Three-dimensional Nanofiber Network Research Articles

Catalytic activity of Cu2O nanoparticles supported on cellulose beads prepared by emulsion–gelation using cellulose/LiBr solution and vegetable oil

Nanocatalysts tend to aggregate and are difficult to recycle, limiting their practical applications. In this study, an environmentally friendly method was developed to produce cellulose beads for use as supporting materials for Cu-based nanocatalysts. Cellulose beads were synthesized from a water-in-oil emulsion using cellulose dissolved in an LiBr solution as the water phase and vegetable oil as the oil phase. Upon cooling, the gelation of the cellulose solution produced spherical cellulose beads, which were then oxidized to introduce surface carboxyl groups. These beads (diameter: 95–105 μm; specific surface area: 165–225 m2 g−1) have a three-dimensional network of nanofibers (width: 20–30 nm). Furthermore, the Cu2O nanoparticles were loaded onto oxidized cellulose beads before testing their catalytic activity in the reduction of 4-nitrophenol using NaBH4. The apparent reaction rate constant increased with increasing loading of Cu2O nanoparticles and the conversion efficiency was >90 %. The turnover frequency was 376.2 h−1 for the oxidized cellulose beads with the lowest Cu2O loading, indicating a higher catalytic activity compared to those of other Cu-based nanoparticle-loaded materials. In addition to their high catalytic activity, the cellulose beads are reusable and exhibit excellent stability.

Self-Exfoliation of Flake Graphite for Bioinspired Compositing with Aramid Nanofiber toward Integration of Mechanical and Thermoconductive Properties

HighlightsA self-grinding exfoliation strategy that depends on mutual shear friction between flake graphite particles is successfully developed to prepare pristine graphene with largely enhanced yield and productivity.Bioinspired assembly of pristine graphene nanosheets to an interconnected aramid nanofiber network is achieved by a continuous sol-gel-film transformation strategy and generates a flexible yet highly thermoconductive film.Flexible yet highly thermoconductive materials are essential for the development of next-generation flexible electronic devices. Herein, we report a bioinspired nanostructured film with the integration of large ductility and high thermal conductivity based on self-exfoliated pristine graphene and three-dimensional aramid nanofiber network. A self-grinding strategy to directly exfoliate flake graphite into few-layer and few-defect pristine graphene is successfully developed through mutual shear friction between graphite particles, generating largely enhanced yield and productivity in comparison to normal liquid-based exfoliation strategies, such as ultrasonication, high-shear mixing and ball milling. Inspired by nacre, a new bioinspired layered structural design model containing three-dimensional nanofiber network is proposed and implemented with an interconnected aramid nanofiber network and high-loading graphene nanosheets by a developed continuous assembly strategy of sol–gel-film transformation. It is revealed that the bioinspired film not only exhibits nacre-like ductile deformation behavior by releasing the hidden length of curved aramid nanofibers, but also possesses good thermal transport ability by directionally conducting heat along pristine graphene nanosheets.Graphical abstract

Praseodymium based double-perovskite cathode nanofibers for intermediate temperature solid oxide fuel cells (IT-SOFC)

In this work, praseodymium based double perovskite oxide, PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF) cathode nanofibers were fabricated by electrospinning for intermediate-temperature solid oxide fuel cell (IT-SOFC). The three-dimensional nanofiber network cathode provides high porosity coupled with the large interfacial contact areas, leading to an improved oxygen reduction reaction (ORR) kinetics. The nanofiber cathode showed a polarization resistance of ∼0.025 Ω cm2 at 750 °C. The resulted low resistance is smaller than the commercial LSCF cathode showing polarization resistance of ∼0.041 Ω cm2 at 750 °C. The peak power densities of the PBSCF fibers are ∼2539, 1580 and 993 mW/cm2 at 750, 700, and 650 °C, respectively when compared to LSCF cathode performance of ∼1974, 1304 and 769 mW/cm2 at same temperature. Such promising performance of fiber cathode IT-SOFCs in this study was achieved via careful selection of the catalyst structure and composition, electrode structure for efficient ORR, and through process optimizations. The results obtained in this study illustrates that one-dimensional nanostructures obtained by adopting nanofibers could be an appropriate cathode material choice for intermediate-temperature SOFCs suitable for high power density applications such in automotive, stationary, APU and aviation fields.

