## 4 Synthesis of Graphene and Its Derivatives
The rehybridization of one or more carbon atoms by condensation, addition, nucleophilic substitution, and electrophilic addition reactions leads to the installation of scattering sites in graphene [128]. Several functional groups such as alkyl, amino, and hydroxyl groups can be covalently bonded onto graphene, where it acts as chemical switches to introduce other functional molecules such as polymers. In addition, this method has a control on the carrier density and bandgap engineering according to the type of the added functional groups and the degree of functionalization. For instance, the binding of acceptor functional groups or donor functional groups gives rise to the p-type semiconductor and an n-type semiconductor, respectively [128]. In contrast to the covalent functionalization method, the noncovalent approach is achieved through dispersion and electrostatic forces, van der Waals, and hydrophilic and hydrophobic interactions, which do not affect the intrinsic and electronic properties of graphene but can adjust the morphology while providing new chemical groups on the graphene structure [128; 129]. In this method, surface functionalization with various molecules is achieved in several ways, such as surfactant adsorption, wrapping of polymers as well as interaction with biomolecules and metal nanoparticles. For instance, noncovalent functionalization of graphene with nanoparticles and nanowires is used in non-enzymatic glucose sensing with remarkable detection limits [128]. In brief, several reduction strategies are used for the synthesis of rGO, through three main simultaneous procedures: (1) Oxidation methods, (2) exfoliation techniques, (3) reduction process. The oxidation methods, [PERSON], Staudenmaier, and Hummer's (improved or modified) methods, produce graphite oxide from bulk graphite. Recently, the modified [PERSON]'s method is mainly used, considering the low reaction time (<3 h), while the other mentioned methods take up to (12 h) or several days. Consequently, the exfoliation by ultrasonication is carried and then followed by reduction strategies including (a) thermally, (b) chemically, (c) electrochemically, (d) photocatalytically, (e) hydrothermally, (f) solvothermally, (g) voltage-inducely as well as microwave irradiation methods [98]. Moreover, the hydrothermal method is a simple strategy for the reduction of graphite oxide to rGO, where the degree of the reduction is dependent on the reaction temperature due to the effect of temperature on the interlayer distance, thus, as the temperature increases the reduction of graphite oxide increases to synthesize more rGO. The solvothermal process is advantageous because of the high yield, relatively low C:O ratio, and mild synthesis conditions as well as the produced graphene-like sheet that exhibit highly resistive properties, considering the high amount of residual functional groups that are produced; however, the produced yield expressed irreversible defects, besides an ample number of oxygen-containing functional groups [130]. For instance, [PERSON] et al. used the hydrothermal method for the fabrication of reduced graphene oxide/CuO, which is utilized in non-enzymatic glucose sensors. In 2017, a novel ultrasensitive non-enzymatic glucose sensor was fabricated by an in situlayered 3D CuS/RGO/CuS nanocomposite on Cu foam by the one-step hydrothermal-assisted process. The electrochemical properties of the produced composite, CuS/RGO/CuS/Cu, was evaluated by cyclic voltammetry and amperometry techniques and utilized as an electrode regarding its high sensitivity of 26.66 \(\upmu\)A mM\({}^{-1}\) cm\({}^{-2}\) to glucose sensing and low detection limit of 0.5 \(\upmu\)m [131]. Furthermore, the microwave irradiation method is a novel approach for speeding up the synthesis of graphene and enhancing the quality of the yield. The principle of this method is the utilization of high-frequency wave on graphite or other carbon materials, which lead to the speed elevation of their temperature, through dipolar polarization and ionic condition.