Imprinting effect

Fig. 17. Chromatographic confirmation of the imprinting effect. Applying (top) polymer ( anti-1 MIP ) imprinted with 11-a-hydroxyprogesterone ( T ),and (bottom) polymer ( anti-9 MIP ) imprinted with 11-deoxycortisol ( 9 ). Component 4 progesterone. Reprinted with permission from Ye L, Yu Y, Mosbach K (2001) Analyst 126 (Advance Article). Copyright 2001 The Royal Society of Chemistry... 

Rampey AM, Umpleby RJ, Rushton GT, Iseman JC, Shah RN, Shimizu KD. Characterization of the imprint effect and the influence of imprinting conditions on affinity, capacity, and heterogeneity in molecularly imprinted polymers using the Freundlich isotherm-affinity distribution analysis. Anal Chem 2004 76 1123-1133. 

A good imprinting effect relies on the formation of a rather rigid macromolecu-lar network, hence it requires highly crosslinked polymers therefore, any preparation method for beaded MIPs must be compatible with the presence of a high amount of crosslinker. 

A good imprinting effect also relies on the existence of stable assemblies between the template molecule and the functional monomers in the course of the polymerisation process therefore, any preparation method for beaded MIPs must preserve such template-monomer assemblies. 

The first study proving the possibility to obtain an imprinting effect in microgels was reported by the group of Wulff in 2001 [73]. In this investigation, highly... 

The nature of the functional monomer can sometimes be of importance. For example, if the MIP monolith is to be used as stationary phase for electrochromatography, the presence of charged groups may be essential for the generation of an electroosmotic flow. These can be simply generated by the use of MAA as the functional monomer [171], but in some cases a combination with other monomers was necessary to improve the imprinting effect [172,173]. 

Besides the choice of monomers and solvent, the ratio of template molecule to functional monomer not only affects the imprinting effect [174] but also the morphology of MIP monoliths. Several authors have observed differences in the monolith structure (polymer morphology, pore size distribution, flow characteristics) between the non-imprinted control polymer and the MIP, derived from the presence of the template [158,175]. For example, an MIP imprinted with ceramide III was compared... 

The preceding results indicated that subtle geometrical structures of amino acids and peptides were imprinted in Ti02-gel matrices. It is conceivable that the imprinting effect is not restricted to geometrical differentiations. 

The volume fractions of fully functional pores as a function of template size for the different preparation conditions are plotted in Fig. 2a. In contrast to the total number of imprinted pores, it appears as if ideal solution conditions hardly favor solvation of the functional sites on the templates, and any deviation from such conditions increases the imprinting effect. However, not only do optimum conditions exist for a given size of the templates (typified by a peak at a particular r), optimum conditions that determine the excluded volume of the monomers as well as the phase segregation tendency appear to exist. Comparison of curves in Fig. 2a show that for r >2, xv=5 proves to be more efficient than both Xv=2 and ifv=8. In addition, mixing interactions (/f<0) are favorable for small templates, while segregating interactions are favorable for larger templates. 

However, also in this case enantio-selectivities never exceeded the values obtained with the oxazaborolidine in solution, probably because of diffusional limitations within the polymer support, which enhanced the contribution of the non-selective, direct borane reduction of the ketone. In spite of the rather low imprinting effects obtained in these initial attempts, we feel that this approach still represents a most interesting application of molecularly imprinted polymers in catalysis and deserves further attention in the near future. 

Takagi s group also examined the origin of the imprinting effect through FT-IR and ESR [17]. Three types of Cu(II)-loaded microspheres were prepared and named Sample 1, 2, and 3 in the following way. Non-imprinted, as well as Cu(II)-imprinted microspheres (0.15 g) were added to a Cu(II) solution to make the Cu(II)-loaded microspheres. They were named Sample 1 and 2, respectively. In addition, Cu(II)-loaded microspheres, II in Scheme 9.6, were separated from the second-step polymerisation mixture by simple centrifugation. The obtained microspheres were dried under vacuum and named Sample 3. 

Cu(II) spectral intensities for Samples 1-3 were in the order of Sample 3 > Sample 2 > Sample 1. Although ESR spectrometry in general is semi-quantitative in nature, the order of the spectral intensity was consistent with the amount of Cu(II) on the microspheres as determined by the Cu(II) adsorption study. The imprinting effect or an enhanced Cu(II) adsorption behaviour as indicated by the metal adsorption test is in a sense indirect, because it only shows the decrease in Cu(II) concentration in a test solution. Meanwhile, the presented ESR spectra directly indicated such an enhanced Cu(II) adsorption and complex chemical structure of the adsorbed Cu(II) species in the Cu(II)-imprinted microspheres. 

The imprinting effects of MIPs prepared with optically active compounds as the print molecules are readily demonstrated by chromatographic evaluations. For example, when the L-enantiomer of an amino acid derivative is used as the print species, a column packed with the resulting polymer will retain the L-enantiomer longer than the o-enantiomer and vice versa when the o-enantiomer is used as the print molecule. Reference polymers prepared with the racemate or without print molecule will not be able to resolve the racemate. A steroselective memory is hence induced in the polymers by the print molecules and is in many cases very precise. 

To be able to attribute the binding of an MIP to an imprinting effect it is of utmost importance to show that specific recognition sites have been formed due to the presence of the print molecules during the polymerisation. This is usually done by comparisons with appropriate reference polymers. Polymers prepared without print molecules are not always the best choice, since the physical properties (surface area, porosity, etc.) of these polymers often differ from those of imprinted polymers. Reference polymers prepared with the optical antipode or a racemic mixture as the print species are preferred. The selectivity will be reversed when using the optical antipode and a racemic mixture will give a polymer incapable of separating the two enantiomers (unless the monomer(s) is/are chiral). 

There are many other conducting and redox polymers which could be investigated for imprinting effects. We expect this to become something of a growth area in the field. 

The adsorption analysis can be performed using the experimental sensor effect data from gravimetric QCM measurements. The frequency shift, which is proportional to the partial pressure of the analyte, is correlated to the number of specific incorporation sites in the linear range of the isotherm slope. Additional evidence for specific interactions between the analyte and the polymer matrix can be demonstrated using infrared spectroscopic analysis [19]. NMR [4,7] can also provide information about imprinting effects. 

The presented methods of examination focus on a comprehensive understanding of the imprinting effect. However, a reliable rule-based mechanism to predict the potential of target MIP structures for sensory applications is still not available and often, only experimental evaluations of MIPs provide useful rules of thumb for the development of selective coatings. 

Low-weight organic molecules, such as volatile organic compounds (VOCs) [25], e.g. hydrocarbons without functionalities or anaesthetics, can be used as print molecules for non-covalent MIPs. If the print molecule is a suitable organic solvent, the print molecule itself is the porogen during the polymerisation process. Enhanced imprinting effects are promoted by n-n interactions between aromatic moieties in monomers and analytes, such as polycyclic aromatic hydrocarbons (PAHs) or aromatic VOCs (xylene or toluene, for example). 

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