(Invited) Recent Advances in Structural Engineering of Nanoporous Anodic Alumina and Applications
Authors: Marsal Lluis F. , Acosta Laura Karen , Ferré-Borrull Josep 
Published: December 2020

In the last years structurally engineered nanoporous materials have attracted an increasing interest due to their potential applications in different research fields such as nanophotonic, biotechnology, sensing, catalysis, drug delivery, tissue engineering, batteries, membrane separation, and so on. Among other, nanoporous anodic alumina (NAA) has become an interesting material as a result of their outstanding set of properties, cost-competitive fabrication processes and fully scalable process compatible with conventional micro- and nanofabrication technology.
Nanoporous anodic alumina is obtained by the electrochemical etching of aluminum. Under specific anodization condition presents a self-ordered hexagonal pore distribution of parallel cylindrical nanopores with pore diameters between 5 and 300 nm. Its geometric characteristics such as pore diameter, pore length and separation distance can be tuned by the anodization conditions (voltage, current density, temperature and electrolyte concentration) [1-2]. Their chemical resistance, thermal stability, and intrinsic photoluminescence are some of the outstanding properties of NAA
Recently, different anodization and post-anodization treatments (etching and annealing) have been developed to create new structures and pore geometries such as modulated, funnel-like, serrated-like, tip-like, etc. The application of periodic variations of current or voltage during the anodization is transferred to the material as the periodic variation of the pore diameter and consequently, it is possible design, 2D and 3D structures. In particular, it is very interesting the design photonic structures with tunable stop bands within the UV-VIS-NIR range by applying cycling anodization processes [3-4]. Another interesting example of structural engineering is the fabrication of anodic alumina nanotubes by pulse anodization performed in galvanostatic conditions. The length of nanotubes and inner and outer diameters are tailor-engineered by varying pulse period and current density level [5].
In addition, its effective surface area makes of NAA an interesting platform for sensing and loading and releasing of active agents. For example, the surface of NAA can be easily functionalized to be selective for specific molecules and covered with biodegradable, chemical, or pH responsive agents to trigger and regulate the release. Another approach is to decorate the surface with magnetic or metallic nanoparticles. The surface modification of nanoporous anodic alumina can provide new properties and functionalities that can be advantageously used besides the structural engineering for innovative applications.
In this talk, we will present recent advances in the design and fabrication of nanostructures based on nanoporous anodic alumina. We introduce different electrochemical approaches to modify the pore geometry during or after the fabrication processes. We will show how different nanoporous anodic alumina structures can be designed with particular photonic properties and how engineered porous anodic alumina nanotubes can be chemically modified and decorated with magnetic nanoparticles. Finally, we present different applications of nanoporous anodic alumina for optical sensing, drug delivery and cell culture.
References
[1] A. Santos, L. Vojkuvka, M. Alba, V. S. Balderrama, J. Ferre-Borrull, J. Pallares, L. F. Marsal, Physica Status Solidi (A) Applications and Materials Science, 2012, 209, 2045.
[2] A. Santos, J. Ferre‐Borrull, J. Pallares, L.F. Marsal, Physica Status Solidi (a), 2011, 208, 668.
[3] L.K. Acosta, F Bertó-Roselló, E Xifre-Perez, CS Law, A Santos, J. Ferre-Borull, L.F. Marsal, ACS Applied Materials & Interfaces 2020, 12, 19778.
[4] L.K. Acosta, F. Bertó-Rosello, E. Xifre-Perez, A. Santos, J. Ferre-Borrull, L. F. Marsal, ACS Applied Materials and Interfaces, 2019, 11, 3360.
[5] J.T. Domagalski, E. Xifre-Perez, A. Santos, J. Ferre-Borrull, L.F. Marsal, Microporous and Mesoporous Materials, 2020, 303, art. no. 110264.