Photoluminescence Properties of Graphene Oxide in Non-Aqueous Solvents

Detailed attention to the interaction between graphene oxide (GO) and various organic fluorophores has been documented in literature as a result of which the impact of GO on the photophysical properties of the fluorophores is well known to the scientific community. However, the photoluminescence (PL) properties of GO in polar aprotic solvents are yet to be established. In this article, the PL properties of GO in polar aprotic solvents using various spectroscopic techniques have been reported. n-π* transition due to the C=O bonds in the sp³ hybrid regions and π-π* transition due to C=C bonds in the sp² hybrid are prominent in GO. The presence of quasi-molecules within sp²-sp³ domains acts as PL centers located in the sp³ matrixes of GO are responsible for the PL properties. This study showcases the presence of multiple emissive states of GO in polar aprotic solvents and conveys the fact that the PL properties of GO are very much wavelength-dependent.     

Oxo‐Functionalized Graphene as a Cell Membrane Carrier of Nucleic Acid Probes Controlled by Aging

We applied a fluorescein‐containing oligonucleotide (ON) to probe surface properties of oxidized graphene (oxo‐G) and observed that graphene‐like patches are formed upon aging of oxo‐G, indicated by enhanced probe binding and by FTIR spectroscopic analysis. By using a recently developed fluorogenic endoperoxide (EP) probe, we confirmed that during the aging process the amount of EPs on the oxo‐G surface is reduced. Furthermore, aging was found to strongly affect cell membrane carrier properties of this material. In particular, freshly prepared oxo‐G does not act as a carrier, whereas oxo‐G aged for 28 days at 4 °C is an excellent carrier. Based on these data we prepared an optimized oxo‐G, which has a low‐defect density, binds ONs, is not toxic, and acts as cell membrane carrier. We successfully applied this material to design fluorogenic probes of representative intracellular nucleic acids 28S rRNA and β‐actin‐mRNA. The results will help to standardize oxidized graphene derivatives for biomedical and bioanalytical applications.     

Cytotoxicity Profile of Highly Hydrogenated Graphene

Graphene and its graphene‐related counterparts have been considered the future of advanced nanomaterials owing to their exemplary properties. An increase in their potential applications in the biomedical field has led to serious concerns regarding their safety and impact on health. To understand the toxicity profile for a particular type of graphene utilized in a given application, it is important to recognize the differences between the graphene‐related components and correlate their cellular toxicity effects to the attributed physiochemical properties. In this study, the cytoxicity effects of highly hydrogenated graphene (HHG) and its graphene oxide (GO) counterpart on the basis of in vitro toxicological assessments are reported and the effects correlated with the physiochemical properties of the tested nanomaterials. Upon 24 h exposure to the nanomaterials, a dose‐dependent cellular cytotoxic effect was exhibited and the HHG was observed to be more cytotoxic than its GO control. Detailed characterization revealed an extensive C?H sp³ network on the carbon backbone of HHG with few oxygen‐containing groups, as opposed to the presence of large amounts of oxygen‐containing groups on the GO. It is therefore hypothesized that the preferential adsorption of micronutrients on the surface of the HHG nanomaterial by means of hydrophobic interactions resulted in a reduction in the bioavailability of nutrients required for cellular viability. The nanotoxicological profile of highly hydrogenated graphene is assessed for the first time in our study, thereby paving the way for further evaluation of the toxicity risks involved with the utilization of various graphene‐related nanomaterials in the real world.     

