Graphene-Based Materials for Biosensors: A Review <s> Liquid Exfoliation <s> Fully exploiting the properties of graphene will require a method for the mass production of this remarkable material. Two main routes are possible: large-scale growth or large-scale exfoliation. Here, we demonstrate graphene dispersions with concentrations up to approximately 0.01 mg ml(-1), produced by dispersion and exfoliation of graphite in organic solvents such as N-methyl-pyrrolidone. This is possible because the energy required to exfoliate graphene is balanced by the solvent-graphene interaction for solvents whose surface energies match that of graphene. We confirm the presence of individual graphene sheets by Raman spectroscopy, transmission electron microscopy and electron diffraction. Our method results in a monolayer yield of approximately 1 wt%, which could potentially be improved to 7-12 wt% with further processing. The absence of defects or oxides is confirmed by X-ray photoelectron, infrared and Raman spectroscopies. We are able to produce semi-transparent conducting films and conducting composites. Solution processing of graphene opens up a range of potential large-area applications, from device and sensor fabrication to liquid-phase chemistry. <s> BIB001 </s> Graphene-Based Materials for Biosensors: A Review <s> Liquid Exfoliation <s> We have demonstrated a method to disperse and exfoliate graphite to give graphene suspended in water-surfactant solutions. Optical characterization of these suspensions allowed the partial optimization of the dispersion process. Transmission electron microscopy showed the dispersed phase to consist of small graphitic flakes. More than 40% of these flakes had <5 layers with approximately 3% of flakes consisting of monolayers. Atomic resolution transmission electron microscopy shows the monolayers to be generally free of defects. The dispersed graphitic flakes are stabilized against reaggregation by Coulomb repulsion due to the adsorbed surfactant. We use DLVO and Hamaker theory to describe this stabilization. However, the larger flakes tend to sediment out over approximately 6 weeks, leaving only small flakes dispersed. It is possible to form thin films by vacuum filtration of these dispersions. Raman and IR spectroscopic analysis of these films suggests the flakes to be largely free of defects and oxides, although X-ray photoelectron spectroscopy shows evidence of a small oxide population. Individual graphene flakes can be deposited onto mica by spray coating, allowing statistical analysis of flake size and thickness. Vacuum filtered films are reasonably conductive and are semitransparent. Further improvements may result in the development of cheap transparent conductors. <s> BIB002 </s> Graphene-Based Materials for Biosensors: A Review <s> Liquid Exfoliation <s> Following the astonishing discoveries of fullerenes and carbon nanotubes in earlier decades, the rise of graphene has recently triggered an exciting new area in the field of carbon nanoscience with continuously growing academic and technological impetus. Currently, several methods have been proposed to prepare graphenes, such as micromechanical cleavage, thermal annealing of SiC, chemical reduction of graphite oxide, intercalative expansion of graphite, bottom-up growth, chemical vapor deposition, and liquid-phase exfoliation. Especially this latter top-down approach is very appealing from a chemist’s point of view for the following reasons: i) it is direct, simple, and benign producing graphenes just by solvent treatment of graphite powders, and ii) the as-obtained sheets form colloidal dispersions in the solvents used for the exfoliation, thereby enabling their manipulation into various processes, like mixing, blending, casting, impregnation, spin-coating, or functionalization. The key parameter for suitable solvents is that the solvent–graphene interactions must be at least comparable to those existing between the stacked graphenes in graphite. To that end, Coleman and coworkers have successfully demonstrated this concept using N-methylpyrrolidone, N,N-dimethylacetamide, g-butyrolactone, 1,3-dimethyl-2-imidazolidinone, and benzyl benzoate as <s> BIB003 </s> Graphene-Based Materials for Biosensors: A Review <s> Liquid Exfoliation <s> This paper describes the electron transfer properties of graphene nano-sheets (GNSs) immobilised on pyrolysed photoresist film (PPF) electrodes. The former are produced by the dispersion and exfoliation of graphite in dimethylformamide, and they are characterised using transmission electron microscopy, scanning electron microscopy and Raman spectroscopy. Cyclic voltammetry and electrochemical impedance spectroscopy are used to quantify the effect of the GNSs on electrochemical surface area and on electron transfer kinetics. Compelling evidence is reported in relation to the importance of edge-plane sites and defects in the promotion of electron transfer at carbon nanostructures. A novel ascorbic acid (vitamin C) sensor is presented based on the PPF/GNS system, which is effective in the range 0.4 to 6.0 mM, with a 0.12 mM detection limit. The selectivity of the sensor is demonstrated using a commercially available vitamin C supplement. This is the first report of the electrochemical properties of graphene nano-sheets produced using