# Supramolecular Materials as Solid-Phase Microextraction Coatings in Environmental Analysis
## 4 Covalent Organic Frameworks (COFs)
### COFs as SPME Coatings for Environmental Applications
#### 4.3.3 Extraction of Polybrominated Diphenyl Ethers and Polyhalogenated Biphenyls  
Polybrominated diphenyl ethers (PBDEs) and polybrominated biphenyls (PBBs) have been widely used as flame retardants in manufacturing since the mid-1990s; now, their use is strictly regulated due to their toxicity, high persistence in the environment, and impact on ecosystems [133]. Owing to their potential ecotoxicological effects on organisms, sensitive methods based on gas chromatography-negative chemical ion mass spectrometry (GC-NCI-MS) and COF-based SPME coatings have been developed to increase both selectivity and extraction efficiency, allowing for the detection of these pollutants at trace levels. In this context, a TpPa-based coating was developed by [PERSON] and coworkers [116] using a solvothermal reaction followed by physical adhesion on the fiber support by means of a silicone sealant. The material showed a flower-shaped morphology, high porosity, and a BET surface of 625 m\({}^{2}\)/g. The coating was tested for the DI-SPME-GC-NCI-MS analysis of five model PBDEs, allowing for the extraction of the analytes via hydrophobic, van del Waals, and \(\pi-\pi\) interactions, with EFs in the 2035(\(\pm\)22)-6859(\(\pm\)67) range. The selectivity of the coating was explained on the basis of the size complementarity between the pores of the material (approximately 1.25 nm) and the PBDEs (average size 0.81 nm). Validation of the optimized method resulted in LOQs in the 0.019-0.074 ng/L range and RSDs < 10%. The analysis of environmental water samples did not show the presence of the analytes in groundwater and drinking water, whereas concentration levels in the 0.50-2.18 ng/L range were reported in pond water. [PERSON] et al. developed a coating based on polyimide and a TAPB-DMTP COF (PI@TAPB-DMTP) supported on SS wires for PBDE extraction from water samples [100]. The etched SS fibers were pretreated with APTES to obtain an amino-functionalized surface to be used for the grafting of DMTP, followed by in situ growth of the COF at room temperature. A PI protective layer was finally deposited  
Figure 8: Schematic illustration of the (**a**) synthesis and (**b**) coating and SPME-CFDI-MS procedure for analysis of TBBPA analogs, reprinted with permission from [98]. onto the fiber (Figure 9). The characterization of the coating revealed a very high BET surface (1880 m\({}^{2}\)/g) and the presence of mesopores with an average diameter of 2.2 nm, suitable for hosting the analytes. The PI@TPB-DMTP coating exhibited a high extraction performance with EFs in the 1470-3555 ng/L range, benefiting from strong hydrophobic effects, \(\pi\)-\(\pi\) stacking, enhanced surface area, and porosity. The optimized DI-SPME-GC-NCI-MS method was validated for the analysis of six PBDEs, obtaining LOQs in the low ng/l level, i.e., 0.027-0.063 ng/L, with RSDs lower than 9%. Despite the sub-trace detection limits, all the investigated PBDEs were below the LODs in the analyzed river, lake, and wastewater samples. The use of poly(ionic liquid)s (PILs) to functionalize the surface of COFs was proposed by [PERSON] et al. [117]: two vinyl-decorated COF analogs (COF-\(\beta\) and COF-\(\gamma\)) were synthesized by solvothermal treatment, and then photo-initiated polymerization was performed to synthesize two series of poly(1-vinyl-3-methylimidazolium bis ((trifluoromethylomethyl)sulfonyl)imide)-based hybrids (COF-PIL) with different mass ratios of IL/COF. Finally, the SPME coating was obtained by physical deposition using a silicone sealant. The use of co-polymerization to obtain covalently composited hybrids resulted in stable PIL@COF bonds. Interestingly, the COF-\(\beta\) and COF-\(\gamma\) hybrids showed different morphologies, namely, spherical and sheet stacking with a hierarchical porous structure, with PIL decorating the pores and interlayer volume. Despite the presence of PIL decreasing the BET surface area of the materials, the functionalization generally improved the extraction performance of the coating.