## 2 Fundamentals of Ion-Imprinted Polymers
### Components of the IIP
#### 2.3.4 Crosslinking Agent  
According to [PERSON] and [PERSON] [62], the crosslinking agent performs three functions in the synthesis of polymers, which correspond to controlling the morphology of the polymer; stabilizing the binding sites with ionic recognition capability; and conferring mechanical stability to the polymer matrix. Crosslinking agent also contains one or more vinyl groups in the structure such as the functional monomer. Mono-vinylated generally forms straight chains, while bi-, tri-, or tetra-(multifunctional) form branched chains with different molecular arrangements. [PERSON] and coworkers synthesized an IIP for Er\({}^{3+}\) (Erbium) using different functional monomers (styrene, 2-hydroxyethyl methacrylate (HEMA), and methyl methacrylate (MMA)) and crosslinking agents (divinylbenzene (DVB) and ethylene glycol dimethacrylate (EGDMA)), which were named as Styrene-DVB, HEMA-EGDMA and MMA-EGDMA [69]. Among them, the IIP containing Styrene-DVB showed better results in terms of preconcentration factor, adsorption capacity, and selectivity towards Er\({}^{3+}\) in the presence of other ions such as Y (Yttrium), Dy (Dysprosium), Ho (Holmium) and Tm (Ytterbium). This outcome can be attributed to the greater rigidity and denser polymeric chain of the Styrene-DVB polymer as a result of \(\pi\)-\(\pi\) stacking interactions from aromatic rings when compared to aliphatic chains present in HEMA-EGDMA and MMA-EGDMA. Figure 6 illustrates the crosslinking agents often used in the synthesis of IIP. As seen in Table 2, an overview of the previously published works showed that EGDMA (bifunctional crosslinker) is the most used crosslinking agent in the synthesis of IIP, due to its ability to form thermally and mechanically stable polymers; in addition, the production of porous materials [1; 62]. High proportions of crosslinking reagent must be used about the functional monomer, in general, 1:4 (mol/mol). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type of Solvent** & **Solvent** & **Boiling Point (\({}^{\circ}\)C)** & **Dielectric Constant (\&)** & **Dipole Moment (\&)** \\ \hline \multirow{2}{*}{Non-polar} & Toluene [53] & 111.6 & 2.38 & 0.36 \\ \cline{2-5}  & Chloroform [38; 50] & 61.1 & 4.81 & 1.04 \\ \hline \multirow{4}{*}{Arotic polar} & Dichloromethane (DCM) & 40.0 & 9.1 & 1.60 \\ \cline{2-5}  & Tetrahydrofuran (THF) & 65.0 & 7.5 & 1.75 \\ \cline{2-5}  & Dimethylformamide (DMF) [42] & 153.0 & 38.0 & 3.82 \\ \cline{2-5}  & Acetonitrile (ACN) [46; 47] & 81.6 & 37.5 & 3.92 \\ \cline{2-5}  & Dimethylsulfoxide (DMSO)[60] & 189.0 & 46.5 & 3.96 \\ \hline \multirow{4}{*}{Protic Polar} & Methanol (MeOH) [39; 41] & 64.6 & 33.0 & 1.70 \\ \cline{2-5}  & Ethanol (EtOH) [34; 40; 43; 51] & 78.2 & 30.0 & 1.69 \\ \cline{1-1} \cline{2-5}  & Acetic Acid (HAc) & 118.0 & 6.2 & 1.74 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Porogenic solvents most used in the synthesis of IIP. Recently, [PERSON] and [PERSON] synthesized IIP for Ni\({}^{2+}\) and Co\({}^{2+}\) using EGDMA or TRIM [2,2-bis(hydroxymethyl)]butanol trimethacrylate] as crosslinking agents [70]. However, the authors did not show the influence of the chemical structure of crosslinking agents on the adsorptive properties of imprinted polymers. Nonetheless, TRIM is expected to give the polymer higher rigidity, because it is a trifunctional crosslinker, i.e., it has three interaction sites in its structure that allow it to bond strongly to the polymer matrix [71, 72], when compared to the bifunctional EGDMA crosslinker. Thus, the more rigid the polymer matrix, the greater the stability of the binding sites, facilitating the interaction with the metal ion and mass transfer (adsorption capacity). It is important to point out that tri- or tetra-functional crosslinking agents such as 2,2-bis(hydroxymethyl)butanol trimethacrylate (TRIM), pentaerythitol triacrylate (PETRA), and pentaerythitol tetraacrylate (PETEA) [73] have been explored in the synthesis of imprinted polymers.