Photoregulation of Ion Permeation through a Polyelectrolyte Multilayer

Dec 29, 2008 - The tilt angle of azobenzene chromophores in amorphous, liquid crystalline, and Langmuir−Blodgett films can be manipulated in any dir...
0 downloads 0 Views 993KB Size
Download PDF
Langmuir 2009, 25, 1767-1771

1767

Photoregulation of Ion Permeation through a Polyelectrolyte Multilayer Membrane by Manipulating the Chromophore Orientation Surjith K. Kumar,† Jousheed Pennakalathil,† Tae-Hyun Kim,† Kyuwon Kim,† Jong-Ku Park,‡ and Jong-Dal Hong*,† Department of Chemistry, UniVersity of Incheon, 177 Dohwa-dong, Nam-gu, Incheon 402-749, Korea, and Korea Institute of Science and Technology, Seoul 130-650, Korea ReceiVed October 8, 2008. ReVised Manuscript ReceiVed NoVember 19, 2008 Toward the realization of nanoscale device control, we report a novel method for photoregulation of ion flux through a polyelectrolyte multilayer membrane by chromophore orientation that is adjusted either by illumination at normal incidence or by slantwise irradiation at an angle of 10° with respect to the surface. Our results indicate that the chromophore reorientation caused by the slantwise irradiation controls the effective pore size and, consequently, the transport behavior on the nanoscale. The slantwise illumination, which includes six EZE photoisomerization cycles generated by alternately irradiating with ultraviolet (λ ) 360 nm) and visible (λ ) 450 nm) light, reversibly switches the orientation of E-azobenzene in the membrane between 53 ( 2° (high tilt) and 17 ( 5° (low tilt) with respect to the surface. The novel feature of this light-gated valve system is its extremely long-lived open-switch state; this behavior stands in contrast to that of other systems based on labile photoisomers, which tend to instantly return to the thermodynamically stable state.

Introduction Among the known addressable nanoscale devices, photoinduced molecular switches have opened a particularly challenging and appealing research area for applications in the development of “gates” for externally modulating the ion flow through both biological and artificial membranes. A typical methodological example of the reversible ion-transport control is the incorporation of photochromic groups in a membrane. Here, photoisomerization is thought to affect several processes including ion binding,1,2 ion flow rate of channel proteins,3-5 creation of polar sites,6 and alteration of pore sizes inside matrixes.7-10 Azobenzene embedded in a membrane is the most frequently investigated photochromic group and is known to control ion flow in various ways. First, it undergoes remarkable size changes during E/Z isomerization, a characteristic that has been used for controlled sizeselective mass transport (electron, gas, inorganic and organic ions).8,9,11 Second, photoisomerization of azobenzene derivatives may sometimes lead to a marked phase transition between the crystal and liquid states, which can change the ion mobilities and therefore the conductivities of composite films.12,13 Third, the isomerization of azobenzene can be used as a “moving arm” which allows the blocker to reach the pores of the membrane in * To whom correspondence should be addressed. Telephone: 82-32-7708234. Fax: 82-32-770-8238. E-mail: [email protected]. † University of Incheon. ‡ Korea Institute of Science and Technology. (1) Kimura, K.; Kaneshige, M.; Yokoyama, M. J. Chem. Soc., Chem. Commun. 1994, 1103. (2) Yamaguchi, T.; Ito, T.; Sato, T.; Shinbo, T.; Nakao, S.-I. J. Am. Chem. Soc. 1999, 121, 4078. (3) Lien, L.; Jaikaran, D. C. J.; Zhang, Z.; Woolley, G. A. J. Am. Chem. Soc. 1996, 118, 12222. (4) Kocer, A.; Walko, M.; Meijberg, W.; Feringa, B. L. Science 2005, 309, 755. (5) Banghart, M.; Borges, K.; Isacoff, E.; Trauner, D.; Kramer, R. H. Nat. Neurosci. 2004, 7, 1381. (6) Anzai, J.; Osa, T. Tetrahedron 1994, 50, 4039. (7) Sato, T.; Kijima, M.; Shiga, Y.; Yonezawa, Y. Langmuir 1991, 7, 2330. (8) Lei, Y.; Hurst, J. K. Langmuir 1999, 15, 3424. (9) Liu, N. G.; Dunphy, D. R.; Atanassov, P.; Bunge, S. D.; Chen, Z.; Lopez, G. P.; Boyle, T. J.; Brinker, C. J. Nano Lett. 2004, 4, 551. (10) Kumar, S. K.; Hong, J.-D. Langmuir 2008, 24, 4190. (11) Sata, T.; Shimokawa, Y.; Matsusaki, K. J. Membr. Sci. 2000, 171, 31.

