Other Solid-State Membranes

AgjS can also be used as a matrix for other metal sulfides, notably CuS, PbS, and CdS, giving membranes responsive to the second metal. The electrode can be regarded as responding to the equilibrium activity of Ag , which is given by 

If the solution contains but not Ag, the electrode obeys the Nemst equation for M++. 

Hansen, Lamm, and Ruzicka described the Selectrode (trademark of Radiometer A/S), which involves a specially treated graphite electrode in direct contact with an electroactive surface. Typical is the halide electrode, involving a layer of silver halide and sulfide in direct contact with a graphite electrode that has been rendered hydrophobic by treatment with Teflon. Another example is the Cu(II) electrode, based on a similar preparation using CuS or CuSe. The Selectrodes couple an electronic conductor directly to the solid-state detector, which is an ionic conductor. Althou the mechanism of coupling is not clear. Buck suggested that an interfacial potential can be communicated through solid-state equilibria to the electronic conductor without the intervention of a redox process. 

Typical response curves of metal sulfide membrane electrodes in pure aqueous solutions show a linear relation with log C, with the slope expected from the Nemst 

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Other types of solid-state membranes include single crystals of sparsely soluble salts and are often called heterogeneous membranes, in which the insoluble salt is embedded in some inert polymer matrix. Obviously, in order for these membranes to be at equilibrium they should be in a saturated solution. In practice, these membranes are used in solutions that are below saturation. In that case, the insoluble salt slowly dissolves. 

In contrast, in most ion-selective membranes the charge conduction is done by ions. Thus, a mismatch between the charge-transfer carriers can exist at the noble metal/membrane interface. This is particularly true for polymer-based membranes, which are invariably ionic conductors. On the other hand, solid-state membranes that exhibit mixed ionic and electronic conductivity such as chalcogenide glasses, perovskites, and silver halides and conducting polymers (Lewenstam and Hulanicky, 1990) form good contact with noble metals. 

Subsequent to these early developments of alloy electrocatalysts in the PAFC technology, have been attempts to use the same in pefluorinated sulfonic acid fuel cells (solid state membranes such as Nation from Dupont, Dow, Asahi and others). Yeagei has reviewed the effect of different electrolytes on the ORR electrocatalysis. The summaiy of this work was that the solid state peifluorinated acid environment offered a significant advantage over phosphoric acid. These were... 

In the present chapter, the relationship between the electrode potential and the activity of the solution components in the cell is examined in detail. The connection between the Galvani potential difference at the electrode solution interface and the electrode potential on the standard redox scale is discussed. This leads to an examination of the extrathermodynamic assumption which allows one to define an absolute electrode potential. Ion transfer processes at the membrane solution interface are then examined. Diffusion potentials within the membrane and the Donnan potentials at the interface are illustrated for both liquid and solid state membranes. Specific ion electrodes are described, and their various modes of sensing ion activities in an analyte solution discussed. The structure and type of membrane used are considered with respect to its selectivity to a particular ion over other ions. At the end of the chapter, emphasis is placed on the definition of pH and its measurement using the glass electrode. 

Ion-selective membranes attain their permselectivity from ion-exchange, dissolution, or complexation phenomena. Different types of membranes are available for the construction of ion-selective electrodes glass and other solid state rods (crystals), liquid or polymer ion ecchangers, or dissolved ionophores. Many electrodes are commercially available with selec-tivities for different ions, mainly H, alkali metal cations, heavy metal ions, and halides or pseudohalides. Also gas-sensing electrodes may be constructed from an ion-selective electrode and a gas-permeable membrane [182]. Ion selective electrodes and gas-selective electrodes... 

The application of potentiometric detection in ion-chromatography is favoured by the progress in the field of membrane ion-selective electrodes (ISE). The electrodes with solid-state membranes were mostly employed for determination of halides, pseudohalides and some other anions binding silver ions. The use of fluoride electrode in multidetector, chromatographic system offers very low detection limit of 1.2 ng fluoride in injected samples. Application of bromide electrode in the same system provides even five-fold better detectability. The same level of detectability was reported by Butler and Gershey for iodide with iodide ISE. In the system with preconcentration step the detectability can be lowered by an order... 

