Nanoporous Filters Research Articles

Shape-Transformable, Fusible Rodlike Swimming Liquid Metal Nanomachine.

The T-1000 liquid metal terminator, which can transform and self-repair, represents a dream for decades that robots can fundamentally change our daily life. Until now, some large-scale liquid metal machines have been developed. However, there is no report on nanoscaled liquid metal machines and their biomedical applications. We describe here a shape-transformable and fusible rodlike swimming liquid metal nanomachine, based on the biocompatible and transformable liquid metal gallium. These nanomachines were prepared by a pressure-filter-template technology, and the diameter and length could be controlled by adjusting the nanoporous templates, filter time, and pressure. The as-prepared liquid gallium nanomotors display a core-shell nanorod structure composed of a liquid gallium core and solid gallium oxide shell. Upon exposure to an ultrasound field, the generated acoustic radiation force in the levitation plane can propel them to move autonomously. The liquid metal nanomachine can actively seek cancer cells and transform from a rod to a droplet after drilling into cells owing to the removal of gallium oxide layers in the acidic endosomes. These transformed nanomachines could fuse together inside cells and photothermally kill cancer cells under illumination of near-infrared light. Such acoustically propelled shape-transformable rodlike liquid metal nanomachines have great potential for biomedical applications.

Back Cover: Development of a low flow-resistive charged nanoporous membrane in a microchip for fast electropreconcentration

DOI: 10.1002/elps.201800093 The cover picture shows the anion-selective electropreconcentration process of fluorescein (green, charge of −2) and TRITC-albumin (red, charge of −10) using a poly-SS membrane as the nanoporous charge-selective filter. The electropreconcentration only concentrates same charged analytes as the nano-filter surface charge. One of the inevitable phenomena due to the charge-selectivity is the limit in maximum concentration of the preconcentrated sample plug, which is limited by the counter-ionic strength of the buffer. Although all experimental conditions were identical, the TRITC-albumin plug grows faster (∽5 times) because the higher valency results in lower maximum concentration.

Optimum design for cylindrical-shaped nanoporous filtration membrane

The design of cylindrical-shaped nanoporous filtration membranes is optimized. Across the membrane thickness, two kinds of concentric nanopores respectively with the radii R0 and R1 (R0 ≤ R1) are manufactured. The nanopore with the radius R0 is for filtration, while the nanopore with the radius R1 is for reducing the flow resistance and increasing the flux of the membrane. The optimum R1/R0 value was obtained for yielding the smallest flow resistance of the membrane. This optimum R1/R0 value is dependent on the radius R0, the depth of the filtration pore and the passing liquid-pore wall interaction. For increasing the flux of the membrane, the depth of the filtration pore should be as small as possible.

Filtration-guided assembly for patterning one-dimensional nanostructures

Tremendous progress has been made in synthesizing various types of one-dimensional (1D) nanostructures (NSs), such as nanotubes and nanowires, but some technical challenges still remain in the deterministic assembly of the solution-processed 1D NSs for device integration. In this work we investigate a scalable yet inexpensive nanomaterial assembly method, namely filtration-guided assembly (FGA), to place nanomaterials into desired locations as either an individual entity or ensembles, and form functional devices. FGA not only addresses the assembly challenges but also encompasses the notion of green nanomanufacturing, maximally utilizing nanomaterials and eliminating a waste stream of nanomaterials into the environment. FGA utilizes selective filtration of 1D NSs through the open windows on the nanoporous filter membrane whose surface is patterned by a polymer mask for guiding the 1D NS deposition. The modified soft-lithographic technique called blanket transfer (BT) is employed to create the various photoresist patterns of sub-10-micron resolution on the nanoporous filter membrane like mixed cellulose acetate. We use single-walled carbon nanotubes (SWCNTs) as a model 1D NS and demonstrate the fabrication of an array pattern of homogeneous 1D NS network films over an area of 20 cm2 within 10 min. The FGA-patterned SWCNT network films are transferred onto the substrate using the adhesive-based transfer technique, and show the highly uniform film thickness and resistance measurements across the entire substrate. Finally, the electrical performance of the back-gated transistors made from the FGA and transfer method of 95% pure SWCNTs is demonstrated.

