Nanoporous Alumina Membranes Research Articles

High Spin Polarization in Co2FeSn Heusler Nanowires for Spintronics

Cylindrical nanowires made of Co2FeSn Heusler alloy with high spin polarization have been synthetized via template-assisted electrochemical deposition in nanoporous anodic alumina membranes. Their ...

Super-fast fabrication of self-ordered nanoporous anodic alumina membranes by ultra-hard anodization

Although the fast fabrication of long-range ordered porous anodic alumina (PAA) membranes has already been realized by hard anodization, achieving super-fast oxide growth rates has been still very challenging. Herein, we report a new generation of the anodization process, called ultra-hard anodization (UHA), in which the mean growth rate of self-ordered PAA reached 2900 µm/h in the first 80 s of the process, resulting in a membrane thickness of 58 µm. By comparison, the same thickness can be obtained using the conventional hard anodization and mild anodization processes in 2400 s and 10 h, respectively. While the current density (J) reaches a high value of 2400 mA/cm2, burning and dielectric breakdown phenomena are avoided by controlling the barrier layer temperature and the diffusion length. In addition, the mechanism of oxide growth is explained on the basis of the high field theory and the diffusion-limited path of ionic species through the nanopores, taking into account the effect of temperature and sprinkling rate of electrolyte on J. By blending 0.3 M oxalic acid with phosphoric or sulfuric acid separately, the UHA process is also performed at target voltages ranging from 85 to 150 V, thereby allowing for a wide range of interpore distances. From a practical point of view, we synthesize high aspect ratio polymer nanowires (up to 320 µm) in the resulting membranes. Our findings open a new approach for the super-fast fabrication of the self-ordered PAA, facilitating the development of nanodevices and their commercial applications.

FePd nanowires modified with cyclodextrin. Characterization and catalytic properties evaluated by a model reduction reaction

FePd and Pd nanowires (NWs), were synthesized by electrodeposition into a nanoporous alumina membrane and superficially functionalized with a modified β-cyclodextrin (βCDMOD14). A complete description is provided, at the molecular level, of the modified surface supported by FT-IR and XPS spectroscopy data. FePd NWs@βCDMOD14 showed excellent catalytic activity in the reduction reaction of 4-nitrophenol with NaBH4, being 10 times higher than NWs without βCDMOD14 and even better than Pd NWs@βCDMOD14.

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.

Synthesis of carbon nanowalls on the surface of nanoporous alumina membranes by RI-PECVD method

This work is devoted to the synthesis of carbon nanowalls on the surface of a nanoporous aluminum oxide membrane by radical-injection plasma enhanced chemical vapor deposition method. Nanoporous alumina oxide membranes with different morphology and thickness, which were obtained by the method of two-stage electrochemical anodization, were used as a substrate. For comparative analysis, carbon nanowalls were also obtained on the surface of a silicon substrate and aluminum foil. The synthesized nanostructures were investigated by using scanning electron microscopy and Raman spectroscopy. The dependence of the morphology and height of carbon nanowalls on the pore size and the thickness of the alumina membrane, respectively, was revealed.

Hexagonal diameter in cadmium sulfide/anodic alumina nanoporous bi-layer membrane by a sol–gel spin coating and their sensing application

Cadmium sulfide/anodic alumina nanoporous bi-layer film was designed for the first time by two-step anodization followed by a sol–gel spin coating method. The chemical composition, morphological, electrical and structural properties of the AA and Cd/AA nanoporous membranes have been examined. The morphological investigation of AA layer demonstrates the development of an exceptionally ordered hexagonal exhibit of nanoporous anodic alumina of pore diameter ~ 55 nm and inter-pore distance ~ 125 nm. Though, CdS/AA demonstrated the deposition of nanoporous CdS layer from agglomerated CdS nanoparticles on the surface of the membrane of AA. The energy-dispersive X-ray spectrum of CdS/AA outlined signs of O, Al, Cd, and S, which shows its high purity. The X-ray diffraction affirmed the development of cubic-phase CdS in the favored (220) orientation with normal ~ 86.2 nm crystallite size. The present voltage and resistance–concentration attributes of the CdS/AA layer as a glucose biosensor are fitted well with second-order polynomial behavior (correlation coefficient = 0.981). The glucose concentration changed from 0 to 400 mg/dl. In light of the upsides of the fabrication approach, this new age of minimal effort biosensors can be utilized as a guarantee building block for nanophotonic and nanoelectronic gadgets.

