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Preparation of free-suspended polyelectrolyte multilayer films using an alginate scaffold and their ion permeability
Materials Science and Engineering C 32 (2012) 2649–2653
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Short communication
Preparation of free-suspended polyelectrolyte multilayer films using an alginate scaffold and their ion permeability Katsuhiko Sato, Takuto Shiba, Jun-ichi Anzai ⁎ Graduate School of Pharmaceutical Sciences, Tohoku University, Aramaki, Aoba-ku, Sendai 980‐8578, Japan
a r t i c l e
i n f o
Article history: Received 26 September 2011 Received in revised form 22 May 2012 Accepted 11 June 2012 Available online 15 June 2012 Keywords: Layer-by-layer film PEM film Free-suspended PEM film Ion permeability
a b s t r a c t Free-suspended polyelectrolyte multilayer (PEM) films were prepared over an aperture (diameter, 1 or 3 mm) in a glass plate to study ion permeation across the PEM films. PEM films were successfully constructed by layer-by-layer deposition of poly(allylamine hydrochloride) and poly(styrene sulfonate) on the surface of glass plate whose aperture had been filled with an alginate gel, followed by dissolution of the alginate gel in an ethylenediaminetetraacetic acid solution. PEM films thus prepared showed selective permeability for inorganic salts such as NaCl, KCl, CaCl2, Na2SO4, K3[Fe(CN)6], and K4[Fe(CN)6]. The results were rationalized based on the size and charge of the ions and the thickness and surface charges of films. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The development of polyelectrolyte multilayer (PEM) films is currently a focal subject in the fields of materials science and engineering [1–4]. PEM films have been successfully prepared by an alternate layer-by-layer (LbL) deposition of oppositely charged polyelectrolytes including synthetic and natural polymers on the surface of a solid support [5–9]. PEM films have been used for constructing electronic and optical sensors [10–13], drug delivery systems [14–17], stimuli-sensitive systems [18–22], molecular architectures [23–27], etc. PEM films have been used also for constructing selective layers for separation and purification of gases and ions [28–30]. Early studies showed that monovalent and multivalent ions can be effectively separated using PEM films deposited on the surface of porous supports [31–33]. For example, PEM films composed of poly(allylamine hydrochloride) (PAH) and poly(styrenesufonate) (PSS) exhibited high flux for monovalent ions while fluxes for divalent and trivalent ions were significantly suppressed. The results were rationalized in terms of enhanced Donnan exclusion as well as hindered diffusion of multivalent ions. PEM films may be thus promising in future applications to ion selective layers of sensors and separators. In the works cited above, PEM films were deposited on solid supports with micro-pores (pore diameter: 20–200 nm) because of a fragile nature of PEM films (thickness of the PEM films was estimated to be less than 100 nm) [31,32].
⁎ Corresponding author. E-mail address: [email protected] (J. Anzai). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2012.06.012
However, the evaluation of ion permeability of support-free PEM films would be of significant importance. For this purpose, in the present study, we have prepared PEM films on the surface of glass supports with 1 or 3 mm aperture and studied ion permeability of the films. The effective area of the PEM films developed in the present study is significantly higher than that of the supported PEM films reported previously [31–33]. It has been reported that free-standing PEM films can be prepared using decomposable thin films as sacrificial layer [34–36]. For example, Ono and Decher prepared freestanding PEM films composed of PAH and PSS using a sacrificial thin films consisting of poly(acrylic acid) (PAA) and poly(ethylene glycol) (PEG) [34]. They constructed (PAA−PEG)−(PAH−PSS) binary architecture on a solid surface followed by dissolving the (PAA−PEG) layer by pH change, resulting in peeling off of free-standing PAH−PSS films. Robust PEM films can be produced by cross-linking or introduction of nanoparticles [35,36]. However, ion permeability of such freestanding PEM films has not yet evaluated probably due to fragility of the films. 2. Experimental 2.1. Materials Poly(sodium styrenesufonate) [PSS; average molecular weight (MW), ~5000,000] was purchased from Scientific Polymer Products, Inc. (NY, USA). An aqueous solution (20%) of poly(allylamine hydrochloride) [PAH; MW, ~150,000] was obtained from Nittobo Co. (Tokyo, Japan). Fluorescein-modified PAH (F-PAH) was synthesized by the reaction of 100 mg PAH and 16.2 mg fluoresceinisothiocyanate as reportedly [37]
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K. Sato et al. / Materials Science and Engineering C 32 (2012) 2649–2653
ALG gel
ALG
PAH
Repeat
PSS
EDTA
Fig. 1. Schematic illustration for the preparation of free-suspended PEM film.
(about 3 mol% amino groups in PAH was modified with fluorescein residue, as determined by UV-visible spectrometry). All other reagents used are of the highest grade available.
