Structural tuning of photoluminescence in nanoporous anodic alumina by hard anodization in oxalic and malonic acids

Nanoporous Alumina Support Covered by Imidazole Moiety-Based Ionic Liquids: Optical Characterization and Application

This work analyzes chemical surface and optical characteristics of a commercial nanoporous alumina structure (NPAS) as a result of surface coverage by different imidazolium-based ionic liquids (1-butyl-3-metylimidazolium hexafluorophosphate, 3-methyl-1-octylimidazolium hexafluorophosphate, or 1-ethyl-3-methylimidazolium tetrafluoroborate). Optical characteristics of the IL/NPAS samples were determined by photoluminescence (at different excitation wavelengths (from 300 nm to 400 nm), ellipsometry spectroscopy, and light transmittance/reflectance measurements for a range of wavelengths that provide information on modifications related to both visible and near-infrared regions. Chemical surface characterization of the three IL/NPAS samples was performed by X-ray photoelectron spectroscopy (XPS), which indicates almost total support coverage by the ILs. The IL/NPAS analyzed samples exhibit different photoluminescence behavior, high transparency (<85%), and a reflection maximum at wavelength ~380 nm, with slight differences depending on the IL, while the refractive index values are rather similar to those shown by the ILs. Moreover, the illuminated I−V curves (under standard conditions) of the IL/NPAS samples were also measured for determining the efficiency energy conversion to estimate their possible application as solar cells. On the other hand, a computational quantum mechanical modeling method (DFT) was used to establish the most stable bond between the ILs and the NPAS support.

Photoluminescence properties of anodic aluminum oxide films formed in a mixture of malonic acid and oxalic acid

In this work, porous anodic aluminum oxide (AAO) films were fabricated by anodization in an electrolyte mixture with various concentration ratios of malonic acid and oxalic acid at room temperature. The photoluminescence (PL) properties of the AAO films before and after annealing from 300 °C to 650 °C in air or vacuum conditions were investigated, showing a strong PL band in the range of 300 - 550 nm. We observed a weak PL in the AAO film formed in the malonic acid electrolyte, while the films fabricated using an electrolyte mixture showed strong PL emissions, exhibiting a maximum. The broad PL band was decomposed into three Gaussian sub-bands, where the first two sub-bands could be attributed to the luminescence center oxygen vacancies (F⁺ and F defect centers), while the latter transformed from malonic impurities and oxalic impurities. More interestingly, the redshift of the PL bands occurred with increasing oxalic acid concentration, and the PL wavelength and intensity could be modulated by varying the concentration ratios in the malonic acid and oxalic acid electrolyte mixture.

Surface Modification of Nanoporous Anodic Alumina during Self-Catalytic Atomic Layer Deposition of Silicon Dioxide from (3-Aminopropyl)Triethoxysilane

Changes associated to atomic layer deposition (ALD) of SiO₂ from 3-aminopropyl triethoxysilane (APTES) and O₃, on a nanoporous alumina structure, obtained by two-step electrochemical anodization in oxalic acid electrolyte (Ox sample) are analysed. A reduction of 16% in pore size for the Ox sample, used as support, was determined by SEM analysis after its coverage by a SiO₂ layer (Ox+SiO₂ sample), independently of APTES or O₃ modification (Ox+SiO₂/APTES and Ox+SiO₂/APTES/O₃ samples). Chemical surface modification was determined by X-ray photoelectron spectroscopy (XPS) technique during the different stages of the ALD process, and differences induced at the surface level on the Ox nanoporous alumina substrate seem to affect interfacial effects of both samples when they are in contact with an electrolyte solution according to electrochemical impedance spectroscopy (EIS) measurements, or their refraction index as determined by spectroscopic ellipsometry (SE) technique. However, no substantial differences in properties related to the nanoporous structure of anodic alumina (photoluminescent (PL) character or geometrical parameters) were observed between Ox+SiO₂/APTES and Ox+SiO₂/APTES/O₃ samples.

