Efficient Production of Graphene through a Partially Frozen Suspension Exfoliation Process: An Insight into the Enhanced Interaction Based on Solid-Solid Interfaces

Liquid phase exfoliation (LPE) offers a promising path for scalable graphene production, but struggles with high energy consumption and low yield, with over 99.99% of the input energy wasted. Here, we present an energy-efficient approach for producing graphene via partially frozen-suspension exfoliation (PFE). As opposed to traditional liquid-solid interfaces, the solid-solid interface enhances shear strength between the frozen solvent and graphite from about 40 N m-2 to 105 N m-2. Additionally, the suspension flow transitions from turbulent to laminar, aligning graphite parallel to the flow direction and conducive to the effective utilization of shear force. Compared to conventional liquid-phase exfoliation (LPE), PFE improves energy efficiency by 102∼103 times. Furthermore, a production rate of 5 g h-1 has been achieved in a 10 L tank at an ultralow shear rate of 3 × 102 s-1.

Electrochemical Regeneration of Highly Stable and Sustainable Cellulose/Graphene Adsorbent Saturated with Dissolved Organic Dye

Electrochemical regeneration of adsorbents presents a cost-effective and environmentally friendly approach. Yet, its application to 3D structured adsorbents such as cellulose/graphene-based aerogels remains largely unexplored. This study introduces a method for producing these aerogels, highlighting their significant adsorption capacity for dissolved organic pollutants and resilience during electrochemical regeneration. By adjusting the ratio of hydrophobized cellulose nanofibers to graphene, the aerogels demonstrate a tunable adsorption capacity, ranging from 56 to 228 mg/g. Hydrophobization using oleic acid is vital for maintaining the aerogels' structural stability in water. Notably, the aerogels maintain structural integrity and efficiency over at least 18 electrochemical regeneration cycles, underscoring their potential for long-term environmental applications. The increase in adsorption capacity observed after regeneration cycles, approximately 10-20% by the fifth cycle, is attributed to electrochemical surface roughening and the creation of new adsorption sites. The tunability and durability of these aerogels offer a sustainable solution for adsorption with electrochemical regeneration technology.

On the interface between biomaterials and two-dimensional materials for biomedical applications

Two-dimensional (2D) materials have garnered significant attention due to their ultrathin 2D structures with a high degree of anisotropy and functionality. Reliable manipulation of interfaces between 2D materials and biomaterials is a new frontier for biomedical nanoscience and combining biomaterials with 2D materials offers a promising way to fabricate innovative 2D biomaterials composites with distinct functionality for biomedical applications. Here, we focus exclusively on a summary of the current work in the interface investigation of 2D biomaterials. Specifically, we highlight extraordinary features that make 2D materials so desirable, as well as the molecular level interactions between 2D materials and biomaterials that have been studied thus far. Furthermore, the approaches for investigating the interface characteristics of 2D biomaterials are presented and described in depth. To capture the emerging trend in mass manufacturing of 2D materials, we review the research progress on biomaterial-assisted exfoliation. Finally, we present a critical assessment of newly developed 2D biomaterials in biomedical applications.

Bio-Surfactant Assisted Aqueous Exfoliation of High-Quality Few-Layered Graphene

Realizing the efficacy of the liquid-phase exfoliation technique to obtain a greater quantity of graphene, this study demonstrates a cost-effective technique of bio-surfactant-assisted liquid-phase exfoliation of few-layer graphene (FLG) with a low defect ratio. An ultrasonic bath without any toxic chemicals or chemical modification was employed to exfoliate the graphene at room temperature. Several state-of-the-art characterization techniques such as TEM, AFM, XRD UV-Vis, and Raman spectroscopy were used to confirm the presence of the graphene. The dispersion exhibits a typical Tyndall scattering to the red laser beam. After a 7-h sonication of the dispersion, followed by a centrifugation frequency of 500 rpm for half an hour, the graphene concentration was found to be 1.2 mg/mL. The concentration decreases monotonically with an increase in the frequency, as a higher frequency causes sedimentation of the larger flakes or removes the adsorbed surfactant molecules from the graphene structures that collapse the graphene sheets into the graphite. The presence of an amino acid head-group in the surfactant facilitated exfoliation in an aqueous solution at well below the critical micelle concentration (CMC) of the surfactant. The product demonstrates all characteristic features of an FLG system. The TEM and AFM image reveals large-area graphene with a wrinkle-free surface; these morphological properties are confirmed by XRD and Raman spectroscopy. This study suggests that a sonication-induced process with a biocompatible surfactant can produce a cheap, large-surface-area graphene system for a wide range of applications. Moreover, the use of a probe sonicator as an alternative to the bath-type sonicator, together with the demonstrated technique, may reduce the time needed, and leads to a manifold increase in the yield.

