Neutral ion carrier

In contrast to ISEs with neutral ion carriers in the membrane, not even qualitative rules have been formulated for the solvent effect on the behaviour of ISEs with ion-exchanger ions in a liquid membrane. A basic condition for the ion-exchanger ions is that they be strongly hydrophobic. It must hold for the standard Gibbs energy of transfer of the ion-exchanger ion X and the deter-minand Y that... 

A membrane can be either a liquid or a solid. Its electrical properties arise when it allows transport of an ion of one charge but not that of another. Membranes are usually sufficiently thick that one can distinguish an inside region and two outer boundary regions which are in contact with electrolyte solutions. Two types of membranes are considered here (1) membranes of solid and glassy materials (2) liquid membranes with dissolved ion-exchanging ions or neutral ion carriers (ionophores). In fact all of these membranes are involved in ion exchange. It is important to understand how this process affects the potentials which develop in the system at both sides of the membrane. 

Nahir TM, Buck RP, Transport processes in membranes containing neutral ion carriers, positive ion complexes, negative mobile sites, and ion pairs. J. Phys. Chem. 1993 97 12363-12372. 

A potassium opto-sensor was recently described [75] for the continuous determination of electrolytes. Certain fluorescent dyes respond to an electric potential at the interface between the aqueous and lipid phases. This potential is created by the neutral ion carrier. The lipid layer is formed on a glass support by the Langmuir-Blodgett thin-fllm technique. This layer incorporates Rhodamine B as a dye and valinomycin as the carrier. The lipid membrane is made of arachidic acid. The fluorescence intensity decreases when this layer is exposed to potassium ions (linearity between 0.01 and 10 mM). This optode is also sensitive to sodium ions [76]. The selectivity factor of potassium in comparison with sodium ions varies from 10 - to 10 , and in relation to ammonium ions by 10. Interferences can be compensated for by a reference optode. However, better selectivity is obtained with new lipid membrane compositions (octadecan-l-ol-valinomycin) [77]. Tetralayers (Figure 17-9) give a maximum response for K". The K /Na selectivity is about 10 in a wide range (0.01-100 mM). 

A broadly applicable sensing scheme has been discovered by Simon s group [77 b]. It makes use of neutral ion carriers, that is, substances capable of specifically recognizing ions such as K +, NH, or Ca ". When the carrier transports such an ion into a PVC membrane, a proton is simultaneously released from a protonated dye in order to maintain electro-neutrality. The release of the proton from the dye results in a change in color which is monitored. 

Potentiometry ISEs for the determination of ionized (free) magnesium are now available. They use membranes with neutral ion carriers (e.g., modified ETH 5220), which, however, lack specificity with respect to calciiun. Therefore, magnesium and calcium must be measured simultaneously in the same analytical portion (with different electrodes) and magnesium concentration is calculated from this. 

The versatility of ISEs was enhanced considerably by the introduction of membranes containing neutral ion carriers (ionophores). The first ISE of this type, with a membrane containing valinomycin and selective for potassium ions, was described by Stefanac and Simon in 1966. There are many liquid chemical systems that interact highly selectively with ions through, e.g., ion exchange, ion association, or solvent extraction. Practically useful ISEs based on these systems and on neutral ionophores have been obtained due to the gradual perfection of the technology of plasticized poly(vinyl chloride) (PVC) matrix membranes. 

With neutral ion carriers, the coefficient of selectivity is given by... 

Neutral carrier liquid- or solvent-polymeric membrane electrodes, in which the membrane is usually formed from an organic solution of an electrically neutral, ion-selective complexing agent (neutral ion carriers, neutral ionophores) held in an inert polymer matrix. These membrane electrodes make use of the outstanding inherent ion selectivity of certain natural and synthetic ionophores. 

Samec, Z., D. HomoUca, and V. Marecek, Charge transfer between two immiscible electrolyte solutions. Part 8. Transfer of aUcah and alkaline earth metal cations across the water-nitrobenzene interface facihtated by synthetic neutral ion carriers, J Electmanal Chem, Vol. 135, (1982) p. 265. 

The behavior of ionic liquids as electrolytes is strongly influenced by the transport properties of their ionic constituents. These transport properties relate to the rate of ion movement and to the manner in which the ions move (as individual ions, ion-pairs, or ion aggregates). Conductivity, for example, depends on the number and mobility of charge carriers. If an ionic liquid is dominated by highly mobile but neutral ion-pairs it will have a small number of available charge carriers and thus a low conductivity. The two quantities often used to evaluate the transport properties of electrolytes are the ion-diffusion coefficients and the ion-transport numbers. The diffusion coefficient is a measure of the rate of movement of an ion in a solution, and the transport number is a measure of the fraction of charge carried by that ion in the presence of an electric field. 

Recently, Shinkai and Manabe achieved the active transport of K+ using a new type of carrier 39 derived from diaza crown ether43, 44). The ionophore forms the zwitter-ionic species 39b, which is most lipophilic among other species (39a, 39c), at about neutral pH region, and it acts as effective ion carrier in the active transport... 

Although one might think of neutral anion carriers for use in membrane electrodes, I cannot offer good suggestions. It is possible, however, to use charged ligands (classical ion exchangers) as membrane components. This makes accessible the measurement of, for example, Cl- and HCO in blood serum, without serious mutual interference. 

Plasmid/cationic carrier complexes have been proposed to internalize into the endosome and initiate the destabilization of endosomal membranes. This destabilization would induce diffusion of anionic lipids from the external layer of the endosomal membrane into the complexes and form charge neutralized ion pairs with the cationic lipids. Destabilization and/or fusion of the complex with the plasma membrane would permit the same anionic lipids to diffuse to the surface, as would fusion with the endosomal membrane. Release of the condensed DNA from the cationic lipid in the endosome is likely to generate a mechanical or osmotic stress that raptures the endosomal bilayer and releases DNA into the cytoplasm. In contrast, DNA release from complexes on the cell surface might be unable to stress the membrane to a degree sufficient to rapture. 

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