Anodic Aluminum Oxide Nanochannels Research Articles

Structural transformation of nanorods in artificial nanochannels: Influence of nanochannel size and mass fraction of homopolymer

In this work, various nanorods (NRs) with different structures by self-assembly of copolymer polystyrene-b-poly (4-vinylpyridine) (PS36·5K-b-P4VP16K, SVP-16) and homopolystyrene (hPS3.35k) in cylindrical anodic aluminum oxide (AAO) templates were obtained. Structural transformation in NRs was investigated by varying the AAO nanochannel size and the mass fraction of hPS3.35k. As the mass fraction of hPS3.35k increases, a typical structure change of NRs can be observed, from a continuous concentric lamellar shape to intermittent irregular blocks and finally to dense spherical structures. The AAO channel size also significantly influences the internal structure of the NRs and the arrangement of the P4VP particles, and the ionic current changes remarkably after the self-assembly of SVP-16/hPS3.35k with different weight fractions of hPS3.35k (φhPS). Furthermore, mesoporous nanorods with controlled structures can be obtained by selective swelling of the micro-regional fraction of BCPs in organic solvents. This work provides valuable insights into the structural transformation of self-assembled polymers in artificial nanochannels, benefiting understanding the underlying mechanism of ion transport.

Biomimetic mesoporous carbon-silica/AAO asymmetric nanochannel array for electrochemical sensing of K+ in rat brain microdialysates and serum

Acquirement of chemical expression in practical brain system is vital to understand the molecular mechanism involved in physiological and pathological processes in brain. Though nanochannels have been demonstrated to be promising platform for electrochemical sensor, it is a great challenge for nanochannels to be employed in practical brain biofluid. In this work, we rationally designed and created the biomimetic asymmetric nanochannels for sensing of K+ through integrating in situ modification of a two-component mesoporous carbon-silica (MCS) thin film with a pore size of ∼3.6 nm at anodic alumina nanochannel array (AAO) with the ∼40 nm pores (denoted as MCS/AAO). Apparent rectification phenomenon in such functionalized nanochannel array was achieved based on diode-like ion transport. Then, 4′-aminobenzeno-18-crown-6 (SP) was selected to be chemically decorated at MCS/AAO as the specific recognition for K+ (SP/MCS/AAO). The developed SP/MCS/AAO exhibited good selectivity towards K+ detection against the coexisting interferences in brain, and possessed a good linear response to K+ concentration in the range of 0.5–10 mM with a detection limit of 0.1 mM. Combined with microdialysis technique, the variation of K+ was successfully determined in rat brain microdialysates and serums. Compared with normal rats, the concentration of K+ was found to be greatly decreased in the cerebral microdialysates and serum of rats with hypertensive model (SHR). This work unveiled a powerful platform for K+, and promised to be extended to design new strategy for detecting other chemical species, in particular non-electroactive species in biofluid related to physiological and pathological events.

Target-triggered cascade signal amplification in nanochannels: An ingenious strategy for ultrasensitive photoelectrochemical DNA bioanalysis

Artificial solid-state nanochannels have aroused intense interests in biosensors and bioelectronics because of their special architectures. Herein, we pioneered an ingenious approach of target-triggered cascade signal amplification in porous anodic aluminum oxide (AAO) nanochannels for ultrasensitive photoelectrochemical (PEC) DNA bioanalysis. In the design, AAO nanochannels were modified initially with capture DNA (cDNA) and then incorporated with a photoelectrode, yielding the desired architecture of highly ordered nanoarrays on top of the signal transducer. For target DNA (tDNA) probing, exonuclease III (Exo-III) mediated target recycling (ETR) was first activated to generate plenty of output DNA (oDNA) fragments. After oDNA and the conjugate of Au-labeled probe DNA (Au-pDNA) were anchored within the nanochannels via DNA hybridization, in-situ synthesis of Ag shells on tethered Au nanoparticles was conducted. The resulting large-sized Au@Ag core-shell nanostructure within the nanochannels would cause conspicuous blocking effect to hinder the transportation of electrons accessing the photoelectrode. Since the signal inhibition was directly related to tDNA concentration, an innovative nanochannels PEC DNA assay was exploited and qualified for ultrasensitive detection. The anti-interference ability of this platform was also emphasized by the split AAO membrane for biological incubation without participation of the photoelectrode. This featured nanochannels PEC strategy with cascade amplification launched a novel detecting platform for trace levels of DNA, and it could spark more inspiration for a follow-up exploration of other smart nanochannels PEC bioassays.

