Molecular Imprinted Polymer - Molecular Imprinting Technique (State of The Art and Perspectives)

Molecular Imprinting Technique (State of The Art and Perspectives)

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 to 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) crosslinker(s), 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.

The self-assembly method has advantages in the fact that it forms a more natural binding site, and also offers additional flexibility in the types of monomers that can be polymerized. The covalent method has its advantages in generally offering a high yield of homogeneous binding sites, but first requires the synthesis of a derivatized imprint molecule and may not imitate the "natural" conditions that could be present elsewhere. Over the recent years, interest in the technique of molecular imprinting has increased rapidly, both in the academic community and in the industry. Consequently, significant progress has been made in developing polymerization methods that produce adequate MIP formats with rather good binding properties expecting an enhancement in the performance or in order to suit the desirable final application, such as beads, films or nanoparticles. One of the key issues that have limited the performance of MIPs in practical applications so far is the lack of simple and robust methods to synthesize MIPs in the optimum formats required by the application. Chronologically, the first polymerization method encountered for MIP was based on "bulk" or solution polymerization. This method is the most common technique used by groups working on imprinting especially due to its simplicity and versatility. It is used exclusively with organic solvents mainly with low dielectric constant and consists basically of mixing all the components (template, monomer, solvent and initiator) and subsequently polymerizing them. The resultant polymeric block is then pulverized, freed from the template, crushed and sieved to obtain particles of irregular shape and size between 20 and 50 µm. Depending on the target (template) type and the final application of the MIP, MIPs are appeared in different formats such as nano/micro spherical particles, nanowires and thin film or membranes. They are produced with different polymerization techniques like bulk, precipitation, emulsion, suspension, dispersion, gelation, and multi-step swelling polymerization. Most of investigators in the field of MIP are making MIP with heuristic techniques such as hierarchical imprinting method. The technique for the first time was used for making MIP by Sellergren et al. for imprinting small target molecules. With the same concept, Nematollahzadeh et al. developed a general technique, so-called polymerization packed bed, to obtain a hierarchically structured high capacity protein imprinted porous polymer beads by using silica porous particles for protein recognition and capture.