Investigation of Simultaneous Cation-Π and Π–Π Stacking Interactions on Graphene and Some Bent Graphenes as Curved Surfaces of Carbon Nanohorns

Computational quantum chemistry methods are used to study simultaneous cation-π and π–π stacking interactions with a graphene sheet and on the inner and outer faces of some bent graphenes as curved surfaces of carbon nanohorns (CNHs). Structural parameters and energy data of ternary benzene–graphene–Na⁺ and benzene-bent graphene–Na⁺ complexes are studied. Also, effects of charge transfer and aromaticity are estimated to determine how changes in the structure influences the above interactions. The results indicate that the graphene curvature leads to structural changes affecting simultaneous interactions of the Na+ cation and benzene with bent graphenes. Also, the results show that although π–π stacking is a weak interaction, it can manipulate the order of binding energies in complexes involving both mentioned interactions and affect drug delivery abilities of these systems.     

Electrochemistry at chemically modified graphenes

Electrochemical applications of graphene are of great interest to many researchers as they can potentially lead to crucial technological advancements in fabrication of electrochemical devices for energy production and storage, and highly sensitive sensors. There are many routes towards fabrication of bulk quantities of chemically modified graphenes (CMG) for applications such as electrode materials. Each of them yields different graphene materials with different functionalities and structural defects. Here, we compare the electrochemical properties of five different chemically modified graphenes: graphite oxide, graphene oxide, thermally reduced graphene oxide, chemically reduced graphene oxide, and electrochemically reduced graphene oxide. We characterized these materials using transmission electron microscopy, Raman spectroscopy, high-resolution X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and cyclic voltammetry, which allowed us to correlate the electrochemical properties with the structural and chemical features of the CMGs. We found that thermally reduced graphene oxide offers the most favorable electrochemical performance among the different materials studied. Our findings have a profound impact for the applications of chemically modified graphenes in electrochemical devices.     

Nucleic acid functionalized graphene for biosensing

There is immense demand for complex nanoarchitectures based on graphene nanostructures in the fields of biosensing or nanoelectronics. DNA molecules represent the most versatile and programmable recognition element and can provide a unique massive parallel assembly strategy with graphene nanomaterials. Here we demonstrate a facile strategy for covalent linking of single stranded DNA (ssDNA) to graphene using carbodiimide chemistry and apply it to genosensing. Since graphenes can be prepared by different methods and can contain various oxygen containing groups, we thoroughly investigated the utility of four different chemically modified graphenes for functionalization by ssDNA. The materials were characterized in detail and the different DNA functionalized graphene platforms were then employed for the detection of DNA hybridization and DNA polymorphism by using impedimetric methods. We believe that our findings are very important for the development of novel devices that can be used as alternatives to classical techniques for sensitive and fast DNA analysis. In addition, covalent functionalization of graphene with ssDNA is expected to have broad implications, from biosensing to nanoelectronics and directed, DNA programmable, self-assembly.     

Effects of heteroatoms on the electronic,sensor, and adsorption properties of graphene

The effects of doping heteroatoms on the structure, electronic and adsorption properties of graphene are investigated using density functional theory calculations. Six different doped graphenes (with Al, B, Si, N, P, and S) are considered, and to obtain the interaction and adsorption properties, three sulfur-containing molecules (H₂S, SO₂, and thiophene) were interacted with selected graphenes. The adsorption energies (E _ad) in the gas phase and solvents show the exothermic interaction for all complexes. The maximum E _ad values are observed for aluminum doped graphene (AG) and silicon doped graphene (SiG), and adsorption energies in the solvent are not so different from those in the gas phase. NBO calculations show that the AG and SiG complexes have the highest E ⁽²⁾ interaction energies and simple graphene (G) and nitrogen doped graphene (NG) have the least E ⁽²⁾ energies. Population analyses show that doping heteroatoms change the energy gap. This gap changes more during the interaction and these changes make these structures useful in sensor devices. All calculated data confirm better adsorption of SO₂ by graphenes versus H₂S and thiophene. Among all graphenes, AG and then SiG are the best adsorbents for these structures.     