Surfactant-Free Stabilization of Aqueous Graphene Dispersions Using Starch as a Dispersing Agent

Attention to graphene dispersions in water with the aid of natural polymers is increasing with improved awareness of sustainability. However, the function of biopolymers that can act as dispersing agents in graphene dispersions is not well understood. In particular, the use of starch to disperse pristine graphene materials deserves further investigation. Here, we report the processing conditions of aqueous graphene dispersions using unmodified starch. We have found that the graphene content of the starch-graphene dispersion is dependent on the starch fraction. The starch-graphene sheets are few-layer graphene with a lateral size of 3.2 μm. Furthermore, topographical images of these starch-graphene sheets confirm the adsorption of starch nanoparticles with a height around 5 nm on the graphene surface. The adsorbed starch nanoparticles are ascribed to extend the storage time of the starch-graphene dispersion up to 1 month compared to spontaneous aggregation in a nonstabilized graphene dispersion without starch. Moreover, the ability to retain water by starch is reduced in the presence of graphene, likely due to environmental changes in the hydroxyl groups responsible for starch-water interactions. These findings demonstrate that starch can disperse graphene with a low oxygen content in water. The aqueous starch-graphene dispersion provides tremendous opportunities for environmental-friendly packaging applications.

Dispersant-assisted liquid-phase exfoliation of 2D materials beyond graphene

The extensive research on liquid-phase exfoliation (LPE) performed in the last 10 years has enabled a low cost and mass scalable approach to the successful production of a range of solution-processed 2-dimensional (2D) materials suitable for many applications, from composites to energy storage and printed electronics. However, direct LPE requires the use of specific solvents, which are typically toxic and expensive. Dispersant-assisted LPE allows us to overcome this problem by enabling production of solution processed 2D materials in a wider range of solvents, including water. This approach is based on the inclusion of an additive, typically an amphiphilic molecule, designed to interact with both the nanosheet and the solvent, enabling exfoliation and stabilization at the same time. This method has been extensively used for the LPE of graphene and has been discussed in many reviews, whilst little attention has been given to dispersant-assisted LPE of 2D materials beyond graphene. Considering the increasing number of 2D materials and their potential in many applications, from nanomedicine to energy storage and catalysis, this review focuses on the dispersant-assisted LPE of transition metal dichalcogenides (TMDs), hexagonal boron nitride (h-BN) and less studied 2D materials. We first provide an introduction to the fundamentals of LPE and the type of dispersants that have been used for the production of graphene, we then discuss each class of 2D material, providing an overview on the concentration and properties of the nanosheets obtained. Finally, a perspective is given on some of the challenges that need to be addressed in this field of research.

Biolayer interferometry-SELEX for Shiga toxin antigenic-peptide aptamers & detection via chitosan-WSe2 aptasensor

We report biolayer interferometry based in-vitro selection technique (BLI-SELEX) for fishing out specific aptamers against E. coli Shiga toxin subtypes viz., stx1 & stx2 via epitopic peptides. BLI-SELEX is a one-step technique for rapidly generating aptamers against protein biomarkers in a microtiter plate format, obliterating the need for multiple enrichment rounds to harvest high-affinity aptamers as in conventional SELEX. Two unique aptamers selected against stx1 & stx2 with picomolar K_d (~47 pM & ~29 pM, respectively) were successfully used to fabricate voltammetric diagnostic assay via immobilization onto chitosan exfoliated 2D tungsten diselenide (WSe₂) nanosheet platform. These aptamers modified nanosensors showed high sensitivity of ~ 5.0 μA ng^-1 mL, a dynamic response range from 50 pg mL^-1 to 100 ng mL^-1, with a detection limit of 44.5 pg mL^-1 & 41.3 pg mL^-1 for stx subtypes, respectively and showed low cross-reactivity in spiked urine, serum and milk samples. The synergistic effect of selective aptamers & high sensitivity imparted by 2D transition metal dichalcogenide (TMD) highlights the superior potential of a fabricated nanosensor for bacterial toxin detection.