Fabrication of a nano-porous electrode film filter using a novel Cu-Cr dealloying and its application for a sensor

Fabrication of a nano-porous electrode film filter using a novel Cu-Cr dealloying and its application for a sensor

Controlled degrees of oxidation of nanoporous graphene filters for water purification using an aqueous arc discharge

Graphene-based nanofiltration membranes with different degrees of oxidation were produced using a novel aqueous arc discharge process for water purification applications. By adjusting the current level during the arc discharge the degree of oxidation was controlled from 28.1% at 1 A (Gr1A) to 53.9% at 4 A (Gr4A). Over 70% of the resulting graphene sheets had less than 10 layers. The effect of the degree of graphene oxidation on water purification was evaluated by monitoring pressure-driven water flux and the self-diffusion of ions across the membrane. The water flux through the graphene membranes was proportionally increased with pressure, which showed the highest water flux (362.3 L h−1 bar−1 m−2) at 31.09 kPa. Membrane thickness affected the self-diffusion rates of ions. Compared to a commercial polyamide membrane, the transported concentration of Na+ decreased by 67.7% (0.58 mM h−1) and 64.5% (0.55 mM h−1) for the Gr1A and Gr4A manufacturing processes, respectively where the membrane was 1120 nm thick. The ability to control the chemical and physical properties of the graphene membranes enhances their water purification performance and demonstrates their potential for use in practical water purification applications.

Tunable, Strain-Controlled Nanoporous MoS₂ Filter for Water Desalination.

The deteriorating state of global fresh water resources represents one of the most serious challenges that scientists and policymakers currently face. Desalination technologies, which are designed to extract potable water from the planet's bountiful stores of seawater, could serve to alleviate much of the stress that presently plagues fresh water supplies. In recent decades, desalination methods have improved via water-filtering architectures based on nanoporous graphene filters and artificial membranes integrated with biological water channels. Here, we report the auspicious performance (in simulations) of an alternative nanoporous desalination filter constructed from a MoS2 nanosheet. In striking contrast to graphene-based filters, we find that the "open" and "closed" states of the MoS2 filter can be regulated by the introduction of mechanical strain, yielding a highly tunable nanopore interface. By applying lateral strain to the MoS2 filter in our simulations, we see that the transition point between "open" and "closed" states occurs under tension that induces about 6% cross-sectional expansion in the membrane (6% strain); the open state of the MoS2 filter demonstrates high water transparency and a strong salt filtering capability even under 12% strain. Our results thus demonstrate the promise of a controllable nanoporous MoS2 desalination filter, wherein the morphology and size of the central nanopore can be precisely regulated by tensile strain. These findings support the design and proliferation of tunable nanodevices for filtration and other applications.

Rational engineering of nanoporous anodic alumina optical bandpass filters.

Herein, we present a rationally designed advanced nanofabrication approach aiming at producing a new type of optical bandpass filters based on nanoporous anodic alumina photonic crystals. The photonic stop band of nanoporous anodic alumina (NAA) is engineered in depth by means of a pseudo-stepwise pulse anodisation (PSPA) approach consisting of pseudo-stepwise asymmetric current density pulses. This nanofabrication method makes it possible to tune the transmission bands of NAA at specific wavelengths and bandwidths, which can be broadly modified across the UV-visible-NIR spectrum through the anodisation period (i.e. time between consecutive pulses). First, we establish the effect of the anodisation period as a means of tuning the position and width of the transmission bands of NAA across the UV-visible-NIR spectrum. To this end, a set of nanoporous anodic alumina bandpass filters (NAA-BPFs) are produced with different anodisation periods, ranging from 500 to 1200 s, and their optical properties (i.e. characteristic transmission bands and interferometric colours) are systematically assessed. Then, we demonstrate that the rational combination of stacked NAA-BPFs consisting of layers of NAA produced with different PSPA periods can be readily used to create a set of unique and highly selective optical bandpass filters with characteristic transmission bands, the position, width and number of which can be precisely engineered by this rational anodisation approach. Finally, as a proof-of-concept, we demonstrate that the superposition of stacked NAA-BPFs produced with slight modifications of the anodisation period enables the fabrication of NAA-BPFs with unprecedented broad transmission bands across the UV-visible-NIR spectrum. The results obtained from our study constitute the first comprehensive rationale towards advanced NAA-BPFs with fully controllable photonic properties. These photonic crystal structures could become a promising alternative to traditional optical bandpass filters based on glass and plastic.