the extended E configuration and to retract again in the shorter Z configuration.3,5,14 Among other compounds, spiropyran has been recently introduced to control ion flow through a lipid membrane containing a salt channel protein. This material undergoes a light-induced charge separation process that leads to the reversible opening and closing of nanometer-sized pores during photoisomerization between the electrically neutral spiropyran and the zwitterionic merocyanine forms.4 Polyelectrolyte multilayer (PEM) films, which are uniquely suitable for salt separation,15,16 have been recently investigated for use as gating membranes with good responses to various stimuli, such as variations of pH,17 salts,18 and temperature.19 We recently introduced an optical molecular switch that controls the ion flow through a PEM film spin-assembled on a nanoporous alumina support by means of the light-induced reversible Z/E isomerization of azobenzene, a process that is used to generate strong voluminous expansions (switch on) and contractions (switch off) of the membrane matrixes.10 The expansions are believed to enlarge the pores inside the membrane as a result of the bent molecular structure, thus making ion transport easier, whereas the contraction process restores the original dense state of the membrane matrix, thereby restraining the ion flow. However, a typical characteristic of the optical switches based on reversible isomerizations of photoreactive chromophores, in particular azobenzene, is their short-standing open states due to (12) Kimura, K.; Suzuki, T.; Yokoyama, M. J. Phys. Chem. 1990, 94, 6090. (13) Kimura, K.; Morooka, H.; Yokoyama, M. J. Appl. Polym. Sci. 1991, 43, 1233. (14) Osman, P.; Martin, S.; Milojevic, D.; Tansey, C.; Separovic, F. Langmuir 1998, 14, 4238. (15) (a) Balachandra, A. M.; Dai, J.; Bruening, M. L. Macromolecules 2002, 35, 3171. (b) Hong, S. U.; Malaisamy, R.; Bruening, M. L. Langmuir 2007, 23, 1716. (16) Pyrasch, M.; Toutianoush, A.; Jin, W.; Schnepf, J.; Tieke, B. Chem. Mater. 2003, 15, 245. (17) (a) Hiller, J.; Rubner, M. F. Macromolecules 2003, 36, 4078. (b) Park, M.-K.; Deng, S.; Advincula, R. C. J. Am. Chem. Soc. 2004, 126, 13723. (18) (a) Antipov, A. A.; Sukhorukov, G. B.; Mohwald, H. Langmuir 2003, 19, 2444. (b) Ito, Y.; Inaba, M.; Chung, D. J.; Imanishi, Y. Macromolecules 1992, 25, 7313. (19) (a) Jaber, J. A.; Schlenoff, J. B. Macromolecules 2005, 38, 1300. (b) Kharlampieva, E.; Kozlovskaya, V.; Tyutina, J.; Sukhishvili, S. A. Macromolecules 2005, 38, 10523.

10.1021/la803316s CCC: $40.75  2009 American Chemical Society Published on Web 12/29/2008

1768 Langmuir, Vol. 25, No. 3, 2009

Kumar et al.

