Abstract 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.

中文翻译：

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pH 敏感自旋探针和标签的 EPR 揭示了高温退火阳极氧化铝膜的内部纳米孔表面的静电性能

摘要阳极氧化铝 (AAO) 膜是一种多功能纳米材料，它结合了陶瓷的化学稳定性和机械稳定性能与均匀的纳米级组织，可以调整到理想的孔径和长度。AAO 基材具有高表面积，大分子和小分子都很容易接触，使这些纳米结构特别适合许多可能的应用。例子包括宏观排列的生物膜和多相催化底物的模板化自组装。为了进一步开发此类应用，人们希望表征和调整内孔表面的静电特性以及纳米通道内的局部酸度。这里，我们使用小分子（可电离的氮氧化物）的电子顺磁共振 (EPR) 光谱作为纳米通道中平均局部酸度和孔隙表面附近局部静电势的报告器。前者是通过测量这种分子探针在 AAO 纳米通道中的水相中扩散的 EPR 光谱来实现的，而对于后者，氮氧化合物共价连接到氧化铝表面的羟基上。我们表明，纳米通道内的局部酸度与本体溶液的 pH 值相比，通过将孔径减小到大约 1.48 个 pH 单位而增加了 ≈ 1.48。31 纳米。此外，在将膜首先退火至 700 °C 然后至 1200 °C 时，所制备的 AAO 的正表面电荷可以减少，甚至转变为负表面电荷。对于制备的 AAO，对于共价连接到 AAO 并位于距离表面约 0.5 nm 的氮氧化物标记，局部静电势达到 ψ= (163 ± 5) mV。总的来说，我们证明了被 AAO 纳米孔限制的水体积的酸基特性可以通过改变孔径来调整。71 到 31 nm 或通过热退火来切换表面电荷的符号。这些观察结果提供了一种简单而可靠的方法，可以为依赖酸碱平衡的特定应用定制这些通用的高表面积纳米材料。对于共价连接到 AAO 并位于距离表面约 0.5 nm 的氮氧化物标记，局部静电势达到 ψ= (163 ± 5) mV。总的来说，我们证明了被 AAO 纳米孔限制的水体积的酸基特性可以通过改变孔径来调整。71 到 31 nm 或通过热退火来切换表面电荷的符号。这些观察结果提供了一种简单而可靠的方法，可以为依赖酸碱平衡的特定应用定制这些通用的高表面积纳米材料。对于共价连接到 AAO 并位于距离表面约 0.5 nm 的氮氧化物标记，局部静电势达到 ψ= (163 ± 5) mV。总的来说，我们证明了被 AAO 纳米孔限制的水体积的酸基特性可以通过改变孔径来调整。71 到 31 nm 或通过热退火来切换表面电荷的符号。这些观察结果提供了一种简单而可靠的方法，可以为依赖酸碱平衡的特定应用定制这些通用的高表面积纳米材料。我们证明了由 AAO 纳米孔限制的水体积的酸基特性可以通过改变孔径来调整。71 到 31 nm 或通过热退火来切换表面电荷的符号。这些观察结果提供了一种简单而可靠的方法，可以为依赖酸碱平衡的特定应用定制这些通用的高表面积纳米材料。我们证明了由 AAO 纳米孔限制的水体积的酸基特性可以通过改变孔径来调整。71 到 31 nm 或通过热退火来切换表面电荷的符号。这些观察结果提供了一种简单而可靠的方法，可以为依赖酸碱平衡的特定应用定制这些通用的高表面积纳米材料。