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.     

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.     

Highly Intact and Pure Oxo‐Functionalized Graphene: Synthesis and Electron‐Beam‐Induced Reduction

Controlling the chemistry of graphene is necessary to enable applications in materials and life sciences. Research beyond graphene oxide is targeted to avoid the highly defective character of the carbon framework. Herein, we show how to optimize the synthesis of oxo‐functionalized graphene (oxo‐G) to prepare high‐quality monolayer flakes that even allow for direct transmission electron microscopy investigation at atomic resolution (HRTEM). The role of undesired residuals is addressed and sources are eliminated. HRTEM provides clear evidence for the exceptional integrity of the carbon framework of such oxo‐G sheets. The patchy distribution of oxo‐functionality on the nm‐scale, observed on our highly clean oxo‐G sheets, corroborates theoretical predictions. Moreover, defined electron‐beam irradiation facilitates gentle de‐functionalization of oxo‐G sheets, a new route towards clean graphene, which is a breakthrough for localized graphene chemistry.     

Sulfur‐Functionalized Graphene Oxide by Epoxide Ring‐Opening

The treatment of graphene oxide (GO) with potassium thioacetate followed by an aqueous work‐up yields a new material via the ring‐opening of the epoxide groups. The new material is a thiol‐functionalized GO (GO‐SH) which is able to undergo further functionalization. Reaction with butyl bromide gives another new material, GO‐SBu, which shows significantly enhanced thermal stability compared to both GO and GO‐SH. The thiol‐functionalized GO material showed a high affinity for gold, as demonstrated by the selective deposition of a high density of gold nanoparticles.     

Thermal Disproportionation of Oxo-Functionalized Graphene

Graphene production by wet chemistry is an ongoing scientific challenge. Controlled oxidation of graphite introduces oxo functional groups; this material can be processed and converted back to graphene by reductive defunctionalization. Although thermal processing yields conductive carbon, a ruptured and undefined carbon lattice is produced as a consequence of CO₂ formation. This thermal process is not understood, but it is believed that graphene is not accessible. Here, we thermally process oxo-functionalized graphene (oxo-G) with a low (4–6 %) and high degree of functionalization (50–60 %) and find on the basis of Raman spectroscopy and transmission electron microscopy performed at atomic resolution (HRTEM) that thermal processing leads predominantly to an intact carbon framework with a density of lattice defects as low as 0.8 %. We attribute this finding to reorganization effects of oxo groups. This finding holds out the prospect of thermal graphene formation from oxo-G derivatives.     

Sodium 1‐Naphthalenesulfonate‐Functionalized Reduced Graphene Oxide Stabilizes Silver Nanoparticles with Lower Cytotoxicity and Long‐Term Antibacterial Activity

Silver nanoparticles (AgNPs) are increasingly used in daily life for their antibacterial properties, but their low stability and high cytotoxicity hamper practical applications. In this work, sodium 1‐naphthalenesulfonate‐functionalized reduced graphene oxide (NA‐rGO) was used as a substrate for AgNPs to produce a AgNP‐NA‐rGO hybrid. This hybrid showed substantially higher antibacterial activity than polyvinyl pyrrolidone(PVP)‐stabilized AgNPs, and the AgNPs on NA‐rGO were more stable than the AgNPs on PVP, resulting in long‐term antibacterial effects. More importantly, this hybrid showed excellent water solubility and low cytotoxicity, suggesting the great potential application as sprayable reduced graphene oxide based antibacterial solutions.     

Edge‐Selectively Functionalized Graphene Nanoplatelets

Graphene, as a single layer of graphite, is currently the focal point of research into condensed matter owing to its promising properties, such as exceptional mechanical strength, high thermal conductivity, large specific surface area, and ultrahigh electron‐transport properties. Therefore, various physical and chemical synthetic procedures to prepare graphene and/or graphene nanoplatelets have been rapidly developed. Specifically, the synthesis of edge‐selectively functionalized graphene (EFG) has been recently reported by using simple and scalable approaches, such as “direct” Friedel‐Crafts acylation reactions in a mild acidic medium and a mechanochemical ball‐milling process. In these approaches, chemical functionalization predominantly take place at the edges of the graphitic layers via the covalent attachment of targeted organic “molecular wedges”. In addition, the distortion of the crystalline structures in the basal plane, which is beneficial for preserving the unique properties of the graphitic framework, can be minimized. In addition, the efficient exfoliation of graphene can be achieved, owing to the strong repulsive forces from the covalently linked wedges and strong shear forces during the reaction. Furthermore, EFG shows promising potential in many useful applications, such as highly conductive large‐area films, metal‐free electrocatalysts for the oxygen‐reduction reaction (ORR), and as additives in composite materials with enhanced properties. Herein, we summarize the recent progress and general aspects of EFG, including synthesis, reaction mechanism, properties, and applications.     

