Developing Monolithic Nanoporous Gold with Hierarchical Bicontinuity Using Colloidal Bijels

3D-Printing of Hierarchical Porous Copper-Based Metal-Organic-Framework Structures for Efficient Fixed-Bed Catalysts

Metallic structures with hierarchical open pores that span several orders of magnitude are ideal candidates for various catalyst applications. However, porous metal materials prepared using alloy/dealloy methods still struggle to achieve continuous pore distribution across a broad size range. Herein, we report a printable copper (Cu)/iron (Fe) composite ink that produces a hierarchical porous Cu material with pores spanning over 4 orders of magnitude. The manufacturing process involves four steps: 3D-printing, annealing, dealloying, and reannealing. Because of the unique annealing process, the resulting hierarchical pore surface becomes coated with a layer of Cu-Fe alloy. This feature imparts remarkable catalytic ability and versatile functionality within fixed bed reactors for 4-nitrophenol (4-NP) reduction and Friedländer cyclization. Specifically, for 4-NP reduction, the porous Cu catalyst demonstrates an excellent reaction rate constant (kapp = 86.5 × 10-3 s-1) and a wide adaptability of the substrate (up to 1.26 mM), whilst for Friedländer cyclization, a conversion over 95% within a retention time of only 20 min can be achieved by metal-organic-framework-decorated porous Cu catalyst. The utilization of dual metallic particles as printable inks offers valuable insights for fabricating hierarchical porous metallic structures for applications, such as advanced fixed-bed catalysts.

Nanoporous Gold: From Structure Evolution to Functional Properties in Catalysis and Electrochemistry

Nanoporous gold (NPG) is characterized by a bicontinuous network of nanometer-sized metallic struts and interconnected pores formed spontaneously by oxidative dissolution of the less noble element from gold alloys. The resulting material exhibits decent catalytic activity for low-temperature, aerobic total as well as partial oxidation reactions, the oxidative coupling of methanol to methyl formate being the prototypical example. This review not only provides a critical discussion of ways to tune the morphology and composition of this material and its implication for catalysis and electrocatalysis, but will also exemplarily review the current mechanistic understanding of the partial oxidation of methanol using information from quantum chemical studies, model studies on single-crystal surfaces, gas phase catalysis, aerobic liquid phase oxidation, and electrocatalysis. In this respect, a particular focus will be on mechanistic aspects not well understood, yet. Apart from the mechanistic aspects of catalysis, best practice examples with respect to material preparation and characterization will be discussed. These can improve the reproducibility of the materials property such as the catalytic activity and selectivity as well as the scope of reactions being identified as the main challenges for a broader application of NPG in target-oriented organic synthesis.

Bicontinuous interfacially jammed emulsion gels with nearly uniform sub-micrometer domains via regulated co-solvent removal

Porous materials possess numerous useful functions because of their high surface area and ability to modulate the transport of heat, mass, fluids, and electromagnetic waves. Unlike highly ordered structures, disordered porous structures offer the advantages of ease of fabrication and high fault tolerance. Bicontinuous interfacially jammed emulsion gels (bijels) are kinetically trapped disordered biphasic materials that can be converted to porous materials with tunable features. Current methods of bijel fabrication result in domains that are micrometers or larger, and non-uniform in size, limiting their surface area, mechanical strength, and interaction with electromagnetic waves. In this work, scalable synthesis of bijels with uniform and sub-micrometer domains is achieved via a two-step solvent removal process. The resulting bijels are characterized quantitatively to verify the uniformity and sub-micrometer scale of the domains. Moreover, these bijels have structures that resemble the microstructure of the scale of the white beetle Cyphochilus. We find that such bijel films with relatively small thicknesses (<150 μm) exhibit strong solar reflectance as well as high brightness and whiteness in the visible range. Considering their scalability in manufacturing, we believe that VIPS-STRIPS bijels have great potential in large-scale applications including passive cooling, solar cells, and light emitting diodes (LEDs).

