Graphene oxide and graphene flakes as stabilizers and dispersing aids

Preparation and characterization of silanized graphene oxide based polyacrylate composites in situ copolymerization

In the present work, graphene oxide (GO) was initially prepared by the modified Hummers' method and then surface modification with 3-Methacryloxypropyltrimeth- oxysilane (MPS) was carried out. The silanized GO based polyacrylate (PA) composite emulsion was fabricated via in situ copolymerization. The covalent bonds formed between GO and PA matrix were proposed to improve the dispersion of MPS-GO in composites. FTIR spectra, Raman spectra, XPS and XRD data confirmed that oxidation and modification were occurred, and oxygen-containing functional groups and CC groups were introduced on the side of GO, respectively. Two kinds of structures were observed in composite latexes, and the average diameter of composite latexes (107 nm) was larger than that of PA latexes (87 nm). FTIR spectra also disclosed that reactive MPS-GO had already successfully copolymerized with the PA matrix. AFM images demonstrated that wrinkled GO nanosheets were homogeneously dispersed and incorporated into the PA matrix. The water contact angle (WCA) was found increasing as the addition of MPS-GO, although the composite films exhibited obvious hydrophilicity with increasing the content of MPS-GO.

Fabrication of poly (styrene-acrylate)/silver nanoparticle-graphene oxide composite antibacterial by in situ Pickering emulsion polymerization

Graphene oxide (GO) nanosheets decorated with silver nanoparticles (AgNPs) were easily synthesized by chemical reduction, and the prepared nanocomposite was then used as a stabilizer in the Pickering emulsion polymerization of poly (styrene-acrylate) to prepare PSA/AgNPs-GO composites. The AgNPs-GO nanocomposites were fully characterized by TEM, FTIR, Raman, SEM and XPS, which demonstrated that about 5-30 nm AgNPs with spherical, octahedral and cubic structures were decorated on the surface of wrinkled GO nanosheets. TEM photos and EDS spectrum of composites showed that the transparent GO nanosheets decorated with AgNPs were covered on the surface of PSA latexes and the AgNPs were uniformly dispersed on the surface of the PSA latexes without aggregation. The average diameter of composite latexes was obviously bigger than that of PSA latexes. However, the role of surfactant and the properties of hydrophilicity decreased the average diameter and WCA of composites while increasing the additions of AgNPs-GO nanocomposites. AFM images disclosed that wrinkled GO nanosheets decorated with AgNPs dispersed on the surface of composite films. XPS data proved clearly that silver was present only in metallic form and migration occurred during film-formation. TGA curves confirmed the composite film displayed better thermal stability than that of PSA. The results of antibacterial activity revealed that composite films had exhibited antibacterial properties against both E. coli and S. aureus, and the latter showed better antibacterial efficacy than the former. The nano-silver polyacrylate coatings with antibacterial activity explored in current work have wide application in the fields of wood coatings, leather finishing, and so on.

Improved Dispersibility of Graphene in an Aqueous Solution by Reduced Graphene Oxide Surfactant: Experimental Verification and Density Functional Theory Calculation

It is difficult to disperse graphene flakes well in an aqueous solution while maintaining conductivity due to its high hydrophobicity. Herein, we demonstrated that a well-dispersed state of graphene in an aqueous solution was realized by using reduced graphene oxide (rGO) with a suitable content of oxygen-functional groups. A rGO-dispersed graphene (rGO/G) film was fabricated from the graphene dispersion with good conductivity by using rGO with a C/O ratio of 2.48 as the surfactant. Also, the prepared rGO/G aerogel has a broad prospect. Density functional theory calculation revealed that the strong electrostatic repulsion, which was more potent than the van der Waals force and the π-π interaction, was the primary driving force promoting the dispersibility of graphene in an aqueous solution. Furthermore, the repulsion of the rGO/G dispersion decreased with the reduction of the oxygen-functional groups of rGO. Therefore, applying rGO with an appropriate content of oxygen-functional groups is an alternative option to improve the dispersibility of graphene in an aqueous medium while maintaining its original properties, from which many potential applications could be expected.

