Suchergebnisse

Anodic aluminum oxide (AAO) membranes are versatile nanomaterials that combine the chemically stable and mechanically robust properties of ceramics with homogeneous nanoscale organization that can be tuned to desirable pore diameters and lengths. The AAO substrates feature high surface area that is readily accessible to large and small molecules, making these nanostructures uniquely suited for many possible applications. Examples include templated self-assembly of macroscopically aligned biological membranes and substrates for heterogeneous catalysis. For further development of such applications, one would like to characterize and tune the electrostatic properties of the inner pore surface as well as the local acidity within the nanochannels. Here, we employed electron paramagnetic resonance (EPR) spectroscopy of a small molecule – ionizable nitroxide – as a reporter of the average local acidity in the nanochannels and the local electrostatic potential in the immediate vicinity of the pore surface. The former was achieved by measuring EPR spectra of this molecular probe diffusing in an aqueous phase confined in the AAO nanochannels while for the latter the nitroxide was covalently attached to the hydroxyl group of the alumina surface. We show that the local acidity within the nanochannels is increased by as much as ≈1.48 pH units vs. the pH of bulk solution by decreasing the pore diameter down to ca. 31 nm. Furthermore, the positive surface charge of the as-prepared AAO could be decreased and even switched to a negative surface charge upon annealing the membranes first to 700 °C and then to 1200 °C. For as-prepared AAO, the local electrostatic potential reaches ψ= (163 ± 5) mV for the nitroxide label covalently attached to AAO and located about 0.5 nm away from the surface. Overall, we demonstrate that the acid-based properties of the aqueous volume confined by the AAO nanopores pores can be tuned by either changing the pore diameter from ca. 71 to 31 nm or by thermal annealing to switch the sign of the surface charge. These observations provide a simple and robust means to tailor these versatile high-surface-area nanomaterials for specific applications that depend on acid-base equilibria. © 2020 Elsevier B.V. ; E.G.K., L.S.M., D.P.T. acknowledge the financial support of the Program 211 of the Government of the Russian Federation no. 02.A03.21.0006 , RFBR grant 18-29-12129mk , and the State Task from the Ministry of Science and Higher Education of the Russian Federation no. 0836-2020-0058 . Fabrication and SEM characterization of AAO, least-squares fitting of EPR spectra and the final preparation of the manuscript were supported by U.S. DOE Contract DE-FG02-02ER15354 to AIS. IAK acknowledges support from the Ministry of Science and Higher Education of the Russian Federation grant No. 14.W03.31.0034 for the nitroxide synthesis. NR-dimethylamine derivative has been synthesized in the context of State Task of the Institute of Organic Synthesis named after Academician I. Ya. Postovsky of the Ural Branch of the Russian Academy of Sciences (Pr. AAAA- A19-119012490006-1 ).