Enhanced Capillary Wicking through Hierarchically Porous Constructs Derived from Bijel Templates

Liquid capillarity through porous media can be enhanced by a rational design of hierarchically porous constructs that suggest sufficiently large liquid pathways from an upper-level hierarchy as well as capillary pressure enabled by a lower hierarchy. Here, we demonstrate a material design strategy utilizing a new class of self-assembled soft materials, called bicontinuous interfacially jammed emulsion gels (bijels), to produce hierarchically porous copper, which enables the unique combination of unprecedented control over both macropores and mesopores in a regular, uniform, and continuous arrangement. The dynamic droplet topologies on the hierarchically copper pores prove the significant enhancement in liquid capillarity compared to homogeneous porous structures. The role of nanoscale morphology in liquid infiltration is further investigated through environmental scanning electron microscopy, in which wetting through the mesopores occurs at the beginning, followed by liquid transport through macropores. This understanding on capillary wicking will allow us to design better hierarchically porous media that can address performance breakthroughs in interfacial applications, ranging from battery electrodes, cell delivery in biomedical devices, to capillary-fed thermal management systems.

Facile fabrication of hierarchically nanostructured gold electrode for bio-electrochemical applications

Nanoporous gold (NPG) is one of the most extensively investigated nanomaterials owing to its tunable pore size, ease of surface modification, and range of applications from catalysis, actuation, and molecular release to the development of electrochemical sensors. In an effort to improve the usefulness of NPG, a simple and robust method for the fabrication of hierarchical and bimodal nanoporous gold electrodes (hb-NPG) containing both macro-and mesopores is reported using electrochemical alloying and dealloying processes to engineer a bicontinuous solid/void morphology. Scanning electron microscopy (color SEM) images depict the hierarchical pore structure created after the multistep synthesis with an ensemble of tiny pores below 100 nm in size located in ligaments spanning larger pores of several hundred nanometers. Smaller-sized pores are exploited for surface modification, and the network of larger pores aids in molecular transport. Cyclic voltammetry (CV) was used to compare the electrochemically active surface area of the hierarchical bimodal structure with that of the regular unimodal NPG with an emphasis on the critical role of both dealloying and annealing in creating the desired structure. The adsorption of different proteins was followed using UV-vis absorbance measurements of solution depletion revealing the high loading capacity of hb-NPG. The surface coverage of lipoic acid on the hb-NPG was analyzed using thermogravimetric analysis (TGA) and reductive desorption. The roughness factor determinations suggest that the fabricated hb-NPG electrode has tremendous potential for biosensor development by changing the scaling relations between volume and surface area which may lead to improved analytical performance. We have chosen to take advantage of the surface architectures of hb-NPG due to the presence of a large specific surface area for functionalization and rapid transport pathways for faster response. It is shown that the hb-NPG electrode has a higher sensitivity for the amperometric detection of glucose than does an NPG electrode of the same geometric surface area.

Bijel rheology reveals a 2D colloidal glass wrapped in 3D

We present rheological evidence demonstrating the glass-like nature of bicontinuous interfacially jammed emulsion gels (bijels). Under small amplitude oscillatory shear, bijels exhibited rheological signatures akin to α and β relaxation that are also invariable to interfacial tension changes, behaviors which are reminiscent of caged particle dynamics found in colloidal glasses, and well described by a previously reported adaptation of mode-coupling theory for colloidal glass rheology. Guided by their rheological signatures and supported by particle detachment and attraction energy approximations, we rationalize that bijels can be represented as 2-dimensional (2D) colloidal glasses that percolate in 3-dimensional (3D) space, and attractive interactions are not required for their stability. To provide further support for this conjecture, we qualitatively compare the rheology of bijels and a capillary suspension that is stabilized by strong, rigid capillary bridges between the particles, beyond their limit of linear viscoelasticity. Our results demonstrate that the strong adsorption of particles to the continuous interface and the lack of strong attractive interparticle forces enable recovery by interfacial tension into new jammed configurations after shear deformation. These behaviors are qualitatively different from those in the capillary suspension, where the breaking of attractive interparticle bonds results in dramatic changes to the microstructure and rheology over a narrow range of shear amplitudes. Our findings unveil bijels as 2D colloidal glasses weaving in 3D space and establish that interparticle attractions are not required for stability in bijels, and interfacial jamming alone is sufficient to impart viscoelasticity and gel-like rheology to these materials.