Porous Alumina Membrane-Based Electrochemical Biosensor for Protein Biomarker Detection in Chronic Wounds.

A label-free electrochemical detection platform for the sensitive and rapid detection of Flightless I (Flii) protein, a biomarker of wound chronicity, has been developed using nanoporous anodic alumina (NAA) membranes modified with Flii antibody recognition sites. The electrochemical detection is based on the nanochannel blockage experienced upon Flii capture by immobilized antibodies within the nanochannels. This capture impedes the diffusion of redox species [[Fe(CN)6]4−/3−] toward a gold electrode attached at the backside of the modified NAA membrane. Partial blockage causes a decrease in the oxidation current of the redox species at the electrode surface which is used as an analytical signal by the reported biosensor. The resulting biosensing system allows detection of Flii at the levels found in wounds. Two types of assays were tested, sandwich and direct, showing <3 and 2 h analysis time, respectively, a significant reduction in time from the nearly 48 h required for the conventional Western blot assay. Slightly higher sensitivity values were observed for the sandwich-based strategy. With faster analysis, lack of matrix effects, robustness, ease of use and cost-effectiveness, the developed sensing platform has the potential to be translated into a point-of-care (POC) device for chronic wound management and as a simple alternative characterization tool in Flii research.

TiN, ZrN, and HfN Nanoparticles on Nanoporous Aluminum Oxide Membranes for Solar-Driven Water Evaporation and Desalination

Water desalination via thermal evaporation using plasmonic nanostructures which harness and convert solar irradiation to provide the requisite heat input is gaining interest as a scalable and susta...

Compact and efficient gas diffusion electrodes based on nanoporous alumina membranes for microfuel cells and gas sensors.

This work describes the fabrication, characterization and evaluation of thin gas diffusion electrodes (GDEs) that are capable of electrocatalyzing the reaction of dissolved gases operating at high diffusion limiting current densities and with fast response times. Nanoporous alumina membranes (NAMs) were used as supports of metal electrocatalysts. NAMs were hydrophobized by silanization and coated on one side with Pt, either over the whole alumina surface by sputtering or just onto the pore openings by local chemical deposition. The Pt-modified NAM-based GDEs were evaluated for hydrogen oxidation. They operated by exposing their coated side to the electrolyte solution and the hydrophobic uncoated side to the flowing gas. Due to the NAM hydrophobicity, flooding of the pores by the electrolyte was diminished, so they were quickly filled by the circulating gas. Simulations of the process on single-pore GDEs showed that the dissolved gas diffused into the solution both radially (ideal pore) and linearly (partially flooded pore), reaching the electrocatalyst film placed right around the pore, which guaranteed a fast mass transport. Hydrogen oxidation operated on these NAM-based GDEs at a steady state with limiting current densities as high as 0.5 A cm-2, which were attained in less than 5 s and were proportional to H2 concentrations over a wide range. Thus, their potential use in microfuel cells and gas sensors was demonstrated.

Magnetization reversal mechanisms in Fe/NiO bilayers grown onto nanoporous alumina membranes and Si wafers

