Sp2 Domains Research Articles

Structural Evolution of Reduced Graphene Oxide of Varying Carbon sp2 Fractions Investigated via Coulomb Blockade Transport

We investigate the structural evolution of reduced graphene oxide (RGO) sheets with carbon sp2 fractions varying from 55 to 80% using low-temperature Coulomb blockade (CB) transport. At 4.2 K, all RGO sheets exhibit a complete suppression of current (CB) below a threshold voltage Vt, the value of which decreased from 3.34 to 0.25 V with increasing carbon sp2 fraction. From the temperature-dependent Vt, we calculate an effective charging energy and individual graphene domain size of 160 meV and 1.34 nm at 55% carbon sp2 fractions, respectively. These values are 20 meV and 4.18 nm at 80% carbon sp2 fractions, respectively. This implies that with increasing reduction, newly formed sp2 domains increase the effective size of the graphene domain. For an applied voltage V > Vt, the current I follows a scaling law I ∼ [(V – Vt)/Vt]α where the scaling parameter α increases from 2.11 to 3.40 with increasing sp2 fraction, suggesting that increasing sp2 fraction creates more topological defects on the RGO. Our report provides a much desired insight into the structural evolution of RGO sheets.

Graphene Quantum Dots from a Facile Sono-Fenton Reaction and Its Hybrid with a Polythiophene Graft Copolymer toward Photovoltaic Application

A new and facile approach for synthesizing graphene quantum dots (GQDs) using sono-Fenton reaction in an aqueous dispersion of graphene oxide (GO) is reported. The transmission electron microscopy (TEM) micrographs of GQDs indicate its average diameter as ∼5.6 ± 1.4 nm having a lattice parameter of 0.24 nm. GQDs are used to fabricate composites (PG) with a water-soluble polymer, polythiophene-g-poly[(diethylene glycol methyl ether methacrylate)-co-poly(N,N-dimethylaminoethyl methacrylate)] [PT-g-P(MeO2MA-co-DMAEMA), P]. TEM micrographs indicate that both P and PG possess distinct core-shell morphology and the average particle size of P (0.16 ± 0.08 μm) increases in PG (0.95 ± 0.45 μm). Fourier transform infrared and X-ray photoelectron spectrometry spectra suggest an interaction between -OH and -COOH groups of GQDs and -NMe2 groups of P. A decrease of the intensity ratio of Raman D and G bands (ID/IG) is noticed during GQD and PG formation. In contrast to GO, GQDs do not exhibit any absorption peak for its smaller-sized sp(2) domain, and in PG, the π-π* absorption of polythiophene (430 nm) of P disappears. The photoluminescence (PL) peak of GQD shifts from 450 to 580 nm upon a change in excitation from 270 to 540 nm. PL emission of PG at 537 nm is quenched, and it shifts toward lower wavelength (∼430 nm) with increasing aging time for energy transfer from P to GQDs followed by up-converted emission of GQDs. Both P and PG exhibit semiconducting behavior, and PG produces an almost reproducible photocurrent. Dye-sensitized solar cells (DSSCs) fabricated with an indium-titanium oxide/PG/graphite device using the N719 dye exhibit a short-circuit current (Jsc) of 4.36 mA/cm(2), an open-circuit voltage (Voc) of 0.78 V, a fill factor of 0.52, and a power conversion efficiency (PCE, η) of 1.76%. Extending the use of GQDs to fabricate DSSCs with polypyrrole, both Voc and Jsc increase with increasing GQD concentration, showing a maximum PCE of 2.09%. The PG composite exhibits better cell viability than the components.

Propagative Exfoliation of High Quality Graphene

High quality graphene materials that readily disperse in water or organic solvents are needed to achieve some of the most ambitious applications. However, current synthetic approaches are typically limited by irreversible structural damages, little solubility, or low scalability. Here, we describe a fundamental study of graphene chemistry and covalent functionalization patterns on sp2 carbon lattices, from which a facile, scalable synthesis of high quality graphene sheets was developed. Graphite materials were efficiently exfoliated by reductive, propagative alkylation. The exfoliated, propagatively alkylated graphene sheets (PAGenes) not only exhibited high solubility in common solvents such as chloroform, water, and N-methyl-pyrrolidone, but also showed electrical conductivity as high as 4.1 × 103 S/m, which is 5 orders of magnitude greater than those of graphene oxides. Bright blue photoluminescence, unattainable in graphene, was also observed. We attribute the rise of blue photoluminescence in PAGenes to small on-graphene sp2 domains created by the propagative covalent chemistry, which may expand from graphene edges or existing defect sites leaving sp2-hybridized patches interlaced with sp3-hybridized regions. The intact sp2 domains enable effective electrical percolation among different graphene layers affording the observed high electrical conductivity in PAGene films.

