Potential-modulated Fluorescence Research Articles

Ion transfer mechanism of fluorescence-labeled octa-arginine on model biomembrane surfaces

The interfacial behavior of a synthesized fluorescent polycationic cell-penetrating peptide, octa-arginine covalently labeled with carboxyfluorescein (FAM-R8), was investigated through spectroelectrochemical analysis at the bare and phospholipid-modified water|1,2-dichloroethane (DCE) interfaces. The potential modulated fluorescence (PMF) analysis indicated that the adsorption of FAM-R8 from the aqueous side of the bare interface occurred in the wide potential range at lower FAM-R8 concentrations, while the ion transfer across the interface proceeded at the positive edge of the polarizable potential window as the concentration increased. The complicated interfacial reaction behavior of FAM-R8 was further observed at the biomimetic interface in the presence of a phospholipid layer, where the adsorption potential of FAM-R8 was positively shifted as compared with the bare interface. Moreover, the ion transfer process of FAM-R8 became predominant when the surface excess of FAM-R8 was substantially increased at the biomimetic interface. The FAM-R8 concentration and its interaction with the phospholipid layer were found to play an important role for the membrane permeation mechanism.

Ion transfer and adsorption of water-soluble metal complexes of 8-hydroxyquinoline derivatives at the water|1,2-dichloroethane interface

The transfer mechanism and adsorption state of water-soluble 8-quinolinolate complexes were studied at the water|1,2-dichloroethane interface by electrochemical and spectroelectrochemical techniques. The interfacial affinities of the metal complexes of 8-hydroxyquinoline-5-sulfonate (QS) were estimated as AlQS33−, CuQS22− >ZnQS22−. Potential modulated fluorescence spectroscopy revealed the potential-driven process of fluorescent QS complexes, where Al(III) and Zn(II) complexes were transferred across the interface accompanied by the adsorption at the aqueous side of the interface. The adsorption state and preferential molecular orientation of these complexes were analyzed in detail by polarization-modulation total internal reflection fluorescence (PM-TIRF) spectroscopy. The PM-TIRF results showed that the square-planar 1:2 complexes, AlQS2− and ZnQS22−, were oriented relatively in parallel to the interface and approximately identical to the aqueous species. The adsorption behavior of the Zn(II) complex of tridentate 8-hydroxyquinoline-2-carboxylate (QC) ligand was also investigated, and ZnQC22− exhibited a strong interfacial affinity with intermediate spectral features between the aqueous and organic species.

Mechanistic Analysis of Ion Association between Dendrigraft Poly-l-lysine and 8-Anilino-1-naphthalenesulfonate at Liquid|Liquid Interfaces.

Molecular association between biocompatible dendritic polymers, dendrigraft poly-l-lysines (DGLs), and an anionic fluorescent probe, 8-anilino-1-naphthalenesulfonate (ANS-), was studied at the polarized water|1,2-dichloroethane (DCE) interface. The fluorescence intensity of ANS measured in aqueous solution was enhanced by the coexistence of DGLs over a wide pH range (2 < pH < 10), where ANS and DGL exist as a monoanionic form and a polycation, respectively. The voltammetric responses indicated that the positively charged DGLs were adsorbed at the water|DCE interface, whereas ANS- was transferred across the interface accompanied by the adsorption process. The interfacial behavior of the DGL-ANS associates was analyzed by potential-modulated fluorescence (PMF) spectroscopy. The PMF results demonstrated that the ion association between DGLs and ANS at the water|DCE interface is strongly affected by the applied potential and the dendritic generation of DGL. By applying appropriate potentials, the ANS anion was dissociated from its ion associate with DGLs at the interface and transferred into the organic phase, whereas DGLs remained in the aqueous phase. The Gibbs free energy of ion association (Δ GD···ANS) was estimated for the second-fourth generation DGLs (DGL-G2-G4) and the G4 polyamidoamine (PAMAM) dendrimer as a control. The highest stability of the DGL-G4-ANS associate manifested itself through Δ GD···ANS: DGL-G4-ANS (>G4 PAMAM dendrimer-ANS) > DGL-G3-ANS > DGL-G2-ANS. The results elucidated the efficient anion-binding ability of higher generation DGLs and its potential dependence at the liquid|liquid interface.

