Non-covalent imprinting

Most MIPs show a heterogeneous distribution of binding sites and can be considered as polyclonal in their nature. In non-covalent imprinting, the amorphous material contains binding sites which are not identical because they may have different cross-linking density or accessibility. Moreover, the monomer (M) and the template molecule (A) may form complexes of different stoichiometry (MnA) in the pre-polymerization mixture [5]... 

Both of these MIP preparation procedures have their advantages and limitations [20, 21]. For instance, the size of the analyte molecule is not a discriminating criterion in covalent imprinting since the template selectively determines the interaction sites. In contrast, non-covalent imprinting has the advantage of being simpler since an additional synthetic step is not required to introduce the template into the polymer matrix. Moreover, removal of the template via extraction with the suitable solvent solution is simple and mostly complete for the non-covalent imprinting. 

The polymeric adsorbents are usually prepared by variations of two-phase suspension processes. These refer to systems where microdroplets of monomers and solvent are converted into solid beads upon polymerisation. In the case where the monomers are not water soluble, as in the case of styrene-based polymers and many methacrylate-based polymers, the monomers, a solvent and a droplet stabiliser are suspended as droplets by stirring in water and then polymerised (o/w suspension polymerisation). The particle size and dispersity can be influenced by the stirring speed and the type of stabiliser. So far, only a few examples of the preparation of imprinted polymers in bead format have been described [4-8] and these are thoroughly reviewed in Chapter 12. In non-covalent imprinting, the main limitation to the use of these techniques is that the imprinting method often requires the use of polar partly water soluble monomers or templates in combination with less polar water insoluble components. Use of the o/w suspension method... 

As mentioned above, the non-covalent imprinting approach is somewhat more general than its cousin-techniques. It is also the technique which has been used exclusively for the preparation of molecularly imprint-based CEC separation systems so far. 

Fig. 16.7. CEC-separation of D,L-phenylalanine on a column containing imprints of L-phe-nylalanine anilide. This demonstrates the interesting strategy of using a structurally similar compound to the analyte as the imprint molecule for the MIP preparation. Due to their low solubility in the pre-polymerisation mixture, amino acids are normally not amenable to non-covalent imprinting. Reprinted from [69] Copyright (1997), with permission from Elsevier Science,...

Covalent and non-covalent imprinting using fluorophores as functional monomers... 

Fig. 20.10. Fluorescent functional monomers for covalent and non-covalent imprinting.

A 2,6-disubstituted pyridine derivative was also synthesised and incorporated into an EDMA-based (the porogen was not disclosed) non-covalently imprinted polymer. The resulting polymer was reportedly sensitive to solvent, pH and the presence of organic additives in solution. The example provided shows that cyclohexane suspensions of the polymer exhibit no diminution of fluorescence upon the addition of anisole, but show approximately 15% quenching in the presence of an equivalent concentration of phenol. Approaches such as these, wherein the fluorescent properties of the MIP itself are altered by interaction with the analyte, should prove extremely useful. 

The tailored arrangement in non-covalent imprints is achieved by polymerisation in the presence of the template (analyte). The self-assembling process with non-covalent imprinting is performed mainly via hydrogen bonds and weaker forces, such as van der Waals interactions, which leads to binding sites with more heterogeneous affinities when compared to covalent imprints. The incorporated... 

In summary, non-covalently imprinted polymers offer a universal tool for sensor technology, besides the main applications in HPLC and solid phase extraction. The examples discussed above are but a few of the potential applications of smart chemosensory devices coated with non-covalent MIPs. The strategy of non-covalent imprinting is highly appropriate for sensory applications. Antibody-like... 

Water is incompatible with most non-covalent imprinting procedures... 

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