Nanoporous Alumina Research Articles

Flow through Alumina Nanopores Bearing Responsive Polymer Brushes

Flow through Alumina Nanopores Bearing Responsive Polymer Brushes

Non-equilibrium Effects of Polymer Dynamics under Nanometer Confinement: Effects of Architecture and Molar Mass.

The non-equilibrium dynamics of linear and star-shaped cis-1,4 polyisoprenes confined within nanoporous alumina is explored as a function of pore size, d, molar mass, and functionality (f = 2, 6, and 64). Two thermal protocols are tested: one resembling a quasi-static process (I) and another involving fast cooling followed by annealing (II). Although both protocols give identical equilibrium times, it is through protocol I that it is easier to extract the equilibrium times, teq, by the linear relationships of the characteristic peak frequencies with time and rate, respectively, as log(fmax) = C1 - k log(t) and log(fmax) = C2 + λ log(β). Both thermal protocols establish the existence of a critical temperature (at Tc, where k → 0 and λ → 0) below which non-equilibrium effects set-in. The critical temperature depends on the degree of confinement, 2Rg/d, and on molecular architecture. Strikingly, establishing equilibrium dynamics at all temperatures above the bulk, Tg, requires 2Rg/d ∼ 0.02, i.e., pore diameters that are much larger than the chain dimensions. This reflects non-equilibrium configurations of the adsorbed layer that extent away from the pore walls. The equilibrium times depend strongly on temperature, pore size, and functionality. In general, star-shaped polymers require longer times to reach equilibrium because of the higher tendency for adsorption. Both thermal protocols produced an increasing dielectric strength for the segmental and chain modes. The increase was beyond any densification, suggesting enhanced orientation correlations of subchain dipoles.

Dynamic compaction of aluminum with nanopores of varied shape: MD simulations and machine-learning-based approximation of deformation behavior

We compare two machine-learning-based approaches, artificial neural network (ANN) and micromechanical model with automatic Bayesian identification of the model parameters, in application to mimicking the deformation behavior of nanoporous aluminum extracted from molecular dynamics (MD) simulations. Reference data are generated by means of MD simulation of both hydrostatic and uniaxial deformation with compression of representative volume elements of aluminum single crystal with nanopores of spherical, cubic and cylindrical shapes at the temperatures of 300, 500, 700 and 900 K. Several typical sizes of the nanopores are considered: The smallest one corresponds to the initial porosity less than 1%, while the largest one gives the initial porosity in the range of 30–50%. Plastic collapse of nanopores in all cases occurred by the mechanism of emission of partial Shockley dislocation half-loops from the pore surface. The emission of initial loop occurred earlier in the case of cylindrical pore; however, this did not lead to an explosive increase in the number of dislocations in the system at this stage. In general, flat free surfaces of pores are less subjected to the dislocation nucleation than the rounded ones. A new physically-based micromechanical model of the plastic compaction of nanoporous metal is formulated with accounting of different pore shapes and anisotropy of the compaction process. Both tested machine-learning approaches show an adequate approximation of MD data. The developed ANN and parameterized micromechanical model are applied to simulate the propagation of a shock wave in nanoporous aluminum in comparison with direct MD simulations of this process; this comparison shows an adequate description of the shock wave structure by means of both approaches incorporated into continuum mechanics modeling. Thus, the developed machine-learning-based approaches can be applied as constitutive equations of nanoporous aluminum in macroscopic simulations of the dynamic compaction and shock-wave processes in this material.

A Micropowered Chemoresistive Sensor Based on a Thin Alumina Nanoporous Membrane and SnxBikMoyOz Nanocomposite.

This work presents and discusses the design of an efficient gas sensor, as well as the technological process of its fabrication. The optimal dimensions of the different sensor elements including their deformation were determined considering the geometric modeling and the calculated moduli of the elasticity and thermal conductivity coefficients. Multicomponent SnxBikMoyOz thin films were prepared by ionic layering on an anodic alumina membrane and were used as gas-sensitive layers in the sensor design. The resistance of the SnxBikMoyOz nanostructured film at temperatures up to 150 °C exceeded 106 Ohm but decreased to 104 Ohm at 550 °C in air. The sensitivity of the SnxBikMoyOz composite to concentrations of 5 and 40 ppm H2 at 250 °C (10 mW) was determined to be 0.22 and 0.40, respectively.

Experimental observation of Berreman modes in an uniaxial anisotropic nanoporous alumina film on aluminum substrate.

