Understanding Ionic Flow in Nanofluidic Blind‐Hole Anodic Aluminum Oxide (AAO) Membranes for Osmotic Energy Generation

Highly ordered silver nanowire with a diameter of 10 ㎚ was arrayed by electroless deposition in a porous anodic aluminum oxide (AAO) membrane. The AAO membrane was fabricated electrochemically in an oxalic acid solution via a two-step anodization process, while growth of the silver nanowire was initiated by using electroless deposition at the long-range-ordered nanochannels of the AAO membrane followed by thermal reduction of a silver nitrate aqueous solution by increasing the temperature up to 350℃ for an hour. An additional electro-chemical procedure was applied after the two-step anodization to control the pore size and channel density of AAO, which enabled us to fabricate highly-ordered silver nanowire on a large scale. Electroless deposition of silver nitrate aqueous solution into the AAO membrane and thermal reduction of silver nanowires was performed by increasing the temperature up to 350℃ for 1 h. The morphologies of silver nanowires arrayed in the AAO membrane were investigated using SEM. The chemical composition and crystalline structure were confirmed by XRD and EDX. The electrolessdeposited silver nanowires in AAO revealed a well-crystallized self-ordered array with a width of 10 ㎚.

A novel bottom imprint method to fabricate high-quality Si [100] nanowire arrays is demonstrated. This new approach combines the functions of a high-ordering AAO template as a stamp and template simultaneously. By the protective polymer layer in the hot imprint, the vertical 40 nm Si nanowire arrays grow epitaxially on the Si substrate with a narrow size distribution Detailed facts of importance to specialist readers are published as "Supporting Information". Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Recently, anodic aluminum oxide (AAO) membranes with ordered and monodispersed nanopores have attracted significant attention because the pore sizes and lengths of the AAO membranes can be systematically controlled by changing the anodization parameters. Asymmetries of geometry, wettability, charge, and polarity in nanoporous AAO membranes are now highly desired for applications such as drug delivery, membrane filtration, and catalysis. Although the asymmetries in AAO membranes have been widely studied, AAO membranes whose nanopores simultaneously possess more than two asymmetric properties still need to be developed. In this work, we present a simple method to fabricate asymmetric AAO membranes by combining the post-treatments with the three-step anodization method. The asymmetries of the AAO membranes simultaneously possessing larger nanopores with higher hydrophobicity and smaller nanopores with lower hydrophobicity are characterized by scanning electron microscopy, water contact angle measurements, and energy-dispersive X-ray spectroscopy. Using the asymmetric AAO membranes, nanowires of polystyrene and poly(styrene-block-dimethylsiloxane) with double-sized diameters are also fabricated via the melt and solvent wetting methods, exhibiting that the AAO membranes with double-sized diameters can be used as the templates for generating double-sized polymer nanowires.

The architecture-dependent wettability of three-dimensional (3D), porous anodic aluminium oxide (AAO) membranes with varying surface morphologies including hierarchical, mesh and honeycomb nanostructures is reported. The surface morphology and underlayer structure play different roles in regulating the wetting behaviour of the AAO membranes. For the mild AAO membranes, the wetting behaviour of the ultra-thin top layer is dominated by the surface morphology in which the water contact angles (WCAs) of the AAO membranes with hierarchical, mesh and honeycomb structures are approximately 113.7° ± 4.6°, 94.9° ± 0.7° and 98.8° ± 5.8°, respectively. The wetting behaviour of the 3D, layered AAO membranes is dominated by both the surface morphology and the underlayer structure. Notably, the WCA of the mild AAO membrane with a layered hierarchical structure increases in the second layer (increase in the hole density). The WCAs of the three kinds of layered hard AAO membranes decrease in the second layer (increase in the hole depth) and then decrease slowly or increase in the third transition layer (decrease in the hole density). The WCAs of all the AAO membranes decrease linearly at different rates with the formation of the ordered bottom layer. The above results can facilitate the engineering of nanostructures for controlling the surface wetting behaviour of materials and devices for applications in multiple fields.

