# Supramolecular Materials as Solid-Phase Microextraction Coatings in Environmental Analysis
## 2 Supramolecular-Based Coatings: Deposition Methods
As for dual-stage deposition, different approaches have been proposed, including physical adhesion, the sol-gel technique, and chemical cross-linking [7; 29; 30; 32]. The physical adhesion of a pre-synthesized material is the most commonly applied procedure [7; 29; 30; 31; 32]. Besides fiber pretreatment, the coating process consist of several steps: (i) the inert surface is immersed in an adhesive material (e.g., silicone-based sealants or epoxy planes) or pre-coated with a polymeric layer (usually polyacrylamide-PAN); (ii) the fiber is dipped in the fine powdered supramolecular material to obtain a coating with the intended thickness (multiple dipping cycles can be required); (iii) an additional layer of adhesive material can be applied to increase the mechanical stability of the fiber; and (iv) the fiber is dried and thermally conditioned to promote coating hardening, curing, and allowing for the evaporation of both the residual solvents and organic sub-products. Steps i-iii can be merged when the pretreated fiber is directly immersed into a solution containing the suspended material without the need of an adhesive medium [29; 30; 31]. In this case, the immersion-heating stages are usually performed multiple times to obtain the intended coating thickness. In physical adhesion, the adhesive (or the suspending solvent) used for developing the coating has a major impact on the SPME performance, affecting the thermal and mechanical stability of the material, as well as the resistance to organic solvents. The main advantage of this approach relies on its simplicity, cost-effectiveness, and versatility. Since the synthesis of the material is not affected by the deposition, microwave- or ultrasound-assisted reactions can be applied to obtain the supramolecular material [30]. The main drawback of this approach is related to the presence of adhesives or polymeric  
Figure 2: Schematic representation of the ALD process, edited from [PERSON] et al. [34]. substances that could clog the pores or change the surface chemistry of the adsorbent, thus reducing the surface area or affecting the extraction efficiency and selectivity. Concerning the sol-gel technique, it involves the embedding of the supramolecular materials into a gel network for coating deposition [29; 30; 35; 36]. The pre-synthesized supramolecular material can be either dispersed in a sol-gel suspension or deposited after the gel formation onto the SPME support. The produced coatings are characterized by high homogeneity and uniformity as well as enhanced thermal, mechanical, and chemical resistance compared to the use of adhesives. In addition, the sol-gel process usually occurs under very mild conditions, and the thickness, composition, and morphology of the coating can be finely tuned by changing the concentration of the precursors, the dipping rate, the number of dipping cycles, and the drying temperature. The main drawback of this approach relies on the encapsulation of the supramolecular framework within the gel network, which can lead to the pores clogging and possible interactions between the silica gel and the compounds present in the sample. Chemical cross-linking is based on the chemical bonding of the pre-synthesized supramolecular material with the fiber surface [25; 29; 30]. The fiber is pre-treated to obtain carboxylic or amino groups at the surface to be used as binding sites for the immobilization of the material. In this context, the frameworks are linked to the fiber with strong covalent bonds, thus providing high thermal and mechanical stability. Since no additive is required, selectivity is improved, and the extraction/desorption kinetics are higher than those achieved by the coatings obtained using other dual-stage deposition processes. However, chemical cross-linking could be a complex procedure and the amount of loaded material and the coating thickness could present a serious challenge to be tuned during coating deposition.