Imprint techniques

Some restrictions of this molecular imprinting technique are obvious. The template must be available in preparative amounts, it must be soluble in the monomer mixture, and it must be stable and unreactive under the conditions of the polymer-... 

A number of studies have recently been devoted to membrane applications [8, 100-102], Yoshikawa and co-workers developed an imprinting technique by casting membranes from a mixture of a Merrifield resin containing a grafted tetrapeptide and of linear co-polymers of acrylonitrile and styrene in the presence of amino acid derivatives as templates [103], The membranes were cast from a tetrahydrofuran (THF) solution and the template, usually N-protected d- or 1-tryptophan, removed by washing in more polar nonsolvents for the polymer (Fig. 6-17). Membrane applications using free amino acids revealed that only the imprinted membranes showed detectable permeation. Enantioselective electrodialysis with a maximum selectivity factor of ca. 7 could be reached, although this factor depended inversely on the flux rate [7]. Also, the transport mechanism in imprinted membranes is still poorly understood. 

As a consequence one might expect that the future needs to rely on hybrid elements which arise from advanced UV-and electron-beam lithography, from imprint techniques or automated and parallelized nanomanipulation techniques, like dip-pen lithography or focused ion-beam techniques in combination with supramolecular approaches for the assembly of molecular inorganic/organic hybrid system. Nevertheless, it is evident for any kind of chemical approach that falling back onto the present-day... 

Molecular imprinting has recently attracted considerable attention as an approach to the preparation of polymers containing recognition sites with predetermined selectivity. The history and specifics of the imprinting technique pioneered by Wulff in the 1970s have been detailed in several excellent review articles [122-124]. Imprinted monoliths have also received attention as stationary phases for capillary electrochromatography. 

Various novel imprinting techniques have also been presented recently. For instance, latex particles surfaces were imprinted with a cholesterol derivative in a core-shell emulsion polymerization. This was performed in a two-step procedure starting with polymerizing DVB over a polystyrene core followed by a second polymerization with a vinyl surfactant and a surfactant/cholesterol-hybrid molecule as monomer and template, respectively. The submicrometer particles did bind cholesterol in a mixture of 2-propanol (60%) and water [134]. Also new is a technique for the orientated immobilization of templates on silica surfaces [ 135]. Molecular imprinting was performed in this case by generating a polymer covering the silica as well as templates. This step was followed by the dissolution of the silica support with hydrofluoric acid. Theophylline selective MIP were obtained. 

Some limitations of this molecular imprinting technique are obvious the template must be available in preparative amounts, it must be soluble in the monomer mixture and it must be stable and unreactive under the conditions of the polymerization. The solvent must be chosen considering the stability of the monomer-template assemblies and it should result in the porous structure necessary for rapid kinetics of the template interaction with the binding sites. If these criteria are satisfied, a robust material capable of selectively rebinding the template can be easily prepared and evaluated in a short period of time. 

Stationary phases with specific molecular recognition properties for D,L-enantiomers of peptides have been tailored using the molecular imprinting technique. A template molecule is added to suitable monomer(s), the system is polymerized, and the chiral template molecule is washed out [128]. 

The most popular and commonly used chiral stationary phases (CSPs) are polysaccharides, cyclodextrins, macrocyclic glycopeptide antibiotics, Pirkle types, proteins, ligand exchangers, and crown ether based. The art of the chiral resolution on these CSPs has been discussed in detail in Chapters 2-8, respectively. Apart from these CSPs, the chiral resolutions of some racemic compounds have also been reported on other CSPs containing different chiral molecules and polymers. These other types of CSP are based on the use of chiral molecules such as alkaloids, amides, amines, acids, and synthetic polymers. These CSPs have proved to be very useful for the chiral resolutions due to some specific requirements. Moreover, the chiral resolution can be predicted on the CSPs obtained by the molecular imprinted techniques. The chiral resolution on these miscellaneous CSPs using liquid chromatography is discussed in this chapter. 

The CSPs prepared by the molecular imprint technique have also been used for chiral resolution by CEC [98-100]. Lin et al. [91] synthesized L-aromatic amino acid-imprinted polymers using azobisnitriles with either photoinitiators or thermal initiators at temperatures ranging from 4°C to 60° C. Methacrylic acid (MAA) was used as the functional monomer and ethylene glycol dimethacrylate (EDMA) was used as the cross-linker. The resulting polymers were ground and sieved to a particle size less than 10 pm, filled into the capillary columns, and used for enantiomeric separations of some amino acids at different temperatures. The relationships of separation factor and column temperatures were demonstrated to be linear between the logarithm of the separation factors and the inverse of the absolute temperature (Fig. 24). The authors also compared the obtained chiral resolution with the chiral resolution achieved by HPLC and reported the best resolution on CEC. The chromatograms of the chiral resolution of dl-... 

In spite of the development of more successful and reliable CSPs (Chaps. 2-8), these miscellaneous types of CSP have their role in the field of the chiral resolution also. The importance of these CSPs ties in the fact that they are readily available, inexpensive, and economic. Moreover, these CSPs can be used for some specific chiral resolution purpose. For example, the CSP based on the poly(triphenylmethyl methacrylate) polymer can be used for the chiral resolution of the racemic compounds which do not have any functional group. The CSPs based on the synthetic polymers are, generally, inert and, therefore, can be used with a variety of mobile phases. The development of CSPs based on the molecularly imprinted technique has resulted in various successful chiral resolutions. The importance and application of these imprinted CSPs lies in the fact that the chiral resolution can be predicted on these CSPs and, hence, the experimental conditions can be designed easily without greater efforts. Because of the ease of preparation and the inexpensive nature of these CSPs, they may be useful and effective CSPs for chiral resolution. Briefly, the future of these types of CSP, especially synthetic polymers and polymers prepared by the molecularly imprinted technique, is very bright and will increase in importance in the near future. 

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. 

MATSUI J, KATO T, TAKF.UCHT T, SUZUKI M, YOKOYAMA K, TAMIYA E and KARUBE I (1993), Molecular recognition in continuous polymer rods prepared by a molecular imprinting technique , Anal Chem, 65, 2223-2224. 

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... 

The imprinting technique resembles the formation of antibodies from haptens. Indeed, an imprinting-like mechanism was formerly postulated for the formation of... 

Some important metal oxide materials that have used molecular and supramole-cular templates to direct structure formation are the zeolites and related semi-crystalline aluminosilicates. In this section we shall discuss the use of ammonium cations that direct formation of microporous zeolites and finish with some of the possibilities that exist with the use of surfactant systems and molecular aggregates to create mesoporous structure. Excellent books and reviews are suggested for additional reading into the detailed description of the art [58-60]. The intention of this section is to briefly introduce this area and describe the types of materials being produced using various imprinting techniques in metal oxide materials. 

---