Functional monomers analyte

Molecular imprinting can be accomplished in two ways (a), the self assembly approach and (b), the preorganisation approach3. The first involves host guest complexes produced from weak intermolecular interactions (such as ionic or hydrophobic interaction, hydrogen bonding) between the analyte molecule and the functional monomers. The self assembled complexes are spontaneously formed in the liquid phase and are sterically fixed by polymerisation. After extraction of the analyte, vacant recognition sites specific for the imprint are established. Monomers used for self assembly are methacrylic acid, vinylpyridine and dimethylamino methacrylate. 

Figure 1. 1, Functional monomers 2, Cross-linker 3, Analyte a, Self-assembly or Preorganisation b, Polymerisation c, Removal of the analyte.

FIGURE 1.49 Principle of molecular imprinting.169 1 = functional monomers 2 = cross-linking monomer 3 = molecule whose imprint is desired (molecular template). In (A), 1 and 2 form a complex with 3 and hold it in position in (B), polymerization involving 1 2 occurs and the template (imprint molecule) is held in the polymeric structure in (C) and (D) the imprint molecule is removed leaving a cavity complementary to its size and shape into which a target analyte of similar dimensions can fit. (Reproduced with permission from Taylor Francis.)... 

A very important issue in the synthesis of MIPs is the study of the pre-organized complexes formed between the functional monomer(s) and the template. Preferably, this complex should be sufficiently stable to withstand polymerization conditions on the one hand and satisfactorily labile to allow both facile release of a template after polymerization and fully reversible rebinding of an analyte on the other. 

Table 1 Structural formulas of analytes and functional monomers, accompanied by characteristic values of merit, used in the UV-vis MIP sensors Analyte Functional monomer Value of merit References... 

Analyte Functional monomer Value of merit References... 

All structures of both functional monomers and analytes along with characteristic values of merit presented within this section are compiled in Table 1. 

For instance, aromatic solvent vapours were determined with polyurethane MIPs combined with SAW transducers [124]. That is, first, the hydrophilic quartz surface of SAW was hydrophobized with NW-dimethylaminotrimethylsilane. Then a solution for polymerization was prepared by mixing functional monomers, such as 4,4 -dihydroxydiphenyldimethylmethane, 4,4 -diisocyanatodiphenylmethane and 30% 2,4,4 -triisocyanatodiphenylmethane, with the 1,3,5-trihydroxybenzene crosslinker in the ethyl acetate or ethanol template used also as the solvent for polymerization. Subsequently, the hydrophobized resonator surface was spin-coated with an aliquot of this solution. Finally, the free-radical polymerization has been initiated thermally to form a polyurethane MIP film. The desired vapour concentration and relative humidity of the analyte were achieved by mixing dry air and saturated steam with solvent vapours generated with thermoregulated bubblers. 

Interestingly, the analytes of smaller sizes than that of the functional monomer used for MIP preparation were determined based on the MIP-PZ sensing. For instance, acetaldehyde was determined by using the methacrylate functional monomer for MIP preparation [154]. The fabricated MIP-PZ chemosensor was 11-fold more selective for acetaldehyde than for acetone. 

Analyte Functional monomer Cross-linking Polymerization... 

Herbicides, like demestryn, have been determined using a chemosensor based on the MIP film recognition and capacitive transduction [193, 194]. In this determination, photografting polymerization has been demonstrated as an efficient procedure for fabrication of a capacitive chemosensor. This procedure involved immobilization of an initiator on the electrode. In this case, first, an alkanethiol monolayer was self-assembled on the gold electrode. This monolayer was perfectly dielectric. Then, an MIP film was deposited on top of this monolayer by photo-radical polymerization in the acetone solution with benzophenone, 2-acrylamido-2-methyl-1-propane sulphonic acid, /V./V -iriethylenediacrylamide and demestryn used as the initiator, cross-linker, functional monomer and template, respectively. Subsequently, the template was extracted with methanol. The capacitance decreased by 20% upon binding the demestryn analyte by the MIP film. Similarly, a creatine chemosensor was constructed [194],... 

In the molecular imprinting technique, a cross-linked polymer matrix is formed around a target analyte (the template). The precursor mixture contains a functional monomer which can interact with the template molecule by covalent or non-covalent bonding. After the polymerisation process, the functional groups are held in position by the polymer backbone and the template molecule is removed. The residual binding sites are complementary to the target molecules in size and shape. 

---