In this Letter, we demonstrate experimentally and verify numerically the excitation of Berreman modes that propagate in a dielectric film of uniaxial anisotropic nanoporous alumina grown on an aluminum substrate. It is an air-dielectric-metal asymmetric polaritonic system with a real part of the effective permittivity having a value near zero. The modes are excited at a wavelength lower than the epsilon-near-zero wavelength region. Minimum reflection is observed for the mid-infrared p-polarized light, while maximum reflection is observed for the s-polarized light. The experimental results are numerically reproduced for both p- and s-polarized light and confirm the excitation of Berreman modes in the system. At the exciting wavelength, the field is confined in the dielectric region near the air-dielectric interface. The reported system is straightforward and can be easily fabricated over a large scale and is helpful in a variety of mid-infrared applications such as thermal management systems, sensors, passive radiative cooling devices, nonlinear applications, and terahertz frequency generation.

Strongly Quantum-Confined Perovskite Nanowire Arrays for Color-Tunable Blue-Light-Emitting Diodes.

Color tunability of perovskite light-emitting diodes (PeLEDs) by mixed halide compositional engineering is one of the primary intriguing characteristics of PeLEDs. However, mixed halide PeLEDs are often susceptible to color red-shifting caused by halide ion segregation. In this work, strongly quantum-confined perovskite nanowires (QPNWs) made of CsPbBr3 are grown in nanoporous anodic alumina templates using a closed space sublimation process. By tuning the pore size with atomic layer deposition, QPNWs with a diameter of 6.6 to 2.8 nm have been successfully obtained, with continuous tunable photoluminescence emission color from green (512 nm) to pure blue (467 nm). To better understand the photophysics of QPNWs, carrier dynamics and the benefit of alumina passivation are studied and discussed in detail. Eventually, PeLEDs using various diameters of CsPbBr3 QPNWs are successfully fabricated with cyan color (492 nm) PeLEDs, achieving a record high 7.1% external quantum efficiency (EQE) for all CsPbBr3-based cyan color PeLEDs. Sky blue (481 nm) and pure blue (467 nm) PeLEDs have also been successfully demonstrated, respectively. The work here demonstrates a different approach to achieve quantum-confined one-dimensional perovskite structures and color-tunable PeLEDs, particularly blue PeLEDs.

Fabrication of FePt nanowires through pulsed electrodeposition into nanoporous alumina templates

Fabrication of FePt nanowires through pulsed electrodeposition into nanoporous alumina templates

Structural Engineering of the Barrier Oxide Layer of Nanoporous Anodic Alumina for Iontronic Sensing.

The hemispherical barrier oxide layer (BOL) closing the bottom tips of hexagonally distributed arrays of cylindrical nanochannels in nanoporous anodic alumina (NAA) membranes is structurally engineered by anodizing aluminum substrates in three distinct acid electrolytes at their corresponding self-ordering anodizing potentials. These nanochannels display a characteristic ionic current rectification (ICR) signal between high and low ionic conduction states, which is determined by the thickness and chemical composition of the BOL and the pH of the ionic electrolyte solution. The rectification efficiency of the ionic current associated with the flow of ions across the anodic BOL increases with its thickness, under optimal pH conditions. The inner surface of the nanopores in NAA membranes was chemically modified with thiol-terminated functional molecules. The resultant NAA-based iontronic system provides a model platform to selectively detect gold metal ions (Au3+) by harnessing dynamic ICR signal shifts as the core sensing principle. The sensitivity of the system is proportional to the thickness of the barrier oxide layer, where NAA membranes produced in phosphoric acid at 195 V with a BOL thickness of 232 ± 6 nm achieve the highest sensitivity and low limit of detection in the sub-picomolar range. This study provides exciting opportunities to engineer NAA structures with tailorable ICR signals for specific applications across iontronic sensing and other nanofluidic disciplines.

Construction of fluorescent anodic aluminum oxide films coloured by 3,4:9,10-perylene tetracarboxylic salt

Perylene derivatives are known for their strong fluorescence and high quantum yields, which makes them an ideal candidate for preparing fluorescent anodic aluminum oxide. However, their application of perylene as a fluorescent dye is hampered by its poor water solubility which resulting the fluorescence quenching in water or on the surface of nanometer pores. Herein, we present a fabrication process for preparation of fluorescent films. In the process, the water-soluble 3,4:9,10-perylene tetracarboxylic (PTC4−) potassium salt was utilized as fluorescent dye and introduced into anodic aluminum oxide nanopores. To avoid fluorescence quenching, the influence factors in-fabrication process (i.e., pretreation, colouring and sealing) were discussed. Additionally, the possible fluorescence quenching mechanism of the PTC4− in the nanopores was also proposed. Our fabrication process for preparation of fluorescent anodic aluminum oxide films was similar to the method applied in mass industrial production and the raw materials are easily available and inexpensive, highlighting the application prospect of our method.

