Chemically Modified Graphene Research Articles

Graphene platforms for the detection of caffeine in real samples

The analysis of food components is of high importance due to food safety and security. Here the electrochemical detection of caffeine was performed on different chemically modified graphene (CMG) surfaces carrying diverse amount of defects and oxygen functionalities. The analytical performances of graphite oxide (GPO), graphene oxide (GO), and electrochemically reduced graphene oxide (ERGO) were compared for the first time for the detection of caffeine. It was found that ERGO showed the most favourable analytical parameters, such as lower oxidation potential, sensitivity, linearity and reproducibility of the response. ERGO was then used for the analysis of real samples. Caffeine levels of soluble coffee, teas and energetic drinks were measured without the need of any sample pre-treatment. Our findings are very important to gain more insight into the applicability of different graphene materials to real samples for sense-and-act analysis.

A Rapid and Efficient Self‐Healing Thermo‐Reversible Elastomer Crosslinked with Graphene Oxide

A self-healing thermo-reversible elastomer is synthesized by cross-linking a hydrogen bonding polymer network with chemically-modified graphene oxide. This nanocomposite allows for both rapid and efficient self-healing (in only several minutes) at room temperature, without the need for any external stimuli (e.g., heating or light exposure), healing agents, plasticizers or solvents.

Macroporous Graphene Oxide–Polymer Composite Prepared through Pickering High Internal Phase Emulsions

Macroporous polymer-graphene oxide (GO) composites were successfully prepared using Pickering high internal phase emulsion (HIPE) templates. GO flakes were modified by the cationic surfactant cetyltrimethylammonium bromide (CTAB) and used as the stabilizer of water-in-oil (W/O) Pickering emulsions. CTAB-modified GO is effective at stabilizing W/O Pickering HIPEs, and the lowest GO content is only about 0.2 mg mL(-1) (relative to the volume of the oil phase). The close-cell morphology of the resulting poly-Pickering HIPEs is observed, and the void size of the porous polymers is tuned by varying the concentration of GO. Three-dimensional macroporous chemically modified graphene (CMG) monoliths with a high specific surface area of about 490 m(2) g(-1) were obtained after removing the cellular polymer substrates through calcination. The micropores were also found in CMGs, which may be caused by the decomposition of CTAB adsorbed on the surface of GO.

Salting-out as a scalable, in-series purification method of graphene oxides from microsheets to quantum dots

Chemically modified graphenes (CMGs) are used to reinforce composites and as platforms for sensing, electrodes in batteries, conductive films for displays, and others. However, performance of CMG materials is often limited by their physicochemical heterogeneity. Here, we report that simple addition of ammonium sulfate to heterogeneous CMG mixtures, results in effective, scalable, and in-series purification, by salting-out precipitation. The mechanism is based on differences in the dispersibility of the individual graphene oxide (GO). At relatively low ammonium sulfate concentrations (∼10mM), large, GOs with predominant hydrophobic domains are precipitated. As the salt concentration increases, smaller and less GOs with predominant hydrophobic domains are purified, and eventually graphene quantum dots (GQDs) are isolated. The method described herein was inspired by a well-known protein purification process called ‘salting-out,’ in which proteins of different sizes and surface charge, successively become indispersible over a narrow range of salt concentrations. The salting-out process for GOs is simple and scalable, and can be used to achieve large-scale purification.

Control of Superhydrophilic and Superhydrophobic Graphene Interface

Superhydrophobic and superhydrophilic properties of chemically-modified graphene have been achieved in larger-area vertically aligned few-layer graphene nanosheets (FLGs), prepared on Si (111) substrate by microwave plasma chemical vapor deposition (MPCVD). Furthermore, in order to enhance wettability, silicon wafers with microstructures were fabricated, on which graphene nanosheets were grown and modified by a chemical method to form hydrophilic and hydrophobic structures. A superhydrophilic graphene surface (contact angle 0°) and a superhydrophobic graphene surface (contact angle 152.0°) were obtained. The results indicate that the microstructured silicon enhances the hydrophilic and hydrophobic wettabilities significantly.