2.2. Preparation of PEM films A glass disc (diameter, 20 mm; thickness, 1 mm) with an aperture (diameter, 1 or 3 mm) was thoroughly rinsed before use. The glass disc was placed on a glass plate and the aperture was filled with an appropriate volume of the mixture of 2% alginic acid (ALG) solution and 10% CaCl2 solution (2:3 by volume) to construct an ALG gel scaffold (the gelation was completed in an approximately 15 min). The ALG gel-filled glass disc was then immersed in 2 mg/mL PAH solution (0.1 M Tris–HCl buffer containing 1% CaCl2, pH 7.0) for 15 min to cover the ALG gel surface with a PAH layer through an electrostatic affinity and rinsed in the working buffer for 5 min. In a similar manner, the next PSS layer was deposited on the ALG gel by immersing the PAH-covered ALG gel in 2 mg/mL PSS solution (0.1 M Tris–HCl buffer containing 1% CaCl2, pH 7.0) for 15 min and rinsed in the working buffer for 5 min. The alternate deposition of PAH and PSS layers was repeated to construct PEM films with a desired number of layers. After deposition of PEM film, the glass disc was immersed in 0.2 M ethylenediaminetetraacetic acid (EDTA) solution for 30 min to dissolve the ALG scaffold. It is known that ALG gels cross-linked with Ca 2+ ion can be dissolved in the solution containing EDTA by chelating the Ca 2+ ion [38,39]. Then, PEM film deposited on one side of the glass disc was carefully removed. Fig. 1 schematically illustrates the procedure for constructing the PAH-PSS PEM film.
2.3. Absorption spectra of PEM films PEM films were prepared using F-PAH and PSS in a similar manner on a glass plate (1 × 9 × 40 mm 3) with an aperture (diameter, 1 mm) for measuring the absorption spectra. One side of the glass plate was coated with black paints before use to block the incident light so that absorption spectrum of PEM films located over the aperture is recorded exclusively. The absorption spectra of PEM films were recorded in 0.1 M Tris–HCl buffer containing 1% CaCl2, pH 7.0, using a UV-visible absorption spectrophotometer (UV-3100PC, Shimadzu Co., Kyoto, Japan).
2.4. Ion permeation The apparatus used for measuring ion permeation is shown in Fig. 2. One of the chambers contained salt solution (source phase) and the other chamber was filled with distilled water (receiving phase). The volume of the solution in each side was 50 mL. The conductivity of solution in the receiving phase was monitored using a conductivity cell (3552-10D, Horiba Co., Kyoto, Japan) to determine the concentration of salt transported across the film. Both source and receiving phase solutions were gently stirred to minimize concentration polarization at the film-solution interfaces. The effect of osmotic flow on the solution volume was negligible under the experimental conditions. All measurements were carried out at an ambient temperature (ca. 20 °C).
A
B
Absorbance
2.4
2.0
e d c b a
1.6
1.2 450
500
550
600
650
700
Wavelength / nm Fig. 2. U-shaped glass cell for measuring ion permeation. (A) Source phase (50 mL). (B) Receiving phase (50 mL).
Fig. 3. Absorption spectra of PEM films composed of F-PAH and PSS. The number of layers: 3 (b), 5 (c), 7 (d), and 10 (e). The spectrum a is that of glass plate recorded without PEM film.
K. Sato et al. / Materials Science and Engineering C 32 (2012) 2649–2653
3. Results and discussion
3.2. Ion permeation Ion permeability of PEM films thus prepared was evaluated using the U-shaped glass cell illustrated in Fig. 2. Fig. 5A shows typical results for the permeation of NaCl, KCl, CaCl2, and Na2SO4 across 5bilayer (PAH-PSS)5 film as a function of time. The concentration of salts in the receiving phase linearly increased with time, suggesting steady-state ion permeation across the PEM film occurred. This implies that the change in the salt concentration in the source phase is negligibly small during the measurement. In fact, judging from the conductivity, transported ions across the film within the experimental time scale (i.e., 120 min) is less than 1 μmol, which corresponds to ca. 0.02% of the amounts of salts added in the source phase. Consequently, the concentration of salts in source phase is practically constant during the permeation and thus we can estimate ion permeability of the salts across the PEM film from the slope of the linear plots. The transfer of NaCl across the aperture (1 mm diameter) in the glass disc without PEM film was also evaluated under the same experimental conditions (Fig. 5B). NaCl was transported rapidly across the aperture and the concentration of NaCl in the two phases reached nearly equilibrium after 120 min. The source phase contained 0.1 M NaCl solution and the receiving phase was filled with distilled water before permeation (the volume of each phase was 50 mL). Consequently, both phases should contain 2.5 mmol NaCl in the equilibrium. The plot gave a curved line not a straight line because of the decreased concentration of NaCl in the source phase during the transport. Note here that the salt transport across the aperture of the glass disc without PEM film may originate from convection of the solutions not from diffusion of NaCl because the solutions in both phases were stirred constantly throughout the experiment. Fig. 6 compares ion permeability across (PAH-PSS)5 and (PAHPSS)5PAH films (film diameters, 1 mm for Fig. 6A and B and 3 mm for Fig. 6C and D). Permeability of KCl and NaCl was high, whereas
Fig. 4. A photo of the (PAH-PSS)5 PEM film suspended over a 3 mm aperture in a glass disc.
