Anal. Chem. 1999, 71, 842-848
Dynamic Fluorescence Spectroscopic Study on the Microstructures in Ion-Exchange Resin Particles Haeng-Boo Kim, Satoshi Habuchi, and Noboru Kitamura*
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
The fluorescence dynamics of Rhodamine B (RhB) and excitation energy transfer from RhB to Malachite Green (MG) in cation-exchange resin particles were studied with special reference to elucidating effects of the cross-linking density of the resin on the fluorescence dynamics. In the cation-exchange resin, the fluorescence lifetime and rotational relaxation time of RhB were very slow compared to those determined in an aqueous solution, suggesting strong binding of the dye to the ion-exchange group of the resin. Therefore, the spatial distribution characteristics of the dye in the resin particle was used as a measure for that of the ion-exchange group. The apparent structural dimension determined by the fractal dimension analysis of energy-transfer quenching of RhB by MG, representing the distribution characteristics of the dye in the resin, was smaller than Euclidean dimension (3) and decreased from 2.8 to 2.6 with decrease in the cross-linking density of the resin from 8 to 2%. The origin of the fractal-like distribution of the dye and, thus, that of the ion-exchange group was discussed. An ion exchanger is one of the fundamental materials for separation sciences and has been used widely in various fields.1 The most important class of ion exchangers is organic ionexchange resins since they show high performances in chemical and mechanical stabilities, an exchange capacity, and an ionexchange rate. So far, a variety of organic resins with different ion-exchange properties have been developed and used for various purposes such as purification of water, simultaneous collection and separation of metal ions, catalysts, dehydrating agents, and so on. In addition to such applications, the organic resins have been applied to separating biomaterials with relatively large molecular weights: peptides and proteins.2 Organic ion-exchange resins are commonly made of three-dimensionally cross-linked polymer chains (matrix), a typical example being a styrenedivinylbenzene copolymer. Although the matrix itself is inert to an ion-exchange reaction, the ion-exchange abilities of the resin (1) Helfferich, F. Ion Exchange; McGraw-Hill: New York, 1962. (b) Samuelson, O. Ion Exchange Separations in Analytical Chemistry; John Wiley: New York, 1963. (c) Harland, C. E. Ion Exchange: Theory and Practice; The Royal Society of Chemistry: Cambridge, 1994. (d) Seno, M., Abe, M., Suzuki, T., Eds. Ion Exchange; Kohdansha: Tokyo, 1993 (in Japanese). (2) For example: (a) Melis, S.; Markos, J.; Cao, G.; Morbidelli, M. Ind. Eng. Chem. Res. 1996, 35, 1912-1920. (b) Guan-Sajonz, H.; Sajonz, P.; Zhong, G.; Guiochon, G. Biotechnol. Prog. 1996, 12, 380-386. (c) Bowen, W. R.; Moran, E. B. Biotechnol. Bioeng. 1995, 48, 559-572.
842 Analytical Chemistry, Vol. 71, No. 4, February 15, 1999
can be tuned by a variation of the nature of an ion-exchange group, the cross-linking density of the matrix, and so forth. It is known that an ion-exchange process is controlled by diffusion of ions, either inside a resin particle or in the diffusion film surrounding a particle.1 For applications of resins to separation sciences and catalysis, an elucidation of the factors governing diffusion of an ion inside a resin is of primary importance. For example, the diffusion rate of an ion in a resin is expected to be influenced by the three-dimensional network of the polymer chain, so that microstructures of the matrix including ion/matrix and ion/ion-exchange group interactions are worth elucidating in detail. In contrast to inorganic exchangers such as a zeolite, organic resins made of flexible random networks of polymer chains are elastic. Furthermore, when an ion-exchange resin is immersed in water, the resin takes up a considerable amount of water leading to swelling of the polymer. Clearly, these properties make it very difficult to study microstructures in the resin and the relevant roles in ion-exchange processes. One of the promising methodologies to study microstructures in polymeric resins is a fluorescence probe method, since fluorescence properties of a probe molecule such as intensity, maximum wavelength, and decay kinetics are strongly dependent on