Figure 1. (a, left) Schematic view of a multilayer film comprised of azacrown multi-ion HNA and PSS on a porous support. (b, right) Graphic illustration of the photoresponsive behavior of the PEM membrane as an optical ion switch.

labile Z-isomers that tend to return rapidly to the thermodynamically stable E-isomers. An intriguing question arises here regarding how to hold up the open state of an optical switch prior to a switch-off command. The three-dimensional manipulation of the azobenzene orientation in a membrane via slantwise irradiation with nonpolarized light is considered a good possibility, provided the mass-permeation rate through the membrane would change along with the chromophore alignment. The tilt angle of azobenzene chromophoresinamorphous,liquidcrystalline,andLangmuir-Blodgett films can be manipulated in any direction with the slantwise irradiation of light.20 In this paper, we approach two fundamental questions to establish a new type of optical gate for ion flow through a membrane: (1) To what extent is it possible to control the orientation (tilt angle) of the azobenzene chromophores tethered to the azacrown-type multication 1,4,10-[3-(4-(4′methoxyphenylazo)-2-nitrophenoxy)propyl]-1,4,7,10,13,16-hexamethylhexaazacyclooctadecane (HNA) lying relatively flat on the film surface [sandwiched between negatively charged polyelectrolyte poly(sodium styrenesulfonate) (PSS) layers (see Figure 1a) by means of slantwise irradiation? (2) How do the changes in the chromophore orientation affect the pore sizes and the ion mobilities utilized to reversibly control the ion flow through a PEM membrane (as schematically illustrated in Figure 1b) ?

Results and Discussion Film Morphology on the Porous Support. The topographical morphology of a PEM composed of 20 PSS/HNA bilayers on a nanoporous alumina membrane was investigated using fieldemission scanning electron microscopy (FE-SEM). At first, the top surface of the porous alumina membrane was completely covered with a pinhole-free smooth film, whereas the crosssectional state contained unblocked free pores (see Figure 2). The thickness of the 20 PSS/HNA bilayers on the alumina was estimated to be 19.4 nm by performing ellipsometric measurements of the multilayer film on a silicon substrate.21 (20) (a) Ichimura, K. Chem. ReV. 2000, 100, 1847. (b) Han, M.; Ichimura, K. Macromolecules 2001, 34, 82. (c) Ubukata, T.; Seki, T.; Ichimura, K. J. Phys. Chem. B 2000, 104, 4141. (d) Han, M.; Ichimura, K. Macromolecules 2001, 34, 90. (21) Kumar, S. K.; Park, J.-K.; Hong, J.-D. Langmuir 2007, 23, 5093.

Figure 2. FE-SEM (a) top and (b) cross-sectional images of a nanoporous alumina membrane coated with a PEM film composed of 20 PSS/HNA bilayers by means of an electrostatic self-assembly technique.

Three-Dimensional Manipulation of the Chromophore Orientation. Irradiation of a polymer film containing azobenzene with linearly polarized light is known to cause an alteration of the molecular axis of the compound, giving rise to dichroism during the following photoorientation processes: Photochemical E-to-Z isomerization, photochemical Z-to-E isomerization, thermal Z-to-E isomerization, and angular redistribution. All these processes are connected by rotational diffusion resulting from thermally induced Brownian motion.22 In general, the angularselective photon absorption by azobenzene molecules follows the photoselection rule cos 2ϑ, where ϑ is the angle between the electric dipole transition moment and the electric-field vector of the light leading to molecular reorientation.23,24 Wendorff et al. provided the first report on polarization photochromismswith the generation of holographic gratingssin layers of liquid crystalline (LC) polymers containing azobenzene side chains using Ar gas laser beams at 488 nm.25 The level of photobirefringence is reduced markedly in the early stages of alternating irradiation with UV light and visible light due to the homeotropic reorientation of the azobenzene. Ultimately, no light absorption occurs upon prolonged photoirradiation of the azobenzene(22) (a) Sekkat, Z.; Dumon, M. Synth. Met. 1993, 54, 373. (b) Sekkat, Z.; Wood, J.; Knoll, W J. Phys. Chem. 1995, 99, 17226. (23) Michl, J.; Thulstrup, E. W. Spectroscopy with Polarized Light; VCH Publishers: New York, 1995; p 198. (24) Dumont, M. Mol. Cryst. Liq. Cryst. 1996, 282, 437. (25) (a) Eich, M.; Wendorff, J. H.; Reck, B.; Ringsdorf, H. Makromol.Chem., Rapid Commun. 1987, 8, 59. (b) Eich, M.; Wendorff, J. H. Makromol. Chem., Rapid Commun. 1987, 8, 467.

