# Supramolecular Materials as Solid-Phase Microextraction Coatings in Environmental Analysis
## 4 Covalent Organic Frameworks (COFs)
### COFs as SPME Coatings for Environmental Applications
#### 4.3.2 Extraction of Phenols and Derivatives  
Phenols, phenolic compounds, and synthetic derivatives are characterized by very different chemical and physical properties. The main challenge in their detection is related to their hydrophilicity and low concentrations in environmental matrices. In this context, COFs as SPME coatings could be promising materials to address both enrichment and selectivity issues [97; 113; 114; 115]. Recent studies investigated the extraction capabilities of two different COFs, namely a spherical TPB-DMTP-COF [113] and a piperazine-linked, copper-doped phthalocyanine metal covalent organic framework (CuPc-MCOF) [114], to be used as SPME coatings for the extraction of phenols and chlorophenols (CPhs). The spherical TPB-DMTP-COF was synthesized in an organic solvent at room temperature and deposited onto etched SS wires by means of a silicone sealant. A uniform spherical superstructure (1-2 \(\upmu\)m) with a strong \(\pi\)-\(\pi\) stacking architecture between the layers, a pore size of 2.54 nm, and a BET surface of 1560 m\({}^{2}\)/g, higher than that of solvothermal synthesized COFs, were the main features of the developed coating [113]. The SPME fibers were applied for the HS extraction of phenols and CPhs, obtaining EFs in the 1741-4265 range. The adsorption was mostly driven by hydrophobicity and steric hindrance effects, with the formation of \(\pi\)-\(\pi\), van der Waals interactions, and hydrogen bonds between the analytes and the material. Validation demonstrated LOQs and recoveries in the 0.016-0.050 ng/L and in the 81-116% ranges, respectively, with RSDs \(\leq\) 10%. When the HS-SPME-GC-MS/MS method was applied for the analysis of environmental samples, no analyte was detected in drinking and reservoir water, whereas underground water samples were contaminated by CPhs (in the 0.6-22.7 ng/L range). [PERSON] and coworkers synthesized a CuPc-MCOF coating via Buchwald-Hartwig cross-coupling and deposited it onto etched SS fibers using a polyimide resin. The coating was used for the EE-SPME of CPhs from water and biological samples, namely oysters and prawns [114]. The high hydrophobicity of the COF-based material was ascribed to its conjugated structure and abundant fluorine atoms, whereas the presence N and of Cu atoms within the framework provided free metal sites for coordination binding. During EE-SPME, the analytes underwent mass transfer, thus increasing the enrichment of chlorophenols onto the fiber coating, acting as an electrode. The extraction performance of the developed coating was better than that of commercially available SPME fibers, with EFs in the 339-988 range. The EE-SPME-GC-MS/MS method was validated, obtaining LOQs at low ng/L levels, linearity over three orders of magnitude, and good precision (RSD \(<\) 9%). Finally, the method was applied for the analysis of seawater samples, detecting trichlorophenols in a 6.8-30.1 ng/L range with a recovery rate ranging from 90% to 113%. Similarly, when oysters and prawns were analyzed, 2,4-dichlorophenol,2,4,5-trichlorophenol, 2,4,6-trichlorophenol, and 2,4,5,6-tetrachlorophenol were detected at low ng/L levels. [PERSON] and coworkers developed a TpBD COF based on an MC approach and the coating was obtained either by in situ growth or physical deposition onto SS wires [115]. Depending on the deposition procedure, different morphologies were obtained: in situ deposition resulted in a highly porous structure with a uniform section, whereas a less uniform coating with a smooth cross-section characterized by a less porous structure was observed for physical deposition, probably due to pore obstruction by the silicone adhesive. Both coatings were tested for the HS-SPME-GC-MS analysis of 2-nitrophenol, 2,6-dimethylphenol, 2,4-dimethylphenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol. The extraction performance of the in situ grown coating was superior to both the physically deposited COF and 85 um PA commercially available fibers, mainly due to the formation of \(\pi\)-\(\pi\) interactions and hydrogen bonding.