Efficient exfoliation of UV-curable, high-quality graphene from graphite in common low-boiling-point organic solvents with a designer hyperbranched polyethylene copolymer and their applications in electrothermal heaters

The use of stabilizer with designer structures can effectively promote graphite exfoliation in common solvents to render functionalized graphene desirable for their various applications. Herein, a hyperbranched polyethylene copolymer, HBPE@Py@Acryl, simultaneously bearing multiple pyrene terminal groups and pendant acryloyl moieties has been successfully synthesized from ethylene with a Pd-diimine catalyst based on unique chain walking mechanism. The unique structural design of the HBPE@Py@Acryl makes it capable of effectively promote graphite exfoliation in a series of common, low-boiling-point organic solvents, e.g. CHCl₃, to render stable graphene dispersions with concentrations effectively adjustable by changing feed concentrations of graphite and polymer or sonication time. Meanwhile, it can be irreversibly adsorbed on the exfoliated graphene surface based on the π-π interactions between them to concurrently render acryloyl-functionalized graphene free of structural defects, with majority (92.7%) of them having a thickness of 2-3 layers. This allows us to obtain graphene electrothermal films simply by filtration and UV irradiation, which exhibit outstanding stability in use. The action mechanism of the HBPE@Py@Acryl as stabilizer for promoting graphite exfoliation and the role of UV irradiation on improving the stability in use of resulting graphene films have been elucidated.

Stirred Not Shaken: Facile Production of High-Quality, High-Concentration Graphene Aqueous Suspensions Assisted by a Protein

A simple method to produce record concentrations (up to 10 mg mL^-1) of high-quality aqueous graphene suspensions by using an ordinary benchtop magnetic stirrer is reported. The shear rates employed here are almost 10 times less than those in previous reports, and graphene is efficiently separated from unexfoliated graphite during the synthesis. Systematic optimization of synthesis parameters, such as pH, protein concentration, temperature, stirrer speed, and volume of solution, afforded efficient conversion (100%) of graphite to graphene-aqueous suspensions. The synthesis is readily scaled-up with a continuous flow reactor where the graphene is produced and separated 24/7, with little or no human intervention. Raman spectroscopy confirmed little to no sp³ or oxidative defects, and that the graphene nanosheets consisted of three to five layers. The graphene suspensions were coated on aluminum and tested for thermal conductivity applications. The thermal conductivity of our graphene sample was calculated to be 684 W m^-1 K^-1, a value greater than that of a commercial sample. The activation energy measured for shear exfoliation by stirring was found to be over 45 billion times smaller than the corresponding thermal activation energy, affording physical insight into the process. We hypothesize that stirring selectively populates translational states that are necessary for exfoliation and thus requires far less energy than conventional exfoliation methods, where the energy is uniformly distributed among all available modes. Therefore, an efficient, convenient, and inexpensive method for graphene production in limited-resource settings is reported here.

Method of ultrasound-assisted liquid-phase exfoliation to prepare graphene

Graphene is a two-dimensional material with unique structure and excellent properties. After first being successfully prepared in 2004, it rapidly became a research hotspot in the fields of materials, chemistry, physics, and engineering. Currently, there are many methods for preparing graphene, such as ball milling method, chemical oxidation-reduction, chemical vapor deposition, and liquid-phase exfoliation. Among these methods, liquid-phase exfoliation is the most important preparation method. In this paper, ultrasound-assisted liquid-phase exfoliation is systematically studied. The output power and frequency of the ultrasonic crusher used in the experiment are 100 W and 20 kHz, respectively. Results show that ultrasonic waves can affect the size and thickness distribution of graphene sheets; ultrasound-assisted deoxycholic acid sodium aqueous solution has a good exfoliation effect. In addition, the effects of the 3 liquid-phase systems on preparing graphene are studied, including organic solvent system, aqueous surfactant system, and ionic liquids system; the improvement efforts for ultrasound-assisted liquid-phase exfoliation method are discussed including the exploration of new solvents and optimization of exfoliation process. The application of auxiliary agent-assisted liquid-phase exfoliation method is also discussed.