microenvironments.3 Particularly, it is well-known that timeresolved measurements provide valuable information about microstructures. For example, the lifetime or rotational relaxation time of an excited molecule can sense the mobility of the probe molecule in a medium as well as interactions between the molecule and the surrounding medium. Excitation energy transfer has been also widely employed to study heterogeneous systems,4 since the separation distance and orientation between donor and acceptor molecules are very sensitive to microstructures.5 As one of the very important advantages of the energy-transfer technique, it is worth noting that information about the morphology of a medium and the distribution characteristics of a probe molecule can be obtained on the basis of the fractal dimension analysis of the fluorescence decay kinetics.6 Indeed, fractal dimension analysis has been applied successfully to studying various microhetero(3) Slavik, J. Fluorescent Probes in Cellular and Molecular Biology; CRC Press: Boca Raton, FL, 1994. (b) Winnik, M. A., Ed. Photophysical and Photochemical Tools in Polymer Science; Reidel: Dordrecht, The Netherlands, 1986. (4) Meer, B. W. van der; Coker, G., III; Chen, S.-Y. S. Resonance Energy Transfer. Theory and Data; VCH Publishers: New York, 1994. (b) Agranovich, V. M.; Galanin, M. D. Electronic Excitation Energy Transfer in Condensed Matter; North-Holland: New York, 1982. (5) Zhao, C.-L.; Wang, Y.; Hruska, Z.; Winnik, M. A. Macromolecules 1990, 23, 4082-4087. (b) Kim, H.-B.; Winnik, M. A. Macromolecules 1995, 28, 2033-2041. 10.1021/ac980868w CCC: $18.00
© 1999 American Chemical Society Published on Web 01/20/1999
geneous systems such as LB films,7 vesicles,8 latex beads,9 block polymers,10 silica,11 and so on,4,12 and accepted as a very useful method. Previously, we reported in situ measurements of ion-exchange/ diffusion processes of a cationic fluorescence dye (Rhodamine B, RhB) in single resin particles on the basis of laser trappingmicrospectroscopy and confocal fluorescence microspectroscopy techniques and demonstrated the spatial and temporal concentration profiles of the dye in single resin microparticles with diameters of 10-20 µm.13 Also, we showed that a technique of direct excitation energy transfer from RhB to Malachite Green (MG) was a powerful means to study ion-exchange/diffusion processes in cation-exchange resins.14 However, the study reported earlier was based on steady-state fluorescence measurements, so that detailed information about effects of microstructures of the resin on ion-exchange/diffusion processes could not be obtained. To clarify microstructures of the resin and the relevant roles in ion-exchange/diffusion processes, we studied the fluorescence dynamics of RhB in cation-exchange resins with special reference to elucidating effects of the cross-linking density of the resin on the fluorescence dynamics. On the basis of the results of fluorescence dynamics (lifetime and rotational relaxation time of RhB) and energy-transfer dynamics, we discuss microstructures of ion-exchange resin particles. EXPERIMENTAL SECTION Chemicals and Sample Preparation. The properties of the cation-exchange resins used in this study (Mitsubishi Chemicals, MCI-GEL CK0XS or CK0XA where X ) 2, 4, or 8) are summarized in Table 1. These resins are made of a styrene-divinylbenzene copolymer and possess SO3- groups as an ion-exchange site. X (2, 4, or 8) represents the cross-linking density of the resin (F in %), and an increase in X implies a decrease of the free volume in the polymer resin. The resins (Na+ form) were washed with water several times, until the pH of the washed solution became 7, and then dried in air at room temperature. The diameter of the resin particle was determined experimentally under an optical microscope. The size distribution of the particle was very narrow, as shown in Table 1, so that both single and ensemble measurements of the particles would provide analogous results. (6) Klafter, J.; Blumen, A. J. Chem. Phys. 1984, 80, 875-877. (b) Blumen, A.; Klafter, J.; Zumofen, G. J. Chem. Phys. 1986, 84, 1397-1401. (c) Levits, P.; Drake, J. M.; Klafter, J. J. Chem. Phys. 1988, 89, 5224-5236. (7) Yamazaki, I.; Tamai, N.; Yamazaki, T. J. Phys. Chem. 1990, 94, 516-525. (8) Tamai, N.; Yamazaki, T.; Yamazaki, I.; Mizuma, A.; Mataga, N. J. Phys. Chem. 1987, 91, 