Free‐Radical‐Promoted Conversion of Graphite Oxide into Chemically Modified Graphene

The preparation of chemically modified graphene (CMG) generally involves the reduction of graphite oxide (GO) by using various reducing reagents. Herein, we report a free-radical-promoted synthesis of CMG, which does not require any conventional reductant. We demonstrated that the phenyl free radical can efficiently promote the conversion of GO into CMG under mild conditions and produces phenyl-functionalized CMG. This pseudo-"reduction" process is attributed to a free-radical-mediated elimination of the surface-attached oxygen-containing functionalities. This work illustrates a new strategy for preparing CMG that is alternative to the conventional means of chemical reduction. Furthermore, the phenyl-functionalized graphene shows an excellent performance as an electrode material for lithium-battery applications.

Material Processing of Chemically Modified Graphene: Some Challenges and Solutions

Graphene-based sheets show promise for a variety of potential applications, and researchers in many scientific disciplines are interested in these materials. Although researchers have developed many ways of generating single atomic layer carbon sheets, chemical exfoliation of graphite powders to graphene oxide (GO) sheets followed by deoxygenation to form chemically modified graphene (CMG) offers a promising route for bulk scale production. The materials processing, which we broadly define as the physical and chemical means to tailor a material's chemical and microstructures, enables us to control the properties in bulk CMG materials. For example, by processing CMG sheets in different solvents, we can make thin films, blend CMG sheets with other materials, and modify them by chemical reactions. Materials processing methods also allow us to control the interactions between CMG sheets for the assembly of large scale two- or three-dimensional structures with desirable microstructures. This Account highlights a few problems associated with large scale production and processing of GO and CMG. First, we briefly discuss the potential fire risk of GO and CMG when alkaline salt byproducts are not completely removed. These impurities can catalyze carbon combustion. We introduce an improved purification procedure that effectively removes the byproducts and speeds up the production. Next, we address the challenges of imaging GO and CMG sheets on common substrates such as glass and plastics using standard microscopy methods. We have introduced a new technique fluorescence quenching microscopy (FQM), which allows us to observe graphene-based sheets with both high throughput and high contrast on arbitrary substrates and even in solution. Then we focus on how to prevent aggregation in CMG. Aggregation greatly reduces the material processability and accessible surface area, which degrades the material properties. We introduce two strategies to reduce aggregation by (i) reducing the lateral dimension of the sheets to nanometer range to enhance their colloidal stability and (ii) crumpling the sheets into paper ball-like, fractal-dimensional particles to make them aggregation-resistant in both solvents and solid state, even after mechanical compression. Solutions to these material processing challenges can pave the way for further research and development. We hope that the tools and strategies presented in this Account can facilitate the processing and property control of this promising material.

Electroactive nanoparticle directed assembly of functionalized graphene nanosheets into hierarchical structures with hybrid compositions for flexible supercapacitors

Hierarchical structures of hybrid materials with the controlled compositions have been shown to offer a breakthrough for energy storage and conversion. Here, we report the integrative assembly of chemically modified graphene (CMG) building blocks into hierarchical complex structures with the hybrid composition for high performance flexible pseudocapacitors. The formation mechanism of hierarchical CMG/Nafion/RuO2 (CMGNR) microspheres, which is triggered by the cooperative interplay during the in situ synthesis of RuO2 nanoparticles (NPs), was extensively investigated. In particular, the hierarchical CMGNR microspheres consisting of the aggregates of CMG/Nafion (CMGN) nanosheets and RuO2 NPs provided large surface area and facile ion accessibility to storage sites, while the interconnected nanosheets offered continuous electron pathways and mechanical integrity. The synergistic effect of CMGNR hybrids on the supercapacitor (SC) performance was derived from the hybrid composition of pseudocapacitive RuO2 NPs with the conductive CMGNs as well as from structural features. Consequently, the CMGNR-SCs showed a specific capacitance as high as 160 F g(-1), three-fold higher than that of conventional graphene SCs, and a capacitance retention of >95% of the maximum value even after severe bending and 1000 charge-discharge tests due to the structural and compositional features.

