## 3 Sample Preparation Techniques and Development of Extraction Sorbents
### Solid-Based Extraction
#### 3.2.3 Solid-Phase Extraction (SPE)
The application of ISs for organophosphorus pesticides has been studied by [PERSON] et al. [88]. The study revealed that 13 OPPs were successfully extracted using an in-house monoclonal antibody immobilised on CNBr-activated Sepharose 4B from a water sample with adequate recovery (60.2-107.1%) and low limit of quantification (0.01-0.13 ng mL\({}^{-1}\)). This work justifies that ISs are capable of simultaneous extraction of OPPs. The advantage of using IS is that the extraction and isolation of target compounds from complex matrices could be performed in one step, thereby eliminating the co-extraction of interferences problem due to a high degree of antibodies' selectivity [88]. However, despite the high degree of selectivity shown for specific analytes, IS is not a promising sorbent in environmental analysis. This is because IS requires high cost for its production, involves tedious preparation, is time consuming, and has less availability and low capacity [89]. Identifying suitable antibodies for the sorbent is the main difficulty [86]. Besides that, it is probably due to the fact that there are multiclasses of pollutants differing in structures and polarities. Carbon-based nanomaterials such as fullerenes, graphene, and carbon nanotubes (CNTs) are commonly used for analytical purposes, and these materials have demonstrated their capabilities as excellent SPE sorbent due to giant \(\pi\)-electronic structure [87; 88]. Recently, a graphene-based sorbent has prompted significant interest due to its promising advantages such as a high surface area, high adsorption capacity, it could be easily functionalized, and its facile preparation in comparison to CNTs [75; 90]. A study conducted by [PERSON] et al. [76] reported that the SPE method developed using graphene as sorbent coupled with GC-MS showed good LODs (0.04-0.35 ng mL\({}^{-1}\)) and satisfactory recoveries, ranging from 69.8 to 106.2% for determination of OPPs in apple juices. However, several drawbacks of graphene such as strong hydrophobicity, \(\pi\)-\(\pi\) stacking, and van der Waals forces led to restacking and aggregation between graphene sheets, which resulted in the reduction of graphene surface area and, consequently, decrease in adsorption efficiency. Thus, graphene is not a practical SPE sorbent [91; 92]. Therefore, modification of graphene has been initiated to alleviate this issue, by which graphene oxide and three-dimensional (3D) graphene that possess higher surface area and more polar moieties (i.e., hydroxyl and carbonyl groups) were developed [93; 94]. [PERSON] et al. [8] successfully enriched OPPs in water samples using SPE cartridge packed with 3D graphene aerogel prior to GC-MS analysis, and achieved low detection limits (LODs), ranging from 0.12-0.58 ng mL\({}^{-1}\) with excellent recoveries (93.8-104.2%) for all targeted analytes. Hence, these studies indicated that a graphene-based sorbent is a promising material for the extraction of OPPs from different matrices. Silica-based materials such as methyltrimethoxysilane, tetraethoxysilane, and polym dimethylsiloxane have been used as sol-gel precursors to synthesise sorbent materials. The sol-gel technique is commonly used to prepare SPME- and SBSE-coating fiber for OPPs' determination. As of 2011, the application of a sol-gel technique for the preparation of SPE sorbent was limited. However, [PERSON] et al. [20] introduced a novel organic-inorganic sol-gel hybrid MTMOS-TEOS as SPE sorbent for the extraction of OPPs from water and fruits' samples prior to detection using GC-MS. The developed method showed low LOD (0.5-0.9 pg mL\({}^{-1}\)) and high recovery toward the targeted analytes because of the sol-gel hybrid mesoporous nature and high surface area. Besides that, the good performance of the method was also probably due to the use of GC-MS as the detection system. This is because GC-MS provides higher sensitivity and possesses good anti-interference effect and better accuracy.