3503-3507. (9) Nakashima, K.; Duhamel, J.; Winnik, M. A. J. Phys. Chem. 1993, 97, 1070210707. (b) Pekcan, O ¨ .; Egan, L. S.; Winnik, M. A.; Croucher, M. D. Macromolecules 1990, 23, 2210-2216. (10) Tcherkasskaya, O.; Ni, S.; Winnik, M. A. Macromolecules 1996, 29, 610616. (b) Tcherkasskaya, O.; Ni, S.; Winnik, M. A. Macromolecules 1996, 29, 4241-4246. (c). Tcherkasskaya, O.; Spiro, J. G.; Ni, S.; Winnik, M. A. J. Phys. Chem. 1996, 100, 7114-7121. (d) Tcherkasskaya, O.; Ni, S.; Winnik, M. A. Macromolecules 1997, 30, 2623-2632. (e) Yekta, A.; Spiro, J. G.; Winnik, M. A. J. Phys. Chem. B 1998, 102, 7960-7970. (11) Pines-Rojanski, D.; Huppert, D.; Avnir, D. Chem. Phys. Lett. 1987, 139, 109115. (b) Rojanski, D.; Huppert, D.; Bale, H. D.; Dacai, X.; Schmidt, P. W.; Farin, D.; Seri-Levy, A.; Avnir, D. Phys. Rev. Lett. 1986, 56, 2505-2508. (12) Dewey, T. G.; Datta, M. Biophys. J. 1989, 56, 415-420. (13) Kim, H.-B.; Hayashi, M.; Nakatani, K.; Kitamura, N.; Sasaki, K.; Hotta, J.; Masuhara, H. Anal. Chem. 1996, 68, 409-414 (correction: 1996, 68, 1987). (b) Kitamura, N.; Hayashi, M.; Kim, H.-B.; Nakatani, K. Anal. Sci. 1996, 12, 49-54. (14) Kim, H.-B.; Habuchi, S.; Hayashi, M.; Kitamura, N. Anal. Chem. 1998, 70, 105-110.
Table 1. Properties of the Cation-Exchange Resins Used in This Study Fa/% CK08S CK04S CK02A
8 4 2
diameter dryb/µm wetb/µm 10.7 ( 1 8.5 ( 1 13.3 ( 1
11.6 ( 1 12.2 ( 1 21.6 ( 2
ion-exchange capacityc/ mequiv/mL (mequiv/particle) 2.02 (1.65 × 10-9) 1.24 (1.18 × 10-9) 0.70 (3.69 × 10-9)
a Cross-linking density. b Determined under an optical microscope in this study. c Taken from the data sheet supplied from Mitsubishi Chemicals.
Figure 1. Block diagram of a dynamic fluorescence spectroscopy system: OPA, optical parametric amplifier; SBC, Soleil-Babinet compensator; S, sample; P, polarlizer; DP, depolarlizer; MC, monochromator; PMT, photomultiplier; PD, pin photodiode.
RhB (Tokyo Kasei; Ace grade) and MG (Kanto Chemicals; G grade) were used as supplied. The purity of the dye was checked by comparing the molar absorptivity at a maximum wavelength with the reported value14 (RhB and MG) and also by the fluorescence lifetime in a dilute aqueous solution (RhB). Water was obtained with a Toraypure LV-08 (Toray; conductivity >17 MΩ cm). RhB-adsorbed resins (RhB-resin) were prepared by soaking each resin (2 mg, dry weight) in an aqueous RhB solution (100 mL, initial concentration [RhB]0 ) 2.3 × 10-8 M) for one week. After filtration, the RhB-resin was dried in air. A 2-mg sample of the RhB-resin was then soaked in 4 mL of an aqueous MG solution for 3 h. The concentration of the MG solution ([MG]0) was set at 1 × 10-6-6 × 10-6 M, depending on F of the resin. To avoid photodegradation of MG, the pH of the MG solution was adjusted to 4 by HCl. Spectroscopic Measurements. Fluorescence decay profiles of RhB in the resin were obtained by using a time-correlated single-photon counting system in Figure 1. Laser pulses (800 nm) from a mode-locked Ti:sapphire laser (Coherent, Mira model 900F), pumped by a diode laser (Verdi), were amplified by an Ar+ laser (Innova 300) pumped-regenerative amplifier (RegA model 9000). Optical parametric amplification (model 9400) of the output pulses gave 532-nm laser pulses (repetition rate, 100 kHz; fwhm, 200-fs autocorrelation trace) as an excitation light source. By using a Soleil-Babinet compensator (Melles Griot), the polarized direction of the excitation laser beam was set at a magic angle (35.3°) or vertical direction for fluorescence decay or dynamic anisotropy measurements, respectively. A polarizer was set in front of a detector system for dynamic anisotropy measurements. Fluorescence from the sample was detected by using a microchannelAnalytical Chemistry, Vol. 71, No. 4, February 15, 1999
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Table 2. Concentrations of RhB in the Resin Particles
CK08S CK04S CK02A
[RhB]0/10-8 M
absa
[RhB]Rb/10-4 M
2.3 2.3 2.3
0.063 0.043 0.033
4.9 3.2 1.4
a A 2-mg sample of the dry resin was soaked in 100 mL of an aqueous RhB solution. Absorbance of RhB in the particle was determined by a laser trapping-absorption microspectroscopy system reported earlier.13 b The RhB concentration in the particle was calculated on the basis of molar absorptivity at the maximum wavelength (565 nm; 1.1 × 105 M-1cm-1) and the diameter of the particle being assumed to be equal to the optical path length.