Confined amphipathic ionic-liquid regulated anodic aluminum oxide membranes with adjustable ion selectivity for improved osmotic energy conversion

To attain carbon neutrality and carbon peaking, there is an urgent need to convert the vast amount of blue energy present between seawater and river water into usable electricity. Reverse electrodialysis based on ion-exchange membranes is a promising way to efficiently achieve osmotic energy conversion. Anodic aluminum oxide (AAO) membranes are frequently used for osmotic energy harvesting because of their uniform nanopore channels, high flux, and excellent stability. However, the existing surface modification methods are complex and inefficient. In this study, an amphiphilic ionic liquid was selected to modify a porous anodic alumina membrane via simple capillary insertion. Due to the abundance of pH-dependent amphiphilic OH groups on the surface of AAO pore channels, the ionic liquids not only provide abundant surface charge but can also intelligently adjust its surface charge to different environments. In addition, it fills the AAO nanochannels to provide a continuous ion transport network. The modified hybrid membrane achieves efficient and stable osmotic energy conversion performance. This simple and feasible strategy paves the way for further improvements in commercial membranes.

Electrochemical Olfactory Sensor for Odorants Using Porous Anodic Alumina (PAA) Nanochannels with Coupled Human Odorant Binding Proteins

By mimicking biological olifactory systems, sensors have been reported for the identification of important odorants at low concentration levels in complex environments. Herein a rapid and label-free olfactory sensor based on porous anodic alumina (PAA) nanochannels and the human odorant binding protein s(OBP2b) is reported for odorants. The OBP2b proteins were assembled on the surface of the PAA nanochannels by a Schiff base reaction and the recognition and connection of target with the OBP2b proteins led to steric hindrance inside the PAA nanochannels, resulting in a decreased anodic current due to the indicator, potassium ferricyanide. Using the PAA nanochannels with 55 nm pore diameter as the optimal reaction platform and the OBP2b proteins as the recognition element, the chronoamperometry allowed sensitive determination of ethyl acetate, lauric acid, and vanillin. Furthermore, odorants released by fruit samples were also detected by the developed sensor. Consequently, this sensitive and simple strategy provides an efficient and promising approach for environmental and biological sensing.

Integrating Ordered Two-Dimensional Covalent Organic Frameworks to Solid-State Nanofluidic Channels for Ultrafast and Sensitive Detection of Mercury.

Grafting specific recognition moieties onto solid-state nanofluidic channels is a promising way for selective and sensitive sensing of analytes. However, the time-consuming interaction between recognition moieties and analytes is the main hindrance to the application of nanofluidic channel-based sensors in rapid detection. Here, we show the integration of ordered two-dimensional covalent organic frameworks (2D COFs) to solid-state nanofluidic channels to achieve rapid, selective, and sensitive detection of contaminants. As a proof of concept, a thiourea-linked 2D COF (JNU-3) as the recognition unit is covalently bonded on the stable artificial anodic aluminum oxide nanochannels (AAO) to fabricate a JNU-3@AAO-based nanofluidic sensor. The rapid and selective interaction of Hg(II) with the highly ordered channels of JNU-3 allows the JNU-3@AAO-based nanofluidic sensor to realize ultrafast and precise determination of Hg(II) (90 s) with a low limit of detection (3.28 fg mL-1), wide linear range (0.01-100 pg mL-1), and good precision (relative standard deviation of 3.8% for 11 replicate determination of 10 pg mL-1). The developed method was successfully applied to the determination of mercury in a certified reference material A072301c (rice powder), real water, and rice samples with recoveries of 90.4-99.8%. This work reveals the great potential of 2D COFs-modified solid-state nanofluidic channels as a sensor for the rapid and precise detection of contaminants in complicated samples.

