Photonic Properties of Structurally-Engineered Nanoporous Anodic Alumina and Application to Biosensing
Authors: Ferré-Borrull Josep , Xifré-Pérez Elisabet , Acosta Laura Karen , Pallares Josep , Marsal Lluis F 
Published: February 2020

Nanoporous anodic alumina (NAA) is a material with growing interest in nanotechnology because of its high versatility and ease of preparation[1]. By electrochemical etching of aluminum in acidic electrolytes and under the adequate conditions of composition, anodization voltage or current and temperature, the NAA pores grow perpendicular to the aluminum surface in a self-arranged matrix. Pore radius and interpore distances can be modulated in the range of a few tens of nanometers and up to a few hundreds. A wide range of applications has been demonstrated in sensing[2], drug delivery[3], cell culture[4] or photovoltaics[5].
Technologies for NAA have evolved to tailor material properties to the desired applications, specifically photonic properties with a view to sensing[6]. These structures are applied to analytes such as glucose, proteins or antibodies, usually accomplished by reflection interference spectroscopy. This technique permits to monitor real-time changes in the optical properties of the NAA as they are infiltrated by solutions in a flow cell.
Photonic properties are obtained by engineering the pore shape. For instance, by combining sequentially anodization steps with wet etching steps, two-layer systems can be obtained that provide self-calibration in sensing[7]. On the other hand, periodic modulation of the applied anodization voltage between two different values leads to periodic in-depth variations of pore morphology, and consequently to the formation of successive layers with different effective refractive index giving rise to distributed-Bragg reflectors (DBRs)[8]. Precise control of the anodization times for each voltage level and temperature permits to obtain high refractive index contrasts between the different layers in the DBR and consequently, wide photonic stop bands at the desired wavelengths in the spectrum.
In this communication we report the more recent results in biosensing with NAA-engineered structures, focusing on NAA-based rugate filters (NAA-RF). These are obtained by the application of a periodic sinusoidal anodization current, composed of a constant offset current together with a sinusoidal component, which produce an in-depth continuous periodic variation in pore diameter. The consequent continuous variation in refractive index results in a 1-D photonic crystal structure with a stop band tunable within the UV-VIS-IR range. Figure 1 shows a SEM cross-section of a NAA-RF, where the pore modulation can be recognized.
It has been shown that different sinusoidal components can be overlapped in the anodization current profile[9]. With this, photonic structures with several stop bands can be produced. Furthermore, such stop bands can be tailored at any position in the spectrum what permits to propose new applications such as information encoding. Nevertheless, this overlap of sinusoidal components has the drawback that the refractive index contrasts that can be achieved have to be shared among the different sinusoidal components, what results in stop bands with low reflectance.
In this work, we study the preparation of NAA-RF based on the successive application of the different sinusoidal components. We call these structures stacked NAA-RF, since the different photonic crystals grow separately in the NAA. Figure 2 shows three selected sections of the applied sinusoidal current and of the measured electrochemical cell voltage. Figure 3 shows the spectrum resulting from such applied anodization current. Three clear stop bands with reflectances as high as 40% can be observed. In the communication we will report our results on the tunability of the stop bands and its sensitivity in biosensing applications.
ACKNOWLEDGMENT
This work was supported by the Spanish MINECO (TEC2015-71324-R), the Catalan authority AGAUR (2014SGR1344), and ICREA under the ICREA Academia Award.
REFRENCES
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Figure 1