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.     

KH-570功能化石墨烯的制备与表征

采用Hummers法对天然石墨进行氧化处理制备了氧化石墨烯,通过γ-甲基丙烯酰氧丙基三甲氧基硅烷与氧化石墨烯反应得到功能化氧化石墨烯,然后在水合肼的作用下制备了功能化石墨烯。未烘干的功能化石墨烯在超声处理下,能稳定分散在体积比为9∶1(V/V)的乙醇/水、丙酮/水或N,N-二甲基甲酰胺/水的混合溶剂中。用傅立叶变换红外光谱、原子力显微镜、X射线光电子能谱及X射线衍射对样品结构、形貌进行了分析。结果表明,KH-570上的硅氧烷与氧化石墨烯上的羟基发生了反应,经水合肼还原后,功能化石墨烯的无序度增加,层间距也比功能化氧化石墨烯的缩小了。功能化石墨烯在DMF/水中呈高度剥离状态,片层厚度为1.1～2.3 nm。     

Growth of PtRu Clusters on Ru(0001)‐Supported Monolayer Graphene Films

We report on results of a detailed scanning tunnelling microscopy study on the formation, size and size distribution, and internal structure of small bimetallic PtRu clusters on a graphene monolayer film supported on a Ru(0001) substrate. These clusters, with sizes around ～15 (Ru) or ～40 (Pt) atoms per cluster at the lowest coverage, are interesting model systems for the catalytic behaviour of small metal PtRu particles, for example for application in electrocatalytic oxidation reactions. The clusters were generated by sequential deposition of the two metals at room temperature. The data reveal a distinct influence of the deposition sequence on the cluster formation process, with Ru pre‐deposition followed by Pt deposition leading to predominantly bimetallic clusters, possibly with a core–shell‐type structure, while the reverse sequence results in co‐existent mono‐ and bimetallic clusters, where the latter are likely to intermix at the interface. The observations are related to the nucleation process of the respective metals on the templated surface, and the 2D growth behaviour of the two metals.     

n‐Type Reduced Graphene Oxide Field‐Effect Transistors (FETs) from Photoactive Metal Oxides

Graphene is of considerable interest as a next‐generation semiconductor material to serve as a possible substitute for silicon. For real device applications with complete circuits, effective n‐type graphene field effect transistors (FETs) capable of operating even under atmospheric conditions are necessary. In this study, we investigated n‐type reduced graphene oxide (rGO) FETs of photoactive metal oxides, such as TiO₂ and ZnO. These metal oxide doped FETs showed slight n‐type electric properties without irradiation. Under UV light these photoactive materials readily generated electrons and holes, and the generated electrons easily transferred to graphene channels. As a result, the graphene FET showed strong n‐type electric behavior and its drain current was increased. These n‐doping effects showed saturation curves and slowly returned back to their original state in darkness. Finally, the n‐type rGO FET was also highly stable in air due to the use of highly resistant metal oxides and robust graphene as a channel.     

Graphene oxide‐induced polymerization and crystallization to produce highly conductive polyaniline/graphene oxide composite

Graphene oxide (GO)–polyaniline (PANI) composite is synthesized by in situ polymerization of aniline in the presence of GO as oxidant, resulting in highly crystalline and conductive composite. Fourier transform infrared spectrum confirms aniline polymerization in the presence of GO without using conventional oxidants. Scanning electron microscopic images show the formation of PANI nanofibers attached to GO sheets. X‐ray diffraction (XRD) patterns indicate the presence of highly crystalline PANI. The sharp peaks in XRD pattern suggest GO sheets not only play an important role in the polymerization of aniline but also in inducing highly crystalline phase of PANI in the final composite. Electrical conductivity of doped GO–PANI composite is 582.73 S m^?1, compared with 20.3 S m^?1 for GO–PANI obtained by ammonium persulfate assisted polymerization. The higher conductivity appears to be the result of higher crystallinity and/or chemical grafting of PANI to GO, which creates common conjugated paths between GO and PANI. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1545–1554     

β‐Cyclodextrin Functionalized Nanoporous Graphene Oxides for Efficient Resolution of Asparagine Enantiomers

Efficient resolution of racemic mixture has long been an attractive but challenging subject since Pasteur separated tartrate enantiomers in 19^th century. Graphene oxide (GO) could be flexibly functionalized by using a variety of chiral host molecules and therefore, was expected to show excellent enantioselective resolution performance. However, this combination with efficient enantioselective resolution capability has been scarcely demonstrated. Here, nanoporous graphene oxides were produced and then covalently functionalized by using a chiral host material‐β‐cyclodextrin (β‐CD). This chiral GO displayed enantioselective affinity toward the l ‐enantiomers of amino acids. In particular, >99 % of l ‐asparagine (Asn) was captured in a racemic solution of Asn while the adsorption of d ‐enantiomer was not observed. This remarkable resolution performance was subsequently modelled by using an attach‐pull‐release dynamic method. We expect this preliminary concept could be expanded to other chiral host molecules and be employed to current membrane separation technologies and finally show practical use for many other racemates.     

