# 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
This behavior was ascribed to the increased affinity between the COF surface and the adsorbed PBDEs, able to interact via weak hydrogen bonding, \(\pi\)-\(\pi\) stacking, and Van der Waals forces. The COF-\(\gamma\)-PIL exhibited a superior extraction capability compared to the COF-\(\beta\)-PIL: this behavior could be explained considering that the sheet-like stacking morphology featured shorter diffusion paths and broader pore size distribution, improving the accessibility of adsorption sites and mass transfer processes. The COF-\(\gamma\)-PIL-based coating also demonstrated a higher extraction capability when compared to commercially available coatings, namely, 100 \(\upmu\)m PDMS, 65 \(\upmu\)m PDMS/DVB, and 75 \(\upmu\)m CAR/PDMS fibers, providing EFs in the 913-3625 range. Finally, the DI-SPME-GC-MS method was validated, obtaining LOQs in the 0.0070-0.046 ng/L range and excellent precision (RSDs < 8%), and successfully applied for the analysis of river, lake, and seawater samples. [PERSON] and coworkers developed a coating based on TAPB--2,5-dimethoxyterephthaldehyde (DMTP) COF using 2,5-dimethoxybenzaldehyde (DBA) as a modulator for the extraction of PBBs [118]. The material was fabricated at room temperature and deposited onto etched SS wires by means of a silicone sealant. The use of a modulator allowed the optimization of the COF microstructure in terms of specific surface area (BET values in the 1545-1611 m\({}^{2}\)/g  
Figure 9: Schematic representation of the synthesis of the PI@TPB-DMTP coating, reprinted with permission from [100]. range, using different modulator percentages), surface hydrophobicity (contact angles higher than 90\({}^{\circ}\) with DBA higher than 40%), and morphology (in terms of particle size). The material containing 40% DBA showed EFs higher than those obtained using the unmodulated COF (in the 4400-11,360 range). Adsorption occurred through \(\pi\)-\(\pi\) interactions, hydrophobic effects, and electrostatic interactions between the material and the analytes. The optimized HS-SPME-GC-MS method was validated, achieving LOQs in the range of 0.12-0.94 ng/L, precision with RSDs \(<\) 8%, and recoveries in the 80-120% range, thus allowing the determination of 3-bromo-1,1'-biphenyl and 4,4'-dibromo-1,1'-biphenyl at concentration levels in the 0.33-1.1 and 2.6-3.0 ng/L ranges, respectively. COF-based SPME coatings have also been developed for the selective extraction of PCBs [99; 121; 122]. [PERSON] and coworkers developed a TpPa-functionalized PAN membrane (PAN-SiO\({}_{2}\)@TpPa) [121]: the COF was grown in situ on the PAN-SiO\({}_{2}\) electrospun nanofiber membrane, which was deposited onto etched SS fibers using a resin glue. Functionalization provided the COFs with improved properties in terms of BET area (33 m\({}^{2}\)/g vs. 108 m\({}^{2}\)/g for PAN-SiO\({}_{2}\) and TpPa@PAN-SiO\({}_{2}\), respectively), average pore size (35.1 \(\pm\) 5.1 vs. 9.8 \(\pm\) 1.3 nm) and pore volume (0.009 vs. 0.104 cm\({}^{3}\)/g). The TpPa@PAN-SiO\({}_{2}\) coating showed extraction capabilities up to three times higher than the unfunctionalized membrane for the six investigated PCBs. The adsorption was obtained by the synergistic effect of hydrophobic interaction, \(\pi\)-\(\pi\) stacking, and hydrogen bonding. Under optimized conditions, the HS-SPME-GC-CCD method exhibited LOQs in the 0.1-5 ng/L range, EFs in the 2714-3949 range, and precision with RSDs\(<\) 9%. No PCB could be detected in river, lake, and seawater samples, whereas the analysis of spiked matrices resulted in recoveries in the 73-126% range. [PERSON] et al. synthesized a chlorine-functionalized COF coating onto etched SS wires pretreated with APTES [122]: the COF was grown in situ using Tp, 2,5-dichloro-1,4-phenylenediamine, mesitylene, and 1,4-dioxane, obtaining a chlorinated analogue of TpPa-1. Using the chlorine-functionalized COF, EFs in the 699-4281 range were achieved, higher than those obtained using the TpPa-1 and commercially available polymeric coatings (100 \(\upmu\)m PDMS and 50/30 \(\upmu\)m DVB/CAR/PDMS fibers).