Structural Engineering of the Barrier Oxide Layer of Nanoporous Anodic Alumina for Iontronic Sensing

Abstract

The hemispherical barrier oxide layer (BOL) closing the bottom tips of hexagonally distributed arrays of cylindrical nanochannels in nanoporous anodic alumina (NAA) membranes is structurally engineered by anodizing aluminum substrates in three distinct acid electrolytes at their corresponding self-ordering anodizing potentials. These nanochannels display a characteristic ionic current rectification (ICR) signal between high and low ionic conduction states, which is determined by the thickness and chemical composition of the BOL and the pH of the ionic electrolyte solution. The rectification efficiency of the ionic current associated with the flow of ions across the anodic BOL increases with its thickness, under optimal pH conditions. The inner surface of the nanopores in NAA membranes was chemically modified with thiol-terminated functional molecules. The resultant NAA-based iontronic system provides a model platform to selectively detect gold metal ions (Au3+) by harnessing dynamic ICR signal shifts as the core sensing principle. The sensitivity of the system is proportional to the thickness of the barrier oxide layer, where NAA membranes produced in phosphoric acid at 195 V with a BOL thickness of 232 ± 6 nm achieve the highest sensitivity and low limit of detection in the sub-picomolar range. This study provides exciting opportunities to engineer NAA structures with tailorable ICR signals for specific applications across iontronic sensing and other nanofluidic disciplines.

Nanoporous Anodic Alumina Photonic Crystals for Sunlight Harvesting Applications: A Perspective

“…[24][25][26] In their pioneering studies, they utilized nanoindentation by lithographically produced master stamps to pattern the surface of aluminum substrates with highly ordered indentations, the function , nanopore diameter or filling factor-D P , interpore distance or lattice constant-D Int , and nanopore length-L P ), and top view (scale bar: 1 μm), bottom view (1 μm), and general and magnified (inset) cross-sectional view (scale bars: 250 and 50 nm, respectively) field emission scanning electron microscopy (FEG-SEM) images of NAA produced by the two-step anodization process in 0.3 M oxalic acid electrolyte at 40 V. Adapted with permission. [232] Copyright 2022, American Chemical Society. b) Fabrication diagram describing the two-step anodization process with: aluminum substrate, nanopore nucleation (initial stage of the first anodization step), nanopore growth, chemical wet etching to remove the sacrificial anodic layer, and nanopore development (second anodization step).…”

“…In a pioneering study, Fu et al used NAA–FPIs as a passive radiative cooling optical coating. [ 232 ] The structure of NAA films was optimized to achieve high absorbance with nearly no loss in the far‐IR atmospheric window. The authors reported a cooling power density for this system in the range of 64 W m −2 at ambient temperature and 70% humidity under a sunlight radiance of AM1.5.…”

“…In a similar study, Liu et al developed a composite optical film with NAA–FPIs filled with silica nanoparticles. [ 232 ] The resultant film technology is capable of reducing the temperature to 8.9 and 8.4 °C below the ambient air temperatures at night and day, respectively. Despite these advances, there remains a broad range of opportunities to integrate NAA‐based PC structures for solar‐to‐thermal energy conversion processes.…”

“…Structure and fabrication of NAA. a) Schematics describing the main geometric features of NAA (i.e., nanopore diameter or filling factor-D P , interpore distance or lattice constant-D Int , and nanopore length-L P ), and top view (scale bar: 1 μm), bottom view (1 μm), and general and magnified (inset) cross-sectional view (scale bars: 250 and 50 nm, respectively) field emission scanning electron microscopy (FEG-SEM) images of NAA produced by the two-step anodization process in 0.3 M oxalic acid electrolyte at 40 V. Adapted with permission [232]. Copyright 2022, American Chemical Society.…”

“…b) Fabrication diagram describing the two-step anodization process with: aluminum substrate, nanopore nucleation (initial stage of the first anodization step), nanopore growth, chemical wet etching to remove the sacrificial anodic layer, and nanopore development (second anodization step). Adapted with permission [232]. Copyright 2022, American Chemical Society.…”