Figure 2. Fluorescence (RhB, dotted line) and absorption spectra (RhB, broken line; MG, solid line) of single, laser-trapped dyeadsorbed resin particles (CK08S) dispersed in water. The absorbance was normalized to that at the peak wavelength, and the absolute values were summarized in Tables 2 and 3.
Table 3. Concentrations of MG in the Resin Particles
CK08S CK04S
plate photomultiplier (Hamamatsu, R3809U-50) equipped with a monochromator (Jobin Ybon, H-20, monitor wavelength 585 nm) and analyzed by a single-photon counting module (Edinburgh Instruments, SPC-300). During spectroscopic measurements, an aqueous suspension (4 mL) of the dye-adsorbed resin (2 mg) in a quartz 1-cm cuvette was stirred gently by using a magnetic stirrer. The excitation pulse profile was measured at 532 nm by using dye-free resins as a scattering material. The response time of the system was ∼30 ps. Since the amount of the resin particles in the sample solution was very low and the size of the particle was as small as 10 µm, scattering of the excitation pulses by the particles did not disturb fluorescence decay profile measurements at 585 nm. Decay profiles were analyzed by using an iterative nonlinear least-squares deconvolution method. Absorption and fluorescence spectra of the dye-adsorbed single resin particles dispersed in water were measured by a laser trapping-microspectroscopy system reported earlier.13-15 All the experiments were performed under aerobic conditions in a temperature-controlled room (21 °C). RESULTS AND DISCUSSION Absorption and Fluorescence Spectra of the Dye in the Resin Particles: Interaction between the Dye and the IonExchange Group. Figure 2 shows absorption and fluorescence spectra of RhB and MG adsorbed on single, laser-trapped CK08S resin particles dispersed in water. The maximum wavelengths and band shapes of both absorption and fluorescence spectra were independent of the cross-linking density of the resin so that the data for CK04S and CK02A were not shown here. The fluorescence of MG in the resin was very weak and negligible compared to that of RhB. The absorption maximum wavelength of RhB (565 nm) or MG (637 nm) in the particle was shifted to a longer wavelength compared to that in the relevant dilute aqueous solution (554 or 617 nm for RhB or MG, respectively). The fluorescence maximum of RhB in the particle (592 nm) was also shifted to the red by 10 nm relative to that in water (582 nm). Despite the spectral shift of the dye in the resin, the band shapes observed in the resin and water agreed quite well with each other. An analogous spectral shift has been observed when RhB or MG (15) Kim, H.-B.; Yoshida, S.; Kitamura, N. Anal. Chem. 1998, 70, 51-57.
844 Analytical Chemistry, Vol. 71, No. 4, February 15, 1999
CK02A
[MG]0/10-6 M
absa
[MG]Rb/10-3 M
1.21 2.34 3.66 2.34 4.82 6.12 3.66 4.82 6.12
0.16 0.31 0.48 0.21 0.43 0.55 0.25 0.33 0.42
1.70 3.29 5.14 2.13 4.39 5.57 1.42 1.87 2.38
a A 2-mg sample of the RhB-resin was soaked in 4 mL of an aqueous MG solution. Absorbance of MG in the particle was determined by a laser trapping-absorption microspectroscopy system reported earlier.13 b The MG concentration in the particle was calculated on the basis of the molar absorptivity at the maximum wavelength (637 nm; 8.1 × 104 M-1 cm-1) and the diameter of the particle being assumed to be equal to the optical path length.