We report on the magnetization reversal measured in two ferromagnetic/antiferromagnetic (F/AF) bilayer series: Fe/NiO/Al2O3 (nanoporous membranes, series N) and Fe/NiO/Si (wafers, series W). The Fe and NiO layers were deposited by pulsed laser ablation and magnetron sputtering, respectively. In both series different oxygen partial pressures were implemented in the Ar plasma during the NiO growth. The morphologies of both series (as imaged by atomic force microscopy) reflect those of their substrates and, particularly, the series N samples exhibit a six-fold columnar growth around each one of the membranes nanopores. The in-plane hysteresis loops measured upon field cooling the samples down to different temperatures in the range from 50 K up to 290 K evidenced i) 50 K, 0% oxygen coercivities that decreased markedly in both series samples with the increase of the Fe layer thickness (particularly the Fe 5 nm, series N sample exhibited a coercivity larger than the Fe magneto-crystalline anisotropy field), ii) a decrease of the coercivity with the increase of the NiO deposition oxygen partial pressure, observed in both series independently of the Fe layer thickness, iii) low temperature coercivities larger in the series N samples than in the series W ones. Our data are analyzed in correlation to the deposits morphology and in terms of the occurrence of either propagation mediated reversal (collective mode linked to spatially averaged interactions at the F/AF interface) or localized switching (defect ruled mechanism taking place in a spatially confined environment). It is concluded that i) the magnetization reversal mechanism active in series W samples corresponds to a weak pinning regime propagation of walls interacting with uncompensated moments at the F/AF interface, ii) in series N samples, the magnetization reversal does not involve propagation, and iii) in the latter series the reversal events are spatially restricted to the dot-like tops of the NiO columns surrounding the membrane pores.

Tunable rectifications in nanofluidic diodes by ion selectivity of charged polystyrene opals for osmotic energy conversion

An ion-selective layer based on self-assembled polystyrene opals is integrated with an alumina nanoporous membrane to form nanofluidic diodes.

Rod-Cell-Mimetic Photochromic Layered Ion Channels with Multiple Switchable States for Controllable Ion Transport.

The controllable ion transport in the photoreceptors of rod cells is essentially important for the light detection and information transduction in visual systems. Herein, inspired by the photochromism-regulated ion transport in rod cells with stacking structure, layered ion channels have been developed with a visual photochromic function induced by the alternate irradiation with visible and UV light. The layered structure is formed by stacking spiropyran-modified montmorillonite 2D nanosheets on the surface of an alumina nanoporous membrane. The visual photochromism resulting from the photoisomerization of spiropyran chromophores reversibly regulates the ion transport through layered ion channels. Furthermore, the cooperation of photochromism and pH value achieves multiple switchable states of layered ion channels for the controllable ion transport mimicking the biological process of the visual cycle. The ion transport properties of these states are explained quantitatively by a theoretical calculation based on the Poisson and Nernst-Plank (PNP) equations.

All Electrodeposited Nanowire Array Based Tandem Junction Photoelectrochemical Devices

Large-scale solar fuel production using semiconductor based photoelectrochemical (PEC) system requires inexpensive, efficient and stable devices that can provide photovoltages sufficient to overcome both the thermodynamic and overpotential losses of the redox reaction. Here we report all electrodeposited multi-junction PEC devices for solar fuel production using serially connected CdSe based and CdTe based Schottky barrier junctions with photovoltages exceeding 1.0 V. These PEC devices are synthesized inside nanoporous alumina membranes with each nanoscale solar conversion units physically and electronically isolated from each other, making the entire device fault tolerant. The alumina membranes also serve to protect the semiconductor from chemical and PEC corrosion.

Carbon Dots-Modified Nanoporous Membrane and Fe3O4@Au Magnet Nanocomposites-Based FRET Assay for Ultrasensitive Histamine Detection.

Histamine can be formed by enzymatic decarbonylation of histidine, which is an important indicator of seafood quality. A rapid and sensitive assay method is necessary for histamine monitoring. A fluorescence resonance energy transfer (FRET) assay system based on a carbon dot (CD)-modified nanoporous alumina membrane and Fe3O4@Au magnet nanocomposites has been developed for histamine detection in mackerel fish. CDs immobilized on nanoporous alumina membranes were used as donors, which provided a fluorescence sensing substrate for histamine detection. Fe3O4@Au magnet nanocomposites can not only act as acceptors, but also concentrate histamine from fish samples to increase detection sensitivity. Histamine was detected by the fluorescence signal changes of CDs capturing histamine by an immune reaction. The fluorescence signals of CDs were quenched by Fe3O4@Au magnet nanocomposites via the FRET mechanism. With an increase of histamine, the fluorescence intensity decreased. By recording fluorescence spectra and calculating intensity change, histamine concentration can be determined with a limit of detection (LOD) of 70 pM. This assay system can be successfully applied for histamine determination in mackerel fish to monitor the fish spoilage process in different storage conditions. It shows the potential applications of CDs-modified nanoporous alumina membranes and Fe3O4@Au magnet nanocomposites-based biosensors in the food safety area.

