# 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
Compared to classical fiber extraction, the NTD proved to be a more robust microextraction tool, characterized by a higher sorption capacity and extraction efficiency. The validation of the COF-packed NTD-GC-FID method allowed for the achievement of LODs in the 0.01-0.05 ng/g range, excellent linearity over five orders of magnitude, and RSDs always lower than 13%. Finally, a \(\upSigma\)PAH concentration in the 0.86-2.3 \(\upmu\)g/g range was obtained when soil samples collected from the neighboring ground of gas stations were analyzed. Another interesting solution for the extraction of PAHs was based on the development of hybrid MOF/COF materials, which combine the outstanding features of MOF and COF structures, integrating the rigidity and high surface area of MOFs with the high flexibility and tunability of the COF surface. Different MOF/COF SPME coatings were obtained for the HS-SPME-GC-FID determination of PAHs from contaminated soil samples [108; 109; 110]. [PERSON] and coworkers developed a Zn-MOF/COF coating by solvothermal synthesis of the MOF precursor, followed by a further functionalization using ethylenediamine and a hydrothermal treatment with melamine and 1,4-benzenedicarboxylate to prepare the new MOF/COF hybrid material [108]. A similar approach was followed by [PERSON] at al. to obtain a porous Cu-MOF/COF coating using an amino-functionalized Cu-MOF as the starting material [109]. The coatings were obtained by the physical deposition of the material onto SS fibers using an epoxy glue. These synthetic strategies resulted in the COF assembly within the pores of the amino-MOF by in situ condensation using the amino groups present on the MOF surface as linkers to obtain stable covalent bonds between the MOF and the COF. The hybrid MOF/COF coatings showed higher extraction efficiency than both commercial fibers and individual materials, being able to interact with PAHs via \(\pi\)-\(\pi\), van der Waals, and hydrophobic interactions. In addition, the presence of nitrogen atoms in the framework was exploited to promote electron donor-acceptor interactions with \(\pi\)-donor aromatic compounds, whereas the Zn or Cu metal sites could interact with the Lewis bases of the condensed aromatic rings. The HS-SPME-GC-FID method proposed by [PERSON] et al. was validated for the analysis of six PAHs, obtaining LODs below 1 ng/g and RSDs! 12%. The method was then applied for the analysis of contaminated soil samples, detecting the analytes up to 70 ng/g [108]. Compared to the previous study, the HS-SPME-GC-FID method proposed by [PERSON] et al. was characterized by lower LODs (in the 0.1-0.5 ng/g range), and higher precision, with RSDs below 8%, allowing for the determination of the investigated analytes in environmental samples at concentration levels up to 839 ng/g [109]. A hybrid COF/MOF (2 DTP/MIL-101-Cr) coating was developed by [PERSON] et al. to be used for the extraction of BTEX and six model PAHs [110]. The MIL-101 MOF was obtained by hydrothermal synthesis, whereas the 2D COF was synthesized through metal-assisted and solvent-mediated synthesis. The hybrid material was obtained by ultrasonically mixing the two components, resulting in the embedding of the MOF within the COF framework. The material was physically deposited onto an SS needle to be used as an SPME coating for the in-syringe, vacuum-assisted HS-SPME (IS-VA-HS-SPME). The VA-HS-SPME relies on the application of a reduced pressure to improve the transfer rate of high-boiling-point components from the sample matrix to the headspace, effectively increasing the extraction efficiency of semi-volatile compounds (Figure 7). The hybrid material exhibited higher extraction performances than both the individual materials and commercially available PDMS and CAR/PDMS fibers. The optimized VA-HS-SPME-GC-FID method was validated, obtaining LOQs in the 7.1-17 ng/g and 0.23-3.7 ng/g ranges for BTEX and PAHs, respectively, with RSDs below 16%.