# Applications of Graphene-Based Nanomaterials in Environmental Analysis
## 1 Introduction  
Oftentimes, the concentration of the target compounds may be lower than the limits of detection (LOD) even for the most advanced instruments, especially for trace or ultra-trace analytes in complex matrices. With sample preparation, we eliminate the above problem by preconcentrating the low concentrations of the molecules, thus enhancing the sensitivity and rendering it possible to detect them [1]. While this step is time-consuming and sometimes costly, there are green approaches which lower the cost and the time needed to pretreat a sample. Some extra benefits that sample preparation provides are the removal of contaminants or substances that interfere with the compounds of interest (cleanup), the dissolution of a solid sample into an aqueous media or the chemical modification with the proper reagents so as to obtain better analytical parameters (lower LODs, higher recoveries, higher selectivity and sensitivity, improved accuracy and repeatability and etc.) [2].  
Lately, there has been a huge interest in graphene (G) and graphene-based nanomaterials, with scientists all over the world studying and publishing a variety of topics regarding its properties and its use in the sample pretreatment of a plethora of samples, matrices and target analytes [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36]. Some of these applications include wastewater treatment [37], such as carbon dioxide adsorption [38], photocatalysis [39] graphene oxide membranes [40] and so forth. Graphene is a single layer of two dimensions of sp\({}^{2}\) hybridized carbon atoms arranged into a honeycomb grid. Due to this formation, G shows extraordinary properties, such as high mechanical strength, excellent optical, great electronic and thermal properties with a vast surface area of 2630 m\({}^{2}\)/g [35, 36]. The combination of its high surface area and its hexagonal array with delocalized \(\pi\)-\(\pi\) electron interactions renders it ideal for the adsorption of aromatic compounds and their derivatives, which also intrigues the analytical chemists to use it as a sorbent [41]. The most common way of producing G is via the chemical oxidation and sonication of graphite to graphene oxide (GO) with the Hummers method and its subsequent reduction of GO to reduced G (rGO) (Figure 1) [42, 43]. Organic analytes can be adsorbed onto G with the development of potent \(\pi\)-\(\pi\) interactions betweenthe carbon rings of the analytes and the hexagonal carbons of G [44]. GO provides a variety of functional groups, making electrostatic interactions possible and enabling hydrogen bonding possible for the organic molecules that contain N- and/or O- functional groups. Other possible interactions between the analytes and the nanoparticles are dispersion forces, dative bonds and hydrophobicity [45]. G and GO are oftentimes modified so as to be more selective towards the compounds of interest or to enhance their adsorption properties and recovery rates, as seen in the applications that will be discussed.  
The purpose of this review is to highlight some environmental applications in environmental analysis of graphene-based nanomaterials or nanocomposites that utilize the properties of GO or rGO and the ones of their guest components (e.g., Fe\({}_{3}\)O\({}_{4}\)), to show how the most used nanocomposites are synthesized and present their advantages versus common sorbents used in Solid Phase Extraction (SPE) or Magnetic Solid Phase Extraction (MSPE) (Figure 2).  
Figure 1: Synthesis of graphene oxide (GO) and reduced G (rGO) from graphite powder.  
Figure 2: Steps of Magnetic Solid Phase Extraction (MSPE).