Spin-spin and spin-orbit interactions in nanographene fragments: a quantum chemistry approach

The relativistic behavior of graphene structures, starting from the fundamental building blocks--the poly-aromatic hydrocarbons (PAHs) along with other PAH nanographenes--is studied to quantify any associated intrinsic magnetism in the triplet (T) state and subsequently in the ground singlet (S) state with account of possible S-T mixture induced by spin-orbit coupling (SOC). We employ a first principle quantum chemical-based approach and density functional theory (DFT) for a systematic treatment of the spin-Hamiltonian by considering both the spin-orbit and spin-spin interactions as dependent on different numbers of benzene rings. We assess these relativistic spin-coupling phenomena in terms of splitting parameters which cause magnetic anisotropy in absence of external perturbations. Possible routes for changes in the couplings in terms of doping and defects are also simulated and discussed. Accounting for the artificial character of the broken-symmetry solutions for strong spin polarization of the so-called "singlet open-shell" ground state in zigzag graphene nanoribbons predicted by spin-unrestricted DFT approaches, we interpolate results from more sophisticated methods for the S-T gaps and spin-orbit coupling (SOC) integrals and find that these spin interactions become weak as function of size and increasing decoupling of electrons at the edges. This leads to reduced electron spin-spin interaction and hence almost negligible intrinsic magnetism in the carbon-based PAHs and carbon nanographene fragments. Our results are in agreement with the fact that direct experimental evidence of edge magnetism in pristine graphene has been reported so far. We support the notion that magnetism in graphene only can be ascribed to structural defects or impurities.     

Graphene: The New Two‐Dimensional Nanomaterial

Every few years, a new material with unique properties emerges and fascinates the scientific community, typical recent examples being high‐temperature superconductors and carbon nanotubes. Graphene is the latest sensation with unusual properties, such as half‐integer quantum Hall effect and ballistic electron transport. This two‐dimensional material which is the parent of all graphitic carbon forms is strictly expected to comprise a single layer, but there is considerable interest in investigating two‐layer and few‐layer graphenes as well. Synthesis and characterization of graphenes pose challenges, but there has been considerable progress in the last year or so. Herein, we present the status of graphene research which includes aspects related to synthesis, characterization, structure, and properties.     

Interaction of cationic liposomes with cell membrane models

The colloidal dispersion stability of nano-sized graphene sheets in supercritical fluid (SCF) media is very important for developing SCF-based exfoliation and dispersion technologies for stabilization and solubilization of graphenes. We carried out molecular dynamics simulations to elucidate the stability mechanism of graphene in supercritical CO(2) (scCO(2)). The potential of mean force (PMF) between two graphene nanosheets in scCO(2) was simulated, and the effect of scCO(2) density and temperature on the PMF behavior has been investigated. The simulation results demonstrate that there exists a free energy barrier between graphenes in the scCO(2) fluid, possibly obstructing the aggregation of graphenes. The single-layer confined CO(2) molecules between the graphene sheets can induce a dominating repulsion interaction between graphene sheets. At higher scCO(2) fluid density, there are more confined CO(2) molecules within the interplate regions, resulting in a stronger repulsive free energy barrier. The effect of temperature on the PMF is relatively minor. The scCO(2) solvent structure shows layered confined arrangement in the interfacial region near the graphene nanosheets, which is correlated well with the PMF profile curve.     

Inherent Electrochemistry and Activation of Chemically Modified Graphenes for Electrochemical Applications

Graphene research is currently at the frontier of electrochemistry. Many different graphene‐based materials are employed by electrochemists as electrodes in sensing and in energy‐storage devices. Because the methods for their preparation are inherently different, graphene materials are expected to exhibit different electrochemical behaviors depending on the functionalities and density of defects present. Electrochemical treatment of these “chemically modified graphenes” (CMGs) represents an easy approach to alter surface functionalities and consequently tune the electrochemical performance. Herein, we report a preliminary electrochemical characterization of four common chemically modified graphenes, namely: graphene oxide, graphite oxide, chemically reduced graphene oxide, and thermally reduced graphene oxide. These CMGs were compared with graphite as a reference material. Cyclic voltammetry was used to ascertain the chemical functionalities present and to understand the potential ranges in which the materials were electroactive. Electrochemical treatment with either an oxidative or a reductive fixed potential were then carried out to activate these chemically modified graphenes. The effects of such electrochemical treatments on their electrocatalytic properties were then investigated by cyclic voltammetry in the presence of well‐known redox probes, such as [Fe(CN)₆]^4?/3?, Fe^3+/2+, [Ru(NH₃)₆]^2+/3+, and ascorbic acid. Thermally reduced graphene oxide exhibited the best electrochemical behavior amongst all of the CMGs, with the fastest rate of heterogeneous electron transfer (HET) and the lowest overpotentials. These findings will have far‐reaching consequences for the evaluation of different CMGs as electrode materials in electrochemical devices.     

