Porous Anodic Alumina Surface Research Articles

Boosting of Surface Anion Density on Porous Anodic Alumina Surface—A Key Factor to Enhance Moisture Sensor Operation to Critical Sub-RH Level

Humidity sensing at low RH%, its mechanism, and even lowering further minimum detection limit has no clear answer as of now. Porous anodic alumina (PAA), though an excellent hygroscopic material; still lacks a comprehensive theory on how to lower the minimum detection limit (LOD) to the sub-RH level. In this report, in addition to the pore morphology design, the effect of electrolyte driven anion concentration on PAA surface has been suitably fine-tuned by increasing the attachment of hydroxyl bond density. The organic solvent like glycerol mixed with the anodizing electrolyte has been chosen in proper concentration; leading to enhance the degree of ionic attachment to the PAA surface, hence increase in the sensitivity and the lower detection limit. A lower detection limit of 0.5 RH% is obtained with optimized glycerol concentration, examined through FTIR spectroscopy, which is a direct consequence of increased surface anion concentration that facilitates the adsorption of water vapor molecules on the surface. The developed sensor shows wide detection range 0.5-99 RH%, ultrahigh sensitivity of 275.4 pF/RH%, very fast response- and recovery time of 3 s and 15 s and negligible drift over cycling over 30 times for 0-11.4 RH%. The sensor provides a low-cost alternative solution specifically, to industries having low moisture monitoring requirements e.g., tea and lithium battery manufacturing industry.

Fabrication of Orderly Porous Anodic Alumina Optical Waveguide Sensor via Interface Hydrophilic Treatment for the Detection of Small and Large Molecules

AbstractIn this paper, an orderly porous anodic alumina (PAA) optical waveguide sensor is presented with a simple and low‐cost fabrication method. Both the gold film and PAA film undergo interface hydrophilic treatment. The refractive index (RI) sensitivity and resolution for the first order of transverse magnetic (TM1) mode reached 64.67 deg RIU−1 and 1.73 × 10−6 RIU, respectively. Small and large molecules are used to verify its sensing feasibility. For the detection of small molecule, the glucose molecule is used and its limit of detection (LOD) of 39.03 mg L−1, that is 0.22 × 10−3 m, is achieved. In addition, bovine serum albumin (BSA) is used as a model of large molecule to adsorb onto the surface of PAA film due to the characteristic of BSA molecule. The results show that the adsorption of BSA onto the PAA surface belongs to 1:1 model of Langmuir adsorption and the LOD reached 2.67 × 10−12 m based on the threefold standard deviation theory and Langmuir fitting. Further, in the experiment of fluorescence enhancement, the LOD as low as 770.72 × 10−15 m for BSA/Cy5 is obtained, owing to the high adsorption capacity of PAA nanostructure and field enhancement of the PAA waveguide.

Ultrasensitive and Label-Free Detection of Cell Surface Glycan Using Nanochannel-Ionchannel Hybrid Coupled with Electrochemical Detector.

In this work, asymmetric nanochannel-ionchannel of porous anodic alumina (PAA) coupled with electrochemical detector was used for sensitive and label-free detection of cell surface glycan. The amplified ionic current caused by array nanochannels as well as the ionic current rectification (ICR) caused by asymmetric geometry endows PAA with sensitive ionic current response. Functionalized with the special molecular probe, the constructed nanofluidic device can be used for selective recognition and detection of glycan in a real-time and label-free format. In addition, due to the subnanosize of ionchannels, the probe immobilization and glycan recognition is carried out on the outer surface of PAA, avoiding the blockage of PAA nanochannel by samples, which promises the reproducibility and accuracy of the present method toward bioanalysis. Results show that the glycan concentration ranging from 10 fM to 10 nM can be successfully detected with a detection limit of ∼10 aM, which is substantially lower than most previous works. The designed strategy provides a valuable platform for sensitive and label-free detection of cell surface glycan, which acts as a promising candidate in pathological research and cancer diagnosis.

