Anodic Aluminum Oxide
Porous Anodic Aluminum Oxide or AAO is a self-organized material with honeycomb-like structure formed by high density arrays of uniform and parallel pores. The diameter of the pores can be as low as 5 nanometers and as high as several hundred nanometers, and length can be controlled from few tens of nanometers to few hundred micrometers. Porous AAO is formed by electrochemical oxidation (anodization) of aluminum in acid electrolytes in the conditions that balance the growth and the localized dissolution of aluminum oxide to form arrays of nanometer-sized pores. In the absence of such dissolution, dense anodic aluminum oxide films are formed with limited thickness.
Anodizing aluminum has been widely used since early last century for corrosion protection and as decorative coatings. The porous nature of anodic alumina films was discovered in the 1930s  and further elaborated in the 1950s-1970s. Processes for producing anodic aluminum oxide membranes using chromic acid, sulfuric acid, oxalic acid, or phosphoric acid appear in a patent attributed to Alan W. Smith of the Boeing Company in 1974.
The formation of anodic aluminum oxide with highly ordered 2D hexagonal porous structure was demonstrated for the first time in 1995. Further empirical search of anodization parameters shown, that AAO with ordered hexagonal structure can be obtained solely within the narrow regions of processing conditions. Nature of these self-organization windows was explained only in 2017. It was shown, that the formation of the long-range ordered porous structure occurs when all pores grow with equal rate (when anodization rate is limited by kinetics of the barrier layer dissolution at the pore bases or by diffusion of ions in pores).
Starting in the late 1980s, owing to uniform nanostructure, AAO began to attract interest in the area of nanotechnology, in particular as a template for deposition of the uniform arrays of nanowires. Since several key publications on using AAO for bottom-up templated nanofabrication appeared by the mid-1990s, AAO became widely recognized and very popular platform for design and synthesis of high density arrays of nanostructures (nanowires, nanotubes) and functional nanocomposites.
AAO-based nanomaterials have a broad range of applications, from nanoelectronics and magnetic storage media to photonics and energy conversion to nanoporous substrates and nanotags for bioanalysis. The number of AAO-related publications in this area increased exponentially from 1990 to 2005, with over 75% of the papers focused on use of AAO in nanotechnology, and continues to grow rapidly.
The significance of AAO in science and technology is underpinned by the fact that its structure and chemistry could be controllably engineered at the nanoscale over very large areas and in practical formats, enabling development of new materials and products with desired properties and functionality. For e.g. anodized alumina membranes have been used as a platform / transducer for chemical / biological sensors. Protein molecules like thrombin have been detected using AAO membranes.
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