Ion-Permselective Polyelectrolyte Multilayer Membrane Installed with

Apr 28, 2010 - ‡Korea Institute of Science and Technology, Seoul 130-650, Korea. Received February 17, 2010. Revised Manuscript Received April 18, 2...
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Ion-Permselective Polyelectrolyte Multilayer Membrane Installed with a pH-Sensitive Oxazine Switch Jousheed Pennakalathil,† Tae-Hyun Kim,† Kyuwon Kim,† Kyoungja Woo,‡ Jong-Ku Park,‡ and Jong-Dal Hong*,† †

Department of Chemistry, University of Incheon, 12-1 Songdo-dong Yeonsu-gu, Incheon 406-772, Korea, and ‡ Korea Institute of Science and Technology, Seoul 130-650, Korea Received February 17, 2010. Revised Manuscript Received April 18, 2010

This paper describes a pH-responsive multilayer film composed of two layered components, namely poly(allylamine hydrochloride) (PAH) and a copolymer of acrylic acid and [1,3]oxazine-modified acrylate (POA). The oxazine ring is an acidochromic chromophore and opens to form either cationic 3H-indolium or anionic hemiaminal in a pH-dependent manner. This structural transition was used to generate a net positive or negative charge on the membrane for selective ion permeation. Interestingly, the reversible oxazine ring opening and closing proceeded smoothly without significant steric hindrance in the multilayer film comprising 10 PAH/POA bilayers. The pH-responsive permselectivity for cationic and anionic probe molecules was demonstrated using a POA monolayer film adsorbed electrostatically onto an aminofunctionalized ITO electrode. The origin of the excellent ion-transport selectivity in the 1 nm ultrathin POA membrane is discussed in terms of alternating charges of the aromatic amphoteric group, oxazine, in the polyeletrolyte membrane.

1. Introduction Development of stimuli-responsive porous membranes has been extensively pursued in the past few decades for potential applications, including controlled release, selective separation, chemical sensors, and biosensors. The external stimuli that controls the permeation/separation properties of these membranes are the environmental parameters such as pH,1-6 temperature,7-9 *To whom all correspondence should be addressed: Tel 82-32-835-8234, Fax 82-32-835-0762, e-mail [email protected]. (1) Okahata, Y.; Ozaki, K.; Seki, T. J. Chem. Soc., Chem. Commun. 1984, 519– 521. (2) Ito, Y.; Inaba, M.; Chung, D. J.; Imanishi, Y. Macromolecules 1992, 25, 7313–7316. (3) Liu, Y.; Zhao, M.; Bergbreiter, D. E.; Crooks, R. M. J. Am. Chem. Soc. 1997, 119, 8720–8721. (4) Zhang, H. J.; Ito, Y. Langmuir 2001, 17, 8336–8340. (5) Park, M.-K.; Deng, S.; Advincula, R. C. J. Am. Chem. Soc. 2004, 126, 13723. (6) Lee, D.; Nolte, A. J.; Kunz, A. L.; Rubner, M. F.; Cohen, R. E. J. Am. Chem. Soc. 2006, 128, 8521–8529. (7) Iwata, H.; Oodate, M.; Uyama, Y.; Amemiya, H.; Ikada, Y. J. Membr. Sci. 1991, 55, 119–130. (8) Park, Y. S.; Ito, Y.; Imanishi, Y. Langmuir 1998, 14, 910–914. (9) Chu, L. Y.; Li, Y.; Zhu, J. H.; Chen, W. M. Angew. Chem., Int. Ed. 2005, 44, 2124–2127. (10) Sato, T.; Kijima, M.; Shiga, Y.; Yonezawa, Y. Langmuir 1991, 7, 2330– 2335. (11) 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, 551–554. (12) Kocer, A.; Walko, M.; Meijberg, W.; Feringa, B. L. Science 2005, 309, 755– 758. (13) Kumar, S. K.; Hong, J.-D. Langmuir 2008, 24, 4190–4193. (14) Kumar, S. K.; Pennakalathil, J.; Kim, T.-H.; Kim, K.; Park, J.-K.; Hong, J.-D. Langmuir 2009, 25, 1767–1771. (15) Bhaskar, R. K.; Sparer, R. V.; Himmelstein, K. J. J. Membr. Sci. 1985, 24, 83–96. (16) Nishizawa, M.; Menon, V. P.; Martin., C. R. Science 1995, 268, 700. (17) Lee, S. B.; Martin, C. R. J. Am. Chem. Soc. 2002, 124, 11850–11851. (18) Ito, T.; Hioki, T.; Yamaguchi, T.; Shinbo, T.; Nakao, S.; Kimura, S. J. Am. Chem. Soc. 2002, 124, 7840–7846. (19) Chu, L. Y.; Yamaguchi, T.; Nakao, S. Adv. Mater. 2002, 14, 386–389. (20) Mika, A. M.; Childs, R. F.; Dickson, J. M.; Mccarry, B. E.; Gagnon, D. R. J. Membr. Sci. 1995, 108, 37–56. (21) Kontturi, K.; Mafe, S.; Manzanares, J. A.; Svarfvar, B. L.; Viinikka, P. Macromolecules 1996, 29, 5740–5746. (22) Ito, Y.; Ochiai, Y.; Park, Y. S.; Imanishi, Y. J. Am. Chem. Soc. 1997, 119, 1619–1623.

