Chemical structures of hydrazine-treated graphene oxide and generation of aromatic nitrogen doping

Engineered Carbon-Nanomaterial-Based Electrochemical Sensors for Biomolecules

The study of electrochemical behavior of bioactive molecules has become one of the most rapidly developing scientific fields. Biotechnology and biomedical engineering fields have a vested interest in constructing more precise and accurate voltammetric/amperometric biosensors. One rapidly growing area of biosensor design involves incorporation of carbon-based nanomaterials in working electrodes, such as one-dimensional carbon nanotubes, two-dimensional graphene, and graphene oxide. In this review article, we give a brief overview describing the voltammetric techniques and how these techniques are applied in biosensing, as well as the details surrounding important biosensing concepts of sensitivity and limits of detection. Building on these important concepts, we show how the sensitivity and limit of detection can be tuned by including carbon-based nanomaterials in the fabrication of biosensors. The sensing of biomolecules including glucose, dopamine, proteins, enzymes, uric acid, DNA, RNA, and H2O2 traditionally employs enzymes in detection; however, these enzymes denature easily, and as such, enzymeless methods are highly desired. Here we draw an important distinction between enzymeless and enzyme-containing carbon-nanomaterial-based biosensors. The review ends with an outlook of future concepts that can be employed in biosensor fabrication, as well as limitations of already proposed materials and how such sensing can be enhanced. As such, this review can act as a roadmap to guide researchers toward concepts that can be employed in the design of next generation biosensors, while also highlighting the current advancements in the field.

Self-Supplied Nano-Fusing and Transferring Metal Nanostructures via Surface Oxide Reduction

Here, we demonstrate that chemical reduction of oxide layers on metal nanostructures fuses junctions at nanoscale to improve the opto-electrical performance, and to ensure environmental stability of the interconnected nanonetwork. In addition, the reducing reaction lowers the adhesion force between metal nanostructures and substrates, facilitating the detachment of them from substrates. Detached metal nanonetworks can be easily floated on water and transferred onto various substrates including hydrophobic, floppy, and curved surfaces. Utilizing the detached metal nanostructures, semitransparent organic photovoltaics is fabricated, presenting the applicability of proposed reduction treatment in the device applications.

Three dimensional nitrogen-doped graphene hydrogels with in situ deposited cobalt phosphate nanoclusters for efficient oxygen evolution in a neutral electrolyte

A hybrid electrode of cobalt phosphate (CoPi) on nitrogen doped graphene hydrogels was fabricated by hydrothermal treatment of graphene oxide followed by CoPi electrodeposited in situ, which showed excellent performance toward oxygen reaction in a neutral electrolyte.

Chemical functionalization of N-doped carbon nanotubes: a powerful approach to cast light on the electrochemical role of specific N-functionalities in the oxygen reduction reaction

Two different synthetic approaches to carbon nanotube N-decoration/doping are used to foster the hypothesis of a unique N-configuration (N-pyridinic) at work in the ORR.

Electrophoretic separation and deposition of metal–graphene nanocomposites and their application as electrodes in solar cells

Synthesis of metal nanoparticle–graphene composites without the use of any stabilising ligands has enabled the nanoparticle surface to be available for electron transfer reactions in the development of new counter electrodes for solar cells.

Coordination Chemistry of [Co(acac)2 ] with N-Doped Graphene: Implications for Oxygen Reduction Reaction Reactivity of Organometallic Co-O4 -N Species

Hybridization of organometallic complexes with graphene-based materials can give rise to enhanced catalytic performance. Understanding the chemical structures within hybrid materials is of primary importance. In this work, archetypical hybrid materials are synthesized by the reaction of an organometallic complex, [Co(II) (acac)2 ] (acac=acetylacetonate), with N-doped graphene-based materials at room temperature. Experimental characterization of the hybrid materials and theoretical calculations reveal that the organometallic cobalt-containing species is coordinated to heterocyclic groups in N-doped graphene as well as to its parental acac ligands. The hybrid material shows high electrocatalytic activity for the oxygen reduction reaction (ORR) in alkaline media, and superior durability and methanol tolerance to a Pt/C catalyst. Based on the chemical structures and ORR experiments, the catalytically active species is identified as a Co-O4 -N structure.