Capillary Performance of Nanoporous Aluminum Braided Wicks Prepared by Anodic Oxidation

With the rapid development of two-phase heat exchangers, the further improvement of the capillary performance of their internal wick faces a great challenge. As an important technology in the surface treatment of aluminum alloys, anodic oxidation has been widely used to develop various functional nanostructures. In this study, nanopores with diameters of 30–40 nm were fabricated on the surface of aluminum fibers through anodic oxidation under an oxalic acid system. Results showed that anodizing increased the specific surface area of the aluminum braid by 163 times, and changed its surface wettability from hydrophobic to superhydrophilic. A significant reduction in the effective capillary radius can substantially increase the capillary force of aluminum braids on the basis of capillary theory. Therefore, the nanoporous aluminum braids can be used as a novel wick in the vapor chamber to improve its capillary performance. Capillary rate-of-rise tests with ethanol and acetone were performed to characterize the capillary of this novel wick structure. Infrared thermal imaging was utilized to monitor the capillary rise of aluminum braided wicks. The capillary force of the anodized wicks was greater than that of a normal wick, and the maximum capillary rise height was 81 mm. The nanoporous aluminum braided wicks prepared by anodizing could be applied in heat transfer.

The Influence of Thermal Treatments on Anchor Effect in NMT Products.

The anchor effect in nanomolding technology (NMT) refers to the effect that polymer nanorods in nanopores on metal surfaces act as anchors to firmly bond the outside polymer components onto the metal surface. In this work, the influences of thermal treatments on the anchor effect are studied at microscopic level from the perspective of interfacial interaction by a model system (poly(n-butyl methacrylate) (PBMA) and alumina nanopore composite). The differential scanning calorimeter and fluorescence results indicate that the formation of a dense polymer layer in close contact with the pore walls after proper thermal treatments is the key for a strong interfacial interaction. Such polymer layers were formed in NMT products composed of PBMA and aluminum after slow cooling or annealing, with an up to eighteen-fold improvement of the interfacial bonding strength. The polymer chains near the nanopore walls eliminate the thermal stress induced by the mismatch of thermal expansion coefficients through relaxation over time and remain in close proximity with the pore walls during the cooling process of nanomolding. The above dynamic behaviors of the polymer chains ensure the formation of stable interfacial interaction, and then lead to the formation of the anchor effect.

Production of nanoporous alumina membranes modified with multi‐walled carbon nanotube, polyacrylamide and polystyrene and use in wastewater treatment

AbstractIn this study, the nanofiltration materials having high selectivity, high filtration and mechanical resistance have been developed for the removal of organic contaminants in wastewater. Al2O3‐NP membranes were modified with multi‐walled carbon nanotube (MWCNT), polyacrylamide‐MWCNT (PA‐MWCNT) and polystyrene‐MWCNT (PS‐MWCNT) to provide better performance in the treatment of wastewater. The evaluation of the separation performance of the prepared membranes was carried out at 25°C using a vacuum filtration unit. Congo red (CR) and methylene blue (MB) dyes were selected as model pollutants. The results showed that the flux value was higher in the unmodified membrane, and the Al2O3‐NP membrane modified with MWCNT exhibited high removal efficiency among the modified membranes.

Glass Transition Dynamics of Poly(phenylmethylsiloxane) Confined within Alumina Nanopores with Different Atomic Layer Deposition (ALD) Coatings

Dielectric spectroscopy (DS) and differential scanning calorimetry (DSC) were employed to study the effect of changes in the surface conditions on the segmental dynamics of poly(phenylmethylsiloxane) confined in alumina nanopores (AAO). The inner surface of the pores was modified by using the atomic layer deposition technique. Coated membranes include 5 nm thick layers of hafnium oxide, titanium oxide, and silicon oxide, exhibiting different wetting properties. Modification of the surface conditions dramatically affects the interfacial interactions between the polymer and confining surface. The interfacial energy calculations indicate a decrease of γSL value from 18.7 mN/m in SiO2-coated to 0.5 mN/m in HfO2-coated nanopores. The results of the dielectric relaxation study demonstrate that the segmental relaxation time of confined PMPS 2.5k depends on the thermal treatment protocol and the hydrophilic/hydrophobic character of the pore walls. From calorimetric measurements, we found that the two glass transitions events are still observed, even in the absence of strong interfacial interactions. Values of both Tgs do not depend strongly on the chemical nature of the surface. In this way, changes in the glass-transition behavior of the tested polymer confined in ALD-coated nanopores cannot be rationalized in terms of the polymer/substrate interfacial energy. Eliminating strongly adhered surfaces does not eliminate the puzzling two-Tgs effect seen in cylindrical nanopores.

