Realisation and advanced engineering of true optical rugate filters based on nanoporous anodic alumina by sinusoidal pulse anodisation

Abstract

This study is the first realisation of true optical rugate filters (RFs) based on nanoporous anodic alumina (NAA) by sinusoidal waves. An innovative and rationally designed sinusoidal pulse anodisation (SPA) approach in galvanostatic mode is used with the aim of engineering the effective medium of NAA in a sinusoidal fashion. A precise control over the different anodisation parameters (i.e. anodisation period, anodisation amplitude, anodisation offset, number of pulses, anodisation temperature and pore widening time) makes it possible to engineer the characteristic reflection peaks and interferometric colours of NAA-RFs, which can be finely tuned across the UV-visible-NIR spectrum. The effect of the aforementioned anodisation parameters on the photonic properties of NAA-RFs (i.e. characteristic reflection peaks and interferometric colours) is systematically assessed in order to establish for the first time a comprehensive rationale towards NAA-RFs with fully controllable photonic properties. The experimental results are correlated with a theoretical model (Looyenga-Landau-Lifshitz - LLL), demonstrating that the effective medium of these photonic nanostructures can be precisely described by the effective medium approximation. NAA-RFs are also demonstrated as chemically selective photonic platforms combined with reflectometric interference spectroscopy (RIfS). The resulting optical sensing system is used to assess the reversible binding affinity between a model drug (i.e. indomethacin) and human serum albumin (HSA) in real-time. Our results demonstrate that this system can be used to determine the overall pharmacokinetic profile of drugs, which is a critical aspect to be considered for the implementation of efficient medical therapies.

Rational engineering of nanoporous anodic alumina optical bandpass filters

“…the thickness of the individual NAA-BPFs forming the structure of bi-NAA-BPFs) results in a slight blue shift of the position of the characteristic transmission bands of NAABPFs. 55 As commented before (vide supra), each NAA-BPF in the structure of bi-NAA-BPFs was produced with an anodisation time of 12 h. It is worthwhile mentioning that the blue shift observed in the 500 s band for the NAA-BPF500-1100 (i.e. from 395 ± 1 nm to 342 ± 1 nm) was less marked than that of NAA-BPF500-700 and NAA-BPF500-900 due to the merger of the first order band of 500 s and the second order band of 1100 s. Table S1 (ESI) summarises the obtained results for all the bands in bi-NAA-BPFs.…”

Realisation and optical engineering of linear variable bandpass filters in nanoporous anodic alumina photonic crystals

“…As reported in our previous studies, NAA-GIFs produced by SPA feature a layered nanoporous structure. 35,39 Fig. 2 compiles representative SEM images of NAA-GIFs produce by SPA under the conditions specified in our study.…”

“…NAA-GIFs were fabricated by sinusoidal pulse anodisation (SPA) under galvanostatic conditions following the process reported elsewhere. 35,39 40,41 The temperature of the electrolyte solution (T an ) was kept at -1⁰C throughout the anodisation process to overcome the dissolution of the oxide film. The SPA process started with an initial step at 1.120 mA cm -2 for 1 h to achieve a homogeneous oxide growth rate.…”

Real-Time Binding Monitoring between Human Blood Proteins and Heavy Metal Ions in Nanoporous Anodic Alumina Photonic Crystals

“…A layer of APTES molecules was then deposited onto the inner surface of NAA-PCs through chemical vapor deposition to provide amine functional groups. 23 These groups were then activated by GTA via the aldehyde functionality of GTA. Blood protein molecules were selectively immobilized onto the GTA-APTES-activated surface of NAA-PCS via N-terminus covalent binding with the aldehyde functionality, where the amine moiety in the N-terminus of blood proteins reacts with the aldehyde group of GTA to form an imine.…”