Salt transported / µmol
Fig. 3 shows absorption spectra of PEM films prepared by the layer-by-layer deposition of F-PAH and PSS. The films showed absorption maxima around 500 nm originating from fluorescein moiety in FPAH. The intensity of the peaks increased with the increasing number of depositions, suggesting F-PAH and PSS were successfully deposited in a layer-by-layer fashion. It is noted that the deposition of at least 3 bilayers, (F-PAH-PSS)3, was prerequisite for constructing free-suspended PEM film due to fragility of thinner films. The enhanced baseline for the spectra is ascribed to a slightly turbid nature of the PEM films. Fig. 4 shows a photo of typical PAH-PSS PEM film suspended over a 3 mm aperture. These data clearly show that free-suspended PEM films composed of PAH and PSS can be prepared using the ALG scaffold.
A
1.0
a
0.8
b 0.6 0.4
c
0.2
d 0 0
30
60
90
120
90
120
Time / min 2.5
Salt transported / mmol
3.1. Preparation of PEM film
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B
2.0 1.5 1.0 0.5 0 0
30
60
Time / min Fig. 5. (A) Permeation of KCl (a), NaCl (b), CaCl2 (c), and Na2SO4 (d) across (PAH-PSS)5 film with 1 mm diameter. (B) Permeation of NaCl across the aperture of 1 mm diameter in the glass disc without PEM film. Source phase (50 mL): 0.1 M salt solution, receiving phase (50 mL): H2O.
permeation of other multivalent ion-containing salts was significantly suppressed for both the (PAH-PSS)5 and (PAH-PSS)5PAH films. This is basically in line with the results reported for PEM films deposited on porous supports. Tieke and coworkers reported that permeation rates of NaCl and KCl across polyelectrolyte PEM films are significantly higher than those of multivalent ion-containing salts such as MgCl2, BaCl2, Na2SO4, and K4[Fe(CN)6] [32,33]. The higher permeability of NaCl and KCl can be rationalized based on smaller size and lower charge of the ions. One possible barrier to ion permeation across the PEM film may be Donnan exclusion to ions at the surface of PEM film because the outermost surface of the PEM film always contains excess charges. It is most likely that NaCl and KCl undergo Donnan exclusion less effectively than multivalent ions. The suppressed CaCl2 permeation is ascribable to enhanced Donnan exclusion due to the divalent nature, though a possible effect of a trace level of contaminated ALG in the film cannot be fully excluded. A significant role of Donnan exclusion can be found in the permeation of K3[Fe(CN)6] and K4 [Fe(CN)6], in which the permeation was highly suppressed. Another possible factor affecting the ion permeation is diffusion rate in the PEM film, which probably relates to the effective size of ions. It is clear that sizes of Na +, K +, and Cl − ions are smaller than the other ions tested. These considerations support higher permeability of NaCl and KCl. Extremely suppressed permeation of multivalent Fe(CN)63− and Fe(CN)64− ions also supports this view. It may be interesting to evaluate the effects of surface charges of PEM films on the permeation rate of salts, which was not clearly discussed in previous reports [31–33]. We have found that ion permeation evidently depends on the polarity of the surface charge of the PEM films. Permeability of NaCl, KCl, and CaCl2 across the (PAH-PSS)5 film is higher than those across the (PAH-PSS)5PAH film. Two mechanisms may be plausible for the suppressed permeation in the (PAH-PSS)5PAH
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0.03
mol h -1
A
1.0 0.8
Permeation rate /
Permeation rate /
mol h -1
1.2
0.6 0.4 0.2
B
0.02
0.01
0
0 NaCl
KCl
C2aCl
K3[Fe(CN)6]
Na2SO4
mol h -1
mol h -1
Permeation rate /
10
Permeation rate /
C 15
5
0
K4[Fe(CN)6]
D 0.2
0.1
0 NaCl
KCl
CaCl2
Na2SO4
K3[Fe(CN)6]
K4[Fe(CN)6]
Fig. 6. Permeability of salts across (PAH-PSS)5 (open columns) and (PAH-PSS)5PAH films (filled columns). Data in (A) and (B) were collected using PEM films with 1 mm diameter, while PEM films with 3 mm diameter were used for collecting the data in (C) and (D).
