Ionic Current Rectification of Porous Anodic Aluminum Oxide (AAO) with a Barrier Oxide Layer

Ultrathin Ionic Diodes with Electrostatically Heterogeneous Hybrid Interfaces of Nanoporous SiO2 Nanofilms and Polymer Layer-by-Layer Multilayers

Nanofluidic ionic diodes have attracted much attention due to their unique functions as unidirectional ion transportation ability and promising applications from molecular sensing, and energy harvesting to emerging neuromorphic devices. However, it remains a challenge to fabricate diode-like nanofluidic systems with ultrathin film thickness <100 nm. Herein the formation of ultrathin ionic diodes from hybrid nanoassemblies of nanoporous (NP) SiO2 nanofilms and polyelectrolyte layer-by-layer (LbL) multilayers is described. Ultrathin ionic diodes are prepared by integrating polyelectrolyte multilayers onto photo-oxidized NP SiO2 nanofilms obtained from silsesquioxane-containing block copolymer thin films as a template. The obtained ultrathin ionic diodes exhibit ion current rectification (ICR) properties with high ICR factor = ≈20 under low ionic strength and asymmetric pH conditions. It is concluded that this ICR behavior arises from effective ion accumulation and depletion at the interface of NP SiO2 nanofilms and LbL multilayers attributed to high ion selectivity by combining the experimental data and theoretical calculations using finite element methods. These results demonstrate that the hybrid nano assemblies of NP SiO2 nanofilms and polyelectrolyte LbL multilayers have potential applications for (bio)sensing materials and integrated ionic circuits for seamless connection of human-machine interfaces.

Controllable Fabrication and Rectification of Bipolar Nanofluid Diodes in Funnel-Shaped Si3 N4 Nanopores

Solid-state nanopores attract widespread interest, owning to outstanding robustness, extensive material availability, as well as capability for flexible manufacturing. Bioinspired solid-state nanopores further emerge as potential nanofluidic diodes for mimicking the rectification progress of unidirectional ionic transport in biological K+ channels. However, challenges that remain in rectification are over-reliance on complicated surface modifications and limited control accuracy in size and morphology. In this study, suspended Si3 N4 films of only 100 nm thickness are used as substrate and funnel-shaped nanopores are controllably etched on that with single-nanometer precision, by focused ion beam (FIB) equipped with a flexibly programmable ion dose at any position. A small diameter 7 nm nanopore can be accurately and efficiently fabricated in only 20 ms and verified by a self-designed mathematical model. Without additional modification, funnel-shaped Si3 N4 nanopores functioned as bipolar nanofluidic diodes achieve high rectification by simply filling each side with acidic and basic solution, respectively. Main factors are finely tuned experimentally and simulatively to enhance the controllability. Moreover, nanopore arrays are efficiently prepared to further improve rectification performance, which has great potential for high-throughput practical applications such as extended release of drugs, nanofluidic logic systems, and sensing for environmental monitoring and clinical diagnosis.

Understanding the Intrinsic Rectification Properties of Nanoporous Anodic Alumina by Selective Chemical Etching

The distribution of oxygen and aluminum vacancies across the hemispherical barrier oxide layer (BOL) of nanoporous anodic alumina (NAA) relies intrinsically on the electric field-driven flow of electrolytic species and the incorporation of electrolyte impurities during the growth of anodic oxide through anodization. This phenomenon provides new opportunities to engineer BOL's inherited ionic current rectification (ICR) fingerprints. NAA's characteristic ICR signals are associated with the space charge density gradient across BOL and electric field-induced ion migration through hopping from vacancy to vacancy. In this study, we engineer the intrinsic space charge density gradient of the BOL of NAA under a range of anodizing potentials in hard and mild anodization regimes. Real-time characterization of the ICR fingerprints of NAA during selective etching of the BOL makes it possible to unravel the distribution pattern of vacancies through rectification signals as a function of etching direction and time. Our analysis demonstrates that the space charge density gradient varies across the BOL of NAA, where the magnitude and distribution of the space charge density gradient are revealed to be critically determined by anodizing the electrolyte, regime, and potential. This study provides a comprehensive understanding of the engineering of ion transport behavior across blind-hole NAA membranes by tuning the distribution of defects across BOL through anodization conditions. This method has the potential to be harnessed for developing nanofluidic devices with tailored ionic rectification properties for energy generation and storage and sensing applications.