脱合金化法を用いたナノポーラス電極膜フィルタの製作とセンサへの応用

脱合金化法を用いたナノポーラス電極膜フィルタの製作とセンサへの応用

Dendrimers in Nanoscale Confinement: The Interplay between Conformational Change and Nanopore Entrance.

Hyperbranched dendrimers are nanocarriers for drugs, imaging agents, and catalysts. Their nanoscale confinement is of fundamental interest and occurs when dendrimers with bioactive payload block or pass biological nanochannels or when catalysts are entrapped in inorganic nanoporous support scaffolds. The molecular process of confinement and its effect on dendrimer conformations are, however, poorly understood. Here, we use single-molecule nanopore measurements and molecular dynamics simulations to establish an atomically detailed model of pore dendrimer interactions. We discover and explain that electrophoretic migration of polycationic PAMAM dendrimers into confined space is not dictated by the diameter of the branched molecules but by their size and generation-dependent compressibility. Differences in structural flexibility also rationalize the apparent anomaly that the experimental nanopore current read-out depends in nonlinear fashion on dendrimer size. Nanoscale confinement is inferred to reduce the protonation of the polycationic structures. Our model can likely be expanded to other dendrimers and be applied to improve the analysis of biophysical experiments, rationally design functional materials such as nanoporous filtration devices or nanoscale drug carriers that effectively pass biological pores.

Effect of current density on silicon surface in electrochemical etching

A simple and reliable fabrication technique for producing nanoporous filters is presented. The nanoporous filter plays an important role in biomedical microelectromechanical systems applications, especially in filtering out waste and solute from inside human blood. Nanosized components in the biological fluid are filtered using silicon membranes that are controlled by nanosized pores. The technique explored was the electrochemical etching (ECE) process of silicon. This approach starts with thinning the bulk silicon until only several micrometres thick using the KOH process and then carry out ECE to produce pores. The yield of the process was a 3 µm thick nanoporous silicon membrane with pore sizes of less than 100 nm. This physical characteristic enables the membrane to filter all the waste and solute particles of less than 100 nm. Owing to this simple and reliable method, the development of nanoporous silicon membrane can be used in nanofiltration applications especially in an artificial kidney.

Nanostructural Engineering of Nanoporous Anodic Alumina for Biosensing Applications.

Modifying the diameter of the pores in nanoporous anodic alumina opens new possibilities in the application of this material. In this work, we review the different nanoengineering methods by classifying them into two kinds: in situ and ex situ. Ex situ methods imply the interruption of the anodization process and the addition of intermediate steps, while in situ methods aim at realizing the in-depth pore modulation by continuous changes in the anodization conditions. Ex situ methods permit a greater versatility in the pore geometry, while in situ methods are simpler and adequate for repeated cycles. As an example of ex situ methods, we analyze the effect of changing drastically one of the anodization parameters (anodization voltage, electrolyte composition or concentration). We also introduce in situ methods to obtain distributed Bragg reflectors or rugate filters in nanoporous anodic alumina with cyclic anodization voltage or current. This nanopore engineering permits us to propose new applications in the field of biosensing: using the unique reflectance or photoluminescence properties of the material to obtain photonic barcodes, applying a gold-coated double-layer nanoporous alumina to design a self-referencing protein sensor or giving a proof-of-concept of the refractive index sensing capabilities of nanoporous rugate filters.

On the Origin of Mesoscale Structures in Aqueous Solutions of Tertiary Butyl Alcohol: The Mystery Resolved

We have performed a detailed experimental study on aqueous solutions of tertiary butyl alcohol which were a subject of long-standing controversies regarding the puzzling presence of virtually infinitely stable large-scale structures in such solutions occurring at length scales exceeding appreciably dimensions of individual molecules, referred to also as mesoscale structures. A combination of static and dynamic light scattering yielding information on solution structure and dynamics and gas chromatography coupled with mass spectrometry yielding information on chemical composition was used. We show that tertiary butyl alcohol clearly exhibiting such structures upon mixing with water does not contain any propylene oxide, which was previously considered as a source of these structures (an impurity expected to be present in all commercial samples of TBA). More importantly, we show that no mesoscale structures are generated upon addition of propylene oxide to aqueous solutions of TBA. The ternary system TBA/water/propylene oxide exhibits homogeneous mixing of the components on mesoscales. We show that the source of the mesoscale structures is a mesophase separation of appreciably more hydrophobic compounds than propylene oxide. These substances are explicitly analytically identified as well as their disappearance upon filtering out the mesoscale structures by nanopore filtration. We clearly show which substances are disappearing upon filtration and which are not. This enables us to estimate with rather high probability the chemical composition of the mesoscale structures. Visualization of large-scale structures via nanoparticle tracking analysis is also presented. Video capturing the mesoscale particles as well as their Brownian motion can be found in the Supporting Information .