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photoirradiation,10-14 electric field,15-17 concentration of chemical species,18,19 and ionic strength.2,20-22 In particular, pHresponsive membranes have been investigated most intensively due to their potential uses in a broad array of technologies, including bioseparation of amino acids23,24 or ionic drugs.25 The membranes are usually modified with pH-sensitive chemical groups (e.g., weak acids4,26,27 and weak acid/base pairs3,5,28,29 or polypeptides22,30,31) that are influenced by the local pH and electrolyte concentration. For instance, by varying the solution pH, the ionization state of a membrane containing carboxylic acid/amino groups was tuned such that the membrane had an excess positive or negative charge.3,5 Consequently, the membranes possessed either anion- or cation-permselective functions. These membranes could be reversibly switched between cation- and anionpermitting states in response to the pH of the solution. Control over the rate of water permeation through a membrane was also demonstrated using porous membranes, the pore sizes of which reversibly increased or decreased due to swelling/deswelling behavior in the polyelectrolyte multilayers.6 Another such variable pore size membrane employed the conformational changes of polypeptide brushes that were dependent on the solution pH.22,30,31 Polyelectrolyte multilayer films deposited onto a solid substrate by means of electrostatic self-assembly32 are promising (23) Minagawa, M.; Tanioka, A. J. Colloid Interface Sci. 1998, 202, 149–154. (24) Barboui, M.; Guizard, C.; Luca, C.; Albu, B.; Hovnanian, N.; Palmeri, J. J. Membr. Sci. 1999, 161, 193–206. (25) Jimbo, T.; Ramı´ rez, P.; Tanioka, A.; Mafe, S.; Minours, N. J. Colloid Interface Sci. 2000, 225, 447–454. (26) Cheng, Q.; Brajter-Toth, A. Anal. Chem. 1996, 68, 4180–4185. (27) Chun, K.-Y.; Stroeve, P. Langmuir 2001, 17, 5271–5275. (28) Jimbo, T.; Tanioka, A.; Minoura, N. Langmuir 1998, 14, 7112–7118. (29) Lee, S. B.; Martin, C. R. Chem. Mater. 2001, 13, 3236–3244. (30) Ito, Y.; Ochiai, Y.; Park, Y. S.; Imanishi, Y. Langmuir 2000, 16, 5376–5381. (31) Hollman, A. M.; Bhattacharyya., D. Langmuir 2002, 18, 5946–5952. (32) (a) Decher, G.; Hong, J.-D. Makromol. Chem., Macromol. Symp. 1991, 46, 321–327. (b) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/211, 831– 835. (c) Lee, S.-S.; Lee, K.-B.; Hong, J.-D. Langmuir 2003, 19, 7592–7596. (33) (a) Sullivan, D. M.; Bruening, M. L. Chem. Mater. 2003, 15, 281–287. (b) Kotov, N. A.; Magonov, S.; Tropsha, E. Chem. Mater. 1998, 10, 886–895. (c) Lev€asalmi, J.-M.; McCarthy, T. J. Macromolecules 1997, 30, 1752–1757. (d) Ackern v., F.; Krasemann, L.; Tieke, B. Thin Solid Films 1998, 327-329, 762–766.

