Acid–Base Properties of Nanoconfined Volumes of Anodic Aluminum Oxide Pores by EPR of pH-Sensitive Spin Probes

Abstract

Anodic aluminum oxide (AAO) ceramic membranes with macroscopically aligned and hexagonally packed nanopore architecture are attractive substrates for forming nanotubular lipid bilayers as well as sorption and catalytic media because of a tunable pore diameter, robust pore structure, and low fabrication cost. Here we employed continuous wave X-band (9 GHz) EPR of two pH-sensitive nitroxide radicals to assess acid−base properties AAO membranes prepared from low-cost commercial grade aluminum and compared those with commercial Anodisc membranes from Whatman, Ltd. The AAO membranes with pore diameters ≥58 ± 8 nm showed essentially the same pH inside the pores, pH int , as the bulk external solution, pH ext , over the 0.1−3.0 M range of ionic strength. However, the apparent pK a of nitroxide probes inside the pores deviated from the bulk values for the nanopores of smaller diameters of ca. 29 and 18 nm. Specifically, for the latter nanopores the values of pH int were found to be 0.5−0.8 pH unit lower than the bulk pH ext . An increase in acidity of the bulk solution led to a steady decrease of the negative charge on inner surface of the 38 nm nanopores and its recharge from a negative to a positive value at pH 4.7 ± 0.1, corresponding to the point of zero charge (pzc). Overall, the EPR titration method described here could assist in characterization of meso-and nanoporous membranes for catalytic and sorption applications as well as act a support medium for self-assembled biomembrane systems.

Experimental and Theoretical Study of Enhanced Vapor Transport through Nanochannels of Anodic Alumina Membranes in a Capillary Condensation Regime

“…These membranes possess a through porosity represented by a closely packed system of cylindrical channels that can be easily formed by anodic oxidation of metallic aluminum. Anodic oxidation enables control over channel diameter and interpore distance in a wide range of values (pore diameter from 15 to 200 nm; interpore distance from 50 to 500 nm; porosity 10–30%). − Moreover, the thickness of anodic aluminum oxide (AAO) films can be regulated precisely using conventional electrochemistry methods by controlling total electric quantity . Variation of anodization voltage during AAO film formation enables preparation of asymmetric membranes with hierarchical porous structures .…”

Acidic and Electrosurface Properties of Binary TiO2-SiO2 Xerogels Using EPR of pH-Sensitive Nitroxides

“…Sealing of the samples inside the quartz tube prevented water evaporation from the pores. Detailed descriptions of EPR titration experiments and analysis of EPR spectra can be found in [26,31,38].…”

The Microenvironment in Immobilized Enzymes: Methods of Characterization and Its Role in Determining Enzyme Performance

“…The alternative is to label the support material, which is, in general, the preferred approach for the preparation of internally sensitive material (Figure 3A). Diverse examples of luminescence labeling can be found in the literature: entrapment in alginate beads [52,53]; covalent incorporation into PEG microparticles [54] and membranes [55]; ionic adsorption on amine-activated silica surfaces [21]; covalent incorporation into porous silica or aluminum oxide pores [56,57]; and encapsulation in PVA hydrogel beads [58].…”