Block Copolymers Self-Assembly Allows Obtaining Tunable Micro or Nanoporous Membranes or Depth Filters Based on PMMA; Fabrication Method and Nanostructures

An environmentally friendly method to produce tunable bulk micro or nanoporous structures is presented. Nano- and micro-porous structures are obtained by gas dissolution foaming using CO2, following the pattern of polymer blends between poly(methylmethacrylate) (PMMA) and a triblock copolymer, namely poly(methylmethacrylate)-co-poly(butyl acrylate)-co-poly(methyl methacrylate) (MAM) blends nanostructuration. Membranes or depth filters can be produced by this method, varying the amount of the triblock copolymer in the blend. Furthermore, main characteristics, such as the average pore size and porosity can be finely tuned by adjusting the process parameters (temperature or pressure) during gas foaming.

Chapter 17 - NPC Mimics: Probing the Mechanism of Nucleocytoplasmic Transport

In vitro mimics of cellular machines have been recently engineered and utilized to investigate processes within cells. These devices can provide novel insights into biological mechanisms and have the potential to improve biotechnological processes such as separation. In particular, several devices have been designed to mimic translocation through nuclear pore complexes (NPCs). We describe here the fabrication of a biomimetic NPC using nanoporous filters lined with FG-repeats of proteins that create a selectivity barrier. We show the utility of this nanoselective filter as a testbed for the investigation of nucleocytoplasmic transport and demonstrate that this device closely reproduces key features of trafficking through the NPC.

Fabrication and testing of nanoporous Si3N4 optical filters for gas sensing applications

We demonstrate the concept of nanoporous multilayer optical filters as optical gas sensors, based on plasma-enhanced chemical vapor deposited Si3N4 films with controlled porosity. In situ real-time spectroscopic ellipsometry, associated with the Bruggeman effective medium approximation is employed for characterizing the film microstructure, particularly the total volume fraction of porosity (fv), and the fraction of open porosity (fo). In order to do so, the effective refractive index (neff) of the as-grown films is evaluated both in vacuo and when exposed to ethanol vapor. We find that fv=34% and fo=27% for porous films, while fv=13% and fo=2% for dense films. Additionally, adsorption isotherms of ethanol are performed and the resulting curves are interpreted using the Kelvin equation as well as the Dubinin–Radushkevich model, allowing us to estimate the average micropore size (L≈2nm) and the micropore/mesopore volumes (46%/54% of fo). Infrared ellipsometry is then applied for the characterization of the film chemical structure and, particularly, for the study of heat treatment-induced chemical stabilization. Finally, porous/dense Si3N4 quarterwave stacks are implemented in a Fabry–Perot filter that is tested as a prototype sensor in a simple configuration. We observe high sensitivity to model vapors such as ethanol (10ppm) and ammonia (28ppm), as well as fast response times (≈10s), good reproducibility, and complete signal regeneration after heat treatment.

Mesoscale phenomena in solutions of 3-methylpyridine, heavy water, and an antagonistic salt

We have investigated controversial issues regarding the mesoscale behavior of 3-methylpyridine (3MP), heavy water, and sodium tetraphenylborate (NaBPh4) solutions by combining results obtained from dynamic light scattering (DLS) and small-angle neutron scattering (SANS). We have addressed three questions: (i) what is the origin of the mesoscale inhomogeneities (order of 100 nm in size) manifested by the "slow mode" in DLS? (ii) Is the periodic structure observed from SANS an inherent property of this system? (iii) What is the universality class of critical behavior in this system? Our results confirm that the "slow mode" observed from DLS experiments corresponds to long-lived, highly stable mesoscale droplets (order of 100 nm in size), which occur only when the solute (3MP) is contaminated by hydrophobic impurities. SANS data confirm the presence of a periodic structure with a periodicity of about 10 nm. This periodic structure cannot be eliminated by nanopore filtration and thus is indeed an inherent solution property. The critical behavior of this system, in the range of concentration and temperatures investigated by DLS experiments, indicates that the criticality belongs to the universality class of the 3-dimensional Ising model.