Control the size and surface chemistry of graphene for the rising fluorescent materials

Fluorescent graphene-based materials, labelled as a sort of fluorescent carbon-based nanomaterial, have drawn increasing attention in recent years. When the size and structure of graphene were controlled properly, photoluminescence was induced in graphene, resulting in the so-called fluorescent graphene (FG). FG has a size-, defect-, and wavelength-dependent luminescence emission, which is similar to traditional semiconductor-based quantum dots. Moreover, with excellent chemical stability, fine biocompatibility, low toxicity, up-conversion emission, pH-sensitivity and resistance to photobleaching, FG promises to offer substantial applications in numerous areas: bioimaging, photovoltaics, sensors, etc. Currently, research works have allowed FG to be produced by many approaches ranging from simple oxidation of graphene to cutting carbon sources and organic synthesis from small molecules. In this Feature Article, we summarize the reported fluorescent graphenes, with emphasis on their category, properties, synthesis and applications. Meanwhile, we give a perspective on their subsequent developments and compare the features of FG and other fluorescent carbon-based materials.     

Sonochemical preparation of functionalized graphenes

A convenient sonochemical method is described for the preparation of polystyrene functionalized graphenes starting from graphite flakes and a reactive monomer, styrene. Ultrasonic irradiation of graphite in styrene results in the mechanochemical exfoliation of graphite flakes to single-layer and few-layer graphene sheets combined with functionalization of the graphene with polystyrene chains. The polystyrene chains are formed from sonochemically initiated radical polymerization of styrene and can make up to ~18 wt % of the functionalized graphene, as determined by thermal gravimetric analysis. This one-step protocol can be generally applied to the functionalization of graphenes with other vinyl monomers for graphene-based composite materials.     

Epitaxial assembly of graphene on face (0001) of silicon carbide: Modeling by semiempirical methods

The epitaxial growth of graphene on silicon face (0001) of silicon carbide is simulated using the semiempirical methods of quantum chemistry. The experimental conditions for the epitaxial growth of graphene on SiC, at which the probability of seams and similar defects appearing is reduced to a minimum, are formulated. Possible ways of the emergence of reconstructions of the singular carbon and silicon SiC faces during the synthesis of graphenes are investigated as a test of the approach’s efficiency. It is noted that simulation reproduces the reconstruction periods experimentally determined for both faces, and yields the most likely atomic arrangements in cases where the experimental formula of the superstructure allows different versions of such arrangements.     

Theoretical investigation on the performance of simple and doped graphenes for the surface adsorption of various ions and water desalination

In this work, the abilities of simple and heteroatom-doped graphenes for the adsorption of several ions (sodium, calcium, magnesium, chloride, carbonate, sulfate, and nitrate) were studied using theoretical methods. The results of this work could be helpful for future studies in the water desalination projects because the considered ions are the major existing ions in water. The structures of all graphenes and their complexes with these ions were optimized in the gas phase and water (using PCM model). Moreover, the interaction energies in the gas phase and water, atomic charges, second-order perturbation energies (from NBO calculations), and QTAIM results were obtained to provide more evidences related to these interactions. The obtained energies showed more favorable interactions in water versus than those in the gas phase. Among all graphenes, aluminum-doped graphene showed the highest and nitrogen-doped graphene showed the least adsorption energies. QTAIM calculations indicated the noticeable electron densities in bond critical points for all interactions, and the Laplacians of electron densities confirmed nonbonding nature for the most of interactions. Population analyses revealed that the HOMO-LUMO gap in doped graphene was increased in comparing with simple graphene that make them as potential candidates for detection of these ions.     

