Nanoporous Anodic Alumina Films Research Articles

Facile one-step nanoporous alumina for modifying the surface optical properties of ITO glass

Structure color is a useful technique with good brightness, variuos color, and friendly environmental properties, compared with the dyes and pigments colors. However, the structure colors are based on the micro- or nano structures, which is still a challenge for fabricating the desired colors. Anodic aluminum oxide (AAO) is a common nanomaterial for fabricating the structure colors because of the properties of hardness, stability, and the self-assembly. However, current techniques mostly manufactured the structure color on the aluminum foils or the alloy, which limited these techniques from the glass-based applications. Additionally, current technologies usually require a time-consuming fabrication process lasting several tens of hours. In this study, a facile method is proposed for producing structure colors on transparent ITO glass through AAO coating with a short fabrication time. The vibrant structure colors result from constructive interference of visible light due to the nanoporous AAO film on the ITO glass substrate. Furthermore, a correlation between the anodization time of AAO and color is established, enabling the prediction of anodization time to achieve the desired color without resorting to a trial-and-error approach. A spectrum of colors within the visible light range, spanning wavelengths from 420 to 740 nm, is successfully demonstrated on the ITO glass using this uncomplicated one-step anodization process.

Human Papilloma Virus DNA Detection in Clinical Samples Using Fluorogenic Probes Based on Oligonucleotide Gated Nanoporous Anodic Alumina Films.

In this work, fluorogenic probes based on oligonucleotide capped nanoporous anodic alumina films are developed for specific and sensitive detection of human papilloma virus (HPV) DNA. The probe consists of anodic alumina nanoporous films loaded with the fluorophore rhodamine B (RhB) and capped with oligonucleotides bearing specific base sequences complementary to genetic material of different high-risk (hr) HPV types. Synthesis protocol is optimized for scale up production of sensors with high reproducibility. The sensors' surfaces are characterized by scanning electron microscopy (HR-FESEM) and atomic force microscopy (AFM) and their atomic composition is determined by energy dispersive X-ray spectroscopy (EDXS). Oligonucleotide molecules onto nanoporous films block the pores and avoid diffusion of RhB to the liquid phase. Pore opening is produced when specific DNA of HPV is present in the medium, resulting in RhB delivery, that is detected by fluorescence measurements. The sensing assay is optimized for reliable fluorescence signal reading. Nine different sensors are synthesized for specific detection of 14 different hr-HPV types in clinical samples with very high sensitivity (100%) and high selectivity (93-100%), allowing rapid screening of virus infections with very high negative predictive values (100%).

Thin nanoporous anodic alumina film on aluminium for passive radiative cooling

We demonstrate a simple, low-cost, and passive radiative cooler based on a monolithic design consisting of thin nanoporous anodic alumina (NAA) films grown on aluminium sheets. The NAA/Al structure maintains a high broadband reflectivity close to 98 $$\%$$ within the solar spectrum (0.4–2.2 $$\mu $$ m) and simultaneously exhibits a high average emissivity of 88 $$\%$$ within the atmospheric infrared (IR) transmission window of 8–13 $$\mu $$ m with the peak IR emission approaching 99 $$\%$$ at a wavelength of 10 $$\mu $$ m. Optical modelling of the system using optical parameters of the materials confirms that the high solar reflectance arises due to the transparent nature of NAA and high reflectivity of bottom Al, while the large thermal IR emissivity arises from the interference effects of the NAA film and the high absorption of IR light due to phonon resonances in alumina at wavelength larger than 10 $$\mu $$ m. Further, we estimate the average cooling power of NAA/Al to be about 136 W $$\hbox {m}^{-2}$$ at ambient temperature even after including the contribution to heat input from external non-radiative processes. This robust and lightweight NAA/Al can be projected as an excellent alternative to optical solar reflectors used in spacecraft for thermal heat management and rooftop cooling green technologies.

