Molecular imprinting in situ

In situ molecular imprinting can be defined as a technique for preparing imprinted polymers in a place where the polymers are subsequently utilised. Imprinted polymers prepared by an in situ technique, therefore, require no subse-... 

The in situ molecular imprinting protocol employing dispersion polymerisation has some advantageous features. The dispersion polymer can be removed from a column and re-packed when a column is damaged after repeated use. Back-pressure of agglomerated polymer particles is less problematic therefore, this in situ method can be applied to a wider range of analytical techniques. Here, two applications of in situ dispersion polymer, solid phase extraction (SPE) and CE are described. 

In situ molecular imprinting was applied to prepare imprinted polymer-filled or modified capillaries usable in CE and CEC (see also Chapter 16). To date, attempts... 

CEC was demonstrated in the analysis of /i-adrenergic antagonists using capillaries modified with propranolol-imprinted polymer. In situ molecular imprinting was performed via photo-polymerisation at — 20°C within a capillary that was premodified with 3-methacryloxypropyltrimethoxysilane. The time for UV irradiation was carefully determined so as to obtain a polymer coating of appropriate thickness. Enantioseparation of the racemate of propranolol was successfully demonstrated with a separation factor of 1.12 and a resolution factor of 1.26. 

Fig, 13.7. Batch-type in situ molecularly imprinted procedure for preparing a library of artificial antibody polymers. 

In situ molecular imprinting is a convenient way to prepare imprinted polymers. Here, imprinted polymers are prepared in a place where the polymers are subsequently utilized. In general, moleculariy imprinted polymers are prepared by bulk polymerization, and block polymers obtained are broken to pieces, ground, sieved and packed in a column. These experimental procedures are extremely tedious and time-consuming. The procedure also results in polymer particles of irregular size and shape, which may have a negative influence on column efficiency. 

Qu P, Lei JP, Ouyang RZ, Ju HX (2009) Enantiose-paration and amperometric detection of chiral compounds by in situ molecular imprinting on the microchannel wall. Anal Chem 81 9651-9656... 

Recently, an in-depth review on molecular imprinted membranes has been published by Piletsky et al. [4]. Four preparation strategies for MIP membranes can be distinguished (i) in-situ polymerization by bulk crosslinking (ii) preparation by dry phase inversion with a casting/solvent evaporation process [45-51] (iii) preparation by wet phase inversion with a casting/immersion precipitation [52-54] and (iv) surface imprinting. 

Several selective interactions by MIP membrane systems have been reported. For example, an L-phenylalanine imprinted membrane prepared by in-situ crosslinking polymerization showed different fluxes for various amino acids [44]. Yoshikawa et al. [51] have prepared molecular imprinted membranes from a membrane material which bears a tetrapeptide residue (DIDE resin (7)), using the dry phase inversion procedure. It was found that a membrane which contains an oligopeptide residue from an L-amino acid and is imprinted with an L-amino acid derivative, recognizes the L-isomer in preference to the corresponding D-isomer, and vice versa. Exceptional difference in sorption selectivity between theophylline and caffeine was observed for poly(acrylonitrile-co-acrylic acid) blend membranes prepared by the wet phase inversion technique [53]. 

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

New in situ techniques with enhancements in sensitivity and selectivity (sensors based on molecular imprinted polymers)... 

Molecularly imprinted polymers with a variety of shapes have also been prepared by polymerizing monoliths in molds. This in situ preparation of MIPs was utilized for filling of capillaries [20], columns [21], and membranes [22, 23]. Each specific particle geometry however needs optimization of the respective polymerization conditions while maintaining the correct conditions for successful imprinting. It would be advantageous to separate these two processes, e.g., to prepare a molecularly imprinted material in one step, which then can be processed in a mold process in a separate step to result the desired shape. 

Unfortunately, to date, this technique has received little attention from the molecular imprinting community and only one report of a dispersion polymerisation method had appeared until very recently [26]. This is probably better classified as a precipitation polymerisation, since random aggregates were produced rather than beads. No colloidal stabilisers were included in this procedure. The aggregates were made in situ in chromatography columns, which avoided the need to grind and sieve the polymer and pack the columns. Due to the rather polar nature of the solvent mixtures used (cyclohexanol, dodecanol, isopropanol), good imprints were only achieved for compounds which interact strongly with functional monomer... 

Fig. 13.3. In situ procedure for preparing molecularly imprinted polymer rods.

Another in situ preparation of molecularly imprinted columns employs dispersion polymerisation, whereby agglomerated polymer particles are obtained [16]. The procedure is similar to the rod preparation a mixture of the chemicals for the polymer preparation, such as a template, a functional monomer, a cross-linker, a porogen and an initiator is put in a column and heated to effect polymerisation. This method also requires polar solvents, such as cyclohexanol-dodecanol and isopropanol-water, to obtain aggregated polymer particles of well-defined micro-sises. A crucial difference with the rod preparation lies in the volume of the porogen used larger volumes of porogens are used in dispersion polymerisation. 

Batch use of imprinted polymers has been applied in the evaluation of polymers by saturation binding tests and in the applications of molecularly imprinted sorbent assays [1,23,24]. In a common procedure, imprinted polymers obtained as blocks were crushed, ground and sieved to prepare sized polymer particles. The resultant particles were then distributed into each vial and recovered by filtration after use. Recently, a new batch-type in situ procedure has been reported. It utilises a polymer coating prepared on an inner surface at the bottom of a vial and allows direct assessment of the polymers. In this section, this type of in situ preparation of imprinted polymers and an application to combinatorial chemistry are described. 

A molecularly imprinted polymer is an artificial antibody that is tailored for a specific molecule. The in situ technique introduces another attractive feature, in that the imprinted polymer is tailor-made for a specific application with a minimum of experimental steps, emphasising the original statement that imprinted polymers can be easily and simply prepared and applied. It is also shown that the in situ method is useful for designing a molecular imprinting system and would be helpful for mechanistic studies of molecular imprinting and recognition. Thus, advances in the basics and applications of molecular imprinting can be expected by this technique. 

The first reported preparation of capillary columns containing MIPs utilised a thermally initiated dispersion polymerisation procedure [58], The functional monomer MAA and the cross-linking monomer EDM A were used. Agglomerates of micrometre-sized globular polymer particles were claimed to be prepared in situ in the capillary. Molecular imprinting of L-phenylalanine anilide, pentamidine and benzamidine was undertaken. A pH-dependent retardation of pentamidine over benzamidine in the pentamidine capillary was observed, while the opposite. 

An alternative approach to effect chiral discrimination is to use the technique of molecular imprinting, the subject of this book. This technique, sometimes also referred to as template polymerisation, results in synthetic polymers of predetermined selectivity. Receptor-like binding sites are tailor-made in situ by the copolymerisation of cross-linkers and functional monomers, which are interacting with... 

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