Ion—ionophore complex formation

The main classes of plasticizers for polymeric ISEs are defined by now and comprise lipophilic esters and ethers [90], The regular plasticizer content in polymeric membranes is up to 66% and its influence on the membrane properties cannot be neglected. Compatibility with the membrane polymer is an obvious prerequisite, but other plasticizer parameters must be taken into account, with polarity and lipophilicity as the most important ones. The nature of the plasticizer influences sensor selectivity and detection limits, but often the reasons are not straightforward. The specific solvation of ions by the plasticizer may influence the apparent ion-ionophore complex formation constants, as these may vary in different matrices. Ion-pair formation constants also depend on the solvent polarity, but in polymeric membranes such correlations are rather qualitative. Insufficient plasticizer lipophilicity may cause its leaching, which is especially undesired for in-vivo measurements, for microelectrodes and sensors working under flow conditions. Extension of plasticizer alkyl chains in order to enhance lipophilicity is only a partial problem solution, as it may lead to membrane component incompatibility. The concept of plasticizer-free membranes with active compounds, covalently attached to the polymer, has been intensively studied in recent years [91]. 

The treatment of partition equilibrium was further generalized to the cases in the presence of ion-pair formation [19] and ion-ionophore complex formation [21]. An important corollary of this theory of partition equilibrium based on standard ion transfer potentials of single ions is to give a new interpretation to liquid extraction processes. Kakutani et al. analyzed the extraction of anions with tris(l,10-phenan-throline) iron(II) cation from the aqueous phase to nitrobenzene [22], which demonstrated the effectiveness of the theory and gave a theoretical backbone for ion-pair extraction from an electrochemical point of view. 

Theoretical insight into the interfacial charge transfer at ITIES and detection mechanism of this type of sensor were considered [61-63], In case of ionophore assisted transport for a cation I the formation of ion-ionophore complexes in the organic (membrane) phase is expected, which can be described with the appropriate complex formation constant, /3ILnI. 

Conversely, the fundamentals for the UDL he on the coextraction of counterions into the membrane therefore, the membrane is no longer permselective (Donnan failure) [9]. Ideally, when the ionophores are saturated by ions, the ion-ionophore complex functions as an ion-exchanger and the membrane shows an anion Nernstian response. The UDL can be estimated from the membrane composition, formation constant and coextraction coefficients obtained from the so-called sandwich membrane method [73]. 

Importantly, the unbiased selectivity coefficients are thermodynamically meaningful. When an interfering ion, such as tetraethylammonium ion for cation-selective electrodes, does not bind to an ionophore, the selectivity coefficient for an ion of interest against such an interfering ion can be used to determine the formation constant of the ion-ionophore complexes (9). For example, when a neutral ionophore forms complexes with an ion, i, and does not bind to an interfering ion, j, the formation constant is given as... 

There appear to be two major ways by which ionophores aid ions to cross membrane barriers. Ionophores such as valinomycin and nonactin enclose the cation such that the outside of the complex is quite hydro-phobic (and thus lipid-soluble). The transport behaviour thus involves binding of the cation at the membrane surface by the antibiotic, followed by diffusion of the complexed cation across the membrane to the opposite surface where it is released. Such carrier type ionophores can be very efficient, with one molecule facilitating the passage of thousands of ions per second. A prerequisite for efficient transport by this type of ionophore is that both the kinetics of complex formation and dissociation be fast. 

The ionophoric properties of ( )-37 with a variety of alkaline and alkaline-earth cations have been reported, and K+ ions were found to bind preferentially (effective complex-formation constant log KclT = 5.4 0.2) over the other alkali-metal ions [55],... 

Liquid membranes with an ionophore L function via complex formation between the ionophore and an ion in the aqueous phase. When this ion is a monovalent cation M" " the membrane phase contains the cation in the form ML" ", that is, completely complexed with the ionophore. The other components of the membrane are a hydrophobic solvent S which constitutes the majority of the liquid phase and a hydrophobic anion A. The concentration of free cation M within the membrane is very small but must be considered in assessing the Donnan equilibria on each side. The description of the membrane system is... 

According to Katano and Senda [15,16], the transfer of Pb ions in the presence of citrate in W facilitated by 1,4,7,10,13,16-hexathiacyclo-octadecane is limited by the dissociation reaction of Pb + ions from their complexes with citrate in W, while the transfer of Pb ions across the interface and the complex formation of Pb ions with the ionophore in O are fast. The quantitative analyses of linear-sweep voltammograms and normal-pulse polarograms consistently show that the entire process is described by a CE mechanism and that the dissociation and association rate constants of the Pb -citrate complex are... 

The elucidation of the spatial structures of ionophores and their ion complexes is essential for understanding the detailed mechanisms of their biological action. From this point of view, the nature of the conformational rearrangements accompanying complex formation is of special interest, because these very conformational changes contribute considerably to ion binding selectivity. For that reason, this review will focus on comparisons between ligand conformations in both the com-plexed and the uncomplexed state whenever data on both are available. 

Upon complex formation, the ionophores undergo considerable conformational rearrangements, mainly resulting in a turning of the ester carbonyl oxygens toward the interior of the molecule. Just as in the case of valinomycin, but in contrast to beauvericin, the central ligand cavity serves as binding site in all the ion complexes... 

Lipophilic ionophores are essential to achieving a high sensing selectivity with liquid membrane ISEs. As explained above, ion-exchanger-based membranes always show the same selectivity pattern that follows the solvation energies of the ions, but iono-phore-based membranes may show very different selectivities. This is achieved by the formation of strong complexes between the extracted analyte ion and the ionophore in the membrane. Complex formation constants have been recently determined in the membrane phase in the range of 10 for monovalent to for divalent ions. With ion-... 

To illustrate which components are necessary to prepare an ISE membrane, let us again go back to a simple extraction experiment as it was similarly described in Section 3.1.1. Consider an aqueous potassium chloride solution equilibrated with an immiscible organic phase containing an electrically neutral ionophore for K+, that is, a receptor compound that binds the potassium ion selectively. How does the phase boundary potential between these two phases depend on the KCl concentration in the aqueous phase Upon equilibration of the two phases, some KCl will be present in the organic phase (Figure 5). For low amounts of KCl in the system, the potassium ions in the organic phase will be present in the form of ionophore complexes, and there will be an excess of free ionophore, L. In comparison to the concentration of the ionophore complex, the organic phase concentration [K+] of free potassium ions that are not bound by the ionophore is very low and can be calculated from the formation constant, of the potassium ion complex, [LK+] ... 

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