Pr-Based Nanofiber Cathode for Intermediate Temperature SOFCs

In this work, praseodymium based double perovskite (PrBa0.5Sr0.5Co1.5Fe0.5O5+\U0001d6ff) - PBSCF nanofibers cathodes were fabricated by electrospinning technique for intermediate-temperature solid oxide fuel cells (IT-SOFCs).The optimized three-dimensional nanofiber network cathode provides high porosity coupled with the higher interfacial contact area, leading to a faster oxygen reduction reaction (ORR) kinetics.EIS analysis of the nanofiber structured cathode resulted in a polarization resistance of ~0.025 Ω cm2 at 750 °C. Resulted low resistance is smaller than the commercial LSCF cathode showing polarization resistance of ~0.041 Ω cm2 at 750 °C. The peak power densities of the PBSCF fibers are ~2539, 1580 and 993 mW/ cm2 at 750, 700, and 650 °C, respectively when compared to LSCF cathode performance of ~ 1974, 1304 and 769 mW/ cm2 at same temperature. Such promising performance of fiber cathode IT-SOFCs in this study was achieved via careful selection of the catalyst structure and composition, electrode structure for efficient ORR, and perhaps through process optimizations. The results obtained in this study illustrates that one-dimensional nanostructures obtained by adopting nanofibers can be an appropriate cathode material choice for intermediate-temperature SOFCs suitable for automotive applications.

Sustainable Cellulose-Nanofiber-Based Hydrogels.

Hydrogel materials have many excellent properties and a wide range of applications. Recently, a new type of hydrogel has emerged: cellulose nanofiber (CNF)-based hydrogels, which have three-dimensional nanofiber networks and unique physical properties. Because CNFs are abundant, renewable, and biodegradable, they are green and eco-friendly nanoscale building blocks. In addition, CNF-based hydrogel materials exhibit excellent mechanical properties and designable functions by different preparation methods and structure designs, demonstrating huge development potential. In this Perspective, we summarize the recent progress in the development of CNF-based hydrogels and introduce their applications in elastic hydrogels, ionic conduction, water purification, and biomedicine, highlighting future trends and opportunities for the further development of CNF-based hydrogels as emerging materials systems.

Cellulose Nanofiber-Reinforced Ionic Conductors for Multifunctional Sensors and Devices.

Ionic conductors are normally prepared from water-based materials in the solid form and feature a combination of intrinsic transparency and stretchability. The sensitivity toward humidity inevitably leads to dehydration or deliquescence issues, which will limit the long-term use of ionic conductors. Here, a novel ionic conductor based on natural bacterial cellulose (BC) and polymerizable deep eutectic solvents (PDESs) is developed for addressing the abovementioned drawbacks. The superstrong three-dimensional nanofiber network and strong interfacial interaction endow the BC-PDES ionic conductor with significantly enhanced mechanical properties (tensile strength of 8 × 105 Pa and compressive strength of 6.68 × 106 Pa). Furthermore, compared to deliquescent PDESs, BC-PDES composites showed obvious mechanical stability, which maintain good mechanical properties even when exposed to high humidity for 120 days. These materials were demonstrated to possess multiple sensitivity to external stimulus, such as strain, pressure, bend, and temperature. Thus, they can easily serve as supersensitive sensors to recognize physical activity of humans such as limb movements, throat vibrations, and handwriting. Moreover, the BC-PDES ionic conductors can be used in flexible, patterned electroluminescent devices. This work provides an efficient strategy for making cellulose-based sustainable and functional ionic conductors which have broad application in artificial flexible electronics and other products.

Metal ions modulation of the self-assembly of short peptide conjugated nonsteroidal anti-inflammatory drugs (NSAIDs).