Preparation and characterization of corn starch/soy protein biocomposite film reinforced with graphene and graphene oxide nanoplatelets

Corn starch (CS) and soy protein isolate (SPI), as inexpensive, abundant, and biodegradable materials, can chemically interact well with each other to produce biofilms. However, to overcome some of their physical and mechanical limitations, it is preferred to use their composite form, employing reinforcing materials. In this study, initially, graphene (G) and graphene oxide (GO) were synthesized by a green method. Then, to enhance the polymer blend final properties, the effects of adding G and GO in the range of 0.5 to 2 wt% on physical and mechanical properties of starch/protein blend were investigated. The results showed that the presence of 0.5‐wt% G and 2‐wt% GO significantly increased the modulus of starch/protein film from 252 to 578 and 449 MPa, respectively. In addition, the thermal stability of CS/SPI/GO (2 wt%) composite film was 50°C to 60°C more than that of the pure starch/protein film. On the other hand, G‐reinforced composite films tended to decline water diffusion compared with the pure polymer film. In addition, the composite film with 2‐wt% GO content had the lowest oxygen permeation rate (3.48 cm³ μm/m²d kpa) among the other composite films.     

Synthesis of polydopamine‐coated graphene–polymer nanocomposites via RAFT polymerization

Graphene‐polymer nanocomposites have significant potential in many applications such as photovoltaic devices, fuel cells, and sensors. Functionalization of graphene is an essential step in the synthesis of uniformly distributed graphene‐polymer nanocomposites, but often results in structural defects in the graphitic sp² carbon framework. To address this issue, we synthesized graphene oxide (GO) by oxidative exfoliation of graphite and then reduced it into graphene via self‐polymerization of dopamine (DA). The simultaneous reduction of GO into graphene, and polymerization and coating of polydopamine (PDA) on the reduced graphene oxide (RGO) surface were confirmed with XRD, UV–Vis, XPS, Raman, TGA, and FTIR. The degree of reduction of GO increased with increasing DA/GO ratio from 1/4 to 4/1 and/or with increasing temperature from room temperature to 60 °C. A RAFT agent, 2‐(dodecylthiocarbonothioylthio)?2‐methylpropionic acid, was linked onto the surface of the PDA/RGO, with a higher equivalence of RAFT agent in the reaction leading to a higher concentration of RAFT sites on the surface. Graphene‐poly(methyl methacrylate), graphene‐poly(tert‐butyl acrylate), and graphene‐poly(N‐isopropylacrylamide) nanocomposites were synthesized via RAFT polymerization, showing their characteristic solubility in several different solvents. This novel synthetic route was found facile and can be readily used for the rational design of graphene‐polymer nanocomposites, promoting their applications. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3941–3949     

Regeneration of a Conjugated sp2 Graphene System through Selective Defunctionalization of Epoxides by Using a Proven Synthetic Chemistry Mechanism

Graphene is a promising material capable of driving technological advancement. It is, however, a challenge to obtain pristine graphene in large quantities given the limitation of current synthetic methods. Among the numerous methods available, the chemical approach provides an optimistic outlook and has garnered much interest within the graphene community as a potential alternative. One of the most crucial steps of the chemical approach is the chemical reduction of graphene oxide as this dictates the final quality of the graphene sheets. In recent years, much of the focus has shifted to the usage of established reducing agents or oxygen removal reagents, frequently applied in organic chemistry, onto a graphene oxide platform. Herein, the selective removal of epoxide groups and subsequent regeneration of disrupted conjugated sp² system is highlighted, based on the synergistic effect of indium and indium(I) chloride. The morphological, structural, and electrical properties of the resulting graphene were fully characterized with X‐ray photoelectron, Fourier transform IR, solid‐state ¹³C NMR, and Raman spectroscopy; thermogravimetric analysis; scanning electron microscopy; and conductivity measurements. The as‐prepared graphene showed a tenfold increase in conductivity against conventional graphene treated with hydrazine reducing agent and demonstrated a high dispersion stability in ethanol. Moreover, the selective defunctionalization of the epoxide groups provides opportunities for potential tailoring of graphene properties for prospective applications.     