Ion Permeation through PEM Membrane

Langmuir, Vol. 25, No. 3, 2009 1769

Figure 4. Cyclic photoswitching of chloride and sulfate ion permeations through a PEM comprised of 20 PSS/HNA bilayers by alternately changing irradiation direction for the low-tilt (closed state) and high-tilt (open state) orientations of the chromophores. Figure 3. Changes in orientation (tilt angle) of chromophores in a multilayer film composed of 10 PSS/HNA bilayers as a function of incident photoirradiation angle.

containing polymer film when the light comes from the direction normal to the surface. This is due to the fact that the transition moment of the chromophore molecule lies parallel to the propagation direction of light. On the other hand, slantwise irradiation results in a tilting of the molecular axis of the azobenzene molecule in the direction of the incident light. These results provide a convenient way to manipulate the threedimensional orientation of the photochromic molecules by simply varying the incidence direction of the light used for photoisomerization, irrespective of whether the light is polarized.26 Analysis of a virgin multilayer film by polarized UV/visible spectroscopy indicates that there is no preferred orientation for the chromophores dispersed in an isotropic state. The averaged alignment (tilt angle) of the transition moment of E-azobenzene in the virgin film with respect to the surface was found to be 26 ( 2°, which indicates a strong trendency of the chromophore toward planar orientation. Photocontrol of the molecular orientation in a PEM composed of 10 PSS/HNA bilayers was investigated by changing the irradiation direction of nonpolarized light. Reorientation of the azobenzene chromophores in a selected direction was completed after several EZE photoisomerization cycles performed by alternately irradiating the PEM with nonpolarized UV light and visible light. The tilt angle of the chromophores in the PEM membrane could be linearly adjusted between 53 ( 2° and 17 ( 5° by varying the irradiation direction with respect to the surface from normal to 10° (see Figure 3). Obviously, the tilt angle of the chromophores does not correlate exactly with the irradiation direction because of the limited alignment freedom of the chromophores, which are tethered to the azacrown core and sandwiched between polyelectrolyte layers. The tilt angle of the chromophores could be reversibly adjusted between the maximal and the minimal values. Optical Switch on Ion Permeation. Here, we focus on illustrating whether the manipulation of three-dimensional chromophore orientation can affect the ion permeation through a PEM membrane composed of 20 PSS/HNA bilayers on a nanoporous alumina support. To achieve this, we investigated the transport of chloride and sulfate ions through the PEM membrane by monitoring the increase of conductivity in the receiving phase for 90 min. Note that the detailed experimental procedure and the calculation of the permeation rate (PR) are described in the Supporting Information. First, the permeation rate (PR) of the chloride ion was measured to be (7.89 ( 0.02) × 10-7 cm · s-1 for a PEM membrane with a chromophore tilt (26) Ichimura, K; Morino, S.; Akiyama, H. Appl. Phys. Lett. 1998, 73, 921.

Figure 5. Permeation rate of chloride, bromide, iodide, and sulfate in the closed (9) and open states (b) of the ion switch built in the PEM membrane composed of 20 PSS/HNA bilayers. Table 1. Ion-Size Effect on the Permeation Rate through the PEM Membrane Comprised of 20 PSS/HNA Bilayers permeation rate (×107 cm/s)

ions ClBrISO42-

low tilt (closed) 7.89 ( 0.02 6.92 (0.01 6.31 ( 0.01 5.93 ( 0.01

high tilt (open) 8.04( 0.03 7.13 ( 0.03 6.46 ( 0.01 6.28 ( 0.01

high/low tilt

aqueous ionic radiia (Å)

1.02 1.03 1.02 1.06

1.56 1.80 2.10 2.18

a The ionic radii of anions in aqueous media were obtained from the literature (chloride, bromide, and iodide;27 sulfate28).

