Potentiometric membrane dyes

Loew LM (1994) Characterization of potentiometric membrane dyes. In Blank M, Vodyanoy I (eds) Biomembrane electrochemistry. Washington DC, American Chemical Society... 

POTENTIOMETRIC membrane DYES are employed to study cell physiology. The pioneering work of Cohen and his co-workers (1-3) led to the availability of a large number of organic dyes whose spectral properties are sensitive to changes in membrane potential. The applications of these dyes to a variety of problems in cell biology and neuroscience were reviewed in a series of chapters in a recent book (4). The aim of this chapter is to review the characteristics of these dyes as determined in model and cell membranes. 

When the feed solution is 1 mM in KCl and 0.5 mM in the cationic dye methylene blue and the receiver solution is 1 mM KCl, the initially colorless receiver solution turns blue due to transport of the cationic dye across the membrane [71]. In contrast, when the feed solution is 1 mM in KCl and 5 mM in KMn04 (Mn04 is red) and the receiver solution is 1 mM KCl, the receiver solution remains colorless [71]. These experiments provide simple visual evidence that this membrane transports a large cation but does not transport a much smaller anion. We have used potentiometric measurements to explore the nature of this cation permselectivity. 

This system behaves like a nonpolarizable interface. The salt concentration ratio will not be affected by potential applied from an extraneous source. The equilibrium potential depends only on the standard potentials of transfer of the ions in particular, it does not depend on the initial concentrations (ca and cp) nor is it a function of the phase volumes. Therefore, if only one salt is present in a LL system, the system is not amenable to potentiometric studies. It is thus essential that a supporting electrolyte be present to observe a potentiometric response of a third ion. The need to have a supporting electrolyte is similar to the need of immobilized ions in an ion exchanger membrane of an ion-selective electrode it also explains why it is essential that a supporting electrolyte or physiological concentration of salts must be present in measurements that employ fluorescent dyes. 

Slow Dyes. Slow dyes generally operate by a potential-dependent partitioning between the extracellular medium and either the membrane or the cytoplasm. This redistribution of dye molecules is effected via the interaction of the voltage with the ionic charge on the dye. Unlike fast potentiometric indicators, slow redistribution dyes must be charged. Three chromophore types have yielded useful slow dyes cyanines, oxonols, and rhodamines. Each of these chromophores has special features that suit different kinds of experimental requirements. A set of important slow dyes is depicted in Chart III. 

To determine if a dye will be a useful potentiometric probe in biological applications, information on its chemical, physical, spectral, and toxicological properties must be gathered in addition to data on the sensitivity to membrane potential. A number of model membrane systems are employed to characterize dye properties as well as some simple biological preparations. 

Z. Kormosh, I. Hunka, Y. Bazel, Potentiometric determination of diclofenac in pharmaceutical formulation by membrane electrode based on ion associate with base dye, Chinese... 

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