Retraction Note: Porous Aluminum Oxide for Medical Applications Including Targeted Drug Delivery

Anodizing of aluminum and its alloys is widely investigated and used for corrosion protection, electronic devices, and micro-/nanostructure fabrication. Anodizing of aluminum in acidic solutions causes formation of porous aluminum oxide films, which consists of numerous hexagonal cells perpendicular to the aluminum substrate, and each cell has nanoscale pores at its center. Recently, highly ordered porous aluminum oxide has been widely investigated for various novel nanoapplications. In this review article, we introduce the fundamentals of anodic oxide films including barrier and porous oxides. Then, we summarize the electrolyte species used so far for porous oxide fabrication and describe the self-ordering conditions during anodizing in these electrolyte solutions. Fabrication of highly ordered porous oxides through the vertical section can be achieved by a two-step anodizing and nanoimprint technique. Various nanoapplications based on the ordered porous oxide are also introduced.

A new image of porous aluminium oxide

The fabrication of a matrix of highly ordered vertically aligned carbon tubes synthesized using a porous anodic aluminum oxide template is considered. The effect of synthesis regimes on the order and topological characteristics of carbon tubes is investigated. The effect of structural and morphological changes in the porous aluminum oxide which take place during the high-temperature synthesis of carbon tubes on the origin and growth of the latter is discussed.

Spectroscopic ellipsometry and scanning electron microscopy (SEM) experiments are employed to characterize porous aluminum oxide obtained by anodization of thin aluminum films. Rutherford backscattering spectra and x-ray diffraction experiments provide information on the composition and the structure of the samples. Results on our thin film samples with a well-defined geometry show that anodization of aluminum is reproducible and results in a porous aluminum oxide network with randomly distributed, but perfectly aligned cylindrical pores perpendicular to the substrate. The ellipsometry spectra are analyzed using an anisotropic optical model, partly based on the original work by Bruggeman. The model adequately describes the optical response of the anodized film in terms of three physically relevant parameters: the film thickness, the cylinder fraction, and the nanoporosity of the aluminum oxide matrix. Values of the first two quantities, obtained from fitting the spectra, are in perfect agreement with SEM results, when the nanoporosity of the aluminum oxide matrix is taken into account. The validity of our optical model was verified over a large range of cylinder fractions, by widening of the pores through chemical etching in phosphoric acid. While the cylinder fraction increases significantly with etch time and etchant concentration, the nanoporosity remains almost unchanged. Additionally, based on a simple model considering a linear etch rate, the concentration dependence of the etch rate was determined.

A comprehensive review of the current level of research in the development of intelligent hybrid nanosystems based on porous inorganic materials, in particular porous alumina, has been carried out. Porous materials are of great interest for the development of controlled drug delivery platforms due to their high effective surface area and controlled pore size. One of the main determinants of the overall pore drug loading and release profile is pore geometry. Three-dimensional pore structures with complex pore geometries and increasing surface area are promising platform designs for sustained drug release. But there are certain structural and financial difficulties in their manufacture. Porous anodic alumina, which is easily and economically produced by electrochemical anodization, allows complex and reproducible three-dimensional pore geometries to be obtained. The unique physical and chemical properties of porous anodic alumina make this material a promising platform for controlled drug release. Porous anodic alumina has a highly ordered pore distribution, and its well-known electrochemical fabrication methods allow precise control of pore spacing, diameter, length, and pore geometry. The purpose of this work is to analyze the current level of research in the development of intelligent hybrid nanosystems based on porous alumina with different characteristics for use in medicine. The presented works have shown interesting opportunities for the development of intelligent, multifunctional optical devices with a microchip design. The design of implantable biosensors with the ability to monitor biological systems in vivo and in real time is promising for the use of porous anodic alumina immunosensors.

The results of an investigation into the production of porous aluminum oxide, which is a typical self-organized material, by anodic oxidation of aluminum in acid solutions are presented. The functional dependences of the impact of the formation conditions on the structural parameters of porous anodic aluminum oxide, including pore diameter and oxide-cell size, are studied.

Electrophoretic deposition of PTFE particles on porous anodic aluminum oxide film and its tribological properties

We report the successful attempt of electrodeposition of polypyrrole (PPy) on the electrochemically developed porous oxide surface of aluminium alloy. Corrosion protection properties were assessed by potentiodynamic measurements and electrochemical impedance spectroscopy. It is shown that PPy on porous oxide significantly increases the corrosion potential making the surface more noble.

Fabrication of patterned anodic aluminum oxide arrays using a dense layer of barrier aluminum oxide as the anodization mask is described. This fabrication process includes patterning of the aluminum film with a photoresist and brief anodization at a high voltage. The photoresist is then removed and the aluminum film is again anodized at a low voltage to grow porous aluminum oxide. Using this procedure, we are able to fabricate anodic aluminum oxide arrays on silicon wafers consisting of alternating regions of porous aluminum oxide and aluminum metal perpendicular to the silicon substrate.

