Membrane, artificial electrode

The presence of polymer, solvent, and ionic components in conducting polymers reminds one of the composition of the materials chosen by nature to produce muscles, neurons, and skin in living creatures. We will describe here some devices ready for commercial applications, such as artificial muscles, smart windows, or smart membranes other industrial products such as polymeric batteries or smart mirrors and processes and devices under development, such as biocompatible nervous system interfaces, smart membranes, and electron-ion transducers, all of them based on the electrochemical behavior of electrodes that are three dimensional at the molecular level. During the discussion we will emphasize the analogies between these electrochemical systems and analogous biological systems. Our aim is to introduce an electrochemistry for conducting polymers, and by extension, for any electrodic process where the structure of the electrode is taken into account. 

Detection of Li+ in artificial serum with a voltammetric Li-selective electrode in a flowthrough system was demonstrated [64], Lithium salts such as lithium carbonate have been extensively used for treatment of manic depressive and hyperthyroidism disorders. The therapeutic range of Li concentration is generally accepted to be 0.5-1.5mM in blood serum. The authors used normal pulse voltammetry in which a stripping potential was applied between pulses in order to renew the membrane surface and expel all of the extracted ions from the membrane, similar to galvanostatically controlled potentiometric sensors described above. Unfortunately, the insufficient selectivity... 

The unique ability of crown ethers to form stable complexes with various cations has been used to advantage in such diverse processes as isotope separations (Jepson and De Witt, 1976), the transport of ions through artificial and natural membranes (Tosteson, 1968) and the construction of ion-selective electrodes (Ryba and Petranek, 1973). On account of their lipophilic exterior, crown ether complexes are often soluble even in apolar solvents. This property has been successfully exploited in liquid-liquid and solid-liquid phase-transfer reactions. Extensive reviews deal with the synthetic aspects of the use of crown ethers as phase-transfer catalysts (Gokel and Dupont Durst, 1976 Liotta, 1978 Weber and Gokel, 1977 Starks and Liotta, 1978). Several studies have been devoted to the identification of the factors affecting the formation and stability of crown-ether complexes, and many aspects of this subject have been discussed in reviews (Christensen et al., 1971, 1974 Pedersen and Frensdorf, 1972 Izatt et al., 1973 Kappenstein, 1974). 

Ion-selective electrodes are a remarkable product of this approach. Their development can be followed in several circular pathways from natural bioelectric phenomena to artificial membrane systems and back again, to attempts to explain processes at a cellular level. 

The use of synthetic materials that imitate recognition characteristics of biological materials has been explored. Particularly, MIPs can be thought of as viable alternates to replace natural receptors. Due to easier methods of in situ preparation as films on electrode surfaces or in membranes and, hence, easier fabrication, the field of chemosensors featuring artificial receptors has received broad attention showing a pronounced progress. 

Fig. 12.86. Preparation of Pd membrane with artificial voids. (A) indented artificial voids, (B) voids filled with octacosane, (C) microcavities in 5-pm-thick electrodeposited Pd, (D) electrode covered with 100-pm-thick electrodeposited Pd layer. (Reprinted from Z. Minevski, dissertation, Texas A M University, 1995.)...

Channel activity is best studied electrochemically as charged species cross a cell membrane or artificial lipid bilayer. There is a difference in electrical potential between the interior and exterior of a cell leading to the membrane itself having a resting potential between -50 and -100 mV. This can be determined by placing a microelectrode inside the cell and measuring the potential difference between it and a reference electrode placed in the extracellular solution. Subsequent changes in electrical current or capacitance are indicative of a transmembrane flux of ions. 

Electrocatalytic groups such as porphyrins and phthalocyanines that act as supramolecular hosts for different metals and mimic the active sites of various proteins are commonly used in amperometric sensors [66,67]. A biomimetic sensor based on an artificial enzyme or synzyme has been demonstrated [68]. The artificial enzyme used in this study was a synthetic polymer (quaternised polyethyleneimine containing 10% primary amines) which decarboxylated oxaloacetate. The product carbon dioxide was detected potentiometrically via a gas membrane electrode. 

Current-voltage (I-V) curves for these artificial ion channels were obtained by mounting the membrane sample between the two halves of a U-tube conductivity cell [18]. Each half-cell was filled with -5 mL of a 10 mM pH 7.0 phosphate buffer that was also 100 mM in KCl. A Ag/AgCl reference electrode was inserted into each half-cell solution, and a Keithley instruments... 

Figure 8.29. Ion transport mechanisms through lipid membranes in living cells. There are principally two kinds of transport protein (a) channel proteins, that is, a channelforming ionophore, and (b) carrier proteins, that is, a mobile ion carrier ionophore. The phenomena observed in living cells have much in common with those in artificial polymer membrane ion-selective electrodes. (From Widmer, 1993.)...

Scheller et al. (1986a) combined polyurethane-immobilized LOD with an Au/Pd-sputtered carbon electrode. The electrode modification permits H2O2 to be electrochemically oxidized at a potential as low as +450 mV, where interferences by other anodically oxidizable compounds such as NADH and ascorbic acid are largely reduced (Fig. 57). This increased selectivity also enables the measurement of LDH activity. The sensor has been introduced in an FIA manifold. A sample frequency of 200Ai (Fig. 58) and a CV below 1% were obtained with this setup. A platinum electrode with LOD covalently bound to a nylon membrane has been employed for continuous blood lactate determination in an artificial pancreas by Mascini et al. (1985b, 1987). 

The development of a portable and rugged sensing device requires that the selective recognition element be directly interfaced to the physical transducer. In the case of electrochemical transducers based on artificial BLMs, this entails stabilization of the assembly onto an electrode. The stabilization method must allow the membrane to retain characteristics of molecular mobility and fluidity which are essential for transduction and should provide sufficient ruggedness to permit use over an extended period of time (several months) without severe alteration of the response characteristics of the membrane. 

An extension of artificial membranes for ion selective electrochemical work was the construction of biomimetic ion channel sensors [65]. These devices were based on Langmuir-Blodgett deposition of charged lipid membranes onto a glassy carbon electrode. This work indicated that a conductive zone can be opened reversibly by a stimulant-membrane interaction by surface charge alterations. This work has demonstrated how the concept of the conductivity measurement could be extended to the more common and useful technique of cyclic voltammetry. 

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