Surface membrane potential electrode measurement

The potential of each channel may be composed of two potentials. One is an oxidation-reduction potential generating at the boundary surface between the Ag electrode and the lipid membrane. The other is a Donnan potential at the boundary between the lipid membrane and the aqueous medium or more generally a Gouy-Chapman electrical double-layer potential formed in the aqueous medium [24]. Figure 7 shows a potential profile near the lipid membrane. The oxidation-reduction potential would not be affected by the outer solution in short time, because the lipid membrane had low permeability for water. Then the measured potential change by application of the taste solution is mainly due to the change in the surface electrical potential. 

Ion selective electrodes measure an interface potential at the surface of the electrode The specifier is either generated by a specific property of the surface (glass electrode) or by a selective carrier transporting the ion across a membrane The main problem here as with poten-tiometry in general is the stability of the output signal. Consequently the production of a stable reference electrode junction potential is very critical. 

The second technique involves using a four probe apparatus, similar to that described by Cahan and Wainright. The membrane sample is placed in an PTFE apparatus which is equipped with two platinum strips in contact with the film, as shown in Fig. 1.115. Two platinum electrodes in a fixed geometry (distance of 1.026 cm) were placed on the surface of the film to measure the membrane potential and capacitance. Conductivity measurements could be obtained by utilizing complex impedance plots, which employ a circuit diagram... 

It is universally found that electrical potentials are associated with surfaces in biology. In pre-electrodic days, this called for some special explanation. Many measurements of the so-called membrane potentials showed values in the region of 50-100 mV but occasionally measurements were much lower, or stretched to values of 250 mV " (Table 1). The large edifice of electrophysiology is intimately associated with these potentials. 

For biological samples four different contributions to the standard potential Ef can be observed the contribution of the internal standard potential of the reference electrode ERef the diffusion potential over the liquid junction Ej generated between the sample solution and the reference electrode a potential difference (electrical asymmetry) of the ion-selective membrane after preparation and conditioning Eei and a sample-induced asymmetry of the membrane Eas. For measurements in human blood samples directly the adsorption of sample components at the membrane surface creates a drift associated with the affinity of the membrane to lipids as well as proteins. 

Membrane electrodes are a class of electrodes that respond selectively to ions by the development of a potential difference (a type of junction potential) across a membrane that separates the analyte solution from a reference solution. The potential difference is related to the concentration difference in the specific ion measured on either side of the membrane. These electrodes do not involve a redox reaction at the surface of the electrode as do metallic electrodes. Because these electrodes respond to ions, they are often referred to as ion selective electrodes (ISEs). The ideal membrane allows the transport of only one kind of ion across it that is, it would be specific for the measurement of one ionic species only. As of this writing, there are no specific ISEs, but there are some highly selective ones. Each electrode is more or less selective for one ion therefore, a separate electrode is needed for each species to be measured. In recent years, many different types of membrane electrodes have been developed for a wide variety of measurements. 

As in nature, networks are relatively fault tolerant concerning, for example, changes in synaptic connections. All these effects can be measured by the change of membrane potential during an action potential (cf. Sect. 3.2). This potential has a direct influence on the gate of a field-effect transistor, or, in another device, it influences the capacity between a microelectrode and the axon, which can be measured with a.c.-coupled amplifiers with high input impedances. AU measurement conditions have to be chosen so that no electrochemical reaction takes place at the electrode surface in order to avoid the formation of poisoning chemicals. 

It is well known that the resting and dynamic electrical activity of the brain is a consequence of electrochemical potentials across membranes. Many other aspects of electrochemistry are also familiar in the neurosciences. Hence it may seem paradoxical to have suggested that the electro-analytical techniques are far afield of the mainstream of neurobiology. However, neuronal membrane potentials depend on ionic charge distributions and fluxes insofar as is known, electron current plays no role. Just the opposite is true for electroanalytical techniques—ionic conductance is of minimal importance but electron flow (current) is the essence of the measurement. The electrodes employed do not sense membrane potentials or respond to sodium or potassium fluxes rather, they pass small but finite currents because molecules close to their surface undergo oxidation or reduction. Such electrochemical measurements are called faradaic (because the amount of material converted at the electrode surface can be calculated from Faraday s law). 

From equation (3), besides the charges, the membrane potential is affected by the surface area (A) and the thickness (d), vrhich have been demonstrated, 22 We have measured the capacitance of the glass membrane as a function of pH as discussed in the last part of this paper. The membrane electrodes behave definitely as capacitors. 

The key component of a potentiometric sensor is the ion-selective electrode (ISE), an electrode or electrode assembly with a potential that is dependent on the concentration of an ionic species in the test solution and is used for electroanalysis. This interfacial potential at the electrode surface is caused by the selective ion exchange reaction. Ion-selective electrodes are often membrane type electrodes. The well-known glass pH electrode, which is selectively sensitive to hydrogen ions, is a typical ISE and has been used for years for the measurement of acidity or basicity of aqueous solutions in... 

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