Photoresponsive Ion Gating Function of an Azobenzene

Feb 27, 2008 - Jousheed Pennakalathil and Jong-Dal Hong. ACS Nano 2011 5 (11), 9232-9237. Abstract | Full Text HTML | PDF | PDF w/ Links. Cover Image ...
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Photoresponsive Ion Gating Function of an Azobenzene Polyelectrolyte Multilayer Spin-Self-Assembled on a Nanoporous Support Surjith K. Kumar and Jong-Dal Hong* Department of Chemistry, UniVersity of Incheon, 177 Dohwa-dong, Nam-gu, Incheon 402-749, Korea ReceiVed October 29, 2007. In Final Form: January 4, 2008 The fabrication of a polyelelctrolyte multilayer (PEM) on a porous membrane was successfully improved by using spin-coating electrostatic self-assembly. Surprisingly, the quality of the PEM film obtained on the nanoporous alumina substrate (i.e., its thickness and surface morphology) was comparable to that of a film deposited on silicon. An optical molecular switch that acts as an ion-gating channel was realized using a PEM membrane deposited layer-by-layer on an alumina support. One of the layer components of this device was a poly(acrylamide) copolymer containing an azobenzene chromophore, which is known to reveal strong voluminous expansion and contraction during light-induced reversible cis/trans isomerizations. The permeability of the bulk SO42- ions was found to be sensitive to the changed channel sizes; for instance, the ion-permeation rate of SO42- increased about 1.6 times after UV irradiation of the PEM, whereas that of the Cl- ion increased only 1.2 times. In the study, it was successfully demonstrated that the ion flow through the PEM membrane could be reversibly switched on and off over several azobenzene isomerization cycles.

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 gating channels for externally modulating ion flow and drug release.1 Azobenzene could be regarded as an excellent candidate for an optical molecular switch to control mass transport through a membrane due to well-known light-triggered reversible trans/ cis isomerization.2 The formation of cis isomers accompanies remarkable changes of the size (0.90-0.55 nm), dipole moment (0.5-3.1 D), and geometry (planar-spherical). Only several examples have been found in the literature for photochemically switchable membranes modified with azobenzene. The switchable properties have been used to control size-selective mass transport (electron, gas, inorganic and organic ions) through a membrane. However, it has been shown that the size change of the azobenzene in the trans/cis isomerization affects the pore sizes of the membrane in a controversial way to either enlarge or shrink depending on the local environment of the chromophores and morphology of the membranes. For instance, the enlarged mean free volume of cis isomers appeared to increase the pore sizes of a membrane that is composed of vesicles,3 a cross-linked copolymer of (chloromethyl)styrene and divinylbenzene,4 or a rigid inorganic scaffold with monosized pores, which are modified with azobenzene.5 In contrast, the spherical cis form is known to act rather as a more effective diffusion obstacle than the elongated trans isomer, when azobenzene materials are densely packed inside microporous ceramic or zeolite membrane cavities.6 * To whom correspondence should be addressed. Phone: 82-32-7708234. Fax: 82-32-770-8238. E-mail: [email protected]. (1) Kocer, A.; Walko, M.; Meijberg, W.; Feringa, B. L. Science 2005, 309, 755. (2) Rau, H. In Photochemistry and Photophysics; Rabek, J. F., Ed.; CRC Press Inc.: Boca Raton, FL, 1990; Vol. 4, Chapter 4, p 120. (3) Lei, Y.; Hurst, J. K. Langmuir 1999, 15, 3424. (4) Sata, T.; Shimokawa, Y.; Matsusaki, K. J. Membr. Sci. 2000, 171, 31. (5) 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. (6) Weh, K.; Noack, M.; Ruhmann, R.; Hoffmann, K.; Toussaint, P.; Caro, J. Chem. Eng. Technol. 1998, 21, 5. Weh, K.; Noack, M.; Hoffmann, K.; Schro¨der, K. -P.; Caro, J. Microporous Mesoporous Mater. 2002, 54, 15.

