Molecular imprinted polymer

A Molecularly Imprinted Polymer (MIP) is a polymer that has been processed using the molecular imprinting technique which leaves cavities in polymer matrix with affinity to a chosen "template" molecule. The process usually involves initiating the polymerization of monomers in the presence of a template molecule that is extracted afterwards, thus leaving complementary cavities behind. These polymers have affinity for the original molecule and have been used in applications such as chemical separations, catalysis, or molecular sensors. Published works on the topic date to the 1930s.

Molecular imprinting is, in fact, making an artificial tiny lock for a specific molecule that serve as miniature key. Like plastic receptors the imprinted polymer grabs specific chemicals. Many basic biological processes, from sensing of odors to signaling between nerve and muscle cells, rely on such lock-and-key combinations. For decades, scientists trying to understand these interactions often play locksmith, searching for the right key to fit a particular receptor. Now, the elegance of molecular imprinting in nature has been spurring many scientists to build the locks themselves. They etch a material to create specific cavities which in size, shape and functional groups, fit the target molecule. However, one of the greatest advantages of artificial receptors over naturally occurring ones is freedom of molecular design. Their frameworks are never restricted to proteins, and a variety of skeletons (e.g., carbon chains and fused aromatic rings) can be used. Thus, the stability, flexibility, and other properties are freely modulated according to need. Even functional groups that are not found in nature can be employed in these man-made compounds. Furthermore, when necessary, the activity in response towards outer stimuli (photo-irradiation, pH change, electric or magnetic field, and others) can be provided by using appropriate functional groups. The spectrum of functions is far wider than that of naturally occurring ones. In a molecular imprinting processes, one needs a 1) template, 2) functional monomer(s) 3) cross-linker, 4) initiator, 5) porogenic solvent and 6) extraction solvent. According to polymerization method and final polymer format one or some of the reagent can be avoided.

There are two main methods for creating these specialized polymers. The first is known as self-assembly, which involves the formation of polymer by combining all elements of the MIP and allowing the molecular interactions to form the cross-linked polymer with the template molecule bound. The second method of formation of MIPs involves covalently linking the imprint molecule to the monomer. After polymerization, the monomer is cleaved from the template molecule. The selectivity is greatly influenced by the kind and amount of cross-linking agent used in the synthesis of the imprinted polymer. The selectivity is also determined by the covalent and non-covalent interactions between the target molecule and monomer functional groups. The careful choice of functional monomer is another important choice to provide complementary interactions with the template and substrates. In an imprinted polymer, the cross-linker fulfills three major functions: First of all, the cross-linker is important in controlling the morphology of the polymer matrix, whether it is gel-type, macroporous or a microgel powder. Secondly, it serves to stabilize the imprinted binding site. Finally, it imparts mechanical stability to the polymer matrix. From a polymerization point of view, high cross-link ratios are generally preferred in order to access permanently porous materials and in order to be able to generate materials with adequate mechanical stability.

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