Depth dependent X-ray diffraction of porous anodic alumina films filled with cubic YAlO3:Tb3+ matrix

The presented paper deals with the measurement methodologies of the structural properties of porous anodic alumina (PAA) films filled with YAlO3:Tb3+ composite using X-ray diffraction, atomic force microscopy and scanning electron microscopy. It shows that the deposited material does not uniformly fill the porous volume of the anodic alumina film and the part of it forms a thick layer on the PAA surface. The aim of this work is to show the differences in the XRD response obtained at different angles of incidence of the excitation beam for the PAA/YAlO3:Tb3+ system. Furthermore, this simple approach enables separation of the signal from both regions on the surface and inside the PAA pores, providing more accurate data interpretation. It reveals that the crystallization of the material on the PAA surface and within the pores is different.

Rhodopsin-Like Ionic Gate Fabricated with Graphene Oxide and Isomeric DNA Switch for Efficient Photocontrol of Ion Transport.

Rhodopsin, composed of opsin and isomeric retinal, acts as the primary photoreceptor by converting light into electric signals. Inspired by rhodopsin, we have fabricated a light-regulated ionic gate on the basis of the design of a graphene oxide (GO)-biomimetic DNA-nanochannel architecture. In this design, photoswitchable azobenzene (Azo)-DNA is introduced to the surface of porous anodic alumina (PAA) membrane. With modulation of the interaction between the GO blocker and Azo-DNA via flexibly regulating trans and cis states of Azo under the irradiation of visible and ultraviolet light, alternatively, the ionic gate is switched between ON and OFF states. This newly constructed ionic gate can possess high efficiency for the control of ion transport because of the high blocking property of GO and the rather tiny path within the barrier layer which are both first employed to fabricate ionic gate. We anticipate that this rhodopsin-like ionic gate may provide a new model and method for the investigation of ion channel, ion function, and ion quantity. In addition, because of the advantages of simple fabrication, good biocompatibility, and universality, this bioinspired system may have potential applications as optical sensors, in photoelectric transformation, and in controllable drug delivery.

Nanostructured ZnS : Cu(Mn) Coatings on the Surface of Porous Anodic Alumina for Optical Applications

The results of X-ray diffraction, electron microscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and X-ray photoelectron spectroscopy studies of nanostructured ZnS : Cu(Mn) coatings obtained by ultrahigh vacuum thermal deposition onto the surface of anodic Al2O3 porous matrices are presented. A comparison with data obtained for films on glass substrates is made. Deposition is carried out during the independent evaporation of ZnS powder, metallic Cu, and MnS powder at various temperatures of the evaporation of impurity-ion sources. For the first time, porous nanostructured ZnS : Cu(Mn) films are shown to be formed on the surface of a porous matrix, the structure of which depends on the parameters of the porous structure of the matrices.

Preparation and Antibiofilm Properties of Zinc Oxide/Porous Anodic Alumina Composite Films

The PAA (porous anodic alumina) films were prepared by two-step anodic oxidation after different times, and then the ZnO/PAA composite films were prepared by sol-gel method on their surface. Meanwhile, the ZnO/PAA composite films were characterized by X-ray diffraction (XRD), thermogravimetric/differential thermal analyzer (TG/DTA), Fourier transform infrared spectrometer (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and water contact angle (CA). The antibiofilm properties of ZnO/PAA composite films on Shewanella putrefaciens were measured simultaneously. The results show that the micromorphologies of PAA and ZnO/PAA composite films are affected by second anodization time. ZnO is a hexagonal wurtzite structure, and ZnO particles with a diameter of 10–30 nm attach to the inner or outer surfaces of PAA. After being modified by Si69, the ZnO films translate from hydrophilia to hydrophobicity. The ZnO/PAA film with the optimal antibiofilm properties is prepared on the PAA surface by two-step anodization for 40 min. The adherence of Shewanella putrefaciens is restrained by its super-hydrophobicity, and the growth of biofilm bacteria is inhibited by its abundant ZnO particles.

