# Recent Advances in Extraction and Stirring Integrated Techniques
## 2 Stir Bar Sorptive Extraction
### Selective Coatings in SBSE  
Conventional PDMS lacks extraction selectivity as the analytes are isolated by hydrophobic interactions. However, selectivity is an important analytical feature in extraction methods that becomes critical in the microextraction context where the sorptive capacity is limited. This situation is even more complicated when complex samples comprising hundreds of interferents are processed [6]. Under these circumstances, a selective coating allows the use of the limited sorption capacity to the target analytes, avoiding sorbent saturation by the matrix components. The enhancement of sorption selectivity in SBSE has been accomplished following several strategies, the most important among them being the use of molecularly imprinted polymers (MIPs) or selective molecules. Molecularly imprinted polymers can be defined as polymeric networks containing selective chemical cavities for the target analyte and structurally related compounds. These cavities are ad-hoc prepared by incubation, prior to the polymerization, of a special monomer with the target compound. In these conditions, the polymer grows around the template creating a cavity sterically and chemically compatible with the template. This cavity is released by the washing of the polymer, a critical step. The use of MIPs as SBSE coatings was first proposed by [PERSON] et al. in 2006 [7]. This approach exploited the switchable solubility of nylon in order to create a multi-cavity network towards monocrotophos. A precursor solution containing the template and the polymer was prepared in formic acid where nylon was completely soluble. The stir bar was subsequently immersed into the solution and finally in pure water. The solvent change over induces the gelation of the polymer, which is not soluble in water, around the template molecules creating the selective cavities. The resulting MIP showed a high porosity which was beneficial for the extraction kinetics. Although the previous approach is simple and useful, MIPs-bars have been mainly synthesized by co-polymerization of an appropriate monomer and a crosslinker in the presence of the template molecule. The bar must be previously treated in order to achieve a strong retention of the MIP over its surface which is crucial during the extraction (mechanical stability) and elution (chemical stability) steps. This strategy was followed by [PERSON] et al. in order to fabricate MIP-bar towards ratcopamine [8]. In the last few years, this workflow has been proposed for the extraction of triazines in rice [9], thiabendazole in citrus samples [10], and melamine in powdered milk [11]. The selectivity of MIPs restricts their applicability to compounds with similar chemical structures. In some cases, the analyst may be interested in the extraction of two or more different families of compounds while maintaining a high selectivity level. Dual template MIPs try to address this situation by creating a polymeric network with two types of cavities, each one selective for a family of compounds. The process is quite similar to that previously described although in this case two templates, instead of one, are added to the synthesis medium. This approach has been proposed to create stir bars that are selective towards different estrogenic compounds in water and plastic samples [12]. [PERSON] et al. have also proposed an alternative synthetic method that consists of the preparation of silica particles coated with the MIP which are finally loaded into the stir bar. This approach was applied for the extraction of bisphenol A from water with excellent results [13]. Other chemical structures can be used to enhance the extraction selectivity. In this sense, a polymeric composite containing \(\alpha\)-cyclodextrintrin as a selective element has been reported as an SBSE coating for the extraction of polychlorinated biphenyls (PCBs) from water samples [14]. \(\alpha\)-Cyclodextrin is a cyclic (\(R\)-1,4)-linked oligosaccharide that consist of six glucopyranose subunits that presents a cage-like structure.