Lateral assembly of oxidized graphene flakes into large-scale transparent conductive thin films with a three-dimensional surfactant 4-sulfocalix[4]arene

Graphene is a promising candidate material for transparent conductive films because of its excellent conductivity and one-carbon-atom thickness. Graphene oxide flakes prepared by Hummers method are typically several microns in size and must be pieced together in order to create macroscopic films. We report a macro-scale thin film fabrication method which employs a three-dimensional (3-D) surfactant, 4-sulfocalix[4]arene (SCX), as a lateral aggregating agent. After electrochemical exfoliation, the partially oxidized graphene (oGr) flakes are dispersed with SCX. The SCX forms micelles, which adsorb on the oGr flakes to enhance their dispersion, also promote aggregation into large-scale thin films under vacuum filtration. A thin oGr/SCX film can be shaved off from the aggregated oGr/SCX cake by immersing the cake in water. The oGr/SCX thin-film floating on the water can be subsequently lifted from the water surface with a substrate. The reduced oGr (red-oGr) films can be as thin as 10−20 nm with a transparency of >90% and sheet resistance of 890 ± 47 kΩ/sq. This method of electrochemical exfoliation followed by SCX-assisted suspension and hydrazine reduction, avoids using large amounts of strong acid (unlike Hummers method), is relatively simple and can easily form a large scale conductive and transparent film from oGr/SCX suspension.

Synthesis of strongly fluorescent graphene quantum dots by cage-opening buckminsterfullerene

Graphene quantum dots is a class of graphene nanomaterials with exceptional luminescence properties. Precise dimension control of graphene quantum dots produced by chemical synthesis methods is currently difficult to achieve and usually provides a range of sizes from 3 to 25 nm. In this work, fullerene C60 is used as starting material, due to its well-defined dimension, to produce very small graphene quantum dots (∼2-3 nm). Treatment of fullerene C60 with a mixture of strong acid and chemical oxidant induced the oxidation, cage-opening, and fragmentation processes of fullerene C60. The synthesized quantum dots were characterized and supported by LDI-TOF MS, TEM, XRD, XPS, AFM, STM, FTIR, DLS, Raman spectroscopy, and luminescence analyses. The quantum dots remained fully dispersed in aqueous suspension and exhibited strong luminescence properties, with the highest intensity at 460 nm under a 340 nm excitation wavelength. Further chemical treatments with hydrazine hydrate and hydroxylamine resulted in red- and blue-shift of the luminescence, respectively.

Wet-spun, porous, orientational graphene hydrogel films for high-performance supercapacitor electrodes

Supercapacitors with porous electrodes of graphene macroscopic assembly are supposed to have high energy storage capacity. However, a great number of "close pores" in porous graphene electrodes are invalid because electrolyte ions cannot infiltrate. A quick method to prepare porous graphene electrodes with reduced "close pores" is essential for higher energy storage. Here we propose a wet-spinning assembly approach based on the liquid crystal behavior of graphene oxide to continuously spin orientational graphene hydrogel films with "open pores", which are used directly as binder-free supercapacitor electrodes. The resulting supercapacitor electrodes show better electrochemical performance than those with disordered graphene sheets. Furthermore, three reduction methods including hydrothermal treatment, hydrazine and hydroiodic acid reduction are used to evaluate the specific capacitances of the graphene hydrogel film. Hydrazine-reduced graphene hydrogel film shows the highest capacitance of 203 F g(-1) at 1 A g(-1) and maintains 67.1% specific capacitance (140 F g(-1)) at 50 A g(-1). The combination of scalable wet-spinning technology and orientational structure makes graphene hydrogel films an ideal electrode material for supercapacitors.

Paper-Based N-Doped Carbon Films for Enhanced Oxygen Evolution Electrocatalysis

Cellulous-fiber papers are used as 3D structural templates for the assembly of graphene and graphitic carbon nitrate (g-C3N4) ultrathin nanosheets. The resultant materials, which possess highly active centers, rich porosity, and 3D conductive networks, can catalyze the oxygen evolution reaction with competitive activity and much better durability compared to the benchmark noble metal electrocatalysts (IrO2).

Preparation of graphene/poly(p-phenylenebenzobisoxazole) composite fibers based on simultaneous zwitterion coating and chemical reduction of graphene oxide at room temperature

Graphene reinforced PBO fibers were fabricated based on zwitterion-coated rGO sheets, and a widely applicable mechanism for the acid-catalyzed reduction of GO was proposed.

