Functional-group effects on the ion-exchange properties of organically modified silicates

The ion-exchange and permselectivity of organically modified silicate films prepared from 3-aminopropyltriethoxysilane (APTEOS) have been investigated as a function of film structure and functionality. In this study, APTEOS was hydrolyzed and co-polymerized with either isobutyltrimethoxysilane (BTMOS), phenyltrimethoxysilane (PTMOS), methyltrimethoxysilane (MTMOS), or tetramethoxysilane (TMOS), and the resultant hybrid sol spin cast on the surface of a glassy carbon electrode. The magnitude and rate of the ion-exchange and permselectivity of the modified electrode in pH 4, 0.05 M potassium hydrogen phthalate (KHP) buffer solutions of potassium ferricyanide (Fe(CN)(6)(3-)) and ruthenium hexaammine (Ru(NH3)(6)(3+)) were evaluated via cyclic voltammetry and visible spectrophotometry. In a 1 mM Fe(CN)(6)(3-) solution, the organically modified silicate films showed a ca. 4-10 fold increase in Faradaic current (relative to a bare electrode) after 10-120 min, whereas the voltammetric response of 1 mM Ru(NH3)(6)(3+) was completely or nearly completely suppressed. The magnitude and rate of ion-exchange and permselectivity depend strongly on the amount of ion-exchange sites and the type of modifier introduced into the material. Films prepared from APTEOS and either PTMOS or BTMOS showed significantly faster ion-exchange relative to films prepared with either TMOS or MTMOS, presumably due to a more open silicate framework. In terms of both ion-exchange and permselectivity, films prepared with a 1 : 1 mole ratio of modifier to APTEOS provided the best results. (C) 1999 Elsevier Science B.V. All rights reserved.