White‐Light‐Emitting Edge‐Functionalized Graphene Quantum Dots

Graphene quantum dots (GQDs) have received considerable attention for their potential applications in the development of novel optoelectronic materials. In the generation of optoelectronic devices, the development of GQDs that are regulated in terms of their size and dimensions and are unoxidized at the sp² surfaces is desired. GQDs functionalized with bulky Fréchet’s dendritic wedges at the GQD periphery were synthesized. The single‐layered, size‐regulated structures of the dendronized GQDs were revealed by atomic force microscopy. The edge‐functionalization of the GQDs led to white‐light emission, which is an uncommon feature.     

Amides and Heparin‐Like Polymer Co‐Functionalized Graphene Oxide Based Core @ Polyethersulfone Based Shell Beads for Bilirubin Adsorption

Excessive bilirubin in the body of patient with liver dysfunction or metabolic obstruction may cause jaundice with irreversible brain damage, and new type of adsorbent for bilirubin is under frequent investigation. Herein, graphene oxide based core @ polyethersulfone‐based shell beads are fabricated by phase inversion method, amides and heparin‐like polymer are introduced to functionalize the core‐shell beads. The beads are successfully prepared with obvious core‐shell structure, adequate thermostability and porous shell. Clotting times and protein adsorption are investigated to inspect the hemocompatibility property of the beads. The adsorption of bilirubin is systematically investigated by evaluating the effects of contacting time, initial concentration and temperature on the adsorption, which exhibits improved bilirubin adsorption amount for the beads with amides contained cores or/and shells. It is worth believing that the amides and heparin‐like polymer co‐functionalized core‐shell beads may be utilized in the field of hemoperfusion for bilirubin adsorption.     

Reduced and Surface‐Modified Graphene Oxide with Nonlinear Resistivity

Field‐grading materials (FGMs) are used to reduce the probability for electrical breakdowns in critical regions of electrical components and are therefore of great importance. Usually, FGMs are heavily filled (40 vol.%) with semi‐conducting or conducting particles. Here, polymer‐grafted reduced graphene oxide (rGO) is used as a filler to accomplish percolated networks at very low filling ratios (<2 vol.%) in a semi‐crystalline polymer matrix: poly(ethylene‐co‐butyl acrylate) (EBA). Various simulation models are used to predict the percolation threshold and the flake‐to‐flake distances, to complement the experimental results. A substantial increase in thermal stability of rGO is observed after surface modification, either by silanization or subsequent polymerizations. The non‐linear DC resistivity of neat and silanized rGO and its trapping of charge‐carriers in semi‐crystalline EBA are demonstrated for the first time. It is shown that the polymer‐grafted rGO improve the dispersibility in the EBA‐matrix and that the graft length controls the inter‐flake distances (i.e. charge‐carrier hopping distances). By the appropriate selection of graft lengths, both highly resistive materials at 10 kV mm^‐1 and FGMs with a large and distinct drop in resistivity (six decades) are obtained, followed by saturation. The nonlinear drop in resistivity is attributed to narrow inter‐flake distance distributions of grafted rGO.     

Acetone‐Induced Graphene Oxide Film Formation at the Water–Air Interface

Graphene oxide (GO) is an amphiphilic soft material, which can accumulate at the water–air interface. However, GO sheets diffuse slowly in the aqueous phase because of their large size. It is still challenging to form high quality GO films in a controllable and simple way. In this study, we showed that GO sheets can quickly migrate to the water–air interface and form thin films when a suitable amount of acetone is directly mixed with a GO aqueous dispersion. The film formation rate and surface coverage of GO sheets depend on the volume of acetone added, GO dispersion concentration, and formation time. Among several organic solvents, acetone has its advantage for GO film formation owing to its three properties: a nonsolvent to GO aqueous dispersions, miscible with a GO aqueous dispersion, and fast evaporation. Furthermore, we have found that the film formation also is governed by the size of GO sheets and their oxygen content. Although smaller GO sheets could migrate to the water–air interface faster, the overlapping of small GO sheets and the increase in contact resistance is not desirable. A higher oxygen content in GO sheets could also result in smaller GO sheets. Multilayer GO films can be obtained through layer‐by‐layer dip‐coating. These findings open opportunities in developing simple scalable GO film fabrication processes.