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Figure 1. (a) Molecular structure of the copolymer of acrylic acid and [1,3]oxazine-bearing acrylate (POA). (b) Opening of the [1,3]oxazine ring (OX) in the presence of either HCl or NaOH to form either the cationic 3H-indolium (IC) or the anionic hemiaminal (NA).

candidates membranes for the separation of gases,33 water/ organic mixtures,34 and ions due to their bipolar nature.35 Recently, polyelectrolyte multilayer films, combined with stimuliresponsive functions,5,6,13,14 were introduced, and their development is emerging as a promising field for sophisticated applications, such as filtration systems, membrane-based separation, bioseparation, and sensors. In the present study, we developed a new pH-switchable membrane system comprising poly(allylamine hydrochloride) (PAH) and a copolymer of acrylic acid and [1,3]oxazine-bearing acrylate (POA), the molecular structures of which are given in Figure 1a. The pH-responsive membrane relies on the homogeneous dispersal of the oxazine groups (OX), which are known to open, yielding either a positively charged cation (IC) at low pH or a negatively charged anion (NA) at high pH,36 as shown in Figure 1b. Thus, the net positive or negative charges on the POA membrane can be switched via solution pH to either pass or exclude cationic or anionic probe molecules. It is important to note that the 1 nm ultrathin POA film showed ion-transport selectivities that were comparable to those of membranes composed of amphoteric (aliphatic weak acid/base) groups, reported earlier.3,5

2. Results and Discussion 2.1. Architecture of the Polyelectrolyte Multilayer Membrane. The electrostatic self-assembly characteristics of weak polyelectrolytes during the synthesis of multilayer films have been extensively explored by Rubner’s group and others.37 The morphology of weak polyelectrolyte films is significantly affected by the environmental pH during multilayer assembly due to charge density effects. Poly(acrylic acid), PAA (pKa ≈ 5), and PAH (pKa ≈ 10) contain ionizable carboxylic acids and amines, respectively, that influence the degree of ionization in these weak polyelectrolytes (i.e., the relative number of COO- vs COOH groups for PAA and the relative number of NH3þ vs NH2 groups for PAH), depending on the deposition pH conditions. Thus, the number of ionic bonds (COO- 3 3 3 NH3þ) used to assemble the multilayers may be tuned as desired. The thickness of adsorbed (34) (a) Lenk, W.; Meier-Haack, J. Desalination 2002, 148, 11–16. (b) Toutianoush, A.; Krasemann, L.; Tieke, B. Colloids Surf., A 2002, 198-200, 881–889. (35) (a) Liu, X.; Bruening, M. L. Chem. Mater. 2004, 16, 351–357. (b) Lajimi, R. H.; Abdallah, B. A.; Ferjani, E.; Roudesli, M. S.; Deratani, A. Desalination 2004, 163, 193–202. (36) Tomasulo, M.; Sortino, S.; Raymo, F. M. J. Org. Chem. 2008, 73, 118–126. (37) (a) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213–4219. (b) Chung, A. J.; Rubner, M. F. Langmuir 2002, 18, 1176–1183.

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Figure 2. Ellipsometric thickness versus the number of PAH/POA bilayers, deposited by dip-coating in 2 mM PAH and 3 mM POA solutions at pH 5.0, on a silicon substrate (data fits are shown as broken line).

Figure 3. Topographical 2D (a) and 3D (b) AFM images of the PEM composed of 5 POA/PAH bilayers on silicon. Note that the AFM image recorded in the tapping mode (a) is given in a top view presentation with the lighter areas denoting higher regions and the darker areas representing lower regions.

layers of PAH or PAA depends primarily on the charge density and conformation (segmental population of loops, tails, and trains) of the adsorbing polymer or on the free ionic binding sites available in the previously adsorbed polymer layer. In our case, the pH of the PAH and POA solutions was adjusted to 5.0 for the dip-coating of a solid substrate because most of the [1,3]oxazines remain in closed ring structures under these conditions. The deposition of PAH and PAA monolayer films was accomplished with maximal thickness.37 A fused silica substrate was dip-coated in PAH and POA solutions (pH ≈ 5.0) in a layer-by-layer (LBL) fashion. First, the regular and uniform growth of PAH/POA bilayers, to form a multilayered film, was monitored at the oxazine absorption maximum, 320 nm, by UV-vis spectroscopy (1.5  10-3 for one deposition cycle, data not shown). The LBL deposition of PAH and POA on a silicon wafer was characterized by optical ellipsometry. The thickness of the PAH/POA bilayers increased linearly with the number of adsorption cycles, as shown in Figure 2. The average thickness of each PAH/POA bilayer was determined to be 1.92 ( 0.04 nm. Topographical 2D and 3D AFM images of the polyelectrolyte multilayer (PEM) composed of 5 POA/PAH bilayers, dip-coated on a silicon wafer are shown in parts a and b of Figure 3, respectively. The surface morphology of the PEM on silicon is almost identical to the typical images of multilayer films comprising (38) (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 4. UV-vis absorption spectra of (a) POA in 1 vol % DMSO/H2O (0.02 unit mM) and (b) a multilayer film composed of 10 PAH/POA bilayers after the addition of 1 N NaOH.

