# Supramolecular Materials as Solid-Phase Microextraction Coatings in Environmental Analysis
## 2 Supramolecular-Based Coatings: Deposition Methods  
Different approaches have emerged to develop SPME coatings using supramolecular materials, depending on both the nature of the adsorbent material and on the sampling conditions. It is known that the proper planning of the synthetic strategy plays a key role in guaranteeing the availability of the active binding sites together with the thermal, mechanical, and chemical stability of the coating [7; 29; 30; 31]. The first step to be performed is the activation of the supporting material: to this aim, different treatments have been proposed, including ultrasonic treatment using organic solvents, etching using mineral acids (HF, HCl, HNO\({}_{3}\)), or deposition of a sol-gel coating bearing amino moieties, e.g., (3-aminopropyl) triethoxysilane (APTES). Both the ultrasonic treatment and acidic etching are useful to clean the support of the fiber, remove passive layers, and expose the metal ions/active groups on the support surface to strengthen the adhesion between the coating and the substrate. According to [PERSON] et al. [29], the coating approaches can be divided in two main groups: (i) in situ deposition during framework development, allowing the synthesis of the material directly on the surface of the fiber; (ii) two-stage deposition, including the synthesis of the adsorbent material and the post-synthetic deposition (Figure 1). In situ growth (or deposition from solution), chemical vapor deposition (CVD), and atomic layer deposition (ALD) are the most commonly applied methodologies of in situ deposition. The main advantages of these approaches rely on the development of coatings containing only the supramolecular frameworks, thus increasing the extraction selectivity. In situ growth is based on the immersion of the pretreated fiber in the reacting solution, allowing for the synthesis of the material directly onto its surface [7; 29; 30; 31; 32]. In this context, stainless-steel (SS) supports are preferred to fused silica and quartz substrates, being that these fibers very fragile. Compared to the other in situ approaches, this technique is simpler and does not require the use of expensive equipment. However, it is time-consuming and several parameters, including reaction time, concentration of reagents, and agitation speed, need to be optimized to obtain stable and reproducible coatings [29; 30]. Figure 1: Schematic representation of the different approaches used for the development of supramolecular-based SPME coatings. Reprinted with permission from [PERSON] et al. [29]. CVD involves the deposition of the coating onto a heated substrate surface via the chemical reactions of evaporated precursors [33]. The structure, morphology, dimensions, and orientation of the coating can be finely tuned by changing the deposition conditions. This process is highly reproducible and can be performed at higher rates compared to the solution deposition [29]. Atomic layer deposition (ALD, or ALD-molecular layer deposition) is a nanotechnological modification of CVD, involving a vapor-phase layer-by-layer deposition of the coating materials onto a solid support exploiting gas-surface reactions [34]. The deposition process is divided into sequential reaction stages between the precursors and substrate, leading to the consecutive deposition of reactants (Figure 2): (i) in the first step a precursor (metallic center, polymeric material, or organic linker) is pulsed into a vacuum-heated chamber containing the fiber; (ii) after the deposition of the precursor layer, the unfunctionalized reactants are removed from the chamber; and (iii) the other precursors are purged onto the already-formed layer, leading to a formation of a second layer of the coating. The process is repeated until the desired coating is obtained. ALD provides a unique control on thin film formation in terms of conformality and uniformity, with atomic-level accuracy [29; 34].