film, being Donnan exclusion to the cations at the positively charged surface due to PAH and enhanced film thickness by the addition of terminal PAH layer. On the other hand, opposite trend was observed for Na2SO4, K3[Fe(CN)6], and K4[Fe(CN)6], suggesting predominant effect of surface charge of the PEM film. It is reasonable to assume that SO42−, Fe(CN)63−, and Fe(CN)64− ions undergo a significant Donnan exclusion at the negatively charged surface of (PAH-PSS)5 film. The addition of terminal PAH layer cancelled the Donnan effect, resulting in the enhanced permeability for the multivalent ions. The effects of film thickness on the permeability of NaCl and KCl are shown in Fig. 7. For both NaCl and KCl, the permeation rate decreased with the increasing number of layers, suggesting a predominant role of diffusion of these ions in the film interior for determining the overall permeation rate. In this context, it has been reported that the permeation rate of NaCl across (PAH-PSS)10 film is about a half of that across (PAH-PSS)5 film [32]. The permeation
rate of KCl is always higher than that of NaCl if the film thickness is identical. This may be ascribable to smaller Stokes radius of K + ion (1.25 × 10 − 10 m) than that of Na + ion (1.84 × 10 − 10 m) [40]. Reusability of (PAH-PSS)5 film for permeation of NaCl and CaCl2 was evaluated. Fig. 8 shows relative permeation rates of NaCl and CaCl2 across the (PAH-PSS)5 film determined 5 times alternately. The film was rinsed in distilled water for 1 h after each measurement. Permeation rates for both NaCl and CaCl2 exhibited nearly constant values for the 5 measurements, showing an excellent reusability of the PEM film. In contrast, decreased permeability of NaCl was reported for supported PEM films after repeated use [33]. A longterm stability of the PEM film was also tested. To this goal, NaCl permeability across the (PAH-PSS)5 film was evaluated once a week for 10 weeks (the film was stored in distilled water at 4 °C when not in use). Virtually no deterioration in the NaCl transport was observed after 10 weeks.
1.0
Relative permeation rate
Permeation rate / mol h-1
1.2
0.8 0.6 0.4 0.2
1.0 0.8 0.6 0.4 0.2 0
0 5
10
Number of bilayers
15
Fig. 7. Effects of the film thickness on the rate of permeation of NaCl (open columns) and KCl (filled columns) across (PAH-PSS)n films (n = 5, 10, and 15).
1
2
3
4
5
Number of measurements Fig. 8. Reusability of (PAH-PSS)5 film for the transport of NaCl (open columns) and CaCl2 (filled columns).
K. Sato et al. / Materials Science and Engineering C 32 (2012) 2649–2653
4. Conclusion Free-suspended PEM films with 1 and 3 mm diameter can be prepared using ALG gel scaffold. The PEM films exhibited selective permeation for metal and metal complex ions depending on the ionic size and charge. Film thickness and surface charge significantly affected the permeation rates of ions. The present protocol would be useful for the construction of PEM films for biomedical applications as well as for selective layers in sensors and separators. A further study for improving mechanical stability of the films is now in progress in our laboratory. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
G. Decher, Science 277 (1997) 1232. Z. Tang, Y. Wang, P. Podasiadlo, N.A. Kotov, Adv. Mater. 18 (2006) 3203. K. Ariga, J.P. Hill, Q. Ji, Phys. Chem. Chem. Phys. 9 (2007) 2319. B.G. De Geest, N.N. Sanders, G.B. Sukhorukov, J. Demeester, S.C. De Smedt, Chem. Soc. Rev. 36 (2007) 639. Z. Dai, J.T. Wilison, E.L. Chaikof, Mater. Sci. Eng., C Bio. S. 27 (2007) 402. K. Sato, I. Suzuki, J. Anzai, Langmuir 19 (2003) 7406. T. Noguchi, J. Anzai, Langmuir 22 (2006) 2870. B. Wang, J. Anzai, Langmuir 23 (2007) 7378. L.L. der Mercato, P. Rivera-Gil, A.Z. Abbasi, M. Ochs, C. Ganas, I. Zins, C. Sonnichsen, W.L. Parak, Nanoscale 2 (2010) 458. A. Liu, J. Anzai, Langmuir 19 (2003) 4043. X. Qin, H. Wang, X. Wang, S. Li, Z. Miao, N. Huang, Q. Chen, Mater. Sci. Eng., C Bio. S. 29 (2009) 1453. H. Yao, N. Hu, J. Phys. Chem. 114 (2010) 3380.