Spectra investigation on surface characteristics of graphene oxide nanosheets treated with tartaric, malic and oxalic acids

The surface characteristics of graphene oxide nanosheets (GO) treated respectively with tartaric acid, malic acid and oxalic acid, have been investigated by mainly using optical spectroscopic methods including Fourier transform infrared spectroscopy (FT-IR), Ultraviolet–visible (UV–Vis) absorption and Raman spectroscopy. Additionally, the electrochemical property of the products has also been studied. The data revealed that oxygen-containing groups such as OH, COOH and CO on the GO surface have been almost removed and thus reduced graphene oxide nanosheets (RGN) were obtained. Interestingly, the number of sp2 domains of RGN increases as treated by tartaric acid<malic acid<oxalic acid whereas the steric hindrance (SH) decreases and the ionization constant (IC) differs among these three acids. Furthermore, the specific capacitances (Cs) of GO have been greatly promoted from 2.4Fg−1 to 100.8, 112.4, and 147Fg−1 after treated with tartaric, malic and oxalic acids, respectively. This finding agrees well with the spectra result of the tendency of surface conjugated degree alteration. We claim that the difference in both SH and IC among these acids is the main reason for the diverse surface characteristics as well as the improved Cs of the RGN.

Graphenes for low percolation threshold in electroconductive nylon 6 composites

Moderately functionalized graphenes, prepared by the thermal reduction of graphite oxide, were utilized to prepare electroconductive graphene/nylon 6 composites which exhibited a highest conductivity value of 6.84 × 10−4 S cm−1 for a low carbon incorporation of 0.54 wt% and a lowest percolation threshold of 0.39 wt%, the lowest values reported so far for graphene/nylon 6 composites. The functionality of the graphenes was modulated by the thermal reduction time. The graphitic structure of graphene was strengthened by extended thermal treatment. The main strengthening mechanism in the first 5 min was the generation of new sp2 domains followed by the growth of the domains during the next 5 min. This extended thermal treatment improved the conductivity of the graphene itself as well as the composite loaded with graphene. However, it led to poor dispersion of the graphene in the composites, reduced crystallization of nylon 6 and reduced reinforcement of nylon 6 by graphene. © 2013 Society of Chemical Industry

Nano‐Graphene Oxide: A Potential Multifunctional Platform for Cancer Therapy

Nano-GO is a graphene derivative with a 2D atomic layer of sp² bonded carbon atoms in hexagonal conformation together with sp³ domains with carbon atoms linked to oxygen functional groups. The supremacy of nano-GO resides essentially in its own intrinsic chemical and physical structure, which confers an extraordinary chemical versatility, high aspect ratio and unusual physical properties. The chemical versatility of nano-GO arises from the oxygen functional groups on the carbon structure that make possible its relatively easy functionalization, under mild conditions, with organic molecules or biological structures in covalent or non-covalent linkage. The synergistic effects resulting from the assembly of well-defined structures at nano-GO surface, in addition to its intrinsic optical, mechanical and electronic properties, allow the development of new multifunctional hybrid materials with a high potential in multimodal cancer therapy. Herein, a comprehensive review of the fundamental properties of nano-GO requirements for cancer therapy and the first developments of nano-GO as a platform for this purpose is presented.

Synthesis of Stable Colloidal Suspension of Graphene

In this article, graphite oxide was prepared via Hummers method, then reduced by L-ascorbic acid to obtain colloidal suspension of graphene. The resultant graphene was characterized by FT-IR, UV-vis, Raman, XRD, AFM, TEM in detail, and it was found that the the sp2 domain of graphene oxide is restored efficiently, the suspension is very stable and most of the graphene is single-layer. Compared to hydrazine hydrate, L-ascorbic acid has higher deoxygenation efficiency and inclines to gain few-layer graphene and its stable suspensions.