3D bioprinted GelMA/GO composite induces osteoblastic differentiation

Graft substitute is a mature treatment tool in craniofacial bone repair. However, stress shielding and immutability of structure limit its use in patients with congenital defects. Therefore, a regenerative graft would be best suited for repair. Mesenchymal stem cells (MSCs) have been shown to be feasible in regenerative medicine and the clinical treatment of bone repair. The aim of this study was to propose a strategy that would directly blend graphene oxide (GO) and MSCs with gelatin methacrylate anhydride (GelMA), as bioink, to generate the scaffold for bone regenerative repair. The survival and osteogenic capacity of MSCs in the composite bioink were assessed by cell viability and proliferation assays, along with expression analysis of osteogenesis-related genes and proteins, and targeted immunofluorescence. The introduction of GO to the printing process had no influence on cell printing, viability, or printability of GelMa. However, the GO-involved structure exhibited a positive influence on MSC proliferation, without significantly affecting cell viability. Alkaline phosphatase was expressed more in cells cultured with GO than in those with pure GelMA. In addition, GO promoted the expression of osteogenesis-related genes and proteins, such as osteopontin, osteocalcin, and RUNX2. Collectively, the composite bioink enhanced cell proliferation and adhesion, as well as osteogenic differentiation properties, compared with pure GelMA.

Spontaneous surface adsorption of aqueous graphene oxide by synergy with surfactants

The spontaneous adsorption of graphene oxide (GO) sheets at the air-water interface is explored using X-ray reflectivity (XRR) measurements. As a pure aqueous dispersion, GO sheets do not spontaneously adsorb at the air-water interface due to their high negative surface potential (-60 mV) and hydrophilic functionality. However, when incorporated with surfactant molecules at optimal ratios and loadings, GO sheets can spontaneously be driven to the surface. It is hypothesised that surfactant molecules experience favourable attractive interactions with the surfaces of GO sheets, resulting in co-assembly that serves to render the sheets surface active. The GO/surfactant composites then collectively adsorb at the air-water interface, with XRR analysis suggesting an interfacial structure comprising surfactant tailgroups in air and GO/surfactant headgroups in water for a combined thickness of 30-40 Å, depending on the surfactant used. Addition of too much surfactant appears to inhibit GO surface adsorption by saturating the interface, and low loadings of GO/surfactant composites (even at optimal ratios) do not show significant adsorption indicating a partitioning effect. Lastly, surfactant chemistry is also a key factor dictating adsorption capacity of GO. The zwitterionic surfactant oleyl amidopropyl betaine causes marked increases in GO surface activity even at very low concentrations (≤0.2 mM), whereas non-ionic surfactants such as Triton X-100 and hexaethyleneglycol monododecyl ether require higher concentrations (ca. 1 mM) in order to impart spontaneous adsorption of the sheets. Anionic surfactants do not enhance GO surface activity presumably due to like-charge repulsions that prevent co-assembly. This work provides useful insight into the synergy between GO sheets and molecular amphiphiles in aqueous systems for enhancing the surface activity of GO, and can be used to inform system formulation for developing water-friendly, surface active composites based around atomically thin materials.