Peculiarities of nanocrystalline silicon films growth on porous anodic alumina surface

Peculiarities of nanocrystalline silicon films growth on porous anodic alumina surface

Effect of air-annealing on the morphological, microstructural and optical properties of CdSe NCs grown into porous anodic alumina template

In this work, cadmium selenide nanocrystals (CdSe NCs) were embedded into porous anodic alumina (PAA) template by simple immersion in aqueous solution containing l-cysteine (Cys) functionalized CdSe NCs and water. The prepared samples were heat treated at different temperatures between 150 and 300°C during 30min under air. The effect of thermal treatment under air on the CdSe NCs/PAA has been shown from morphological, microstructural and optical studies. Several characterization techniques such as scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray diffraction (XRD) spectroscopy were used to investigate the surface morphology and microstructural properties of CdSe nanoparticles on the PAA template as function of air-annealing temperature. XRD study shows the transformation phase of deposited CdSe nanoparticles on the PAA template from cubic structure to hexagonal structure at 300°C. The morphology of CdSe nanorods array which are into the PAA surface were exhibited by AFM scan. Optical properties of CdSe NCs into PAA have been studied at different air-annealing temperatures using optical absorption and photoluminescence (PL) spectroscopy. The results showed that the crystal quality was significantly improved with the annealing temperature increase, bringing about an enhancement in PL and a decrease in optical band gap from 2.44 to 2.2eV exhibiting a red shift.

In vitro apatite formation on porous anodic alumina induced by a phosphorylation treatment

In this study, a phosphorylation treatment of porous anodic alumina (PAA) was performed by wet impregnation in phosphoric acid and a subsequent heat treatment. The PAA and phosphorylated PAA specimens were analyzed using a field emission scanning electron microscope, an energy-dispersive X-ray spectrometer, and Fourier transform infrared spectroscopy. The apatite-forming ability of the phosphorylated PAA was evaluated by soaking the specimens in simulated body fluid for 1, 3, and 7 days. The surface microstructures and chemical property changes after soaking in simulated body fluid were again characterized by field emission scanning electron microscope, energy-dispersive X-ray spectrometer, and Fourier transform infrared spectroscopy. Results of this study demonstrated that the functional -PO4 groups introduced onto the PAA surface dramatically promoted the deposition of bone-like apatite on PAA. The results from this study indicated that the phosphorylation treatment of anodic alumina is an effective method for inducing bone-like apatite formation, and this phosphorylated PAA can be a promising candidate to be used as bioactive surface coatings on implant metals and alloys for orthopedic and dental applications.

Tribological Behaviors of Lubricants Modified Nanoporous Anodic Alumina Film

A uniform closely packed hexagonal array of porous anodic alumina (PAA) film with an average pore diameter of ~100 nm and a pore height of ~330 nm was obtained by anodizing in oxalic acid solution. Low surface energy molecules of perfluoropolyether (PFPE) and octadecyltrichlorosilane (OTS) were used to enhance the hydrophobicity of the PAA and aluminum (Al) surface. The tribological performances were investigated in detail. The results showed that the static friction coefficient of PAA was much lower than that of smooth Al, which could be due to the higher hardness of the PAA than that of Al. PAA modified with PFPE was characterized by lower friction coefficient and longer wear life as compared with bare Al, PAA, and PAA modified with OTS. This is caused by the combined effects of flexibility, mobility, and inherent lubricity of PFPE molecules and the nanoporous structure of PAA as a reservoir for lubricants and wear particles.

Inhomogeneous nanostructured honeycomb optical media for enhanced cathodo- and under-x-ray luminescence

Photo-, radio-, and pulse cathodoluminescence spectra from sol-gel derived titania, doped with strontium and terbium, deposited on porous anodic alumina (PAA) films are reported. The morphology and qualitative elemental depth distributions have been examined by transmission electron microscopy, scanning electron microscopy, and radio-frequency glow discharge optical emission spectroscopy. PAA films with pore and cell sizes ranging from 170 to 190 and 240 to 270 nm, respectively, have been generated on aluminum and monocrystalline silicon substrates followed by spin-on sol-gel derived coating with the subsequent thermal treatment. The resultant PAA surface is not coated with a continuous xerogel film; the xerogel is mainly distributed near the pore bases, leaving much of the pore volume unfilled. The xerogel/PAA structures reveal terbium-related luminescence under x-ray excitation and cathodoluminescence. The same xerogels generated on monocrystalline silicon revealed no cathode- or under-x-ray luminescence. Thus, PAA enhances strongly the cathode- and under x-ray luminescence from terbium and strontium-doped titania xerogels confined in the porous matrix. The fabricated structures are considered as a type of low-cost, thin-film convertor of x-rays, and cathode ray irradiation into visible light, with an average cell size of the convertor of about 250 nm.

