Graphene is a two-dimensional carbon material with a honeycomb lattice and Dirac-like low-energy excitations. When Zeeman and spin-orbit interactions are neglected, its Landau levels are fourfold degenerate, explaining the 4e2/h separation between quantized Hall conductivity values seen in recent experiments. In this Letter we derive a criterion for the occurrence of interaction-driven quantum Hall effects near intermediate integer values of e2/h due to charge gaps in broken symmetry states.

Quantum Hall Ferromagnetism in Graphene

Biosensors with high sensitivity, selectivity and a low limit of detection, reaching nano/picomolar concentrations of biomolecules, are important to the medical sciences and healthcare industry for evaluating physiological and metabolic parameters. Over the last decade, different nanomaterials have been exploited to design highly efficient biosensors for the detection of analyte biomolecules. The discovery of graphene has spectacularly accelerated research on fabricating low-cost electrode materials because of its unique physical properties, including high specific surface area, high carrier mobility, high electrical conductivity, flexibility, and optical transparency. Graphene and its oxygenated derivatives, including graphene oxide (GO) and reduced graphene oxide (rGO), are becoming an important class of nanomaterials in the field of biosensors. The presence of oxygenated functional groups makes GO nanosheets strongly hydrophilic, facilitating chemical functionalization. Graphene, GO and rGO nanosheets can be easily combined with various types of inorganic nanoparticles, including metals, metal oxides, semiconducting nanoparticles, quantum dots, organic polymers and biomolecules, to create a diverse range of graphene-based nanocomposites with enhanced sensitivity for biosensor applications. This review summarizes the advances in two-dimensional (2D) and three-dimensional (3D) graphene-based nanocomposites as emerging electrochemical and fluorescent biosensing platforms for the detection of a wide range of biomolecules with enhanced sensitivity, selectivity and a low limit of detection. The biofunctionalization and nanocomposite formation processes of graphene-based materials and their unique properties, surface functionalization, enzyme immobilization strategies, covalent immobilization, physical adsorption, biointeractions and direct electron transfer (DET) processes are discussed in connection with the design and fabrication of biosensors. The enzymatic and nonenzymatic reactions on graphene-based nanocomposite surfaces for glucose- and cholesterol-related electrochemical biosensors are analyzed. This review covers a very broad range of graphene-based electrochemical and fluorescent biosensors for the detection of glucose, cholesterol, hydrogen peroxide (H2O2), nucleic acids (DNA/RNA), genes, enzymes, cofactors nicotinamide adenine dinucleotide (NADH) and adenosine triphosphate (ATP), dopamine (DA), ascorbic acid (AA), uric acid (UA), cancer biomarkers, pathogenic microorganisms, food toxins, toxic heavy metal ions, mycotoxins, and pesticides. The sensitivity and selectivity of graphene-based electrochemical and fluorescent biosensors are also examined with respect to interfering analytes present in biological systems. Finally, the future outlook for the development of graphene based biosensing technology is outlined.

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A review on graphene-based nanocomposites for electrochemical and fluorescent biosensors