Metal ions are essential components that help maintain the processes of normal life, and they can be used to fabricate self-assembled building blocks for peptide derivatives, proteins and nucleic acids. Here, we have developed a novel strategy to construct supramolecular hydrogels modulated using metal cations. Upon introducing a variety of metal ions into aqueous solutions of a gelator (naproxen-FF), including a nonsteroidal anti-inflammatory drug (NSAID) and dipeptide, we obtain stable hydrogels under neutral or alkaline conditions. It is found that these hydrogels with three-dimensional nanofiber networks exhibit excellent mechanical properties and thixotropy, as well as superb responsivity to multiple metal ions. Due to the significance of potassium ions in biological processes, the K-triggered hydrogel has been chosen as a model, and its self-assembly mechanism has been explored via various spectral analysis processes. In addition, the self-assembly performances of peptides are significantly affected by the chemical structures of the gelator molecules. This work provides deep insight into the aggregation mechanism of dipeptide-conjugating drug molecules through introducing a variety of metal ions, laying the foundation for further biological applications.

Metal-Organic Gels Derived from Iron(III) and Pyridine Ligands: Morphology, Self-Healing and Catalysis for Ethylene Selective Dimerization.

Metal-organic gels showing potential application in catalysis have received much concern. In this work, we designed and synthesized two metal-organic gels based on coordination between FeIII and pyridine ligands at room temperature. The gels were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to reveal their assembly structures and morphologies, and it was found the metal-organic gel derived from di-topic ligand was composed of three-dimensional network of nanofibers, while the gel derived from tri-topic ligand was constituted of sponge-like structure with amorphous phase. Rheological analysis showed the gel consisting of nanofiber networks displayed self-healing property. The gels were used as catalysts for selective ethylene dimerization, and the optimum catalysis results of the gel with nanofibers reached the maximal catalytic activity of 1.48×105 g/(mol Fe⋅h) with C4 yield more than 90 %, whereas the sponge-like gel only gave 38 % C4 products at the same condition. The higher dimerization selectivity of the former FeIII gel was attributed to its regular assembly structure and lower steric hindrance of the surface metal sites. Due to its catalytic activity, high selectivity and preparation simplicity, the FeIII gel might be potentially applicable for the preparation of C4 α-olefins.

Directed self-assembly of herbal small molecules into sustained release hydrogels for treating neural inflammation

Self-assembling natural drug hydrogels formed without structural modification and able to act as carriers are of interest for biomedical applications. A lack of knowledge about natural drug gels limits there current application. Here, we report on rhein, a herbal natural product, which is directly self-assembled into hydrogels through noncovalent interactions. This hydrogel shows excellent stability, sustained release and reversible stimuli-responses. The hydrogel consists of a three-dimensional nanofiber network that prevents premature degradation. Moreover, it easily enters cells and binds to toll-like receptor 4. This enables rhein hydrogels to significantly dephosphorylate IκBα, inhibiting the nuclear translocation of p65 at the NFκB signalling pathway in lipopolysaccharide-induced BV2 microglia. Subsequently, rhein hydrogels alleviate neuroinflammation with a long-lasting effect and little cytotoxicity compared to the equivalent free-drug in vitro. This study highlights a direct self-assembly hydrogel from natural small molecule as a promising neuroinflammatory therapy.

Biocompatible Electroactive Tetra(aniline)-Conjugated Peptide Nanofibers for Neural Differentiation.

Peripheral nerve injuries cause devastating problems for the quality of patients' lives, and regeneration following damage to the peripheral nervous system is limited depending on the degree of the damage. Use of nanobiomaterials can provide therapeutic approaches for the treatment of peripheral nerve injuries. Electroactive biomaterials, in particular, can provide a promising cure for the regeneration of nerve defects. Here, a supramolecular electroactive nanosystem with tetra(aniline) (TA)-containing peptide nanofibers was developed and utilized for nerve regeneration. Self-assembled TA-conjugated peptide nanofibers demonstrated electroactive behavior. The electroactive self-assembled peptide nanofibers formed a well-defined three-dimensional nanofiber network mimicking the extracellular matrix of the neuronal cells. Neurite outgrowth was improved on the electroactive TA nanofiber gels. The neural differentiation of PC-12 cells was more advanced on electroactive peptide nanofiber gels, and these biomaterials are promising for further use in therapeutic neural regeneration applications.