Recent Advances in Nanoporous Anodic Alumina: Principles, Engineering, and Applications

The development of aluminum anodization technology features many stages. With the story stretching for almost a century, rather straightforward-from current perspective-technology, raised into an iconic nanofabrication technique. The intrinsic properties of alumina porous structures constitute the vast utility in distinct fields. Nanoporous anodic alumina can be a starting point for: Templates, photonic structures, membranes, drug delivery platforms or nanoparticles, and more. Current state of the art would not be possible without decades of consecutive findings, during which, step by step, the technique was more understood. This review aims at providing an update regarding recent discoveries-improvements in the fabrication technology, a deeper understanding of the process, and a practical application of the material-providing a narrative supported with a proper background.

Advances in Optical Biosensors and Sensors Using Nanoporous Anodic Alumina

This review paper focuses on recent progress in optical biosensors using self-ordered nanoporous anodic alumina. We present the fabrication of self-ordered nanoporous anodic alumina, surface functionalization, and optical sensor applications. We show that self-ordered nanoporous anodic alumina has good potential for use in the fabrication of antibody-based (immunosensor), aptamer-based (aptasensor), gene-based (genosensor), peptide-based, and enzyme-based optical biosensors. The fabricated optical biosensors presented high sensitivity and selectivity. In addition, we also showed that the performance of the biosensors and the self-ordered nanoporous anodic alumina can be used for assessing biomolecules, heavy ions, and gas molecules.

Morphology and photoluminescence of nano-porous anodic alumina membranes obtained in oxalic acid at different anodization potentials

Porous Anodic alumina (PAA) with highly ordered pore geometry serves as an ideal template for future development of nanodevices. In the present work, PAA was prepared by varying anodization potential from 10 to 50 V in oxalic acid at 8 °C with a constant electrolyte concentration of 0.3 M. The influence of anodization potential on the structural and photoluminescence (PL) properties of PAA has been studied. The effect of anodization potential on the main pore characteristics of PAA such as pore diameter, interpore distance, pore density, porosity and circularity was studied. It was observed that circularity of PAA was nearly equal to one in the case of anodization potential of 40 V. Highest regularity ratio (RR) obtained for PAA formed in 40 V and it was calculated by WSxM software. It was noticed that with increasing anodization potential there is a slight change in PL spectra of PAA and shows a strong PL peak in blue region.

Stacked Nanoporous Anodic Alumina Gradient-Index Filters with Tunable Multispectral Photonic Stopbands as Sensing Platforms

This study presents the development and optical engineering of stacked nanoporous anodic alumina gradient-index (NAA-GIFs) filters with tunable multispectral photonic stopbands for sensing applications. The structure of these photonic crystals (PC) is formed by stacked layers of NAA produced with sinusoidally modified effective medium. The progressive modification of the sinusoidal period during the anodization process enables the generation and precise tuning of the characteristic photonic stopbands (PSB) (i.e., one per sinusoidal period in the anodization profile) of these PC structures. Four types of NAA-GIFs featuring three distinctive PSBs positioned within the visible spectral region are developed. The sensitivity of the effective medium of these NAA-GIFs is systematically assessed by measuring spectral shifts in the characteristic PSBs upon infiltration of their nanoporous structure with analytical solutions of d-glucose with several concentrations (0.025-1 M). This study provides new insights into the intrinsic relationship between the nanoporous architecture of these PCs and their optical properties, generating opportunities to fabricate advanced optical sensing systems for high-throughput and multiplexed detection of analytes in a single sensing platform.

