Doping is the most feasible and convenient method to modulate the band structure of graphene from semimetal to p-type or n-type material. In recent years, the chemical vapor deposition methods have been well developed to grow graphene layer with high quality and large area. This paper briefly reviews the recent research progress on doping methods of CVD graphene, including the doping effects by metals, small molecules, chemical reactions and replacement of lattice atoms. The methods of bilayer graphene band regulation as well as the fabrication of graphene p-n junction are also introduced, and the future tendency and potential applications of doped graphene are proposed. For graphene, it is relatively easy to produce p-type doping via surface absorption, exposing pristine graphene in those molecules with electron withdrawing groups (H₂O, O₂, N₂, NO₂, PMMA et al.) will lead to evident p-type doping, and graphene of this kind of p-type doping can rapidly recover to its original state when doping molecules are removed. If boron source was introduced into the CVD growth process of graphene, substitutional p-doping that some carbon atoms in graphene hexagonal lattice are replaced by boron atoms can be formed. Compared to the p-type doping, stable n-type doping is not facile for graphene. It has been proved that some electron-donating molecules such as ammonia, potassium, phosphorus, hydrogen and poly(ethyleneimine) (PEI) can produce n-type doping in graphene through surface electron transfer, but these doping effects are unstable. By introducing nitrogen-containing precursors in growth approach, small part of lattice carbon atoms will be replaced by nitrogen atoms which can result in effectively n-doping effect. Combine the p-type and n-type doping method together, the p-n junction can be produced in mono- or bi-layer graphene, a series of novel functional devices like photothermoelectric devices have been constructed using these hetero-doped graphene p-n junctions.

In this study, a new isothermal signal amplification method is developed for sensitive detection of microRNAs (miRNAs) by integrating the distinct advantages of graphene oxide (GO) for efficient fluorescence quenching of fluorophore-labeled single strand DNA (ssDNA) and double strand (ds)-specific nuclease (DSN) for highly selective digestion of DNA strand in DNA/RNA hybrids. DSN is a nuclease purified from hepatopancreas of Red King crab, which shows a strong preference for cleaving dsDNA and DNA in DNA/RNA hybrid duplexes. On contrast, DSN is practically inactive towards ssDNA or single- or double-stranded RNA. Herein, let-7a is selected as the proof-of-concept target miRNA and a fluorescein-labeled ssDNA probe is designed to be complementary to let-7a. The ssDNA probe, which will not be hydrolyzed by DSN in the absence of let-7a, will be adsorbed on GO via π-π stacking, resulting in efficient fluorescence quenching. When let-7a is introduced, it will hybridize with the ssDNA probe to form a double helix structure (dsDNA). DSN can selectively cleave the DNA oligonucleotides of the DNA/RNA hybrid to produce very small DNA fragments. Let-7a is thus released and will hybridize with another ssDNA probe again, which will be further cleaved by DSN. In this manner, each let-7a molecule can specifically trigger various cycles of hybridization and DSN cleavage of fluorescent ssDNA to yield numerous small fragments of DNA oligonucleotides. It should be noted that the π-π stacking interaction between GO and the very small DNA fragments bearing the fluorophores will be remarkably weakened, making the fluorescence maintained. Therefore, the DSN-mediated cycling of fluorescent ssDNA cleavage greatly amplifies the fluorescence signal for miRNA detection. Under the optimized experimental conditions, the fluorescence signal is proportional linearly to the concentration of let-7a in the range from 100 pmol/L to 5 nmol/L, and the detection limit is calculated to be 60 pmol/L (3σ). Furthermore, this proposed approach can also be applied to the simultaneous detection of multiplex miRNA targets.

Porous graphene material, named as MWRGO, has been prepared by microwave-assisted reduction method. Extensive characterizations indicate that graphene oxide was effectively reduced and MWRGO has a porous and disordered stacking structure. It has a special surface area of 461.6 m²/g with pore size centered at 0.67 nm. H₂ and CO₂ adsorption properties of MWRGO were investigated, showing a H₂ uptake of 0.52 wt% at 77 K and 1 atm and an absolute adsorption amount as high as 10.7 wt% at a higher pressure of 60 bar. The amount of CO₂ adsorption at 273 K and 1 atm is 7.1 wt%.