is adsorbed on a polystyrene-latex surface.9a The surface of the latex consists of both hydrophobic polystyrene domain and dispersely located sulfate groups originated from a radical polymerization initiator, Na2S2O8. Both RhB and MG are cationic dyes so that the sulfate group acts as a strong adsorption site for the dye, and this has been shown to be the primary origin of the spectral shift.9a The resins used in this study possess sulfonate groups as ion-exchange sites. Therefore, it is safely concluded that the red shifts of the absorption and fluorescence spectra in the particle are caused by binding of the dye to the ion-exchange group in the resin. Absorbance of RhB or MG at the maximum wavelength determined for single resin particles is summarized in Tables 2 and 3. The results in Table 2 demonstrate explicitly that the absorbance of RhB in the particle decreases from 0.063 to 0.033 with a decrease in the F value from 8 to 2%. An analogous tendency was observed for the MG-adsorbed resin, although a direct comparison between the data was difficult owing to the different [MG]0 for each sample (Table 3). The results are explained by swelling of the resin particles in water. On the basis of the data in Table 1, the swelling ratio of the particle defined as the ratio of the wet diameter to the dry diameter can be calculated to be 1.08, 1.43, or 1.62 for CK08S, CK04S, or CK02A, respectively. These values indicate that the lower the F of the resin, the larger is the swelling ratio. This implies that the amount of water taken up by the resin particle increases with decreasing F value, which might influence diffusion and/or molecular motions of the dye in the resin, as discussed later. Under the present experimental conditions, the dye is distributed uniformly in the whole of the resin
particle as reported previously.13a,15 Thus, the dye concentration in the single resin particle ([RhB]R) can be calculated as the data are included in Table 2. The [RhB]R value corresponds to the amount of RhB in each particle: 4.0 × 10-16, 3.0 × 10-16, or 7.3 × 10-16 mol/particle for CK08S, CK04S, or CK02A, respectively. Similarly, the amount of MG in each particle has been estimated from [MG]R in Table 3 to be on the order of 10-15 mol/particle. The above calculations demonstrate that the total amount of the dye (RhB and MG) adsorbed on each particle is far below the ion-exchange capacity of the resin (Table 1). Nonetheless, the concentration of the dye in the particle is on the order of 10-4 M (Tables 2 and 3), which may not be dilute enough to prevent interactions between the dye molecules. It is known that RhB or MG in water is likely to produce a ground-state dimer showing a new absorption maximum at around 520 or 580 nm, respectively. The dimer formation constant (K) has been also reported to be 1500 or 140 M-1 for a RhB-16 or MG-dimer,17 respectively. If one assumes that the environment around RhB molecules in the resin particle is similar to that in water, the K value predicts that ∼10% of the RhB molecules in the particle produces a dimer. However, dimer formation of RhB is not confirmed from the absorption spectrum in Figure 2. The same conclusion is obtained for MG in the resin. The absence of dimer formation indicates that the dye molecules are distributed at isolated sites in the resin, probably because of binding to the ion-exchange group. As a crude estimation, the average separation distance between the ionexchange groups is 13-10 Å as calculated from the ion-exchange capacity (0.7-2.0 mequiv/mL in Table 1). In the actual experiments, the amount of the dye doped in the resin is below the ion-exchange capacity as described above, so that the distance between the dye molecules will be larger than this value. Nevertheless, the average distance of 10-13 Å suggests the possibility of dimer formation even at a lower concentration than that calculated from the ion-exchange capacity, since the dye molecules are supposed to bind strongly to the ion-exchange groups. The absence of dimer formation will thus be indicative of a random distribution of the sulfonate groups in the resin. Fluorescence Dynamics of RhB in the Resin Particle: Molecular Motion of RhB. The fluorescence dynamics of RhB adsorbed on the resin particles dispersed in water has been also studied (not single-particle measurement18); a typical example (CK08S) is shown in Figure 3a together with that in water (b). Each decay profile was fitted satisfactorily by a single-exponential function. The fluorescence lifetime of RhB in the resin was independent of F and that in the resin or water was determined to be 3.8 or 1.7 ns, respectively. It has been reported that RhB in an adsorbed state shows a longer fluorescence lifetime compared to that in a homogeneous solution owing to suppression of the internal rotation of the diethylamino group in RhB. For example, Nakashima et al. reported that the lifetime of RhB adsorbed on polystyrene-latex particles was 3.6 ns9a and Avnir et al. demonstrated that adsorption of RhB on porous silica surfaces rendered (16) Selwyn, J. E.; Steinfeld, J. I. J. Phys. Chem. 1972, 76, 762-774. (17) Yao, H.; Inoue, H.; Ikeda, K.; Nakatani, K.; Kim, H.-B.; Kitamura, N. J. Phys. Chem. 1996, 100, 1494-1497. (18) Fluorescence lifetime measurements on RhB in single resin particles were performed by a laser trapping-time-resolved fluorescence spectroscopy method, whose detail will be reported in a separate publication (Kim, H.B.; Habuchi, S.; Kitamura, N., in preparation). The fluorescence lifetime determined was independent of the size of the particle and F.
