# Magnetic Solid-Phase Extraction of Organic Compounds Based on Graphene Oxide Nanocomposites
## 2 Preparation and Applications of GO for the MSPE of Organic Compounds
### Nanocomposites of Reduced GO with Fe\({}_{3}\)O\({}_{4}\) Nanoparticles  
Reduced graphene oxide (RGO) is a nanomaterial obtained by chemical reduction of graphene oxide that contains less oxygen groups and has properties closer to those of graphene [25; 58]. RGO has various applications such as removal of metals and dyes [59; 60], catalysts [61], electroanalytical sensors [62] etc. Magnetic nanocomposites of reduced graphene oxide have been successfully applied for the MSPE of various analytes from different sample matrices. Due to the combination of the magnetic Fe\({}_{3}\)O\({}_{4}\) nanoparticles and the graphene sheets, the magnetic RGO sorbent shows distinguished properties including good dispersity, high surface area, high adsorption efficiency and good super-paramagnetism [63]. There are different synthetic procedures for the fabrication of RGO/Fe\({}_{3}\)O\({}_{4}\) sorbents. The co-precipitation approach is a multi-step procedure in which GO/Fe\({}_{3}\)O\({}_{4}\) previously prepared by precipitating Fe\({}^{2+}\) and Fe\({}^{3+}\) in the presence of GO is reduced with the addition of hydrazine hydrate [64; 65]. For the solvothermal approach, graphite oxide is exfoliated in diethylene glycol under sonication to produce graphene oxide while ferric chloride with sodium acetate are also dissolved in diethylene glycol. In this case, diethylene glycol was both solvent and reducing agent. Accordingly, the GO dispersion is added into the second solution and the mixture was sonicated and heated at 190 \({}^{\circ}\)C in an autoclave [64; 66]. The RGO/Fe\({}_{3}\)O\({}_{4}\) can be also prepared through the hydrothermal method, which is similar to the solvothermal but instead of organic solvents that are used in the solvothermal method, water is used as a solvent in the hydrothermal approach. For this purpose, a salt of Fe\({}^{3+}\) and sodium hydroxide is added to an aqueous solution of graphene oxide and the mixture is heated in an autoclave for a certain time span [64; 67]. Figure 3 shows the scanning electron micrographs (SEM) of RGO/Fe\({}_{3}\)O\({}_{4}\) prepared by solvothermal (_a_), hydrothermal (_b_) and co-precipitation (_c_) methods. Sudan dyes have been extracted from tomato sauce and chili-containing foods with RGO/Fe\({}_{3}\)O\({}_{4}\) prior to their determination by HPLC-DAD [68]. The developed MSPE procedure was simple, economic and provided satisfactory extraction recoveries and LOD values. Magnetic RGO/Fe\({}_{3}\)O\({}_{4}\) has been also used for the MSPEe of bisphenol A from water samples prior to its determination by HPLC-UV. The MSPE technique was coupled with dispersive liquid-liquid microextraction (DLIME) in order to utilize the benefits of both sample preparation techniques. The magnetic sorbent was separated conveniently and rapidly from the sample matrix and it was found to be reusable for at least 12 repeated cycles [69]. In 2016, [PERSON] et al. developed a microwave-assisted synthesis of reduced graphene oxide decorated with magnetite and gold nanoparticles. Gold nanoparticles offer the benefits of chemical stability, biocompatibility, satisfactory magnetic properties and possibility for chemical modification. The RGO/Fe\({}_{3}\)O\({}_{4}\)@Au sorbent was used for the MSPEe of organochlorine pesticides from seawater samples prior to their determination with GC-MS [70]. A magnetic polyethyleneimine functionalized reduced graphene oxide nanocomposite was synthesized and used for the MSPEe of polar non-steroidal anti-inflammatory drugs from water samples [71] and polar acidic herbicides from rice [72]. The modification of RGO with polyethyleneimine changed the polarity of RGO to some extent and offered more active sites for the adsorption of the polar target analytes. In 2018, [PERSON] et al. synthesized RGO/Fe\({}_{3}\)O\({}_{4}\)-carbon nanotubes composite via a simple and green one-pot solvothermal approach.