Nanoporous Anodic Alumina Photonic Crystals for Optical Chemo- and Biosensing: Fundamentals, Advances, and Perspectives

Optical sensors are a class of devices that enable the identification and/or quantification of analyte molecules across multiple fields and disciplines such as environmental protection, medical diagnosis, security, food technology, biotechnology, and animal welfare. Nanoporous photonic crystal (PC) structures provide excellent platforms to develop such systems for a plethora of applications since these engineered materials enable precise and versatile control of light⁻matter interactions at the nanoscale. Nanoporous PCs provide both high sensitivity to monitor in real-time molecular binding events and a nanoporous matrix for selective immobilization of molecules of interest over increased surface areas. Nanoporous anodic alumina (NAA), a nanomaterial long envisaged as a PC, is an outstanding platform material to develop optical sensing systems in combination with multiple photonic technologies. Nanoporous anodic alumina photonic crystals (NAA-PCs) provide a versatile nanoporous structure that can be engineered in a multidimensional fashion to create unique PC sensing platforms such as Fabry⁻Pérot interferometers, distributed Bragg reflectors, gradient-index filters, optical microcavities, and others. The effective medium of NAA-PCs undergoes changes upon interactions with analyte molecules. These changes modify the NAA-PCs' spectral fingerprints, which can be readily quantified to develop different sensing systems. This review introduces the fundamental development of NAA-PCs, compiling the most significant advances in the use of these optical materials for chemo- and biosensing applications, with a final prospective outlook about this exciting and dynamic field.

Nanoporous Anodic Alumina Surface Modification by Electrostatic, Covalent, and Immune Complexation Binding Investigated by Capillary Filling

The fluid imbibition-coupled laser interferometry (FICLI) technique has been applied to detect and quantify surface changes and pore dimension variations in nanoporous anodic alumina (NAA) structures. FICLI is a noninvasive optical technique that permits the determination of the NAA average pore radius with high accuracy. In this work, the technique is applied after each step of different surface modification paths of the NAA pores: (i) electrostatic immobilization of bovine serum albumin (BSA), (ii) covalent attachment of streptavidin via (3-aminipropyl)-triethoxysilane and glutaraldehyde grafting, and (iii) immune complexation. Results show that BSA attachment can be detected as a reduction in estimated radius from FICLI with high accuracy and reproducibility. In the case of the covalent attachment of streptavidin, FICLI is able to recognize a multilayer formation of the silane and the protein. For immune complexation, the technique is able to detect different antibody-antigen bindings and distinguish different dynamics among different immune species.

Advances in Nanoporous Anodic Alumina-Based Biosensors to Detect Biomarkers of Clinical Significance: A Review

There is a strong and growing demand for compact, portable, rapid, and low-cost devices to detect biomarkers of interest in clinical and point-of-care diagnostics. Such devices aid in early diagnosis of diseases without the need to rely on expensive and time-consuming large instruments in dedicated laboratories. Over the last decade, numerous biosensors have been developed for detection of a wide range of clinical biomarkers including proteins, nucleic acids, growth factors, and bacterial enzymes. Various transduction techniques have been reported based on biosensor technology that deliver substantial advances in analytical performance, including sensitivity, reproducibility, selectivity, and speed for monitoring a wide range of human health conditions. Nanoporous anodic alumina (NAA) has been used extensively for biosensing applications due to its inherent optical and electrochemical properties, ease of fabrication, large surface area, tunable properties, and high stability in aqueous environment. This review focuses on NAA-based biosensing systems for detection of clinically significant biomarkers using various detection techniques with the main focus being on electrochemical and optical transduction methods. The review covers an overview of the importance of biosensors for biomarkers detection, general (surface and structural) properties and fabrication of NAA, and NAA-based biomarker sensing systems.

Sustained, Controlled and Stimuli-Responsive Drug Release Systems Based on Nanoporous Anodic Alumina with Layer-by-Layer Polyelectrolyte

Controlled drug delivery systems are an encouraging solution to some drug disadvantages such as reduced solubility, deprived biodistribution, tissue damage, fast breakdown of the drug, cytotoxicity, or side effects. Self-ordered nanoporous anodic alumina is an auspicious material for drug delivery due to its biocompatibility, stability, and controllable pore geometry. Its use in drug delivery applications has been explored in several fields, including therapeutic devices for bone and dental tissue engineering, coronary stent implants, and carriers for transplanted cells. In this work, we have created and analyzed a stimuli-responsive drug delivery system based on layer-by-layer pH-responsive polyelectrolyte and nanoporous anodic alumina. The results demonstrate that it is possible to control the drug release using a polyelectrolyte multilayer coating that will act as a gate.