Molecular association between flavin derivatives and dendritic polymers at the water|1,2-dichloroethane interface

The ion transfer and adsorption mechanism of flavin derivatives, riboflavin (RF) and flavin mononucleotide (FMN), at the polarized water|1,2-dichloroethane (DCE) interface were studied in the presence of the fourth generation (G4) amino-terminated polyamidoamine (PAMAM) dendrimer or hyperbranched bis-MPA polyester-64-hydroxyl (HBP). The flavin derivatives associated with the positively charged G4 PAMAM dendrimer both in the aqueous solution and at the water|DCE interface. Spectroelectrochemical analysis through potential-modulated fluorescence spectroscopy demonstrated that the dendrimer-bound flavin derivatives were transferred across the water|DCE interface in the positive potential region, while the interfacial adsorption of flavin derivatives in the negative potential region was effectively inhibited by the competitive adsorption of the neutral G4 HBP molecules.

Interfacial Behavior of Fluorescent Species Associated with Dendritic Polymers at Polarized Liquid|Liquid Interfaces

Charge transfer and adsorption reactions at interfaces between two immiscible electrolyte solutions (ITIES) have been investigated as a model of mass-transport and enzymatic reactions on biomembrane systems. In particular, the ion-partitioning property of bio-related materials in the liquid|liquid biphasic system is significantly important to evaluate their pharmacokinetics in vivo. Dendritic polymers such as dendrimer and hyperbranched polymer have unique properties as compared with conventional polymers. For instance, various organic molecules and metal ions have been reported to be accommodated into the internal cavity and captured on the peripheral region of dendrimers by hydrophobic or electrostatic interactions. The dendritic polymers have therefore the potential ability as the molecular capsule and/or carrier for the drug delivery systems (DDS), separation and detection regents. Recently, we have been reported that the interfacial association between the polyamidoamine (PAMAM) dendrimer and ionic species is strongly dependent on pH, the Galvani potential difference (Δw o φ) and generation of the dendrimer [1, 2]. In the present study, we investigated the interfacial behavior of flavin derivatives, i.e. riboflavin (RF) and flavin mononucleotide (FMN), in the presence of the generation 4 (G4) PAMAM dendrimer or hyperbranched bis-MPA polyster-64-hydroxyl (HBP) at the water|1,2-dichloroethane (DCE) interface. The interfacial mechanism was analyzed through the potential modulated fluorescence (PMF) spectroscopy [3]. In PMF measurements, the ion transfer and adsorption processes at ITIES are distinguishable through the complex analysis of ac fluorescence signals as a function of ac-potential modulation. The PMF responses obtained for RF were decreased by the addition of the PAMAM dendrimer. The real and imaginary components of PMF were obtained as positive and negative signs at Δw o φ = 0.33 V and vice versa at 0.36 V in the presence of the dendrimer. These PMF responses were associated with the ion transfer and adsorption of the dendrimer incorporating RF at the interface. In addition, the PMF responses arisen from the flavin derivatives were drastically weakened in the presence of HBP. These PMF results indicated that the PAMAM dendrimers effectively incorporate the flavin derivatives into the internal cavity. On the other hand, HBP strongly adsorbed at the interface inhibited the adsorption process of the flavin derivatives. The encapsulation behavior of anionic species in a cationic dendrigraft poly-L-lysine (DGL) was also investigated by using an anionic lipophilic probe, 8-anilino-1-naphthalenesulonic acid (ANS). The absorption and fluorescence spectra of ANS measured in aqueous solution were significantly affected by the addition of DGL, where the fluorescence intensity of ANS was increased in the presence of higher generation DGLs at 2 < pH < 10. In the PMF analysis, the real and imaginary components of PMF were obtained as negative and positive signs at the ion transfer potential of ANS−(Fig. 1). The PMF responses were negatively shifted by the third generation DGL (DGL-G3). These results indicated that ANS was effectively encapsulated in DGL even at the interface. H. Sakae, H. Nagatani, K. Morita, H. Imura, Langmuir, 30, 937 (2014).H. Sakae, H. Nagatani, H. Imura, Electrochim. Acta, 191, 631 (2016).H. Nagatani, T. Sagara, Anal. Sci., 23, 1041 (2007). Figure 1

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).

Ion transfer and adsorption behavior of ionizable drugs affected by PAMAM dendrimers at the water|1,2- dichloroethane interface