Improvement of Photoluminescence of Graphene Quantum Dots with a Biocompatible Photochemical Reduction Pathway and Its Bioimaging Application

As a rising star in the family of fluorescent material, graphene quantum dots (GQDs) have attracted great attention because of their excellent properties such as high photostability against photobleaching and blinking, biocompatibility, and low toxicity. Herein, blue luminescent GQDs were prepared by photo-reducing GQDs with isopropanol. After photochemical reduction, the increasing of sp(2) domains and the formed hydroxyl in pGQDs can enhance the photoluminescence of GQDs. The quantum yield of the photo-reduced GQDs (pGQDs) was increased 3.7 fold. Because of its less negative surface charges and lower cytotoxicity than chemical reduced GQDs (cGQDs), the pGQDs were more easily uptaken by cells. This work may provide a simple and green pathway to enhance the QY of GQDs with satisfactory biocompatibility as fluorescent nanoprobes.

Validation of the functional and social performance – DSF-84 checklist: preliminary study

Purpose: To develop, implement and validate an instrument for assessing the functional and social performance of young male adults with lower limb amputees based on the international classification of functionality, incapacity, and health. Methods: Developed the instrument, the items were grouped into domains (organic aspects – OA, daily activities – DA, performance components – PC, social participation – SP and environmental factors – EF) for statistical analysis. The implementation of the instrument was filmed for validation. Four assessors watched the films on two occasions and gave scores. Intra-class correlation was used to evaluate intra- and inter-rater reproducibility and to the internal consistency was calculated by Cronbach’s alpha and the criterion validity was assessed by Student’s t-test and the Mann–Whitney U-test. Results: The results showed good reliability in the scores for OA, DA, PC and SP domains and a reasonable reliability for the EF domain. The differences between assessors performed by the analysis of variance were not significant. The reliability intra-rater, performed through the test–retest method, showed in all domains high levels of intra-rater correspondence. Conclusions: The results show the validity and reliability of DSF-84 to young male adults with amputation of the lower limb, being useful for this population.Implications for RehabilitationIn this study have been developed, implemented, and validated an instrument (DSF-84) for assessing the functional and social performance of young male adults with lower limb amputees based on the ICF.The results show the validity and reliability of DSF-84 to young male adults with amputation of the lower limb, being useful for this population.

The extended growth of graphene oxide flakes using ethanol CVD

We report the extended growth of Graphene Oxide (GO) flakes using atmospheric pressure ethanol Chemical Vapor Deposition (CVD). GO was used to catalyze the deposition of carbon on a substrate in the ethanol CVD with Ar and H2 as carrier gases. Raman, SEM, XPS and AFM characterized the growth to be a reduced GO (RGO) of <5 layers. This newly grown RGO possesses lower defect density with larger and increased distribution of sp(2) domains than chemically reduced RGO. Furthermore this method without optimization reduces the relative standard deviation of electrical conductivity between chips, from 80.5% to 16.5%, enabling RGO to be used in practical electronic devices.

Photoluminescent nanographitic/nitrogen-doped graphitic hollow shells as a potential candidate for biological applications.

Water-dispersible graphitic hollow spheres were synthesized using a soft chemical route under hydrothermal conditions by glucose carbonization using a magnetite/silica-encapsulated core-shell sphere as a template. Carbonization on the templates happens as the magnetite core is partially or completely eliminated depending on the reaction conditions. Therefore, nano-sized graphitic hollow spheres or magnetite-core-encapsulated graphitic shells could be obtained. Also nitrogen-doped graphitic spheres were synthesized by a hydrothermal reaction. The graphitic and nitrogen-doped graphitic spheres show wavelength dependent photoluminescence in 300-600 nm range. The photoluminescence seems to depend on the fraction of the sp2 domains and N-doping, therefore, tunable PL emission can be achieved by controlling the nature of sp2 sites. In addition the cellular uptake of the graphitic hollow spheres was evaluated in human HeLa cells, demonstrating its main localization in the cytoplasm. A blue fluorescence signal was the most intensively observed in the cellular uptake process, although some green and red fluorescence was also observed. Since the cores of Fe3O4 could be completely or partly eliminated in a controllable way, it can be used as a magnetic resonance imaging agent. In addition, their easily modifiable hydrophilic surfaces for multi-functionality and hydrophobic voids covered by oxidized graphite make them promising candidates for applications in cellular photo-imaging and targeted drug delivery.

Synthesis and photoluminescence of three-dimensional europium-complexed graphene macroassembly

Here, we report a novel three-dimensional (3D) europium-complexed reduced graphene oxide (Eu–RGO (3D)) macroassembly with a microporous structure by a facile one-pot self-assembly process under very mild conditions. Europium (Eu) ions complex well with the 3D graphene through the oxygen functional groups. Apart from the typical red emission from the trivalent Eu ions, the as-produced Eu–RGO (3D) exhibits blue emission at ca. 440 nm and green emission at ca. 544 nm, which are derived from the tuned PL emission of newly formed sp2 domains after introducing Eu ions into the lattice of the reduced graphene oxide. The Eu–RGO (3D) macroassembly has also been demonstrated to quench the photoluminescence (PL) of organic dyes such as Rhodamine-B. The interesting dual optical functionalities confirm its potential applications in the fields of biosensing as well as environmental protection.

