# Supramolecular Materials as Solid-Phase Microextraction Coatings in Environmental Analysis
## 4 Covalent Organic Frameworks (COFs)
### Synthetic Procedures  
Similarly to MOFs, the morphology and properties of COFs strongly depend on both the organic building blocks and synthetic conditions, including reaction medium, time, temperature, and type of catalyst [30; 32; 93; 94]. According to the dimensions of the building blocks, they can be classified into 2D structures and 3D consolidated networks. Two-dimensional COFs are based on rigid conjugated planar macrocyclic molecules, such as porphyrin, thiophene, phthalocyanine, and tetrazolium, presenting strong interlayer interactions, mainly \(\pi\)-\(\pi\) stacking. Both 2D and 3D COFs include different active functional groups, namely, B-O, C=N, C-N, and C=C [30; 32; 93]. They can be synthesized via boron or nitrogen condensation, triazine-based trimerization, metal-catalyzed coupling reactions, and Schiff base reactions between organic monomers.  
Boron-based COFs consist mainly of boronated ester COFs, obtained through covalent interaction between catechol and polyfunctional boric acid. The main features of these materials are low density, high specific surface, and multiple binding sites; however, they suffer from low chemical stability, being that these COFs are usually unstable in humid environments and in air [12; 93; 95]. COFs based on Schiff base linkages encompass different organic ligands with substituents on the amine moiety, including amines, hydrazones,azines, and \(\beta\)-ketoenamines. Among them, imine-based COFs are based on the formation of C=N linkage though the condensation reaction between amino groups and aldehydes in the presence of a Lewis acid catalyst. These COFs are stable both in common organic solvents and water, providing superior stability compared to boron-based COFs. Imide-based COFs are based on the condensation between an amine and an anhydride at high temperatures. Amine-type COFs are less stable and require a laborious synthesis, which largely limits their application. Recently, novel \(\beta\)-ketoenamine-linked COFs have emerged, featuring high chemical and thermal stability owing to the presence of irreversible bonds, which cannot be hydrolyzed in water, acidic or basic environments [96; 97]. Another important class is the triazine-based COFs, which are characterized by surface areas up to 2390 m\({}^{2}\)/g and high thermal and chemical stability. Finally, C=C linkage-based COFs have been developed, exploiting less reversible or irreversible poly-condensations processes: 2D sp\({}^{2}\) C=C COFs have been obtained by condensation of aldehydes and benzyl cyanides in the presence of a base catalyst [32; 93].  
Up to date, different synthetic strategies have been proposed to produce COF-based SPME coatings. The most applied technique is solvothermal synthesis, due to the use of high-pressure conditions to promote the formation of the crystalline framework. Additional approaches have been suggested, among which microwave-assisted and sonochemical syntheses are gaining increasing interest, reducing the reaction time and preserving the porosity of the framework. Ionothermal synthesis has been also proposed for the preparation of triazine-based COFs: despite the green and solventless approach, this strategy requires very harsh reaction conditions and has limited applications due to the poor control of the crystallinity [32]. Finally, mechanochemical (MC) synthesis is a new mild, simple,and rapid approach to synthetize COFs by grinding reaction: high yields, great thermal stability, and promising crystallinity are the key features of this synthetic strategy, although limitations in terms of porosity and superficial area have been demonstrated [20; 32; 93].