© 2020 The Author(s). We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a 'hands-on' approach, providing practical details and procedures as derived from literature as well as from the authors' experience, in order to enable the reader to reproduce the results. Section I is devoted to 'bottom up' approaches, whereby individual constituents are pieced together into more complex structures. We consider graphene nanoribbons (GNRs) produced either by solution processing or by on-surface synthesis in ultra high vacuum (UHV), as well carbon nanomembranes (CNM). Production of a variety of GNRs with tailored band gaps and edge shapes is now possible. CNMs can be tuned in terms of porosity, crystallinity and electronic behaviour. Section II covers 'top down' techniques. These rely on breaking down of a layered precursor, in the graphene case usually natural crystals like graphite or artificially synthesized materials, such as highly oriented pyrolythic graphite, monolayers or few layers (FL) flakes. The main focus of this section is on various exfoliation techniques in a liquid media, either intercalation or liquid phase exfoliation (LPE). The choice of precursor, exfoliation method, medium as well as the control of parameters such as time or temperature are crucial. A definite choice of parameters and conditions yields a particular material with specific properties that makes it more suitable for a targeted application. We cover protocols for the graphitic precursors to graphene oxide (GO). This is an important material for a range of applications in biomedicine, energy storage, nanocomposites, etc. Hummers' and modified Hummers' methods are used to make GO that subsequently can be reduced to obtain reduced graphene oxide (RGO) with a variety of strategies. GO flakes are also employed to prepare three-dimensional (3d) low density structures, such as sponges, foams, hydro- or aerogels. The assembly of flakes into 3d structures can provide improved mechanical properties. Aerogels with a highly open structure, with interconnected hierarchical pores, can enhance the accessibility to the whole surface area, as relevant for a number of applications, such as energy storage. The main recipes to yield graphite intercalation compounds (GICs) are also discussed. GICs are suitable precursors for covalent functionalization of graphene, but can also be used for the synthesis of uncharged graphene in solution. Degradation of the molecules intercalated in GICs can be triggered by high temperature treatment or microwave irradiation, creating a gas pressure surge in graphite and exfoliation. Electrochemical exfoliation by applying a voltage in an electrolyte to a graphite electrode can be tuned by varying precursors, electrolytes and potential. Graphite electrodes can be either negatively or positively intercalated to obtain GICs that are subsequently exfoliated. We also discuss the materials that can be amenable to exfoliation, by employing a theoretical data-mining approach. The exfoliation of LMs usually results in a heterogeneous dispersion of flakes with different lateral size and thickness. This is a critical bottleneck for applications, and hinders the full exploitation of GRMs produced by solution processing. The establishment of procedures to control the morphological properties of exfoliated GRMs, which also need to be industrially scalable, is one of the key needs. Section III deals with the processing of flakes. (Ultra)centrifugation techniques have thus far been the most investigated to sort GRMs following ultrasonication, shear mixing, ball milling, microfluidization, and wet-jet milling. It allows sorting by size and thickness. Inks formulated from GRM dispersions can be printed using a number of processes, from inkjet to screen printing. Each technique has specific rheological requirements, as well as geometrical constraints. The solvent choice is critical, not only for the GRM stability, but also in terms of optimizing printing on different substrates, such as glass, Si, plastic, paper, etc, all with different surface energies. Chemical modifications of such substrates is also a key step. Sections IV-VII are devoted to the growth of GRMs on various substrates and their processing after growth to place them on the surface of choice for specific applications. The substrate for graphene growth is a key determinant of the nature and quality of the resultant film. The lattice mismatch between graphene and substrate influences the resulting crystallinity. Growth on insulators, such as SiO2, typically results in films with small crystallites, whereas growth on the close-packed surfaces of metals yields highly crystalline films. Section IV outlines the growth of graphene on SiC substrates. This satisfies the requirements for electronic applications, with well-defined graphene-substrate interface, low trapped impurities and no need for transfer. It also allows graphene structures and devices to be measured directly on the growth substrate. The flatness of the substrate results in graphene with minimal strain and ripples on large areas, allowing spectroscopies and surface science to be performed. We also discuss the surface engineering by intercalation of the resulting graphene, its integration with Si-wafers and the production of nanostructures with the desired shape, with no need for patterning. Section V deals with chemical vapour deposition (CVD) onto various transition metals and on insulators. Growth on Ni results in graphitized polycrystalline films. While the thickness of these films can be optimized by controlling the deposition parameters, such as the type of hydrocarbon precursor and temperature, it is difficult to attain single layer graphene (SLG) across large areas, owing to the simultaneous nucleation/growth and solution/precipitation mechanisms. The differing characteristics of polycrystalline Ni films facilitate the growth of graphitic layers at different rates, resulting in regions with differing numbers of graphitic layers. High-quality films can be grown on Cu. Cu is available in a variety of shapes and forms, such as foils, bulks, foams, thin films on other materials and powders, making it attractive for industrial production of large area graphene films. The push to use CVD graphene in applications has also triggered a research line for the direct growth on insulators. The quality of the resulting films is lower than possible to date on metals, but enough, in terms of transmittance and resistivity, for many applications as described in section V. Transfer technologies are the focus of section VI. CVD synthesis of graphene on metals and bottom up molecular approaches require SLG to be transferred to the final target substrates. To have technological impact, the advances in production of high-quality large-area CVD graphene must be commensurate with those on transfer and placement on the final substrates. This is a prerequisite for most applications, such as touch panels, anticorrosion coatings, transparent electrodes and gas sensors etc. New strategies have improved the transferred graphene quality, making CVD graphene a feasible option for CMOS foundries. Methods based on complete etching of the metal substrate in suitable etchants, typically iron chloride, ammonium persulfate, or hydrogen chloride although reliable, are time- and resourceconsuming, with damage to graphene and production of metal and etchant residues. Electrochemical delamination in a low-concentration aqueous solution is an alternative. In this case metallic substrates can be reused. Dry transfer is less detrimental for the SLG quality, enabling a deterministic transfer. There is a large range of layered materials (LMs) beyond graphite. Only few of them have been already exfoliated and fully characterized. Section VII deals with the growth of some of these materials. Amongst them, h-BN, transition metal tri- and di-chalcogenides are of paramount importance. The growth of h-BN is at present considered essential for the development of graphene in (opto) electronic applications, as h-BN is ideal as capping layer or substrate. The interesting optical and electronic properties of TMDs also require the development of scalable methods for their production. Large scale growth using chemical/physical vapour deposition or thermal assisted conversion has been thus far limited to a small set, such as h-BN or some TMDs. Heterostructures could also be directly grown.

