## 2 Molecular Imprinting  
Molecular imprinting is based on the polymerization of a functional monomer and a cross-linker around a template molecule. Briefly, a selected template molecule and a functional monomer interact covalently or non-covalently and develop complexes. Then, polymerization takes place between the developed template-functional monomer complexes and a cross-linker. After the polymerization the template molecule is removed, leaving a polymer with imprinted sites complementary to the molecular structure and the functional groups of the template. The imprinted sites are available for binding the molecules with the same or similar structure as the template [36; 37; 38]. Molecular imprinting is schematically presented in Figure 1.  
The essential components of a molecular imprinting process are the template molecule, the functional monomer and the cross-linker. The polymerization initiator and the porogenic solvent  
Figure 1: The template molecule and the functional monomer interact. The template-monomer complexes and a cross-linker polymerize. The template molecule is removed to provide a polymer with imprinted sites. are also important. Furthermore, polymerization is conducted under a stream of an inert gas such as nitrogen to ensure oxygen removal. For the selection of the template molecule specific criteria should be met. The functional groups of the template should be able to interact with the functional monomers and not hinder the polymerization reaction, while the template molecule should be chemically stable during polymerization. Usually, the templates are small organic molecules, such as pesticides, pharmaceuticals, sugars, amino acids and peptides [39; 40]. Functional monomers consist of a recognition unit that interacts with the template and a polymerizable unit that is polymerized with the cross-linker. The recognition site of the selected functional monomer should develop strong interactions with the functional groups of the template molecule to form a complex before polymerization takes place. Functional monomers such as acrylic acid, methacrylic acid (MAA), methyl methacrylate, 2-(trifluoromethyl)acrylic acid (TFMAA), styrene, 4-vinylpyridine (4-VP), acrylamide (AM), methacrylamide, 2-hydroxyethyl methacrylate (HEMA) and 3-aminopropyltriethoxysilane (APTES) are used in non-covalent imprinting [36; 39; 40]. The cross-linker polymerizes with the polymerizable unit of the functional monomer to form a highly cross-linked polymer around the template-monomer complex that remains firm after the template removal. The selectivity and the binding capacity of the developed MIP depend on the amount of the selected cross-linker. Insufficient amount of the cross-linker results in mechanically unstable poorly cross-linked MIPs, one the other hand increased amount results in MIPs with reduced imprinted sites. Cross-linkers such as ethylene glycol dimethacrylate (EGDMA), trimethylopropane methacrylate (TRIM), divinylbenzene (DVB) and tetramethylene dimethacrylate are used in non-covalent imprinting [36; 39; 40]. The selection of the porogenic solvent is also important in molecular imprinting. The porogen influences the interaction between the template and the monomer, serves as the dispersion media and affects the pore forming during the polymerization reaction. In non-covalent imprinting less polar or non-polar solvents such as toluene, acetonitrile and chloroform are preferred [39; 40]. Most MIPs are prepared by free radical polymerization that is initiated either thermally or photochemically with the addition of a peroxy or an azo compound. The most frequently used azo compound is azobisisobutyronitrile (AIBN) that is used at temperature range of 50-70 \({}^{\circ}\)C [39]. After the polymerization reaction is complete, template removal is essential before the prepared MIPs can be used in any application.