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

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, pHint, as the bulk external solution, pHext, over the 0.1-3.0 M range of ionic strength. However, the apparent pKa 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 pHint were found to be 0.5-0.8 pH unit lower than the bulk pHext. 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. © 2016 American Chemical Society.This work was supported by the Russian Foundation for Basic Research (Grant 14-03-00898), the Program 211 of the Government of the Russian Federation No. 02.A03.21.0006, and the State Task of the Ministry of Education (Russian Federation) No. 4.1626.2014/K and 2014/239. Fabrication and SEM characterization of AAO and the final preparation of the manuscript were supported by U.S. DOE Contract DE-FG02-02ER15354 to A.I.S. The authors are thankful to Prof. Igor A. Grigor''ev and Dr. Igor A. Kirilyuk (Institute of Organic Chemistry of the Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia) for synthesis of pH-sensitive nitroxide radicals