2D transition metal dichalcogenide nanosheets for photo/thermo-based tumor imaging and therapy

Two-dimensional (2D) graphene-like nanomaterials show wide applications in the fields of nanodevices, sensors, energy materials, catalysis, drug delivery, bioimaging, and tissue engineering. Recently, many studies have been focused on the synthesis and application of 2D transition metal dichalcogenide (TMD) nanosheets for various biomedical applications. In particular, 2D TMD nanosheets exhibit great advantages for tumor imaging and therapy compared to some traditional nanomaterials due to their high specific surface area, good biocompatibility, easy modification, and ultrahigh light and heat conversion efficiency. In this review, we summarize the recent advances in the synthesis, modification, and photo/thermo-based tumor imaging and therapy of 2D TMD nanosheets. The important studies on tumor bioimaging with TMD nanosheets, such as X-ray computed tomography, magnetic resonance imaging, and photoacoustic imaging, are demonstrated and discussed. In another section, the physical photothermal and photodynamic therapies as well as the pharmacological therapy of tumors with TMD nanosheet-based nanohybrids are introduced. It is expected that this work will be valuable for readers to understand the synthesis and modification of TMD nanosheets to design novel 2D functional nanomaterials for photo/thermo-based tumor imaging and therapy in one aspect, and in another aspect will extend the applications of TMD-based nanomaterials in materials science, analytical science, electrocatalysis, tissue engineering, and others.

Scalable exfoliation and dispersion of two-dimensional materials - an update

The preparation of dispersions of single- and few-sheet 2D materials in various solvents, as well as the characterization methods applied to such dispersions, is critically reviewed. Motivating factors for producing single- and few-sheet dispersions of 2D materials in liquids are briefly discussed. Many practical applications are expected for such materials that do not require high purity formulations and tight control of donor and acceptor concentrations, as required in conventional Fab processing of semiconductor chips. Approaches and challenges encountered in exfoliating 2D materials in liquids are reviewed. Ultrasonication, mechanical shearing, and electrochemical processing approaches are discussed, and their respective limitations and promising features are critiqued. Supercritical and more conventional liquid and solvent processing are then discussed in detail. The effects of various types of stabilizers, including surfactants and other amphiphiles, as well as polymers, including homopolymeric electrolytes, nonionic polymers, and nanolatexes, are discussed. Consideration of apparent successes of stabilizer-free dispersions indicates that extensive exfoliation in the absence of dispersing aids results from processing-induced surface modifications that promote stabilization of 2D material/solvent interactions. Also apparent paradoxes in "pristineness" and optical extinctions in dispersions suggest that there is much we do not yet quantitatively understand about the surface chemistry of these materials. Another paradox, emanating from modeling dilute solvent-only exfoliation by sonication using polar components of solubility parameters and surface tension for pristine graphene with no polar structural component, is addressed. This apparent paradox appears to be resolved by realizing that the reactivity of graphene to addition reactions of solvent radicals produced by sonolysis is accompanied by unintended polar surface modifications that promote attractive interactions with solvent. This hypothesis serves to define important theoretical and experimental studies that are needed. We conclude that the greatest promise for high volume and high concentration processing lies in applying methods that have not yet been extensively reported, particularly wet comminution processing using small grinding media of various types.

Alginic Acid-Aided Dispersion of Carbon Nanotubes, Graphene, and Boron Nitride Nanomaterials for Microbial Toxicity Testing

Robust evaluation of potential environmental and health risks of carbonaceous and boron nitride nanomaterials (NMs) is imperative. However, significant agglomeration of pristine carbonaceous and boron nitride NMs due to strong van der Waals forces renders them not suitable for direct toxicity testing in aqueous media. Here, the natural polysaccharide alginic acid (AA) was used as a nontoxic, environmentally relevant dispersant with defined composition to disperse seven types of carbonaceous and boron nitride NMs, including multiwall carbon nanotubes, graphene, boron nitride nanotubes, and hexagonal boron nitride flakes, with various physicochemical characteristics. AA’s biocompatibility was confirmed by examining AA effects on viability and growth of two model microorganisms (the protozoan Tetrahymena thermophila and the bacterium Pseudomonas aeruginosa). Using 400 mg·L⁻¹ AA, comparably stable NM (200 mg·L⁻¹) stock dispersions were obtained by 30-min probe ultrasonication. AA non-covalently interacted with NM surfaces and improved the dispersibility of NMs in water. The dispersion stability varied with NM morphology and size rather than chemistry. The optimized dispersion protocol established here can facilitate preparing homogeneous NM dispersions for reliable exposures during microbial toxicity testing, contributing to improved reproducibility of toxicity results.