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[13] B.Y. Wu, S.H. Hou, F. Yin, Z.X. Zhao, Y.Y. Wang, X.S. Wang, Q. Chen, Biosens. Bioelectron. 22 (2007) 2854. [14] C.M. Nolan, M.J. Serpe, L.A. Lyon, Biomacromolecules 5 (2004) 1940. [15] S. Tomita, K. Sato, J. Anzai, J. Colloid Interface Sci. 326 (2008) 35. [16] K. Yoshida, K. Sato, J. Anzai, J. Mater. Chem. 20 (2010) 1546. [17] K. Sato, K. Yoshida, S. Takahashi, J. Anzai, Adv. Drug Deliv. Rev. 63 (2011) 809. [18] F. Boulmedais, C.S. Tang, B. Keller, J. Vörös, Adv. Funct. Mater. 16 (2006) 63. [19] K. Sato, D. Kodama, Y. Naka, J. Anzai, Biomacromolecules 7 (2006) 3302. [20] Y. Egawa, R. Hayashida, J. Anzai, Langmuir 23 (2007) 13146. [21] K.C. Wood, N.S. Zacharie, D.J. Schmidt, S.N. Wrightman, B.J. Andaya, P.T. Hammond, Proc. Nat. Acad. Sci. 105 (2008) 2280. [22] K. Sato, Y. Imoto, J. Sugama, S. Seki, H. Inoue, T. Odagiri, T. Hoshi, J. Anzai, Langmuir 21 (2005) 797. [23] D.M. Lynn, Adv. Mater. 19 (2007) 4118. [24] J. Anzai, Y. Kobayashi, Langmuir 16 (2000) 2851. [25] L. Liu, Z. Chen, S. Yang, X. Jin, X. Lin, Sens. Actuators, B 129 (2008) 218. [26] H. Yao, N. Hu, J. Phys. Chem. B 114 (2010) 9926. [27] T. Hoshi, S. Akase, J. Anzai, Langmuir 18 (2002) 7024. [28] T.R. Farhat, J.B. Schlenoff, Langmuir 17 (2001) 1184. [29] P. Stoeve, V. Vasquez, M.A.N. Coelho, J.F. Rabolt, Thin Solid Films 284 (1996) 708. [30] S.K. Kumar, J.D. Hong, Langmuir 24 (2008) 4190. [31] J.J. Harris, J.L. Stair, M.L. Bruening, Chem. Mater. 12 (2000) 1941. [32] L. Krasemann, B. Tieke, Languir 16 (2000) 287. [33] A. Toutianoush, B. Tieke, Mater. Sci. Eng., C 22 (2002) 135. [34] S.S. Ono, G. Decher, Nano Lett. 6 (2006) 592–598. [35] J. Park, J. Kim, S. Lee, J. Bang, B.J. Kim, Y.S. Kim, J. Cho, J. Mater. Chem. 19 (2009) 4488–4490. [36] C. Jiang, S. Markutsya, V.V. Tsukruk, Adv. Mater. 16 (2004) 157–161. [37] J. Goicoechea, C.R. Zamarreno, I.R. Matias, Sens. Actuators, B 132 (2008) 305. [38] H. Zhu, R. Srivastava, M.J. McShane, Biomacromolecules 6 (2005) 2221. [39] K. Sato, N. Hoshina, J. Anzai, Polym. Bull. 68 (2012) 891. [40] P.C.F. Pau, J.O. Berg, W.G. McMillan, J. Phys. Chem. 94 (1990) 2671.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Short communication
Preparation of free-suspended polyelectrolyte multilayer films using an alginate scaffold and their ion permeability Katsuhiko Sato, Takuto Shiba, Jun-ichi Anzai ⁎ Graduate School of Pharmaceutical Sciences, Tohoku University, Aramaki, Aoba-ku, Sendai 980‐8578, Japan
a r t i c l e
i n f o
Article history: Received 26 September 2011 Received in revised form 22 May 2012 Accepted 11 June 2012 Available online 15 June 2012 Keywords: Layer-by-layer film PEM film Free-suspended PEM film Ion permeability
a b s t r a c t Free-suspended polyelectrolyte multilayer (PEM) films were prepared over an aperture (diameter, 1 or 3 mm) in a glass plate to study ion permeation across the PEM films. PEM films were successfully constructed by layer-by-layer deposition of poly(allylamine hydrochloride) and poly(styrene sulfonate) on the surface of glass plate whose aperture had been filled with an alginate gel, followed by dissolution of the alginate gel in an ethylenediaminetetraacetic acid solution. PEM films thus prepared showed selective permeability for inorganic salts such as NaCl, KCl, CaCl2, Na2SO4, K3[Fe(CN)6], and K4[Fe(CN)6]. The results were rationalized based on the size and charge of the ions and the thickness and surface charges of films. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The development of polyelectrolyte multilayer (PEM) films is currently a focal subject in the fields of materials science and engineering [1–4]. PEM films have been successfully prepared by an alternate layer-by-layer (LbL) deposition of oppositely charged polyelectrolytes including synthetic and natural polymers on the surface of a solid support [5–9]. PEM films have been used for constructing electronic and optical sensors [10–13], drug delivery systems [14–17], stimuli-sensitive systems [18–22], molecular architectures [23–27], etc. PEM films have been used also for constructing selective layers for separation and purification of gases and ions [28–30]. Early studies showed that monovalent and multivalent ions can be effectively separated using PEM films deposited on the surface of porous supports [31–33]. For example, PEM films composed of poly(allylamine hydrochloride) (PAH) and poly(styrenesufonate) (PSS) exhibited high flux for monovalent ions while fluxes for divalent and trivalent ions were significantly suppressed. The results were rationalized in terms of enhanced Donnan exclusion as well as hindered diffusion of multivalent ions. PEM films may be thus promising in future applications to ion selective layers of sensors and separators. In the works cited above, PEM films were deposited on solid supports with micro-pores (pore diameter: 20–200 nm) because of a fragile nature of PEM films (thickness of the PEM films was estimated to be less than 100 nm) [31,32].