Generation of B-Doped Graphene Nanoplatelets Using a Solution Process and Their Supercapacitor Applications

Chemically modified graphene (CMG) nanoplatelets have shown great promise in various applications due to their electrical properties and high surface area. Chemical doping is one of the most effective methods to tune the electronic properties of graphene materials. In this work, novel B-doped nanoplatelets (borane-reduced graphene oxide, B-rG-O) were produced on a large scale via the reduction of graphene oxide by a borane-tetrahydrofuran adduct under reflux, and their use for supercapacitor electrodes was studied. This is the first report on the production of B-doped graphene nanoplatelets from a solution process and on the use of B-doped graphene materials in supercapacitors. The B-rG-O had a high specific surface area of 466 m(2)/g and showed excellent supercapacitor performance including a high specific capacitance of 200 F/g in aqueous electrolyte as well as superior surface area-normalized capacitance to typical carbon-based supercapacitor materials and good stability after 4500 cycles. Two- and three-electrode cell measurements showed that energy storage in the B-rG-O supercapacitors was contributed by ion adsorption on the surface of the nanoplatelets in addition to electrochemical redox reactions.

Controllable coverage of chemically modified graphene sheets with gold nanoparticles by thermal treatment of graphite oxide with N,N-dimethylformamide

We describe a simple chemical method to reduce and functionalize graphite oxide by reaction with dimethylformamide under controlled heating. Our experiments suggest that the reaction conditions assist the decomposition of the solvent to produce dimethylamine molecules that can react with the oxygen-rich functional groups covering the surface of the exfoliated layers of graphene, therefore generating chemically modified graphene (CMG). These N-functionalities have been next used as anchoring points for the grafting of Au nanoparticles. Given that the functionalization extent can be controlled with the temperature and reaction time, our approach can be considered as a straightforward route for the controlled decoration of CMG layers with gold nanoparticles.

Safety Considerations for Graphene: Lessons Learnt from Carbon Nanotubes

Many consider carbon nanomaterials the poster children of nanotechnology, attracting immense scientific interest from many disciplines and offering tremendous potential in a diverse range of applications due to their extraordinary properties. Graphene is the youngest in the family of carbon nanomaterials. Its isolation, description, and mass fabrication has followed that of fullerenes and carbon nanotubes. Graphene's development and its adoption by many industries will increase unintended or intentional human exposure, creating the need to determine its safety profile. In this Account, we compare the lessons learned from the development of carbon nanotubes with what is known about graphene, based on our own investigations and those of others. Despite both being carbon-based, nanotubes and graphene are two very distinct nanomaterials. We consider the key physicochemical characteristics (structure, surface, colloidal properties) for graphene and carbon nanotubes at three different physiological levels: cellular, tissue, and whole body. We summarize the evidence for health effects of both materials at all three levels. Overall, graphene and its derivatives are characterized by a lower aspect ratio, larger surface area, and better dispersibility in most solvents compared to carbon nanotubes. Dimensions, surface chemistry, and impurities are equally important for graphene and carbon nanotubes in determining both mechanistic (aggregation, cellular processes, biodistribution, and degradation kinetics) and toxicological outcomes. Colloidal dispersions of individual graphene sheets (or graphene oxide and other derivatives) can easily be engineered without metallic impurities, with high stability and less aggregation. Very importantly, graphene nanostructures are not fiber-shaped. These features theoretically offer significant advantages in terms of safety over inhomogeneous dispersions of fiber-shaped carbon nanotubes. However, studies that directly compare graphene with carbon nanotubes are rare, making comparative considerations of their overall safety and risk assessment challenging. In this Account, we attempt to offer a set of rules for the development of graphene and its derivatives to enhance their overall safety and minimize the risks for adverse reactions in humans from exposure. These rules are: (1) to use small, individual graphene sheets that macrophages in the body can efficiently internalize and remove from the site of deposition; (2) to use hydrophilic, stable, colloidal dispersions of graphene sheets to minimize aggregation in vivo; and (3) to use excretable graphene material or chemically-modified graphene that can be degraded effectively. Such rules can only act as guidelines at this early stage in the development of graphene-based technologies, yet they offer a set of design principles for the fabrication and safe use of graphene material that will come in contact with the human body. In a broader context, the safety risks associated with graphene materials will be entirely dependent on the specific types of graphene materials and how they are investigated or applied. Therefore, generalizations about the toxicity of "graphene" as a whole will be inaccurate, possibly misleading, and should be avoided.

