Polymer-membrane ISE

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]. 

The phenomena observed in living cells have much in common with those in artificial polymer membrane ISEs. In membrane electrodes, ionophores allow the net movement of ions in a membrane only down their electrochemical gradients. The equilibrium state is reached when the electrochemical gradient becomes zero and the cell potential reaches its final equilibrium value net transport no longer occurs. 

Polymer membrane ISEs are employed for monitoring pH and for measuring electrolytes, including Na% CL, Ca % Li, Mg h and CO (for total CO2 measurements). They are the predominant class of potentiometric electrodes used in modern clinical analysis instruments. 

Fig. 2, Structures of some neutral carrier ionophores used in the preparation of liquid and polymer membrane ISEs [From ref. (S5) with permission. Copyright (1982), Pergamon Press.]...

Fig. 3. Fabrication of polymer membrane ISEs (a) dropwise addition of casting solution to glass ring resting on glass slide (b) ring loosely covered, THF evaporates (c) ion-selective polymeric membrane forms on the slide (d) membrane removed from slide and small disk cut out and (e) pasted onto a piece of PVC tubing (f) a Ag/AgCl wire and internal reference solution complete electrode.

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. 

Most ionophores used in liquid/polymer membrane ISEs today are electrically neutral in their uncomplexed form and assume the charge of the analyte ion when complexed. In the membrane, therefore, the following complexation equilibrium exists between ionophore L and the analyte I" " ... 

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

In addition to solid-state electrodes, other ISEs operate with a polymer membrane, with a good example being the calcium electrode described below in Section 3.5.2.3. 

In both types of liquid-membrane ISEs, the membrane acts as an inunis-cible phase boundary between the aqueous and non-aqueous solutions inside the ISE (see the schematic diagram presented in Figure 3.13). In order to minimize mixing, the liquid membrane is held in place by an inert, porous material such as a rigid glass frit or a flexible synthetic polymer - the choice will depend on the manufacturer rather than on experimental considerations. 

New polymer membrane-based ISEs for nitrate and carbonate exhibit detection limits and selectivities that may be applicable for ocean measurements. In addition, a number of these ISEs can be used as internal transducers for the design of useful potentiometric gas sensors. For example, dissolved C02 can be detected potentiometrically by using either a glass membrane electrode or a polymer-based carbonate ISE, in conjunction with an appropriate reference electrode, behind an outer gas permeable membrane. Novel differential pC02 sensors based on two polymer membrane-type pH sensors have also been developed recently. 

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. 

In polymer-based ISEs, electrical contact between the membrane and inner reference electrode is made via an inner filling electrolyte. This type of ISE is the most common and they are usually referred to as liquid contact ISEs or very often simply ISEs. On the other hand, the contact can be obtained by the substitution of the aqueous inner solution with another polymeric material, to produce so-called solid-contact ISEs Table 2.1 provides current achievements in trace level... 

The use of ISEs with ion-selective membranes based on plasticized PVC, as well as glass pH electrodes, is limited to the analysis of aqueous solutions. On the other hand, sensors based on conducting polymer membranes are usually insoluble in organic solvents, which extends the range of possible applications. Electrosynthesized polypyrrole doped with calcion works as a Ca2+ sensor that can be applied as indicator electrode in the titration of Ca2+ with NaF in mixed solvents, such as water-methanol (1 1) and water-ethanol (1 1) [52], Another example is the use of polyaniline as indicator electrode in order to follow the acid-base precipitation titration of trimeprazine base with tartaric acid in isopropanol solution (see Procedure 5). 

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. 

ISEs Based on Cobyrinates. Vitamin B12 is hydrophilic and, therefore, it is necessary to modify it chemically in order to use it as ionophore in polymer membrane-based ISEs. Different hydrophobic derivatives that lack the nucleotide part of the vitamin (cobyrinates) have been prepared for this purpose. These compounds, although structurally similar to vitamin B12, have a quite different coordination behavior from that of the vitamin (7). 

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. 

Most ISEs are based on purely physicochemical and non-catalytic recognition elements solid membranes with fixed ionic sites (e.g. the glass pH electrode), ion-exchange polymer membranes or plasticised hydrogel membranes incorporating ionophores [9], Silicon oxide or metal oxides act as the recognition element in pH-ISFETs, gas-sensitive FETs, solid-state electrolyte, solid-state semiconductor and many conductometric gas sensors. 

One useful aspect of the ISE approach is the ease with which selectivity testing can be performed. Once the polymer is employed as the active ingredient in a polymer membrane electrode, the binding can be examined by measuring the potential of a cell as outlined below. We have seen that the selectivity obtained by batch extraction procedures gives the same affinity series as that measured by using the polymer in an electrode [11]. 

In these devices polymer materials containing specific ingredients constitute the backbone of the film covering the electrochemical transducer. Here we deal with a liquid membrane, because the organic solvent provides the medium in which the ions permeate across the membrane. The polymer membrane ion-selective electrodes (ISE) and their ion transport across the membrane function similarly as the ion transport across the membranes of living cells (Figure 8.29). We follow the presentation given by Widmer (1993). 

Glass membrane and polymer membrane electrodes are two types of ISEs that are commonly used in chnical chemistry applications. 

Figure 4-3 Structures of common ionophores used to fabricate polymer membrane type ISEs for clinical analysis.

Studies have demonstrated that the ultimate detection limits of polymer membrane type ISEs are controlled in part by the leakage of analyte ions, from the internal solution to the outer surface of the membrane, and into the sample phase in close contact with the membrane. Hence, much lower limits of detection can often be achieved by decreasing the concentration of the primary analyte ion within the internal solution of the electrode. Further, this leakage of analyte ions, coupled with an ion-exchange process at the membrane sample interface when assessing the selectivity of the membrane over other ions, can often yield a measured... 

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... 

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. 

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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