Towards tuning the modality of hierarchical macro-nanoporous metals by controlling the dealloying kinetics of close-to-eutectic alloys

Largely inspired by nature, hierarchical porous materials are attractive for a wide range of applications as they provide a unique combination of transport and interfacial properties. Hierarchical macro-nanoporous metals (HMNPM) are of particular interest due to their high thermal and electrical conductivities, high volumetric macroporosity as well as their strong capillary forces, and large surface area due to their nanopores. However, tuning the porosity of HMNPMs remains challenging and often requires complex multi-step synthesis methods. Here we demonstrate that controlling the dealloying kinetics of close-to-eutectic alloys allows the selective tuning of the porosity of a hierarchical metal from tens of nanometers to hundreds of micrometers. This was demonstrated by dealloying the Cu-Mg-Zn alloy of close-to-eutectic composition to develop trimodal hierarchical macro-nanoporous copper with an impressive porosity of 94 vol% in the form of macroscopic self-supporting bulk samples. A combination of dealloying experiments and density functional theory calculations indicate that while selective corrosion of chemical phases in the Cu-Mg-Zn alloy is triggered according to their Volta potential, the kinetics can be altered by confinement and non-homogeneity effects. The obtained insights into the kinetics of close-to-eutectic alloy dealloying can be used to develop other hierarchical porous metals with tunable porosity and controlled shape.

Phase Inversion-Based Technique for Fabricating Bijels and Bijels-Derived Structures with Tunable Microstructures

Bicontinuous interfacially jammed emulsion gels ("bijels") are a new class of soft matter containing two interpenetrating continuous phases. They have great potential for applications in many areas. However, difficulties in fabricating bijels and controlling structural features of interest have posed severe barriers to their wide applications. In this study, a phase inversion-based technique was developed for fabricating bijels and bijels-derived structures. The effects of varying the composition of casting solutions for the fabrication of bijels on the porosity, oil-to-water percentage, and domain size of bijels were investigated. Composite bijels prepared from two organic monomers were also made, demonstrating the flexibility of the phase inversion-based technique for the fabrication of bijels. Interestingly, the incorporation of a second monomer into the casting solution also affected the porosity and domain size of bijels formed, which may provide a new strategy for the controlled fabrication of bijels. Doxorubicin hydrochloride (DOX, as a model drug)-loaded bijels-derived hybrid hydrogels comprising two continuous phases were successfully made, with one phase being cross-linked alginate that carried the drug. Controlled release of DOX from the bijels-derived structures could be achieved. In vitro degradation study indicated that cross-linking of alginate in bijels-derived hybrid hydrogels controlled alginate degradation, thereby affecting the DOX release behavior. Our current work has provided a facile and reproducible protocol for the controlled fabrication of bijels and bijels-derived structures, which facilitates expanding their applications in the biomedical field.

Rheological Behavior and in Situ Confocal Imaging of Bijels Made by Mixing

Bijels (bicontinuous interfacially jammed emulsion gels) have the potential to be useful in many different applications due to their internal connectivity and the possibility of efficient mass transport through the channels. Recently, new methods of making the bijel have been proposed, which simplify the fabrication process, making commercial application more realistic. Here, we study the flow properties of bijels prepared by mixing alone using oscillatory rheology combined with confocal microscopy and also squeezing flow experiments. We found that the bijel undergoes a two-step yielding process where the first step corresponds to the fluidizing of the interface, allowing the motion of the structure, and the second step corresponds to the breaking of the structure. In the squeeze flow experiments, the yield stress of the bijel is observed to show a power law dependence on squeezing speed. However, when stress in excess of yield stress is plotted against shear rate, all the different squeeze flow data show a superposition.