High‐Performance NO2 Sensors Based on Chemically Modified Graphene

Covalently grafting reduced graphene oxide (rGO) sheets with sulfophenyl or ethylenediamine groups can produce chemically modified graphene (CMG) for fabricating high-performance gas sensors. The NO(2) sensors based on these CMGs exhibit sensitivities 4 to 16 times higher than that of a sensor based on rGO. They also show excellent selectivity and repeatability without the aid of UV-light or thermal treatment.

ChemInform Abstract: Nanocomposites and Macroscopic Materials: Assembly of Chemically Modified Graphene Sheets

AbstractReview: 230 refs.

Engineering Graphene Mechanical Systems

We report a method to introduce direct bonding between graphene platelets that enables the transformation of a multilayer chemically modified graphene (CMG) film from a "paper mache-like" structure into a stiff, high strength material. On the basis of chemical/defect manipulation and recrystallization, this technique allows wide-range engineering of mechanical properties (stiffness, strength, density, and built-in stress) in ultrathin CMG films. A dramatic increase in the Young's modulus (up to 800 GPa) and enhanced strength (sustainable stress ≥1 GPa) due to cross-linking, in combination with high tensile stress, produced high-performance (quality factor of 31,000 at room temperature) radio frequency nanomechanical resonators. The ability to fine-tune intraplatelet mechanical properties through chemical modification and to locally activate direct carbon-carbon bonding within carbon-based nanomaterials will transform these systems into true "materials-by-design" for nanomechanics.

Structural transformations in chemically modified graphene

In this article, we review our efforts to continuously tune mechanical and thermal properties in multilayer chemically modified graphene (CMG) films. An alteration of the graphene lattice by functional groups, by defects created during reduction, or by defect re-crystallization is used to control CMG mechanical and thermal properties. We attribute a notable increase in Young's modulus and film strength to an emerging network of sp2–sp3 crosslinks established between graphene layers. Control over the film stress and strength enabled us to dramatically improve the performance of radio frequency CMG resonators by fine tuning the fabrication process.

3D Macroporous Graphene Frameworks for Supercapacitors with High Energy and Power Densities

In order to develop energy storage devices with high power and energy densities, electrodes should hold well-defined pathways for efficient ionic and electronic transport. Herein, we demonstrate high-performance supercapacitors by building a three-dimensional (3D) macroporous structure that consists of chemically modified graphene (CMG). These 3D macroporous electrodes, namely, embossed-CMG (e-CMG) films, were fabricated by using polystyrene colloidal particles as a sacrificial template. Furthermore, for further capacitance boost, a thin layer of MnO(2) was additionally deposited onto e-CMG. The porous graphene structure with a large surface area facilitates fast ionic transport within the electrode while preserving decent electronic conductivity and thus endows MnO(2)/e-CMG composite electrodes with excellent electrochemical properties such as a specific capacitance of 389 F/g at 1 A/g and 97.7% capacitance retention upon a current increase to 35 A/g. Moreover, when the MnO(2)/e-CMG composite electrode was asymmetrically assembled with an e-CMG electrode, the assembled full cell shows remarkable cell performance: energy density of 44 Wh/kg, power density of 25 kW/kg, and excellent cycle life.

