## 2 Fundamentals of Ion-Imprinted Polymers
### Components of the IIP
#### 2.3.2 Ligand  
The ligand used in the synthesis of IIP is characterized by containing in the structure the vinyl group (-CH=CH2), hence its name of vinylated ligand. This ligand is also named bifunctional monomer because it has the function of forming a complex with the metal ion, to later incorporate this into the polymeric network by participating in the polymerization process through the vinyl group [35]. The functional monomer choice depends on the chemical nature of the metal ion. [PERSON] (1963) in his work \"Hard and Soft Acid and Bases\", known as the HSAB theory [61], argued that hard acids will preferentially react with hard or intermediate bases; and soft acids with soft or intermediate bases, in which the terms \"hard\" and \"soft\" are associated with the polarizability of the reactants. Among the most functional monomers used in the IIP synthesis is the methacrylic acid (MAA) [44; 45; 46; 47; 48; 49; 50; 51], first proposed by [PERSON] and [PERSON] in 1980 [31], whose electron donor is the atom of oxygen. Basic monomers such as 1-vinylimidazole (1-VID) and 4-vinylpyridine (4-VP) are also frequently used in the synthesis of IIP. However, the complex formed with the metal ion is via a nitrogen atom. Figures 2-4 show the structure of the most important functional monomers classified as acids, basic, and neutral, respectively [62]. Figure 3: Main basic functional monomers. Figure 2: Main acid functional monomers. Other functional monomers used in the synthesis of IIP [38; 52; 55; 56; 57] were found, which have in their structure the oxygen, nitrogen, or sulfur atom as electron donors (Figure 5). To improve the efficiency of the metal ion complexation, the vinylated ligand can be added together with non-vinylated ligands, which contain functional groups with greater affinity for the metal ion. This process is known in the context of ion-imprinted polymers as \"Trapping\". In the IIP synthesis using the Trapping process, the metal ion is  
Figure 4: Main neutral functional monomers. Figure 5: Other functional monomers were used in the IIP synthesis. initially mixed with the non-vinylated ligand to form the complex. Then, the functional monomer (vinylated ligand) is added to complete the complexation. Due to the absence of polymerizable groups (vinyl group) in the structure of the non-vinylated ligand, it is not chemically bonded to the polymeric network, however, it is incorporated into the polymer matrix when interacting with the functional monomer through intermolecular forces (Hydrogen bonding), as well as by the interaction with the metal ion. [PERSON] and coworkers synthesized an IIP for the K\({}^{+}\) ion using the non-vinylated ligand dicyclohexyl 18C6 [44], whereas [PERSON] and coworkers added 1,5-diphenylcarbazone to complex the Pb\({}^{2+}\) ion [46]. In other papers, the non-vinylated ligand Morin (3,5,7,2\({}^{\prime}\),4\({}^{\prime}\)-pentahydroxflavone), Dz (Ditizone), and PAR (4-(2-pyridylazo)-resercinol) were used to complex the Pb\({}^{2+}\), Ni\({}^{2+}\), and Cu\({}^{2+}\) ions, respectively [48; 50; 63]. After the removal of the metal ion, the authors propose that the non-vinylated ligand remains in the polymeric network due to hydrogen bond interactions with the functional monomer [64]. Other non-vinylated ligands containing sulfur as a donor atom and used in the IIP synthesis are diphenylthiocarbazone (Dithizone), thiosemicarbazide, and acetaldehyde thiosemicarbazone [35]. On the other hand, it deserves to point out that in addition to the chemical nature of the functional monomer, the stoichiometric relationship between the metal ion and the functional monomer also interferes in the chemical imprinting process.