than conventional ISEs, but suffered from severe potential drifts, resulting in poor reproducibility. The origin of the CWE potential instabilities is now believed to be the formation of a thin aqueous layer between membrane and metal [95], The dominating redox process in the layer is likely the reduction of dissolved oxygen, and the potential drift is mainly caused by pH and p02 changes in a sample. Additionally, the ionic composition of this layer may vary as a function of the sample composition, leading to additional potential instabilities. 

Symmetrical placement of the ion-selective membrane is typical for the conventional ISE. It helped us to define the operating principles of these sensors and most important, to highlight the importance of the interfaces. Although such electrodes are fundamentally sound and proven to be useful in practice, the future belongs to the miniaturized ion sensors. The reason for this is basic there is neither surface area nor size restriction implied in the Nernst or in the Nikolskij-Eisenman equations. Moreover, multivariate analysis (Chapter 10) enhances the information content in chemical sensing. It is predicated by the miniaturization of individual sensors. The miniaturization has led to the development of potentiometric sensors with solid internal contact. They include Coated Wire Electrodes (CWE), hybrid ion sensors, and ion-sensitive field-effect transistors. The internal contact can be a conductor, semiconductor, or even an insulator. The price to be paid for the convenience of these sensors is in the more restrictive design parameters. These must be followed in order to obtain sensors with performance comparable to the conventional symmetrical ion-selective electrodes. 

This membrane is normally employed in what is called a symmetrical configuration , to form the classical ISE which employs an internal reference solution, as sketched in Figure 30.8. This membrane can also be used directly fixed on a solid contact, per example a metallic wire or a screen-printed substrate, in the coated-wire configuration [77]. The proposed Procedure 45 (see in CD accompanying this book) suggests the use of coated wire sensors, as these are the easiest to construct. 

The fluid nature of PTC cocktails allows them to conform to the shape of a surface. Consequently, on evaporation of the tetrahydro-furan the PTC sensor film is left as a particular contour. This has allowed improved designs of ISEs, e.g., coated wires/epoxy, tubular flow-through, micro and all solid-state epoxy models. Thus, a lithiun sensor cast on top of a small epoxy base in a flow injection system is suitable for the assay of lithium in the saliva of manic depressive patients (Beswick,C.W., MDody,G.J., Thomas,J.D.R., diversity of Wales College of Cardiff, unpublished data). 

Coated-wire electrode (CWE)-type devices for in vivo monitoring have also been described. For example, workers at General Electric Inc. (N3), have patented a catheter for in vivo pH measurements which was based on the approach used by the Miles workers to prepare the previously mentioned K CWE. The ISE portion of the pH catheter consisted of a Ag/AgCl wire coated first with a hydrophilic polymer containing bufifer components and chloride ions and then with a polymer membrane containing a H" carrier molecule. A second tube with appropriate Ag/AgCl wire and electrolyte served as the external reference element. Once again, stable potentials can only be obtained if the osmolarity of the hydrophilic polymer layer matches that of whole blood. 

FIG. 1 Schematic cross sections of selected microelectrode configurations, (a) Nomenclature for parts of microelectrode, (b) Na+-sensitive microelectrode (22), (c) recessed-tip Na+-sensitive microelectrode (27), (d) liquid ion-exchanger micropipette electrode (38), (e) coated wire electrode (16), (f) flow-through ISE (e.g., NOVA 6, Boehringer ISE 2020), (g) micro-capillary glass electrode of tubular shape (e.g., Radelkis OP-266), (h) planar sensor fabricated by microelectronic technology (93), (i) ISFET sensor (94). 

Figure 19.3 Measurement set-up of ion-specific electrodes. Schematic of a measuring cell with a ISE targeted for the fluoride ion using a double junction reference electrode. From the two sides of the membrane two equilibria are produced, LaF, - LaF -F F . To avoid osmosis of KCI into the solution, the reference electrode is surrounded by a separate chamber which contains an auxiliary non-interfering electrolyte. KNO, IM difficult to oxidize or to reduce, is often used for F , CP, P, CN , or Ag+. The measurement requires a high impedance mihivoltmeter (type pH-meter). Right, two examples of simple to construct coated wire electrodes . A wire forms an ohmic contact with the membrane (ISE for fluorine with a LaF, crystal and ISE for chloride with frit).