Pillararene‐Based Nanochannels for Para‐Xylene Separation from Xylene Isomers

AbstractIn industrial production, the continuous, efficient and energy‐saving separation of alkylaromatic compounds has always been a challenge. Herein, we designed and synthesized the functional pillararene (TP5A) via a thiol‐alkene click reaction. We modified TP5A molecules into the surface of anodic aluminum oxide nanochannels via a simple two‐step condensation reaction for fabricating TP5A channel, confirming by the X‐ray photoelectron spectroscopy, contact angle, laser scanning confocal microscopy, energy‐dispersive X‐ray spectroscopy, Fourier transform infrared spectroscopy and current‐voltage curves. This biomimetic nanochannels can be used to separate para‐xylene from its structural isomers. We speculate that the host‐guest interaction between TP5A and para‐xylene is responsible for the emergence of selectivity, confirmed by the molecular simulation. This study provides a potential approach to membrane separation and may also contribute to the development of artificial nanofluids.

Effect of channel diameter and homopolymer content on ionic transport properties of the confined Self-Assembled polymers in anodic aluminum oxide template

Ion transport properties of the self-assembled block copolymer/homopolymer blends in cylindrical anodic aluminum oxide (AAO) nanochannels was reported in this paper. Polystyrene-b-poly(4-vinylpyridine)/homopolystyrene (PS-b-P4VP/hPS) nanostructures were prepared by self-assembly in AAO membranes with different channel diameters. Ionic transport properties of the self-assembled structures with different PS-b-P4VP/hPS ratios in AAO nanochannels were systematically investigated. The weight ratio of hPS was proven to play a significant role in the structural transition of the assemblies and the ionic transport properties in nanochannels. In addition, the channel diameter of the AAO membranes was also proven to play an important role in the ion transport properties. The proposed system showed high pH-dependent and good stability, and regulation of ion transport inside artificial nanochannels could be achieved by tuning pH values. This work provides valuable information for understanding the mechanism of ion transport for self-assembled polymers in artificial nanochannels and can promote the construction of nanofluidic systems for many applications.

Current-perpendicular-to-plane giant magnetoresistance in Co/Cu multilayered nanocylinders electrodeposited into anodized aluminum oxide nanochannels with ultra-large aspect ratio

An anodized aluminum oxide (AAO) thick film with numerous nanochannels was synthesized by a cathodic exfoliation technique from a metallic aluminum rod. The nanochannel diameter D and length L were about 60 nm and 60 µm, respectively. The aspect ratio L/D reached ~ 1000. A rectangular pulsed potential deposition technique was applied to electrochemically grow the multilayered nanocylinders with alternating Cu and Co thin layers in the AAO nanochannels. The Co/Cu bilayer thickness was decreased to around 7 nm at the minimum by controlling the pulsed potential and duty cycle during the electrodeposition process. An electrochemical in situ single contact process was demonstrated by monitoring the observed current until a Co/Cu multilayered nanocylinder reached the nanoporous Au film on the AAO thick film. The resistance of the Co/Cu multilayered nanocylinder was determined to range from 1 to 2 kΩ, which corresponded well with a theoretical estimation. Current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) of a Co/Cu multilayered nanocylinder with ~ 6000 bilayers (Cu = 2.4 nm, Co = 7.1 nm) reached up to 31.5% at room temperature. Valet–Fert model agreed well with the present study concerning the effect of Cu-layer thickness on CPP-GMR.

CPP-GMR performance of electrodeposited metallic multilayered nanowires with a wide range of aspect ratios

Effect of the diameter on the CPP-GMR performance of electrodeposited Co/Cu multilayered nanowires was systematically investigated using our home-made anodized aluminum oxide (AAO) templates with the nanochannel diameter ranging from 35 to 130 nm. The nanochannel diameter was precisely controlled by tuning the anodization voltage ranging from 20 to 110 V. The Co/Cu multilayered nanowires with the aspect ratio of ~1000 were synthesized in the AAO nanochannels by applying a rectangular pulsed potential electrodeposition technique using an acidic aqueous solution containing Co2+ and Cu2+ ions. An ideal alternating Co/Cu multilayered structure with each layer thickness of several nanometers was clearly observed in the nanowires with the diameter ranging from 35 to 95 nm. The multilayered nanowires with the diameter ranging from 35 to 70 nm were spontaneously magnetized in the axial direction due to the preferential crystal orientation of hcp-Co (002) which was induced by the magnetic shape anisotropy. With decreasing the diameter down to 35 nm, the coercivity increased up to 1.37 kOe while the squareness reached 0.73 at the diameter of 50 nm. The CPP-GMR value reached 23.4% at room temperature in the Co/Cu multilayered nanowires with the diameter of 70 nm.