Formation and Entrapment of Tris(8-hydroxyquinoline)aluminum from 8-Hydroxyquinoline in Anodic Porous Alumina

The formation and entrapment of tris(8-hydroxyquinoline)aluminum (Alq₃) molecules on the surface of anodic porous alumina (APA) immersed in an ethanol solution of 8-hydroxyquinoline (HQ) were investigated by absorption, fluorescence, and Raman spectroscopies. The effects of the selected APA preparation conditions (galvanostatic or potentiostatic anodization method, anodizing current and voltage values, one- or two-step anodizing process, and sulfuric acid electrolyte concentration) on the adsorption and desorption of Alq₃ species were examined. Among the listed parameters, sulfuric acid concentration was the most important factor in determining the Alq₃ adsorption characteristics. The Alq₃ content measured after desorption under galvanostatic conditions was 2.5 times larger than that obtained under potentiostatic ones, regardless of the adsorbed quantities. The obtained results suggest the existence of at least two types of adsorption sites on the APA surface characterized by different magnitudes of the Alq₃ bonding strength. The related fluorescence spectra contained two peaks at wavelengths of 480 and 505 nm, which could be attributed to isolated Alq₃ species inside nanovoids and aggregated Alq₃ clusters in the pores of APA, respectively. The former species were attached to the adsorption sites with higher binding energies, whereas the latter ones were bound to the APA surface more weakly. Similar results were obtained for the Alq₃ species formed from the HQ solution, which quantitatively exceeded the number of the Alq₃ species adsorbed from the Alq₃ solution. Alq₃ molecules were formed in the HQ solution during the reaction of HQ molecules with the Al³⁺ ions in the oxide dissolution zone near the oxide/electrolyte interface through the cracks and the Al³⁺ ions adsorbed on surface of pore and cracks. In addition, it was suggested that HQ molecules could penetrate the nanovoids more easily than Alq₃ species because of their smaller sizes, which resulted in higher magnitudes of the adsorption.

Color filters based on a nanoporous Al-AAO resonator featuring structure tolerant color saturation

Reflection type subtractive tri-color filters, enabling metal-thickness tolerant high color saturation, were proposed and demonstrated capitalizing on a nanoporous metal-dielectric-metal (MDM) resonant structure, which comprises a cavity made of self-assembled nanoporous anodic aluminum oxide (AAO), sandwiched between an Al film of the same nanoporous configuration and a highly reflective aluminum (Al) substrate. For the proposed filter, the output color was easily determined by controlling the resonance wavelength via the thickness of the porous AAO cavity. In particular, the spectral response was deemed to exhibit a near-zero resonant dip, thereby achieving enhanced color saturation, which was stably maintained irrespective of the thickness of the porous Al film, due to its reduced effective refractive index. In order to manufacture the proposed color filters on a large scale, a porous Al film of hexagonal lattice configuration was integrated with an identically porous self-assembled AAO layer, which has been grown on an Al substrate. For the realized tri-color filters for cyan, magenta, and yellow (CMY), having a 15-nm Al film, near-zero reflection dips were observed to be centered at the wavelengths of 436, 500, and 600 nm, respectively. The resulting enhanced color saturation was stably maintained even though the variations were as large as 10 nm in the metal thickness.

