Molecular Encapsulation and Association of Ionic Species with Dendrimers at Polarized Liquid|Liquid Interfaces

Abstract

Charge transfer and ion partitioning mechanisms at an interface between two immiscible electrolyte solutions (ITIES) have been extensively studied for separation sciences, pharmacokinetic analysis, and mass transport in vivo. Dendrimers are unique and nontraditional polymers with a well-defined macromolecular architecture consisting of a core, iterative branch units (interior), and terminal groups. Dendrimers have been demonstrated to be capable of encapsulating various molecules and examined as molecular capsule or container, in which a variety of organic molecules and metal ions can be accommodated in the internal cavity through electrostatic and hydrophobic interactions. The most widely studied poly(amidoamine) (PAMAM) dendrimer is constructed based on an ethylenediamine core and amidoamine branch units with various terminal groups. The net charge on the PAMAM dendrimer depends on the protonation equilibria of tertiary amines in the interior and terminal groups. The electrostatic interaction between the dendrimer and ionic species, thus, is considerably affected by the pH condition. This talk will present the characteristic interfacial behavior of amino- and carboxylate-terminated PAMAM dendrimers at the polarized water|1,2-dichloroethane (DCE) interface. The fundamental ion transfer reaction of the dendrimer was successfully analyzed by means of conventional voltammetric techniques and potential modulated fluorescence spectroscopy [1]. The spectroelectrochemical analysis clearly demonstrated that the interfacial mechanism of the dendrimer involves transfer and adsorption processes depending on the pH condition and the Galvani potential difference [2]. The molecular encapsulation and association of fluorescent species such as anilinonaphthalenesulfonates, porphyrin derivatives and ionizable drugs with the PAMAM dendrimer were investigated in detail at ITIES. The anionic dye species exhibited electrostatic association behavior with the positively charged dendrimer and the stability of the dendrimer-anionic dye associates was varied as a function of pH [3]. Although the dendrimers stably associated with anionic dye species under acidic conditions, the dye anions were released from the dendrimer at its transfer potential and then transferred across the interface. A negative shift of the transfer potential of dye anion compared to its intrinsic transfer potential was observed for each ion association system. The ion association stability between the dendrimer and dye species was quantitatively estimated from the shift of the transfer potential. The photoreactivity of the dendrimer−zinc(II) porphyrin associates was also studied at the polarized water|DCE interface [4]. The photocurrent generated through the heterogeneous photoreduction of the water-soluble zinc(II) porphyrin by a lipophilic ferrocene derivative in the organic phase was drastically enhanced by the formation of the dendrimer−zinc(II) porphyrin associates. Furthermore, the association property of several ionizable drug molecules with the dendrimer was examined at ITIES. While the spectroscopic data exhibited no specific association between ionizable drugs and the dendrimers in the bulk solution phase, the interfacial behavior of drug molecules was affected by the addition of the dendrimers, indicating that the dendrimers interact with the drug molecules only in the interfacial region [5]. These findings demonstrated that the ionic-partitioning and interfacial reactivity of ionic species can readily be modified through the ion association with the dendrimer. In particular, the membrane permeability and reactivity of ionizable drugs could be controlled by the dendrimer. References (a) H. Nagatani, “In Situ Spectroscopic Characterization of Porphyrins at Liquid Interfaces”, in Handbook of Porphyrin Science, Vol. 34, K. M. Kadish, K. M. Smith, R. Guilard, eds., World Scientific Publishing, Singapore, Chapter 176 (2014); (b) H. Nagatani, T. Sagara, Anal. Sci., 23, 1041 (2007). H. Nagatani, T. Ueno, T. Sagara, Electrochim. Acta, 53, 6428 (2008). (a) H. Nagatani, T. Sakamoto, T. Torikai, T. Sagara, Langmuir, 26, 17686 (2010); (b) H. Sakae, H. Nagatani, K. Morita, H. Imura, Langmuir, 30, 937 (2014). H. Nagatani, H. Sakae, T. Torikai, T. Sagara, H. Imura, Langmuir, 31, 6237 (2015). H. Sakae, H. Nagatani, H. Imura, Electrochim. Acta, 191, 631 (2016).