Surface Electrostatics and Peptide Binding to Lipid Bilayer of Defined Curvature

Many proteins are known to have strong binding preferences for curved lipid bilayer surfaces. This property of proteins to sense membrane curvature has a number of implications in cellular processes including endocytosis, exocytosis, and vesicle trafficking as well as pathogenic processes associated with viral infections or protein aggregation disorders. Here we employ small unilamellar vesicles (SUVs) that are sized by extrusion through nanoporous filters as a model of convex membranes with accessible outer surface. For concave structures we developed nanopore-confined tubular lipid bilayers with tunable curvature radii ranging from ca. 40 to 250 nm. The nanotubular bilayers are formed by lipid self-assembly inside cylindrical nanopores of anodic aluminum oxide (AAO). These systems impose essentially no restrictions on the lipid composition and have the inner surface of negative curvature accessible for solute molecules and peptide binding. Using these model structures we show that the surface electrostatic potential of lipid bilayers is affected by the curvature to a rather large degree. For example, for SUVs composed of negatively charged lipids the magnitude of the surface potential increased with bilayer bending from ca. −106 mV for 100 nm SUV to −166 mV for 30 nm SUVs. These measurements were carried out by spin probe EPR method using recently synthesized lipids having pH-reporting nitroxides covalently tethered to the lipid polar head. EPR titration experiments were followed by the measurement of the lipid vesicle electrophoretic mobility and peptide binding. Overall, the data indicate that the bilayer bending affects the local electrostatic potential in a rather large degree providing a likely biophysical mechanism for affecting protein binding to lipid membranes without adjusting lipid composition. Supported by U.S. DOE Contract DE-FG02-02ER15354.

Modeling Stem/Progenitor Cell-Induced Neovascularization and Oxygenation Around Solid Implants

Tissue engineering constructs and other solid implants with biomedical applications, such as drug delivery devices or bioartificial organs, need oxygen (O(2)) to function properly. To understand better the vascular integration of such devices, we recently developed a novel model sensor containing O(2)-sensitive crystals, consisting of a polymeric capsule limited by a nanoporous filter. The sensor was implanted in mice with hydrogel alone (control) or hydrogel embedded with mouse CD117/c-kit+ bone marrow progenitor cells in order to stimulate peri-implant neovascularization. The sensor provided local partial O(2) pressure (pO(2)) using noninvasive electron paramagnetic resonance signal measurements. A consistently higher level of peri-implant oxygenation was observed in the cell-treatment case than in the control over a 10-week period. To provide a mechanistic explanation of these experimental observations, we present in this article a mathematical model, formulated as a system of coupled partial differential equations, that simulates peri-implant vascularization. In the control case, vascularization is considered to be the result of a foreign body reaction, while in the cell-treatment case, adipogenesis in response to paracrine stimuli produced by the stem cells is assumed to induce neovascularization. The model is validated by fitting numerical predictions of local pO(2) to measurements from the implanted sensor. The model is then used to investigate further the potential for using stem cell treatment to enhance the vascular integration of biomedical implants. We thus demonstrate how mathematical modeling combined with experimentation can be used to infer how vasculature develops around biomedical implants in control and stem cell-treated cases.

THE DEPENDENCE OF ALUMINA NANOWIRE FORMATION FROM POROUS ANODIC ALUMINA MEMBRANES ON THE ETCHING SOLUTION

We describe variations in the structure and formation mechanism of alumina nanowires (ANWs) formed from porous anodic alumina (PAA) membranes, depending on the composition of the etching solution. Two types of ANWs were synthesized by etching PAA membranes in etching solutions containing H 3 PO 4 or NaOH . Not only did the ANW formation mechanism vary, but also the morphological properties and the surface chemical composition of the ANWs varied with the etching solution used. These results suggest that the ANW surfaces may be optimized by the appropriate selection of etching solution. The optimization of ANWs may potentially advance, for example, water purification technological developments because ANWs have, in recent studies, provided more effective filtration performances than nanoporous alumina membrane filters.