Membrane-Supported ITIES

In 1991, Kihara et al. pioneered the concept of voltammetry for organic liquid membranes with two ITIES in series, that is, for a system aqueous electrolyte (AEl)-organic electrolyte-aqueous electrolyte (AE2) [292,293], Using a 6-elec-trode cell, they have clearly shown that, as we polarize a liquid membrane, AE2-AEl, the two polarized ITIES in series, are coupled by the equation of continuity of current, and the overall potential window is the sum of the two individual potential windows in series. In 1998, Beriet and Girault used this approach to selectively transfer metal ions for metal recovery [294], 

In 2002, Uhneanu et al. used a thin organic layer that is supported by a porous hydrophobic membrane such as porous Teflon or poly vinylidenedifluoride (PVDF), or sandwiched between two aqueous dialysis membranes [295]. With this setup, they showed that the transfer of highly hydrophilic ions at one interface can be studied by limiting the mass transfer of the other ion-transfer reaction at the other interface. They have also shown that cyclic voltammetry for coupled ion-transfer reactions at the two interfaces in series is analogous to cyclic voltammetry for electron-transfer reactions studied by Stewart et al. [207], as the diffusion equations of the reactants and products are analogous, and as the overall Nernst equation for the coupled ion transfer equal to the two individual Nemst equations for ion distribution is also analogous to the Nemst equation for the heterogeneous ET. 

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Ishimatsu R, Kim J, ling P, Striemer CC, Fang DZ, Fauchet PM et al (2010) Ion-selective permeability of an ultrathin nanoporous silicon membrane as probed by scanning electrochemical microscopy using micropipet-supported ITIES tips. Anal Chem 82(17) 7127-7134. doi 10.1021/acl005052... 

Ion transport through the single nanopores of a pnc-Si membrane was imaged by using nanoscale SECM to quantitatively demonstrate an unprecedentedly high spatial resolution of 30 nm. In this nanoscale SECM imaging, a nanopipet-supported ITIES tip approached to a distance of down to... 

FIGURE18.4 The permeability of a pnc-Si membrane, k, plotted against the diffusion coefficients of probe ions in the aqueous phase, The solid line represents the best fit of Equation 18.2 for monovalent ions. The permeability to Arixtra and protamine was measured with 0.10, 0.03, and 0.01 M phosphate buffer at pH 7.0 while the buffer concentration was 0.10 M for monovalent probe ions. (Adapted with permission from Ishimatsu, R., Kim, J., Jing, R et al., Ion-selective permeability of a ultrathin nanopore silicon membrane as probed by scanning electrochemical microscopy using micropipet-supported ITIES tips. Anal. Chem., 82, 7127-7134, 2010. Copyright 2010 American Chemical Society.)... 

Electron-transfer reactions at ITIES resemble electron-transfer reactions across biological membranes, which adds a special interest. Also, in contrast to homogeneous electron-transfer reactions, they allow a separation of the reaction products. So it is disappointing to report that only very few experimental investigations of electron-transfer reactions at ITIES have been performed. This is mainly due to the fact that it is difficult to find systems where the reactants do not cross the interface after the reaction in addition, side reactions with the supporting electrolyte can be a problem. 

In these kinds of systems, the polarization phenomenon is effective at the two interfaces involved (see also Sect. 2.3.2). Specifically, in membrane systems comprising two ITIES, this behavior is achieved when the membrane contains a hydrophobic supporting electrolyte and the sample aqueous solution (the inner one) contains hydrophilic supporting electrolytes, and there is no common ion between any of the adjacent phases. In this case, the potential drop cannot be controlled individually and the processes taking place at both interfaces are linked to each other by virtue of the same electrical current intensity. 

The mechanism by which n-3 fatly acids are able to reduce the cholesterol level is still unclear, although some hypotheses have been advanced. For example, Bourre (1991) claimed that a-linolenic acid controls the composition of nerve membranes. Salem (1995) proposed that docosahexaenoic acid [DHA or (22 6n-3)] controls the composition and functions of the neuronal membrane. We recently reviewed a number of studies that provided support for apreferred n-3/n-6 ratio of 1 4 (Yehuda, et al., 1993). Itis possible that such a ratio optimizes uptake into the brain and its eventual incorporation into the neuronal membranes. 

The mass transfer through the non-catalytic support should also be taken into account, especially when, as occurs in most of the studied catalytic membranes, the support is quite thick (e.g., 1-2 mm). Effective diffusiv-ity through a porous membrane can be simply evaluated by correcting the binary diffusivity by the support porosity and tortuosity ... 

Similar to solid/liquid electrochemistry, important advantages can be obtained by replacing a macroscopic interface between two immiscible electrolyte solutions (ITIES) with a liquid/liquid nanointerface. In 1986, Taylor and Girault introduced micrometer-sized liquid/liquid interface supported at the tip of a pulled glass pipette. Nanoscale ITIES and their arrays have later been formed by using nanopipettes, nanopores, and porous membranes. Electrochemistry of nanopores and porous membranes is surveyed in Chapter 11 our focus here is on the ITIES supported at the tip of a nanometer-sized pipette. 

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