Electrical monitoring of infection biomarkers in chronic wounds using nanochannels

Chronic wounds represent an important healthcare challenge in developed countries, being wound infection a serious complication with significant impact on patients’ life conditions. However, there is a lack of methods allowing an early diagnosis of infection and a right decision making for a correct treatment. In this context, we propose a novel methodology for the electrical monitoring of infection biomarkers in chronic wound exudates, using nanoporous alumina membranes. Lysozyme, an enzyme produced by the human immune system indicating wound infection, is selected as a model compound to prove the concept. Peptidoglycan, a component of the bacterial layer and the native substrate of lysozyme, is immobilized on the inner walls of the nanochannels, blocking them both sterically and electrostatically. The steric blocking is dependent on the pore size (20–100 nm) and the peptidoglycan concentration, whereas the electrostatic blocking depends on the pH. The proposed analytical method is based on the electrical monitoring of the steric/electrostatic nanochannels unblocking upon the specific degradation of peptidoglycan by lysozyme, allowing to detect the infection biomarker at 280 ng/mL levels, which are below those expected in wounds. The low protein adsorption rate and thus outstanding filtering properties of the nanoporous alumina membranes allowed us to discriminate wound exudates from patients with both sterile and infected ulcers without any sample pre-treatment usually indispensable in most diagnostic devices for analysis of physiological fluids.Although size and charge effects in nanochannels have been previously approached for biosensing purposes, as far as we know, the use of nanoporous membranes for monitoring enzymatic cleavage processes, leading to analytical systems for the specific detection of the enzymes has not been deeply explored so far. Compared with previously reported methods, our methodology presents the advantages of no need of neither bioreceptors (antibodies or aptamers) nor competitive assays, low matrix effects and quantitative and rapid analysis at the point-of-care, being also of potential application for the determination of other protease biomarkers.

Functional gradient films on aluminum alloy with high absorption efficiencies and damage thresholds for inertial confinement fusion applications

Functional gradient films with high absorption efficiencies and damage thresholds are crucial for eliminating stray light to decrease laser-induced damage in optical systems. Recent advances related to aluminum-based functional films have attracted much attention. However, the fabrication of large area of metal frame need to satisfy low cost, absorb stray light and high stability. A three-layer functional gradient film is designed here. The bottom barrier layer is non-porous alumina, the middle absorption layer has CdSe-quantum dots embedded in anodic aluminum oxide nanopores, and the anodic aluminum film was sealed by a silica layer at the outer side. It could be mass produced at low cost on large areas on aluminum-alloy substrates and have high damage thresholds. The average measured absorbance of light was over 95% over the range 300–750 nm. Moreover, the prepared process has capability to be operated with large scale and high efficiency in an industrial plant. The aluminum-based functional gradient films has its potential to be used in large sized laser facility manufacturing like building inertial-confinement fusion systems to eliminate stay light and improve system cleanliness.

Oligonucleotic Probes and Immunosensors Based on Nanoporous Anodic Alumina for Screening of Diseases

AbstractThis paper is an overview of the recent advances in nanoporous anodic alumina sensing platforms based on the interaction of oligonucleotides and antibodies with bioelements (e.g., virus, bacteria, or biomarkers) that can cause or identify diseases. Nanoporous alumina is the common platform material for all these sensing devices that have an enormous potential for the next‐generation bioanalytical systems. Nanoporous alumina is a cost‐effective material with morphological flexibility that provides high sensitivity to the sensing systems due to its large surface‐to‐volume ratio. Immunosensors transduce the signal produced by the specific antibody–antigen interaction. They are widely used for clinical analysis due to the high selectivity and sensitivity of their immunoreaction, transduced by different methods (electrochemical, optical, piezoelectric, etc). Another important, outstanding and more recently type of biosensors are based on the oligonucleotic functionalization of nanoporous alumina. Oligonucleotides can bind to a wide variety of elements (small organic molecules, proteins, or cells) with high selectivity, specificity, and affinity, even higher to those of antibodies. The basic concepts and mechanisms of their sensing methods for health care biosensing are introduced. Their transduction methods and great potential applications will be evaluated and classified in detail, together with a discussion of possible future trends.

