## 1 Introduction
Then, after adding a cross-linking agent and carrying out the polymerization reaction, a cross-linked polymer is formed, from which, in the next step, the template ion or molecule is removed, and three-dimensional cavities are created with a shape corresponding to the template and capable of repeatedly adsorbing and desorbing it with high selectivity [18]. In recent years, such materials have been most often used in applications such as beds in the SPE technique to concentrate Eu, Ce, Nd, Dy, and Y from wastewaters, human serum, and plasma, and as sensors [17; 19], to improve the specific identification of artemisinin [20], for Cu\({}^{2+}\) detection [21], and for the capture and purification of hemoglobin [22], among others. In the literature, there are various classifications of IIP synthesis methods, the most popular of which divides them into four groups, namely (1) cross-linking of linear polymer chains carrying groups capable of forming complexes with ions, (2) chemical immobilization of ligands with a vinyl moiety in the presence of a cross-linking agent, (3) surface imprinting involving a ligand with or without a vinyl group, and (4) entrapment of a non-vinylated ligand inside the polymer network [11]. In the case of IIPs for REEs, the fourth method proved to be the most effective [23]. Several components are needed to obtain IIPs by the fourth method, such as a template ion, which is usually a metal ion [24], functional monomers containing vinyl groups, such as 1-vinylimidazole (1-VI), 2-hydroxyethyl methacrylate (HEMA), methyl methacrylate (MMA), acrylamide (AAm), methacrylate acid (MAA), 4-vinylpyridine (4-VP), and acrylic acid (AA) [24; 25; 26; 27; 28; 29; 30; 31; 32]. In addition, the physical entrapment of various non-vinylated ligands (such as dithizone (DZ), diphenylcarbazide (DPC), and 8-hydroxyquinoline (8-HQ)) in the IIP networks generated by copolymerization of vinylated ligands with a cross-linking agent [31; 32] is also used. Other key IIP components are cross-linkers used to stabilize the IIP network and provide appropriate mechanical stability, flexibility, mass transfer efficiency, appropriate arrangement, and a number of recognition sites per unit mass of the IIP. The cross-linkers can be classified into two groups, i.e., those containing vinyl groups that can react with functional monomers (e.g., ethylene glycol dimethacrylate (EGDMA), poly(ethylene glycol) diacrylate (PEGDA), \(N\),\(N^{\prime}\)-methylenebisacrylamide (MBAAm), trimethylolpropane trimethacrylate (TMPTMA), and divinylbenzene (DVB)) and those that are used for cross-linking linear or hyperbranched polymers (e.g., glutaraldehyde (GA) and epichlorohydrin (ECH)) [24; 31; 32]. The synthesis of IIPs also involves the use of initiators, the most commonly used among them being azobisisobutyronitrile (AIBN), and porogens, which provide a suitable polymerization reaction environment with an appropriately selected polarity and dielectric constant (e.g., methanol, toluene, dimethyl sulfoxide (DMSO), \(N\),\(N\)-dimethylformamide (DMF), acetonitrile, acetic acid (CH\({}_{3}\)COOH), trichloromethane (CHCl\({}_{3}\)), etc.). In the final step of the IIP development, it is necessary to remove template ions or molecules using suitable reagents, such as HCI, H\({}_{2}\)SO\({}_{4}\), HNO\({}_{3}\), or ethylenediaminetetraacetic acid (EDTA) and thiourea in the case of using functional monomers that are unstable in an acidic environment [32]. In addition to the abovementioned methods, used most frequently for the synthesis of IIPs, there are some others involving the use of sol-gel, surface imprinting, stimuli-responsive imprinting, dual template imprinting, a combination of various ion imprinting technologies, and free radical polymerization [24]. Free radical polymerization is used more often than the other methods mentioned above, and it may involve bulk, precipitation, suspension, or emulsion polymerization [24; 32].