## 2 Selectivity
### MIPs as Alternative Biosorbent Candidates  
One of the hot topics of recent research is the reuse of some compounds existed as pollutants in environment. These compounds (molecules, ions, complexes, _etc._) are of high-added value and it will be ideal to selectively bind them with any environmental application and reuse them in their initial or modified form. The latter could be achieved using selective biosorbents with a special technique, which is called molecular recognition. Molecular recognition is realized with molecular imprinting which is an emerging technique to create high affinity polymeric matrices (MIPs) for target molecules. Many papers in literature refer monomers (or their derivatives) used in molecular imprinting. Detailed studies (as that of [PERSON] and [PERSON]) presents especially those used in non-covalent technique [26]. Some of the widely used monomers are: (i) methacrylic acid or acrylic acid; (ii) 4-vynilpyridine; (iii) diethylaminoethyl methacrylate; (iv) acrylamides; (v) 2-hydroxyethyl methacrylate; (vi) acrylamide, _etc._ The other crucial reagent in MIP preparation is the cross-linker which samely presents variety. There are the acrylated-based cross-linkers (ethyleneglycol dimethacrylate, trimethylolpropane trimethacrylate, _etc._), those which are structurally based on styrene and its derivatives (_i.e._, divinylbenzene), and those which are soluble in water (ethylenebisacrylamide). Molecular recognition is a process occurred everywhere. We can call that molecular recognition exists when two molecules has the spatially same geometry but each molecule can selectively interact with only one functional site [27]. Therefore, a specific technology has been bloomed and re-designed in the last 40 years, called molecular imprinting (MI). The whole process is based on adsorption technology, which is already one of the most successful techniques for pollutants removal [28, 29, 30, 31, 32, 33, 34]. MI is not a recent science. The first track of imprinting was recorded in 1930 from [PERSON] [35]. His target was to prepare silica gels (not only one but a series of those). Finally, he understood that the silica prepared showed selective capacity (binding) for a particular solvent in which the synthesis has been done. In 1955, a senior student of [PERSON], [PERSON], observed that after the removal of \"patterning\" dye the silica would re-bind the same dye in preference to the others [36]. However, in 1972, a step change in molecular imprinting occurred when the group of [PERSON] reported that they had successfully prepared a molecularly imprinting organic polymer (MIP) [37], using what is now termed a \"covalent approach\", to prepare an organic molecularly imprinted polymer capable of discriminating between the enantiomers of glyceric acid. The technique, which is widely used for MIP preparation, is based on reactions of condensation that can be characterized as reversible. Characteristic examples are the use of (i) boronate ester [38]; (ii) ketal/acetal [39]; (iii) formation of Schiff's base [40], in order to synthesize different combinations between templates and monomers. In more recent decades (1970s and 1980s), the research group of [PERSON] used this technique/approach with different templates and prepared numerous specialized MIPs. The second major breakthrough in organic polymer imprinting occurred in 1981 when [PERSON] and [PERSON] reported that they had prepared an organic MIP using non-covalent interactions only [41]. This approach was termed the \"non-covalent approach\", as opposed to the covalent approach favored by [PERSON], and it was this, with its simple, seemingly trivial methodology that triggered the explosion in molecular imprinting, occurring during the 1990s. Contrarily, the non-covalent technique is based on some attractive bonds among the molecules (hydrogen bonding, ion-pairs, forces of dipole-dipole, van der Waals bonds). All the aforementioned forces produce and stabilize the template molecule and the monomer (after selection) in the solution (solvent).