Size-controllable zinc oxide nanowires fabricated via the combination of die-casting and oxidation process

Herein, we demonstrate a facile approach in the fabrication of one-dimensional ZnO nanowires (NWs) from the vacuum die-casting method in assistance with anodic aluminum oxide (AAO) template. The casting method enables molten Zn to embed uniformly over the AAO nanomould under high pressure and temperature of 500 °C. Besides, the significant influence of the oxidation time and temperature stimulates the ZnO NWs growth inside the AAO nanochannels. The oxidation in the nanochannels was profoundly controlled by the inward diffusion of oxygen atoms and the ZnO grows perpendicular towards the surface which obeys the parabolic law. The TEM and XRD results revealed the ZnO wurtzite crystalline structure with preferential orientation along the c-axis. The relationship between ZnO geometry and function of oxidation parameters is demonstrated by microscopic and spectroscopic analysis. The obtained NWs from the chemical etching process resulted in high crystalline, homogeneous, and high-aspect ratio ZnO NWs in relationship with the AAO pore dimension. Further, the as-prepared ZnO NWs electrical properties were measured in a four-electrode system through the single nanowire device fabricated by FIB. The obtained current-voltage characteristic curves exhibited hysterias loop in consequence with the space charge limited current effect of ZnO NW, which confirms its potential to be used in practical electronic applications.

Rapid identification of nicotine in electronic cigarette liquids based on surface-enhanced Raman scattering.

Nicotine-containing electronic cigarette liquid (e-liquid) is prohibited in many countries, creating requirements for rapid detection approaches for on-site inspection or screening for large amounts of samples. Here, we demonstrate a simple way to identify nicotine using surface-enhanced Raman scattering (SERS) with substrates made of silver nanoparticle arrays imbedded in anodic aluminum oxide nanochannels (Ag/AAO). Compared with the reported colloidal nanoparticle-based SERS, that required serial dilutions to enable colloid aggregation in the viscous e-liquid, a small amount of undiluted e-liquid sample can be directly added onto our solid-phase Ag/AAO substrate without any pre-treatment. The sensitivity of our SERS measurements is 2–3 orders of magnitude higher than that required for identification of nicotine in e-liquid, which is typically around 1000–18,000 ppm. Using such nanoparticle array-based SERS, we have tested 22 commercially available e-liquid products, using the corresponding gas chromatography-mass spectrometry (GC–MS) reports as the reference. The SERS measurements were done within one hour and successfully identified 20 samples. Only 2 samples showed SERS interference from ingredients that were not suitable for SERS analysis.

A Novel Biosensor Based on Molybdenum Disulfide (MoS2 ) Modified Porous Anodic Aluminum Oxide Nanochannels for Ultrasensitive microRNA-155 Detection.

Artificial photoresponsive nanochannels have attracted widespread attention because of their capacity to achieve ion transport through light modulation. Herein, a biosensor for ultrasensitive miRNA-155 detection is devised based on molybdenum disulfide (MoS2 ) modified porous anodic aluminum oxide (AAO) photoresponsive nanochannels by atomic layer deposition (ALD). According to the optimized experimental results, when the cycles of ALD, the wavelength, and the power of the excitation laser are 70 cycles, 450 nm, and 80 mW, respectively, the most supreme photocurrent performance of these photoresponsive nanochannels are obtained. AAO nanochannels modified with MoS2 can work as a photoelectrochemical (PEC) biosensor by generating photoexcitation current; what is more, the high channel density in AAO can magnify the ion current signal response effectively by aggrandizing the flux of electroactive species. By using AAO photoresponsive nanochannels with an average diameter of 150 nm as PEC biosensor, an ultrasensitive detection record ranging from 0.01 fM to 0.01 nM with a detection limit of 3 aM can be achieved. This work not only proposes a simple method for manufacturing semiconductor photoresponsive nanochannels, but also exhibits great potential in the ultrasensitive detection of biomolecules.

