## 2 Fundamentals of Ion-Imprinted Polymers  
### History  
Despite this review focusing on Ion-Imprinted Polymers (IIP), it is important to know first all of the Molecularly Imprinted Polymers (MIP) history, since with the latter the concept of chemical imprinting is born. The chemical imprinting technique emerged from the concept of several biomolecular interactions such as substrate-enzyme, drug-receptor, and antigen-antibody, in which the biomacromolecules (enzyme, receptor, and antibody) have receptor sites capable of selectively binding to a molecule (substrate, drug, and antigen) in the presence of other molecules with similar structure. However, among the systems mentioned, the selective antigen-antibody interaction has been the most cited to explain the concept of chemical imprinting, due to the process of how the artificial antibody is formed on the antigen [1,2]. According to [PERSON] (1940) in his work \"The theory of antibody formation,\" the antigen (template molecule) is inhibited by an artificial antibody. The process of inhibition or neutralization occurs when the antibody's polypeptide chain (glycoprotein) is shaped around the antigen, generating a highly selective complementary configuration for the antigen [2,27], in which each antibody can only inhibit the antigen for which it was created. From the complementary configuration existing between the antigen-antibody, [PERSON] (1949) synthesized silica gel in the presence of methyl orange and its homologs (ethyl, \(n\)-propyl, and \(n\)-butyl) to study the selectivity of the adsorbent material [28]. [PERSON] concluded that by removing the dye from the silica, there is a creation of complementary binding sites to the respective dye, so that, when this material is used in adsorption processes for the dyes mentioned above, the silica showed greater affinity for the compound in which it was synthesized. Thus, [PERSON]'s work was the first publication associating silica as a molecularly imprinted material. Hence, the adsorbent materials development with the ability to selectively retain the analyte has become of great interest to many researchers. In this context, [PERSON] (1972) [29] and [PERSON] (1972) [30] worked independently in the preparation of organic imprinted polymers, adding into the synthesis vinyl monomers that contained functional groups capable of selectively binding to a molecule. These polymers were named Molecularly Imprinted Polymers (MIP). [PERSON] and [PERSON] presented the organic polymer synthesis with selective sites for the enantiomeric separation of glyceric acid racemates using the 2,3-op-vinylphenyl boronic ester (MF) functional monomer, while [PERSON] and [PERSON] synthesized a polymer with the monomer polyethylenimine (PEI) in the presence of methyl orange dye [29]. Both [PERSON] and [PERSON] and [PERSON] and [PERSON] confirmed that the prepared polymers showed a high selective capacity for template molecules (MM). However, due to the covalent bond between the template molecule (MM) and the functional monomer (MF), it was difficult to remove the MM from the binding site, requiring the use of hydrolysis (cleavage of bonds) in most situations to facilitate the elution process [1]. [PERSON] and [PERSON] (1980) proposed a simple strategy, named \"host-guest polymerization\", for the synthesis of an imprinted polymer based on non-covalent interactions, in which the host-guest relationship (Template monomer--Molecule functional) was materialized during polymerization [31]. The functional monomers were chosen for their non-covalent interaction (ionic, hydrogen bonding, hydrophobic, charge transfer) with MM, so that, through a simple wash, varying the pH value, ionic strength, solvent, among other parameters, the removal of the MM became easier, and the binding sites were preserved, obtaining three-dimensional cavities complementary to the MM. In the 1990s, [PERSON] et al. (1995) [32] synthesized an imprinted polymer for cholesterol using 4-vinylphenol carbonate as a functional monomer.