Figure 3. Fluorescence decay profiles of RhB in (a) the resin particles (CK08S, dispersed in water) and (b) water. The solid curve shows the best fit to a single-exponential function.
Figure 4. Fluorescence anisotropy decay profiles of RhB in (a) the resin particles (CK08S, dispersed in water) and (b) water. The solid curve shows the best fit by eq 1.
the lifetime to be 3.2-4.8 ns.11 Therefore, the relatively long fluorescence lifetime of RhB in the cation-exchange resin indicates strong binding of the dye to the ion-exchange group, analogous to adsorption on the anionic sites of a latex or porous silica. Further information about the molecular motions of RhB or interactions between the dye and the ion-exchange group can be obtained through a fluorescence depolarization study. Fluorescence depolarization is one of the powerful techniques for studying the tumbling or rotational motion of a fluorescent molecule in a picosecond to nanosecond time regime and has been applied to studying microstructures in various heterogeneous systems.3 Dynamic fluorescence anisotropy (r(t)) is defined as in eq 1, where
r(t) ) (i|(t) - i⊥(t))/(i|(t) + 2i⊥(t)) ) r0 exp(-t/τrot) (1) i|(t) and i⊥(t) are the parallel and perpendicular components of a fluorescence decay, respectively, r0 is initial anisotropy () r(0)), and τrot is the rotational correlation time of a probe molecule. As a typical example, the result obtained for the RhB-CK08S sample is shown in Figure 4a together with that determined in water (b). In water, r(t) was best fitted by a single-exponential function, giving τrot ) 0.19 ns and r0 ) 0.39. The values agreed Analytical Chemistry, Vol. 71, No. 4, February 15, 1999
845
very well with the reported values (τrot ) 0.18 ns and r0 ) 0.38).19 As can be seen in Figure 4, on the other hand, r(t) obtained for RhB-CK08S did not decay over 10 ns after excitation and was constant at r0 ) 0.38 in the time range studied.20 Analogous results were obtained for CK04S and CK02A. It is noteworthy that the excited lifetime of RhB in the resin is 3.8 ns. The results in Figure 4 clearly indicate that the rotational motion of RhB is strongly inhibited in the resin, probably due to tight binding of RhB to the sulfonate group. All of the results mentioned above support strong interactions of the dye with the ion-exchange group. Furthermore, the singleexponential fluorescence decay in Figure 3 and the absence of a fast decay component in r(t) (Figure 4) indicate that a contribution of free dye molecules, not bound to the ion-exchange group, to the fluorescence characteristics is negligibly small. Thus, we conclude that most of the dye molecules are bound strongly to the ion-exchange groups in the resin and its molecular motion is inhibited completely, at least within the excited-state lifetime of the dye. It is important to note that the conclusion is valid for all the samples irrespective of F of the resin. The binding mode of the dye to the ion-exchange group is concluded to be not affected by the nature of the matrix itself: F and the swelling ratio. Analysis of Excitation Energy-Transfer Dynamics in the Resin Particles. Our previous experiments by steady-state fluorescence spectroscopy demonstrated that excitation energy transfer from RhB to MG could be used as a direct measure of the ion-exchange/diffusion processes of the ion in resin particles.14 To obtain further detailed information about the microstructures in the resin particle, therefore, we performed dynamic measurements on energy transfer. As discussed in the previous sections, the dyes are strongly bound to the ion-exchange groups. In such a case, energy-transfer dynamics (i.e., decay profile of an energy donor; ID(t)) cannot be analyzed by a simple exponential function, but should be discussed based on a modified Klafter-Blumen equation,6,9 fractal dimension analysis.
ID(t) ) B1 exp[-(t/τD) - P(t/τD)dh/6] + B2 exp(-t/τD) (2)
The first term of the right-hand side in eq 2 represents the contribution of energy-transfer quenching to the overall decay profile (ID(t)), while the second term corresponds to the decay component not influenced by the acceptor. B1 and B2 are the fraction of each decay component: B1 + B2 ) 1. τD is the excitedstate lifetime of a donor (RhB) in the absence of MG. P is a term related to the acceptor concentration in the resin. dh is a concentration-independent parameter sensitive to the local geometry of the donor-acceptor distribution in the resin. The B1 term in eq 2 has been originally derived to describe energy-transfer dynamics on fractal lattices in restricted geometries, and dh is equal to a fractal dimension. Thus, dh is often referred as the apparent structural dimension of a system and, the dh and P parameters involve information about the spatial distribution characteristics (19) Visser, A. J. W. G.; Vos, K.; Van Hoek, A.; Santema, J. S. J. Phys. Chem. 1988, 92, 759-765. (20) As a simple estimation, the rotational correlation time of the resin particle itself with the diameter of ∼10 µm is on the order of 1 s. On the nanosecond time scale, therefore, the rotational motion of the resin particle itself does not influence fluorescence dynamic anisotropy.
