# Recent Advances in Extraction and Stirring Integrated Techniques
## 2 Stir Bar Sorptive Extraction
### The Potential of Nanoparticles in SBSE  
Nanoparticles (NPs) can be defined as those particles that present one or more dimensions in the nanometric range (using 100 nm as an arbitrary reference), which provide them with special properties not observed in the bulk material [16]. The use of NPs in sample treatment has been the focus of extensive research during the last decade [17; 18], since their properties are different than the classical materials. In SBSE, NPs may play two different and, in some cases, complementary roles. On the one hand, they can act as the active sorptive phase introducing new interaction chemistries that boost the extraction of the target analytes. On the other hand, the inclusion of NPs in the polymeric SBSE coating avoids the normal stacking of the polymer network, creating a more porous structure which makes the diffusion of the analytes easier with evident kinetics benefits. Nanoparticles can be classified according to different criteria. Considering their chemical composition, they can be divided into inorganic or carbon based NPs. In this sense, inorganic NPs have been proposed as components of SBSE coatings. [PERSON] et al. reported the use of zirconia NPs as the sorptive phase for the extraction of polar organophosphorous compounds in water samples [19]. The location of ZrO\({}_{2}\) NPs, with sizes in the range of 10-20 nm, over a PDMS phase generates a rough coating with a superficial area of 103 m\({}^{2}\)/g and a pore size of 5 nm. Figure 1 shows an SEM picture of the dumbbell-shaped stir bar and a closer view of the coating. ZrO\({}_{2}\) NPs prevail over PDMS in the interaction with the polar analytes, which can be extracted by three mechanisms (cation/anion/ligand exchange) depending on the working pH. The enrichment factors (742-1583) were excellent. Magnetic nanoparticles have also found applications in SBSE in an elegant approach proposed by [PERSON] et al. (2012), which combines the benefits of classical dispersive phase microextraction and SBSE. The extraction workflow, which is schematically shown in Figure 2, consists of several subsequent steps. Initially, the oleic acid coated CoFe\({}_{2}\)O\({}_{4}\) NPs are attached to the stir bar by magnetic forces. Once the stir bar is introduced into the sample, the bar is stirred at high velocities inducing the detachment of the magnetic nanoparticles (MNPs) which are dispersed into the sample, extracting the target analytes. After that, the stirring rate is reduced inducing again the attachment of the MNPs on the bar for the final elution. This approach reduces the treatment time as the isolation takes place under a perfect dispersion of the sorbent. In addition, it is versatile since virtually any MNP can be applied. In fact, the same authors have evaluated the potential of magnetic nylon 6 composites under this format ([PERSON], 2017). Carbon based nanomaterials can develop special interaction chemistries that can be used to enhance the extraction selectivity. The capacity of carbon nanotubes (CNTs) to interact by \(\pi\)-\(\pi\) bonds with analytes containing aromatic domains has been exploited in SBSE (S  
Figure 1: Scanning electron microscopy (SEM) picture of the (a) dumbbell-shaped stir bar (magnification 40\(\times\)) and (b) the surface of the coating (magnification 150\(\times\)). Reproduced with permission from ([PERSON], 2017), copyright Elsevier, 2012. Figure 2: Scheme of stir bar sorptive-dispersive microextraction mediated by magnetic nanoparticles. Reproduced with permission of the Microextraction Tech blog. CNTs in the sorptive phase can be done in several ways. [PERSON] et al. (2017) introduced amino-modified CNTs in a PDMS matrix, where they are entrapped, in order to make the extraction of polar phenols feasible. The presence of CNTs in the polymer creates a rougher surface with a higher extraction capacity.