Studies have shown that polyelectrolyte multilayer (PEM) membranes possess a unique suitability for salt7,8 and gas9 separations and pervaporation from water/organic solvent mixtures10 and as reverse osmosis membranes.11 Recently, much effort has been devoted to developing PEMs for gating membranes, which respond to various stimuli such as pH,12 salt,13 and temperature.14 To our best knowledge, the development of a PEM functionalized with a light-gated ion switch has not been reported before, although it is very important question how the size and dipole moment changes of the azobenzene during trans/ cis photoisomerization affect the pore size of the PEM which is composed of three-dimensionally cross-linked matrixes due to the salt formation among the layer components, positively and negatively charged polyelectrolytes. Thus, this paper aims to functionalize a PEM membrane with light-activated channels that allow rapid, remote, noninvasive, and reversible control. The idea is to use photochromic azobenzene derivatives as layer components of the PEM; these species have been extensively studied as model photoswitching compounds.15 Here, the fabrication of a PEM on a porous support is also attempted, for the first time, by using spin-coating electrostatic self-assembly (SCESA),16 a technique which has been shown not only to facilitate the rapid fabrication of the layers but also (7) Pyrasch, M.; Toutianoush, A.; Jin, W.; Schnepf, J.; Tieke, B. Chem. Mater. 2003, 15, 245. (8) (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. (9) (a) Levasalmi, J.-M.; McCarthy, T. J. Macromolecules 1997, 30, 1752. (b) Sullivan, D. M.; Bruening, M. L. Chem. Mater. 2003, 15, 281. (10) (a) Krasemann, L.; Tieke, B. J. Membr. Sci. 1998, 150, 23. (b) Sun, L.; Baker, G. L.; Bruening, M. L. Macromolecules 2005, 38, 2307. (11) Jin, W.; Toutianoush, A.; Tieke, B. Langmuir 2003, 19, 2550. (12) (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. (13) (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. (14) (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. (15) Dugave, C.; Demange, L. Chem. ReV. 2003, 103, 2475. (16) (a) Cho, J.; Char, K.; Hong, J.-D.; Lee, K.-B. AdV. Mater. 2001, 13, 1076. (b) Lee, S. S.; Hong, J.-D.; Kim, C. H.; Kim, K.; Koo, J. P.; Lee, K.-B. Macromolecules 2001, 34, 5358.

10.1021/la7033615 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/27/2008

Ion Gating Function of an Azobenzene PEM

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Scheme 1. Structures of the Polyelectrolytes Used in the Present Study

to allow precise control of the film thickness on a solid substrate. The multilayered membranes were obtained on a porous alumina support by the layer-by-layer (LBL) deposition of a negatively charged poly(styrenesulfonate) (PSS) and positively charged polyacrylamide copolymer containing a photochromic chromophore, 2-nitro-4′-methoxyazobenzene (PNA; Scheme 1). The synthesis and detailed characterization of PNA were described previously.17 The basic operation of the ion gate membrane is schematically illustrated in Figure 1a: UV irradiation on the PEM induces trans-to-cis isomerization of the nitroazobenzene species incorporated into the layer component PNA, which is supposed to enlarge pores inside the membrane as a result of the bent molecular structure, thus making ion transport easier. Subsequent exposure to visible light restores the original dense state of the membrane matrix, which in turn restrains the ion flow. The 15 PSS/PNA bilayer film appears transparent with a uniformly yellow color (Figure 1b). Experimental Section Materials. Poly(allylamine hyhrochloride) (PAH; Mw ) 70 000), poly(styrenesulfonate, sodium salt) (PSS; Mw ) 70 000), and ι-carrageenan (CAG) were purchased from Aldrich and KCl and K2SO4 (ACS grade) 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 ananlysis of poly{N-[3-(4-(4′-methoxyphenylazo)-2-nitrophenoxy)propyl]-N-[3-(dimethylamino)propylmethacrylamide} (PNA) was described elsewhere in a previous paper.17 Deionized water (Milli-Q, >18.2 MΩ·cm-1) was employed for preparation of polyelectrolyte solutions and spin-assembled multilayer films and also for ion permeation studies. Spin-Coating Electrostatic Self-Assembly (SCESA) on a Porous Alumina Support. The porous alumina supports were cleaned as described in an earlier paper.18 The cleaned alumina substrates were dipped for a short period (10 min) into water (pH ≈ 5) before film deposition, and then they were fixed on a flat solid plate layered with mica by means of a clear tape. For LBL deposition of polyelectrolytes, ca. 0.5 mL of PSS solution (5 mM in H2O) was poured onto the cleaned alumina support, and then the support was spun at a speed of 5000 rpm for 20 s. Subsequently, 1 mL of Milli-Q deionized water was put on the substrate and then the substrate spun again at 5000 rpm for 20s to remove the weakly bound polyelectrolyte. The washing steps were repeated three times. After that, the substrate was further rinsed in ∼1 mL of water three times, 1 min each time, to guarantee the removal of the nonspecifically bound polyelectrolytes on the surface. The subsequent film deposition using 1 mM PNA (1:20 DMSO/H2O) solution adopted a procedure identical to the former one. Note that regular multilayer growth of PSS/PNA bilayers on the alumina support was estimated from their UV/vis absorbance spectra of the multilayers on fused silica and from their ellipsometrical thickness on a silicon wafer. Methods. The thickness of the multilayer films on silicon wafers was measured by using a real-time spectroscopic ellipsometer (NanoView, SMG-1000) equipped with a Xe arc lamp (350-820 (17) Kumar, S. K.; Hong, J.-D.; Lim, C.-K.; Park, S.-Y. Macromolecules 2006, 39, 3217. (18) 18. Nagale, M.; Kim, B. Y.; Bruening, M. L. J. Am. Chem. Soc. 2000, 122, 11670.