Synthesis of Silica Nanotubes with Orientation Controlled Mesopores in Porous Membranes via Interfacial Growth

Silica nanotubes with mesopores perforating the wall were synthesized on the channel surface of porous anodic alumina (PAA) or polycarbonate membranes via interfacial growth. Transmission and scanning electron microscopy characterizations indicated that the mesopore orientation can be regulated from perpendicular to the wall to circling the axis when the thickness of the silica nanotube wall increases from about 15 to 40–60 nm, without any modification of the channel surface or external forces, just by optimizing the amount of the tetraethoxysilane confined in the channels of PAA. Mesopores in the silica nanotube wall arrange hexagonally when the diameter of the silica nanotubes is larger than 200 nm.

Nanoporous Waveguide Sensor with Optimized Nanoarchitectures for Highly Sensitive Label-Free Biosensing

Label-free optical biosensors have attracted much attention, and nanoporous metal-oxide membranes with uniform pore structure and diameter are promising candidates for platforms of label-free optical biosensors. However, development of such sensors with high sensitivity still remains challenging. In this paper, we report on the remarkably enhanced sensitivity of a label-free nanoporous optical waveguide (NPWG) sensor composed of a porous anodic alumina (PAA) waveguiding film and an aluminum cladding film. The enhanced sensitivity was achieved by engineering nanostructures and tuning optical properties of the PAA film. Careful tuning of the porosity, pore density, thickness, and refractive index of the PAA film could significantly improve the sensitivity of the NPWG sensor toward adsorption of bovine serum albumin (BSA) onto the PAA surface, and the optimized sensor responded to the adsorption of BSA with an extraordinarily large red shift (>300 nm) of a waveguide mode due to the large adsorption capacity of the PAA film and the inherently high sensitivity of the waveguide mode. The Fresnel calculations suggested that the potential sensitivity of the NPWG sensor was much higher than that of the conventional surface plasmon resonance (SPR) sensors.

Mechanical properties at nanometric scale of alumina layers formed in sulphuric acid anodizing under burning conditions

A mechanical study (hardness, H and Young's modulus, E) at nanometric scale of the distribution of mechanical properties across the surface of porous anodic alumina (PAA) films formed during aluminium anodizing in sulphuric acid under burning conditions is presented. Statistical methods have been employed to extract the mechanical properties of the protruding oxide structures (POS) observed in the burning areas and the results have been compared with those obtained from the standard PAA (S-PAA) of the non-burning areas. The results indicated that H and E of the POS are 3.8 ± 0.3 and 80 ± 4 GPa, respectively, while those of the S-PAA are 6.8 ± 0.5 and 125 ± 8 GPa. Thus, the hardness and Young's modulus of POS are factors of about 1.6–1.8 times lower than those of S-PAA, indicating that POS have lower mechanical properties associated with their different chemical composition and structure.

A novel porous anodic alumina based capacitive sensor towards trace detection of PCBs

To detect polychlorinated biphenyls (PCBs), a novel porous anodic alumina (PAA) based capacitive sensor working at room temperature has been developed. The parallel nanopores of PAA not only provide large surface area for PCB adsorption, but also benefit for the enhancement of capacitive response. By dropping the PCB methanol solution on the surface of PAA, it is convenient to load PCB into the nanopores by solvent evaporation. 3,3′,4,4′-tetrachlorobiphenyl (PCB77) was chosen as a typical sample of PCBs to investigate the sensing properties of the capacitive sensor. The capacitance of the PAA membrane shows remarkable enhancement after PCB77 solution was loaded while no significant change can be seen after the pure solvent was loaded. The capacitive sensor also shows good response to PCB77 even in the presence of the interferent of benzene. The sensing mechanism has been qualitatively discussed based on a parallel plate capacitor model. The detection limit is down to 8 × 10 −8 M towards PCB77. The novel PAA based capacitive sensor exhibits great potential for practical application in trace detection of PCBs.