Nanopore sequencing is among the most promising technologies to achieve the goals of the "$1,000 Genome". Two major types of nanopores have been extensively investigated, protein and silicon-based solid-state nanopores. However, protein pores are short-lived and the length of solid-state nanopores is much larger than the distance between adjacent bases, resulting in incapability to discriminate individual bases along the single-stranded DNA molecules. In this paper, we report λ-DNA translocations through graphene nanopore. A large nanopore with diameter of about 30 nm on silicon nitride substrates were first fabricated using focused ion beam (FIB) system under the beam current of 2 pA, accelerating voltage of 30 kV and sculpting time of 1 s. Individual graphene membranes were suspended onto the substrates to cover the large pore, and nanopores with a diameter less than 10 nm are sculpted in the graphene sheet by focused electron beam (FEB) from a transmission electron microscope (TEM) under a 400 kx magnification times and 300 kV accelerating voltage at 1×10⁵～5×10⁵ A/m² current density for 2～3 min. The edges of these graphene nanopores became smoother and sharper when the temperature was increased to about 450 ℃, which might help to lower interactions between graphene nanopores and the analytes. The signals of DNA translocation through graphene nanopores had been recorded using a patch clamp amplifier at 10 kHz sampling frequency filtered at 5 kHz via an integrated four-pole low-pass Bessel filter. Analyses of the DNA translocation current traces indicated that different conformations of DNA molecules may exist during entrance into nanopores. In addition, our overall detection platform had a low noise amplitude of around 10 pA, which allowed more sensitive signal detection. Taken together, our observations demonstrate that graphene nanopores are feasible for DNA sensing, leading a forward step towards single-molecule DNA sequencing using monolayered graphene nanopores.

Graphene materials are materials with a flat mono/few layer of carbon atoms tightly packed to a two-dimensional honeycomb lattice. Graphene materials are expected to be applied in lithium ion batteries due to their unique structural, mechanical and electrical properties. As an anode material, the charge/discharge characteristics of graphene materials is similar to those of low-temperature soft carbon materials, such as high capacity, low initial efficiency and large voltage hysteresis. Although attractive results have been achieved for graphene as anode materials for LIBs, detailed lithium storage mechanisms are still not clear. The effects of the following several structural parameters including disorder degree, surface area, micropores, interlayer spacing, C/O ratio and layer number on the lithium storage properties are discussed. Thermally reduced graphene materials with a highly disordered structure and high surface area has exceptionally high reversible capacity. Micropores in graphene materials have a great impact on their electrochemical performance. Although these micropores can provide additional sites for increased reversible lithium storage, it can also results in severe capacity fading and voltage hysteresis. Oxygen functional groups and larger interlayer spacing may provide higher reversible capacity of graphene, but the micropores and defect-based reversible storage may be the main contribution. Effect of layer number on lithium storage mechanisms of graphene and the conclusion are still in debate. Graphene with rich oxygen functional groups is a promising cathode material with high capacity and rate performance for lithium storage. High specific capacity of graphene cathode is mainly ascribed to lithiation reaction of oxygen functional groups, such as epoxide and carbonyl groups. Lithiation of oxygen functional groups still requires further study for a full understanding. Based on the lithium storage characteristics of graphene anode and cathode, lithium ion capacitors with high energy density and graphene composite cathode materials for lithium ion batteries may be designed and developed in the future. Graphene based lithium ion capacitors facilitate the reversible lithium storage, which significantly improves the energy density of lithium ion capacitors compared to those of conventional systems based on activated carbon. LiFePO₄ modified with graphene layers has reached 208 mAh/g in specific capacity. The excess capacity is attributed to the reversible reduction-oxidation reaction between the lithium ions of the electrolyte and the exfoliated graphene flakes.

