Incorporation of Nanoparticles into Polyelectrolyte Multilayers via

Jun 15, 2009 - ... were carried out on a JEM-2010 microscope operating at 100.0 kV. ..... Nolte , A. J., Rubner , M. F., and Cohen , R. E. Langmuir 20...
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Incorporation of Nanoparticles into Polyelectrolyte Multilayers via Counterion Exchange and in situ Reduction Xingjie Zan†,‡ and Zhaohui Su*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, and ‡ Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun, 130022, P. R. China Received May 10, 2009. Revised Manuscript Received June 2, 2009

We report a general method for incorporation of nanoparticles into polyelectrolyte multilayer (PEM) thin films by utilizing the excess charges and associated counterions present in the PEMs. Silver ions were introduced directly into multilayers assembled from poly(diallyldimethylammonium chloride) (PDDA) and poly(styrene sulfonate) (PSS), (PDDA/PSS)n, by a rapid ion exchange process, which were then converted into silver nanoparticles via in situ reduction to create composite thin films. The size and the content of the nanoparticles in the film can be tuned by adjusting the ionic strength in the polyelectrolyte solutions used for the assembly. Spatial control over the distribution of the nanoparticles in the PEM was achieved via the use of multilayer heterostructure containing PDDA/PSS bilayer blocks assembled at different salt concentrations. Because excess charges and counterions are always present in any PEM, this approach can be applied to fabricate a wide variety of composite thin films based on electrostatic self-assembly.

Introduction The fabrication of composite thin films containing metal nanoparticles has attracted much interest recently due to their potential applications in various fields such as photonics,1,2 catalysis,3-6 magnetics,7-9 and antimicrobial coatings.10,11 A frequently utilized method to create these composite films involves fabricating organic thin films via layer-by-layer (LbL) assembly of polyelectrolytes and subsequent functionalization of these polyelectrolyte multilayer (PEM) films. Since its rediscovery in 1990s by Decher et al.,12 the LbL assembly has quickly become a popular technique for thin film preparation because of its simplicity, robustness, and versatility.13 Various methods have been developed for incorporation of metal nanoparticles in the PEMs. For example, Bruening and coworkers assembled complexes of polyethylenimine with Pd(II) or Ag(I) with poly(acrylic acid) (PAA) to introduce metal ions into PEMs, and by in situ reduction obtained metal nanoparticles embedded in the PEMs, which exhibited catalytic activity for various chemical reactions.3-6 Rubner and co-workers reported a more general approach for in situ nanoparticle synthesis in PEMs based on weak polyelectrolytes, in particular PAA. They showed that by controlling * To whom all correspondence should be addressed. Phone: (+86)043185262854; fax: (+86)0431-85262126; e-mail: [email protected]. (1) Nolte, A. J.; Rubner, M. F.; Cohen, R. E. Langmuir 2004, 20, 3304. (2) Wang, T. C.; Cohen, R. E.; Rubner, M. F. Adv. Mater. 2002, 14, 1534. (3) Bhattacharjee, S.; Bruening, M. L. Langmuir 2008, 24, 2916. (4) Dai, J. H.; Bruening, M. L. Nano Lett. 2002, 2, 497. (5) Kidambi, S.; Bruening, M. L. Chem. Mater. 2005, 17, 301. (6) Kidambi, S.; Dai, J. H.; Li, J.; Bruening, M. L. J. Am. Chem. Soc. 2004, 126, 2658. (7) Sun, S. H.; Anders, S.; Thomson, T.; Baglin, J. E. E.; Toney, M. F.; Hamann, H. F.; Murray, C. B.; Terris, B. D. J. Phys. Chem. B 2003, 107, 5419. (8) Ziolo, R. F.; Giannelis, E. P.; Weinstein, B. A.; Ohoro, M. P.; Ganguly, B. N.; Mehrotra, V.; Russell, M. W.; Huffman, D. R. Science 1992, 257, 219. (9) Sohn, B. H.; Cohen, R. E.; Papaefthymiou, G. C. J. Magn. Magn. Mater. 1998, 182, 216. (10) Grunlan, J. C.; Choi, J. K.; Lin, A. Biomacromolecules 2005, 6, 1149. (11) Wang, Q.; Yu, H.; Zhong, L.; Liu, J.; Sun, J.; Shen, J. Chem. Mater. 2006, 18, 1988. (12) Decher, G. Science 1997, 277, 1232–1237. (13) Zhang, X.; Chen, H.; Zhang, H. Y. Chem. Commum. 2007, 1395.