Surface Modification of an Alumina‐Based Bioceramic for Cement Application

AbstractBiograde zirconia toughened alumina (ZTA) has found wide application in load bearing endoprosthetic implants due to high strength, fracture toughness, and wear resistance. In order to enhance bonding to acrylic bone cement (BC) for implants, fixation modification of ZTA with a thin layer of porous anodic alumina (PAA) was investigated. An Al‐layer of approximately 500 nm was sputtered on the ZTA substrate which subsequently was electrochemically oxidized by anodic polarization in H2C2O4 or H3PO4 solution. PAA layers with a total porosity ranging from 11 to 30%, mean pore spacing of 90–200 nm and pore diameters of 30–110 nm were prepared. Compared to unmodified ZTA/BC interface (≈ 30 MPa), the PAA modified specimens (ZTA/PAA/BC) achieved a significantly higher interface bonding strength (≈ 60 MPa) measured by four point bending on composite beam specimens. While crack propagation in the unmodified ZTA/BC specimen was found to proceed along the interface, fracture analysis on the ZTA/PAA/BC specimens showed a mixed mode fracture with part of the fracture propagation localized along the PAA/BC interface and part through BC. Thus, pore structure controlled mechanical interlocking is expected to offer a high potential for applying PAA surface modification to improve biomaterial to BC bonding.

Self-Ordering of Cell Arrangement in Porous Anodic Alumina

A convincing interpretation to hexagonal prism ordered-arrangement and self-ordering cell in porous anodic alumina (PAA) is absent up to now. Based on the growth model of oxygen bubble mould effect (OBME) for PAA, a satisfactory explanation for the growth process of hexagonal cells is proposed. The columnar pores and hexagonal cells result from the oxide growth embracing oxygen bubbles. The avalanche electron multiplication at critical thickness dc leads to electronic current which gives rise to the evolution of oxygen gas under anion-contaminated alumina (ACA) layer. The holes on the surface are usually irregular whereas the pores under the surface layer (ACA layer) are big and regular. The thickness of the barrier oxide layer remains constant due to continuous releasing of the oxygen bubbles at the critical thickness. The self-ordering of cell arrangement and the ordered morphology are related to the dissolving process of the ACA layer on PAA surface.

Low cost fabrication of diamond nano-tips on porous anodic alumina by hot filamentchemical vapor deposition and the field emission effects

A novel growth method for diamond nano-tips on porous anodic alumina (PAA) byhot filament chemical vapor deposition has been investigated for the first time.Before diamond film deposition could be carried out, nano-diamond particleswere deposited onto the pore surface of PAA by electrophoretic deposition at thenucleation sites. Diamond nano-tips were then successfully formed by the shape ofporous alumina. The results show that a diamond nano-tip with a smaller radiushas superior effects of field emission. Its threshold field and current density are1.2 V µm − 1 and4.0 mA cm − 2, respectively, for the case of a tip radius of about 20–30 nm.

Petal-Like Morphology on the Surface of Porous Anodic Alumina

The formation process of a petal-like morphology on the surface of porous anodic alumina (PAA) is discussed in detail. During the anodizing process, the electronic current is produced within the growing oxide, which results in the oxygen evolution at the pore bottom. The pressure of the oxygen bubbles increases along with the anodizing process, and their high pressure acts as a driving-force of the micro-gas-flow, resulting in the micro-liquid-flow in the pores of PAA. The micro-liquid-flow can flow into each other between a center pore and the nearest neighboring pores. The nanogroove between two pores can be formed due to the dissolving effect during the process of micro-liquid-flow between the two pores. This leads to the formation of the petal-like morphology on the PAA surface. As the micro-liquid-flow leaves off the pore bottom, there a local vacuum is formed. This local vacuum behaves as a driving-force of the micro-liquid-flow, making the electrolyte renovated in the nanopores. The renovated electrolyte can provide enough anions or impurity centers, which are the cause of the generation of the electronic current. The self-organizing for the petal-like morphology on PAA surface is mainly dependent upon the high pressure of the oxygen bubbles and the local vacuum produced at the pore bottom. The present results may help us to understand the nature of the self-organization in the porous anodic oxides.