Bacterial Cellulose Supported Alumina Catalyst for Ethanol Dehydration

An ultrafine three-dimensional nanofiber network structure of a very high porosity endows bacterial cellulose (BC) to function as support for heterogeneous catalysis. A novel catalyst of BC supported alumina (Al2O3) was developed by soaking purified BC hydrogel in aluminum nitrate (Al(NO3)3) aqueous solution, dehydration (hot air drying and freeze-drying) and calcination. The Al/BC catalysts reveal interior meso–macro porous structures with average pore diameters in the range of 17–34 nm. The catalytic activities were examined through an ethanol dehydration reaction in the gas phase at atmospheric pressure in the range of 200–400 °C. The effects of acidic metal loading and dehydration methods were investigated. Increasing Al loading from 12 to 50% resulted in a decreased surface area but an increase in pore size. At the same Al loading, the catalysts with a dehydration process by hot air drying presented higher Al concentrations on the outer surface compared with those by freeze-drying. At high temperature of 400 °C, Al/BC catalysts with 25 wt% Al loading and dehydrated by freeze drying (25Al/BC FD) and 50 wt% Al loading, dehydrated by hot air drying (50Al/BC TD) exhibited the highest ethanol conversions of 65.7–66.4% and ethylene yields of 43.26–44.24%, respectively, whereas at low temperature of 200 °C, Al/BC catalysts with 25 wt% Al loading with either dehydration method exhibited the highest diethyl ether yields of 40.02–41.60%.

Aldehyde Approach to Hydrophobic Modification of Chitosan Aerogels.

Biobased nanofiber aerogels are ones of the attractive emerging materials in the fields of biochemistry and materials chemistry, but their poor humidity stability due to high hydrophilicity has limited their practical uses. In this paper, a new series of hydrophobic nanofibrous aerogels made from regenerated chitosan and alkyl aldehydes were prepared via a simple one-pot reaction followed by supercritical drying. Hexanal-modified chitosan aerogel shows excellent hydrophobicity with a water contact angle of ∼136°, a low density of 0.04-0.07 g cm-3, and structurally homogeneous three-dimensional nanofiber network at the nanoscale. Systematic investigations using various alkyl aldehydes revealed that pentanal-modified aerogel has similar high hydrophobicity and low density compared to the hexanal-modified material, while heptanal- and octanal-modified aerogels show drastic shrinkage during gelation. The aldehyde modification also suppresses permeation of water droplets into aerogel monoliths as well as reducing shrinkage under high humidity conditions.

Nanofiber-Structured TiO2 Nanocrystals as a Scattering Layer in Dye-Sensitized Solar Cells

We developed a scattering layer composed of TiO2 nanocrystals assembled into a densely packed three-dimensional network of nanofibers to localize light within a photoanode used in dye sensitized solar cells (DSSCs). The electro-netting approach was applied to obtain polyamide 6 nanofibers with bi-modal diameter distribution, followed by solvothermal synthesis for the coating of TiO2 nanocrystals on the polymer template. The resulting nanofiber-structured scattering layer (NFSL) is composed of TiO2 nanofibers (200–300 nm in diameter) supporting an ultrathin nanofiber network (diameters within 10–50 nm) and exhibits strong light scattering in the visible range (400 to 700 nm). This NFSL was applied on top of a transparent active TiO2 layer (TL) forming the photoanode in DSSCs. The performance of the bi-layered photoanode was compared to its analogue, fabricated with commercial scattering layers containing different sizes of nanoparticles. The DSSCs assembled with the NFSL showed an 18% enhancement in power conversion efficiency (PCE) compared to that of DSSCs whose photoanode contained only a TL. This enhancement factor was improved up to 31% when the bi-layered structure was post-treated with TiCl4. The PCE improvement was mainly associated with the light harvesting efficiency within the photoanode because of scattering from the NFSL and increased dye adsorption due to the addition of this top layer. These conclusions were inferred from diffuse reflectance behavior, dye loading measurements, external quantum efficiency and electrochemical properties. Our work demonstrates a promising approach without the requirement of time consuming and complicated procedures for the fabrication of a densely packed 3D nanofiber network scattering layer for diverse energy conversion devices and photocatalytic applications.