Kinetics of Anodic Oxidation of Aluminum and Titanium: Formation of Porous Alumina and Titanium Oxide Nanotube Layers

The synthesis of nanocomposite carbon material with an ordered structure in a porous anodic aluminum oxide template is considered. A template with a highly ordered structure provides the formation of an array of aligned carbon tubes with high aspect ratios (higher than 1000) in polycrystalline anodic aluminum oxide. The obtained material is studied using scanning and transmission electron microscopy, Raman scattering spectroscopy, Auger electron spectroscopy, and infrared and X-ray photoelectron spectroscopy.

Porous aluminum oxide is popular for its unique honeycomb nanoscale structure. However, the state of the original aluminum surface plays a huge role in the formation of such a structure. The paper considers options for preliminary surface treatment, in particular, different polishing methods, and their effect on the geometry of the resulting porous layer.

Introduction. Nanostructured porous anodic aluminum oxide is used as a matrix, where it is often necessary to introduce impurities of various metals into its structure. In this case, an effective solution is the electrochemical deposition of metals into the pores of anodic aluminum oxide. However, metals can be deposited on the cathode from aqueous solutions of salts in which the electrode potential is electronegative and less than the potential for hydrogen evolution in solutions. Materials and methods. The anodizing process takes place in two stages: preliminary anodizing and anodizing itself. Before preliminary anodization, experimental samples were coated with a chemically resistant varnish for separation between the electrolyte and air. Then the samples prepared in this way were dried for 4–6 hours, after which the second stage of anodization was carried out. Results. It has been revealed that substrate inhomogeneities can be reduced by slowing down the oxide growth process or by using pore blanks. Using similar methods, a highly ordered structure can be achieved. A more dense arrangement of pores can be achieved through the use of strong acids, forming a barrier layer of thinner thickness with more penetration paths. It is worth noting that when using such acids, the number of inhomogeneities becomes not such an important parameter. Discussion. Films were obtained with greater chemical strength and a thickness of 84-100 microns, a pore diameter of 0.15-0.3 microns. By carrying out additional etching, the pore diameter was increased to 0.6 µm. Etching of PHA can be carried out in an alkali solution in an anhydrous solution of ethyl alcohol. Also, pores can be dissolved by reanodizing in a solution of phosphoric acid. The barrier layer grows to 100 A, removal is carried out with dilute hydrofluoric acid. The resulting PHAs have a fairly porous, homogeneous structure with an average effective pore size of 0.45-0.65 μm. Conclusion. Conditions have been developed for the joint deposition of two metals from aqueous solutions at close values of their electrode potentials, such as iron and cobalt. The conditions for the deposition of copper from copper sulfate into the pores of porous aluminum oxide have been established at 25 – 70 t/l of sulfate and 7÷12% sulfuric acid. Resume. The research results can be useful in obtaining films of porous anodic aluminum oxide, which can be used as substrates for creating solar-electric energy converters based on materials with a perovskite structure. This is especially true for the Republic of North Ossetia-Alania and for the mountainous territories of the North Caucasus. The use of solar energy converters will make it possible to more efficiently electrify populated areas in mountainous areas.

Patterned arrays with alternating regions of the metallic phase and porous aluminum oxide are fabricated by using a dense layer of barrier aluminum oxide as a mask for porous-type anodization of 99.5% Al and 0.5% Cu films. The pore curvature at the interface between the metallic phase and porous aluminum oxide is investigated as a function of anodization voltage. The degree of anisotropy, which is defined as a ratio between vertical and lateral propagation of pores, increases slightly as anodization voltage increases. The pore curvature compromises the mask transfer during anodization due to the lateral pore propagation under the anodization mask. For miniaturization of regions composed of Al-Cu, it is important to establish the width of anodization mask required to preserve the metallic phase between two regions of porous aluminum oxide. Our results show that a wide anodization mask is necessary during porous-type anodization of thick Al-Cu films at a chosen set of anodization conditions (40 V, , 5°C).

A simple and effective method for the synthesis of aluminum oxy-hydroxide nanofiber is demonstrated. In this method, porous anodic oxide film of aluminum (porous anodic alumina) is used to synthesize the nanofibers, where it acts both as a precursor as well as the template. Initially, the porous oxide film is hydrothermally treated for the formation of hydroxide (boehmite or pseudoboehmite) inside the oxide pores, and then the porous oxide is selectively etched from the film, using tri-sodium citrate solution, leaving the hydroxide intact. The method results in highly uniform aluminum oxy-hydroxide nanofibers. The size (diameter) of the nanofiber was controlled by controlling the size of the pores in the anodic oxide film.