Graphene: One Material, Many Possibilities—Application Difficulties in Biological Systems

Table 1

Two main approaches with different methods of graphene synthesis [19].

Method	Description	Advantages/disadvantages
Top-down methods
Micromechanical cleavage	First method used to isolate graphene from graphite using adhesive tape. Involves repeated cleavage and yields mono-, di-, and few-layer graphene.	High quality graphene sheets. Slow method, used mainly for study of the graphene properties.
Electrochemical exfoliation	Exfoliation of graphite as a sacrificial electrode and collecting graphene from the electrolyte solution (e.g., surfactant, H2SO4-KOH).	Produces a mixture of different thicknesses of graphite flakes with the possibility to isolate few-layer graphene by centrifugation. Surfactant molecules are difficult to remove and influence the electrical and electrochemical properties of graphene.
Solvent-based exfoliation	Solvent-assisted or thermal exfoliation methods for the production of graphene from GICs.; Exfoliation of unmodified graphite via sonication in solvents.; Exfoliation of graphite oxide.; (Mostly used method of solvent exfoliation and reduction to obtain graphene including Hummers [81] method to synthesize graphite oxide.)	Expensive and hazardous solvents or surfactant molecules are difficult to remove and affect properties of graphene. Increased concentration of graphene is accompanied by the decrease in flake size and increase in defect contamination.; Exfoliation of graphite oxide results in rGO with different properties than pristine graphene due to high levels of defects induced by the harsh conditions of the production process.
Unzipping carbon nanotubes	Few-layer graphene synthesis through unzipping single or multiwall carbon nanotubes using wet chemistry methods or physical methods.	The unzipping results in graphene nanoribbons with different widths considered as quasi-one-dimensional material with different properties than pristine graphene.
Bottom-up methods
Epitaxial growth	The formation of graphene on silicon carbide (SiC) at high temperatures (>1000°C) and generally in ultrahigh vacuum (UHV) conditions or in different atmospheres by the preferential sublimation of silicon from SiC surface and graphitization of the carbon atoms left behind.	Relatively high quality but rarely ≤2 layers of graphene. Possible transfer of graphene from SiC substrates. SiC substrates are commercially available but are too expensive for commercial applications.
Chemical vapor deposition (CVD)	Formation of graphene films by the high temperature carbon pyrolysis in gaseous atmosphere on metal substrates. The optimum conditions of the process depend on the chosen metal substrate. Possible synthesis of graphene nanosheets without preparing the substrate.	Production of monolayer graphene, but the number of layers strongly depends on various process factors.