⁎ Corresponding author. E-mail address: [email protected] (J. Anzai). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2012.06.012
However, the evaluation of ion permeability of support-free PEM films would be of significant importance. For this purpose, in the present study, we have prepared PEM films on the surface of glass supports with 1 or 3 mm aperture and studied ion permeability of the films. The effective area of the PEM films developed in the present study is significantly higher than that of the supported PEM films reported previously [31–33]. It has been reported that free-standing PEM films can be prepared using decomposable thin films as sacrificial layer [34–36]. For example, Ono and Decher prepared freestanding PEM films composed of PAH and PSS using a sacrificial thin films consisting of poly(acrylic acid) (PAA) and poly(ethylene glycol) (PEG) [34]. They constructed (PAA−PEG)−(PAH−PSS) binary architecture on a solid surface followed by dissolving the (PAA−PEG) layer by pH change, resulting in peeling off of free-standing PAH−PSS films. Robust PEM films can be produced by cross-linking or introduction of nanoparticles [35,36]. However, ion permeability of such freestanding PEM films has not yet evaluated probably due to fragility of the films. 2. Experimental 2.1. Materials Poly(sodium styrenesufonate) [PSS; average molecular weight (MW), ~5000,000] was purchased from Scientific Polymer Products, Inc. (NY, USA). An aqueous solution (20%) of poly(allylamine hydrochloride) [PAH; MW, ~150,000] was obtained from Nittobo Co. (Tokyo, Japan). Fluorescein-modified PAH (F-PAH) was synthesized by the reaction of 100 mg PAH and 16.2 mg fluoresceinisothiocyanate as reportedly [37]
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K. Sato et al. / Materials Science and Engineering C 32 (2012) 2649–2653
ALG gel
ALG
PAH
Repeat
PSS
EDTA
Fig. 1. Schematic illustration for the preparation of free-suspended PEM film.
(about 3 mol% amino groups in PAH was modified with fluorescein residue, as determined by UV-visible spectrometry). All other reagents used are of the highest grade available.
2.2. Preparation of PEM films A glass disc (diameter, 20 mm; thickness, 1 mm) with an aperture (diameter, 1 or 3 mm) was thoroughly rinsed before use. The glass disc was placed on a glass plate and the aperture was filled with an appropriate volume of the mixture of 2% alginic acid (ALG) solution and 10% CaCl2 solution (2:3 by volume) to construct an ALG gel scaffold (the gelation was completed in an approximately 15 min). The ALG gel-filled glass disc was then immersed in 2 mg/mL PAH solution (0.1 M Tris–HCl buffer containing 1% CaCl2, pH 7.0) for 15 min to cover the ALG gel surface with a PAH layer through an electrostatic affinity and rinsed in the working buffer for 5 min. In a similar manner, the next PSS layer was deposited on the ALG gel by immersing the PAH-covered ALG gel in 2 mg/mL PSS solution (0.1 M Tris–HCl buffer containing 1% CaCl2, pH 7.0) for 15 min and rinsed in the working buffer for 5 min. The alternate deposition of PAH and PSS layers was repeated to construct PEM films with a desired number of layers. After deposition of PEM film, the glass disc was immersed in 0.2 M ethylenediaminetetraacetic acid (EDTA) solution for 30 min to dissolve the ALG scaffold. It is known that ALG gels cross-linked with Ca 2+ ion can be dissolved in the solution containing EDTA by chelating the Ca 2+ ion [38,39]. Then, PEM film deposited on one side of the glass disc was carefully removed. Fig. 1 schematically illustrates the procedure for constructing the PAH-PSS PEM film.
2.3. Absorption spectra of PEM films PEM films were prepared using F-PAH and PSS in a similar manner on a glass plate (1 × 9 × 40 mm 3) with an aperture (diameter, 1 mm) for measuring the absorption spectra. One side of the glass plate was coated with black paints before use to block the incident light so that absorption spectrum of PEM films located over the aperture is recorded exclusively. The absorption spectra of PEM films were recorded in 0.1 M Tris–HCl buffer containing 1% CaCl2, pH 7.0, using a UV-visible absorption spectrophotometer (UV-3100PC, Shimadzu Co., Kyoto, Japan).
2.4. Ion permeation The apparatus used for measuring ion permeation is shown in Fig. 2. One of the chambers contained salt solution (source phase) and the other chamber was filled with distilled water (receiving phase). The volume of the solution in each side was 50 mL. The conductivity of solution in the receiving phase was monitored using a conductivity cell (3552-10D, Horiba Co., Kyoto, Japan) to determine the concentration of salt transported across the film. Both source and receiving phase solutions were gently stirred to minimize concentration polarization at the film-solution interfaces. The effect of osmotic flow on the solution volume was negligible under the experimental conditions. All measurements were carried out at an ambient temperature (ca. 20 °C).
