# Supramolecular Materials as Solid-Phase Microextraction Coatings in Environmental Analysis
## 4 Covalent Organic Frameworks (COFs)
### COFs as SPME Coatings for Environmental Applications
#### 4.3.1 Extraction of Polycyclic Aromatic Hydrocarbons and Nitroaromatic Compounds  
COFs have been proposed as novel SPME coatings for the extraction of PAHs from water and soil samples, based on either HS or DI modes. Recent studies investigated the HS extraction performances of different COF coatings toward low-molecular-weight PAHs [94; 105; 106; 107; 104]. [PERSON] and coworkers developed a hydrazone-based covalent organic framework (BTCH-PTA-COF) coating by the sol-gel technique [101]. The adsorption of PAHs within the framework was mainly attributed to the \(\pi\)-\(\pi\) and hydrophobic interactions between the target compounds and the benzene rings and C=N bonds present within the COF. The material was characterized by a high surface area (1460 m\({}^{2}\)/g), mesoporousstructure, and high hydrophobicity. Under the optimal extraction conditions, the BTCH-PTA-COF-coated fiber demonstrated a high extraction efficiency for PAHs in a water sample, with EFs in the 767-1411 range, LOQs in the 0.10-0.15 \(\upmu\)g/L range, RSDs always lower than 9.6%, and recovery rates between 88.5 and104.8%, allowing for the sensitive analysis of target pollutants in river water samples. A facile and layer-by-layer self-assembly was proposed by [PERSON] et al. to develop under very mild conditions a thin SPME film based on imine COFs (TFPA-TAPP-COF) [105]. The TFPA-TAPP-COF coating fiber consisted of tetra(4-aminophenyl) porphyrin and tris (4-formyl phenyl) amine: the use of very mild synthetic conditions together with good mechanical strength, remarkable thermal stability (up to 260 \({}^{\circ}\)C), and high extraction efficiency towards PAHs were the main features of the proposed material. The synthetic procedure allowed a close control over both the thickness of the coating and the number of assembled layers. The HS-SPME extraction performance of the novel COF material was assessed by extracting five model PAHs from aqueous samples, achieving LODs in the 6-24 ng/L range and RSDs \(<\) 10%. Finally, the HS-GC-FID method was applied for the analysis of spiked river water samples, obtaining recoveries in the 80.3-102.6% range. Another porphyrin-based COF was synthesized through a Schiff base reaction by [PERSON] and coworkers [106] and deposited onto SS wires using a silicone sealant. An average narrow pore size of 8.7 nm was achieved, leading to an improvement in the selectivity of the material. PAH extraction was mostly related to \(\uppi\)-\(\uppi\) stacking, van der Waals interactions, and weak hydrogen bond formation between the amino group of the COF and the hydrogen atoms in the PAHs. The developed HS-SPME method showed LOQs in the range of 0.5-10 \(\upmu\)g/L and good inter-day and fiber-to-fiber repeatability, with RSDs \(<\) 9%. Finally, the HS-SPME-GC-FID method was applied for the extraction of five model low-molecular-weight PAHs from both lake water and soil samples, obtaining recoveries in the 68-99% and 41-105% ranges, respectively. Recently, [PERSON] et al. developed a COF based on 1,3,5-tris(4-aminophenyl)benzene and trimesoyl chloride (TAPB-TMC-COF) by amide coupling at room temperature. The coating was obtained the by physical deposition of the material onto etched SS wires and used for the HS-SPME-GC-MS determination of low-molecular-weight PAHs in water samples [107]. Analyte adsorption was mostly due to \(\uppi\)-\(\uppi\) stacking, with EFs in the 819-2420 range. The developed method exhibited good sensitivity (LODs in the 0.29-0.94 ng/L range) and linearity over three orders of magnitude. The HS-SPME-GC-MS method was applied for the analysis of PAHs from spiked river, pond, and wastewater samples, obtaining RSDs \(<\) 8% and recoveries in the 79-105% range, although a significant matrix effect was demonstrated for the wastewater samples. An imine-based COF-SCU1 material was prepared through a one-step synthesis and packed into a stainless-steel needle to be used as a needle trap device (NTD) for the extraction of PAHs from soil samples [94].