chromophore-containing polyelectrolytes on a smooth surface of the substrate, which exhibits a granular structure, as reported previously in the literature.13,38 As shown in the sectional analysis, the roughness of the multilayer film fluctuated between about -2.00 and þ2.00 nm. The root-mean-squared surface roughness of the PEM on the silicon was determined to 0.88 nm. 2.2. Ring Opening and Closing of POA in Solution and Multilayer Assemblies. As described in detail by Raymo et al.,36 the [1,3]oxazine ring opened to generate an anionic hemiaminal, in the presence of tetrabutylammonium hydroxide, or the 3Hindolium cation, in the presence of trifluoroacetic acid (see Figure 1b). The absorption maximum for POA in a 1 vol % DMSO/H2O solution (0.02 unit mM) at pH 5.42 was found to be 320 nm, which corresponded to the characteristic absorption band of oxazine, indicating that the rings remained closed without any significant influence of the neighboring carboxylic groups, as shown in Figure 4a. The transformation that occurred as the pH was changed from 5.42 to 13.04 was accompanied by a decrease in the [1,3]oxazine absorption band at 320 nm and an increase in the hemiaminal absorption band at 410 nm. The intensity of the absorbance at 320 nm gradually transferred to 410 nm band with an isosbestic point at 350 nm upon the addition of 1 N NaOH. The spectral shift was found to be completed at pH 13.04. On the other hand, the oxazine ring gradually opened to yield the cationic indolium through the addition of 1 N HCl, with a corresponding increase in the absorbance at 320 nm. The spectral change was completed near pH 1.78 (see Figure S1 in the Supporting Information). The successive transformation of the absorption band, from 320 to 410 nm, was observed in the UV/vis spectrum of a multilayer film composed of 10 PAH/POA bilayers over a pH range of 7.5-10.5, as shown in Figure 4b. Note that partial dissociation of the layer components from the multilayered film was observed, when the pH of the dipping solution exceeded 10.5. The pH-responsive reversible switching of IC / OX / NA was a requirement for the practical use of oxazine in multilayered membranes that selectively control ion transport. The acidochromic transformation of OX was tested by dipping the multilayer film (5 PAH/POA bilayers) alternately in aqueous solutions of pH 2.50 and 10.50 for 1 min each, with a washing step between dips. Figure 5 demonstrates that switching between the cationic IC and anionic NA was steady and regular over 10 cycles without observation of steric hindrance in the multilayer film. Now, we studied the pH-dependent ring-opening kinetics of [1,3]oxazine in solution and in the film deposited on a fused silica using UV/vis spectrometry. The absorbance change at the maximum absorbance of the ring-opened oxazine; cationic 3Hindolium (λmax=320 nm) in 0.1 mM POA solution and in a film Langmuir 2010, 26(13), 11349–11354

Figure 5. Cyclic switching of [1,3]oxazine in a multilayer assembly (5 PAH/POA bilayers) between IC and NA structures by alternately dipping in aqueous solutions of pH = 2.50 or 10.50, monitored spectroscopically at 410 nm, the absorption maximum for the anionic hemiaminal NA. Washing steps were inserted between alternate dipping steps.