A fluorogenic capped mesoporous aptasensor for gluten detection

Celiac disease is a complex and autoimmune disorder caused by the ingestion of gluten affecting almost 1% of global population. Nowadays an effective treatment does not exist, and the only way to manage the disease is the removal of gluten from the diet. Owing the key role played by gluten, clear and regulated labelling of foodstuff and smart methods for gluten detection are needed to fight frauds on food industry and to avoid the involuntary ingestion of this protein by celiac patients. On that scope, the development of a novel detection system of gluten is here presented. The sensor consists of nanoporous anodic alumina films loaded with a fluorescent dye and capped with an aptamer that recognizes gliadin (gluten’s soluble proteins). In the presence of gliadin, aptamer sequences displace from the surface of anodic alumina resulting in pore opening and dye delivery. The dispositive shows a limit of detection (LOD) of 100 μg kg−1 of gliadin, good selectivity and a detection time of approximately 60 min. Moreover, the sensor is validated in real food samples. This novel probe allows fast gluten detection through a simple signalling process with potential use for food control.

Evaluation of the influence of synthesis conditions on the morphology of nanostructured anodic aluminum oxide coatings on Al 1050

Currently, the use of nanostructured films of anodic aluminum oxide (AAO) arouses great interest for a variety of nanotechnological applications. Previous studies have shown that the most important parameters in the anodic synthesis of nanoporous AAO films are the pretreatment of the substrate, the anodization voltage, and the nature, concentration and temperature of the electrolyte. To advance the development of nanomaterials, which are increasingly specific according to their functionality, in a simple and inexpensive way, it is essential to study the combined effect of the synthesis variables on the morphological properties of the coatings. The objective of this work was to study and predict the combined effect of the concentration and temperature of oxalic acid and the anodization voltage in the ordering, the pore diameter, the interpore distance, the pore density and the thickness of nanostructured films of AAO synthesized at 20, 30, 40 and 60 V and 20, 30 and 40 °C in 0.3 and 0.9 M oxalic acid, from the Al 1050 alloy. For this, surface images were obtained by optical microscopy and scanning electron microscopy, the latter being processed through the Fast Fourier Transform, and non-linear regressions were performed with the data obtained.The results proved which anodic synthesis conditions have the greatest influence on the ordering and each of the morphological parameters of the AAO films. The optimum voltage and temperature of the self-ordering regime for both oxalic acid concentrations tested were 40 V and 20 °C. The combined effect of the synthesis variables allowed obtaining coatings with a pore diameter of 27–150 nm, an interpore distance of 51–160 nm, a pore density of 35 × 108–390 × 108 and a thickness of 10–130 μm.

Label-free detecting oligonucleotide hybridization melting temperature in real-time with a reflectometric interference spectroscopy–based nanosensor system

The accurate determination of melting temperature (Tm) of oligonucleotide hybridization is the primary for various biosensors based on the technique of using surface-tethered nucleic acids as probes. In this paper, a new type of nanosensor based on the reflectometric interference spectroscopy (RIFS) was proposed to determine the Tm in real time. A composite solid nanostructure, i.e., a nanoporous anodic alumina film with a 15 nm top layer of gold sputtered on surface, was employed as the substrate of the RIFS-based nanosensor, and a 30 bp oligonucleotide molecule was bond to the inner wall of pores and hybridized, and then its Tm was detected with raising temperature. The experimental data showed that there were obvious signs of dsDNA molecules denatured and melted into ssDNA molecules at 95 ℃, which was highly consistent with the theoretical analysis. The cost-effective RIFS nanosensor has excellent thermal and physicochemical stabilities, higher determination accuracy, and great potential in the field of biomolecule research.

Synthesis and Morphological Characterization of Nanoporous Aluminum Oxide Films by Using a Single Anodization Step

Nanoporous anodic aluminum oxide (AAO) films play an important role in nanotechnology due to their easily adjustable morphological properties and wide range of applications. Thus, a deep and systematic characterization of the morphological properties of these coatings is essential. The most important variables in the synthesis of nanoporous AAO films include the anodization voltage, nature, concentration and temperature of the electrolyte, which, combined, result in pores of different sizes and geometries. In the present work, AA 1050 alloy was used to synthesize AAO films, using 0.3 and 0.9 M oxalic acid as the electrolyte and combining different electrolyte temperatures (20, 30 and 40 °C) and anodizing voltages (30, 40 and 60 V), with the aim to correlate the morphological properties of the coatings with the synthesis parameters of a single anodization step. The coatings obtained were characterized by optical microscopy and scanning electron microscopy, determining pore diameter, interpore distance, pore density and coating thickness. The results showed that, by varying the anodic synthesis conditions, it is possible to obtain coatings with a pore diameter between 21 and 97 nm, an interpore distance between 59 and 138 nm, pore density between 2.8 × 1010 and 5.4 × 109 pores/cm2 and thicknesses between 15 and 145 µm. In this way, the right combination of synthesis variables allows synthesizing AAO coatings with morphological characteristics best suited to each particular application.