Neuron-Inspired Nanofluidic Biosensors for Highly Sensitive and Selective Imidacloprid Detection

Pesticides have caused concerns about food safety due to their residual effects in vegetables and fruits. Imidacloprid, as the frequently used neonicotinoid pesticide, could harm cardiovascular and respiratory function and cause reproductive toxicity in humans. Therefore, reliable methods for portable, selective, and rapid detection are desirable to develop. Herein, we report a neuron-inspired nanofluidic biosensor based on a tyrosine-modified artificial nanochannel for sensitively detecting imidacloprid. The functional tyrosine is modified on the outer surface of porous anodic aluminum oxide to rapidly capture imidacloprid through π-π interactions and hydrogen bonds. The integrated nanofluidic biosensor has a wide concentration range from 10-8 to 10-4 g/mL with an ultralow detection limit of 6.28 × 10-9 g/mL, which outperforms the state-of-the-art sensors. This work provides a new perspective on detecting imidacloprid residues as well as other hazardous pesticide residues in environmental and food samples.

Essence of the Giant Reduction of Power Density in Osmotic Energy Conversion in Porous Membranes: Importance of Testing Area

Harvesting osmotic energy through nanofluidic devices with diverse materials has received considerable attention in recent years. Often, a small testing area on a membrane was chosen to assess its power performance by calculating power density as output power per effective area. Since the choice of this testing area is arbitrary, and it is usually quite small, the result obtained can be too optimistic. There is a need to come up with a common standard so that the performance of a device/membrane can be assessed reasonably. In this study, we systematically investigate the power density as a function of testing area in nanoporous anodic-aluminum-oxide membranes. Through changing the aperture size of substrates, we clearly show that the obtained power density decreases drastically with increasing testing area. For instance, the power density acquired from the testing area of μm2-scale can be five orders of magnitude larger than that from the pristine membrane of cm2-scale. We also advance simulations by building a 3D model to simulate osmotic-driven ion transport in the multichannel system. The result of modeling agrees with our experimental observation that the power density decreases with increasing number of channels, and the ionic concentration profile reveals that the concentration polarization becomes serious as the number of channels increases. Our result highlights the importance of effective area on testing the power performance in nanofluidic devices.

Switchable biomimetic nanochannels for on-demand SO2 detection by light-controlled photochromism

In contrast to the conventional passive reaction to analytes, here, we create a proof-of-concept nanochannel system capable of on-demand recognition of the target to achieve an unbiased response. Inspired by light-activatable biological channelrhodopsin-2, photochromic spiropyran/anodic aluminium oxide nanochannel sensors are constructed to realize a light-controlled inert/active-switchable response to SO₂ by ionic transport behaviour. We find that light can finely regulate the reactivity of the nanochannels for the on-demand detection of SO₂. Pristine spiropyran/anodic aluminium oxide nanochannels are not reactive to SO₂. After ultraviolet irradiation of the nanochannels, spiropyran isomerizes to merocyanine with a carbon‒carbon double bond nucleophilic site, which can react with SO₂ to generate a new hydrophilic adduct. Benefiting from increasing asymmetric wettability, the proposed device exhibits a robust photoactivated detection performance in SO₂ detection in the range from 10 nM to 1 mM achieved by monitoring the rectified current.

Sustainable power generation for at least one month from ambient humidity using unique nanofluidic diode

The continuous energy-harvesting in moisture environment is attractive for the development of clean energy source. Controlling the transport of ionized mobile charge in intelligent nanoporous membrane systems is a promising strategy to develop the moisture-enabled electric generator. However, existing designs still suffer from low output power density. Moreover, these devices can only produce short-term (mostly a few seconds or a few hours, rarely for a few days) voltage and current output in the ambient environment. Here, we show an ionic diode–type hybrid membrane capable of continuously generating energy in the ambient environment. The built-in electric field of the nanofluidic diode-type PN junction helps the selective ions separation and the steady-state one-way ion charge transfer. This directional ion migration is further converted to electron transportation at the surface of electrodes via oxidation-reduction reaction and charge adsorption, thus resulting in a continuous voltage and current with high energy conversion efficiency.

Energy harvesting of humidity present in air can be used for the development of clean energy sources and self-sustained systems. The authors propose a nanofluid energy conversion system with integrated ionic diode-type hybrid membrane for energy generation in environmental moisture.

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.

Aquaporin-Inspired CPs/AAO Nanochannels for the Effective Detection of HCHO: Importance of a Hydrophilic/Hydrophobic Janus Device for High-Performance Sensing

Probe reactivity has long been considered to play a key role in artificial nanochannel sensors, but systematic studies of membrane wettability on detection performance are currently lacking. Inspired by biological aquaporins, we developed an effective strategy to regulate the hydrophilic/hydrophobic balance by the controllable in situ assembly of coordination polymers (CPs) using BDC-NH₂ on anodic aluminum oxide (AAO) nanochannels to promote HCHO detection. We found that the hydrophobic/hydrophilic balance in CP/AAO heterosomes plays significant roles in the effective detection of HCHO. The hydrophobic AAO barrier layer is necessary to support the confinement effect, while the hydrophilic CP surface is favorable for HCHO to access the channels and then condense with the responsive amine to generate a new imine. The optimized CP/AAO Janus device shows excellent performance in the quantitative analysis of HCHO over a wide range from 100 pM to 1 mM by monitoring the rectified ionic current.