Indirect optical transitions in hybrid spheres with alternating layers of titania and graphene oxide nanosheets

In this report, we studied the optical properties of hybrid spherical structures consisting of alternating nanosheets of titania (TiO(2)) and graphene oxide (GO) prepared by a layer-by-layer self-assembly technique. Compared to samples with only TiO(2) spheres or GO nanosheets, a blue-to-red light emission band emerges and persists in this novel composite material even after it was further reduced through microwave irradiation. From detailed time-resolved measurements and energy-level structure modeling, this unexpected fluorescent feature was attributed to the indirect optical transitions between TiO(2) and the localized sp(2) domains of GO in a charge-separated configuration.

Protein recognition on a single graphene oxide surface fixed on a solid support

We study protein recognition on a graphene oxide (GO) surface using a single GO piece fixed on a solid support. The GO surface is modified by newly designed processes using pyrene as a linker to an sp2 domain in the GO, an aptamer for thrombin recognition, and a probe dye for fluorescence detection. In this system, the dye probe fluorescence, which was initially quenched by GO, is recovered when the aptamer recognizes the corresponding protein. We demonstrate the label-free and selective protein recognition for thrombin. The elementary processes of protein recognition are observed directly with a confocal laser scanning microscope and an atomic force microscope using an identical piece of GO. They indicate that proteins are recognized homogeneously on the modified GO surface. We also show that the recognition system can be installed and operated in microchannel devices.

Size and Sp&lt;sup&gt;2&lt;/sup&gt; Fraction Dependence of Energy Gaps in Carbon Dots

A simple quantitative model is developed to understand the origin of the photoluminescence (PL) behavior of carbon dots. The sp2 fraction and size depend on the energy gaps in carbon dots containing a mixture of sp2 and sp3 bonding. The finite-sized molecular sp2 domains within the carbon-carbon and carbon-oxygen sp3 matrix behave as the luminescence centers and when they are opposed to the surface of carbon dots, surface passivation by organic molecules is required for preventing the occurrence of nonradiative recombination. According to our calculations, highly efficient and tunable PL from carbon dots can be achieved by controlling the nature of sp2 sites.

Mutations in Multiple Domains of Gag Drive the Emergence of In Vitro Resistance to the Phosphonate-Containing HIV-1 Protease Inhibitor GS-8374

GS-8374 is a potent HIV protease inhibitor (PI) with a unique diethyl-phosphonate moiety. Due to a balanced contribution of enthalpic and entropic components to its interaction with the protease (PR) active site, the compound retains activity against HIV mutants with high-level multi-PI resistance. We report here the in vitro selection and characterization of HIV variants resistant to GS-8374. While highly resistant viruses with multiple mutations in PR were isolated in the presence of control PIs, an HIV variant displaying moderate (14-fold) resistance to GS-8374 was generated only after prolonged passaging for >300 days. The isolate showed low-level cross-resistance to darunavir, atazanavir, lopinavir, and saquinavir, but not other PIs, and contained a single R41K mutation in PR combined with multiple genotypic changes in the Gag matrix, capsid, nucleocapsid, and SP2 domains. Mutations also occurred in the transframe peptide and p6* domain of the Gag-Pol polyprotein. Analysis of recombinant HIV variants indicated that mutations in Gag, but not the R41K in PR, conferred reduced susceptibility to GS-8374. The Gag mutations acted in concert, since they did not affect susceptibility when introduced individually. Analysis of viral particles revealed that the mutations rendered Gag more susceptible to PR-mediated cleavage in the presence of GS-8374. In summary, the emergence of resistance to GS-8374 involved a combination of substrate mutations without typical resistance mutations in PR. These substrate changes were distributed throughout Gag and acted in an additive manner. Thus, they are classified as primary resistance mutations indicating a unique mechanism and pathway of resistance development for GS-8374.