Colloid Approach to the Sustainable Top-Down Synthesis of Layered Materials

The successful future of 2D materials, which are crucial for accelerating technology development and societal requirements, depends on their efficient preparation in an economical and ecological way. Herein, we present a significant advance in the top-down exfoliation and dispersion method via an aqua colloid approach. We demonstrate that a broad family of natural oil-in-water emulsification agents with an elevated hydrophilic/lipophilic balance acts in the exfoliation of layered materials and the formation of their concentrated colloids. The concentration exceeds 45 g/L for exfoliated few-layered graphene sheets possessing a micrometer size. The exfoliation of carbon nanofibers provides one of the best known unsupported and N-undoped metal-free catalysts to date in the selective dehydrogenation of ethylbenzene to styrene. Other examples include aqua colloids of exfoliated/dispersed nitrides, carbides, or nanodiamonds.

Controlling the Graphene-Bio Interface: Dispersions in Animal Sera for Enhanced Stability and Reduced Toxicity

Liquid phase exfoliation of graphite in six different animal sera and evaluation of its toxicity are reported here. Previously, we reported the exfoliation of graphene using proteins, and here we extend this approach to complex animal fluids. A kitchen blender with a high-turbulence flow gave high quality and maximum exfoliation efficiency in all sera tested, when compared to the values found with shear and ultrasonication methods. Raman spectra and electron microscopy confirmed the formation of three- or four-layer, submicrometer size graphene, independent of the serum used. Graphene prepared in serum was directly transferred to cell culture media without post-treatments. Contrary to many reports, a nanotoxicity study of this graphene fully dispersed to human embryonic kidney cells, human lung cancer cells, and nematodes (Caenorhabditis elegans) showed no acute toxicity for up to 7 days at various doses (50-500 μg/mL), but prolonged exposure at higher doses (300-500 μg/mL, 10-15 days) showed cytotoxicity to cells (∼95% death) and reproductive toxicity to C. elegans (5-10% reduction in brood size). The origin of toxicity was found to be due to the highly fragmented smaller graphene sheets (<200 nm), while the larger sheets were nontoxic (50-300 μg/mL dose). In contrast, graphene produced with sodium cholate as the mediator has been found to be cytotoxic to these cells at these dosages. We demonstrated the toxicity of liquid phase exfoliated graphene is attributed to highly fragmented fractions or nonbiocompatible exfoliating agents. Thus, low-toxicity graphene/serum suspensions are produced by a facile method in biological media, and this approach may accelerate the much-anticipated development of graphene for biological applications.

Experimental review: chemical reduction of graphene oxide (GO) to reduced graphene oxide (rGO) by aqueous chemistry

The electrical conductivity of reduced graphene oxide (rGO) obtained from graphene oxide (GO) using sodium borohydride (NaBH₄) as a reducing agent has been investigated as a function of time (2 min to 24 h) and temperature (20 °C to 80 °C). Using a 300 mM aqueous NaBH₄ solution at 80 °C, reduction of GO occurred to a large extent during the first 10 min, which yielded a conductivity increase of 5 orders of magnitude to 10 S m^-1. During the residual 1400 min of reaction, the reduction rate decreased significantly, eventually resulting in a rGO conductivity of 1500 S m^-1. High resolution XPS measurements showed that C/O increased from 2.2 for the GO to 6.9 for the rGO at the longest reaction times, due to the elimination of oxygen. The steep increase in conductivity recorded during the first 8-12 min of reaction was mainly due to the reduction of C-O (e.g., hydroxyl and epoxy) groups, suggesting the preferential attack of the reducing agent on C-O rather than C[double bond, length as m-dash]O groups. In addition, the specular variation of the percentage content of C-O bond functionalities with the sum of Csp² and Csp³ indicated that the reduction of epoxy or hydroxyl groups had a greater impact on the restoration of the conductive nature of the graphite structure in rGO. These findings were reflected in the dramatic change in the structural stability of the rGO nanofoams produced by freeze-drying. The reduction protocol in this study allowed to achieve the highest conductivity values reported so far for the aqueous reduction of graphene oxide mediated by sodium borohydride. The 4-probe sheet resistivity approach used to measure the electrical conductivity is also, for the first time, presented in detail for filtrate sheet assemblies' of stacked GO/rGO sheets.