Bijel-templated implantable biomaterials for enhancing tissue integration and vascularization

Mitigation of the foreign body response (FBR) and successful tissue integration are essential to ensuring the longevity of implanted devices and biomaterials. The use of porous materials and coatings has been shown to have an impact, as the textured surfaces can mediate macrophage interactions with the implant and influence the FBR, and the pores can provide space for vascularization and tissue integration. In this study, we use a new class of implantable porous biomaterials templated from bicontinuous interfacially jammed emulsion gels (bijels), which offer a fully percolating, non-constricting porous network with a uniform pore diameter on the order of tens of micrometers, and surfaces with consistent curvature. We demonstrate that these unique morphological features, inherent to bijel-templated materials (BTMs), can enhance tissue integration and vascularization, and reduce the FBR. Cylindrical polyethylene glycol diacrylate (PEGDA) BTMs, along with PEGDA particle-templated materials (PTMs), and non-templated materials (NTMs), were implanted into the subcutaneous space of athymic nude mice. After 28 days, implants were retrieved and analyzed via histological techniques. Within BTMs, blood vessels of increased size and depth, changes in collagen deposition, and increased presence of pro-healing macrophages were observed compared to that of PTM and NTM implants. Bijel templating offers a new route to biomaterials that can improve the function and longevity of implantable devices. STATEMENT OF SIGNIFICANCE: All implanted biomaterials are subject to the foreign body response (FBR) which can have a detrimental effect on their efficacy. Altering the surface chemistry can decrease the FBR by limiting the amount of proteins adsorbed to the implant. This effect can be enhanced by including pores in the biomaterial to allow new tissue growth as the implant becomes integrated in the body. Here, we introduce a new class of self-assembled biomaterials comprising a fully penetrating, non-constricting pore phase with hyperbolic (saddle) surfaces for enhanced tissue integration. These unique morphological characteristics result in dense blood vessel formation and favorable tissue response properties demonstrated in a four-week implantation study.

Hierarchical Nanoporous Copper Architectures via 3D Printing Technique for Highly Efficient Catalysts

Nanoporous metals represent a class of functional materials with unique bicontinuous open porous structural properties, making them ideal candidates for various catalyst applications. However, the pursuit of nanoporous properties, extremely small pores, and high surface area, results in the restriction of mass transport. Herein, a free-standing hierarchical nanoporous Cu material, prepared by a selective laser melting 3D printing technique and a one-step dealloying process, is presented as a highly efficient electrocatalyst for methanol oxidation. It is demonstrated that the digitally controlled hierarchical structure with macro- and nano-scaled pores can be utilized for promoting and directing mass transport as well as for the enhancement of catalytic properties. This work highlights a facile, low-cost, and alternative strategy for hierarchical nanoporous structure design that can be applied to binary, ternary, and quaternary metal alloys for various functional applications.

Nanoporous Metallic Networks: Fabrication, Optical Properties, and Applications

Nanoporous metallic networks are a group of porous materials made of solid metals with suboptical wavelength sizes of both particles and voids. They are characterized by unique optical properties, as well as high surface area and permeability of guest materials. As such, they attract a great focus as novel materials for photonics, catalysis, sensing, and renewable energy. Their properties together with the ability for scaling-up evoke an increased interest also in the industrial field. Here, fabrication techniques of large-scale metallic networks are discussed, and their interesting optical properties as well as their applications are considered. In particular, the focus is on disordered systems, which may facilitate the fabrication technique, yet, endow the three-dimensional (3D) network with distinct optical properties. These metallic networks bridge the nanoworld into the macroscopic world, and therefore pave the way to the fabrication of innovative materials with unique optoelectronic properties.

Jammed Limit of Bijel Structure Formation

Over the past decade, methods to control microstructure in heterogeneous mixtures by arresting spinodal decomposition via the addition of colloidal particles have led to an entirely new class of bicontinuous materials known as bijels. Herein, we present a new model for the development of these materials that yields to both numerical and analytical evaluation. This model reveals that a single dimensionless parameter that captures both chemical and environmental variables dictates the dynamics and ultimate structure formed in bijels. We demonstrate that this parameter must fall within a fixed range in order for jamming to occur during spinodal decomposition, as well as show that known experimental trends for the characteristic domain sizes and time scales for formation are recovered by this model.

Self-assembly of gold nanowire networks into gold foams: production, ultrastructure and applications

Fine-tuning the shape of nanostructured materials through easy and sustainable methods is a challenging task for green nanotechnology.