The transfer and adsorption reactions of ionizable drug molecules, i.e. dipyridamole (DIP), propranolol (PRO) and warfarin (WAR), at the water|1,2-dichloroehtane (DCE) interface were studied in the presence of the carboxylate- terminated generation 3.5 (G3.5) or amino-terminated generation 4 (G4) polyamidoamine (PAMAM) dendrimers. The ionic partition diagram of the ionizable drugs was determined through the voltammetric analysis of ion transfer responses. In the DIP system, the additional voltammetric responses associated with the interfacial adsorption were observed in the positive potential region. Although the spectroscopic features of the drug species in the aqueous solution were hardly affected by the addition of the dendrimers, the ion transfer currents in the DIP and PRO systems were decreased in the presence of the G3.5 PAMAM dendrimer indicating the intermolecular association between the cationic drugs and negatively charged dendrimers in the interfacial region. The interfacial mechanism of the fluorescent DIP species was investigated in detail by potential-modulated fluorescence (PMF) spectroscopy. The PMF results demonstrated that the monoprotonated form, HDIP+, was transferred across the water|DCE interface accompanied by the adsorption process. The interfacial mechanism of the DIP species was significantly modified by the dendrimer, depending on the pH condition. Under acidic conditions, the positively charged G3.5 PAMAM dendrimer adsorbed at the interface effectively prevented the coadsorption of HDIP+. At higher pHs, DIP (or HDIP+) interacted with the hydrophobic interior moiety (or negatively charged periphery) of the dendrimers.

Combined use of two membrane-potential-sensitive dyes for determination of the Galvani potential difference across a biomimetic oil/water interface

The fluorescence behavior of anionic membrane-potential-sensitive dyes, bis-(1,3-dibutylbarbituric acid) trimethine oxonol (DiBAC4(3)) and bis-(1,3-diethylthiobarbituric acid)trimethine oxonol (DiSBAC2(3)), at a biomimetic 1,2-dichloroethane (DCE)/water (W) interface was studied by the mean of potential-modulated fluorescence (PMF) spectroscopy. The respective dyes gave a well-defined PMF signal due to the adsorption/desorption at the DCE/W interface. It was also found that the potentials where the two dyes gave the PMF signals were different by about 100mV. We then attempted a combined use of the two dyes for determination of the Galvani potential difference across the DCE/W interface. When 40μM DiBAC4(3) and 15μM DiSBAC2(3) were initially added to the W phase, distinctly different spectra were obtained for different interfacial potentials. The ratio of the PMF signal intensities at 530 and 575nm (the fluorescence maximum wavelengths for the respective dyes) showed a clear dependence on the interfacial potential. These results suggested the potential utility of the combined use of two dyes for the determination of membrane potentials in vivo.

Spectroelectrochemical Characterization of Dendrimer–Porphyrin Associates at Polarized Liquid|Liquid Interfaces

Molecular encapsulation of anionic porphyrins in NH2-terminated polyamidoamine (PAMAM) dendrimers and the interfacial behavior of the dendrimer-porphyrin associates were studied at the polarized water|1,2-dichloroethane (DCE) interface. Formation of the ion associates was significantly dependent on the pH condition and on generation of dendrimers. 5,10,15,20-Tetrakis(4-sulfonatophenyl)porphyrin (ZnTPPS(4-)) associated with the positively charged fourth-generation (G4) PAMAM dendrimer was highly stabilized in acidic aqueous solution without protolytic demetalation in a wide range of pH values (pH > 2). In contrast to the zinc(II) complex, the free base porphyrin (H2TPPS(4-)) was readily protonated under acidic conditions even in the presence of the dendrimers. In addition, the J-aggregates of diprotonated species, (H4TPPS(2-))n, were preferably formed on the dendrimer. The interfacial mechanism of the dendrimer-porphyrin associates was analyzed in detail by potential-modulated fluorescence (PMF) spectroscopy. PMF results indicated that the dendrimers incorporating porphyrin molecules were transferred across the positively polarized water|DCE interface via adsorption step, whereas the transfer responses of the porphyrin ions released from the dendrimers were observed at negatively polarized conditions. A negative shift of the transfer potential of porphyrin ions compared to the intrinsic transfer potential was apparently observed for each ion association system. The ion association stability between the dendrimer and the porphyrin molecules could be estimated from a negative shift of the transfer potential. ZnTPPS(4-) exhibited relatively strong interaction with the higher generation dendrimer, whereas H2TPPS(4-) was less effectively associated with the dendrimers.

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.

Encapsulation of Anilinonaphthalenesulfonates in Carboxylate-Terminated PAMAM Dendrimer at the Polarized Water|1,2-Dichloroethane Interface

Molecular encapsulation of water-soluble anionic fluorescent dye molecules, 8-anilino-1-naphthalenesulfonic acid (ANS), and its bimolecular derivative (bis-ANS), in the generation 3.5 polyamidoamine (G3.5 PAMAM) dendrimer was investigated in the bulk aqueous phase and at the polarized water|1,2-dichloroethane interface. ANS(-) was electrostatically incorporated in the dendrimer, and the fluorescence enhancement with a blue shift of the emission maximum was observed at pH values <6, where the interior of the dendrimer was positively charged. The fluorescence enhancement of ANS was maximized around pH 3 and then decreased under more acidic conditions. The potential dependences of the molecular encapsulation and the interfacial mechanism were studied in detail by means of potential modulated fluorescence (PMF) spectroscopy. Under acidic conditions, the dendrimer incorporated ANS(-) at the positively polarized interface as well as in the aqueous phase. ANS(-) was released from the dendrimer at the intrinsic transfer potential and independently transferred across the interface. Bis-ANS exhibited relatively strong interaction with the dendrimer over a wide pH range (1 < pH < 8), and a negative shift of the transfer potential was observed under the corresponding pH condition. The PMF 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.