Multipolar Resonances of Ag Nanoparticle Arrays in Anodic Aluminum Oxide Nanochannels for Enhanced Hot Spot Intensity and Signal-to-Background Ratio in Surface-Enhanced Raman Scattering

Hot spots generated through the electrodynamic coupling of the local plasmons in metallic nanoparticles play an important role in enhancing surface-enhanced Raman spectroscopy (SERS) signal. Howeve...

A Dual Antibody-modified Nanochannel Biosensor for Capture and Identification of Exosomes

Since exosomes carry the genetic information of their parental cells, there is an increasing need for new technologies capable of detecting exosomes with high sensitivity and accuracy. In this study, we propose a dual antibody-modified nanochannel biosensor for rapid, sensitive analysis of exosomes. Two antibodies (anti-CD63 and anti-EpCAM) were simultaneously immobilized on the surface of the porous anodic alumina (PAA) nanochannel to combine with proteins overexpressed on the exosomal membrane. As such, exosomes could be efficiently captured by the dual antibody-modified nanochannel. The captured exosomes effectively covered the nanochannel inlet, leading to remarkable block of ion flow through the channel. Based on this principle, the captured exosomes could be sensitively and accurately detected using a picoammeter. It was found that the use of two antibodies greatly increased the capture efficiency of exosomes compared to that of a single antibody. The results showed that exosomes down to 7.3 × 103 particles/mL were successfully detected by the nanochannel. Moreover, exosomes were successfully detected from plasma samples using a nanochannel biosensor modified with dual antibody, and the results showed significant differences in the exosomes content in plasma samples of cancer patients and healthy people, so as to initially distinguish cancer patients from healthy people.

Electrostatic properties of inner nanopore surfaces of anodic aluminum oxide membranes upon high temperature annealing revealed by EPR of pH-sensitive spin probes and labels

Anodic aluminum oxide (AAO) membranes are versatile nanomaterials that combine the chemically stable and mechanically robust properties of ceramics with homogeneous nanoscale organization that can be tuned to desirable pore diameters and lengths. The AAO substrates feature high surface area that is readily accessible to large and small molecules, making these nanostructures uniquely suited for many possible applications. Examples include templated self-assembly of macroscopically aligned biological membranes and substrates for heterogeneous catalysis. For further development of such applications, one would like to characterize and tune the electrostatic properties of the inner pore surface as well as the local acidity within the nanochannels. Here, we employed electron paramagnetic resonance (EPR) spectroscopy of a small molecule – ionizable nitroxide – as a reporter of the average local acidity in the nanochannels and the local electrostatic potential in the immediate vicinity of the pore surface. The former was achieved by measuring EPR spectra of this molecular probe diffusing in an aqueous phase confined in the AAO nanochannels while for the latter the nitroxide was covalently attached to the hydroxyl group of the alumina surface. We show that the local acidity within the nanochannels is increased by as much as ≈1.48 pH units vs. the pH of bulk solution by decreasing the pore diameter down to ca. 31 nm. Furthermore, the positive surface charge of the as-prepared AAO could be decreased and even switched to a negative surface charge upon annealing the membranes first to 700 °C and then to 1200 °C. For as-prepared AAO, the local electrostatic potential reaches ψ= (163 ± 5) mV for the nitroxide label covalently attached to AAO and located about 0.5 nm away from the surface. Overall, we demonstrate that the acid-based properties of the aqueous volume confined by the AAO nanopores pores can be tuned by either changing the pore diameter from ca. 71 to 31 nm or by thermal annealing to switch the sign of the surface charge. These observations provide a simple and robust means to tailor these versatile high-surface-area nanomaterials for specific applications that depend on acid-base equilibria.

Directing the Handedness of Helical Nanofilaments Confined in Nanochannels Using Axially Chiral Binaphthyl Dopants.

