## 7 Sorbents Used in the TFME Technique
### Carbon Nanomaterials
#### 7.5.2 Carbon Nanotubes  
Carbon nanotubes (CNTs) are an allotropic variety of carbon with cylindrical nanostructures that are made up of coiled monolayer sheets of sp\({}^{2}\) carbon atoms linked together via covalent bonds. Depending on the number of layers of carbon atoms that make up the tube, CNTs are divided into single-wall carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes (MWCNTs) consisting of several concentric graphene layers connected via van der Waals forces [90]. SWCNTs are nanostructures with diameters ranging from 1 nm to 10 nm, whereas MWCNT diameters are much larger, ranging from 5 nm to several hundred nanometers [91].  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sorption Phase** & **Preparation** & **Analytic** & **Matrix** & **Detection** & **Ref.** \\ \hline CY-GO-LDH & dip coating & \begin{tabular}{c} non-steroidal \\ anti-inflammatory drugs \\ \end{tabular} & plasma, urine & HPLC-UV & [82] \\ \hline graphene membrane & drop casting & metal ions & water & TXRF & [83] \\ \hline LDH/GO/PVDF & application to a Petri dish & diclofenac & urine & HPLC-UV & [84] \\ \hline GO & ESD & metal ions & water & LIBS & [85] \\ \hline PS/G & electrospinning coating & aldehydes & \begin{tabular}{c} exhaled air \\ condensate \\ \end{tabular} & HPLC & [86] \\ \hline GO/PEG & dip coating & \(\beta\)-blockers & saliva & LC-MS/MS & [87] \\ \hline ZIF-8@GO & dip coating & \
\begin{tabular}{c} macrolides, \\ imcosamides \\ \end{tabular} & honey & UPLC-MS/MS & [88] \\ \hline PAMAM@GO-PVDF &
\begin{tabular}{c} application \\ to a Petri dish \\ \end{tabular} & OPPs & water & HPLC-UV & [89] \\ \hline \hline \end{tabular}
\end{table}
Table 7: Examples of the use of graphene materials in TF-SPME.  
Carbon nanotubes are characterized by high thermal, mechanical, and chemical stability, a specific surface area of between 150 and 1500 m\({}^{2}\) g\({}^{-1}\), as well as high sorption capacity. The large sorption capacity of CNTs is a consequence of their large length-to-width ratio. Unlike activated carbons, carbon nanotubes are not porous materials. Adsorption in porous materials is related to the diffusion of analyte particles deep into the pores of the sorbent, whereas sorption-active sites are located on the surface of CNTs and in the spaces between the rolled graphene layers that form MWCNTs.  
Nanotubes are easily accessible to analyte particles during adsorption. Compared to desorption occurring in the pores of a porous sorbent, the nanotubes enable faster thermal desorption of analytes and solvents. Due to the mechanism of adsorption and desorption, which is not controlled by diffusion, carbon nanotubes are used to concentrate a wide range of analytes [92].  
The hexagonal carbon backbone of graphene layers is responsible for the low affinity of CNTs to polar molecules; as a result, pure carbon nanotubes are used as a sorption material in the microextraction of hydrophobic analytes. Carbon nanotubes, functionalized by oxidation reaction, are used in the microextraction of polar analytes. The redox reaction allows the connection between carbon atoms and -OH, -COOH, and -NH\({}_{2}\) groups, among others.  
Sorption of analytes on pure and functionalized CNTs can occur via hydrogen bonds, \(\pi\)-\(\pi\) interactions, electrostatic forces, and hydrophobic interactions. \(\pi\)-\(\pi\) interactions between graphene layers of CNTs and molecules of analytes containing benzene rings in their structure, enable extraction using carbon nanotubes as a sorption phase of polycyclic aromatic hydrocarbons (PAHs), pharmaceuticals, phthalates, pesticides, and parabens. The extraction of metal ions, positively charged particles, and polar molecules occurs via hydrogen bonds, and electrostatic interactions between them and the functional groups present on the CNT surface [31].  
Table 8 summarizes examples of the application of carbon nanotubes in the TF-SPME technique, which are used to analyze environmental and food samples and have been described in scientific publications.