Fabrication of solid-state nanopores

Nanopores are valuable single-molecule sensing tools that have been widely applied to the detection of DNA, RNA, proteins, viruses, glycans, etc. The prominent sensing platform is helping to improve our health-related quality of life and accelerate the rapid realization of precision medicine. Solid-state nanopores have made rapid progress in the past decades due to their flexible size, structure and compatibility with semiconductor fabrication processes. With the development of semiconductor fabrication techniques, materials science and surface chemistry, nanopore preparation and modification technologies have made great breakthroughs. To date, various solid-state nanopore materials, processing technologies, and modification methods are available to us. In the review, we outline the recent advances in nanopores fabrication and analyze the virtues and limitations of various membrane materials and nanopores drilling techniques.

Unraveling the six stages of the current-time curve and the bilayer nanotubes obtained by one-step anodization of Zr

The application and growth mechanism of anodic TiO2 nanotubes have been a hot topic in the last ten years, but the formation mechanism of anodic ZrO2 nanotubes has rarely been studied. In one-step constant voltage anodization of Al and Ti, the typical current-time curve has three stages. Moreover, the current-time curves of the three stages can last for 10 min or even 10 hours, resulting in a single layer of nanotubes with the same diameter due to the constant voltage in one-step anodization. However, in this paper, it was found for the first time that the three stages of the current-time curve appeared twice in succession during one-step constant voltage anodization of Zr for only 900 seconds, and bilayer nanotubes with increased diameter were obtained. This six-stage current-time curve cannot be explained by classical field-assisted dissolution and field-assisted flow or stress-driven mechanisms. Here, the formation mechanism and growth kinetics of bilayer ZrO2 nanotubes have been clarified rationally by the theories of ionic current, electronic current and oxygen bubble mold. The interesting results presented in this paper are of great significance for revealing the anodizing process of various metals and the formation mechanism of porous structures.

Spectral Engineering of Tamm Plasmon Resonances in Dielectric Nanoporous Photonic Crystal Sensors

Model light-confining Tamm plasmon cavities based on gold-coated nanoporous anodic alumina photonic crystals (TMM-NAA-PCs) with spectrally tunable resonance bands were engineered. Laplacian and Lorentzian NAA-PCs produced by a modified Gaussian-like pulse anodization approach showed well-resolved, high-quality photonic stopbands, the position of which was precisely controlled across the visible spectrum by the periodicity in the input anodization profile. These PC structures were used as a platform material to develop highly reflective distributed Bragg mirrors, the top sides of which were coated with a thin gold film. The resulting nanoporous hybrid plasmonic-photonic crystals showed strong light-confining properties attributed to Tamm plasmon resonances at three specific positions of the visible spectrum. These structures achieved high sensitivity to changes in refractive index, with a sensitivity of ∼106 nm RIU-1. The optical sensitivity of TMM-NAA-PCs was assessed in real time, using a model chemically selective binding interaction between thiol-containing molecules and gold. The optical sensitivity was found to rely linearly on the spectral position of the Tamm resonance band, for both Laplacian and Lorentzian TMM-NAA-PCs. The density of self-assembled monolayers of thiol-containing analyte molecules formed on the surface of the metallic film directly contributes to the dependence of sensitivity on TMM resonance position in these optical transducers. Our findings provide opportunities to integrate TMM modes in NAA-based photonic crystal structures, with promising potential for optical technologies and applications requiring high-quality surface plasmon resonance bands.

The Use of Anodic Oxides in Practical and Sustainable Devices for Energy Conversion and Storage

This review addresses the main contributions of anodic oxide films synthesized and designed to overcome the current limitations of practical applications in energy conversion and storage devices. We present some strategies adopted to improve the efficiency, stability, and overall performance of these sustainable technologies operating via photo, photoelectrochemical, and electrochemical processes. The facile and scalable synthesis with strict control of the properties combined with the low-cost, high surface area, chemical stability, and unidirectional orientation of these nanostructures make the anodized oxides attractive for these applications. Assuming different functionalities, TiO2-NT is the widely explored anodic oxide in dye-sensitized solar cells, PEC water-splitting systems, fuel cells, supercapacitors, and batteries. However, other nanostructured anodic films based on WO3, CuxO, ZnO, NiO, SnO, Fe2O3, ZrO2, Nb2O5, and Ta2O5 are also explored and act as the respective active layers in several devices. The use of AAO as a structural material to guide the synthesis is also reported. Although in the development stage, the proof-of-concept of these devices demonstrates the feasibility of using the anodic oxide as a component and opens up new perspectives for the industrial and commercial utilization of these technologies.