Graphene materials-based chemiluminescence for sensing

Graphene has attracted considerable attention in multidisciplinary research fields and shown various promising applications due to its unique structure and extraordinary physicochemical properties. This review covers the latest advances in graphene materials-based chemiluminescence (CL) for sensing. Chemiluminescence resonance energy transfer and luminescence quenching of graphene materials are discussed. Graphene materials, such as graphene nanosheets, graphene quantum dots, graphene oxide, and reduced graphene oxide have been employed successfully in CL systems in recent years. Graphene materials can be utilized as catalysts, platforms, and energy acceptors to improve the performance of CL. Possible challenges and future perspective on this topic are also presented.     

Mesomeric Effects of Graphene Modified with Diazonium Salts: Substituent Type and Position Influence its Properties

In the last decade, graphene and graphene derivatives have become some of the most intensively studied materials. Tuning of the electronic and electrochemical properties of graphene is of paramount importance. In this study, six diazonium‐modified graphenes containing different functional groups according to the diazonium salt precursor were investigated. These diazonium moieties have a strong mesomeric (resonance) effect and act as either electron‐donating or ‐withdrawing species. Different graphene precursors, such as thermally and chemically reduced graphenes were studied. All the products were characterized in detail by elemental combustion analysis, FTIR spectroscopy, Raman spectroscopy, high‐resolution X‐ray photoelectron spectroscopy (XPS), and cyclic voltammetry. Resistivity and zeta potential measurements were consistent with theoretical (DFT) calculations. The results show that chemical modification of graphene by diazotation strongly influences its properties, creating a huge application potential in microelectronics, energy storage and conversion devices, and electrocatalysis.     

功能化石墨烯材料：定义、分类及制备策略

自2004年被成功制备后,石墨烯因其独特迷人的性质在近十几年来备受关注,同时也引发了二维纳米材料的研究热潮。单原子层厚度的二维结构赋予石墨烯非同寻常的光学、电子学、磁学及力学等性质,使得石墨烯在生物学、医学、化学、物理学和环境科学等多个领域展现出极大的应用潜力。制得注意的是,石墨烯在应用时通常需要进行功能化,调节其组成、大小、形状和结构等,以便于加工处理或满足不同的应用需求。石墨烯功能化方法多样,功能化产物也是种类繁多。然而,到目前为止,石墨烯功能化产物并没有系统全面的分类和精确的定义。因此,本文在系统总结现有石墨烯功能化研究的基础上,给出了石墨烯功能化产物的系统分类、各类的精确定义和相应的制备策略,并通过典型示例进行了详细地阐述。石墨烯功能化的产物统称为“功能化石墨烯材料”,分为两类：“功能化石墨烯”和“功能化石墨烯复合材料”。功能化石墨烯材料的制备可由“自上而下”和“自下而上”两种策略实现。制备策略的选择取决于应用需求。系统分类、精确命名和制备策略的归纳必将有助于功能化石墨烯材料的进一步发展。     

On Oxygen‐Containing Groups in Chemically Modified Graphenes

Reduced graphenes (belonging to the class of chemically modified graphenes, CMG) are one of the most investigated and utilized materials in current research. Oxygen functionalities on the CMG surfaces have dramatic influences on material properties. Interestingly, these functionalities are rarely comprehensively characterized. Herein, the four most commonly used CMGs, mainly electrochemically reduced graphene oxide (ER‐GO), thermally reduced graphene oxide (TR‐GO), and the corresponding starting materials, that is, graphene oxide and graphite oxide, were comprehensively characterized by a wide variety of methods, such as high‐resolution X‐ray photoelectron spectroscopy, electrochemical impedance spectroscopy, UV/Vis spectroscopy, transmission electron microscopy (TEM), and voltammetry, to establish connections between the structures of these materials that carry different oxygen functionalities and their electrochemical behaviors. This was followed by the quantification of the negatively charged oxygen‐containing groups (OCGs) by UV/Vis spectroscopy and of the electrochemically reducible OCGs by voltammetry. Lastly, a biofunctionalization with gold nanoparticle (AuNP)‐modified DNA sequences was performed by the formation of covalent bonds with the carboxylic groups (? COOH) on the CMG surfaces. There was an evident predominance of functionalizable ? COOH groups on the ER‐GO surface, as confirmed by a higher amount of Au detected both with differential‐pulse voltammetry and impedance spectroscopy, coupled with visualization by TEM. We exploited the DNA–Au bioconjugates as highly specific stains to localize and visualize the positions of carboxylic groups. Our findings are very important to clearly identify the presence, nature, and distribution of oxygen functionalities on different chemically modified graphenes.     