Laccase immobilization on nanoporous aluminum oxide for black liquor treatment

Nanoporous aluminum oxide coating has been successfully synthesized using different anodic synthesis conditions and self-assembled nanoporous with variable pore diameter, thickness and porosity were obtained. The Anodic Aluminum Oxide (AAO) coatings exhibited appropriate morphological characteristics which were employed for the Phebia brevispora BAFC 633 Laccase immobilization. Principal component analysis demonstrated that pH, contact time and enzyme concentration were the main variables which explain the total variation among the Laccase immobilization in the samples. The highest enzymatic activity was reported in the sample synthesized to 0.3 M of electrolyte concentration, 40°C of temperature and 30 V of anodization voltage. Laccase specific activity of 4.1 Ucm−2 could be obtained using an enzymatic concentration of 3.209 UL−1, pH 4.39 and contact time of 1.18 h according to the proposed regression model. Laccase/AAO bionanocatalyst can be easily collected and reserved 95.6% of initial activity after four cycles tests and immobilization yield of around 18.1%. Application of the immobilized Laccase for Black Liquor removal was also investigated. The bionanocatalyst showed better pH stability, 40% higher BL removal efficiency compared to free laccase after 40 min of reaction, and the ability to be collected and used up to twice for BL treatment. The novelty of this work contemplates the development of a bionanocatalyst from a nanoporous coating and immobilized Laccases that can be successfully applied for the treatment of effluents contaminated with black liquor.

Development of Carbon Nanotubes (CNTs) Membrane from Waste Plastic: Towards Waste to Wealth for Water Treatment

Plastic, a non-biodegradable material has always been a concern to the environment and people. This single-use item generates waste to landfills and it persists for centuries once disposed. The urge of transforming such material into a highly valuable product has sought attention from many researchers. This study emphasizes on a nanotechnological approach to synthesize vertically-aligned carbon nanotubes (CNTs) on a substrate template using commercially available plastic bags as carbon precursor. CNTs are grown inside a hexagonally arranged nanoporous anodic alumina membranes (NAAMs). CNTs are liberated by wet chemical etching to dissolve the alumina matrix. The resulting CNTs are used as adsorption media filters for water treatment purpose. The high adsorption affinity towards heavy metals, organic matters and microbes, ability to antifouling and self-cleaning function have made CNTs a better choice over others. This article briefly discusses the catalyst-free synthesis, growth mechanism, characterization and functionalization of CNTs for water treatment application.

Application of cavity Maxwell Garnett theory in SPR based fiber optic sensor with porous alumina structure

The present manuscript describes the theoretical understanding of nanoporous alumina based fiber optic sensing structures. The Cavity Maxwell Garnett theory is used to calculate the dielectric functions of the proposed layer. The performance of the proposed sensing structure is evaluated in terms of its sensitivity towards change in the refractive index of the nearby medium. The sharpness of the resonance has also been calculated as an estimation of the performance parameters. It has been observed that the proposed structure is approximately 13 times more sensitive than the conventional fiber optic sensors. The study has further been extended by replacing the nanolayer of aluminum with the nanolayer of the gold. A comparative study has been provided in terms of the efficiency of the fiber optic probe. The effects of change in pore radius, thickness of the adsorbed medium and shell radius have also been studied.

Lasing from Narrow Bandwidth Light-Emitting One-Dimensional Nanoporous Photonic Crystals

Nanoporous anodic alumina (NAA) is an emerging platform material for photonics and light-based applications. However, demonstrations of narrow bandwidth lasing emissions from this optical material remain limited. Here, we demonstrate that narrow bandwidth NAA-based gradient-index filters (NAA-GIFs) can be optically engineered to achieve high-quality visible lasing. NAA-GIFs fabricated by a modified sinusoidal pulse anodization approach feature a well-resolved, intense, high-quality photonic stopband (PSB). The inner surface of NAA-GIFs is functionalized with rhodamine B (RhoB) fluorophore molecules through micellar solubilization of sodium dodecyl sulfate (SDS) surfactant. Systematic variation of the ratio of SDS and RhoB enables the precise engineering of the light-emitting functional layer to maximize light-driven lasing associated with the slow photon effect at the red edge of NAA-GIFs’ PSB. It is found that the optimal surfactant-to-fluorophore ratio, namely, 20 mM SDS to 0.81 mM RhoB, results in a strong, polarized lasing at ∼612 nm. This lasing was characterized by a remarkably high-quality–gain product of ∼536, a Purcell factor of 2.2, a lasing threshold of ∼0.15 mJ per pulse, and a high-quality polarization ratio of ∼0.7. Our results benefit the advancement of the NAA-based lasing technology for a variety of photonic disciplines such as sensing, tweezing, light harvesting, and photodetection.