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Analytical Chemistry, Vol. 71, No. 4, February 15, 1999
Figure 5. Fluorescence decay profiles of RhB in the resin particles (CK08S, dispersed in water): (a) [MG]R ) 0, (b) 1.70 × 10-3, (c) 3.29 × 10-3, and (d) 5.14 × 10-3 M. The solid curve shows the best fit by eq 2, and the fitting parameters are summarized in Table 4. Table 4. Fractal Dimension Analysis of Excitation Energy-Transfer Quenching of RhB by MG in the Cation-Exchange Resinsa
CK08S CK04S CK02A
[MG]R/10-3 M
B1
P
dh
χ2
1.70 3.29 5.14 2.13 4.39 5.57 1.42 1.87 2.38
1.0 1.0 1.0 1.0 1.0 1.0 0.99 0.99 1.0
0.67 1.18 2.09 0.99 1.87 2.22 0.89 1.03 1.27
2.81 2.87 2.84 2.72 2.67 2.69 2.52 2.54 2.63
0.989 1.015 1.095 0.922 1.114 1.099 1.075 0.986 1.039
a The decay curves were analyzed by using an iterative nonlinear least-squares deconvolution program. The accuracy of the fitting was checked by the weighted residuals between calculated and experimental fluorescence decay profiles, the autocorrelation trace of the residuals, and the χ2 - dh plot. See also the main text and Figure 7.
of RhB and MG in the resin particle. On the basis of the fractal dimension analysis, the microstructures in the resin can be discussed. A typical example of the decay profiles of RhB in the resin particles coadsorbed with MG is shown in Figure 5. With increasing MG concentration ([MG]R), the RhB fluorescence decayed faster and the deviation of the decay profile from a singleexponential function became obvious, indicating the energytransfer mechanism in a confined geometry as predicted from eq 2. Actually, a fitting of the decay profile by eq 2 was very successful, and the parameters obtained by the fitting were summarized in Table 4. The important findings are as follows. First, B1 was almost unity, demonstrating that almost of all the excited RhB molecules were under the influence of energy-transfer quenching by MG. In fact, even when the decay profile was fitted with B2 ) 0, the parameters (P and dh) agreed with those in Table 4. Second, dh observed for the sample with a given F value was less than the Euclidean dimension (3) and almost constant within an experimental error irrespective of [MG]R. Third, dh was smaller for the resin with a lower F value: 2.84 ( 0.03, 2.69 ( 0.03, and 2.56 ( 0.05 for CK08S, CK04S, and CK02A, respectively. The latter two results are very important to discuss the microstructures in the resin.
Figure 6. Relationship between P and A1 for (a) CK08S, (b) CK04S, and (c) CK02A. A1 was calculated from [MG]R and the ion-exchange capacity of each resin. The solid lines show one calculated from eq 3.
The validity of the present analysis and the reliability of the observed parameters are worth checking before discussion of the dh values. Since we know [MG]R and the ion-exchange capacity of the resin, the fraction of the ion-exchange site occupied by the acceptor (A1) can be calculated. According to the theory,6 this is given by eq 3, where R0, a, and d are the Fo¨rster distance for
P ) A1(d/dh)Γ(1 - dh/6)(R0/a)dh
(3)
energy transfer (80 Å), the effective molecular radius of RhB (6 Å),10 and the Euclidean dimension, respectively. Equation 3 implies that the parameter P should increase linearly with an increase in A1 if dh is independent of A1. In Figure 6, the P value was plotted against the relevant A1 value. The plot clearly shows a linear relationship between P and A1 for each resin. Furthermore, the relationship between P and A1 calculated from eq 3 (shown by the solid line in Figure 6) agreed very well with the experimental one. These results verify the validity of the present analysis by eq 2. Since R0 and a are constant, it is obvious from Figure 6 and eq 3 that the slope of the plot is determined by dh, and the difference in the slope value between three samples demonstrates the F dependence of dh. A discussion of an error in determining dh is also very important for further arguments on the results. Tcherkasskaya et al.10c reported that an examination of a relationship between the χ2 parameter of a decay fitting and the relevant dh value was very useful to obtain the optimum set of the fitting parameters in eq 2. In the actual procedures, we analyzed the decay profile with a certain dh value to obtain the best fit values of B1, B2, and P. A similar analysis of the profile was repeated with various dh values to obtain a relation between and dh the χ2 values for the relevant fitting. When energy-transfer quenching of RhB proceeds inefficiently, as in the case with a low acceptor (MG) concentration, a χ2 - dh plot does not show a minimum. If energy-transfer quenching takes place efficiently, on the other hand, the plot shows a minimum value, by which a unique dh value can be obtained.10c The χ2 - dh plots obtained for the present low [MG]R samples are shown in Figure 7. The plot demonstrates clearly a minimum χ2 value, by which a unique dh value can be determined. The value determined by such the procedures agreed very well
Figure 7. Relationship between χ2 and d h : open circle, CK08S, [MG]R ) 1.7 × 10-3 M; closed circle, CK02A, [MG]R ) 1.42 × 10-3 M.