Chemical functionalization and characterization of graphene-based materials

This review offers an overview on the chemical functionalization, characterization and applications of graphene-based materials.

Graphene via Molecule-Assisted Ultrasound-Induced Liquid-Phase Exfoliation: A Supramolecular Approach

Graphene is a two-dimensional (2D) material holding unique optical, mechanical, thermal and electrical properties. The combination of these exceptional characteristics makes graphene an ideal model system for fundamental physical and chemical studies as well as technologically ground breaking material for a large range of applications. Graphene can be produced either following a bottom-up or top-down method. The former is based on the formation of covalent networks suitably engineered molecular building blocks undergoing chemical reaction. The latter takes place through the exfoliation of bulk graphite into individual graphene sheets. Among them, ultrasound-induced liquid-phase exfoliation (UILPE) is an appealing method, being very versatile and applicable to different environments and on various substrate types. In this chapter, we describe the recently reported methods to produce graphene via molecule-assisted UILPE of graphite, aiming at the generation of high-quality graphene. In particular, we will focus on the supramolecular approach, which consists in the use of suitably designed organic molecules during the UILPE of graphite. These molecules act as graphene dispersion-stabilizing agents during the exfoliation. This method relying on the joint effect of a solvent and ad hoc molecules to foster the exfoliation of graphite into graphene in liquid environment represents a promising and modular method toward the improvement of the process of UILPE in terms of the concentration and quality of the exfoliated material. Furthermore, exfoliations in aqueous and organic solutions are presented and discussed separately.

Biomolecule-assisted exfoliation and dispersion of graphene and other two-dimensional materials: a review of recent progress and applications

Direct liquid-phase exfoliation of layered materials by means of ultrasound, shear forces or electrochemical intercalation holds enormous promise as a convenient, cost-effective approach to the mass production of two-dimensional (2D) materials, particularly in the form of colloidal suspensions of high quality and micrometer- and submicrometer-sized flakes. Of special relevance due to environmental and practical reasons is the production of 2D materials in aqueous medium, which generally requires the use of certain additives (surfactants and other types of dispersants) to assist in the exfoliation and colloidal stabilization processes. In this context, biomolecules have received, in recent years, increasing attention as dispersants for 2D materials, as they provide a number of advantages over more conventional, synthetic surfactants. Here, we review research progress in the use of biomolecules as exfoliating and dispersing agents for the production of 2D materials. Although most efforts in this area have focused on graphene, significant advances have also been reported with transition metal dichalcogenides (MoS2, WS2, etc.) or hexagonal boron nitride. Particular emphasis is placed on the specific merits of different types of biomolecules, including proteins and peptides, nucleotides and nucleic acids (RNA, DNA), polysaccharides, plant extracts and bile salts, on their role as efficient colloidal dispersants of 2D materials, as well as on the potential applications that have been explored for such biomolecule-exfoliated materials. These applications are wide-ranging and encompass the fields of biomedicine (photothermal and photodynamic therapy, bioimaging, biosensing, etc.), energy storage (Li- and Na-ion batteries), catalysis (e.g., catalyst supports for the oxygen reduction reaction or electrocatalysts for the hydrogen evolution reaction), or composite materials. As an incipient area of research, a number of knowledge gaps, unresolved issues and novel future directions remain to be addressed for biomolecule-exfoliated 2D materials, which will be discussed in the last part of this review.

Supramolecular Approaches to Graphene: From Self-Assembly to Molecule-Assisted Liquid-Phase Exfoliation

Graphene, a one-atom thick two-dimensional (2D) material, is at the core of an ever-growing research effort due to its combination of unique mechanical, thermal, optical and electrical properties. Two strategies are being pursued for the graphene production: the bottom-up and the top-down. The former relies on the use of covalent chemistry approaches on properly designed molecular building blocks undergoing chemical reaction to form 2D covalent networks. The latter occurs via exfoliation of bulk graphite into individual graphene sheets. Amongst the various types of exfoliations exploited so far, ultrasound-induced liquid-phase exfoliation (UILPE) is an attractive strategy, being extremely versatile, up-scalable and applicable to a variety of environments. In this review, we highlight the recent developments that have led to successful non-covalent functionalization of graphene and how the latter can be exploited to promote the process of molecule-assisted UILPE of graphite. The functionalization of graphene with non-covalently interacting molecules, both in dispersions as well as in dry films, represents a promising and modular approach to tune various physical and chemical properties of graphene, eventually conferring to such a 2D system a multifunctional nature.