On oxygen-containing groups in chemically modified graphenes

Reduced graphenes (belonging to the class of chemically modified graphenes, CMG) are one of the most investigated and utilized materials in current research. Oxygen functionalities on the CMG surfaces have dramatic influences on material properties. Interestingly, these functionalities are rarely comprehensively characterized. Herein, the four most commonly used CMGs, mainly electrochemically reduced graphene oxide (ER-GO), thermally reduced graphene oxide (TR-GO), and the corresponding starting materials, that is, graphene oxide and graphite oxide, were comprehensively characterized by a wide variety of methods, such as high-resolution X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, UV/Vis spectroscopy, transmission electron microscopy (TEM), and voltammetry, to establish connections between the structures of these materials that carry different oxygen functionalities and their electrochemical behaviors. This was followed by the quantification of the negatively charged oxygen-containing groups (OCGs) by UV/Vis spectroscopy and of the electrochemically reducible OCGs by voltammetry. Lastly, a biofunctionalization with gold nanoparticle (AuNP)-modified DNA sequences was performed by the formation of covalent bonds with the carboxylic groups (-COOH) on the CMG surfaces. There was an evident predominance of functionalizable -COOH groups on the ER-GO surface, as confirmed by a higher amount of Au detected both with differential-pulse voltammetry and impedance spectroscopy, coupled with visualization by TEM. We exploited the DNA-Au bioconjugates as highly specific stains to localize and visualize the positions of carboxylic groups. Our findings are very important to clearly identify the presence, nature, and distribution of oxygen functionalities on different chemically modified graphenes.     

Structures and electronic states of fluorinated graphene

Functionalized graphenes have been utilized as electronic devices and energy materials. In the present paper, the effects of fluorine-termination of graphene edge on the structures and electronic states of graphene have been investigated by means of density functional theory (DFT) method. It was found that the ionization potential (Ip) and electron affinity of graphene (EA) are blue-shifted by the F-substitution. On the other hand, the excitation energy was red-shifted. The drastic change shows a possibility as electronic devices such as field-effect transistors. The drastic change of electronic states caused by the F-substitution of graphene edge was discussed on the basis of the theoretical results.     

Geographical and Geological Origin of Natural Graphite Heavily Influence the Electrical and Electrochemical Properties of Chemically Modified Graphenes

Natural graphite is an important precursor for the production of chemically modified graphenes in bulk quantities for electrochemical applications. These natural graphites have varying fundamental properties due to the different geological processes and environments at their points of origin, which are expected to affect their chemical reactivity and hence the properties of the derived graphene materials. Four different natural graphites with known geographical and geological origins were exposed to a modified Hummers oxidation method and the resulting graphite oxides were studied. The graphite oxides were shown to have different extents of oxidation and types of oxygen groups, which directly influenced their electrochemical properties. These differences were propagated further in the subsequent chemical reduction of the graphite oxides, and the reduced graphene oxides exhibited significantly different reduction efficiencies and electrical conductivities. These findings show that the choice of natural graphite of known origin is important to synthesize chemically modified graphenes with a desired set of properties.     

Number of graphene layers exhibiting an influence on oxidation of DNA bases: Analytical parameters

This article investigates the analytical performance of double-, few- and multi-layer graphene upon oxidation of adenine and guanine. We observed that the sensitivity of differential pulse voltammetric response of guanine and adenine is significantly higher at few-layer graphene surface than single-layer graphene. We use glassy carbon electrode as substrate coated with graphenes. Our findings shall have profound influence on construction of graphene based genosensors.