# Supramolecular Materials as Solid-Phase Microextraction Coatings in Environmental Analysis
## 5 Supramolecular Macrocycles
### Calixarenes and Cavitands as SPME Coatings  
Calixarenes and cavitands are synthetic macrocycles able to host target guests inside their cavity. They are versatile multidilentate receptors whose dimensions can be properly modified on the basis of number of monomeric units and whose surface properties can be tuned by the introduction of specific substituents surrounding the hydrophobic cavity.  
Calixarenes are a class of synthetic macrocyclic receptors obtained by the condensation of para-substituted phenol and formaldehyde (Figure 11a). Functionalization can be performed on both faces of the macrocycle, exploiting both the hydroxyl group and the para-substituent of phenols to improve the selectivity of the receptor to anchor the macrocycle onto the substrate or to tune its properties, including the opening and flexibility of the cavity [147]. [PERSON] et al. developed an SPME coating based on novel polyurethane  
Figure 10: (**a**) Preparation of \(\beta\)-CD-coated glass fiber; (**b**) IT-SPME-LC-UV method for PAHs analysis. Reprinted with permission from [143].  
polysulfone/cialix[4]arene (PU-PSU/cialx[4]arene) nanofibers [148] to be used for the HS extraction of 1,2,4-trichlorobenzene, 1,2,3-trichlorobenzene, and 1,2,3,4-tetrachlorobenzene. Four calixarene derivatives, namely, cialx[4]arene, sulfonated cialx[4]arene, p-tert-butyl-cialx[4]arene, and calix[6]arene, were synthesized and electrospun with polyurethane and polysulfone directly onto the SPME fiber. The performance of the different coatings was compared in terms of extraction efficiency, showing the PU-PSU/cialx[4]arene had the highest adsorption capability towards chlorobenzes. After optimization, the HS-SPME-gas chromatography-micro electron capture detector (GC-\(\mu\)ECD) method was validated, obtaining LOQs in the 0.4-4.0 ng/L range and RSDs always lower than 10%. Finally, the HS-SPME-GC-\(\mu\)iECD method was applied for the quantitation of the target analytes in tap, river, sewage, and industrial water; since no analyte could be detected, the environmental matrices were spiked with the analytes at concentrations of 40 and 400 ng/L, obtaining recoveries in the 80-106% range.  
Other important synthetic macrocycles are the resorcinarene-based receptors obtained by the condensation of resorcinol and aldehydes (Figure 11b) [147; 149]. As in the case of CDs and calixarenes, the upper rim of the receptors can be functionalized to obtain deeper cavities, tuned polarities, or different complexation capabilities, whereas the substituents at the lower rim are mostly used to tune the macrocycle solubility or to improve the anchoring on a substrate. Very recently, our research group developed and tested a novel cavitand characterized by four benzoquinoxaline walls at the upper rim (BenzoQxCav) as SPME coatings for the extraction of PAHs from water samples (Figure 11c) [150; 151]. Compared to the tetraquinoxaline cavitand, the new receptor was characterized by a 2.5 A deeper hydrophobic cavity, able to host large guests such as PAHs. The coating was obtained by the dip-coating of silica fibers into an epoxy resin followed by dipping into the powdered cavitand. The coating was tested for the extraction of the 16 US EPA priority pollutants PAHs and provided higher enrichment capabilities compared to commercially available PDMS fibers, owing to \(\pi\)-\(\pi\), CH-\(\pi\) interactions, and analyte engulfment into the hydrophobic cavity. After optimization, the method was validated, obtaining LOQs in the 0.09-1.01 ng/L range, a four order of magnitude linear range, and RSDs \(\leq\) 18%. Finally, the DI-SPME-GC-MS method was applied for the extraction of PAHs from snow samples from Antarctica and the Alps, obtaining \(\Sigma\)PAHs in the 32.2-49.5 ng/L and 10.19-59.13 ng/L ranges, respectively.  
Figure 11: Representation of the structures of (**a**) calix[4]arene, (**b**) resorcinarene-based macrocycle, and (**c**) BenzoQxCav.