1-D nanoporous anodic alumina rugate filters by means of small current variations for real-time sensing applications

Abstract

A rugate filter based on nanoporous anodic alumina was fabricated using an innovative sinusoidal current profile with small current variation. The resulting structure consisted of highly parallel pores with modulations of the pore diameter along the pore axis and with no branching. The effect of the period time and the pore widening post-treatment was studied. From reflectance measurements, it was seen that the position of the reflection band can be tuned by adjusting the period time and the width by pore-widening post-treatments. We tested one of the rugate filters by infiltrating the structure with EtOH and water in order to evaluate its sensing capabilities. This method allows the fabrication of complex in-depth modulated nanoporous anodic alumina structures that open up the possibility of new kinds of alumina-based optical sensing devices.

Optical Properties of Nanoporous Anodic Alumina and Derived Applications

“…Another widespread approach to obtain photonic properties from p-AAO structures is the application of a periodicity to one of the anodization parameters (usually anodization voltage [87], although others as current [63] or temperature [88] have been demonstrated). This periodicity produces a modulation with the depth of the structure porosity.…”

“…However, under the appropriate conditions (such as mild anodization in phosphoric acid electrolytes or hard anodization in oxalic acid electrolytes [58]) the interpore distances can reach values near the wavelength of visible or UV light and give rise to structure-related photonic properties. Furthermore, multilayer porous structures can be achieved by many several methods [59][60][61][62][63]. Photonic properties related to one-dimensional nanostructures such as the distributed Bragg reflectors (DBR) arise when such multilayers have thicknesses of the order of magnitude of the wavelength.…”

Electrochemical methods for template-assisted synthesis of nanostructured materials

“…MA pulses are applied, followed by intervals at 0 V. During the discontinuous anodization period, the temperature of the acid electrolyte is 35°C, so that the anodic alumina is etched along the tube, thus leading to local widening of the alumina nanochannels. Modulation of nanopore diameter can also be achieved by a small sinusoidal variation in the density of the current (0.5-3.5 mA/ cm 2 ) applied during anodization, which induces corrugated alumina pore walls (Macias et al, 2014). …”

Mesoporous alumina as a biomaterial for biomedical applications

“…By adequately controlling the pore morphology in the depth of the PAA using variable anodiza-tion conditions pore diameter variations or controlled pore branching are obtained. Such structures constitute optical structures such as Fabry-Pérot (F-P) cavities [81] or distributed Bragg-reflectors (DBR) [77,78,82].…”

“…The position in the reflectance spectrum of such bands can be used as the transduction mechanism for sensing, without the need of the Fourier transform step. The sensitivity of this strategy has been assessed from different kinds of solution such as glucose [82,83] and [109] or alcohols and alkanes [110]. Table 1 summarizes the optical biosensing methods reported in this review, indicating, where appropriate, the minimum measured concentration, the sensitivity or the limit of detection.…”