CO tolerance of Pt and PtSn intermetallic electrocatalysts on synthetically modified reduced graphene oxide supports

Electrochemical studies demonstrated the ability to modify the catalytic activities of graphene supported Pt and PtSn nanoparticle electrocatalysts by altering the nature of the metal-support interactions.

A facile method of synthesizing ammonia modified graphene oxide for efficient removal of uranyl ions from aqueous medium

Adsorption of uranyl ions on NH₃ modified graphene oxide at pH 6.

Graphene-based materials for flexible supercapacitors

The recent advances in developing graphene-based materials for flexible supercapacitors are summarized in this review.

Strategy and mechanism for controlling the direction of defect evolution in graphene: preparation of high quality defect healed and hierarchically porous graphene

In this paper, a novel approach for controlling the direction of defect evolution in graphene through intercalation of organic small molecules into graphite oxide (GO) combined with a one-pot microwave-assisted reaction is reported. By using ethanol as intercalator, the bulk production of high quality graphene with its defects being satisfactorily healed is achieved. The repair of defects using extraneous carbon atoms and the hybrid state of these carbon atoms are definitely demonstrated using isotopic tracing studies with (13)C-labeled ethanol combined with (13)C solid-state NMR. The defect healed graphene shows excellent crystallinity, extremely low oxygen content (C : O ratio of 23.8) and has the highest sheet conductivity (61 500 S m(-1)) compared to all other reported graphene products derived from GO. By using methanol or benzene as intercalators, hierarchically porous graphene with a self-supported 3-dimensional framework (∼917 m(2) g(-1)) containing both macropores and mesopores (2-5 nm) is obtained. This graphene possesses a distinctive amorphous carbon structure around the edge of the nanopores, which could be conducive to enhancing the lithium storage performance (up to 580 mA h g(-1) after 300 cycles) when tested as an anode of lithium ion batteries, and might have promising applications in the field of electrode materials, catalysis, and separation, and so on. The mechanism involved for the controlled defect evolution is also proposed. The simple, ultrafast and unified strategy developed in this research provides a practical and effective approach to harness structural defects in graphene-based materials, which could also be expanded for designing and preparing other ordered carbon materials with specific structures.

Graphene wrapping as a protective clamping layer anchored to carbon nanofibers encapsulating Si nanoparticles for a Li-ion battery anode

Carbon nanofibers encapsulating Si nanoparticles (CNFs/SiNPs) were prepared via an electrospinning method and chemically functionalized with 3-aminopropyltriethoxysilane (APS) to be grafted onto graphene oxide (GO). As a result, the thin and flexible GO, which exhibits a negative charge in aqueous solution, fully wrapped around the APS-functionalized CNFs with a positive surface charge via electrostatic self-assembly. After the formation of chemical bonds between the epoxy groups on GO and the amine groups in APS via an epoxy ring opening reaction, the GO was chemically reduced to a reduced graphene oxide (rGO). Electrochemical and morphological characterizations showed that capacity loss by structural degradation and electrolyte decomposition on Si surface were significantly suppressed in the rGO-wrapped CNFs/SiNPs (CNFs/SiNPs@rGO). Superior capacities were consequently maintained for up to 200 cycles at a high current density (1048 mA h g(-1) at 890 mA g(-1)) compared to CNFs/SiNPs without the rGO wrapping (304 mA h g(-1) at 890 mA g(-1)). Moreover, the resistance of the SEI layer and charge transfer resistance were also considerably reduced by 24% and 88%, respectively. The described graphene wrapping offers a versatile way to enhance the mechanical integrity and electrochemical stability of Si composite anode materials.

Structure evolution of graphene oxide during thermally driven phase transformation: is the oxygen content really preserved?

A mild annealing procedure was recently proposed for the scalable enhancement of graphene oxide (GO) properties with the oxygen content preserved, which was demonstrated to be attributed to the thermally driven phase separation. In this work, the structure evolution of GO with mild annealing is closely investigated. It reveals that in addition to phase separation, the transformation of oxygen functionalities also occurs, which leads to the slight reduction of GO membranes and furthers the enhancement of GO properties. These results are further supported by the density functional theory based calculations. The results also show that the amount of chemically bonded oxygen atoms on graphene decreases gradually and we propose that the strongly physisorbed oxygen species constrained in the holes and vacancies on GO lattice might be responsible for the preserved oxygen content during the mild annealing procedure. The present experimental results and calculations indicate that both the diffusion and transformation of oxygen functional groups might play important roles in the scalable enhancement of GO properties.