Influence of nanoholes array geometrical parameters on magnetic properties of Dy–Fe antidot thin films

Nanoscale artificially engineered spintronic materials could be used to enlarge the storage density of magnetic recording media. For this purpose, magnetic nanostructures such as antidot arrays exhibiting high uniaxial magnetic anisotropy are new contestants in the field of ultrahigh density magnetic data storage devices. In this context, we focus on the synthesis of nanostructured magnetic materials consisting of Dy–Fe alloyed antidot thin films, deposited onto the surface of nanoporous alumina membranes served as patterned templates. Noticeable variations of in the in-plane magnetic anisotropy have been observed by modifying the layer thickness at both microscopic and macroscopic scales. The microscopic magnetic properties have been locally studied by Nano-MOKE magnetometry. For thinner antidot samples with 15, 20 and 25 nm in thickness, a tri-axial in-plane magnetic anisotropy has been detected. Meanwhile, for thicker antidot samples (40–60 nm of layer thickness), an in-plane uniaxial magnetic anisotropy has been noted. We attribute these changes in the magnetic anisotropy to the strong correlation between the edge-to-edge distance among adjacent nanoholes, W, and the local magnetic anisotropy of antidot samples. The effective magnetic anisotropy exhibits an unexpected crossover from the in-plane to out-of-plane direction due to the increasing of the effective perpendicular magnetic anisotropy with varying the layer thickness of antidot thin films. Therefore, we detected a critical layer thickness, t = 25 nm for the Dy–Fe alloy antidot arrays, at which the appearance of the perpendicular magnetization is observed. Furthermore, an enhancement in the Curie temperature of the antidot arrays compared to the continuous thin films has been obtained. We attribute these effects to the complex magnetization reversal processes and the high thermal stability of the hexagonal structure of antidot arrays. These findings can be of high interest for the development of novel magnetic sensors and for thermo-magnetic recording patterned media based on template-assisted deposition techniques.

Anomalous in-plane coercivity behaviour in hexagonal arrangements of ferromagnetic antidot thin films

Abstract An anomalous magnetic behaviour has been observed for transition metal (Fe, Co and Ni) based antidot nanostructures by modifying only the lattice geometry of the nanoholes array. A series of ferromagnetic, FM, antidot arrays have been fabricated by depositing them onto nanoporous alumina membranes with different pore diameters, d, varying in the range between 32 ± 2 and 93 ± 1 nm and maintaining fixed the inter-holes distance, Dint = 103 ± 2 nm, and layer thickness, t = 20 nm, but reducing the edge-to-edge separation between adjacent antidots, (W = Dint – d). A noticeable change of the in-plane coercivity dependence with W has been observed with an in-plane critical edge to edge distance, WC//, at which the in-plane coercivity behaviour with W is changed. In addition, for antidot samples with large W the in-plane hysteresis loops show single-step magnetic behaviour. Meanwhile, the INP hysteresis loops with W

Magnetic Nanoparticles Enhance Pore Blockage-Based Electrochemical Detection of a Wound Biomarker.