A novel lipid based carbonaceous nanocomposite, 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (sodium salt) (POPG) modified graphene (GP) (POPG-GP), was designed and synthesized by a non-covalent method. The nanocomposite was endowed with excellent properties of the two independent components, such as the biocompatibility of POPG and the outstanding electric properties of graphene. Fourier transform infrared (FT-IR) spectra, ultraviolet-visible (UV-vis) absorption spectra, transmission electron microscopy (TEM) were utilized to characterize the structure, morphology and surface property of the as synthesized POPG-GP. It has been found that the modification of POPG on GP could not only assist the dispersion of graphene in aqueous solution, but also endow it with negatively charged, which was favorable for the further immobilization of model enzyme via self-assembly. Based on the electrostatic interaction, the positively charged horseradish peroxide (HRP) could be immobilized onto the surface of POPG-GP to form HRP/POPG-GP/GC electrode. UV-vis and FT-IR spectroscopies were used to monitor the assembly process and demonstrated that HRP had been immobilized without denaturation. The HRP/POPG-GP/GC electrode could commendably realize the direct electron transfer (DET) between electrode and redox enzyme with good electrochemical performance. Moreover, such modified electrode also showed good electrocatalytic response toward the detection of H₂O₂ with high sensitivity, wide linear range, excellent stability and reproducibility. The linear response range for the HRP/POPG-GP/GC was 3.5～210 μmol/L (R=0.999). The detection limit and the sensitivity of the HRP/POPG-GP/GC electrode was calculated to be 1.17 μmol/L (S/N=3) and 356.6 mA·cm^-2·M^-1, respectively. The apparent Michaelis-Menten constant K_m was estimated to be 0.45 mmol/L, indicating a high affinity of HRP to H₂O₂ on POPG-GP. The experiment results demonstrated that POPG-GP not only provided a biocompatible microenvironment for the immobilized HRP, but also supplied a necessary pathway for its direct electron transfer. Therefore, such biocompatible nanocomposite had potential applications in the field of biosensors.

Graphene, a monolayer of carbon atoms packed into a two-dimensional crystal structure, attracted intense attention owing to its unique structure and optical, electronic properties. Raman spectroscopy is a quick and precise method in material science and has been employed for many years to investigate material properties. It can be used to investigate the electronic band structure, the phonon energy dispersion and the electron-phonon interaction in graphene systems. In probing graphene's properties, Raman spectroscopy is considered to be a reliable method. In this review, we highlight recent progress of studying graphene structure using Raman spectroscopy. First, on the basis of systematically analyzing the phonon dispersion of graphene, the typical Raman scattering features of graphene, such as G band, G' band, and D band, and the basic physical process are introduced. Using these Raman fingerprints, we can quickly and directly distinguish the layer thickness of graphene, determine the edge chirality and monitor the type and density of defects in graphene. Second, stacking disorder will significantly modify the optical properties and interlayer coupling stretch of few-layer graphene so that the Raman features of graphene will be strongly influenced not only in the G band intensity but also in the intensity, lineshape and the frequency of G' band. According to the peak position, width, and intensity of the Raman G band and G' band in graphene, we also discuss the influence of doping, substrate, temperature, and strain on the electronic structure of graphene. Finally, we introduce the second order overtone and combination Raman modes and the low frequency Raman feature (shear and layer breathing mode) in graphene, and discuss the dependence of these peaks on the structure of graphene.

Graphene is a star material due to its intriguing electronic, mechanical, thermal and chemical properties and many potential applications. For most of these potential applications, the synthesis of high-quality graphene layers in large scale is highly desired. In the past 10 years, many methods of synthesizing graphene have been developed and explored extensively. Among them, transition metal (TM)-catalyzed chemical vapor deposition (CVD) method stands out for its numerous advantages. As a typical two-dimensional crystal, the growth of graphene must follow the classical crystal growth theory. Here, we introduce three aspects of graphene CVD growth mechanism based on the classical crystal growth theory and the density functional theory (DFT) calculations. (1) The nucleation process and nucleation rate of graphene on metal terrace and near a step edge. On the basis of the predicated very large nucleation barrier, we have proposed a strategy of using the seeded growth method to grow large-area single crystal graphene. (2) Application of Wulff construction in graphene CVD growth. Based on the investigations of graphene edge structures on metal surface and their formation energies, the equilibrium structures of graphene island can be determined by the theory of Wulff construction. (3) The application of kinetic Wulff construction in graphene CVD growth. A detailed investigation on the structural stability and growth kinetics of graphene on the Cu(111) surface have been systematically investigated. According to the kinetic Wulff construction, the armchair edge which growth fast will gradually disappear and the zigzag edges which grows slowly will eventually dominate the circumference of a growing graphene island. The above discussions and conclusions lead to a deep insight into the CVD graphene growth, which are expected to guide the experimental design of growing large-scale graphene with high-quality.