Mineralization of Calcium Carbonate on Multifunctional Peptide Assembly Acting as Mineral Source Supplier and Template

Crystal phase and morphology of biominerals may be precisely regulated by controlled nucleation and selective crystal growth through biomineralization on organic templates such as a protein. We herein propose new control factors of selective crystal growth by the biomineralization process. In this study, a designed β-sheet Ac-VHVEVS-CONH2 peptide was used as a multifunctional template that acted as mineral source supplier and having crystal phase control ability of calcium carbonate (CaCO3) during a self-supplied mineralization. The peptides formed three-dimensional nanofiber networks composed of assembled bilayer β-sheets. The assembly hydrolyzed urea molecules to one carbonate anion and two ammonium cations owing to a charge relay effect between His and Ser residues under mild conditions. CaCO3 was selectively mineralized on the peptide assembly using the generated carbonate anions on the template. Morphology of the obtained CaCO3 was fiber-like structure, similar to that of the peptide template. The mineralized CaCO3 on the peptide template had aragonite phase. This implies that CaCO3 nuclei, generated using the carbonate anions produced by the hydrolysis of urea on the surface of the peptide assembly, preferentially grew into aragonite phase, the growth axis of which aligned parallel to the direction of the β-sheet fiber axis.

Microrheology and Release Behaviors of Self-Assembled Steroid Hydrogels

A hydrogel is formed by the self-assembly of sodium deoxycholate (NaDC) in aqueous solution with sodium chloride at pH-7.0. The NaDC hydrogel made of the three-dimensional network of nanofibers shows pH-dependent swelling behaviors. Polystyrene particles with a diameter of 100 nm and doxorubicin hydrochloride (DOX) can be easily loaded into the NaDC hydrogel through swelling. By using the loaded polystyrene particles as a light scattering probe, we study the microrheology of the NaDC hydrogel, showing complex viscoelastic properties. The viscous component dominates at both low and high frequencies, while the elastic component dominates in the intermediate range. The cavity size of the nanofiber network can also be estimated to be ~180 nm. We show that the loaded DOX can be slowly released from the hydrogels into aqueous solution. The release profile of DOX is found to depend on the pH value of the solution.

Electrical conductivity of lanthanum oxide based composites containing carbon nanofibers

We have measured the electrical conductivity of lanthanum oxide based composite materials containing different concentrations of carbon nanofibers as additives. The conductivity has been shown to increase sharply (by two orders of magnitude) at carbon nanofiber contents from 2 to 3 wt % owing to the formation of a three-dimensional network of nanofibers in the bulk of the composite. Particular attention has been paid to the morphology of the particles of the constituent components of the composites and to the chemical and phase compositions of the matrix material.

A Novel Glucose/pH Responsive Low-Molecular-Weight Organogel of Easy Recycling

A new phenylboronic acid based gelator was developed to prepare low-molecular-weight organogel (LMOG), which could interact with several solvents to assemble into a three-dimensional nanofiber network. (1)H NMR spectroscopy study suggests that the driving force for the gelation includes hydrogen bonding and π-π stacking. Evaluated by UV-spectroscopy, the gel showed a prompt initial response to glucose at low concentration of 0.012 mmol/mL, which is a critical concentration of venous plasma glucose for diabetes. Significantly, this organogel exhibits excellent sensitivity to glucose among seven sugars tested (i.e., mannitol, galactose, lactose, maltose, sucrose, and fructose). The proposed formation of hydrogen-bonded complexes during the glucose sensing was supported by our energy calculation. Meanwhile, this organogel exhibits pH-response. Importantly, this LMOG could be conveniently recycled and thus be reused.

