## 2 Electrochemical Properties of Graphene, Graphene Oxide (GO), and Reduced Graphene Oxide (rGO)  
Materials with extraordinary electrochemical properties of graphene have appeared because of its bond structure (sp2 bonds) and the configuration (a hexagon configuration) such as a large surface area of about 2630 \(m^{2}g^{-1}\), a controlled bandgap, and high conductivity theoretically about \(0.96\times 10^{6}\,ohm\,cm^{-1}\) (\(200,000\,cm^{2}v^{-1}\,s^{-1}\)_electron mobility_) [38; 39]. While the graphene is a semiconductor with zero bandgap, graphene oxide (GO) and reduced graphene oxide (rGO) have band gaps of about 2.2 and 1.00-1.69 eV, consequently [40]. The highly oriented pyrolytic graphene (HOPG) has an electron transfer rate of the edged plane of graphene that is about 0.01 cm/s, while the basal plane is about below \(10^{-9}\) cm/s. Since the graphene electrochemical properties could be controlled by its edge where the heterogeneous electron transfer (HET) is fast, the graphene nanocomposites are good electrodes for non-enzymatic glucose sensors [41]. A large \(\pi\) bond of poly atoms throughout the entire layer is due to the electrons in the ps2 orbitals that are perpendicular to the plane of the layer of each carbon, which makes graphene possess excellent electrochemical properties such as high conductivity at room temperature and current-voltage characteristics. Moreover, the \(\pi\)-\(\pi\) stacking and hydrophobic interactions enhance the connection of graphene to biomolecules [8; 10; 42]. Solution-processability of graphene is more suitable for biosensing applications, due to its compatibility with biochemical systems and inexpensive cost [7; 8; 10; 43]. In addition to the electronic properties such as semi-metallic properties, a strong bipolar electric field effect with a high concentration of charge carriers, a small overlap of band gaps, and a quantum hall effect, graphene also has unique mechanical properties such as bandgap tenability, high elasticity, and excellent heat transfer [10]. Hence, a variety of graphene-based materials were prepared and successfully used in designing electrochemical biosensors such as graphene oxide (GO) and reduced graphene oxide (rGO) (see Section 5). GO and rGO are widely used in non-enzymatic glucose sensors because of their electrochemical properties. The basal plate of GO including the oxygenated groups (epoxy, hydroxyl) have an electrochemical advantage that increase surface functionalization to attract the target, while the decrease of electrical conductivity is due to insulation from oxygenated groups standing as a barrier. The limit of the insulation directly depends on the oxidation level [44; 45; 46; 47]. In fact, due to the decrease in electrical conductivity, rGO replaced GO in many electrochemical sensors especially non-enzymatic glucose sensors. By reducing the oxygen-containing groups using many approaches such as electrochemically and hydrothermally, that makes the electrochemical properties much better. Furthermore, the electrical properties of rGO are highly dependent on the reduction methods [48; 49; 50; 51]. Table 1 introduces a general idea of the graphene, GO, and rGO properties. Owing to its advantage of the electrochemical properties as mentioned above a vast development of biosensing applications has been recently discovered. Several proposals were established to use graphene-based electrodes for different platforms which could be used to immobilize biomolecules to create the best biosensing parameters. Since the graphene shows an enhancement of signal response due to its electrochemical response, various biosensing applications have been explored [69, 70, 71].