Liquid-Phase Exfoliation of Graphite into Single- and Few-Layer Graphene with α-Functionalized Alkanes

Graphene has unique physical and chemical properties, making it appealing for a number of applications in optoelectronics, sensing, photonics, composites, and smart coatings, just to cite a few. These require the development of production processes that are inexpensive and up-scalable. These criteria are met in liquid-phase exfoliation (LPE), a technique that can be enhanced when specific organic molecules are used. Here we report the exfoliation of graphite in N-methyl-2-pyrrolidinone, in the presence of heneicosane linear alkanes terminated with different head groups. These molecules act as stabilizing agents during exfoliation. The efficiency of the exfoliation in terms of the concentration of exfoliated single- and few-layer graphene flakes depends on the functional head group determining the strength of the molecular dimerization through dipole-dipole interactions. A thermodynamic analysis is carried out to interpret the impact of the termination group of the alkyl chain on the exfoliation yield. This combines molecular dynamics and molecular mechanics to rationalize the role of functionalized alkanes in the dispersion and stabilization process, which is ultimately attributed to a synergistic effect of the interactions between the molecules, graphene, and the solvent.

Graphene Oxide-Assisted Liquid Phase Exfoliation of Graphite into Graphene for Highly Conductive Film and Electromechanical Sensors

Here, we report a new method to prepare graphene from graphite by the liquid phase exfoliation process with sonication using graphene oxide (GO) as a dispersant. It was found that GO nanosheets act a as surfactant to the mediated exfoliation of graphite into a GO-adsorbed graphene complex in the aqueous solution, from which graphene was separated by an additional process. The preparation of isolated graphene from a single to a few layers is routinely achieved with an exfoliation yield of up to higher than 40% from the initial graphite material. The prepared graphene sheets showed a high quality (C/O ∼ 21.5), low defect (ID/IG ∼ 0.12), and high conductivity (6.2 × 10(4) S/m). Moreover, the large lateral size ranging from 5 to 10 μm of graphene, which is believed to be due to the shielding effect of GO avoiding damage under ultrasonic jets and cavitation formed by the sonication process. The thin graphene film prepared by the spray-coating technique showed a sheet resistance of 668 Ω/sq with a transmittance of 80% at 550 nm after annealing at 350 °C for 3 h. The transparent electrode was even greater with the resistance only 66.02 Ω when graphene is deposited on an interdigitated electrode (1 mm gap). Finally, a flexible sensor based on a graphene spray-coating polydimethylsiloxane (PDMS) is demonstrated showing excellent performance working under human touch pressure (<10 kPa). The graphene prepared by this method has some distinct properties showing it as a promising material for applications in electronics including thin film coatings, transparent electrodes, wearable electronics, human monitoring sensors, and RFID tags.

BioGraphene: Direct Exfoliation of Graphite in a Kitchen Blender for Enzymology Applications

A high yielding method for the aqueous exfoliation of graphite crystals to produce high quality graphene nanosheets in a kitchen blender is described here. Bovine serum albumin (BSA), β-lactoglobulin, ovalbumin, lysozyme, and hemoglobin as well as calf serum were used for the exfoliation of graphene. Among these, BSA gave the maximum exfoliation efficiency, exceeding 4mgmL(-1)h(-1) of graphene. Quality of graphene produced was examined by Raman spectroscopy, which indicated 3-5 layer graphene of very high quality and very low levels of defects. Transmission electron microscopy indicated an average size of ~0.5μm flakes. The graphene/BSA dispersions were stable over pH 3.0-11, and at 5°C or 50°C, for more than 2 months. Current approach gave higher rates of BSA/graphene (BioGraphene) in better yields than other methods. Calf serum, when used in place of BSA, also gave high yields of good quality BioGraphene and these preparations may be of direct use for cell culture studies. A simple example of BioGraphene preparation is described that can be adapted in most laboratories, and graphene-adsorbed glucose oxidase is nearly as active as the free enzyme. Current approach may facilitate large-scale production of graphene in most laboratories around the world and it may open new opportunities for biological applications of graphene.