Graphene‐Based Composite with γ‐Fe2O3 Nanoparticle for the High‐Performance Removal of Endocrine‐Disrupting Compounds from Water

Graphene is a 2D sp²‐hybridized carbon sheet and an ideal material for the adsorption‐based separation of organic pollutants. However, such potential applications of graphene are largely limited, owing to their poor solubility and extensive aggregation properties through graphene? graphene interactions. Herein, we report the synthesis of graphene‐based composites with γ‐Fe₂O₃ nanoparticle for the high‐performance removal of endocrine‐disrupting compounds (EDC) from water. The γ‐Fe₂O₃ nanoparticles partially inhibit these graphene? graphene interactions and offer water dispersibility of the composite without compromising much of the high surface area of graphene. In their dispersed form, the graphene component offers the efficient adsorption of EDC, whilst the magnetic iron‐oxide component offers easier magnetic separation of adsorbed EDC.     

The Preparation of Compressible and Fire‐Resistant Sponge‐Supported Reduced Graphene Oxide Aerogel for Electromagnetic Interference Shielding

We here report a facile method to fabricate a sponge‐supported reduced graphene oxide aerogel (S‐RGOA) using a commercial melamine sponge and graphene oxide (GO). Firstly, GO sheets were self‐assembled within the melamine sponge by the assistance of a chemical cross‐linking agent; and then, freeze‐drying and thermal treatment were adopted to prepare S‐RGOA, in which continuous porous reduced graphene oxide (RGO) network formed between the skeleton. The resulting S‐RGOA exhibited a high electromagnetic interference shielding effectiveness (EMI SE) of 20.4‐27.3 dB in 8–12 GHz and the specific EMI SE could reach 1437 dB?cm³g^?1. The mechanical test suggests that the lightweight S‐RGOA is compressible and possesses low energy dissipation. Burning and TGA measurements indicate that S‐RGOA is fire‐resistant and has excellent thermal stability. Our work provides an economical and environmentally‐friendly method to fabricate RGO aerogels for using as electromagnetic interference materials.     

On the role of substituent in noncovalent functionalization of graphene and organophosphor recognition: IQA and SAPT perspective

Despite importance of integrating organic molecules with graphene to fabricate graphene‐based electronic devices, the role of substituents and interface stabilizing forces are poorly understood. In this work, the interactions of 7,7,8,8‐tetracyanoquinodimethane (TCNQ), 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ), hydroquinone (Q), and tetrafluorohydroquinone (TFQ) with graphene have been investigated by means of interacting quantum atoms and SAPT(DFT). In addition, in context of potential design of a graphene‐based sensor for detection of the nerve agent sarin, we studied the interaction of graphene and the organic molecules with the dimethyl methylphosphonate (DMMP)—the molecule that mimics sarin. The results show that the organic molecules attach to graphene via C(sp²)?C(sp²), C(sp²)?C(sp) and H?π bonds. In addition, they trap DMMP via various linkages such as hydrogen, lonepair?π and H?π . The quantum effects play a significant role. The Pauli repulsion is responsible for p‐doping of graphene. The substituents are stabilized on graphene by the exchange‐correlation energy. The fluorination of the benzenoid ring raises the electron‐sharing . The through space and through bond effects of the fluorine atoms (‐F) increase the classical attraction of the cyano groups and benzenoid ring with graphene, respectively. When comparing performance of the ab initio and DFT methods, MP2 predicts too much attraction due to well‐known overestimation of the dispersion energy by the uncoupled dispersion component for benzene rings, while ω B97xD functional and SAPT(DFT) provide weaker interaction energies, in good agreement with each other.     

Controlled Chemistry Approach to the Oxo‐Functionalization of Graphene

Graphene is the best‐studied 2D material available. However, its production is still challenging and the quality depends on the preparation procedure. Now, more than a decade after the outstanding experiments conducted on graphene, the most successful wet‐chemical approach to graphene and functionalized graphene is based on the oxidation of graphite. Graphene oxide has been known for more than a century; however, the structure bears variable large amounts of lattice defects that render the development of a controlled chemistry impossible. The controlled oxo‐functionalization of graphene avoids the formation of defects within the σ‐framework of carbon atoms, making the synthesis of specific molecular architectures possible. The scope of this review is to introduce the field of oxo‐functionalizing graphene. In particular, the differences between GO and oxo‐functionalized graphene are described in detail. Moreover analytical methods that allow determining lattice defects and functional groups are introduced followed by summarizing the current state of controlled oxo‐functionalization of graphene.     

