Covalent molecular imprinting

Molecularly imprinted polymers (MIPs) can be prepared according to a number of approaches that are different in the way the template is linked to the functional monomer and subsequently to the polymeric binding sites (Fig. 6-1). Thus, the template can be linked and subsequently recognized by virtually any combination of cleavable covalent bonds, metal ion co-ordination or noncovalent bonds. The first example of molecular imprinting of organic network polymers introduced by Wulff was based on a covalent attachment strategy i.e. covalent monomer-template, covalent polymer-template [12]. 

Size-related problems may become important for all microsensors. Leakage of sensing materials from a small membrane may lead to rapid deterioration of sensor properties [104], While the lipophilicity of membrane components cannot be increased infinitely, immobilization of ionophore and ion exchanger in the polymer by covalent attachment or molecular imprinting along with utilization of plasticizer-free membranes could help solve the leakage problem. 

The transition state analog (TSA) approach1651 which has proved so successful in the design of enzyme inhibitors and catalytic antibodies lends itself nicely, at least in principle, to the molecular imprinting of polymers. Polymerization carried out in the presence of the TSA, or with the TSA covalently but readily reversibly bound to a monomer, produces a polymer with a number of embedded TSA molecules. If these can be removed under rea-... 

Fig. 1. Concept of molecular imprinting - the non-covalent approach. 1. Self-assembly of template with functional monomers. 2. Polymerization in the presence of a cross-linker. 3. Extraction of the template from the imprinted polymer network. 4. Selective recognition of the template molecule...

TABLE 15.1 Examples of Functional Monomers Used in Covalent Molecular Imprinting Process... 

Scheme 15.4 Molecular imprinting of the tripeptide Lys-Trp-Asp using both covalent and non-covalent interactions. Adapted from Klein et al. (1999). Copyright 1999 Wiley InterScience.

Kempe M, Mosbach K. Chiral recognition of N alpha-protected amino acids and derivatives in non-covalently molecularly imprinted polymers. Int J Peptide Protein Res 1994 44 603 -606. 

Khasawneh MA, Vallano PT, Remcho VT. Affinity screening by packed capillary high performance liquid chromatography using molecular imprinted sorhents. II. Covalent imprinted polymers. J Chromatogr A 2001 922 87-97. 

Other Immobilization Techniques Along with noncovalent and covalent immobilization methods, other techniques have been developed for the preperation of affinity supports. Such methods include entrapment, molecular imprinting, and the use of the ligands as both the support and stationary phase. Although these methods are not as common as the approaches already examined, they have important advantages in some applications [8]. 

Sellergren B (2001) The non-covalent approach to molecular imprinting. In Sellergren B (ed) Molecularly imprinted polymers. Man made mimics of antibodies and their applications in analytical chemistry. Elsevier, Amsterdam... 

Caro E et al (2002) Non-covalent and semi-covalent molecularly imprinted polymers for selective on-line solid-phase extraction of 4-nitrophenol from water samples. J Chromatogr A 963(1-2) 169-178... 

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. 

A novel approach to guest inclusion by dendrimers is the use of molecular imprinting . We will look at molecular imprinted polymers in more detail in Section 14.3. Generally the imprinting technique involves assembly of a covalent polymer - in this case dendrimer - about the target guest which acts... 

Covalent polymers can be imprinted with patterns from small guest molecules at the time of synthesis. These molecular Imprinted polymers (MIPs) are useful in sensing applications. 

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