## 6 Molecular Imprinting by Phase-Inversion  
Bio-sensing systems based on molecularly imprinted membranes (or MIMs), have received considerable attention from researchers in various fields. In these types of systems, the recognition component is represented by the imprinted membranes, which are integrated with a transducer component. The possibility of direct and rapid determination of the target analyte by the interaction with the recognition element was a stimulating factor for the application of such systems, which are excellent alternatives to the classical bio-analytical methods. The biomimetic sensor-based membranes are used for food, environmental and clinical determinations, due to their high sensitivity, specificity and stability [162]. The first reports about the MIMs were published about 30 years ago [163; 164; 165], in which case free-standing membranes or thin polymer membranes were prepared on the surface of a solid support, following classical imprinting recipes [163; 164]. Only one of these studies [164] employed a phase-inversion method starting from linear polymer precursors. Generally, the phase inversion method consists of the use of an already prepared polymer, which is solubilized in a solvent in the presence of the template, so that a complex is formed between the target analyte and the functional polymer (Figure 7). During phase inversion, the template is included in the membrane. The template is extracted afterwards from the solid membrane leading to the formation of a MIM. Phase inversion can be performed using two methods, as follows: (i) wet phase inversion (WPI), in which case the membrane is obtained by coagulation in a coagulation bath, as long as the template is not soluble in the coagulation bath (the solvent diffuses from the polymer solution in the coagulation bath, while the polymer changes to a solid membrane, Figure 7), and (ii) dry phase inversion (DPI), in which case the membrane is obtained by solvent evaporation, as long as the polymer can withstand the heating process (the solvent is evaporated, while the polymer changes to a solid membrane, as shown in Figure 7). Rarely, after the phase inversion, a chemical crosslinking reaction of membranes follows; this is because in most of the cases, the physical crosslinking is sufficient for endowing the MIM with chemical and mechanical stability. The physical crosslinking means that the polymer chains are \"fixed\" by hydrogen bonds or other physical interactions in the presence of a template by solvent evaporation [166] or phase-inversion precipitation [167]. This process is very similar to \"bioimprinting\" in proteins, whereas the structure is \"frozen\" either by lyophilization or by chemical crosslinking in the presence of a template. A range of structures are initially formed when the template interacts with the linear polymer, the heterogeneity of folded structures formed during phase inversion being very similar with the structure formed when the polymer chains crosslink covalently. Usually, the interactions between the template and the polymer are non-covalent for the phase inversion method. Nevertheless, some studies have presented the possibility for the chemical functionalization of a monomer with the template molecules, and this latter monomer is further polymerized to serve as a functional polymer for the subsequent phase inversion. In this case, the template extraction from the MIMs is performed by destroying the covalent bonds between the template and the polymer. The advantage of the phase inversion refers to the fact that it usually implies mild conditions for coagulation without affecting the properties of sensible templates. Thus, phase inversion is very much adequate for preparing MIMs for templates with low chemical, thermal or light stability. Moreover, the functional polymer can be prepared in optimum conditions with no interference from the presence of template molecules, which in some cases can inhibit the polymerization process.