An intermediate-temperature solid oxide fuel cell with electrospun nanofiber cathode

Lanthanum strontium cobalt ferrite (LSCF) nanofibers have been fabricated by the electrospinning method and used as the cathode of an intermediate-temperature solid oxide fuel cell (SOFC) with yttria-stabilized zirconia (YSZ) electrolyte. The three-dimensional nanofiber network cathode has several advantages: (i) high porosity; (ii) high percolation; (iii) continuous pathway for charge transport; (iv) good thermal stability at the operating temperature; and (v) excellent scaffold for infiltration. The fuel cell with the monolithic LSCF nanofiber cathode exhibits a power density of 0.90 W cm−2 at 1.9 A cm−2 at 750 °C. The electrochemical performance of the fuel cell has been further improved by infiltration of 20 wt% of gadolinia-doped ceria (GDC) into the LSCF nanofiber cathode. The fuel cell with the LSCF–20% GDC composite cathode shows a power density of 1.07 W cm−2 at 1.9 A cm−2 at 750 °C. The results obtained show that one-dimensional nanostructures such as nanofibers hold great promise as electrode materials for intermediate-temperature SOFCs.

Functionality in Electrospun Nanofibrous Membranes Based on Fiber's Size, Surface Area, and Molecular Orientation.

Electrospinning is a versatile method for forming continuous thin fibers based on an electrohydrodynamic process. This method has the following advantages: (i) the ability to produce thin fibers with diameters in the micrometer and nanometer ranges; (ii) one-step forming of the two- or three-dimensional nanofiber network assemblies (nanofibrous membranes); and (iii) applicability for a broad spectrum of molecules, such as synthetic and biological polymers and polymerless sol-gel systems. Electrospun nanofibrous membranes have received significant attention in terms of their practical applications. The major advantages of nanofibers or nanofibrous membranes are the functionalities based on their nanoscaled-size, highly specific surface area, and highly molecular orientation. These functionalities of the nanofibrous membranes can be controlled by their fiber diameter, surface chemistry and topology, and internal structure of the nanofibers. This report focuses on our studies and describes fundamental aspects and applications of electrospun nanofibrous membranes.

Self-assembled, photoluminescent peptide hydrogel as a versatile platform for enzyme-based optical biosensors

A self-assembled peptide hydrogel consisting of Fmoc-diphenylalanine has been employed as a biosensing platform through the encapsulation of enzyme bioreceptors (e.g., glucose oxidase or horseradish peroxidase) and fluorescent reporters (e.g., CdTe and CdSe quantum dots). Enzymes and quantum dots (QDs) were physically immobilized within the hydrogel matrix in situ in a single step by simply mixing aqueous solution containing QDs and enzymes with monomeric peptide (Fmoc-diphenylalanine) solution. By using atomic force microscopy and scanning transmission electron microscopy, we observed that the self-assembled peptide hydrogel had a three-dimensional network of nanofibers (with a diameter of approximately 70–90 nm) that physically hybridized with QDs and encapsulated enzyme bioreceptors with a minimal leakage. We successfully applied the peptide hydrogel to the detection of analytes such as glucose and toxic phenolic compounds by using a photoluminescence quenching of the hybridized QDs. The Michaelis–Menten constant ( K M) of the photoluminescent peptide hydrogel was found to be 3.12 mM (GOx for glucose) and 0.82 mM (HRP for hydroquinone), respectively, which were much lower than those of conventional gel materials. These results suggest that the peptide hydrogel is an alternative optical biosensing platform with practical advantages such as simple fabrication via self-assembly, efficient diffusion of target analytes, and high encapsulation efficiencies for fluorescent reporters and bioreceptors.