Without in any way negating the wide-ranging utility of glass membrane electrodes for pH, pNa and pNH measurements and the good solid-state membrane electrodes for fluoride, sulphide and other ions, the main focus of development work of the last twenty years has been in the search for better liquid membrane systems. This has been stimulated by the convenience of PVC matrices for trapping and supporting liquid membranes (32-34,83). 

The length of time over which a chemical sensor can be expected to function reliably can be remarkably great (in the range of years), as in the case of glass-membrane or solid-state membrane electrodes, the lambda probe (Section 28.2.2.3), and the Taguchi gas sensor (see Section 28.2.2.2). On the other hand, a biosensor that depends upon a cascade of enzymes to... 

Inside living cells, enzymes exist in an extremely structured environment, which may have the form of a gel, a membrane or other solid state assemblies. Under these conditions, the behavior of immobilized enzymes towards their inhibitors is very different from that in the homogeneous phase. The mode of enzymatic inhibition is variable from one inhibitor to another, and may also be reversible or irreversible. Inhibition may be observed with enzyme sensors without having to resort to techniques that require long extractions, such as dialysis or gel filtration, which are normally used to separate an inhibitor from a soluble enzyme. The response of the enzyme sensor provides a continuous measure of the activity of the immobilized enzyme, whether or not its inhibitor is present. Enzyme sensors thus allow convenient and rapid study of the inhibition of enzymes and their subsequent reactivation. 

Other useful solid-state electrodes are based on silver compounds (particularly silver sulfide). Silver sulfide is an ionic conductor, in which silver ions are the mobile ions. Mixed pellets containing Ag2S-AgX (where X = Cl, Br, I, SCN) have been successfiilly used for the determination of one of these particular anions. The behavior of these electrodes is determined primarily by the solubility products involved. The relative solubility products of various ions with Ag+ thus dictate the selectivity (i.e., kt] = KSp(Agf)/KSP(Aw)). Consequently, the iodide electrode (membrane of Ag2S/AgI) displays high selectivity over Br- and Cl-. In contrast, die chloride electrode suffers from severe interference from Br- and I-. Similarly, mixtures of silver sulfide with CdS, CuS, or PbS provide membranes that are responsive to Cd2+, Cu2+, or Pb2+, respectively. A limitation of these mixed-salt electrodes is tiiat the solubility of die second salt must be much larger than that of silver sulfide. A silver sulfide membrane by itself responds to either S2- or Ag+ ions, down to die 10-8M level. 

Abstract To understand how membrane-active peptides (MAPs) function in vivo, it is essential to obtain structural information about them in their membrane-bound state. Most biophysical approaches rely on the use of bilayers prepared from synthetic phospholipids, i.e. artificial model membranes. A particularly successful structural method is solid-state NMR, which makes use of macroscopically oriented lipid bilayers to study selectively isotope-labelled peptides. Native biomembranes, however, have a far more complex lipid composition and a significant non-lipidic content (protein and carbohydrate). Model membranes, therefore, are not really adequate to address questions concerning for example the selectivity of these membranolytic peptides against prokaryotic vs eukaryotic cells, their varying activities against different bacterial strains, or other related biological issues. 

Here, we discuss a solid-state 19F-NMR approach that has been developed for structural studies of MAPs in lipid bilayers, and how this can be translated to measurements in native biomembranes. We review the essentials of the methodology and discuss key objectives in the practice of 19F-labelling of peptides. Furthermore, the preparation of macroscopically oriented biomembranes on solid supports is discussed in the context of other membrane models. Two native biomembrane systems are presented as examples human erythrocyte ghosts as representatives of eukaryotic cell membranes, and protoplasts from Micrococcus luteus as membranes... 

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