composed of POA/PAH bilayer on a fused silica was recorded before and after the addition of 1 N HCl solution, as shown in Figure 6a. It was found that the OX-IC isomerization in the solution and the film was completed within about 14 and 5 s, respectively. Besides, the isomerization of oxazine ring to hemiaminal anion (OX-NA) at high pH was also determined from the absorbance change at 410 nm before and after the addition of 1 N NaOH solution, as shown in Figure 6b. The OX-NA isomerization was completed in about 5 s in the film composed of PAH/ POA bilayer, which lays in similar time range of the OX-IC transition. However, the ring-opening process in 0.1 mM POA solution (pH = 10) continues to proceed slowly after first 73% isomerization in 5 s. Note that the volume of the added HCl and NaOH solution was exactly calculated to adjust the pH of POA solution and dip water of the bilayer film to 3 and 10, respectively. For the comparison of the OX-NA or OX-IC isomerization rates of POA in the solution (0.1 mM) and in a thin layer (PAH/ POA), the best fit of the experimental points in Figure 6 was obtained by simulating the absorbance (A) data with the following first-order exponential rise to maximum equation: A(t)=A0 þ A¥(1 - exp(-kt)), where A0 is the initial absorbance, A¥ is the absorbance at infinity, t is time, and k is a rate constant.. The isomerization rates are summarized in Table 1. In general, the isomerizations proceed much faster in the bilayer film than in solution and also the OX-IC isomerization in solution at low pH faster than the OX-NA. 2.3. Ion Permselectivity of the POA Membrane. Ionic permeability through a POA monolayer film deposited on an amino-functionalized ITO electrode (ITO/POA) was electrochemically investigated under different pH conditions using redox DOI: 10.1021/la1007044

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Figure 6. Absorbance of POA in solution (1) and in film (b) at 320 (a) and 410 nm (b) before and after the addition of 1 N HCl and 1 N NaOH, respectively. The red arrow points to the time to add the calculated volume of 1 N HCl and 1 N NaOH to adjust the pH of POA and dip solution of the bilayer film to 3 and 10. Table 1. Summary of the Isomerization Rates for OX-IC and OX-NA in 0.1 mM POA Solution and in Film Composed of PAH/ POA Bilayer at pH 3 and 10, Respectivelya rate constants (k) in 10-3 s-1 pH

solution

PAH/POA film

31.2 ( 1.1 59.8 ( 0.1 47.2 ( 0.5 51.8 ( 0.8 a They were obtained from fitting of the experimental data given in Figure 6. 10 3

probe molecules, anionic Fe(CN)63- or cationic Ru(NH3)63þ. Figure 7 shows cyclic voltammograms of the ITO/POA electrode in a 0.5 M Na2SO4 electrolyte solution containing 5 mM of either Fe(CN)63- or Ru(NH3)63þ, at pH 3.0, 7.0, and 10.0. The electrolyte solutions were buffered at pH 3.0 and 10.0 with acetate or sodium carbonate, respectively. The dependence of voltammogram shape and magnitude on pH clearly demonstrates the distinctive differential gate functions for Fe(CN)63- and Ru(NH3)63þ ions, which are in similar size (6.0 vs 6.2 A˚).5 For instance, the peak current densities resulting from Fe(CN)63- reduction at pH 3.0 and 10.0 were strikingly different (35.3 vs 5.5 μA/cm2), indicating that the POA film was permeable (“on”) to the negatively charged probe, Fe(CN)63-, at pH 3.0, but impermeable (“off”) to the probe at pH 10.0. In contrast, the POA film was closed to the positively charged probe, Ru(NH3)63þ at pH 3.0, but open to the probe at pH 10.0. Note that the peak current density from Ru(NH3)63þ reduction was 1.2 μA/cm2 at pH 3.0. This value increased by a factor of 46-55.0 μA/cm2 at pH 10.0. This responsive behavior was fully reversible with respective to the alternating pH for up to at least 10 cycles (see Figure S2 in the Supporting Information). However, the “off” function with respect to Fe(CN)63- permeation was not as efficient as it was for Ru(NH3)63þ. This trend contrasts that observed in the pHresponsive bipolar polymeric ultrathin films composed of homogeneously dispersed -NH2 and -COOH groups.4,9 The unique characteristics of the OX switch originate from the aromatic nature of the indolium cations, which induced a more homogeneous and intense electrostatic barrier against the identically charged probe ions attempting transport through the membrane. However, the ITO/POA/PAA electrode revealed results that were similar to the ITO/POA behavior in terms of ion selectivity, and the ITO/POA/PAH/POA configuration was slightly worse, probably due to charge neutralization (data not shown). At pH 7, the permeability of the membrane to both cationic and anionic probe molecules decreased to a similar extent with regard to the maximal permeability, indicating no selectivity of the closed ring OX. The peak current densities of Fe(CN)63- and Ru(NH3)63þ were found to be 22.8 and 34.7 μA/cm2, which 11352 DOI: 10.1021/la1007044

Figure 7. Cyclic voltammograms of the ITO electrodes on which were deposited a POA monolayer film in an aqueous 0.5 M Na2SO4 electrolyte solution, containing 5 mM of either Fe(CN)63- or Ru(NH3)63þ buffered at pH 3.0, 10, or unbuffered, pH 7.