In this work, we demonstrate control of the handedness of semicrystalline modulated helical nanofilaments (HNFmods) formed by achiral bent-core liquid crystal molecules by axially chiral binaphthyl-based additives as guest molecules solely under spatial nanoconfinement in anodic aluminum oxide nanochannels. The molecules of the same chiral additives are expelled from the HNFmods in the bulk, and as a result thereof do not affect the handedness or helical pitch of bulk HNFmods, resulting in an HNFmod conglomerate with chirality-preserving growth within each domain. However, under confinement these axially chiral guest molecules, likely embedded in the HNFmod host, do affect the helicity of the HNFmods. The configuration of the axially chiral molecules decides the HNFmod helix handedness and their concentration, and the helix angle is related to the helical pitch of the HNFmods. In addition to local imaging data obtained by scanning electron microscopy, global studies by thin-film circular dichroism spectropolarimetry support the imaging results.

In situ growth of nano-gold on anodized aluminum oxide with tandem nanozyme activities towards sensitive electrochemical nanochannel sensing.

Electrochemical nanochannel sensors have attracted extensive interest due to their potential applications in biosensing systems. In this work, porous anodized aluminum oxide (AAO) nanochannels are coupled with gold nanoparticles (AuNPs) through a polydopamine (PDA)-induced in situ growth process. It is found that the resulting hybrid nanochannel (denoted as Au-PDA-AAO) can act as both glucose oxidase- and peroxidase-like nanozymes to catalyze the cascade reaction involving glucose. To the best of our knowledge, this is the first report on the synthesis of nanozymes in an AAO nanochannel. Moreover, apart from the nanozyme-catalyzed colorimetric reaction, the Au-PDA-AAO nanochannel could simultaneously serve as a sensitive signal reporter for an electrochemical sensing platform. In such an approach, the glucose oxidation reaction boosts the resistance of the Au-PDA-AAO nanochannel towards ion transport based on the H2O2-mediated size enlargement of AuNPs, resulting in the varied transmembrane ionic current signal of the Au-PDA-AAO nanochannel. On the basis of the changed current-potential properties, the label-free detection of glucose can be achieved with a low detection limit, good reproducibility, and high stability. This work demonstrates the feasibility of the incorporation of versatile nanozymes into AAO nanochannels for mimicking multi-enzymatic catalysis reactions and detecting target analytes.

In situ synthesis of a MOFs/PAA hybrid with ultrahigh ionic current rectification.

In recent decades, the ionic current rectification (ICR) property of asymmetric nanochannels has been widely explored in applications of energy conversion, gas separation, water purification and bioanalysis/sensors. How to fabricate nanofluidic devices with a high ICR characteristic remains of critical importance to the development of nanofluidics. Herein, we fabricated an asymmetric MOFs/PAA hybrid via in situ synthesis of a zeolitic imidazole framework (ZIF-90) on porous anodic alumina (PAA) nanochannels. The introduction of asymmetric geometry and charge distribution provides the hybrid with ultrahigh ionic rectification, which can be easily measured using an electrochemical detector. This rectification mechanism is elucidated via finite element simulation, which proves that asymmetric geometry as well as the protonation and deprotonation under varied pH values dominates the ICR property. With the advantages of low cost and facile fabrication while supporting high ionic current rectification, the prepared MOFs/PAA hybrid can be considered as a significant paradigm in nanofluidic systems and has potential applications in the fields of new ionic devices and energy conversion systems.

Creating anodic alumina nanochannel arrays with custom‐made geometry

AbstractPorous anodic alumina oxide (AAO) is one of the most commonly used nanotemplates for growing arrays of nanoparticles, nanowires, nanocomposites, and nanoarchitectures because its pores, which are of a very uniform size, can grow longitudinally into arrays of self‐aligned nanochannels with an extremely high aspect ratio. Furthermore, under specific combinations of anodization voltage and electrolyte, the lateral positions of nanochannels can self‐organize into arrays of two‐dimensional hexagonally close‐packed lattices with domain sizes on the order of few tens of lattice units. The domain size can be greatly increased by prepatterning the Al surface with custom‐designed nanoconcaves prior to the anodization process. The concaves guide the growth fronts of nanochannels and lead to the formation of an ideally long‐range ordered lattice of nanochannel array. Such concaves have been fabricated by many methods, such as stamp imprinting, grating imprinting, and focused ion beam direct writing. In this review, we summarize the development of various methods to create AAO nanochannel arrays with custom‐made geometry and discuss the mechanism responsible for the guiding process.