A
B
Absorbance
2.4
2.0
e d c b a
1.6
1.2 450
500
550
600
650
700
Wavelength / nm Fig. 2. U-shaped glass cell for measuring ion permeation. (A) Source phase (50 mL). (B) Receiving phase (50 mL).
Fig. 3. Absorption spectra of PEM films composed of F-PAH and PSS. The number of layers: 3 (b), 5 (c), 7 (d), and 10 (e). The spectrum a is that of glass plate recorded without PEM film.
K. Sato et al. / Materials Science and Engineering C 32 (2012) 2649–2653
3. Results and discussion
3.2. Ion permeation Ion permeability of PEM films thus prepared was evaluated using the U-shaped glass cell illustrated in Fig. 2. Fig. 5A shows typical results for the permeation of NaCl, KCl, CaCl2, and Na2SO4 across 5bilayer (PAH-PSS)5 film as a function of time. The concentration of salts in the receiving phase linearly increased with time, suggesting steady-state ion permeation across the PEM film occurred. This implies that the change in the salt concentration in the source phase is negligibly small during the measurement. In fact, judging from the conductivity, transported ions across the film within the experimental time scale (i.e., 120 min) is less than 1 μmol, which corresponds to ca. 0.02% of the amounts of salts added in the source phase. Consequently, the concentration of salts in source phase is practically constant during the permeation and thus we can estimate ion permeability of the salts across the PEM film from the slope of the linear plots. The transfer of NaCl across the aperture (1 mm diameter) in the glass disc without PEM film was also evaluated under the same experimental conditions (Fig. 5B). NaCl was transported rapidly across the aperture and the concentration of NaCl in the two phases reached nearly equilibrium after 120 min. The source phase contained 0.1 M NaCl solution and the receiving phase was filled with distilled water before permeation (the volume of each phase was 50 mL). Consequently, both phases should contain 2.5 mmol NaCl in the equilibrium. The plot gave a curved line not a straight line because of the decreased concentration of NaCl in the source phase during the transport. Note here that the salt transport across the aperture of the glass disc without PEM film may originate from convection of the solutions not from diffusion of NaCl because the solutions in both phases were stirred constantly throughout the experiment. Fig. 6 compares ion permeability across (PAH-PSS)5 and (PAHPSS)5PAH films (film diameters, 1 mm for Fig. 6A and B and 3 mm for Fig. 6C and D). Permeability of KCl and NaCl was high, whereas
Fig. 4. A photo of the (PAH-PSS)5 PEM film suspended over a 3 mm aperture in a glass disc.
Salt transported / µmol
Fig. 3 shows absorption spectra of PEM films prepared by the layer-by-layer deposition of F-PAH and PSS. The films showed absorption maxima around 500 nm originating from fluorescein moiety in FPAH. The intensity of the peaks increased with the increasing number of depositions, suggesting F-PAH and PSS were successfully deposited in a layer-by-layer fashion. It is noted that the deposition of at least 3 bilayers, (F-PAH-PSS)3, was prerequisite for constructing free-suspended PEM film due to fragility of thinner films. The enhanced baseline for the spectra is ascribed to a slightly turbid nature of the PEM films. Fig. 4 shows a photo of typical PAH-PSS PEM film suspended over a 3 mm aperture. These data clearly show that free-suspended PEM films composed of PAH and PSS can be prepared using the ALG scaffold.
A
1.0
a
0.8
b 0.6 0.4
c
0.2
d 0 0
30
60
90
120
90
120
Time / min 2.5
Salt transported / mmol
3.1. Preparation of PEM film
2651
B
2.0 1.5 1.0 0.5 0 0
30
60
Time / min Fig. 5. (A) Permeation of KCl (a), NaCl (b), CaCl2 (c), and Na2SO4 (d) across (PAH-PSS)5 film with 1 mm diameter. (B) Permeation of NaCl across the aperture of 1 mm diameter in the glass disc without PEM film. Source phase (50 mL): 0.1 M salt solution, receiving phase (50 mL): H2O.