Liquid crystal size selection of large-size graphene oxide for size-dependent N-doping and oxygen reduction catalysis

Graphene oxide (GO) is aqueous-dispersible oxygenated graphene, which shows colloidal discotic liquid crystallinity. Many properties of GO-based materials, including electrical conductivity and mechanical properties, are limited by the small flake size of GO. Unfortunately, typical sonochemical exfoliation of GO from graphite generally leads to a broad size and shape distribution. Here, we introduce a facile size selection of large-size GO exploiting liquid crystallinity and investigate the size-dependent N-doping and oxygen reduction catalysis. In the biphasic GO dispersion where both isotropic and liquid crystalline phases are equilibrated, large-size GO flakes (>20 μm) are spontaneously concentrated within the liquid crystalline phase. N-Doping and reduction of the size-selected GO exhibit that N-dopant type is highly dependent on GO flake size. Large-size GO demonstrates quaternary dominant N-doping and the lowest onset potential (-0.08 V) for oxygen reduction catalysis, signifying that quaternary N-dopants serve as principal catalytic sites in N-doped graphene.

An ice-templated, pH-tunable self-assembly route to hierarchically porous graphene nanoscroll networks

Porous graphene nanostructures are of great interest for applications in catalysis and energy storage. However, the fabrication of three-dimensional (3D) macroporous graphene nanostructures with controlled morphology, porosity and surface area still presents significant challenges. Here we introduce an ice-templated self-assembly approach for the integration of two-dimensional graphene nanosheets into hierarchically porous graphene nanoscroll networks, where the morphology of porous structures can be easily controlled by varying the pH conditions during the ice-templated self-assembly process. We show that freeze-casting of reduced graphene oxide (rGO) solution results in the formation of 3D porous graphene microfoam below pH 8 and hierarchically porous graphene nanoscroll networks at pH 10. In addition, we demonstrate that graphene nanoscroll networks show promising electrocatalytic activity for the oxygen reduction reaction (ORR).

Dispersibility of reduced alkylamine-functionalized graphene oxides in organic solvents

The alkylamine functionalization of graphene oxide is well known as an efficient approach to prepare reduced functionalized graphene oxide (RFGO) that is highly dispersible in organic solvents. Herein, we systematically investigated the effects of long-chain alkylamine functionalization of graphene oxide on the organic solvent dispersibility and electrical conductivity of RFGO. Three kinds of alkylamines, octylamine, dodecylamine and hexadecylamine, were chosen as functionalization agents. The alkylamine functionalization of graphene oxide was characterized by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis and X-ray diffraction. RFGO using octylamine exhibited the best electrical conductivity of greater than 180 S/m. All of the RFGOs had excellent dispersibility, up to 3.0 mg/mL, in organic solvents, with Hansen solubility parameters in the range of 6.3<(δ(p)+δ(h))<13.7.

Distinct mechanisms of DNA sensing based on N-doped carbon nanotubes with enhanced conductance and chemical selectivity

N-doped capped carbon nanotube (CNT) electrodes applied to DNA sequencing are studied by first-principles calculations. For the face-on nucleobase junction configurations, a conventional conductance ordering is obtained where the largest signal results from guanine according to its high highest occupied molecular orbital (HOMO) level, whereas for the edge-on counterparts a distinct conductance ordering is observed where the low-HOMO thymine provides the largest signal. The edge-on mode is shown to operate based on a novel molecular sensing mechanism that reflects the chemical connectivity between N-doped CNT caps that can act both as electron donors and electron acceptors and DNA functional groups that include the hyperconjugated thymine methyl group.

A novel graphene-polysulfide anode material for high-performance lithium-ion batteries

We report a simple and efficient approach for fabrication of novel graphene-polysulfide (GPS) anode materials, which consists of conducting graphene network and homogeneously distributed polysulfide in between and chemically bonded with graphene sheets. Such unique architecture not only possesses fast electron transport channels, shortens the Li-ion diffusion length but also provides very efficient Li-ion reservoirs. As a consequence, the GPS materials exhibit an ultrahigh reversible capacity, excellent rate capability and superior long-term cycling performance in terms of 1600, 550, 380 mAh g⁻¹ after 500, 1300, 1900 cycles with a rate of 1, 5 and 10 A g⁻¹ respectively. This novel and simple strategy is believed to work broadly for other carbon-based materials. Additionally, the competitive cost and low environment impact may promise such materials and technique a promising future for the development of high-performance energy storage devices for diverse applications.