Role of the SP2 Domain and Its Proteolytic Cleavage in HIV-1 Structural Maturation and Infectivity

HIV-1 buds as an immature, noninfectious virion. Proteolysis of its main structural component, Gag, is required for morphological maturation and infectivity and leads to release of four functional domains and the spacer peptides SP1 and SP2. The N-terminal cleavages of Gag and the separation of SP1 from CA are all essential for viral infectivity, while the roles of the two C-terminal cleavages and the role of SP2, separating the NC and p6 domains, are less well defined. We have analyzed HIV-1 variants with defective cleavage at either or both sites flanking SP2, or largely lacking SP2, regarding virus production, infectivity, and structural maturation. Neither the presence nor the proteolytic processing of SP2 was required for particle release. Viral infectivity was almost abolished when both cleavage sites were defective and severely reduced when the fast cleavage site between SP2 and p6 was defective. This correlated with an increased proportion of irregular core structures observed by cryo-electron tomography, although processing of CA was unaffected. Mutation of the slow cleavage site between NC and SP2 or deletion of most of SP2 had only a minor effect on infectivity and did not induce major alterations in mature core morphology. We speculate that not only separation of NC and p6 but also the processing kinetics in this region are essential for successful maturation, while SP2 itself is dispensable.

Influence of the electrochemical reduction process on the performance of graphene-based capacitors

Electrochemically reduced graphene was used as the key element in the preparation of electric double-layer capacitors where the thickness of the electrode was only a few hundred nano-meters. The resultant electrodes showed different specific capacitances after pre-reduction with scanning potential windows of −1.0 to 1.6V, −1.5 to 0V and −1.0 to 1.0V. Also, a specific capacitance of 246F/g was obtained as the graphene oxide electrode was reduced with an applied potential of −1.0 to 1.0V for 4000s. The influence of the residual oxygen functional groups and sp2 domains in electrochemically reduced graphene were investigated for capacitance performance.

Vinyl‐addition type norbornene copolymer containing sulfonated biphenyl pendant groups for proton exchange membranes

AbstractThe vinyl addition type copolymer poly(butoxymethylene norbornene‐co‐biphenyl oxyhexamethyleneoxymethylene norbornene) (P(BN/BphN)) was synthesized by using bis‐(β‐ketonaphthylimino)nickel(II)/B(C6F5)3 catalytic system. P(BN/BphN) was sulfonated to give sulfonated P(BN/BphN) (SP(BN/BphN)) with concentrated sulfuric acid (98%) as sulfonating agent in a component solvent. The ion exchange capacity (IEC), degree of sulfonation (DS), water uptake, and methanol permeability of the SP(BN/BphN)s were increased with the sulfonated time. The methanol permeability of the SP(BN/BphN) membranes was in the range of 1.8 × 10−7 to 7.5 × 10−7 cm2/s, which were lower than the value 1.3 × 10−6 cm2/s of Nafion®115. The proton conductivity of SP(BN/BphN) membranes increased with the increase of IEC values, temperature, and water uptake. Water uptake of the SP(BN/BphN) membranes was lower than that of Nafion® 115 and leads to low proton conduction. Microscopic phase separation occurred in SP(BN/BphN) membrane and domains containing sulfonic acid groups were investigated by SEM and TEM. SP(BN/BphN) membranes had good mechanical properties, high thermal stability, and excellent oxidative stability. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

The Unconserved Groucho Central Region Is Essential for Viability and Modulates Target Gene Specificity

Groucho (Gro) is a Drosophila corepressor required by numerous DNA-binding repressors, many of which are distributed in gradients and provide positional information during development. Gro contains well-conserved domains at its N- and C-termini, and a poorly conserved central region that includes the GP, CcN, and SP domains. All lethal point mutations in gro map to the conserved regions, leading to speculation that the unconserved central domains are dispensable. However, our sequence analysis suggests that the central domains are disordered leading us to suspect that the lack of lethal mutations in this region reflects a lack of order rather than an absence of essential functions. In support of this conclusion, genomic rescue experiments with Gro deletion variants demonstrate that the GP and CcN domains are required for viability. Misexpression assays using these same deletion variants show that the SP domain prevents unrestrained and promiscuous repression by Gro, while the GP and CcN domains are indispensable for repression. Deletion of the GP domain leads to loss of nuclear import, while deletion of the CcN domain leads to complete loss of repression. Changes in Gro activity levels reset the threshold concentrations at which graded repressors silence target gene expression. We conclude that co-regulators such as Gro are not simply permissive components of the repression machinery, but cooperate with graded DNA-binding factors in setting borders of gene expression. We suspect that disorder in the Gro central domains may provide the flexibility that allows this region to mediate multiple interactions required for repression.