Study of Anticancer Properties of Graphene Oxide–Metal Oxide Composite

Graphene and graphene oxide (GO) have garnered significant attention due to their exceptional properties. GO, enriched with various functional groups such as epoxy, hydroxyl, and carboxylic groups, has exhibited remarkable potential in biomedical applications. The combination of GO with metals has proven to be a promising platform for cellular imaging, with this study focusing on the preparation of diverse hybrids of GO with metal oxides (GO/MO) and their potential as anticancer agents. In this research, GO is functionalized with MOs like TiO₂, Fe₃O₄, and Cu₂O using specific chemical methods and investigated for the anticancer activity for the application as cancer therapeutic agent. The resulting GO/MO hybrids exhibits favorable thermal and mechanical properties. Moreover, their cytotoxicity against human lung cancer cells is assessed in vitro, revealing the promising anticancer activity of GO/MO hybrids. Notably, the GO/Cu₂O hybrid demonstrates particularly high cytotoxicity in human lung cancer cells.     

Alkali‐Metal‐Ion‐Functionalized Graphene Oxide as a Superior Anode Material for Sodium‐Ion Batteries

Although graphene oxide (GO) has large interlayer spacing, it is still inappropriate to use it as an anode for sodium‐ion batteries (SIBs) because of the existence of H‐bonding between the layers and ultralow electrical conductivity which impedes the Na⁺ and e^? transformation. To solve these issues, chemical, thermal, and electrochemical procedures are traditionally employed to reduce GO nanosheets. However, these strategies are still unscalable, consume high amounts of energy, and are expensive for practical application. Here, for the first time, we describe the superior Na storage of unreduced GO by a simple and scalable alkali‐metal‐ion (Li⁺, Na⁺, K⁺)‐functionalized process. The various alkali metals ions, connecting with the oxygen on GO, have played different effects on morphology, porosity, degree of disorder, and electrical conductivity, which are crucial for Na‐storage capabilities. Electrochemical tests demonstrated that sodium‐ion‐functionalized GO (GNa) has shown outstanding Na‐storage performance in terms of excellent rate capability and long‐term cycle life (110 mAh g^?1 after 600 cycles at 1 A g^?1) owing to its high BET area, appropriate mesopore, high degree of disorder, and improved electrical conductivity. Theoretical calculations were performed using the generalized gradient approximation (GGA) to further study the Na‐storage capabilities of functionalized GO. These calculations have indicated that the Na?O bond has the lowest binding energy, which is beneficial to insertion/extraction of the sodium ion, hence the GNa has shown the best Na‐storage properties among all comparatives functionalized by other alkali metal ions.     

Functional Graphene by Thiol‐ene Click Chemistry

Thiol‐ene click reaction was successfully employed for chemical modification of graphene oxide (GO) by one‐step synthesis. Herein, 2,2‐azobis(2‐methylpropionitrile) (AIBN) was used as thermal catalyst and cysteamine hydrochloride (HS?(CH₂)₂?NH₂HCl) was used as thiol‐containing compound, which is incorporated to GO surface upon reaction with the C=C bonds. The hydrochloride acts as protecting group for the amine, which is finally eliminated by adding sodium hydroxide. The modified GO contains both S‐ and N‐containing groups (NS‐GO). We found that NS‐GO sheets form good dispersion in water, ethanol, and ethylene glycol. These graphene dispersions can be processed into functionalized graphene film. Besides, it was demonstrated that NS‐GO was proved to be an excellent host matrix for platinum nanoparticles. The developed method paves a new way for graphene modification and its functional nanocomposites.     

