Cyclodextrin monolayer, permeability

Figure 18. Cyclic voltammograms of 1,4-benzoquinone (p-quinone) as permeability marker. Curve A in the absence of a cyclodextrin monolayer on a buffer solution containing no guest (p.1 M CH3C02Na-CH3C00H, pH 6.0). Curve B in the presence of the condensed monolayer of p-cyclodextrin derivative 41 on a buffer solution containing no guest. Curve C-E in the presence of the condensed monolayer of 41 on a buffer solution containing guest 59 at concentrations of 5.0 x 10", 1.0 x 10 , and 2.0 x 10 M, respectively (reprinted with permission from Anal. Chem. 1993, 65, 930. Copyright 1993 American Chemical Society).

Dependence of Marker Permeability on Surface Pressure The dependence of the cyclic voltammogram area on the surface pressure applied to the cyclodextrin monolayer was examined. By increasing the surfece pressure from 10 to 50 mN m or, in other words, by decreasing the molecular area from 260 to 210 molecule the voltammogram areas for all three markers decreased almost linearly. However, an important point to be emphasized here is that the magnitudes of the decrease were different the decrease for p-quinone was much smaller than that for [Co(phen)3]2+ and [Mo(CN)g]. This is also consistent with the capability of p-quinone and incapability of the other two bulky markers to sterically pass tiirough the P-cyclodextrin cavity. 

Another point to be noted is the fact that a dramatic increase in the voltammogram area upon a decrease in the surface pressure was observed for both of the two bulky markers, regardless of whether the marker is hydrophilic ([Mo(CN)g] ) or hydrophobic ([Co(phen)3]2+). These results indicate that the main factor controlling the permeability through e intramolecular channel is the steric bulkiness rather than the hydrophobicity of the marker. Such an aspect seems to be characteristic of the cyclodextrin monolayer and contrasts to the properties of monolayers of simple alkane derivatives, in which the permeability (through intermolecular voids) is controlled mainly by the hydrophobicity and not the steric bulkiness of the marker (30). 

Figure 19. Schematic representations of the permeation behaviors of electroactive markers through the condensed monolayer of p-cyclodextrin derivative 41 in the presence and absence of the guest, (a) Permeable markers, (b) Nonpermeable markers. ...

Table 6. Selectivity of Guest-Induced Permeability Decrease for a Channel Mimetic Sensing Membrane Composed of a Condensed Monolayer of P-Cyclodextrin Derivative (41) ...

Figure 7.23. Two sorts of molecular channels formed in monolayers of calixarenes or cyclodextrines. a Intermolecular channel, b Intramolecular channel. Permeability for small molecules is controlled by the analyte...

Horizontal Touch Cyclic Voltammetry with a Condensed Monolayer of a Cyclodextrin Derivative. To obtain experimental evidence supporting that such a mode of permeability control is possible, an approach based on horizontal touch cyclic voltammetry was carried out for a condensed monolayer of 4, which was formed at the air/water interface in a Langmuir trough 14), This technique, first used by Fujihira (25) and recently sophisticated by Bard (2d, 27), enables the investigation of the electrochemical properties of oriented monolayers at varying packing densities under an appropriately controlled surface pressure. 

Permeability through the monolayer of cyclodextrin derivative 4 has been estimated from the accessibility of electroactive markers to the electrode surface, which in turn can be estimated from the peak potential and area of cyclic voltammograms (Figure 4). The calculation of the voltammogram area has been made by an integration of either the oxidation or reduction peak that corresponds to the initial process of the redox cycle, i,e, the oxidation peak in the cases of [Co(phen)3]2+ and [Mo(CN)g]4", and the reduction peak in the case of p-quinone. 

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