corresponded to about 65% of the maximal 35.3 μA/cm2 and 63% of the maximal 55.0 μA/cm2, respectively. Note that under neutral conditions most oxazine rings in the multilayer film remained closed. A careful analysis of the oxidation and reduction peak positions allows one to calculate the apparent redox potential, E0 , of the redox probe inside the film, as the average of the oxidation and reduction potentials.39 It appears that E0  increases with the decrease of pH for Fe(CN)63- and the increase of pH for Ru(NH3)63þ (Table 2), respectively. An increase in E0  corresponds to a stabilization of the redox probe, according to the equation Δ(ΔG)=-nFΔE0 , where ΔE0  and Δ(ΔG) are the changes in apparent redox potential, hence the Donnan potential,40 and in the standard free energy of the Fe(CN)64-/Fe(CN)63- or Ru(NH3)62þ/Ru(NH3)63þ couple, (39) Laugel, N.; Boulmedais, F.; Haitami, A. E. E.; Rabu, P.; Rogez, G.; Voegel, J.-C.; Schaaf, P.; Ball, V. Langmuir 2009, 25, 14030–14036. (40) Tagliazucchi, M.; Williams, F. J.; Calvo, E. J. J. Phys. Chem. B 2007, 111, 8105.

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Table 2. Summary of the Apparent Standard Redox Potentials As Obtained from CV

Scheme 1. Synthetic Route to the Copolymer of Acrylic Acid and [1,3]Oxazine-Bearing Acrylate (POA)

redox potential, V (E0 /V/Ag-AgCl) redox probes

pH 3.0

pH 7.0

pH 10.0

Fe(CN)63Ru(NH3)63þ

0.025 ( 0.004 not measurable

0.005 ( 0.003 -0.045 ( 0.012

-0.008 ( 0.001 -0.043 ( 0.010

Figure 8. Schematic illustration of the pH-switchable on/off function of the POA monolayer film on an ITO substrate. At low pH, the film had a net positive charge that excluded cations but passed anions; at high pH the film excluded negatively charged anions but passed cations.

respectively. In the equation, F is the Faraday constant, and n=1 is the number of electrons exchanged between either Fe(CN)64- and Fe(CN)63- or Ru(NH3)62þ and Ru(NH3)63þ. E0  is measured by averaging the redox potential of the same redox couple on the POAcoated ITO electrode. The analysis results consistently support the selective ionic permeability through a POA monolayer film on ITO electrode, which was electrochemically identified under different pH conditions using redox probe molecules, anionic Fe(CN)63- or cationic Ru(NH3)63þ. The PEM is known to tend to confine [Fe(CN)6]3- ion inside the oppositely charged film.41 This is an interesting question about how the [Fe(CN)6]3- or [Ru(NH3)6]3þ ion-confined PEM could affect the selective permeation of the probe ions through POA membrane. However, it was found that the CV of POA film rinsed in neutral unbuffered solution for 5 min, after the switch experiment in probes solution was almost identical to that of the virgin film (data not shown). The result indicates that the probe ions do not tend to remain in about 1 nm thick POA monolayer film after the buffer rinse extracting them due to the reformation of not-charged oxazine ring from cationic indolium or anionic hemiaminal. We attribute the switching behavior in the POA film to the pHsensitive oxazine chromophores in the membrane, which took either the cationic IC or the anionic NA form, by alternately dipping the film in aqueous solutions of pH 3.0 and pH 10.0. The molecular structure conversion produced a monomolecular film with a positive or a negative net charge that rejected ions of the same sign and transported ions of the opposite sign (Donnan exclusion), as shown schematically in Figure 8. In other words, this membrane could be either cation- or anion-selective, depending on the pH of the solution.

3. Conclusion A copolymer (POA) of poly(acrylic acid) and [1,3]oxazinefunctionalized poly(acrylate) was synthesized and used as a layer component for the preparation of a multilayered film composed of 10 PAH/POA bilayers on a solid substrate. The average (41) Noguchi, T.; Anzai, J. Langmuir 2006, 22, 2870–2875.