ISEs can be prepared by simply coating a bare electrode wire with a selective PVC membrane such as those described above. These coated wire electrodes are very easy to prepare and use. A THE solution of the membrane ingredients is simply coated onto the wire and allowed to evaporate. 

Figure 2 Basic types of ISE (A) glass electrode (B) electrode with a solid homogeneous or heterogeneous membrane (C) classical liquid membrane electrode (D) electrode without internal solution (all-solid-state electrode) (E) coated-wire electrode. 1, Internal standard solution 2, internal reference electrode (Ag/AgCI) 3, membrane 4, glass or plastic body of the electrode 5, reservoir of the electroactive substance solution 6, solid-state contact and 7, metal wire.

Ba " ", Cs, NH4", Ag" ") and anions (NOi j Cl , HCOi ). Solid-state ISEs (coated wire electrodes) have also been developed in which the sensitive membrane is coated directly onto a metal wire, usually a silver-silver halide. While these have the advantage of being small and easy to fabricate, they have been noted for their unpredictable properties and suffer from lifetime and stability problems. More sophisticated approaches involve the use of semiconductor planar fabrication technologies to deposit ion-sensitive layers onto semiconductor substrates to produce ion-selective field-effect transistors. These are conceptually very attractive but it has proven very difficult to produce devices as good as the equivalent ISE. 

Fig.l ISEs (a) The solid membrane (b) liquid membrane (c) coated-wire electrode (d) field-effect transistor. The analyte solution is in contact with the ion-selective layers potentials for A-C are measured using an external reference electrode, while the drain current, 7,, is monitored using a current-to-voltage converter in D. 

Efforts to produce a phosphate-selective ISE have been hindered by its diverse spe-ciation and lability in biological samples a recent review describes various potentiometric and amperometric approaches to this problem [22]. Of the potentiometric approaches, selectivity is most often achieved using inorganic or organometal-lic extracting agents such as organotin compounds in liquid-membrane ISEs, cobalt complexes or metallic cobalt in coated-wire and metallic electrodes, respectively. Nickel phosphate, silver phosphate, and mixtures of lead precipitates have also been used in phosphate ISEs. All of these sensors suffer from limited selectivity. Enzyme-based sensors for phosphate have also been a topic of research as yet, however, no commercial phosphate sensor exists [22]. 

The so-called coated wire electrodes (1) are typical examples. A recent review (2) and a book by Freiser (3) give very detailed accounts of the fabrication and range of coated wire sensors studied. Coated wire electrodes are most commonly formed by dipping a copper wire into a polymer solution which, once dry, leaves the metal covered with plastic. The composition of the plastic is often tailored to reproduce compositions which have been used in membrane electrode configurations. A well-documented example is polyvinylchloride (PVC) impregnated with valinomycin (1), which is selective to ions in solution. An extensive collection of ISE literature titles by Moody and Thomas (4) can be used to locate, inter alia, most new devices of this type. 

In coated-wire electrodes (CWE) the membrane is placed directly on the electrode. The electrode can be a wire of noble metal, graphite wire, silver paste or Ag/AgCl electrode, which is the most popular solution. The role of the inner solution of ISE-s can be played in CWE-s by poli(hydroxyethyl methacrylate), polyvinyl alcohol or a hydrogel saturated with NaCl solution, placed between the electrode and the membrane. The membrane stability can be improved through addition of silver complexes to the membrane. The advantage of CWE-s over ISE-s is the possibility of cheap, mass production. The disadvantage of CWE-s is that the membrane can easily unstick from the electrode. Moreover, due to the high contact surface between the membrane and the solution, the membrane contents, i.e., the plasticizer and the ionophore, can be easily washed out into the membrane, which deteriorates the characteristics of the sensor in time. 

Two common forms of soHd-membrane ISEs are the coated-wire electrode and structures made by thick-film technology (screen printing). 

Miniature Shapes. Miniaturization of liquid-membrane ISEs uses the same techniques as are used with solid-membrane electrodes. Coated-wire electrodes are manufactured by dip-coating a metallic wire with a polymer layer of e.g. PVC. The oldest example (Fig. 7.9, left) is a piece of platinum wire soldered to the internal lead of a coaxial cable. The end of the wire was formed into the shape of a ball by melting it in a hot flame. The polymer coating was applied by repeated dipping into the polymer solution (Freiser 1980). 

Figure 7.9. Shapes of miniature liquid-membrane and polymer-membrane ISEs. Left coated-wire elecUode, right sensor in thick-film technology...

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