Abstract
Modifying the diameter of the pores in nanoporous anodic alumina opens new possibilities in the application of this material. In this work, we review the different nanoengineering methods by classifying them into two kinds: in situ and ex situ. Ex situ methods imply the interruption of the anodization process and the addition of intermediate steps, while in situ methods aim at realizing the in-depth pore modulation by continuous changes in the anodization conditions. Ex situ methods permit a greater versatility in the pore geometry, while in situ methods are simpler and adequate for repeated cycles. As an example of ex situ methods, we analyze the effect of changing drastically one of the anodization parameters (anodization voltage, electrolyte composition or concentration). We also introduce in situ methods to obtain distributed Bragg reflectors or rugate filters in nanoporous anodic alumina with cyclic anodization voltage or current. This nanopore engineering permits us to propose new applications in the field of biosensing: using the unique reflectance or photoluminescence properties of the material to obtain photonic barcodes, applying a gold-coated double-layer nanoporous alumina to design a self-referencing protein sensor or giving a proof-of-concept of the refractive index sensing capabilities of nanoporous rugate filters.
Highlights
Nanoporous anodic alumina (NAA) is a material of great interest in nanotechnology, because of its great diversity of applications, as well as its cost-effective and up-scalable production techniques [1,2,3]
The production of NAA has been a known process in industry for long, but it is the discovery of the self-ordering regime of the pores by Masuda and Fukuda [20] that supposed a breakthrough in the nanotechnological applications of this material
It is necessary to perform a pre-anodization step at mild anodization conditions for a short time to obtain a protective oxide layer and increase the voltage gradually until a hard anodization voltage is reached. This protective oxide layer can be an inconvenience in the further application of the NAA, and an alternative method of hard anodization without the formation of the protective layer has been proposed [30] in which the anodization is performed in two steps: a first sacrificial step at hard anodization conditions to obtain adequately self-ordered concavities on the aluminum surface and a second step at which a hard anodization voltage is applied from the very start, but at a very low temperature, low acid concentration and high stirring rate for faster heat diffusion
Summary
Nanoporous anodic alumina (NAA) is a material of great interest in nanotechnology, because of its great diversity of applications, as well as its cost-effective and up-scalable production techniques [1,2,3]. This protective oxide layer can be an inconvenience in the further application of the NAA, and an alternative method of hard anodization without the formation of the protective layer has been proposed [30] in which the anodization is performed in two steps: a first sacrificial step at hard anodization conditions to obtain adequately self-ordered concavities on the aluminum surface and a second step at which a hard anodization voltage is applied from the very start, but at a very low temperature, low acid concentration and high stirring rate for faster heat diffusion This way of beginning the second step promotes the correct nucleation of the pores without the need of a protective layer.
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