Comparison of the electroanalytical performance of chemically modified graphenes (CMGs) using uric acid

We compared the differential pulse voltammetric performance of various chemically modified graphenes (CMGs) to those of glassy carbon (GC) and edge plane pyrolytic graphite (EPPG), using a biologically important analyte, uric acid. Analytical parameters of sensitivity, linearity, repeatability and selectivity were considered. Graphite oxide, graphene oxide, chemically reduced graphene oxide, electrochemically reduced graphene oxide and thermally reduced graphene oxide (TR-GO) were studied. TR-GO showed the highest sensitivity and selectivity, but among the poorest linearity and reproducibility. The other CMGs generally demonstrated more sensitive but less selective and reproducible responses than GC and EPPG. These results will enhance our understanding of the applicability of CMGs in biosensing.

Chemically Modified Graphene/Polyimide Composite Films Based on Utilization of Covalent Bonding and Oriented Distribution

Herein, we have developed a rather simple composite fabrication approach to achieving molecular-level dispersion and planar orientation of chemically modified graphene (CMG) in the thermosetting polyimide (PI) matrix as well as realizing strong adhesion at the interfacial regions between reinforcing filler and matrix. The covalent adhesion of CMG to PI matrix and oriented distribution of CMG were carefully confirmed and analyzed by detailed investigations. Combination of covalent bonding and oriented distribution could enlarge the effectiveness of CMG in the matrix. Efficient stress transfer was found at the CMG/PI interfaces. Significant improvements in the mechanical performances, thermal stability, electrical conductivity, and hydrophobic behavior were achieved by addition of only a small amount of CMG. Furthermore, it is noteworthy that the hydrophilic-to-hydrophobic transition and the electrical percolation were observed at only 0.2 wt % CMG in this composite system. This facile methodology is believed to afford broad application potential in graphene-based polymer nanocomposites, especially other types of high-performance thermosetting systems.

Graphene Nano-Biosensors for Detection of Cancer Risk

Biosensor diagnostics based on bio-functionalized semiconductor devices are an important development in ultrasensitive sensors for early detection of disease biomarkers. Electrochemical devices using chemically modified graphene (CMG) channels are excellent candidates for nanobiosensors. This paper presents the development of novel antibody functionalized epitaxial graphene devices for bio-sensing applications. Epitaxial graphene has been grown on silicon carbide (SiC) substrates under high vacuum and high temperature conditions (1200 – 1700°C). A generic electrochemical surface functionalisation chemistry, which can be used to attach a variety of “bio-receptors” to graphitic surfaces, has been developed. The attached bio-receptors are capable of specific and selective interaction with disease biomarkers. When a target biomarker molecule interacts with the “bio-receptor” functionalized surface, the charge density at that surface is affected. This change can be detected as an electrical signal from the biosensor, enabling highly sensitive (nM) detection of biomarker analytes. This paper reports the fabrication of graphene channel sensors for detection of disease biomarkers.

Impedimetric thrombin aptasensor based on chemically modified graphenes

Highly sensitive biosensors are of high importance to the biomedical field. Graphene represents a promising transducing platform for construction of biosensors. Here for the first time we compare the biosensing performance of a wide set of graphenes prepared by different methods. In this work, we present a simple and label-free electrochemical impedimetric aptasensor for thrombin based on chemically modified graphene (CMG) platforms such as graphite oxide (GPO), graphene oxide (GO), thermally reduced graphene oxide (TR-GO) and electrochemically reduced graphene oxide (ER-GO). Disposable screen-printed electrodes were first modified with chemically modified graphene (CMG) materials and used to immobilize a DNA aptamer which is specific to thrombin. The basis of detection relies on the changes in impedance spectra of redox probe after the binding of thrombin to the aptamer. It was discovered that graphene oxide (GO) is the most suitable material to be used as compared to the other three CMG materials. Furthermore, the optimum concentration of aptamer to be immobilized onto the modified electrode surface was determined to be 10 μM and the linear detection range of thrombin was 10-50 nM. Lastly, the aptasensor was found to demonstrate selectivity for thrombin. Such simply fabricated graphene oxide aptasensor shows high promise for clinical diagnosis of biomarkers and point-of-care analysis.