## 5 Graphene-Based Electrode for Non-Enzymatic Glucose Biosensor  
The emerging nanomaterials-based graphene composition with a non-enzymatic glucose sensor would meet the scientific demands such as cost effectiveness, synthesis approach, biocompatibility, stability, electron-transfer kinetics, and sensitivity [17; 32; 33; 134; 33]. The superlative properties of graphene nanosheets make them a preferable material for developing hybrid nanostructures, which add synergistic features to the sensor, especially, non-enzymatic glucose biosensors [135]. In recent years, various metals, such as platinum (Pt), gold (Au), palladium (Pd), nickel (Ni), and copper (Cu) as well as metal oxides such as Cu\({}_{2}\)O, RuO\({}_{2}\), NiO, Co\({}_{3}\)O\({}_{4}\), and MnO\({}_{2}\) were employed to modify non-enzymatic glucose electrodes [136; 137]. Recently, three-dimensional (3D) electrodes have been employed in the fabrication of non-enzymatic glucose biosensors to enhance the performance by widening the specific surface area. [PERSON] et al. [135] utilized for the first-time hybrid electrodes based on rGO/Ni/ZnO arrays for non-enzymatic biosensing. They have studied the effect of rGO nanosheets on the electrical activity of Ni/ZnO heterostructure. It is shown that the integration of rGO, Ni, and ZnO nanorods exhibit better performance in the electrochemical properties of the biosensor. While Ni/ZnO prisms have a linear range (1-8.1 mM), a sensitivity of 824.34 \(\upmu\)A mM\({}^{-1}\) cm\({}^{-2}\), and a detection limit (0.28 \(\upmu\)M), the utilization of reduced graphene oxide adds advantages of higher sensitivity of 2030 \(\upmu\)A mM\({}^{-1}\) cm\({}^{-2}\) and lower detection limit (0.15 \(\upmu\)M). [PERSON] et al. [138] developed a performance-enhanced (enhanced selectivity and sensitivity) non-enzymatic glucose sensor based on graphene-heterostructure. The proposed sensor is a composite of graphene (G)/platinum oxide (PtO)/n-silicon (Si) heterostructure. While the atomic layer deposition (ALD) was used to deposit the PtO thin film in a N-type silicon wafer at 275 \({}^{\circ}\)C for 50-500 ALD-cycles, the bottom-up approach was used to produce graphene using the Plasma Enhanced Chemical Vapor Deposition (PECVD) at a pressure of 1500 m Torr and 600 \({}^{\circ}\)C. The group studied the effect of the PtO thickness on the catalytic activities which played a significant role in the sensitivity and selectivity. Increasing the PtO layer thickness leads to enhancing the sensitivity by 150% to reach up to 30 \(\upmu\)A/mM.cm\({}^{2}\). [PERSON] et al. [139] proposed a non-enzymatic glucose sensor based on a nickel nanoparticle-atapulgite-reduced graphene oxide-modified glassy carbon electrode. The NiNPs/ATP/RGO-GCE electrocatalysist towards the GO\({}_{x}\) was measured by cyclic voltammetry and amperometric. The group tested the CVs of NiNPs/ATP/RGO-GCE in 0.1 M of NaOH at different scan rates of about 0.020.1 \(V\)\(S^{-1}\). Moreover, they investigated the CVs of NiNPs/ATP/RGO-GCE in 0.1 M of NaOH at a different glucose concentration of about 0-3 mM. Furthermore, the electrochemical performance of GCE, NiNPs/RGO-GCE, and NiNPs/ATP/RGO-GCE was investigated in 0.1 M NaOH at the absence \((1;3;5)\) and presence \((2;4;6)\) of the glucose. The anodic/cathodic potential peaks of GC, NiNPs/RGO-modified, and NiNPs/ATP/RGO-modified electrodes were investigated as 0.322/0.488, 0.292/0.403, and 0.319/0.458 V, respectively. The group demonstrated a sensitive sensor in the real glucose samples of 1414.4 \(\mu A\)\(mM^{-1}\)\(cm^{-2}\), linear range (1-710 \(\upmu\)M), and detection limit (0.37 \(\upmu\)M). [PERSON] et al. [126] reported a composite material of 3D graphene-Co\({}_{3}\)O\({}_{4}\) nanowires as a freestanding electrode for non-enzymatic glucose sensing. The biosensor achieved a sensitivity of 3.39 mA mM\({}^{-1}\) cm\({}^{-2}\) and a LOD of 25 nM, which are nearly six times higher and two times lower than those of the CoO NRs-modified electrode in [PERSON]'s study.