Unveiling the origin of photoluminescence in nanoporous anodic alumina (NAA) obtained by constant current regime

Photoluminescence of Nanoporous Anodic Alumina (NAA) films is characterized by an intense and broad band in the blue region when excited in UV, and its origin and mechanisms are still debatable. In this work, the NAA films were prepared by galvanostatic anodization of electropolished Al foils (99.99%) by applying 5 mA/cm2 for 90 min at 20 °C, in three different electrolytes: oxalic acid, phosphoric acid, and an equimolar mixture of oxalic and phosphoric acids. After anodization, the Al substrate was removed from NAA porous layer in CuCl2 + HCl solution. The NAA membranes were characterized by Scanning Electron Microscopy (SEM) and Rutherford Backscattering Spectroscopy (RBS). The fluorescence measurements were performed in a spectrofluorimeter at λ = 320 nm as excited wavelength. SEM images showed the changes in morphology as being a function of electrolyte composition. The NAA membrane prepared in oxalic acid presented the smallest pore diameter and those prepared in phosphoric acid presented more defects and branched pores. The presence of impurities inside the porous layer structure was investigated by Rutherford Backscattering Analysis using different conditions (default RBS, C resonance using 1.7 MeV protons and H by Forward Recoil Spectrometry) to evaluate their concentration and elemental depth profile. The relative quantities of Al, O, P and C were evaluated. A model of an inner and an outer layer of NAA oxide with different Al/O proportions is proposed based on the results. Incorporation of oxalate ions was not observed in the oxide film galvanostatically prepared in oxalic acid. This is usually attributed to be the origin of the PL emission in NAA films produced in oxalate media. The results showed that the photoluminescence (PL) emission of NAA might be associated with the presence of recombination centers from oxygen defects inside the oxide structure.

Metallized nanoporous anodic alumina films and their applications

The optimal conditions of the aluminum electrochemical anodization and electrochemical and chemical metallization were determined. Metallized nanoporous anodic alumina (NAA) films with ultrahigh/average/low density of pores were obtained using the optimal conditions. The physical and chemical properties of the obtained NAA films were studied by high-resolution scanning electron microscopy and the reflective interference spectra in a wavelength range of 235–735 nm. Possible applications of the obtained NAA films are in micro/nanoscale lasers with indirect electrical pumping by laser diodes; optical interferometric chemical nanosensors; and the selective interference coloration and protection of the metal surface.

IR Scattering by Optically Inhomogeneous Nanoporous Anodic Alumina Films

This paper presents results of thermographic studies of nanoporous alumina films having different geometric parameters of their porous layer (thickness and average pore diameter), which were exposed to thermal radiation. The films have been shown to shield thermal radiation. The present results suggest that nanoporous alumina membranes can be used as heat shields for reducing the thermal contrast of an object and the surrounding background in the IR spectral region.

Image Formation by Ink-Jet Printing in Micro- and Nanoporous Anodic Alumina Film with Capsulating

Image Formation by Ink-Jet Printing in Micro- and Nanoporous Anodic Alumina Film with Capsulating

Investigation of roughness and specular quality of commercial aluminum (6061 alloy) for fabrication of nanoporous anodic alumina films

Nanoporous anodic alumina (NAA), obtained via aluminum anodization, have been used as template for fabrication of nanostructure materials and photonic devices. In order to promote the formation of regular porous and to control the desired properties, high-purity aluminum and pretreatment of Al surfaces are mandatory. This procedure consists in electrochemically removing the external irregular layer of the metal promoting a flat, bright and smooth metallic surface. In this study, the effect of electropolishing parameters on commercial aluminum samples (6061 alloy, 97%) was investigated. This procedure was performed at constant potential in a mixture of perchloric acid and ethyl alcohol solution in different experimental conditions. A factorial design was used to investigate the effect of applied potential, electropolishing time and electrolyte temperature on roughness and specularity of Al substrates. These samples were characterized by Atomic Force Microscopy (AFM) and Diffuse Reflectance Spectroscopy (DRS). All variables presented significant effects on roughness of Al substrates, although electropolishing time presented the most important effect on specularity of these samples. After galvanostatic anodization in oxalic acid solution, the results showed the influence of Al surface roughness on morphology of NAA films. Since all Al samples were anodized under same conditions, the differences observed in micrographs of NAA films were related to roughness of Al substrates, which was affected by electropolishing parameters, mainly the applied potential and electropolishing time.