A highly ordered silver nanorod array (AgNRs) with a diameter of 90 nm has been prepared by chemical reduction from silver nitrate in a porous anodic aluminium oxide (AAO) membrane. The AAO membrane was fabricated electrochemically in an oxalic acid solution via a one-step anodisation process, followed by the growth of silver NR in the long-range ordered nanochannels of the AAO membrane via chemical reduction. The morphologies of AAO membrane and AgNRs with periodic rows of air-pockets encapsulated in the AAO membrane have been investigated using TEM. The crystalline structure and chemical composition have been confirmed by XRD, TEM and EDX techniques. The process, which has three stages, namely, (1) anodising of aluminium foil to form a mesoporous membrane, (2) adsorption of metal ions, then (3) chemical reduction of the adsorbed metal ions to form the nanorod array of AgNRs in the pores of AAO membrane, has not, to the authors’ knowledge, been reported to date.

In recent years, anodic aluminum oxide (AAO) membranes have been widely used as attractive nanoporous materials because of their applications in drug delivery, membrane filtration, and catalysis. Although AAO membranes with anisotropic pore shapes can be achieved, the methods are often complex and costly. In this work, we present an innovative approach to fabricating AAO membranes with controlled pore geometries, focusing on the transition from nonelliptical to elliptical nanopores. Utilizing commercially available aluminum foils of varying thickness and purity, the research demonstrates a cost-effective and scalable method for producing these specialized AAO membranes. The process involves a two-step anodization, where the aluminum foils undergo electropolishing, anodization, and chemical etching, followed by a unique stretching technique that transforms circular concavities into elliptical shapes. This approach successfully produces elliptical AAO membranes with varying pore sizes and aspect ratios, controlled by different electrolytes and pore-widening times. The study further explores the application of these membranes in nanopatterning by creating anisotropic polymer nanorod arrays using polystyrene (PS) and poly(methyl methacrylate) (PMMA), demonstrating the practical utility of the fabricated AAO membranes in nanomaterial synthesis. The research highlights the potential of the stretching approach in AAO membrane fabrication, promising a diverse range of applications in material science and engineering, particularly in fields requiring precise nanopore geometries.

Fabrication of mechanically stable AAO membrane with improved fluid permeation properties

The surface topographies of nanoporous anodic aluminum oxide (AAO) and titanium dioxide (TiO2) membranes have been shown to modulate cell response in orthopedic and skin wound repair applications. In this study, we: (1) demonstrate an improved atomic layer deposition (ALD) method for coating the porous structures of 20, 100, and 200 nm pore diameter AAO with nanometer-thick layers of TiO2 and (2) evaluate the effects of uncoated AAO and TiO2-coated AAO on cellular responses. The TiO2 coatings were deposited on the AAO membranes without compromising the openings of the nanoscale pores. The 20 nm TiO2-coated membranes showed the highest amount of initial protein adsorption via the micro bicinchoninic acid (micro-BCA) assay; all of the TiO2-coated membranes showed slightly higher protein adsorption than the uncoated control materials. Cell viability, proliferation, and inflammatory responses on the TiO2-coated AAO membranes showed no adverse outcomes. For all of the tested surfaces, normal increases in proliferation (DNA content) of L929 fibroblasts were observed over from 4 hours to 72 hours. No increases in TNF-alpha production were seen in RAW 264.7 macrophages grown on TiO2-coated AAO membranes compared to uncoated AAO membranes and tissue culture polystyrene (TCPS) surfaces. Both uncoated AAO membranes and TiO2-coated AAO membranes showed no significant effects on cell growth and inflammatory responses. The results suggest that TiO2-coated AAO may serve as a reasonable prototype material for the development of nanostructured wound repair devices and orthopedic implants.

The preparation of bilayer lipid membranes (BLMs) on solid surfaces is important for many studies probing various important biological phenomena including the cell barrier properties, ion-channels, biosensing, drug discovery and protein/ligand interactions. In this work we present new membrane platforms based on suspended BLMs on nanoporous anodic aluminium oxide (AAO) membranes. AAO membranes were prepared by electrochemical anodisation of aluminium foil in 0.3 M oxalic acid using a custom-built etching cell and applying voltage of 40 V, at 1^oC. AAO membranes with controlled diameter of pores from 30 - 40 nm (top of membrane) and 60 -70 nm (bottom of membrane) were fabricated. Pore dimensions have been confirmed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). AAO membranes were chemically functionalised with 3-aminopropyltriethoxysilane (APTES). Confirmation of the APTES attachment to the AAO membrane was achieved by means of infrared spectroscopy, X-ray photoelectron spectroscopy and contact angle measurements. The Fourier transform infrared (FTIR) spectra of functionalised membranes show several peaks from 2800 to 3000 cm^-1 which were assigned to symmetric and antisymmetric CH₂ bands. XPS data of the membrane showed a distinct increase in C1s (285 eV), N1s (402 eV) and Si2p (102 eV) peaks after silanisation. The water contact angle of the functionalised membrane was 80^o as compared to 20^o for the untreated membrane. The formation of BLMs comprising dioleoyl-phosphatidylserine (DOPS) on APTESmodified AAO membranes was carried using the vesicle spreading technique. AFM imaging and force spectroscopy was used to characterise the structural and nanomechanical properties of the suspended membrane. This technique also confirmed the stability of bilayers on the nanoporous alumina support for several days. Fabricated suspended BLMs on nanoporous AAO hold promise for the construction of biomimetic membrane architectures with embedded transmembrane proteins.