Complementary p- and n-type polymer doping for ambient stable graphene inverter

Graphene offers great promise to complement the inherent limitations of silicon electronics. To date, considerable research efforts have been devoted to complementary p- and n-type doping of graphene as a fundamental requirement for graphene-based electronics. Unfortunately, previous efforts suffer from undesired defect formation, poor controllability of doping level, and subtle environmental sensitivity. Here we present that graphene can be complementary p- and n-doped by simple polymer coating with different dipolar characteristics. Significantly, spontaneous vertical ordering of dipolar pyridine side groups of poly(4-vinylpyridine) at graphene surface can stabilize n-type doping at room-temperature ambient condition. The dipole field also enhances and balances the charge mobility by screening the impurity charge effect from the bottom substrate. We successfully demonstrate ambient stable inverters by integrating p- and n-type graphene transistors, which demonstrated clear voltage inversion with a gain of 0.17 at a 3.3 V input voltage. This straightforward polymer doping offers diverse opportunities for graphene-based electronics, including logic circuits, particularly in mechanically flexible form.

Photothermally controlled gene delivery by reduced graphene oxide-polyethylenimine nanocomposite

Externally stimuli-triggered spatially and temporally controlled gene delivery can play a pivotal role in achieving targeted gene delivery with maximized therapeutic efficacy. In this study, a photothermally controlled gene delivery carrier is developed by conjugating low molecular-weight branched polyethylenimine (BPEI) and reduced graphene oxide (rGO) via a hydrophilic polyethylene glycol (PEG) spacer. This PEG-BPEI-rGO nanocomposite forms a stable nano-sized complex with plasmid DNA (pDNA), as confirmed by physicochemical studies. For the in vitro gene transfection study, PEG-BPEI-rGO shows a higher gene transfection efficiency without observable cytotoxicity compared to unmodified controls in PC-3 and NIH/3T3 cells. Moreover, the PEG-BPEI-rGO nanocomposite demonstrates an enhanced gene transfection efficiency upon NIR irradiation, which is attributed to accelerated endosomal escape of polyplexes augmented by locally induced heat. The endosomal escaping effect of the nanocomposite is investigated using Bafilomycin A1, a proton sponge effect inhibitor. The developed photothermally controlled gene carrier has the potential for spatial and temporal site-specific gene delivery.

25th anniversary article: Chemically modified/doped carbon nanotubes & graphene for optimized nanostructures & nanodevices

Outstanding pristine properties of carbon nanotubes and graphene have limited the scope for real-life applications without precise controllability of the material structures and properties. This invited article to celebrate the 25th anniversary of Advanced Materials reviews the current research status in the chemical modification/doping of carbon nanotubes and graphene and their relevant applications with optimized structures and properties. A broad aspect of specific correlations between chemical modification/doping schemes of the graphitic carbons with their novel tunable material properties is summarized. An overview of the practical benefits from chemical modification/doping, including the controllability of electronic energy level, charge carrier density, surface energy and surface reactivity for diverse advanced applications is presented, namely flexible electronics/optoelectronics, energy conversion/storage, nanocomposites, and environmental remediation, with a particular emphasis on their optimized interfacial structures and properties. Future research direction is also proposed to surpass existing technological bottlenecks and realize idealized graphitic carbon applications.

Pyrolysis of cellulose under ammonia leads to nitrogen-doped nanoporous carbon generated through methane formation

Here, we present a simple one-step fabrication methodology for nitrogen-doped (N-doped) nanoporous carbon membranes via annealing cellulose filter paper under NH3. We found that nitrogen doping (up to 10.3 at %) occurs during cellulose pyrolysis under NH3 at as low as 550 °C. At 700 °C or above, N-doped carbon further reacts with NH3, resulting in a large surface area (up to 1973.3 m(2)/g). We discovered that the doped nitrogen, in fact, plays an important role in the reaction, leading to carbon gasification. CH4 was experimentally detected by mass spectrometry as a product in the reaction between N-doped carbon and NH3. When compared to conventional activated carbon (1533.6 m(2)/g), the N-doped nanoporous carbon (1326.5 m(2)/g) exhibits more than double the unit area capacitance (90 vs 41 mF/m(2)).