Figure 1. (a) Schematic illustration of the photoresponsive behavior of the PEM as an ion switch. (b) Photo of a PEM comprising 15 PSS/PNA bilayers spin-assembled on a porous alumina support. nm), a rotating polarizer, a liquid cell with optical access at an incidence angle of 60°, an analyzer, and a multichannel detection system which can collect 128 data points from 1.577 to 4.843 eV in 20 ms. A self-made computer program calculates the change in polarization of linearly polarized incident light upon reflection from a sample in terms of ∆ and Ψ as a function of the photon energy, where ∆ and Ψ correspond to the changes in the phase and the amplitude of the incident polarized radiation, respectively. ∆ and Ψ determined for both the bare clean substrate and the film were converted to thickness. As usual, the refractive index of the polyelectrolyte film was assumed to be 1.54.19 Electron microscope images of the membrane were obtained using a Hitachi S-4700II field-emission scanning electron microscope. For the cross-sectional images of the membrane, the samples were mechanically cut using a sharp razor, sputter-coated with a few angstroms of platinum, rinsed in methanol, and dried with nitrogen. For field-emission scanning electron microscopy (FE-SEM) imaging, a 5.5 kV accelerating voltage and 10 A beam current were used to obtain fine surface details. The AFM measurements were performed on a Si wafer in air at room temperature by using a Nanoscope IV multimode microscope (Digital Instruments). Using a 125 µm long Si cantilever (Micromash, NSC18/AIBS) with a force constant of 3.5 N/m, topographic images were recorded in tapping mode (1 µm × 1 µm size) at a scan rate of 1.35 Hz. Data were manipulated using Nanoscope III software. Ion Permeation Studies. The dialysis apparatus consists of two glass cells (volume 100 mL), which are interconnected by a 2.5 cm long neck in which the membrane separates the source- and receivingphase sides. The exposed area of the membrane was 2.0 cm2. The receiving phase contains Milli-Q water and is connected to the source phase with the appropriate 0.1 M salt solution of KCl or K2SO4. The receiving-phase solution was stirred to minimize concentration polarization at the membrane surface. Prior to the measurement of ion permeation, the multilayer membrane comprising 10 PNA/PSS bilayers on nanoporous alumina was irradiated with UV light (λ ) 360 nm, p ) 2 mW/cm2) for 1 min, which induces trans-to-cis isomerization of the azobenzene chromophore in the PNA layers, and then it was immediately installed in the dialysis apparatus. The ion conductivity in the receivingphase cell was monitored at room temperature in a 10 min interval for 60 min by means of a conductivity meter (Orion model 115). After the measurement, the membrane was immersed in Milli-Q water for 15 min to remove unwanted salt attached to the membrane. The permeation rate (PR) and selectivity (R) of the ions were then calculated according to the equation11 PR ) (∆Λ/∆t)Λm-1V(Ac)-1 and R ) PR1/PR2, where ∆Λ/∆t is the conductivity change with time (i.e., the slope of a plot of the salt concentration in the receiving cell vs time), Λm is the molar conductivity of the corresponding salt solution, V is the solution volume in the receiving cell, A is the (19) Schlenoff, J. G.; Dubas, S. T. Macromolecules 2001, 34, 592.

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Figure 3. FE-SEM images of the porous alumina support modified with a PEM comprising 15 PSS/PNA bilayers: top side (a) and bottom side (b).