angle of 17 ( 5° (i.e., irradiation at 10° with regard to the film surface). Surprisingly, this permeation rate was found to increase to (8.04 ( 0.03) × 10-7 cm · s-1 upon increasing the tilt angle of the chromophores to 53 ( 2°. Similarly, in the case of sulfate ions, the PR values of (5.93 ( 0.01) × 10-7 and (6.28 ( 0.01) × 10-7 cm · s-1 were obtained for the low and high chromophore tilt angles, respectively. The reversible control of ion permeation through the membrane can be successfully demonstrated by defining the low and high tilt angles of the chromophores in the PEM, as the closed state and open state, respectively (see Figure 4). Note that the value of PR increased about 1.02 and 1.06 times for Cl- and SO42-, respectively, upon reaching attaining the maximum chromophore tilt angle in the PEM. The about 2 and 6% increments in the permeation rate for chlorine and sulfate ions are quite low compared to the typical optical ion switches which have been reported in the literature, for instance, 50% increase (from 1 to 1.5 nS) in conductivity for the open channel,4 and 40% current change (0.62/1.54 µA).9 Therefore, more work needs to be done in order to optimize the function of the ion switch with a proper design of voluminous chromophores, of

1770 Langmuir, Vol. 25, No. 3, 2009

Kumar et al.

Figure 6. Durability of the open (2) and closed (∆) states of the ion switch built in the PEM membrane composed of 20 PSS/HNA bilayers for chloride (a) and sulfate (b) ions.

which reorientations enable generation of larger pores in the membrane. At this stage, it is worth addressing the question of what could be the main cause of the observed increment in the ion-permeation rate through the PEM membrane upon the realigning process of the chromophores to a higher tilt angle. We assume that this increase results from an enlargement of the pore sizes caused by the expansion of the membrane matrix during reorientation of the chromophores to a higher tilt angle. To obtain a qualitative explanation for this hypothesis, we compared the permeability transport through a PEM membrane comprising 20 PSS/HNA bilayers for different ions, such as chloride ion (1.56 Å), bromide ion (1.80 Å), iodide ion (2.10 Å), and sulfate (2.18 Å) in an aqueous medium. In the low-tilt (closed) state, ion permeation decreases with increasing ionic radius, as shown in Figure 5. An equivalent trend is also observed for the high-tilt (open) state of the membrane. These results, in which the reversible on/off states of ion permeation though chromophore photoorientation exhibit the same trend (irrespective of ion sizes), strongly support our hypothesis that the pore sizes in the membrane can be adjusted by changing the chromophore alignment. The higher differences in ion permeability observed between the on and off states of sulfate ion are due to two combined effects present in this divalent sulfate ion, namely, size and electrostatic repulsion. Note that the detailed experimental data summarized in Table 1 are the average values of three different permeation rates obtained from independently prepared samples (see also the Supporting Information). The permeation rates could be reproducibly determined within the narrow error ranges. Since the open states of optical switches based on the photoisomerization of chromophores such as azobenzene and spiropyran have been shown to last only very short times [due to the spontaneous isomerization of unstable isomers to form thermodynamically more stable (closed) ones],4,10 we are particularly interested in studying the durability of the open state of our chromophore-orientation-based optical switch. The stability of the optical switch in the open and closed states was assessed by measuring the permeation rate of chloride through the PEM membrane every 24 h (see Figure 6). We found that the maximum permeation rate of the monovalent chloride ion in the openswitch state decreased gradually during the first 2 days of the observation, thereby reaching a value of about 99.6% of the maximum, and remained constant during a further 4 days of observation. Accordingly, the permeation rate of the photoswitch in the closed-valve state increased gradually during the first 2 days, reaching the value observed for the virgin film ((7.93 ( (27) Heyrovska, R. Chem. Phys. Lett. 2007, 436, 287. (28) Abd-Elwahed, A.; Holze, R. Synth. Met. 2002, 131, 61.

0.01) × 10-7 cm · s-1). This behavior is presumably due to the fact that the optically enforced strain of the membrane matrix (closed state) relaxes to reach the equilibrium state of the virgin film. A similar result was observed for the divalent sulfate ion. Note that the membranes were kept in ultrapure water after each measurement.

