# Supramolecular Materials as Solid-Phase Microextraction Coatings in Environmental Analysis
## 3 Metal-Organic Frameworks
### MOFs as SPME Coatings for Environmental Applications
#### 3.3.1 Extraction of Benzene, Toluene, Ethylbenzene, and Xylenes  
Benzene, toluene, ethylbenzene, and xylenes (BTEX) are volatile non-polar organic compounds mostly derived from petrochemical fuel combustion and used as solvents in the pharmaceutical and dye industries. The International Agency for Research on Cancer (IARC) set benzene in group 1, ethylbenzene in group 2B, and toluene and xylenes in group 3. The main challenge in the detection of these compounds is related to their presence at trace levels in environmental samples, generally interfered with by the overwhelming amounts of aliphatic hydrocarbons [45; 46]. Several MOFs have been proposed to improve the extraction efficiency and selectivity, including Zeolitic Imidazole Frameworks (ZIFs) and Materiaux de l'Institut Lavoisier (MIL). These materials demonstrated both \(\pi\)-\(\pi\) stacking and size complementarity between the analytes and the framework. ZIFs are a class of MOFs that feature divalent metal nodes coordinated by imidazole organic ligands to obtain a material with a tetrahedron structure characterized by enhanced thermal and water stability. These materials have been applied as SPME coatings since 2010 [47]; however, in 2019 [PERSON] and coworkers proposed the use of the single-crystal ordered macroporous (SOM) ZIF-8 as an SPME coating for BTEX extraction from wastewater samples [48]. The material was obtained by synthesizing a polystyrene monolith and a ZIF-8 MOF was grown in its interstices. Then, the polymer was dissolved using dimethylformamide, obtaining the SOM-ZIF-8 material, which was deposited onto SS using a silicone sealant. The obtained coating showed an ordered array of macroopes and was characterized by 2.5-3.1-fold times higher intensities than both the crystalline ZIF-8 and the commercial PDMS fibers. The HS-SPME-GC-FID-based method was validated, obtaining limits of quantitation (LOQs) in the low ng/L level, allowing for BTEX detection in environmental samples. ZIF-8 was also the starting material for developing a superhydrophobic MOF composite material (N\({}_{\text{S}}\)ZIF-8 Si) [49]: after synthesis of Mn\({}_{\text{N}}\)O\({}_{\text{Y}}\) nanosheets, in situ growth of a ZIF-8 MOF was performed on the nanomaterial, followed by deposition of PDMS on its surface. The SPME fiber coating was obtained by physical adhesion using a silicone sealant. The developed material showed excellent thermal stability (up to 450 \({}^{\circ}\)C), superhydrophobic behavior, and significantly higher extraction efficiencies than commercial 30 \(\upmu\)m PDMS due to the combined hydrophobic and \(\pi\)-\(\pi\) stacking effects. Recently, [PERSON] and coworkers [50] used the MIL-101-NH\({}_{2}\) framework, a moisture-sensitive MOF, as a template to develop a carbonaceous coating. After solvothermal synthesis, cobalt and thiourea were encapsulated inside the MOF structure and the material was annealed, obtaining urchin-like nanoporous carbon. The final coating was obtained by physical deposition using a silicone sealant. The extraction performance of the urchin-like nanoporous carbon-based SPME coating was better than that obtained by using commercial PDMS and MIL-derived nanoporous carbon not functionalized with cobalt and thiourea, achieving LOQ values in the 0.28-1.2 ng/L range. Finally, the material provided a very high sampling rate for all the analytes, reaching extraction equilibrium within 2 min. [PERSON] and coworkers developed a polyacrylonitrile/MIL-53(Al)MOF@SBA-15/4,4\({}^{\prime}\)-bipyridinehybrid nanocomposite [51]. The material was based on mesoporous silica obtained by the sol-gel procedure, functionalized with a MIL-53(Al) MOF by solvothermal reaction. The coating was obtained by electrospinningning PAN onto an SS fiber in the presence of the hybrid nanocomposite material. A very porous structure with a high degree of cross-linking and increased adsorption/desorption rates was achieved. Finally, LOQs in the 7.6-20 ng/L range allowed the ultra-trace determination of BTEX in wastewater samples.