In Vitro Biocompatibility of Surface-Modified Porous Alumina Particles for HepG2 Tumor Cells: Toward Early Diagnosis and Targeted Treatment

Porous alumina photoluminescence-inherent particles are produced and proposed for the development of biomarkers detectors and localized treatment of HepG2 cells. Nanoporous alumina particles (NPAPs) are amorphous, consist of hexagonally ordered nanometric pores in an alumina matrix, have high chemical stability in physiological pH, and exhibit a high inherent photoluminescence in the visible spectrum independently of their size, selectable from nanometers to tens of micrometers. The surface of NPAPs is chemically modified using two different functionalization methods, a multistep method with (3-aminopropyl)triethoxysilane (APTES) and glutaraldehyde (GLTA) and a novel simplified-step method with silane-PEG-NHS. Fourier Transform infrared spectroscopy analysis confirmed the proper surface modification of the particles for both functionalization methods. HepG2 cells were cultured during different times with growing concentrations of particles. The analysis of cytotoxicity and cell viability of HepG2 cells confirmed the good biocompatibility of NPAPs in all culture conditions. The results prove the suitability of NPAPs for developing new label-free biomarker detectors and advantageous carriers for localized drug delivery.

Nanoengineered drug-releasing aluminium wire implants: comparative investigation of nanopore geometry, drug release and osteoblast cell adhesion

In this study, drug-releasing aluminium (Al) wire implants featuring nanoporous alumina (NPA) layers produced by different anodization approaches are systematically investigated as potential platforms for localized drug delivery and bone therapy.

Insitu monitored engineering of inverted nanoporous anodic alumina funnels: on the precise generation of 3D optical nanostructures

Herein, we present an innovative approach to generate a novel type of 3D optical nanostructures based on nanoporous anodic alumina (NAA), the so-called inverted nanoporous anodic alumina funnels (INAAFs). This electrochemical approach takes advantage of the differential dissolution rate of NAA with the annealing temperature, which enables the in-depth engineering of nanopores. The structural engineering of these 3D nanostructures is monitored in situ by reflectometric interference spectroscopy through changes in the effective optical thickness of NAA, making it possible to precisely control the formation of INAAFs in real-time. The resulting INAAFs are optical nanostructures featuring a stratified structure of well-defined cylindrical nanopores with increasing pore diameter from top to bottom. As a result of their geometric characteristics and optical properties, INAAFs are envisaged for replicating novel nanostructures and developing optical and photonic devices.

Nanostructural Engineering of Nanoporous Anodic Alumina for Biosensing Applications

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.

Nanoporous Anodic Alumina: A Versatile Platform for Optical Biosensors

Nanoporous anodic alumina (NAA) has become one of the most promising nanomaterials in optical biosensing as a result of its unique physical and chemical properties. Many studies have demonstrated the outstanding capabilities of NAA for developing optical biosensors in combination with different optical techniques. These results reveal that NAA is a promising alternative to other widely explored nanoporous platforms, such as porous silicon. This review is aimed at reporting on the recent advances and current stage of development of NAA-based optical biosensing devices. The different optical detection techniques, principles and concepts are described in detail along with relevant examples of optical biosensing devices using NAA sensing platforms. Furthermore, we summarise the performance of these devices and provide a future perspective on this promising research field.

Tuning nanoporous anodic alumina distributed-Bragg reflectors with the number of anodization cycles and the anodization temperature

The influence of the anodization temperature and of the number of applied voltage cycles on the photonic properties of nanoporous anodic alumina-based distributed-Bragg reflectors obtained by cyclic voltage anodization is analyzed. Furthermore, the possibility of tuning the stop band central wavelength with a pore-widening treatment after anodization and its combined effect with temperature has been studied by means of scanning electron microscopy and spectroscopic transmittance measurements. The spectra for samples measured right after anodization show irregular stop bands, which become better defined with the pore widening process. The results show that with 50 applied voltage cycles, stop bands are obtained and that increasing the number of cycles contributes to enhancing the photonic stop bands (specially for the case of the as-produced samples) but at the expense of increased scattering losses. The anodization temperature is a crucial factor in the tuning of the photonic stop bands, with a linear rate of 42 nm/°C. The pore widening permits further tuning to reach stop bands with central wavelengths as low as 500 nm. Furthermore, the results also show that applying different anodization temperatures does not have a great influence in the pore-widening rate or in the photonic stop band width.