## 2 Selectivity
### Limitations  
The engineering part is based on the selection of the suitable model for fitting of rebinding experiments either of equilibrium (isotherms) or kinetics. However, similarly as in the case of all biosorbents, each system of MIP (adsorbent) and target molecule (adsorbate) is unique. Modifying the conditions in polymer \"cookery\" (synthesis/polymerization, cross-linking, template extraction), the product will be completely different. The latter can be confirmed with scanning electron micrographs (SEM) of the prepared MIPs. For example, a recent study investigated the synthesis for selectively  
Figure 1: Example for the synthesis of Molecular Imprinted Polymers (MIPs) for capturing/binding dye molecules. binding dyes, but having obvious surface changes due to the different solvent used (organic or aqueous) (Figure 2). An important parameter regarding the possible porosity of MIPs is the solvent [45]. The type of solvent (or else \"porogen\") demonstrates some morphological properties of porosity and surface area. There are two possible cases: (i) the use of solvents with low solubility phase, which can be rapidly separated and give adequate large pores, but MIPs with lower surface areas; (ii) the use of solvents with high solubility phase, which can be separated later and give smaller pore size distributions, but MIPs with higher surface area. It does not appear, however, that binding and selectivity in MIPs is dependent on a particular porosity. In fact, optimum results are often obtained when chloroform is used. In this case, the polymers prepared did not present adequate porosity or surface area (BET measurements), they were highly solvated and MIPs made without any porogen did not exhibit any selectivity, because substrate could not access the polymer. Thus, diffusion of substrates through the MIP appears to be fast enough through the chloroform solvated polymers without requiring porosity. Therefore, there is no big chance to repeat the data of literature. Researchers have understood the latter and studied uniquely each system of MIPs and for this reason, they have tested many isotherm models to predict the affinity or theoretical capacity of MIPs. Apart from the synthesis limitations, as described above, another crucial factor which influences the engineering behavior of MIPs is the limitations during extraction of template molecules from the polymeric matrix, after synthesis. A 100% removal of the template was not achievable despite sequential attempts, using mainly Soxhlet apparatus. The latter is presented because some regions of MIP are highly cross-linked and therefore, the insertion/accessibility to them is limited. Another possible scenario is that in many cases, the template molecule does not present enough solubility in the porogen and therefore, cannot participate in interactions with the cavity of MIP [46,47]. The results from the above are not simple: (i) the most possible is a reduction of the total number of cavities suitable for rebinding (Figure 3); (ii) the template can be \"escaped\" during the last step of elution mainly in MIPs used in columns for solid phase extraction [48] or during analysis [49]. Figure 2: SEM micrographs of dye-MIPs using (**a**) aqueous and (**b**) organic solvent. Reproduced with permission from [PERSON] [44], published by Elsevier, 2009. Even in the case of MIPs prepared with non-covalent or self-assembly technique, the binding of the template to the components of the imprinted cavity can be so strong that extraction under drastic conditions is required [51]. Extreme pH or temperature applied for a long time can lead to distortion and even rupture of the cavity during removal, resulting in MIPs of poor selectivity and recovery [46]. Furthermore, changes in the degree of swelling of the MIP network during extraction and subsequent desiccation can result in the collapse of the cavity, sterically hindering the entrance of target molecule, or in a distortion of the binding points or the strength of interactions [52,53].