Conclusions A photoresponsive PEM film was fabricated on a porous alumina support by layer-by-layer deposition of a linear polyelectrolyte (PSS) and a macrocyclic multication containing azobenzene at its side chains (HNA). The FE-SEM studies of the 20 PSS/HNA bilayer films on porous alumina show that their surface morphology is pinhole-free, and comparable to that of films obtained on a silicon substrate. The ellipsometric measurements of the multilayer film on a silicon substrate gave a thickness of 20 PSS/HNA bilayers of 19.4 nm. Selective reorientation of the chromophores in a PEM film comprising 10 PSS/HNA bilayers was demonstrated by alternate irradiation with nonpolarized UV light (λ ) 360 nm) and visible light (λ ) 450 nm) (this was done six times). The average alignment of the transition moment of E-azobenzene (26 ( 2°) in the virgin PEM membrane could be linearly manipulated between 53 ( 2° (high tilt) and 17 ( 5° (low tilt) by changing the irradiation direction from normal to 10° with respect to the surface. The induced tilt angle of the chromophores does not correlate exactly with the irradiation direction, in contrast to the case of LC polymers.26 This might be due to the limited alignment freedom of the chromophores, which are tethered to the side chains of the azacrown core and sandwiched via electrostatic cross-linking between the polyelectrolyte layers. The tilt angle of the chromophores could be reversibly adjusted between its maximal and minimal values. The manipulation of the three-dimensional chromophore orientation in a PEM film comprising 20 PSS/HNA bilayers deposited on a nanoporous alumina support was shown to serve as an optical on/off switch for the ion permeation through the membrane; for instance, the permeation rate (PR) of the chloride ion could be reversibly switched between (7.89 ( 0.02) × 10-7 and (8.05 ( 0.03) × 10-7 cm · s-1 when the tilt angle of the chromophore was adjusted between 17 ( 5° and 53 ( 2° with respect to the film surface. Reversible control of the ion permeation through the membrane was successfully demonstrated (see Figure 4) by defining the low and high tilt angles of the chromophores in the PEM as the closed and the open states, respectively. The permeation ratio through the PEM membrane is strongly correlated with the ion size. In both the low and high tilt states, the ion-permeation rate decreases (in a roughly linear fashion)

Ion Permeation through PEM Membrane

with increasing ionic radius (for chloride, bromide, iodide, and sulfate). These results strongly support the hypothesis that ion permeation through the PEM membrane is mainly controlled by a change in the pore sizes as a result of chromophore reorientation. The most significant result of this research is that it opens up a new possibility to systematically control ion permeability by means of the three-dimensional manipulation of the chromophore orientation. This enables us to maintain the switch-on state, until we intentionally switch off. These results are a clear step toward the development of practical, photogated nanoscale delivery systems, nanotechnology, bioelectronics, and material sciences.

Experimental Section Materials. Poly(styrenesulfonate, sodium salt) (PSS, Mw 70 000) was purchased from Aldrich, and KCl, KBr, KI and K2SO4 (ACS grade) were purchased from Samchun Chemicals. All reagents were used as received. The alumina membranes (pore size ) 0.02 µm) were purchased from Whatman Corp. The synthesis and the detailed structural analysis of 1,4,10-[3-(4-(4′-methoxyphenylazo)-2-nitrophenoxy)propyl]-1,4,7,10,13,16-hexamethylhexaazacyclooctadecane (HNA) were described in a previous publication.21 Deionized water (Milli-Q >18.2 MΩ · cm-1) was employed for preparation of polyelectrolyte solutions and self-assembled multilayer films, and also for ion-permeation studies. Preparation of Polyelectrolyte Multilayer on Porous Alumina Support. The porous alumina supports were cleaned as described in earlier reports.29 The cleaned alumina substrates were dipped for 10 min into water (pH ∼5) before film deposition, and then they were fitted with homemade multilayer build-up cells in order to restrict multilayer formation only on their top sides. For layer-bylayer deposition of polyelectrolytes, the substrate was immersed for 20 min into ca. 2 mL of PSS solution (5 mM in H2O) and then subsequently washed three times with 2 mL of Milli-Q deionized water (pH 4.5) per each use in order to remove nonspecifically adsorbed materials. The deposition of the subsequent HNA film adopted a procedure identical with the former one with using aqueous HNA solution (0.1 mM, pH 4.5). Note that regular multilayer growth of PSS/HNA bilayers on the alumina support was estimated on the basis of the UV/visible absorbance spectra of the multilayers on fused silica and their ellipsometric thickness on a silicon wafer. Methods. Fine surface details and cross-sectional images of multilayer films comprising 20 PSS/HNA bilayers on an alumina membrane were obtained using a field-emission scanning electron microscope (FE-SEM, JEOL, JSM-6700F) with a 5.5 kV accelerating voltage and 10 Å beam current. For the cross-sectional images of the membrane, samples were mechanically cut using a sharp razor, sputter-coated with a few angstroms of platinum, rinsed in methanol, and dried with nitrogen. Three-Dimensional Photoorientation (Tilt Angle) by Slantwise Photoirradiation. The reorientation of azobenzene chromophores in a PEM film composed of 10 PSS/HNA bilayers was completed (29) Nagale, M.; Kim, B. Y.; Bruening, M. L. J. Am. Chem. Soc. 2000, 122, 11670.

