# Supramolecular Materials as Solid-Phase Microextraction Coatings in Environmental Analysis
## 4 Covalent Organic Frameworks (COFs)
### COFs as SPME Coatings for Environmental Applications
#### 4.3.5 Extraction of per- and Polyfluorinated Alkyl Substances  
The use of COFs as SPME coatings for the extraction of PFASs has been proposed as a valid alternative to the use of MOFs, featuring a highly hydrophobic and \(\pi\) electron-rich structure [129; 130; 131]. In this context, a novel dioxin-linked COF (TH-COF) was synthesized by microwave-promoted nucleophilic substitution of 2,3,5,6-tetrafluoro-4-pyridinecarbonitrile with 2,3,6,7,10,11- hexahydrox triphenylene [129]. The COF was then deposited onto SS wires using an epoxy resin and the coating was applied for the DI-SPME-UHPLC-MS/MS analysis of perfluoroalkyl carboxylic acids (C\({}_{4}\)-C\({}_{10}\)), perfluoroobutanesulfonic acid, and perfluorooctanesulfonic acid. The use of microwave conditions strongly increased the BET surface of the material (from 576 m\({}^{2}\)/g to 1254 m\({}^{2}\)/g), obtaining pores with a diameter of 2 nm and a reticulate morphology. PFASs were adsorbed via van der Waals forces and strong hydrogen bonding between the pyridine N atoms of TH-COF and the analytes. Method validation resulted in very low LOQs (0.0066-0.015 ng/L), recoveries in the 89.5-105% range, and good precision, with RSDs \(\leq\) 8%, allowing for the ultra-trace analysis of pollutants in river, drinking, and underground water samples: perfluoropentanoic acid and perfluorobutanesulfonic acid were detected only in the river water samples at a concentration of 0.480 ng/L and 0.451 ng/L, respectively. [PERSON] and coworkers developed a multifunctional COF, featuring an amide group and perfluoroalkyl-chain-functionalized chains (COF-NH-CO-P9), to be used for the extraction of anionic, cationic, and zwitterionic PFASs [130]. The material was obtained under solvothermal conditions and deposited onto SS wires using silicone elastomer. A BET surface of 681 m\({}^{2}\)/g, an interlayer distance of 4.8 A, and an average pore size of 24 A were observed, allowing for a size complementarity with the target analytes. In addition, the functionalization with fluoroalkyl chains increased the hydrophobicity of the surface and was beneficial for the selective enrichment of PFASs, which interacted with the coating via electrostatic, hydrophobic, fluorine\(-\)fluorine, and \(\pi-\)CF interactions, as well as through hydrogen bonding between N\(-\)H\(\cdots\)F and N\(-\)H\(\cdots\)O groups. The EFs for the investigated compounds were in the 66\(-\)160 range, up to 57 times higher than commercially available coatings, the non-fluorinated COF analogue, and the COF-(CF\({}_{3}\))\({}_{2}\) coating described in Section 4.3.4. By coupling DI-SPME with UHPLC-MS/MS, LOQs in the 0.01\(-\)0.65 ng/L range, recovery rates in the 77.1-108% range, and RSDs \(\leq\) 11% were obtained. The method was then applied for the analysis of tap, river, lake, pond, and farmland water, and wastewater samples, detecting total PFAS concentrations in the 14.5-96.3 ng/L range. A triazine-core-based, F-functionalized COF (COF-F-1) was synthesized by [PERSON] and coworkers at room temperature and physically deposited onto an SS probe to be used for the nanoESI-MS analysis of PFASs [131]. After the COF deposition, a chitosan polymeric layer was applied, enhancing the hydrophilicity, biocompatibility, and stability of the SPME probe. The COF-F-1 presented hydrophobic channels, fluorine-rich subunits, and electron-deficient groups containing nitrogen, able to interact with PFASs via fluorine\(-\)fluorine and electrostatic interactions, as well as hydrogen bonding. A good complementarity between the pore dimensions of the coating (2.8 nm) and the molecular sizes of the investigated C\({}_{7}-\)C\({}_{18}\) PFASs (9.4-23.8 A) was observed. The nanoESI-MS method was applied for the analysis of C\({}_{7}-\)C\({}_{10}\) perfluorosulfylsonic acids and C\({}_{7}-\)C\({}_{18}\) perfluoralkyl carboxylic acids, obtaining EFs in the 105\(-\)4538 range, indicating a high enrichment capacity of the developed coating towards PFAS.