Graphene has caught wide attention due to its unique and excellent properties since its first isolation in 2004. Controllable synthesis of graphene with large-area and high-quality is critical for the realization of various graphene based applications. Although scalable graphene could be grown on metal substrates by chemical vapor deposition (CVD) method, as-grown graphene needs to be transferred onto a dielectric layer for further devices construction. The direct synthesis of graphene on dielectric substrates could avoid the damages and contaminations caused by the transfer process. In this review, we provide a comprehensive progress regarding the synthesis of graphene on dielectric substrates including traditional ones (i.e., glass, quartz, amorphous SiO₂, Si₃N₄ and Al₂O₃) and two dimensional hexagonal boron nitride films. The growth techniques based on CVD approach are classified into metal-catalyzed, metal-free and plasma-enhanced CVD. The growth procedures for each technique are first described, and the main results in terms of as-grown graphene sample's properties such as its electron transport, layer number, crystallinity and quality are then discussed. These studies point to the important role of techniques and experimental conditions in tuning various properties of graphene product. With this idea in mind, we summarize the information of growth conditions and graphene-related properties in different cases, which provides a useful reference for comparing and evaluating the advantages and disadvantages of various techniques. Moreover, we discuss the major challenges in this growing field. Although metal catalyzed CVD could achieve the grown graphene placed on the underlying dielectric substrates by the post removal of metal, it still cannot avoid the metal contaminations or damage of graphene. The main limitation for graphene metal-free synthesis on dielectric substrates is associated with very slow growth rate of graphene and the difficult control of defect-free sample. New growth techniques, suitable dielectric substrates or unexplored experimental conditions are expected to be developed to overcome these challenges in future.

Graphene has a unique atom-thick two-dimensional structure and excellent properties, including high conductivity and electron mobility at room temperature, large specific surface area, and excellent mechanical properties. Graphene also possesses a variety of promising electrochemical properties, such as a wide potential window, low charge-transfer resistance, high electrocatalytic activity and fast electron transfer rate. Furthermore, chemically modified graphene materials, particularly graphene oxide (GO) and reduced graphene oxide (rGO), can be produced in a large scale and at low costs. They have good processability and can be assembled, blended or fabricated into macroscopic electrode materials with controlled compositions and microstructures. Thus, graphene and its chemically modified derivatives are unique and attractive electrode materials for electrochemical biosensing. For example, GO is a chemically modified graphene and an important precursor of graphene. GO sheets have a large amount of carboxyl groups at their edges, which can be used to covalently immobilize enzymes, realizing the detection of biomolecules. GO can also enhance the direct charge transfer of protein because of its irreversible adsorption to protein and abundant catalytic sites. However, the oxygen functional groups of GO heavily destroy the conjugated planes of graphene sheets, decreasing the electrical property and limiting the practical applications of GO. Chemical, electrochemical, or thermal reduction can partly restore the conjugated structure, converting GO to conductive rGO. On the other hand, graphene is a material with zero band gap. Doping graphene with heteroatoms can modulate its band gap and improve its electrocatalytic properties. Graphene materials also frequently have to be blended with other functional materials to improve their dispersibility and processibility, enhance their electrochemical activity and/or selectivity. This review will summarize the recent research achievements in electrochemical biosensing based on the electrodes modified with pristine graphene (e.g. GO, rGO, and doped graphene) or graphene composites with biomolecules, polymers, ionic liquids, metal and metal oxide nanoparticles. A perspective of developments in this research field is also provided.