Compressing a spinodal surface at fixed area: bijels in a centrifuge

Bicontinuous interfacially jammed emulsion gels (bijels) are solid-stabilised emulsions with two inter-penetrating continuous phases. Employing the method of centrifugal compression we find that macroscopically the bijel yields at relatively low angular acceleration. Both continuous phases escape from the top of the structure, making any compression immediately irreversible. Microscopically, the bijel becomes anisotropic with the domains aligned perpendicular to the compression direction which inhibits further liquid expulsion; this contrasts strongly with the sedimentation behaviour of colloidal gels. The original structure can, however, be preserved close to the top of the sample and thus the change to an anisotropic structure suggests internal yielding. Any air bubbles trapped in the bijel are found to aid compression by forming channels aligned parallel to the compression direction which provide a route for liquid to escape.

Closed-cell and open-cell porous polymers from ionomer-stabilized high internal phase emulsions

We firstly present a strategy that enables fabrication of both closed-cell and open-cell porous polymers (polyHIPEs) from high internal phase emulsions (HIPEs) stabilized with an ionomer.

Selective capture of glycoproteins using lectin-modified nanoporous gold monolith

The surface of nanoporous gold (np-Au) monoliths was modified via a flow method with the lectin Concanavalin A (Con A) to develop a substrate for separation and extraction of glycoproteins. Self-assembled monolayers (SAMs) of α-lipoic acid (LA) on the np-Au monoliths were prepared followed by activation of the terminal carboxyl groups to create amine reactive esters that were utilized in the immobilization of Con A. Thermogravimetric analysis (TGA) was used to determine the surface coverages of LA and Con A on np-Au monoliths which were found to be 1.31×10(18) and 1.85×10(15)moleculesm(-2), respectively. An in situ solution depletion method was developed that enabled surface coverage characterization without damaging the substrate and suggesting the possibility of regeneration. Using this method, the surface coverages of LA and Con A were found to be 0.989×10(18) and 1.32×10(15)moleculesm(-2), respectively. The selectivity of the Con A-modified np-Au monolith for the high mannose-containing glycoprotein ovalbumin (OVA) versus negative control non-glycosylated bovine serum albumin (BSA) was demonstrated by the difference in the ratio of the captured molecules to the immobilized Con A molecules, with OVA:Con A=2.3 and BSA:Con A=0.33. Extraction of OVA from a 1:3 mole ratio mixture with BSA was demonstrated by the greater amount of depletion of OVA concentration during the circulation with the developed substrate. A significant amount of captured OVA was eluted using α-methyl mannopyranoside as a competitive ligand. This work is motivated by the need to develop new materials for chromatographic separation and extraction substrates for use in preparative and analytical procedures in glycomics.

Numerical simulations of bijel morphology in thin films with complete surface wetting

Bijels are a relatively new class of soft materials that have many potential energy and environmental applications. In this work, simulation results of bijel evolution confined within thin films with preferential surface wetting are presented. The computational approach used is a hybrid Cahn-Hilliard/Brownian dynamics method. In the absence of suspended particles, we demonstrate that the model accurately captures the rich kinetics associated with diffusion-based surface-directed spinodal decomposition, as evidenced by comparison with previous theoretical and simulation-based studies. When chemically neutral particles are included in the films, the simulations capture surface-modified bijel formation, with stabilized domain structures comparable with the experimental observations of Composto and coworkers. Namely, two basic morphologies - bicontinuous or discrete - are seen to emerge, with direct dependence on the film thickness, particle volume fraction, and particle radius.

Stabilizing bijels using a mixture of fumed silica nanoparticles

We demonstrate the fabrication of bicontinuous pickering emulsions (bijels) using “off the shelf” particles.

Porous Gold Films-A Short Review on Recent Progress

Porous gold films have attracted increasing interest over the last ten years due to the unique properties of high specific surface area and electrical conductivity combined with chemical stability and ability to alter the surface chemistry. Several methods have been developed to synthesize porous gold films such as de-alloying, templating, electrochemical, and self-assembling. These porous gold films are used in diverse fields, for example, as electrochemical and Raman sensors or for chemical catalysis. Here, we provide a short review on the progress of porous gold films over the past ten years, including the synthesis and applications of such films.