Bi-Functional Paraffin@Polyaniline/TiO2/PCN-222(Fe) Microcapsules for Solar Thermal Energy Storage and CO2 Photoreduction

A novel type of bi-functional microencapsulated phase change material (MEPCM) microcapsules with thermal energy storage (TES) and carbon dioxide (CO₂) photoreduction was designed and fabricated. The polyaniline (PANI)/titanium dioxide (TiO₂)/PCN-222(Fe) hybrid shell encloses phase change material (PCM) paraffin by the facile and environment-friendly Pickering emulsion polymerization, in which TiO₂ and PCN-222(Fe) nanoparticles (NPs) were used as Pickering stabilizer. Furthermore, a ternary heterojunction of PANI/(TiO₂)/PCN-222(Fe) was constructed due to the tight contact of the three components on the hybrid shell. The results indicate that the maximum enthalpy of MEPCMs is 174.7 J·g^-1 with encapsulation efficiency of 77.2%, and the thermal properties, chemical composition, and morphological structure were well maintained after 500 high-low temperature cycles test. Besides, the MEPCM was employed to reduce CO₂ into carbon monoxide (CO) and methane (CH₄) under natural light irradiation. The CO evolution rate reached up to 45.16 μmol g^-1 h^-1 because of the suitable band gap and efficient charge migration efficiency, which is 5.4, 11, and 62 times higher than pure PCN-222(Fe), PANI, and TiO₂, respectively. Moreover, the CO evolution rate decayed inapparently after five CO₂ photoreduction cycles. The as-prepared bi-functional MEPCM as the temperature regulating building materials and air purification medium will stimulate a potential application.

Osteoblastic differentiation of stem cells induced by graphene oxide-hydroxyapatite-alginate hydrogel composites and construction of tissue-engineered bone

This study aimed to investigate the effect of graphene oxide (GO)-hydroxyapatite (HA)-sodium alginate (SA) composite application in the field of bone tissue engineering. Four scaffold groups were established (SA-HA, SA-HA-0.8%GO, SA-HA-1.0%GO and SA-HA-1.2%GO) and mixed with bone marrow mesenchymal stem cells (BMSCs). Hydrogel viscosity was measured at room temperature, and after freeze-drying and Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) to detect substance crystallinity, the printability of each hydrogel type was measured with a printing grid. Scanning electron microscopy (SEM) was used to observe the internal microstructure of the scaffolds and to evaluate the growth and proliferation of cells on the scaffold. A hollow cylinder was printed to compare the forming effect of the hydrogel bioinks, and cell-hydrogel composites were implanted under the skin of nude mice to observe the effect of the hydrogels on osteogenesis in vivo. Increased GO concentrations led to reduced scaffold degradation rates, increased viscosity, increased printability, increased mechanical properties, increased scaffold porosity and increased cell proliferation rates. In vivo experiments showed that hematoxylin and eosin (HE) staining, Alizarin red staining, alkaline phosphatase staining and collagen type I immunohistochemical staining increased as the implantation time increased. These results demonstrate that GO composites have high printability as bioinks and can be used for bioprinting of bone by altering the ratio of the different components.

The True Amphipathic Nature of Graphene Flakes: A Versatile 2D Stabilizer

The fundamental colloidal properties of pristine graphene flakes remain incompletely understood, with conflicting reports about their chemical character, hindering potential applications that could exploit the extraordinary electronic, thermal, and mechanical properties of graphene. Here, the true amphipathic nature of pristine graphene flakes is demonstrated through wet-chemistry testing, optical microscopy, electron microscopy, and density functional theory, molecular dynamics, and Monte Carlo calculations, and it is shown how this fact paves the way for the formation of ultrastable water/oil emulsions. In contrast to commonly used graphene oxide flakes, pristine graphene flakes possess well-defined hydrophobic and hydrophilic regions: the basal plane and edges, respectively, the interplay of which allows small flakes to be utilized as stabilizers with an amphipathic strength that depends on the edge-to-surface ratio. The interactions between flakes can be also controlled by varying the oil-to-water ratio. In addition, it is predicted that graphene flakes can be efficiently used as a new-generation stabilizer that is active under high pressure, high temperature, and in saline solutions, greatly enhancing the efficiency and functionality of applications based on this material.

