# Supramolecular Materials as Solid-Phase Microextraction Coatings in Environmental Analysis
## 3 Metal-Organic Frameworks
### MOFs as SPME Coatings for Environmental Applications
#### 3.3.2 Extraction of Polycyclic Aromatic Hydrocarbons  
Polycyclic aromatic hydrocarbons (PAHs) are a broad class of compounds consisting of two or more fused aromatic rings, exhibiting a linear, angular, or cluster arrangement. These compounds are characterized by extremely low water solubility, very different vapor pressures, high toxicity, and high bioaccumulation potential; therefore, they are considered as priority pollutants by both the United States Environmental Protection Agency (US EPA) and the European Environment Agency [53]. The main analytical challenge is related to their detection at sub-trace levels in the presence of overwhelming amounts of interfering compounds. ZIF-based coatings were developed and tested for the extraction of PAHs using different deposition approaches or post-synthetic functionalization of the MOFs. In 2019, [PERSON] and coworkers synthesized ZIF-8 MOFs by different approaches, namely solvothermal, stirring, and ball-milling methods, and deposited the MOF-based material onto an SS wire using a dual-stage deposition via the sol-gel technique [54]. The obtained coatings were tested for the direct immersion (DI)--SPME--gas chromatography--mass spectrometry (GC--MS) analysis of 16 US EPA PAHs and 11 nitro-PAHs. The best enrichment capabilities were obtained by using the solvothermally synthesized material, which was also characterized by the highest BET surface area (1390 m\({}^{2}\)/g). Under optimal conditions, LOQs in the 1.1-90.0 ng/L range were obtained, thus assessing the reliability of the method for the detection of PAHs at sub-trace levels. Finally, the analysis of environmental samples highlighted high levels of contamination of river and lake water and industrial wastewater, with PAHs detected in the 30.0(\(\pm\)1.1)-1509(\(\pm\)83) ng/L range. A ZIF-8-based SPME coating was synthetized in situ onto metal alloy wires by CVD and then solvothermally grown by [PERSON] and coworkers [55]. The coating was applied for the extraction of acenaphthene, fluorene, and pyrene as model PAHs together with homosalate, ethylhexyl-saliculate, methyl-antralinate, padimate-O, and ethylhexyl 4-methoxycinamate as personal care product representatives. The DI-SPME-GC-FID method was characterized by limit of detections (LODs) in the 0.6-2 ug/L range and relative standard deviations (RSDs) \(\leq\) 23%; however, a strong matrix effect was observed when wastewater samples were analyzed. [PERSON] and coworkers studied the in situ electrochemical deposition of ZIF-8 onto SS wires pretreated using dopamine under alkaline conditions [56]. A homogeneous porous coating characterized by very high thermal stability (up to 460 \({}^{\circ}\)C) was obtained. The coating was tested for the HS-SPME-GC-FID analysis of eight low-molecular-weight PAHs obtaining LOQs in the 35-179 ng/L range, allowing for the detection of acenaphthene, acenaphthylene, and fluoranthene in lake water at concentration levels in the 200 \(\pm\) 13-360 \(\pm\) 33 ng/L range. [PERSON] et al. developed ZIF-8-based coatings with tunable properties in terms of polarity, porosity, surface area, and conductivity, thus allowing the extraction of either polar or apolar compounds [57]. SS wires coated with PANI deposited by means of cyclic voltammetry were used as an SPME support. Aligned ZnO nanorods were deposited using a dipping -seed-mediated method and then a hydrothermal method was applied for ZIF-8 in situ growth. The developed coating was tested both for the electroenhanced SPME extraction (EE-SPME) of aromatic amines and ionic drugs in spiked standards and for the DI-SPME-GC-FID analysis of six low-molecular-weight PAHs in sewage water samples. Electrochemical deposition was applied for a two-step synthesis to obtain a nickel/titanium alloy coated with cobalt and for the in situ growth of a ZIF-67 coating [58]. The material was then annealed to obtain a Co@ZIF- 67-derived coating to be used for the selective DI--SPME--liquid chromatography--UV detection (LC--UV) of six PAHs.