Potential-modulated fluorescence spectroscopy of the membrane potential-sensitive dye di-4-ANEPPS at the 1,2-dichloroethane/water interface

The spectrofluorometric behavior of a membrane potential-sensitive dye, 1-(3-sulfonatopropyl)-4-[beta-{2-(di-N-butylamino)-6-naphthyl}vinyl]pyridinium betaine (di-4-ANEPPS), at the polarized 1,2-dichloroethane/water interface was studied by means of potential-modulated fluorescence (PMF) spectroscopy. The results, combined with those from cyclic and alternating current voltammetry, clearly suggested that the dye adsorbed at the interface underwent a reorientation with increasing the interfacial potential, giving a well-developed PMF response as well as a voltammetric response. In addition to the PMF response, another PMF response was observed by addition of dilauroyl phosphatidylcholine (DLPC). This additional response was well explained in terms of a reorientation of di-4-ANEPPS at the interface, which would be induced by the potential-dependent desorption of DLPC from the interface. Thus, the present study supported the reorientation/solvatochromic mechanism for the membrane potential-sensitive dye rather than the electrochromic mechanism.

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-Modulation Spectroscopy at Solid/Liquid and Liquid/Liquid Interfaces

Potential-modulation spectroelectrochemical methods at solid/liquid and liquid/liquid interfaces are reviewed. After a brief summary of the basic features and advantages of the methods, practical applications of potential-modulation spectroscopy are demonstrated using our recent studies of solid/liquid and liquid/liquid interfaces, including reflection measurements for a redox protein on a modified gold electrode and fluorescence measurements for various dyes at a polarized water/1,2-dichloroethane interface. For both interfaces, the use of linearly polarized incident light enabled an estimation of the molecular orientation. The use of a potential-modulated transmission-absorption measurement for an optically transparent electrode with immobilized metal nanoparticles is also described. The ability of potential-modulated fluorescence spectroscopy to clearly elucidate the charge transfer and adsorption mechanisms at liquid/liquid interfaces is highlighted.

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.

Interfacial behavior of sulforhodamine 101 at the polarized water/1,2-dichloroethane interface studied by spectroelectrochemical techniques

The transfer mechanism of an amphoteric rhodamine, sulforhodamine 101 (SR101), across the polarized water/1,2-dichloroethane (DCE) interface was investigated using cyclic voltammetry, differential voltfluorometry and potential-modulated fluorescence (PMF) spectroscopy. The voltammetric response for the ion transfer of SR101 monoanion from water to DCE was observed as the diffusion-controlled transfer process. An unusual voltammetric response was found at 0.15 V more negative than the formal transfer potential of SR101(-) (deltaW(O)phi degrees') in the cyclic voltammogram and voltfluorogram. The frequency dependence of the PMF responses confirmed the presence of the adsorption processes at negative potentials. In addition, a further transient adsorption step was uncovered at deltaW(O)phi degrees'. The interfacial mechanism of SR101 is discussed by comparing the results obtained from each technique.

Direct spectroelectrochemical observation of interfacial species at the polarized water/1,2-dichloroethane interface by ac potential modulation technique

Abstract The ion transfer and adsorption behavior of the free base of water-soluble porphyrins were studied at the polarized water/1,2-dichloroethane interface by potential modulated fluorescence (PMF) spectroscopy. The PMF response indicated the presence of adsorption process for all systems depending on the Galvani potential difference. The adsorption from the organic side of the interface was found for cationic meso -tetrakis( N -methylpyridyl)porphyrin (H 2 TMPyP 4+ ) at potentials more negative than its formal ion transfer potential. The emission spectrum for the interfacial species could be obtained successfully by analyzing the dependence of PMF intensity on the wavelength, and the emission maximum wavelength of the interfacial species was significantly different from the bulk species measured in the aqueous and organic phases. It suggests that the solvation structure of interfacial species is modified from both the aqueous and organic bulk species. The presence of adsorption process for anionic porphyrin systems, meso -tetrakis(4-sulfonatophenyl)porphyrin and protoporphyrin IX, was also found by analyzing the PMF responses.

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.