Figure 2. (a) Ellipsometric thicknesses of PNA/PSS multilayers versus the number of layers. (b) FE-SEM cross-sectional image of a PEM comprising 15 PNA/PSS bilayers spin-self-assembled on an alumina support. exposed membrane area, c is the salt concentration in the sourcephase solution, and the subscripts 1 and 2 represent two different ions.

Results and Discussion The LBL deposition of polyelectrolyte layers on a porous alumina support by using SCESA is considered to be a challenge with no guarantee of obtaining a high-quality film. To determine the optimal conditions for the LBL deposition of PNA and PSS on the support, the thickness of a film deposited on silicon was examined by means of optical ellipsometry. The deposition proceeds uniformly and regularly, as shown in Figure 2a, and the thickness of the film composed of 15 PNA/PSS bilayers was determined to be about 43 ( 2.7 nm. The formation of a PEM membrane on the surface of the nanoporous supportswithout penetration into the poresscould be clearly identified from the cross-section FE-SEM image of the PEM (Figure 2b). The film thickness was estimated to be ca. 58 ( 6.9 nm, which is in good agreement with the ellipsometry result, taking into consideration the poorly defined interface between the film and the support. The topographical morphology of the spin-assembled PEM composed of 15 PNA/PSS bilayers on the alumina membrane was investigated with FE-SEM. First, the top surface of the porous alumina membrane is completely covered with the pinhole-free smooth film, whereas the original bottom state contains unblocked free pores (with a size of ∼0.02 µm), as revealed by the FE-SEM image of the PEM comprising the 15 PSS/PNA bilayers (Figure 3). These results demonstrate that SCESA is well suited to the preparation of PEMs on both dense and porous substrate surfaces. The topographical AFM image of the spin-assembled PEM composed of 15 PNA/PSS bilayers on a silicon wafer is shown in Figure 4a for the comparison with that on a nanoporous alumina support (Figure 4b). The surface morphology of the PEM on silicon is almost identical to the typical images of azobenzene polyelectrolyte multilayers on a smooth surface of the substrate, which exhibits a granular structure with uniform size, as reported

Figure 4. Topographical AFM image of the PEM composed of 15 PNA/PSS bilayers on silicon (a) and an alumina support (b). Note that the AFM image recorded in the tapping mode is given in a top view presentation with the lighter areas denoting higher regions and the darker areas representing lower regions.

previously in the literature.20 In contrast, the PEM on the alumina membrane tends to produce larger feature sizes with increased roughness, which directly reflects a highly porous and rough initial surface. Note that the root-mean-squared (rms) surface roughness of the PEM on the silicon and the alumina substrates was determined to be 6.38 and 21.47 nm, respectively. Chloride and sulfate ion transport through PEM comprising 10 PSS/PNA bilayers on a porous alumina support was measured by monitoring the increase of conductivity in the receiving phase for 90 min. After the measurement of ion permeation through the PEM with trans-azobenzene, the PSS/PNA multilayer (20) (a) Hong, J.-D.; Jung, B.-D.; Kim, C. H.; Kim, K. Macromolecules 2000, 33, 7905-7911. (b) Advincula, R.; Park, M.-K.; Baba, A.; Kaneko, F. Langmuir 2003, 19, 654-66.

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Figure 5. Receiving-phase conductivity as a function of time, when the source phase (0.1 M salt) is separated from the receiving phase by a PEM composed of 10 PSS/ PNA bilayers: KCl at the cis state (0) and at the trans state (9), K2SO4 at the cis state (O) and at the trans state (b) of azobenzene.

chromophores upon subsequent exposure to visible light (λ ) 450 nm, p ) 100 mW cm-2). At this stage, an important question is worth considering about whether the higher swelling of the films due to the more hydrophilic nature of the cis isomers could also contribute to the increased permeability of the ions through the PEM membrane upon UV irradiation. The difference between the swelling degrees of the films with trans and cis isomers (10.9 ( 1.0% versus 11.6 ( 0.9%) was found to be negligibly small because of the highly polar character of the polyelectrolytes (see the Supporting Information). Apparently, the size change of the azobenzene in the trans/cis isomerization seems to dominatingly affect the pore sizes of the membrane. Three important issues should be discussed here: First, sulfate ions were much more difficult to permeate through the PEM membrane compared with chloride ions. This might be due to the bulkiness of the sulfate ions and higher rejection of the multipolar PEM.21 Second, the UV irradiation of the PEM membranes has a stronger influence on the permeation rate of SO42- ions than on that of the Cl- ions; this can be attributed to the enlarged pores for the larger sized anions. Third, the permeation rate of the Cl- and SO42- ions in the trans state increased gradually during the cis/trans isomerization cycles. This finding indicates an incomplete recovery of the PEM matrixes caused by the slow relaxation process of the polyelectrolyte networks during the irradiation with the visible light.

