## 7 Sorbents Used in the TFME Technique
### Molecularly Imprinted Polymers  
Molecularly imprinted polymers (MIPs) are an essential group of the most selective sorption phases used in TF-SPME. MIP materials acquire their ability to selectively recognize an analyte during their synthesis, which proceeds with the participation of template molecules, functional monomers, a crosslinking reagent, an initiator, and a solvent. MIP synthesis occurs at high temperatures or by UV-light irradiation. During its course, template molecules combine with functional monomers to form complexes via covalent bonds or hydrogen bonds, \(\pi\)-\(\pi\) interactions, ionic interactions, and van der Waals forces. As a result of the polymerization reaction taking place around these complexes with the participation of a crosslinking agent, the complexes are immobilized in the polymer network. After synthesis, the template is removed, and the complex is degraded. Consequently, molecular cavities are formed with affinities for chemical structures identical or similar to those of the removed template. A scheme of the MIP synthesis is shown in Figure 18.  
The template molecules used for the synthesis of MIP should exhibit chemical stability during the polymerization reaction and have functional groups that do not block the actual polymerization reaction and retain their capacity to interact with the functional monomer. A desirable feature of template molecules is also low molecular weight. Low-molecular-weight molecules are rigid and ensure the formation of permanent molecular cavities in the three-dimensional polymer network. Suitable functional monomers are selected for the template molecules, enabling the formation of chemical interactions leading to the formation of monomer-template transition complexes. Functional monomers also determine the formation of active sites in the MIP polymer that interact with analyte molecules. The cross-linking reagent (cross-linker) forms bonds during MIP synthesis. The formed bounds stabilize the polymer structure, and thanks to them, the cross-linked structure remains stable after the removal of the template molecule. An important factor affecting the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sorption Phase** & **Preparation** & **Analyte** & **Matrix** & **Detection** & **Reference** \\ \hline PmPDA/CNT & electrospinning coating & copper & water, food & FAAS & [51] \\ \hline PPy/PA & electrospinning coating & pesticides & water & GC-MS & [52] \\ \hline PANI & gluing & hormones & urine & HPLC-FLD & [53] \\ \hline PANI-N6 & electrospinning coating & chlorobenzene & water & GC-MS & [54] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Examples of the use of conductive polymers in TF-SPME. Figure 17: Structural formulas of conductive polymers. Figure 18: Synthesis diagram of MIPs [55]. quality of the polymer product is the amount of cross-linking reagent used in the synthesis. Too little of it results in the polymer's mechanical stability being lacking. At the same time, too much of it blocks functional groups and thus influences a reduction in the number of molecular cavities that are selective to the analyte [56]. In the synthesis of molecularly imprinted polymers, solvents of low polarity that generate the formation of large pores are used to provide the polymer material with the largest possible specific surface area. Solvent molecules of low polarity do not interfere with the interactions formed between the polymer and the template molecules, and simultaneously, they make it possible to obtain a material with high selectivity [57]. The MIP-TFME device is obtained by applying several microliters of the prepolymer solution to a silanized microscope slide or stainless substrate, then covering it with a coverslip and exposing it to UV light to initiate polymerization. The obtained polymer thin films are then washed with a suitable solvent to remove the template molecules from the polymer network.