ABSTRACTIn this work, we report a success in fabricating highly-ordered and densely-packed array of silicon nano-needles that are vertically aligned, straight and long, meeting many of the requirements for biomolecular sensing and integration with silicon electronics. Yet, we show that they can be fabricated with a relatively simple and non-lithographic method.In this approach the array of nano-needles of high uniformity in length and diameter are made out of silicon by reactive ion etching (RIE) through either an anodic aluminum oxide (AAO) membrane or an array of metallic nano-dot caps that are evaporated on a silicon wafer using an AAO membrane as mask. The AAO membrane itself is formed non-lithographically via anodization of pure aluminum foil and contains an array of highly-ordered and highly-uniform nano-pores. By using the AAO membrane either directly as an etching mask or as an evaporation mask to deposit metallic nanodots which in turn serve as an etching mask, deep and high aspect-ratio etching is possible to allow the formation of the nanoneedles in a Si substrate.

A facile and efficient approach for pore-opening detection of anodic aluminum oxide membranes

Structure dependent photoluminescence of nanoporous amorphous anodic aluminium oxide membranes: Role of F+ center defects

In this preliminary study we investigate for the first time the biomedical potential of using porous anodic aluminium oxide (AAO) membranes as a cell substrate for culturing the Cercopithecus aethiops (African green monkey) Kidney (Vero) epithelial cell line. One advantage of using the inorganic AAO membrane is the presence of nanometre scale pore channels that allow the exchange of molecules and nutrients across the membrane. The size of the pore channels can be preselected by adjusting the controlling parameters of a temperature controlled two-step anodization process. The cellular interaction and response of the Vero cell line with an in-house synthesised AAO membrane, a commercially available membrane, and a glass control were assessed by investigating cell adhesion, morphology, and proliferation over a 72 h period. The number of viable cells proliferating over the respective membrane surfaces revealed that the locally produced in-house AAO membrane had cells numbers similar to the glass control. The study revealed evidence of focal adhesion sites over the surface of the nanoporous membranes and the penetration of cellular extensions into the pore structure as well. The outcome of the study has revealed that nanometre scale porous AAO membranes have the potential to become practical cell culture scaffold substrates with the capability to enhance adhesion and proliferation of Vero cells.

Cost-effective technique to fabricate a tubular through-hole anodic aluminum oxide membrane using one-step anodization

In this study, Mg-doped CdS nanostructure was deposited onto anodic aluminum oxide (AAO) membrane substrate using sol-gel spin coating method. The AAO membrane was prepared by a two-step anodization process combined with pore widening process. The morphology, chemical composition, and structure of the spin- coated CdS nanostructure have been studied. The morphology of the fabricated AAO membrane and the deposited Mg-doped CdS nanostructure was investigated using scanning electron microscopy (SEM). The SEM of AAO illustrates a typical hexagonal and smooth nanoporous alumina membrane with interpore distance of ~ 100 nm, the pore diameter of ~ 60 nm. SEM of Mgdoped CdS shows porous nanostructured film of CdS nanoparticles. This film well adherents and covers the AAO substrate. The energy dispersive X-ray (EDX) pattern exhibits the signals of Al, O from AAO membrane and Mg, Cd, and S from the deposited CdS. This indicates the high purity of the fabricated membrane and the deposited Mg-doped CdS nanostructure. Using X-ray diffraction (XRD) pattern, Scherrer equation was used to calculate the average crystallite size. Additionally, the texture coefficients and density of dislocations were calculated. The fabricated CdS/AAO was applied to detect glucose of different concentrations. The proposed method has some advantages such as simple technology, low cost of processing, and high throughput. All of these factors facilitate the use of the prepared films in sensing applications.