Potential-modulated Fluorescence Spectroscopy Research Articles

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

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).

Potential-modulated fluorescence spectroscopy of zwitterionic and dicationic membrane-potential-sensitive dyes at the 1,2-dichloroethane/water interface

The previously introduced technique of potential-modulated fluorescence (PMF) spectroscopy was used to study the potential-induced fluorescence change of some different dyes at the polarized 1,2-dichloroethane (DCE)/water (W) interface. A zwitterionic dye (POLARIC 488PPS) showed a PMF response similar to that for the previously studied dye (di-4-ANEPPS) with the same ionic state, and the PMF response was likewise explained by the potential-dependent reorientation of the dye at the DCE/W interface. Though a monocationic dye (POLARIC 488PM) showed no distinct PMF signal, a dicationic dye (di-2-ANEPEQ) showed two relatively weak but detectable PMF signals at lower and higher potential. It has thus been found that the ionic state of a potential-sensitive dye strongly influences the potential-induced reorientation of the dye at the interface and consequently its PMF response. These results support the reorientation/solvatochromic mechanism proposed for "slow" dyes but do not necessarily exclude the electrochromic mechanism proposed for "fast" dyes. PMF spectroscopy would provide useful information on the design of slow dyes for the measurement of the resting potential of cell membranes.

Spectroelectrochemical analysis of ion-transfer and adsorption of the PAMAM dendrimer at a polarized liquid|liquid interface

Abstract The interfacial behavior of the fourth generation polyamidoamine (G4 PAMAM) dendrimer at a water|1,2-dichloroethane (DCE) interface was studied by cyclic voltammetry and potential modulated fluorescence (PMF) spectroscopy. Irregular voltammetric responses were observed at positively polarized interfaces. The cyclic voltammogram was strongly dependent on pH and on the concentrations of the G4 PAMAM dendrimer and the organic supporting electrolyte. PMF spectroscopy was successfully used to analyze the interfacial mechanism of the dendrimer by adding an anionic porphyrin derivative as a fluorescent probe. The results of the PMF measurements demonstrated that the G4 PAMAM dendrimer was transferred across the interface, a process that was accompanied by an adsorption step at pH 7. In contrast, under alkaline conditions, the adsorption process did not seem to be involved in the interfacial behavior.

Potential-Dependent Adsorption of Amphoteric Rhodamine Dyes at the Oil/Water Interface as Studied by Potential-Modulated Fluorescence Spectroscopy

Ion transfer and adsorption of amphoteric rhodamines, that is, Rhodamine B (RB), Rhodamine 19 (R19), and Rhodamine 110 (R110), and a cationic rhodamine, Rhodamine 123 (R123), at a polarized 1,2-dichloroethane/water (DCE/W) interface, were studied by means of cyclic voltammetry and potential-modulated fluorescence (PMF) spectroscopy. For all rhodamines, a well-defined voltammetric wave was obtained and the pH dependence of the reversible half-wave potential (i.e., midpoint potential) was investigated to prepare the ionic partition diagram. Theoretical considerations of the diagrams showed that the voltammetric waves obtained for the amphoteric rhodamines were not due to a simple transfer of the protonated form (R+) but due to the transfer of H+ facilitated by the amphoteric form (R±) in DCE (and partly in W for R110): H+(W) + R±(DCE or W) → R+(DCE). In PMF spectroscopy, the PMF signal due to the adsorption of R+ at the interface could be obtained for RB, R19, and R123, only when the Galvani potential difference across the interface ( φ) was lower than −0.14 V, suggesting a significant role of φ in the interfacial adsorption of the rhodamines. The PMF spectrum obtained for the rhodamines under these conditions suggested that the xanthene ring of the adsorbed species should be located in the DCE phase. Furthermore, the dependence of PMF on the angle of polarization of the excitation beam suggested that the longitudinal axis of the xanthene ring should tilt only by 20−25° with respect to the interface.

Transfer and adsorption of 1-pyrene sulfonate at the water ∣ 1,2-dichloroethane interface studied by potential modulated fluorescence spectroscopy

The transfer mechanism of 1-pyrene sulfonate anion (PSA −) across the polarized water ∣ 1,2-dichloroethane (DCE) interface was investigated by a combination of electrochemical techniques and potential modulated fluorescence (PMF) spectroscopy under total internal-reflection. The dependence of the cyclic voltammogram and the ac voltammogram on the PSA − concentration showed an apparent reversible ion transfer process. However, the PMF responses exhibited a complex dependence on the PSA − concentration, revealing adsorption and dimerization processes taking place at the interface. Analysis of the dynamic spectroelectrochemical responses suggests that PSA − is adsorbed at the interface prior to the transfer step. Upon transferring to the organic phase, PSA − appears to accumulate at the interface undergoing a dimerization reaction.

A Kinetic Model for Adsorption and Transfer of Ionic Species at Polarized Liquid|Liquid Interfaces as Studied by Potential Modulated Fluorescence Spectroscopy

Fundamental expressions for analyzing potential modulated fluorescence (PMF) responses were derived within the framework of a phenomenological model for adsorption and transfer of ionic species across polarized liquid|liquid interfaces. For small periodic perturbations of the Galvani potential difference, PMF signals can be linearized and the contribution of each process can be uncoupled in the frequency domain. The PMF response for kinetically controlled adsorption is expressed as a semicircle in the complex plane in which the characteristic frequency of maximum imaginary component is proportional to the adsorption and desorption rate constants. Considering that the potential dependence of adsorption exhibits opposite sign whether the process take place from the aqueous or organic phase, the corresponding PMF responses appear in different quadrants of the complex plane. The present model delivers useful diagnostic criteria for analyzing the nature of the various processes contributing to the periodic flu...

Adsorption Behavior of Charged Zinc Porphyrins at the Water/1,2-Dichloroethane Interface Studied by Potential Modulated Fluorescence Spectroscopy

The adsorption properties of ionic fluorescent dyes at the polarized water/1,2-dichloroethane interface were studied by potential modulated fluorescence (PMF) spectroscopy under total internal reflection. Analysis of the frequency-dependent fluorescence associated with modulation of the interfacial concentration of the ionic dyes proved to be a rather sensitive approach for separating interfacial phenomena from bulk responses. The combination of PMF and electrochemical techniques allows to uncover differences in the specific interfacial interactions of tris(2,2‘-bipyridyl)ruthenium(II) (Ru(bpy)32+), meso-tetrakis(N-methylpyridyl)porphyrinato zinc(II) (ZnTMPyP4+), and meso-tetrakis(p-sulfonatophenyl)porphyrinato zinc(II) (ZnTPPS4-). While Ru(bpy)32+ shows quasi-reversible ion transfer features, the charged zinc porphyrins exhibit adsorption properties at potential close to the transfer range. The anionic ZnTPPS4- appears to be adsorbed at the interface at potentials more positive than the formal transfer potential. On the other hand, the spectroelectrochemical data show that ZnTMPyP4+ is adsorbed at the interface at potentials either side of the formal transfer potential. Due to the difference in the potential dependence of the adsorption processes, PMF responses associated with interfacial accumulation from the aqueous side exhibit a different phase shift with respect to adsorption steps from the organic side. The experimental results clearly demonstrate that adsorption planes at the organic and aqueous side of the interface are physically distinguishable. Furthermore, PMF dependence on the polarization of the excitation beam allows to estimate average molecular orientation of the adsorbed species.