Graphene, a two-dimensional (2D) atomic crystal composed of single-layer hexagonal mesh of carbon atoms, is one of the most exciting materials being investigated today. Graphene chemistry, the covalent functionalization of graphene as a giant molecule, provides a promising approach to controllably engineer graphene's band structure, create novel graphene derivatives and tailor the interfacial characteristics. One of the great challenges for graphene functionalization originates from its strong chemical stability, thus highly reactive chemical species are needed as the reactants. In recent years, we have been working on the photo-induced free radicals-based photochemistry of graphene, targeting the efficient graphene functionalization for its band structure engineering. Various photochemical modification methods have been developed, such as photochemical chlorination, photochemical methylation, photocatalytic oxidation and bifacially asymmetric functionalization of graphene. The homogeneous and nondestructive photochlorination of graphene could remove the conducting π-bands and open up a band gap in graphene. TiO₂-based photocatalytic oxidation of graphene could realize photochemical tailoring of graphene, including ribbon cutting, arbitrary patterning on any substrate, layer-by-layer thinning, and localized graphene to graphene oxidation conversion. Using photochemical reaction of graphene as a probe, we have investigated the dimension effects on graphene chemistry, including the thickness, stacking order, single- and double-side, and edge dependent reactivity in graphene. After two-step functionalization of graphene, we have fabricated the thinnest Janus disc named Janus graphene, which comprises two kinds of decorations separated by the one-atom-thick carbon layer. It is found that chemical decorations on one side are capable of affecting both chemical reactivity and wettability of the opposite side, indicative of communication between the two grafted decorations separated by a single-layer graphene. In this review, we select several typical examples to demonstrate such kinds of photochemical graphene engineering and its intrinsic 2D reaction characteristics, together with a brief discussion on the future directions, challenges and opportunities in this research area.

Graphene, a two-dimensional crystalline monolayer made of sp²-hybridized carbon atoms arranged in a honeycomb lattice, holds a set of remarkable electronic and physical properties, such as ballistic transport with low resistivity, high chemical stability, and high mechanical strength. By taking advantage of these, in recent years our research group has performed a series of studies for modifying the surfaces of graphene and tuning its properties. These studies can be mainly divided into two categories. First, we opened graphene's band gap to some extent through covalent and/or noncovalent chemical modifications, and installed sensing functions into graphene. In detail, we grafted nitrophenyl group onto graphene through an electrochemical method and methyl group onto graphene by plasma treatment to open its band gap. Also, we assembled lead sulfide or titanium dioxide onto graphene through electron beam evaporation to achieve optical or gas sensing. A rotaxane molecule with a bistable structure was also assembled onto graphene through π-π stacking to obtain optical switches with logic capability. On the other hand, we also fabricated graphene-based nanoelectrodes for making a new-generation molecular electronic devices with diverse functionalities. In detail, we cut graphene using electron beam lithography and reactive ion etching to obtain graphene electrodes. Poly(3-hexyl thiophene) or copper phthalocyanine was spin-coated onto these electrodes to achieve field effect transistors with the high carrier mobility and photoresponsive property. We further developed graphene nanoelectrodes by dash-line lithography, and molecular bridges with different functions were connected between these nanoelectrodes. These single molecule devices can switch their conductance upon exposure to external stimuli, such as metal ion, pH and light. Looking into the future, graphene, as a representative of carbon-based nanomaterials, will continue to play an important role in the area of nano/molecular electronics.

Fibronectin (FN) could be used to modify the transplant of titanium dioxide. However, the hydrophilicity of titanium dioxide may prevent the stable adsorption of protein. Suitable hydrophobic modification of the surface can promote protein adsorption. In this work, all-atom molecular dynamics (MD) simulations were used to study the adsorption of FN on rutile surface, 23% graphene layer modified rutile surface, 92% graphene layer modified rutile surface and the graphite surface. The graphene layer can change the surface chemistry of rutile and break the strong interactions between rutile and water molecules. Parallel tempering Monte Carlo algorithm was used firstly to identify the global-minimum-energy orientation of FN. Subsequently, the orientation and conformation of adsorbed FN on modified titanium dioxide surfaces were studied by MD simulations. The simulation results show that FN can hardly adsorb on the rutile surface. Graphene layer deposited on titanium dioxide can reduce the surface hydrophilicity. When the rutile surface is covered by the graphene layer, FN adsorbs on the surface stably. The specific recognition site of FN faces toward the solution when FN is adsorbed on 23% graphene layer modified rutile surface, which is conducive to the identification of integrin. However, if too much graphene layer deposited on rutile, the specific recognition site of FN would get close to the surface due to the stronger adsorption. The minimum distance between FN and various surfaces can indicate the stability of protein adsorption on surfaces. DSSP analysis shows that the seven β-sheets of FN do not change much in all systems during the 40 ns MD simulations. Due to the deposition of graphene layer, the density of water molecules near the surface decreases. The adsorption energy of FN on different surfaces increases with higher surface graphene composition. Graphene modification could promote the fibronectin adsorption on rutile surfaces. This work can provide some guidance for the design and development of modified implant biomaterials.