A novel pore blockage-based electrochemical immunosensor based on the combination of 100 nm-magnetic nanoparticles (MNPs), as signal enhancers, and 200 nm-pore diameter nanoporous anodic alumina (NAA) membranes, as sensing platform, is reported. A peptide conjugate mimicking flightless I (Flii), a wound healing biomarker, was chosen as target analyte. The sensing platform consists of an anti-Flii antibody (Ab1)-modified NAA membrane attached onto a gold electrode. Anti-KLH antibody (Ab2)-modified MNPs (MNP-Ab2) were used to selectively capture the Flii peptide conjugate in solution. Sensing was based on pore blockage of the Ab1-modified NAA membrane caused upon specific binding of the MNP-Ab2-analyte complex. The degree of pore blockage, and thus the concentration of the Flii peptide conjugate in the sample, was measured as a reduction in the oxidation current of a redox species ([Fe(CN)6]4−) added in solution. We demonstrated that pore blockage is drastically enhanced by applying an external magnetic field at the membrane backside to facilitate access of the MNP-Ab2-analyte complex into the pores, and thus ensure its availability to bind to the Ab1-modified NAA membrane. Combining the pore blockage-based electrochemical magnetoimmunosensor with an externally applied magnetic field, a limit of detection (LOD) of 0.5 ng/ml of Flii peptide conjugate was achieved, while sensing in the absence of magnetic field could only attain a LOD of 1.2 μg/ml. The developed sensing strategy is envisaged as a powerful solution for the ultra-sensitive detection of an analyte of interest present in a complex matrix.

Study on Direct Current Electro-Deposition of Copper Nanowires in Anodic Alumina Membrane Pores

Aluminum alloy plays an important role in industrial applications, but has a high friction coefficient and a high wear rate. On the basis of anodic oxidation treatment on an aluminum alloy surface, copper nanowires are deposited into the anodic alumina membrane pores by direct current (DC) electrolytic treatment to prepare a composite alumina membrane, which significantly improves the tribological properties of the aluminum alloy surface. In this process, in order to obtain a highly-ordered nanoporous alumina membrane with a thin enough barrier layer for further processing, after the first anodic oxidation in phosphoric acid (0.3 mol/L), the obtained alumina membrane is modified by anodizing it in a phosphoric acid-ammonium hexafluorosilicate bath, in combination with a step-by-step voltage drop without oxide removal. By this method, the resistance of the modified alumina membrane is also reduced greatly, which facilitates the deposition of copper nanowires in the sulfate bath. It is found that the composite alumina membrane filled with copper nanowires has a low friction coefficient of about 0.25 and effectively improves the friction condition, giving the surface a self-lubricating property.

Alcohol sensor based on gold-coated nanoporous anodic alumina membrane

We report a refractive index sensor consisting of a gold-coated nanoporous anodic alumina membrane on aluminium substrate that can distinguish between different kinds of alcohols such as methanol and ethanol due to their different refractive indices. The nanoporous volume allows the loading of liquids with low surface energy into its nanopores. Upon dipping one end of the membrane into the alcohol, the entire nanoporous surface experiences wetting. The wavelength shift of the Fabry–Perot resonating modes formed between the gold-coated nanoporous alumina surface and aluminium on the other side due to the changed effective refractive index form the basis of the sensor. The sensitivity of the nanosensor to the refractive index of the loaded liquid is sufficient to distinguish between different alcohols such as methanol, ethanol and isopropanol, and to detect about 5–10% of methanol in a methanol–ethanol mixture.

Enhancing gas separation efficiency by surface functionalization of nanoporous membranes

Here we report evidence for substantial changes in the separation efficiency of nanoporous anodic alumina membranes with nanochannel diameters ranging from 10–100 nm modified with octadecylphosphonic acid in the transitional flow regime. Softening of surface by alkyl groups with a surface density ∼2 groups/nm2 leads to a general permeance decrease in 3–500 times, depending strongly on the penetrant gas nature and the channels diameter. The divergence of the permeance, for different gases, increases with the decreasing diameter of the pores. For a surface-functionalized membrane, with 10-nm channel diameters, it results in n-C4H10/CH4 ideal and mixed gas separation factors up to 32.3 and 9.0 respectively at a n-C4H10 permeance up to 3.5 m3/(m2·bar·h). The effect is related to the changes of the ratio of molecule travelling time to residence time in the adsorbed state, as well as a strong influence of surface saturation by the absorbed molecules on the tangential momentum accommodation coefficient, which is supported by the derived model. Synergetic contribution of these two factors allows to enhance the separation factor of permanent and condensable gases strongly beyond the Knudsen limit, while maintaining a high permeance of porous membranes.