with that in Table 4. The plots obtained for other samples having larger [MG]R also gave clear minimums and, thus, we checked the validity of the dh value. Judging from the results in Figures 6 and 7, we concluded that the variation of dh in 2.84-2.56 with the nature of the sample (F) is meaningful and worth discussing in detail. Distribution Characteristics of the Ion-Exchange Group in the Resin Particle. Organic ion-exchange resins are commonly made of three-dimensional cross-linked polymer networks, and the space among the polymer network (i.e., free volume) is filled by water. Owing to such structures, ion-exchange groups attached to the polymer will be exposed to the water pool in the resin. In the present case, the dye molecule is strongly bound to the ion-exchange group, so that the distribution characteristics of the dye should reflect that of the ion-exchange group in the resin. Therefore, the microstructures in the resin particle can be discussed on the basis of the dh value. The dh value obtained for each resin was smaller than 3 (Euclidean dimension) and approached 3 with increasing F of the resin. In the case of porous silica adsorbed with RhB and MG, Avnir et al. reported that dh was strongly dependent on the pore size of silica and decreased with increasing pore size; dh is 2.82, 2.57, 2.36, and 2.05 for silica having pore diameters of 60, 100, 500, and 5000 Å, respectively.11 The pore size dependence of dh was explained in terms of the spatial irregularity of the pore surface in the scale of the Fo¨rster distance (∼100 Å). In the present case, a decrease in F brings about an increase of the size of the water pool in the resin, and the water pool size increases further upon swelling. Thus, the decrease in dh with decreasing F value is very similar to the results on porous silica, though the chemical and physical properties of silica are essentially different from those of the ion-exchange resin. As a very simple model, one might consider the following explanation for the water pool size dependence of dh. The ion-exchange group is exposed to the water pool, and both RhB and MG are bound tightly to these groups as discussed above. This implies that the dye molecules are located at the periphery of the water pool and excitation energy transfer takes place under such conditions. If one assumes that the water pool is spherical for simplicity, the curvature of the pool becomes relatively flat with increasing water pool size in respect to the molecular size of the dye. Thus, with decreasing F of the resin, the energy-transfer mechanism becomes gradually twoAnalytical Chemistry, Vol. 71, No. 4, February 15, 1999
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dimension-like, which is what is observed in this study. Although this is an oversimplified picture, the decrease in dh with the decrease in F would be explained by such a model. CONCLUSIONS In this paper, we demonstrated that time-resolved fluorescence spectroscopy could provide valuable information about the microstructures of an ion-exchange resin as well as about interactions between an ion and an ion-exchange group. All the results indicated tight binding of RhB and MG to the ion-exchange groups irrespective of the cross-linking density and the degree of swelling of the resin. The fractal dimension analysis of the energy-transfer quenching data demonstrated that the spatial distribution of the ion-exchange group in the resin particle was dependent on the cross-linking density. Such information cannot be obtained by conventional methods, so that we think that the present spectro-
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scopic approach has potential to understand the role of the microstructures in ion-exchange processes. To clarify relationships between the ion-exchange processes and the microstructures of the resins, further investigation is now in progress in our group. ACKNOWLEDGMENT The authors are grateful for Grant-in-Aid from the Ministry of Education, Science, Sports and Culture, Japan (No. 10440218 to H.-B.K. and No. 08404051 to N.K.) for partial support of the research.
Received for review August 4, 1998. Accepted December 5, 1998. AC980868W