Langmuir, Vol. 25, No. 3, 2009 1771 after six EZE photoisomerization cycles carried out by alternate 1 min irradiations at a chosen direction with nonpolarized UV light (λ ) 360 nm, p ) 2 mW · cm-2) and visible (λ ) 450 nm, p ) 100 mW · cm-2) light. An average tilt angle of 53 ( 2° with respect to the film surface was attained for azobenzene in the PEM membrane upon irradiation at normal incidence. This angle changed to 17 ( 5° upon irradiation at 10° with respect to the film surface. The tilt angles of the chromophores in PEM were determined by means of polarized UV/visible spectroscopy following the procedure described in detail elsewhere (see also the Supporting Information).30 Optical Switching on Ion Permeation. The dialysis apparatus consisted of two glass cells (with a volume of 100 mL) connected by a 2.5-cm-long neck connecting a membrane that separates the source-phase and receiving-phase sides. The exposed area of the membrane was 2.0 cm2. The receiving phase contained Milli-Q water and was connected to the source phase through a 0.1 M aqueous salt solution (KCl, K2SO4, etc.). The receiving-phase solution was stirred in order to minimize concentration polarization at the membrane surface. Prior to the ion-permeation measurements, the multilayer membrane (comprising 10 PSS/HNA bilayers on a nanoporous alumina substrate) was irradiated at normal incidence using, alternately, nonpolarized UV light and visible light. This resulted in a tilt angle of the chromophores of 53 ( 2°. This irradiated membrane was then immediately installed in the dialysis apparatus. The ion conductivity in the receiving-phase cell was monitored for 90 min at room teemperature (in a 10 min interval) using a conductivity meter (Orion Model 115). After the measurements, the membrane was kept in Milli-Q water for 30 min to remove any unwanted salt attached to it. Subsequently, it was irradiated again (at an angle of 10° with respect to the film surface) using, alternately, nonpolarized UV light and visible light. This resulted in a tilt angle of the chromophores of 17 ( 5°. Then, the ion-conductivity measurement was performed again, as described above. The permeation rate (PR) of the ions was calculated according to the equation31

PR ) (∆Λ/∆t)Λm-1V(Ac) where (∆Λ/∆t) is the conductivity change with time (i.e., the slope of a plot of the salt concentration in the receiving cell versus time), Λm is the molar conductivity of the corresponding salt solution, V is the solution volume in the receiving cell, A is the exposed membrane area, and c is the salt concentration in the source phase solution.

Acknowledgment. This work was supported by a research grant from the University of Incheon (2007). Supporting Information Available: Detailed experimental procedure and calculation of the permeation rate. This material is available free of charge via the Internet at http://pubs.acs.org. LA803316S (30) Raduge, C.; Papastavrou, G.; Kurth, D. G.; Motschmann, H. Eur. Phys. J., E 2003, 10, 103. (31) Krasemann, L.; Tieke, B. Langmuir 2000, 16, 287.