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thickness of each PAH/POA bilayer was determined to 1.92 ( 0.04 nm by optical ellipsometry. Interestingly, we found that the reversible opening and closing of the [1,3]oxazine ring sandwiched between the top and bottom PAH layers proceeded smoothly depending on the solution pH without significant steric hindrance for at least 10 cycles. The POA contained the ionizable oxazine, which could be charged either positively at pH 3.0 or negatively at pH 10.0. The POA monolayer film was fabricated on an amino-functionalized ITO electrode and showed excellent ion transport selectivity that was reversibly switchable by pH: at pH 3.0, cations were excluded from the film, but anions easily permeated, whereas at pH 10.0, the opposite behavior was obtained. The steady and reversible switching function of the ultrathin POA monolayer film could potentially be exploited in the development of sophisticated nanodevices, such as ion separation, chemical sensor, or biosensor divices.

4. Experimental Section 4.1. Materials. Poly(styrenesulfonate, sodium salt) (PSS, Mw 70 000), poly(allylamine hydrochloride) (PAH, Mw 70 000), ι-carrageenan CAG, acrylic acid, (3-aminopropyl)triethoxysilane (APS), Ru(NH3)6Cl3, and K3Fe(CN)6 were purchased from Sigma-Aldrich and used as received. MeCN, CH2Cl2, and THF were purchased from Samchun Chemicals. MeCN and CH2Cl2 were distilled over CaH2, and THF was distilled over sodium benzophenone ketyl. Buffer solutions were prepared using the following salts: 0.2 M CH3COOH þ 0.004 M CH3COONa (pH 3.2) and 0.025 M Na2CO3 þ 0.025 M NaHCO3 (pH 10.0).

4.2. Synthesis of a Copolymer of Acrylic Acid and [1,3]Oxazine-Bearing Acrylate, POA. The synthetic route to the POA copolymer is shown in Scheme 1. The [1,3]oxazine-bearing acrylate 2 was obtained via the reaction of acryloyl chloride with [1,3]oxazine, which was synthesized as described by Reymo et al.42 The reaction steps and detailed procedures for the synthesis of 2 are described in Scheme S1 of the Supporting Information. The monomer 2 was subjected to radical copolymerization with acrylic acid, using AIBN as the initiator, to achieve the copolymer POA. Acrylic acid 1 (51 mg, 0.5 mL, 0.7 mmol) and acrylate 2 (100 mg, 0.23 mmol) were dissolved in dry THF. AIBN (10 mg) was added to the reaction mixture, which was warmed to 60 C and stirred for 12 h. After completion of polymerization, the polymer was precipitated by dropping the reaction mixture (condensed by evaporation) into cold diethyl ether. The isolated crude polymer POA was dissolved in THF and dropped into cold diethyl ether to remove impurities. This process was repeated three times, and the pure polymer POA was filtered from diethyl ether as a pale yellow powder. Yield: 68%. 1H NMR (DMSO-d6, 40 C 400 MHz): 1.05 (3H, s, CH3), 1.09 (2H, d, CH2), 1.81 (2H, d, CH2), 1.86 (3H, s, CH3), 3.36(1H, t, CH), 3.61 (1H, t, CH), 4.00-5.00 (2H, dd, CH2), (42) Reymo, F. M.; Giordani, S. J. Am. Chem. Soc. 2001, 123, 4651–4652.

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Figure 9. 400 MHz 1H NMR spectrum of POA in DMSO-d6 at