Nanoporous Anodic Aluminum Oxide (AAO) Thin Film Fabrication with Low-Grade Aluminium

Anodic aluminum oxide (AAO) is well known for its nanoscopic structures and its applications in microfluidics, sensors and nanoelectronics. The pore density, the pore diameter, and the interpore distance of an AAO substrate can be controlled by varying anodization process conditions. In this research, the self-organized two-step anodization is carried out with a low-grade (Al6061) aluminium substrate using a 40V voltage at the temperature of 2 to 5 °C. Three experiments are done with the anodization time of 24 hours, 48 hours and 72 hours. The structural features of AAO are characterized by a field emission electron microscope (FE-SEM). The data from FE-SEM show that the average pore diameter increases with the anodization time, and that the Al6061 aluminium substrate can be used to fabricate a nanoporous AAO film with an average pore diameter smaller than 17 nanometers.

Self-Ordering Regimes of Porous Anodic Alumina Layers Formed in Highly Diluted Sulfuric Acid Electrolytes

Nanoporous anodic alumina films with long-range hexagonal order have been obtained from a series of highly diluted sulfuric acid electrolytes. A simple linear relationship was established between the self-ordering voltages and acid concentrations (28, 29, and 30 V for 0.2, 0.1, and 0.05 M electrolytes, respectively). Besides establishing new self-ordering regimes, our experimental work sheds new light on some fundamental principles of honeycomb anodic alumina formation. It suggests that the spontaneous self-organization of a stable nanoscale structure originates from the electrohydrodynamic (EHD) convection rather than from Marangoni-type instability at the anode surface. Theoretical analysis displays a decreasing exponential functional relationship between electrolyte concentration and the critical values of the earlier found electrochemical analogue of Rayleigh number, which can be used for prediction of hexagonal cell pattern in currently unexplored anodizing electrolytes.

Effect of continuous magnetic field on the growth mechanism of nanoporous anodic alumina films on different substrates

The effects induced by an external homogeneous magnetic field on the oxide film growth on aluminum in aqueous solutions of oxalic and sulfuric acid and on surface morphology of the alumina films were studied. Aluminum films of 100 nm thickness were prepared by thermal evaporation on SiO2/Si and glass-ceramic substrates. The pore diameter for oxalic acid alumina films on the SiO2/Si substrate decreased by 0.8 nm, the interpore distance by 5.9 nm, and cell diameter by 6.9 nm if a magnetic field of 0.5 T was applied. When aluminum was anodized in sulfuric acid on the same substrate, the significant changes in parameters of porous structure of alumina, which were similar to the ones in oxalic acid, are firstly observed in stronger magnetic fields (of 0.7 T). On the basis of data obtained in this study and of previous investigation on the negative space charge and thermally activated defects in anodic alumina, we concluded that the intensity of the magnetic field is associated with energy of electron traps and that the changes of cell diameter characterize the trap concentration. The energy of electron traps in oxalic acid alumina films was proved to be smaller than the one in films formed in sulfuric acid, but the concentration of traps was of the same order of magnitude. When the substrate was replaced with the glass-ceramic one, the pore diameter in oxalic acid alumina films increased to ca. 17.6 nm.

Enhanced Reflective Interference Spectra of Nanoporous Anodic Alumina Films by Double Electrochemical Deposition of Chemical Metal Nanoparticles

The effect of selective color reflection enhancement from a double metalized nanoporous anodic alumina (NAA) film surface under proper conditions of electrochemical and chemical metal (Cu—Ag, Cu—Au) deposition has been observed. Selective coloration of the NAA films with high index of the reflection and wide range of the color tones was achieved. The additional chemical deposition of the noble metal leads to the enhancement of the selective reflection ability and the interference contrast of the reflected light. The distribution of the chemically deposited silver nanoparticles on the top surface of electrochemically copper metalized pores of anodized aluminum has been shown by the electron microscopy method. The optical reflection spectra at different angles (10°-85°) of metalized NAA film have been measured, and the effective index of refraction (n ≈ 1.6) and film thickness have been calculated. The effect of the reflected light interference contrast enhancement has been explained as being a result of the more effective separation of the two reflecting surfaces air/Al2O3 in the structure of the film similar to a Fabry-Perot interferometer.

