## 4 Synthesis of Graphene and Its Derivatives
To circumvent graphene-sheet aggregation due to sonication, the surfactant-assisted liquid-phase exfoliation route is carried out using surfactants and biosurfactants as a dispersant and stabilizer that overcomes aggregation by steric and electrostatic repulsion. It involves reducing the high surface tension of water to be compatible with the hydrophobic graphene flakes by the separation of graphite into few-layered graphene flakes, which are coated with surfactant molecules [114]. In 2013, [PERSON] et al. successfully validated the preparation of few-layered graphene dispersion from graphite powder, in a comparatively short-time sonication by combining liquid-phase exfoliation with heat-treatment attempts, which increase the interlayer surface of graphite and facilitate the access of organic molecules to graphite lattice. The pristine graphene was heat-treated in (NMP) and stabilized by adding polyvinyl pyrrolidone (PVP), which leads to the formation of a well-dispersed few-layered flakes by sonication for up to 20 h [114; 115]. Moreover, [PERSON] et al. demonstrated in 2019 a novel approach to produce stable and well-dispersed few-layered graphene dispersion by the exfoliation of hydrothermally expanded graphite through sonication for 3 h and centrifugation. The selected cationic surfactant, hexadecyltrimethylammoniumbromide (CTAB), is a precursor for constructing graphene nanocomposites, which are utilized in promising electrochemical sensors and biosensors. The concentration of the graphene dispersion ranged from 40 to 60 \(\upmu\)g mL\({}^{-1}\), as well as, presented lower (edge/basal) defects of few-layered graphene, up to five layers [114]. Recently, the solution-based chemical exfoliation of graphite is the most promising approach for the construction of biosensor devices including electrochemical biosensors due to its large scale and facile technique besides low cost and high-quality production [98; 99]. Graphene derivatives (GO, rGO) can be produced from graphite by chemical oxidation. For instance, [PERSON]'s method is used to produce graphite oxide. In this method, potassium permanganate is added to a mixture of graphite, sodium nitrate, and concentrated sulfuric acid. During oxidation, small ions embed to bulk graphite oxide and weaken the interaction between the layers. Sonication grants exfoliate GO flakes from graphite oxide sheets and produce a reduced graphene oxide consecutive reduction that can be utilized to enhance electrical properties [99]. In addition, the electrochemical liquid-phase exfoliation of graphite is a method that involves applying an electric current, where commonly HOPG is used as a working electrode in liquid electrolytes such as ammonium sulfate. It has been considered that the electrochemical potential leads to sufficient ion intercalations between graphene flakes to break van der Waals forces and expand their interlayer spacing. Subsequently, gas eruption expands the graphite and results in its exfoliation. Then, flaked-off graphene sheets are collected by vacuum filtration and dispersed in organic solvents such as (DMF) [101; 116]. Radical species that are generated by electrolysis of water and ruin the graphene yield can be reduced by the usage of radical scavengers. The electrochemical exfoliation method is advantageous because of the high yield (75% of one layer and few layers with a lateral scale, 5-10 \(\upmu\)m) besides the stability of the dispersible yield for several weeks without precipitation. Moreover, the fabricated graphene sheets exhibit high quality features with minimum defects, high C:O ratios of \(\sim\)25.3, and electronic mobility values of 405 cm\({}^{2}\) V\({}^{-1}\) S\({}^{-1}\). Furthermore, the production rates can be increased by utilization of a dual graphite electrode (cathode and anode) and applying an alternating current [101]. In situ functionalization of graphene can be induced during the electrochemical exfoliation to give variant graphene-based composite materials [116].