Figure 6. Cyclic photoswitching of the chloride and sulfate ion permeations through a PEM comprising 10 PSS/PNA bilayers by alternate irradiation with UV and visible lights.

Conclusion

membrane was irradiated with UV light for 1 min, which induces trans-to-cis isomerization of the azobenzene chromophore. A wavelength-dependent photostationary state between both trans/ cis isomers of PNA is established after about 50 s of irradiation.17 PNA is a highly photosensitive nitroazobenzene chromophore with relatively fast photoresponsiveness. The receiving-phase conductivity of both KCl and K2SO4 was plotted as a function of the measurement time, as shown in Figure 5. The flux was found to increase in the cis-state PEM for both chloride and sulfate ions, revealing that the trans/cis isomerization of the chromophores attributed to enlargement of the pore sizes in the membrane. The linearity of the conductivity values with time, which occurs with the cis- and trans-state membrane, demonstrates steady-state anion transport where the receiving-phase concentration is negligible compared to that in the source phase. The permeation rate (PR) was measured for the Cl- and SO42ions by using a membrane composed of 35 PNA/PSS bilayers (values of 4.76 × 10-7 and 0.07 × 10-7 cm‚s-1 were obtained, respectively). The selectivity (R) of a membrane with a thickness of about 100 nm was calculated to be 68.9 for the Cl-/SO42system, which is a relatively high value. A light-gated ion switch was realized with a PEM composed of 10 PSS/PNA bilayers deposited on a porous alumina support (see Figure 6). The PEM membrane was irradiated with UV light for 1 min prior to the measurements. The value of PR increased about 1.2 times [from (6.40 ( 0.07) × 10-7 to (7.74 ( 0.07) × 10-7 cm‚s-1] and 1.6 times [from (2.66 ( 0.21) × 10-7 to (4.06 ( 0.09) × 10-7 cm‚s-1] for Cl- and SO42-, respectively. Note that about 10% of the cis isomers in the PEM undergo thermal back-relaxation to the trans state during the measurement in the dark, which is, however, a rather small and negligible amount considering the scope of this experiment. In the case of Cl-, the permeation rate decreased againsalmost reaching its initial valuesas a result of the cis-to-trans isomerization of the azo

The fabrication of PEM membranes on a porous alumina support was improved by using the spin-coating electrostatic method instead of the conventional solution-dip-coating technique. Surprisingly, the film quality of the SCESA films deposited on porous alumina (i.e., their thickness and surface morphology) was comparable to that of the films obtained on a silicon substrate. An optical molecular switch that acts as a gating channel for reversibly opening and closing ion flow through the PEM membrane was successfully realized by using a layer component composed of an azobenzene chromophore, which is known to reveal strong voluminous expansion and contraction during the cis/trans isomerizations. The ion permeation rate of SO42- was found to increase about 1.6 times in the UV-irradiated PEM, an effect that is significantly stronger than that observed for Cl-. Also, the permeability of the bulky SO42- ions was more sensitive to the channel sizes. The newly developed light-activated channels can be applied to fabricate sophisticated modern tools that require a rapid, remote, and noninvasive control, including filtration systems, membrane-based separation units, and sensors. Acknowledgment. This work was supported by a Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2006-311-C00437). We are very grateful to Prof. Il-Sin Ahn at Hanyang University for allowing us to use his ellipsometer and thank Prof. Merlin Bruening (Michigan State University) for his generous help in designing the ion permeation cell. Supporting Information Available: Details of the experimental procedure to determine the change in swelling degree of a polyelectrolyte multilayer membrane upon UV irradiation. This material is available free of charge via the Internet at http://pubs.acs.org. LA7033615 (21) (a) Krasemann, L.; Tieke, B. Langmuir 2000, 16, 287. (b) Harris, J. J.; Stair, J. L.; Bruening, M. L. Chem. Mater. 2000, 12, 1941.