40 C.

6.98-7.00 (2H, m, ArH), 7.40-7.62 (5H, m, ArH), 8.03-8.14 (4H, m, ArH), 12.20 ppm (1H, br, COOH). FTIR: ν (cm-1): 3500 (br, O-H str), 3050 (aromatic C-H str), 1723 (ketone, CdO str), 1350 (-NO2 str), 1300 (C-N str). The molecular structure and composition of the copolymer POA were identified by 1H NMR spectroscopy. The copolymer composition of 2 was estimated to be about 45% based on the clearly separated signals of methylene in the [1,3]oxazine moiety (δ (ppm)=3.32, CH) and acrylic acid (δ (ppm)=3.52, CH) in the 400 MHz 1H NMR spectrum of POA (Figure 9). POA was soluble in THF, DMSO, and DMF but was insoluble in water. However, it should be noted that POA remained stably dissolved in a 1% DMSO/H2O mixture that was obtained by dilution of the DMSO solution with water. 4.3. Electrostatic Self-Assembly. Ultrapure water (Milli-Q, Millipore GmbH, 18.2 MΩ cm) was used for all experiments and cleaning steps. The substrates selected for polyelectrolyte multilayer buildup were quartz glass (12  45 mm2), silicon wafer (2  2 cm2), or ITO glass (20  10 mm2). The quartz glass and silicon wafer were cleaned by ultrasonication in a piranha solution (H2SO4/H2O2 =7:3), followed by cleaning in a mixture of H2O/ H2O2/NH3 (5:1:1) at 80 C for 1 h. The substrates were thoroughly washed with ultrapure water after both steps. Layer-by-layer deposition of polyelectrolytes was initiated on a cleaned substrate (quartz glass or fused silica slide) that was precoated with five PAH/CAG bilayers by spin self-assembly technique (5000 rpm, 20 s) using aqueous solution of PAH (2 unit mM, pH 5.0) and CAG (5 unit mM, pH 6.0). Next, the slide was dipped into an aqueous solution of PAH (2 unit mM, pH 5.0) for 20 min. The loosely adsorbed materials were washed by dipping the slide three times in deionized water for 1 min each. An anionic polyelectrolyte layer was then deposited by dipping the slide in 1 vol % DMSO/H2O solution of POA (3 unit mM, pH 5.0) to obtain the subsequent multilayer deposition. After each adsorption step, the surface of the film was thoroughly dried under a gentle stream of nitrogen gas. The quantity of material deposited

11354 DOI: 10.1021/la1007044

Pennakalathil et al. at each step was determined by absorption on a Perkin-Elmer UV-vis spectrophotometer (Lambda 40). For the cyclovoltammetric experiment, the ITO glass (1  2 cm2) was washed with acetone, ethanol, and deionized water. The cleaned substrate was then dried under a gentle stream of nitrogen. The substrate was immersed in a freshly prepared piranha solution at room temperature for 10 s, thoroughly washed with deionized water, and dried under a gentle stream of nitrogen. Amino-fuctionalization of the ITO surface was performed by dipping in APS (1 mM in toluene) for 45 min. The substrate was stored in a 0.1 M HCl solution to produce a positive surface charge. POA monolayer deposition on the amino-functionalized ITO was carried out manually using a 3 mM POA solution in 1 vol % DMSO/H2O. 4.4. Methods. Nuclear magnetic resonance spectra were recorded on a Bruker (400 MHz) spectrometer. UV/vis absorption spectra were recorded on a spectrometer (Perkin-Elmer, Lambda 40) using a quartz cell with a path length of 10 mm. IR spectra were recorded on an infrared spectrometer (Nicolet, Magnem IR 500). The thickness of the multilayer film mounted on the silicon wafers was measured using a real-time spectroscopic ellipsometer (Ellipso Technology, Elli-SE-F) with a Xe arc lamp (350-820 nm) equipped with a rotating polarizer, a liquid cell with optical access at an incidence angle of 60, an analyzer, and a multichannel detection system. Employing a self-made computer program, the elliptical azimuth and phase angle were calculated for both the cleaned reference substrate and the multilayer films. These experimental values were used to calculate the corresponding film thickness. At least 3-5 sampling points were measured to obtain the average thickness. The AFM measurements were performed on a Si wafer in air at room temperature by using a Nanoscope IV multimode microscope (Veeco). Using a 125 μm long Si cantilever, a below 10 nm of tip radius, and a resonance frequency 320 kHz (Nanoworld) with a force constant of 42 N/m, topographic images were recorded in tapping mode (500 nm  500 nm size) at a scan rate of 0.854 Hz. Data were manipulated using Nanoscope IV software. Cyclic voltammetry measurements were performed using IviumStat Technologies system with a three-electrode cell at a scan rate of 50 mV/s. A platinum wire was used as the counter electrode, and Ag/AgCl (3 M KCl) was used as the reference electrode. Aqueous solution of 0.5 M Na2SO4 containing either Fe(CN)63- or Ru(NH3)63þ were buffered at pH 3.2, 10.0, or not buffered at pH 7.0 to prepare the electrolyte solutions. Note that the pH of the POA solution was adjusted using 1 N HCl or 1 N NaOH. A POA monolayer-coated ITO glass was used as the working electrode. The electrolyte solutions were purged with nitrogen for 20 min prior to performing the experiments.

Acknowledgment. This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2008- 521D00121). Also, the authors gratefully acknowledge the use of AFM in Korea Basic Science Institute (Jeonju Center). Supporting Information Available: Experimental details. This material is available free of charge via the Internet at http:// pubs.acs.org.

Langmuir 2010, 26(13), 11349–11354