Chapter 20
A Physicochemical Study on the Origin of the Imprinting Effect 1
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Tohru Miyajima , Kyoko Sohma , Shin-ichi Ishiguro , Masaki Ando , Shigeo Nakamura , Mizuo Maeda , and Makoto Takagi Downloaded by STANFORD UNIV GREEN LIBR on August 1, 2012 | http://pubs.acs.org Publication Date: May 7, 1998 | doi: 10.1021/bk-1998-0703.ch020
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Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Fukuoka 812-81, Japan Department of Chemical Science and Technology, Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 812-81, Japan
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The origin of the imprinting effects which have been reported for metal ion-templated microspheres with acidic functionalities is probed by examining the acid dissociation properties of non-templated carboxylated resins prepared by emulsion polymerization. The equilibria are compared with those of polyacrylic acid, a linear polymer analog. It is found that: a) a strong and variable electric field is formed at the resin surface due to two-dimensional distribution of the carboxyl groups; b) the salt concentration effect on the acid dissociation equilibria for the resin is much smaller than that for polyacrylic acid, probably because of restricted mobility of the carboxylate groups at the water/polymer interface; and c) the intrinsic acid dissocation reaction at the interface proceeds by a two-step reaction in which hydrogen bonds are formed between two adjacent carboxyl groups. These findings indicate a carboxyl group arrangement which is favorable for the metal ion imprinting. Despite rapid advances in the synthesis of templated polymers, fundamental studies on the origin of the imprinting effect are limited. A combination of several physicochemical factors, such as electrostatic and hydrophobic interactions as well as specific chemical bond formation between the host and guest molecules, e.g. in metal ion coordination, are believed to be of primary importance. To obtain microscopic information concerning such factors, a metal ion-templated resin system was selected for investigation. This system allows several experimental methods to be utilized in evaluating the complexation behavior and enables comparisons to be made with relevant linear polymer analogs which contain the same functionalities. In recent years, complexation by metal ion-templated resins has been extensively studies by our research group (7-9). Kido et al (7,2) first reported a template polymerization technique using oleic acid as the host monomer, divinylbenzene as the matrix-forming monomer, and C u as the metal ion. Rebinding of C u by the templated resin was studied by potentiometric titration. It was found that the complexation reaction was rapid and reversible and high selectivity was achieved with the Cu -templated resin compared to an analogous non-templated resin. Subsequently Tsukagoshi et al (3-5) prepared carboxylated microspheres from methacrylic acid, butyl acrylate, and styrene and demonstrated a surface imprint effect 2+
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©1998 American Chemical Society
In Molecular and Ionic Recognition with Imprinted Polymers; Bartsch, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
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for several metal ion species including C u . Koide et al. (6) utilized 10-(pvinylphenyl)decanoic acid as a host monomer instead of oleic acid and prepared C u imprinted resin which exhibited a surface template effect. Metal ion-templated resins with phosphate groups were prepared by Maeda et al. (7-9). In this case, the host monomers were phosphate surfactants, such as oleylphosphoric acid, oleyl phenylphosphoric acid, and dioleyl-phosphoric acid. The surface template effect observed for these metal ion-selective resins was attributed to multidentate coordination of surface-anchored metallophilic groups to the template metal ions. Since these surface-template resins may be classified as chelating resins, their complexation equilibria can be analyzed on the basis of conventional ion-exchange and polyelectrolyte theories. The major difference between ordinary crosslinked chelating resins and the present surface template resins is the dimensionalitiy of the polyelectrolytes. Polyion gels have a three-dimensional distribution of the functional groups, whereas the functional groups of a surface template resin are located in a two dimensional plane. In addition, the mobility of the functional groups on the twodimensional surface is expected to be considerably lower than that in a threedimensional resin which may result in higher selectivity. The present study was undertaken to obtain fundamental information on the origin of the imprinting effect by a careful examination of the acid dissociation behavior of a carboxylate resin prepared by emulsion polymerization. A non-templated resin was prepared since we believe that the most characteristic features for the imprinting effect are a high surface charge density and multidentate complexation behavior. The importance of these factors can be evaluated from equilibrium studies conducted with the non-templated resin. Experimental. Resin Synthesis. Purified divinylbenzene (20.0 g, Sankyo Chemical Industries, Ltd.), 2.0 g of potassium oleate, 0.20 g of the initiator 2,2'azobis(2,4dimethylvaleronitrile), and 40 mL of distilled water were mixed and stirred under nitrogen for eight hours at 25 °C. The polymer was filtered and washed with 1.0 M hydrochloric acid. This resultant H -form resin was washed with distilled water. The density and average diameter of the particles thus obtained were 1.019 g/mL and U x l O " cm, respectively. +
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Potentiometric Titration. A mixture of 2.0 g of the resin and 30 mL of 0.10 M sodium hydroxide + Cs M sodium chloride solution was titrated with a 0.10 M hydrochloric acid + Cs M sodium chloride solution under nitrogen at 25 °C. The equihbrium pH values were measured potentiometrically with an Orion 91-01 glass electrode and an Orion single junction reference electrode equipped with an Orion Ionalyzer 720A. Just before and after the resin titration, the electrochemical cell was calibrated by a Gran's plot (10). The titrations were conducted automatically with an auto-Buret (APB-118, Kyoto Electronics Manufacturing Company, Ltd., Tokyo) which was controlled by a personal computer. Results and Discussion +
Surface Charge Density. Due to the difficulty in dispersing the H -form resin in the aqueous solution, a back titration procedure was utilized. Since it was expected that a strong electric field would be formed on the resin surface, the titrations were carried out at quite high salt concentrations, i.e. Cs > 1 M , for determination of inflection points in the titration curves. A typical titration curve together with its firstderivative curve, i.e. d(pH)/d(V ) vs. V , obtained at Cs = 3.00 M are shown in Figure 1. The pH decreased with V (the volume of added HCI solution) and shows two inflection points, V and V . The pH decrease in the region 0
