# Supramolecular Materials as Solid-Phase Microextraction Coatings in Environmental Analysis
## 5 Supramolecular Macrocycles
### Cyclodext as SPME Coatings  
CDs are cyclic oligosaccharides containing D-glucopyranoside units bound via \(\alpha\)-(1,4)-glycosidic linkages. The shape is an asymmetrical toroid, with primary and secondary hydroxyl groups situated at the top (primary face, narrower) and at the bottom (secondary face, wider) of the system, respectively, resulting in hydrophilic outer faces and a hydrophobic inner cavity. CDs have been applied for the complexation of different compounds, including ions, volatile, and semi-volatile organic compounds [141; 142]. These macrocyclic receptors are classified on the basis of the number of oligosaccharides, with \(\alpha\)-CD, \(\beta\)-CD, and \(\gamma\)-CD constituted by six, seven, and eight glucopyranose units, respectively. Considering that \(\alpha\)-CDs present a distorted glucose and a smaller cavity and \(\gamma\)-CDs are characterized by higher flexibility and dimensions, resulting in a lower steric barrier, \(\beta\)-CDs are the most commonly used extraction sorbents [141]. CDs can be easily functionalized to improve the complexation capability towards specific classes of compounds, to obtain hybrid nanocomposite materials, or to increase the bonding stability onto the SPME surface. In this context, recent studies have focused on the development of both polymer-based CDs [143] and hybrid nanostructured materials [144; 145; 146]. A \(\beta\)-CD-crosslinked polymer was obtained via in situ functionalization of a glass fiber to be used for an IT-SPME extraction [143]. In particular, the glass fibers were benzylated and functionalized using \(\beta\)-CD via the Friedel-Crafts reaction; the coated fibers were then inserted in an SS tube to be used for the on-line IT-SPME-LC-UV analysis of low- and medium-size PAHs (Figure 10). The extraction capabilities of the developed coating were up to 208 times higher than those obtained using an unfunctionalized glass fiber, achieving EFs in the 2130-2670 range. Method validation provided LOQs in the 12-25 ng/L range and a good precision, with RSDs \(\leq\) 7%. Finally, method reliability was tested for the extraction of bottled lake, soil, and tap water spiked with PAHs in the 3-5 ug/L range, obtaining extraction recoveries in the 80-107% range. In recent studies, multiwalled carbon nanotubes (MWCNTs) were combined with CDs to develop nanostructured hybrid materials [144; 145]. The new coatings featured the high selectivity of the supramolecular receptors combined with enhanced surface area/volume ratio and morphological structure of the MWCNTs. In a study by [PERSON] et al., \(\beta\)-CD-functionalized multiwalled carbon nanotubes and acyl chloride functionalized MWCNTs were synthesized and incorporated into a silica composite by the sol-gel technique [144]. A dip-coating process was performed to deposit the hybrid material onto a polypropylene hollow fiber to be used for the SPME extraction of fluoxetine and northloxetine, an antidepressant drug and its biologically active form, respectively. The \(\beta\)-CD-MWCNT coating provided a superior extraction capability than that obtained by using only the pristine and acyl-functionalized nanotubes, with EFs of 144 and 151 for fluoxetine and northloxetine, respectively. The DI-LC-UV method was validated under optimized conditions, obtaining LOQs of 400 and 300 ng/L and a good precision (RSDs \(<\) 10%). The method was applied for the analysis of well, tap, and river water samples, but none of the investigated analytes were detected in the analyzed samples. Similarly, [PERSON] et al. [145] developed a \(\gamma\)-CD functionalized MWCNT-based material to be used for the extraction of 16 US EPA priority pollutant PAHs from a water sample. In this study, both acidic and hydrogen peroxidepretreatment of the nanotubes were tested, and each of the pretreated MWCNT materials was functionalized using either \(\beta\)- or \(\gamma\)-CDs. Finally, the coatings were obtained by a dip-coating procedure using an epoxy resin.