A molecularly imprinted polymer was prepared by a surface molecular imprinting technique in water-in-oil (W/O) emulsion. In this technique, the solid polymer, which is molecularly imprinted at the internal cavity surface, is prepared by polymerizing W/O emulsions consisting of a water-soluble imprinted molecule, a functional host molecule, an emulsion stabilizer, and a crosslinking agent. Dioleoyl phosphate was used as an emulsion stabilizer, and this compound also acted as a monomer and a host functional group in the imprinted cavity. Divinylbenzene was used as a crosslinker. Tryptophan methyl ester and phenylalanine methyl ester were used as the target template materials. These imprinted polymers exhibited enantiomeric selectivity in absorption experiments, and the maximum separation factor was 1.58. The enantiomeric selectivity with tryptophan methyl ester was higher than that with phenylalanine methyl ester. 1. Introduction Recently, significant attention has been paid to the development of a molecular imprinting technique that enables polymers to mimic biological receptors. This technique is a very useful approach for the fabrication of a matrix with molecular recognition sites, which are formed by the addition of template molecules during the matrix formation process and the removal of the template molecule after the matrix formation [1–5]. Polymers that were prepared by the molecular imprinting technique have attracted much attention as interesting separation tools, especially for high performance liquid chromatography (HPLC). The imprinting technique is conceptually easy to apply to a wide variety of target molecules. Important applications are optical resolutions of amino acids or amino acid derivatives [6–12], direct enantiomeric separation of drugs [13, 14], and separation of sugar or sugar derivatives [4, 15, 16]. The “surface molecular imprinting technique” was proposed to overcome the inapplicability to water-soluble substances, which are important in the biological or biomedical field [17–20]. In this technique, the solid polymer, which is molecularly imprinted at the internal cavity surface, is prepared by polymerizing water-in-oil (W/O) emulsions consisting of a water-soluble imprint molecule, a functional host molecule, an emulsion stabilizer, and a crosslinking agent. The organic-aqueous interface in W/O emulsions is utilized as the recognition field for a target molecule. The target molecule forms a complex with the functional host molecule, while the orientation of the functional host molecule itself is fixed at the oil-water
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