Precise Tuning of Surface Composition and Electron‐Transfer Properties of Graphene Oxide Films through Electroreduction

Graphene materials are generally prepared from the exfoliation of graphite oxide (GO) to graphene oxide, followed by subsequent chemical or thermal reduction. These methods, although efficient in removing most of the oxygen functionalities from the GO material, lack control over the extent of the reduction process. We demonstrate here an electrochemical reduction procedure that not only allows for precise control of the reduction process to obtain a graphene material with a well‐defined C/O ratio in the range of 3 to 10, but also one that is able to tune the electrocatalytic properties of the reduced material. A method that is able to precisely control the amount and density of the oxygen functionalities on the graphene material as well as its electrochemical behaviour is very important for several applications such as electronics, bio‐composites and electrochemical devices.     

Electrochemically Exfoliated Graphene and Graphene Oxide for Energy Storage and Electrochemistry Applications

Top‐down methods are of key importance for large‐scale graphene and graphene oxide preparation. Electrochemical exfoliation of graphite has lately gained much interest because of the simplicity of execution, the short process time, and the good quality of graphene that can be obtained. Here, we test three different electrolytes, that is, H₂SO₄, Na₂SO₄, and LiClO₄, with a common exfoliation procedure to evaluate the difference in structural and chemical properties that result for the graphene. The properties are analyzed by means of scanning transmission electron microscopy (STEM), Raman spectroscopy, and X‐ray photoelectron spectroscopy. We then tested the graphene materials for electrochemical applications, measuring the heterogeneous electron transfer (HET) rates with a Fe(CN)₆^3?/4? redox probe, and their capacitive behavior in alkaline solutions. We correlate the electrochemical features with the presence of structural defects and oxygen functionalities on the graphene materials. In particular, the use of LiClO₄ during the electrochemical exfoliation of graphite allowed the formation of highly oxidized graphene with a C/O ratio close to 4.0 and represents a possible avenue for the mass production of graphene oxide as valid alternative to the current laborious and dangerous chemical procedures, which also have limited scalability.     

Graphene Oxide/Polyacrylamide/Aluminum Ion Cross‐Linked Carboxymethyl Hemicellulose Nanocomposite Hydrogels with Very Tough and Elastic Properties

Development of high‐strength hydrogels has recently attracted ever‐increasing attention. In this work, a new design strategy has been proposed to prepare graphene oxide (GO)/polyacrylamide (PAM)/aluminum ion (Al³⁺)‐cross‐linked carboxymethyl hemicellulose (Al‐CMH) nanocomposite hydrogels with very tough and elastic properties. GO/PAM/Al‐CMH hydrogels were synthesized by introducing graphene oxide (GO) into PAM/CMH hydrogel, followed by ionic cross‐linking of Al³⁺. The nanocomposite hydrogels were characterized by means of FTIR, X‐ray diffraction (XRD), and scanning electron microscopy/energy‐dispersive X‐ray analysis (SEM‐EDX) along with their swelling and mechanical properties. The maximum compressive strength and the Young's modulus of GO_3.5/PAM/Al‐CMH_0.45 hydrogel achieved values of up to 1.12 and 13.27 MPa, increased by approximately 6488 and 18330 % relative to the PAM hydrogel (0.017 and 0.072 MPa). The as‐prepared GO/PAM/Al‐CMH nanocomposite hydrogels possess high strength and great elasticity giving them potential in bioengineering and drug‐delivery system applications.     