Supercritical Fluid-Facilitated Exfoliation and Processing of 2D Materials

Since the first intercalation of layered silicates by using supercritical CO2 as a processing medium, considerable efforts have been dedicated to intercalating and exfoliating layered two-dimensional (2D) materials in various supercritical fluids (SCFs) to yield single- and few-layer nanosheets. Here, recent work in this area is highlighted. Motivating factors for enhancing exfoliation efficiency and product quality in SCFs, mechanisms for exfoliation and dispersion in SCFs, as well as general metrics applied to assess quality and processability of exfoliated 2D materials are critically discussed. Further, advances in formation and application of 2D material-based composites with assistance from SCFs are presented. These discussions address chemical transformations accompanying SCF processing such as doping, covalent surface modification, and heterostructure formation. Promising features, challenges, and routes to expanding SCF processing techniques are described.

Poly(ionic liquid)-Derived N-Doped Carbons with Hierarchical Porosity for Lithium- and Sodium-Ion Batteries

The performance of lithium- and sodium-ion batteries relies notably on the accessibility to carbon electrodes of controllable porous structure and chemical composition. This work reports a facile synthesis of well-defined N-doped porous carbons (NPCs) using a poly(ionic liquid) (PIL) as precursor, and graphene oxide (GO)-stabilized poly(methyl methacrylate) (PMMA) nanoparticles as sacrificial template. The GO-stabilized PMMA nanoparticles are first prepared and then decorated by a thin PIL coating before carbonization. The resulting NPCs reach a satisfactory specific surface area of up to 561 m² g^-1 and a hierarchically meso- and macroporous structure while keeping a nitrogen content of 2.6 wt%. Such NPCs deliver a high reversible charge/discharge capacity of 1013 mA h g^-1 over 200 cycles at 0.4 A g^-1 for lithium-ion batteries, and show a good capacity of 204 mA h g^-1 over 100 cycles at 0.1 A g^-1 for sodium-ion batteries.

2D Particles at Fluid-Fluid Interfaces: Assembly and Templating of Hybrid Structures for Advanced Applications

Fluid-fluid interfaces have widespread applications in personal care products, the food industry, oil recovery, mineral processes, etc. and are also important and versatile platforms for generating advanced materials. In Pickering emulsions, particles stabilize the fluid-fluid interface, and their presence reduces the interfacial energy between the two fluids. To date, most Pickering emulsions stabilized by 2D particles make use of clay platelets or GO nanosheets. These systems have been used to template higher order hybrid, functional materials, most commonly, armored polymer particles, capsules, and Janus nanosheets. This review discusses the experimental and computational study of the assembly of sheet-like 2D particles at fluid-fluid interfaces, with an emphasis on the impact of chemical composition, and the use of these assemblies to prepare composite structures of dissimilar materials. The review culminates in a perspective on the future of Pickering emulsions using 2D particle surfactants, including new chemical modification and types of particles as well as the realization of properties and applications not possible with currently accessible systems, such as lubricants, porous structures, delivery, coatings, etc.

Application of Graphene-Oxide-Modified Polyacrylate Polymer for Controlled-Release Coated Urea

Polyacrylate polymer (PA) was modified with graphene oxide (GO) and the obtained composites were applied as coatings for controlled-release coated urea (CRU). The physicochemical properties of the different PA/GO coatings were characterized in detail and the nitrogen-release characteristics of the obtained CRU samples were determined in water at 25 °C. The experimental results revealed that addition of GO to PA reduced the swelling degree from 83.01% to 46.35% and improved its mechanical properties (the Young’s modulus was improved from 31.52 to 34.97 MPa and the glass transition temperature was increased from 4.21 to 6.11 °C), thus dramatically slowing down the cumulative nutrient release from the CRU fertilizer from 87.25% to 59.71%. These results suggest that GO enhances the properties of PA for CRU applications, which shows that GO-modified PA is a good coating material.

