## 2 Molecular Imprinting Process
### MIPs Physical Forms and Methods  
MIPs can be prepared in a variety of physical forms, using different method, depending on their final application [9, 20]. The conventional approach is to synthesize MIPs in bulk, then grind the resulting polymer and successively sieve the particles into the desired size ranges according to the specific application. This method is the most popular since it is simple; nevertheless, crushing, grinding and sieving to obtain the appropriate particle sizes is tedious and time-consuming and often produces particles that are irregular in size and shape. In addition, some interaction sites are destroyed during grinding, reducing the MIP loading capacity and, since only a portion of the original polymer is used, this method suffers from high consumption of the template molecules. Most imprinting publications are still based on use of this method although the scarce control of the MIP physical form and difficulties in scaling up MIP production are important drawbacks. In order to overcome these problems, alternative methods to prepare novel MIP formats, such as MIP beads, membranes, _in situ_ prepared monoliths, surface imprinting, molecularly imprinted monolayers have been developed in recent years [9]. Regular beads can instead be obtained using precipitation polymerization method. This technique allows the formation of imprinted beads with the same reaction mixture used in the bulk method except for the presence of a higher amount of porogen. Polymer chains will continue to grow, precipitating only when become large enough to be insoluble in the reaction mixture. Then the polymer beads are easily recovered by washing and centrifugation operations. This technique is easy, less time consuming than bulk polymerization and provides regular beads in good yields. The particles diameters decrease, increasing the porogen volume probably due to the formation of less oligomers and nuclei under diluted conditions and with an increase of flow ability of the polymer, thus less monomers and cross-linkers diffuse to the surface and polymerize into the small particles. In a recent work [53], a feasible defined control of MIP size from nanoparticles to microbeads that can be utilized in different applications by changing the reaction conditions was obtained. The change of particle size, while maintaining excellent recognition property, was achieved by varying the ratio of two different cross-linking monomers (DVB and TRIM) in similar precipitation polymerization conditions. Polymers in the range of 130 nm to 2.4 um covering all pore sizes typically obtained with precipitation technique were prepared. [PERSON] and [PERSON] [55] prepared spherical beads in liquid perfluorocarbon by suspension imprinting polymerization in the presence of a stabilizer and a surfactant. More recently [PERSON] _et al._[56], using a polycarbonate-based spiral microflow reactor, demonstrated that in mineral oil, which is less expensive than perfluorocarbon, it is not necessary to add any stabilizer or surfactant to produce monodisperse MIP beads. In recent years molecularly imprinted polymer membranes have attracted significant interest. Porous free-standing molecularly imprinted polymer membranes were synthesized by the method of _in situ_ polymerization using the principle of synthesis of interpenetrating polymer networks and tested in solid-phase extraction of triazine herbicides from aqueous solutions. Addition of oligourethane acrylate provided formation of the highly cross-linked MIP in the form of a free-standing 60 \(\upmu\)m thick flexible membrane [57]. However, for separation purposes low membrane permeability was still the main obstacle for their application in separation processing. [PERSON] _et al_. [58] prepared molecularly imprinted nanotubes supported by a porous alumina membrane. The imprinted nanotubes could be used directly for the separation of biomolecules, without the alumina support being removed.