liquid-phase exfoliation, and it will serve as an important benchmark in the development of inexpensive graphene-based electrodes with high surface area and electro-catalytic activity. <s> BIB004 </s> Graphene-Based Materials for Biosensors: A Review <s> Liquid Exfoliation <s> Due to its unprecedented physical properties, graphene has generated huge interest over the last 7 years. Graphene is generally fabricated in one of two ways: as very high quality sheets produced in limited quantities by micromechanical cleavage or vapor growth or as a rather defective, graphene-like material, graphene oxide, produced in large quantities. However, a growing number of applications would profit from the availability of a method to produce high-quality graphene in large quantities. This Account describes recent work to develop such a processing route inspired by previous theoretical and experimental studies on the solvent dispersion of carbon nanotubes. That work had shown that nanotubes could be effectively dispersed in solvents whose surface energy matched that of the nanotubes. We describe the application of the same approach to the exfoliation of graphite to give graphene in a range of solvents. When graphite powder is exposed to ultrasonication in the presence of a suitable solvent, the powder fragments into nanosheets, which are stabilized against aggregation by the solvent. The enthalpy of mixing is minimized for solvents with surface energies close to that of graphene (∼68 mJ/m(2)). The exfoliated nanosheets are free of defects and oxides and can be produced in large quantities. Once solvent exfoliation is possible, the process can be optimized and the nanosheets can be separated by size. The use of surfactants can also stabilize exfoliated graphene in water, where the ζ potential of the surfactant-coated graphene nanosheets controls the dispersed concentration. Liquid exfoliated graphene can be used for a range of applications: graphene dispersions as optical limiters, films of graphene flakes as transparent conductors or sensors, and exfoliated graphene as a mechanical reinforcement for polymer-based composites. Finally, we have extended this process to exfoliate other layered compounds such as BN and MoS(2). Such materials will be important in a range of applications from thermoelectrics to battery electrodes. This liquid exfoliation technique can be applied to a wide range of materials and has the potential to be scaled up into an industrial process. We believe the coming decade will see an explosion in the applications involving liquid exfoliated two-dimensional materials. <s> BIB005
Liquid exfoliation is a low-cost top-down process for fabricating graphene in solution using an ultrasonic energy source to generate microcavitations and to blast the raw bulk graphite into smaller fragments and thinner layers of graphene. A number of hours are needed to effectively extract the graphite into individual layers. The approach of the liquid exfoliation is carried out in organic solvents such as N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMA), γ-butyrolactone (GBL) and 1,3-dimethyl-2-imidazolidinone (DMEU) BIB002 BIB003 BIB004 BIB001 BIB005 . Recently, studies on the exfoliation and dispersion of graphite have been carried in water/surfactant solutions instead of those organic solvents and their stabilization has been described using DLVO and Hamaker theory BIB002 . The dispersed graphene layers are not supposed to reaggregate due to the coated surfactant which protects against the Coulomb repulsion. The surfactant-coated exfoliated graphene can be stabilized as a colloid. The bounded-molecule surface forms a tail-group in surfactant, a so-called double layer. The possibility of exfoliation arises because of the competition (leading to an energy balance) between the interlayer van der Waals energy and the repulsive solvent-graphene interaction when the surface energy of the solvent is of matched magnitude with that of the dispersed graphene. Figure 3c shows the blasted and exfoliated dispersion of graphite prepared through ultrasonication and mild centrifugation. An individual single-layer graphene from the solution is observed in the transmission electron micrograph, where the selected electron diffraction area shows the characteristic hexagonal rings. This method is very popular due to the simple intercalation, cavitation and exfoliation procedure. The exfoliated graphite layers show high throughput during the production stage. However, prolonged ultrasonication treatment can possibly damage the exfoliated graphene sheets resulting in small size and nano-impurities. The thickness of the exfoliated graphite layers is on the order of 10 nm to 100 nm where they lose the distinctive graphene electronic properties. To improve the properties, microwave radiation and annealing have been used post-exfoliation in the removal of the trapped solvent and air bubbles, significantly expanding the exfoliated graphite layer volume and reducing the nanoscrolling effect. The use of graphene in liquid exfoliation serves as an important benchmark in the development of cost-effective graphene-based transparent electrodes and sensors with high surface area and electrocatalytic activity, optical limiters and mechanical reinforcements for polymer-based composites BIB005 .