Graphene Oxide-Polymer Composite Langmuir Films Constructed by Interfacial Thiol-Ene Photopolymerization

The effective synthesis and self-assembly of graphene oxide (GO) nanocomposites are of key importance for a broad range of nanomaterial applications. In this work, a one-step chemical strategy is presented to synthesize stable GO-polymer Langmuir composite films by interfacial thiol-ene photopolymerization at room temperature, without use of any crosslinking agents and stabilizing agents. It is discovered that photopolymerization reaction between thiol groups modified GO sheets and ene in polymer molecules is critically responsible for the formation of the composite Langmuir films. The film formed by Langmuir assembly of such GO-polymer composite films shows potential to improve the mechanical and chemical properties and promotes the design of various GO-based nanocomposites. Thus, the GO-polymer composite Langmuir films synthesized by interfacial thiol-ene photopolymerization with such a straightforward and clean manner, provide new alternatives for developing chemically modified GO-based hybrid self-assembled films and nanomaterials towards a range of soft matter and graphene applications.

Inverse Pickering Emulsion Stabilized by Exfoliated Hexagonal-Boron Nitride (h-BN)

The formation of inverse Pickering emulsions using exfoliated hexagonal boron nitride (h-BN) as an effective particulate stabilizer without using any surfactants is reported for the first time. The stability and the type of h-BN Pickering emulsions formulated with different BN concentrations and by varying oil/water (o/w) ratios are studied and discussed. First the emulsion structure is analyzed microscopically through optical and epifluorescence microscopy and macroscopically by the study of the rheological behavior. The average droplet size decreases with h-BN concentration whereas the emulsions achieve good stability at 2 wt % BN concentrations and for a 1:1 o/w ratio. In all formulations, the emulsions are of water-in-oil (w/o) type due mainly to the hydrophobicity of h-BN.

An Overview of Pickering Emulsions: Solid-Particle Materials, Classification, Morphology, and Applications

Pickering emulsion, a kind of emulsion stabilized only by solid particles locating at oil-water interface, has been discovered a century ago, while being extensively studied in recent decades. Substituting solid particles for traditional surfactants, Pickering emulsions are more stable against coalescence and can obtain many useful properties. Besides, they are more biocompatible when solid particles employed are relatively safe in vivo. Pickering emulsions can be applied in a wide range of fields, such as biomedicine, food, fine chemical synthesis, cosmetics, and so on, by properly tuning types and properties of solid emulsifiers. In this article, we give an overview of Pickering emulsions, focusing on some kinds of solid particles commonly serving as emulsifiers, three main types of products from Pickering emulsions, morphology of solid particles and as-prepared materials, as well as applications in different fields.

Variable self-assembly and in situ host–guest reaction of beta-cyclodextrin-modified graphene oxide composite Langmuir films with azobenzene compounds

Different composite Langmuir films (GO–CD/N-Azo and GO–CD/PAA-Azo) are prepared via simple interfacial self-assembly process and host–guest reaction, demonstrating variable self-assembly for wide applications.

Preparation and Application of Water-in-Oil Emulsions Stabilized by Modified Graphene Oxide

A series of alkyl chain modified graphene oxides (AmGO) with different alkyl chain length and content was fabricated using a reducing reaction between graphene oxide (GO) and alkyl amine. Then AmGO was used as a graphene-based particle emulsifier to stabilize Pickering emulsion. Compared with the emulsion stabilized by GO, which was oil-in-water type, all the emulsions stabilized by AmGO were water-in-oil type. The effects of alkyl chain length and alkyl chain content on the emulsion properties of AmGO were investigated. The emulsions stabilized by AmGO showed good stability within a wide range of pH (from pH = 1 to pH = 13) and salt concentrations (from 0.1 to 1000 mM). In addition, the application of water-in-oil emulsions stabilized by AmGO was investigated. AmGO/polyaniline nanocomposite (AmGO/PANi) was prepared through an emulsion approach, and its supercapacitor performance was investigated. This research broadens the application of AmGO as a water-in-oil type emulsion stabilizer and in preparing graphene-based functional materials.