High Correlation between Oxidation Loci on Graphene Oxide

Recent experiments have shown the coexistence of both large unoxidized and oxidized regions on graphene oxide (GO), but the underlying mechanism for the formation of the GO atomic structure remains unknown. Now, using density functional calculations, 52 oxidation pathways for local pyrene structures on GO were identified, and a kinetic profile for graphene oxidation with a high correlation between oxidation loci was proposed, which is different from the conventional view, which entails a random distribution of oxidation loci. The high correlation is an essential nature of graphene oxidation processes and can be attributed to three crucial effects: 1) breaking of delocalized π bonds, 2) steric hindrance, and 3) hydrogen‐bond formation. This high correlation leads to the coexistence of both large unoxidized and oxidized regions on GO. Interestingly, even in oxidized regions on GO, some small areas of sp²‐hybridized domains, similar to “islands”, can persist because of steric effects.     

New Insights into the Diels–Alder Reaction of Graphene Oxide

Graphene oxide is regarded as a major precursor for graphene‐based materials. The development of graphene oxide based derivatives with new functionalities requires a thorough understanding of its chemical reactivity, especially for canonical synthetic methods such as the Diels–Alder cycloaddition. The Diels–Alder reaction has been successfully extended with graphene oxide as a source of diene by using maleic anhydride as a dienophile, thereby outlining the presence of the cis diene present in the graphene oxide framework. This reaction provides fundamental information for understanding the exact structure and chemical nature of graphene oxide. On the basis of high‐resolution ¹³C‐SS NMR spectra, we show evidence for the formation of new sp³ carbon centers covalently bonded to graphene oxide following hydrolysis of the reaction product. DFT calculations are also used to show that the presence of a cis dihydroxyl and C vacancy on the surface of graphene oxide are promoting the reaction with significant negative reaction enthalpies.     

Investigation of the Thermal Stability of the Carbon Framework of Graphene Oxide

In this study, we use our recently prepared graphene oxide (GO) with an almost intact σ‐framework of carbon atoms (ai‐GO) to probe the thermal stability of the carbon framework for the first time. Ai‐GO exhibits few defects because CO₂ formation is prevented during synthesis. Ai‐GO was thermally treated before chemical reduction and the resulting defect density in graphene was subsequently determined by statistical Raman microscopy. Surprisingly, the carbon framework of ai‐GO is stable in thin films up to 100 °C. Furthermore, we find evidence for an increase in the quality of ai‐GO upon annealing at 50 °C before reduction. The carbon framework of GO prepared according to the popular Hummers’ method (GO‐c) appears to be less stable and decomposition starts at 50 °C, which is qualitatively indicated by CO₂‐trapping experiments in μm‐thin films. Information about the stability of GO is important for storing, processing, and using GO in many applications.     

Preparation of porous polyimide/in‐situ reduced graphene oxide composite films for electromagnetic interference shielding

Herein we report an easy and efficient approach to prepare lightweight porous polyimide (PI)/reduced graphene oxide (RGO) composite films. First, porous poly (amic acid) (PAA)/graphene oxide (GO) composite films were prepared via non‐solvent induced phase separation (NIPS) process. Afterwards PAA was converted into PI through thermal imidization and simultaneously GO dispersed in PAA matrix was in situ thermally reduced to RGO. The GO undergoing the same thermal treatment process as thermal imidization was characterized with thermogravimetric analysis, Raman spectra, X‐ray photoelectron spectroscopy and X‐ray diffraction to demonstrate that GO was in situ reduced during thermal imidization process. The resultant porous PI/RGO composite film (500‐µm thickness), which was prepared from pristine PAA/GO composite with 8 wt% GO, exhibited effective electrical conductivity of 0.015 S m^?1 and excellent specific shielding efficiency value of 693 dB cm² g^?1. In addition, the thermal stability of the porous PI/RGO composite films was also dramatically enhanced. Compared with that of porous PI film, the 5% weight loss temperature of the composite film mentioned above was improved from 525°C to 538°C. Moreover, tensile test showed that the composite film mentioned above possessed a tensile strength of 6.97 MPa and Young's modulus of 545 MPa, respectively. Copyright © 2016 John Wiley & Sons, Ltd.