Production and processing of graphene and related materials

Recent progress in applications of graphene oxide for gas sensing: A review

Crystalline defects on the nano-scale, which are called artificial pinning centers (APCs), were successfully introduced into high-temperature superconductors (HTS) by nanotechnology, in order to strongly pin the quantized vortices. The critical current densities, Jc, of the HTS films were dramatically improved by APCs. It is possible to form APCs in high-quality epitaxial films, keeping the desired dimensionality, volume fraction, spatial distribution and so on. The in-field Jc of HTS films at 77 K was improved by one order of magnitude compared with previous values using APCs. This technology can be applied to the coated conductor technology in progress, and a high Jc has already been reported. A current outline of the research is described in this review.

https://iopscience.iop.org/article/10.1088/0953-2048/23/1/014001/pdf

Artificial pinning center technology to enhance vortex pinning in YBCO coated conductors

We report on hydrogen adsorption and desorption on titanium-covered graphene in order to test theoretical proposals to use of graphene functionalized with metal atoms for hydrogen storage. At room temperature, titanium islands grow on graphene with an average diameter of about 10 nm. Samples were then loaded with hydrogen, and its desorption kinetics was studied by thermal desorption spectroscopy. We observe the desorption of hydrogen in the temperature range between 400 K and 700 K. Our results demonstrate the stability of hydrogen binding at room temperature and show that hydrogen desorbs at moderate temperatures in line with what is required for practical hydrogen-storage applications.

Hydrogen storage with titanium-functionalized graphene

The quantum Hall effect has been observed in quasi-free-standing monolayer graphene on SiC for the first time. This was achieved by decreasing the carrier density while applying gate voltage in top-gated devices. The charge neutrality point was also clearly observed, which has not been reported in top-gated structures. The mobilities at constant carrier densities did not show apparent temperature dependence up to 300 K, and conductivity was linearly dependent on carrier density. These results indicate that Coulomb scattering induced by charged impurities limits the mobility of quasi-free-standing monolayer graphene up to 300 K.

https://iopscience.iop.org/article/10.1143/APEX.5.125101/pdf

Quantum Hall Effect and Carrier Scattering in Quasi-Free-Standing Monolayer Graphene

We report that mobility in quasi-free-standing monolayer graphene grown on SiC(0001), when compared at the same carrier density, depends on the annealing temperature used for hydrogen intercalation. This was verified by measuring mobility in top-gated devices using quasi-freestanding monolayer graphene obtained by annealing at different temperatures. The density of charged impurities varies with annealing temperature, and it influences transport properties. Our systematic investigation shows that annealing temperatures between 700 and 800 °C are optimum for obtaining high-mobility quasi-free-standing monolayer graphene with the lowest number of charged impurities.

https://iopscience.iop.org/article/10.7567/JJAP.53.04EN01/pdf

Effects of hydrogen intercalation on transport properties of quasi-free-standing monolayer graphene

We report on a new protein sensor built on pristine graphene. Graphene grown by chemical vapor deposition and supported on a SiO2 surface was modified with a sequence consisting of pyrene, DNA aptamer, and dye. Here, an aptamer is a nucleic acid having a specific base sequence that recognizes and forms a complex with a target molecule such as a protein. In our protein sensing system, the recognition of a target protein triggers the fluorescence of a dye tethered on the graphene surface. This is based on the effective fluorescence quenching property of graphene via fluorescence resonance energy transfer (FRET). We first demonstrate that the graphene FRET aptasensor yields fluorescence when there is a target protein in the sample using prostate specific antigen (PSA, a cancer marker) as the target. We confirm that the fluorescence intensities vary depending on the PSA concentrations. We also discuss the selectivity and limit of detection using a microchannel configuration built on a graphene FRET aptasensor...

Graphene FRET Aptasensor

A heating treatment is often used in graphene research to remove adsorbates and resist materials from graphene. Heating graphene followed by air exposure is also known to result in heavy hole doping in graphene, although the role of heating has been unclear. Here, we demonstrate that a practical graphene sample fabricated using the commonly used growth and transfer techniques is unstable against heating in a high vacuum. Structural disorder likely due to defect formation is induced by heating, and the disorder is accompanied by hole doping. Our analysis shows that the main cause of the defect formation is graphene reacting with O2 and H2O molecules inserted between graphene and the substrate. The hole doping caused by air exposure after heating is explained by gas adsorption at the defect sites.

Makoto Takamura

Papers

Hydrogen storage with titanium-functionalized graphene

Quantum Hall Effect and Carrier Scattering in Quasi-Free-Standing Monolayer Graphene

Effects of hydrogen intercalation on transport properties of quasi-free-standing monolayer graphene

Graphene FRET Aptasensor

Structural Instability of Transferred Graphene Grown by Chemical Vapor Deposition against Heating