permeation of other multivalent ion-containing salts was significantly suppressed for both the (PAH-PSS)5 and (PAH-PSS)5PAH films. This is basically in line with the results reported for PEM films deposited on porous supports. Tieke and coworkers reported that permeation rates of NaCl and KCl across polyelectrolyte PEM films are significantly higher than those of multivalent ion-containing salts such as MgCl2, BaCl2, Na2SO4, and K4[Fe(CN)6] [32,33]. The higher permeability of NaCl and KCl can be rationalized based on smaller size and lower charge of the ions. One possible barrier to ion permeation across the PEM film may be Donnan exclusion to ions at the surface of PEM film because the outermost surface of the PEM film always contains excess charges. It is most likely that NaCl and KCl undergo Donnan exclusion less effectively than multivalent ions. The suppressed CaCl2 permeation is ascribable to enhanced Donnan exclusion due to the divalent nature, though a possible effect of a trace level of contaminated ALG in the film cannot be fully excluded. A significant role of Donnan exclusion can be found in the permeation of K3[Fe(CN)6] and K4 [Fe(CN)6], in which the permeation was highly suppressed. Another possible factor affecting the ion permeation is diffusion rate in the PEM film, which probably relates to the effective size of ions. It is clear that sizes of Na +, K +, and Cl − ions are smaller than the other ions tested. These considerations support higher permeability of NaCl and KCl. Extremely suppressed permeation of multivalent Fe(CN)63− and Fe(CN)64− ions also supports this view. It may be interesting to evaluate the effects of surface charges of PEM films on the permeation rate of salts, which was not clearly discussed in previous reports [31–33]. We have found that ion permeation evidently depends on the polarity of the surface charge of the PEM films. Permeability of NaCl, KCl, and CaCl2 across the (PAH-PSS)5 film is higher than those across the (PAH-PSS)5PAH film. Two mechanisms may be plausible for the suppressed permeation in the (PAH-PSS)5PAH
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0.03
mol h -1
A
1.0 0.8
Permeation rate /
Permeation rate /
mol h -1
1.2
0.6 0.4 0.2
B
0.02
0.01
0
0 NaCl
KCl
C2aCl
K3[Fe(CN)6]
Na2SO4
mol h -1
mol h -1
Permeation rate /
10
Permeation rate /
C 15
5
0
K4[Fe(CN)6]
D 0.2
0.1
0 NaCl
KCl
CaCl2
Na2SO4
K3[Fe(CN)6]
K4[Fe(CN)6]
Fig. 6. Permeability of salts across (PAH-PSS)5 (open columns) and (PAH-PSS)5PAH films (filled columns). Data in (A) and (B) were collected using PEM films with 1 mm diameter, while PEM films with 3 mm diameter were used for collecting the data in (C) and (D).
film, being Donnan exclusion to the cations at the positively charged surface due to PAH and enhanced film thickness by the addition of terminal PAH layer. On the other hand, opposite trend was observed for Na2SO4, K3[Fe(CN)6], and K4[Fe(CN)6], suggesting predominant effect of surface charge of the PEM film. It is reasonable to assume that SO42−, Fe(CN)63−, and Fe(CN)64− ions undergo a significant Donnan exclusion at the negatively charged surface of (PAH-PSS)5 film. The addition of terminal PAH layer cancelled the Donnan effect, resulting in the enhanced permeability for the multivalent ions. The effects of film thickness on the permeability of NaCl and KCl are shown in Fig. 7. For both NaCl and KCl, the permeation rate decreased with the increasing number of layers, suggesting a predominant role of diffusion of these ions in the film interior for determining the overall permeation rate. In this context, it has been reported that the permeation rate of NaCl across (PAH-PSS)10 film is about a half of that across (PAH-PSS)5 film [32]. The permeation
rate of KCl is always higher than that of NaCl if the film thickness is identical. This may be ascribable to smaller Stokes radius of K + ion (1.25 × 10 − 10 m) than that of Na + ion (1.84 × 10 − 10 m) [40]. Reusability of (PAH-PSS)5 film for permeation of NaCl and CaCl2 was evaluated. Fig. 8 shows relative permeation rates of NaCl and CaCl2 across the (PAH-PSS)5 film determined 5 times alternately. The film was rinsed in distilled water for 1 h after each measurement. Permeation rates for both NaCl and CaCl2 exhibited nearly constant values for the 5 measurements, showing an excellent reusability of the PEM film. In contrast, decreased permeability of NaCl was reported for supported PEM films after repeated use [33]. A longterm stability of the PEM film was also tested. To this goal, NaCl permeability across the (PAH-PSS)5 film was evaluated once a week for 10 weeks (the film was stored in distilled water at 4 °C when not in use). Virtually no deterioration in the NaCl transport was observed after 10 weeks.
1.0
Relative permeation rate
Permeation rate / mol h-1
1.2
0.8 0.6 0.4 0.2
1.0 0.8 0.6 0.4 0.2 0
0 5
10
Number of bilayers
15
Fig. 7. Effects of the film thickness on the rate of permeation of NaCl (open columns) and KCl (filled columns) across (PAH-PSS)n films (n = 5, 10, and 15).
1
2
3
4
5
Number of measurements Fig. 8. Reusability of (PAH-PSS)5 film for the transport of NaCl (open columns) and CaCl2 (filled columns).
K. Sato et al. / Materials Science and Engineering C 32 (2012) 2649–2653
4. Conclusion Free-suspended PEM films with 1 and 3 mm diameter can be prepared using ALG gel scaffold. The PEM films exhibited selective permeation for metal and metal complex ions depending on the ionic size and charge. Film thickness and surface charge significantly affected the permeation rates of ions. The present protocol would be useful for the construction of PEM films for biomedical applications as well as for selective layers in sensors and separators. A further study for improving mechanical stability of the films is now in progress in our laboratory. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
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