The effect of anodization temperature on optical properties of nanoporous anodic alumina (NAA) films

Nanoporous anodic alumina (NAA), produced by aluminum anodization presents interesting optical effects in luminescence property exhibiting waveguides or Fabry–Pérot interference. This phenomenon, which is used as transducer response in optical sensors, depends on morphological characteristics of the NAA films, such as pore diameter, inter-pore distance and film thickness. In this work, the effect of electrolyte temperature on NAA morphology was investigated and, consequently, how this effect shaped the interference property. For this, all anodizing experimental conditions were kept constant while the electrolyte temperature was changed (5, 10, 15, 20 and 25°C). It was observed an increase in the minimum and stationary current density values during anodization as the electrolyte temperature increased, which were correlated to enlargement of porous oxide layer, since pore diameter, inter-pores distance and porosity remained constant for all studied temperatures. The photoluminescence (PL) spectra showed an increase in the number of fringes and an amplitude decrease with the increase of electrolyte temperature. NAA film thickness was evaluated using optical and morphological parameters and compared to values in the literature. A brief discussion of the results regarding the ratio of film thickness and pore diameter (L/dp) was performed to validate the NAA films with better waveguides property to be used as a platform for optical sensing applications.

Nanoporous anodic aluminum oxide films for UV/vis detection of noble and non-noble metals

In this study, a simple, rapid and inexpensive approach for the screening of heavy metals with photometric reagents was developed based on porous, anodic aluminium oxide (AAO) films, with detection limits of 0.45 mg L−1 (Co2+), 0.25 mg L−1 (Pb2+) and 0.59 mg L−1 (Ni2+).

Facile Fabrication of Ordered Multi-Tiered Hierarchical Porous Alumina Nanostructures with Multiple and Fractional Ratios of Pore Interval toward Multifunctional Nanomaterials

Conventional nanoporous anodic alumina films are composed of one-tiered pore arrays that can only render to one-dimensional (1D) nano-structures if they are used as nano-templates. Hence, three-dimensional (3D) nanoporous alumina films are desirable to dramatically increase the variety of nanostructures by template-fabrication for various potentially enhanced performances. Herein we report a facile approach to fabricate a variety of ordered multi-tiered 3D porous alumina nanostructures with multiple integer (m = 2, 3, 4, 5, 7, 9, and 10) and fractional ratios (n = 1/2 and 1/3) of the pore interval and pore diameter among different tiers on ITO/glass and Al sheets. The multi-tiered nanostructures and the pore interval ratios of the neighboring tiers can be accurately and independently controlled by a tailored multiple stepwise anodization, successively in appropriate acidic electrolytes of sulfuric, oxalic, and/or phosphoric solutions at designed voltages of 15–180 V, corresponding to the multiple or fractional ratios of pore interspacing. Moreover, the multi-tiered hierarchical nanoporous alumina films on ITO/glass substrates exhibited a lower transmittance compared with that of the single-tiered films with ordered cylindrical pores, which can be attributed to the diffusive reflection from the tier interface with different pore intervals.

Variation of the refractive index by means of sulfate anion incorporation into nanoporous anodic aluminum oxide films

The variation in the refractive index of nanoporous anodic aluminum oxide (AAO) films regarding the sulfur anion incorporation is studied in this work. For this purpose, different samples are grown under potentiostatic conditions at different voltages and concentrations of sulfuric acid in the electrolyte. The samples are analyzed by Rutherford backscattering spectroscopy and infrared spectrometry, confirming the presence of sulfate anions and water embedded into the nanoporous AAO films. The incorporation of sulfate ions into the alumina matrix varies from 6.3 up to 11.7% regarding aluminum content. We have studied Fabry–Pérot optical interferences by shining incident monochromatic light in specular reflectance conditions. The reflected monochromatic light waves interact in internal reflectivity generating constructive and destructive interferences known as Fabry–Pérot optical interferences. An iterative method based on the equation for constructive interferences conditions in thin films and Snell equations is developed in order to calculate the refractive index of the nanoporous AAO films as a function of the wavelength. The calculated refractive indices increase when the sulfur content of the nanoporous AAO films decreases. The variation of the calculated refractive index is 0.08 and remains constant in the wavelength range 400–1200 nm.