# Supramolecular Materials as Solid-Phase Microextraction Coatings in Environmental Analysis
## 3 Metal-Organic Frameworks
### MOFs as SPME Coatings for Environmental Applications
#### 3.3.4 Extraction of Poly- and Perfluoroalkyl Substances  
Poly- and perfluoroalkyl substances (PFASs) are a wide class of emerging pollutants, accounting for thousands of synthetic compounds with linear, branched, or cyclic structures, characterized by a hydrophilic terminal, such as a carboxylic or sulfonate group, and a hydrophobic fluorinated alkyl chain. PFASs have been widely used in manufacturing, packaging, the textile and semiconductor industry, or as foams and detergents [69]. Owing to the chemical and biological stability of the C-F bonds, these compounds are extremely persistent in the environment and can be easily accumulated in the tissues of living organisms, thus being considered as a major threat to human life and ecosystems [69]. Both EU and US EPA have set stringent regulations for PFASs, limiting their concentration at trace levels in drinking water and environmental samples. Analytical monitoring of these compounds is a challenging issue due to their unique hydrophobic and oleophobic nature [69]. In this context, MOFs have been proposed as valuable coating materials for PFAS extraction [70; 71; 72; 73]. Four different water-resistant MOFs, namely, ZIF-8, UiO-66, MIL88-A, and Tb\({}_{2}\)(BDC)\({}_{3}\), were investigated as coatings for probe extraction of perfluorooctanoic acid (PFOA) from environmental water, being that this compound is one of the most widely spread PFASs [70]. The coating was obtained by in situ growth of the MOFs onto poly(dopamine) (PDA)-coated SS needles. ZIF-8 and UiO-66 were able to adsorb PFOA inside the pores and exhibited similar binding energies, whereas MIL88-A and Tb\({}_{2}\)(BDC)\({}_{3}\) ZIF-8 were characterized by surface adsorption, being that their pores were too small for hosting the analyte. Due to the lower production costs and higher reproducibility, only ZIF-8 was selected for PFOA quantitation. Finally, a DI-SPME extraction followed by direct analysis by nanoelectrospray ionization-mass spectrometry (nanoESI-MS) method was validated, allowing for very rapid analysis (10 min per sample, including pretreatment and MS analysis) and achieving a LOD value of 11 ng/L. Therefore, the reliability of the method was demonstrated for the purpose of high-throughput screening of PFOA at trace levels. The validated method was applied for the quantitation of PFOA in contaminated samples of tap water, rainwater, and seawater. A strong matrix effect was observed, especially when seawater samples were analyzed, obtaining a sensitivity (7.75 \(\times\) 10\({}^{8}\) M\({}^{-1}\)) higher than that observed in ultrapurewater (2.43 \(\times\) 10\({}^{8}\) M\({}^{-1}\)), probably as a consequence of the alkalinity and ion abundance of seawater. Another strategy to improve the enrichment capability of SPME through electrostatic interactions and hydrogen bond formation between the amine group and the acidic group of PFAAs was based on the functionalization of MOFs with amino groups [71; 72; 73]. In this context, the extraction of eight model PFASs using an amino-functionalized ZIF-8 (NH\({}_{2}\)-ZIF-8) coating followed by HPLC-MS/MS analysis was proposed by [PERSON] and coworkers [71]. The MOF was synthesized by solvothermal reaction and deposited onto quartz fibers by means of PAN. The developed coating showed a higher performance than pristine ZIF-8 and commercial fibers, namely 65 \(\upmu\)m PDMS/DVB and 85 \(\upmu\)m PA fibers. After optimization and validation, the DI-SPME-HPLC-MS/MS method was applied for the analysis of river water, seawater, and wastewater treatment plant effluent samples, allowing for the detection of the target analytes at ng/L levels. In 2022, [PERSON] et al. proposed a modification of UiO-66(Zr) introducing both amino groups on the surface and the end-capping of the material to avoid any interaction with water molecules, thus enhancing the hydrophobic interactions [72]. The NH\({}_{2}\)- UiO-66(Zr) MOF was obtained in a one-step solvothermal synthesis, capped using phenylsilane, and deposited onto the fiber by means of PAN.