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assembly conditions a significant content of nonionized carboxylic acid groups can be incorporated into the PEMs, which are used to bind metal ions via a simple aqueous ion exchange procedure; the metal ions are then subsequently converted into nanoparticles.1,2,14-17 This approach is based on the pH-dependent dissociation of weak acids, where the weak polyelectrolyte such as PAA essentially becomes a copolymer with carboxylate and carboxylic acid units at suitable pH; while the ionized carboxylate groups are responsible for the electrostatic assembly with a polycation and end up forming ion pairs, the remaining nonionized carboxylic acid groups are then available for subsequent binding of metal ions via ion exchange. In addition, spatial control over the nanoparticle distribution in the PEMs can be achieved via the use of multilayer heterostructures containing “nonbinding” bilayer blocks fabricated from fully ionized polyelectrolytes.1,2,14 The key element in the above approach is free (nonpaired) ionic groups in the PEMs. Actually, one needs not to manipulate the pH in order to find free ionic groups in PEMs. It has been well established that in a multilayer buildup process, on each adsorption step surface charge overcompensation occurs, leading to reversal of the surface charge.18,19 Therefore, there always exist excess charged groups and corresponding counterions (salt ions) at the surface of a PEM. The presence of counterions at PEM surfaces has been verified using radiolabeled counterions,19 and it has been reported that counterions can affect layer thickness,20 adsorption kinetics,21 mechanical properties,22 and swelling (14) Joly, S.; Kane, R.; Radzilowski, L.; Wang, T.; Wu, A.; Cohen, R. E.; Thomas, E. L.; Rubner, M. F. Langmuir 2000, 16, 1354. (15) Lee, D.; Rubner, M. F.; Cohen, R. E. Chem. Mater. 2005, 17, 1099. (16) Li, Z.; Lee, D.; Sheng, X. X.; Cohen, R. E.; Rubner, M. F. Langmuir 2006, 22, 9820. (17) Wang, T. C.; Rubner, M. F.; Cohen, R. E. Langmuir 2002, 18, 3370. (18) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592. (19) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626. (20) Salomaki, M.; Tervasmaki, P.; Areva, S.; Kankare, J. Langmuir 2004, 20, 3679. (21) Mermut, O.; Barrett, C. J. J. Phys. Chem. B 2003, 107, 2525. (22) Salomaki, M.; Laiho, T.; Kankare, J. Macromolecules 2004, 37, 9585.

Published on Web 06/15/2009

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properties23 of the multilayers. Recently we demonstrated that counterions at PEM surface can be utilized to modulate the surface wettability via ion exchange with ions of different hydration characteristics.24,25 In the present report, we show that the ion exchange/in situ reduction approach can be extended to PEMs fabricated from strong polyelectrolytes, or in principle any polyelectrolyte, to incorporate nanoparticles into PEMs with no requirement of pH adjustments by utilizing the excess charged groups naturally present in all PEMs.

Experimental Section Materials. Silver nitrate (AgNO3), sodium chloride (NaCl), and sodium boronhydride (NaBH4) were of analytical grade and were purchased from Beijing Chemical Reagents Company. Poly (styrene sulfonate) (PSS, MW ∼70 000) and poly(diallyldimethylammonium chloride) (PDDA, 20 wt % in water, MW ∼200 000350 000) were purchased from Aldrich and were used as received. Polished silicon wafers were purchased from Shanghai Wafer Works Corporation. Water used for rinsing and preparing all solutions was purified with a Millipore Simplicity 185 purification unit (18.2 MΩ cm). Preparation of PDDA/PSS Films. Quartz and silicon wafers were immersed in a boiling piranha solution (3:1 mixture of 98% H2SO4 and 30% H2O2) for 20 min and then rinsed with copious amounts of water. PDDA/PSS multilayers were assembled on cleaned wafers following a literature procedure.26 In brief, PEM films were assembled by sequential dipping of a substrate in PDDA (1.0 mg/mL) and PSS (1.0 mg/mL) aqueous solutions for 30 min each with water rinsing in between each deposition step until the desired number of layers was obtained. All polyelectrolyte solutions contained 3.0 M NaCl unless indicated otherwise. Incorporation of Silver Ion and Synthesis of Silver Nanoparticles in Situ. Silver ions were introduced into the PDDA/ PSS multilayer films by immersing the films in an AgNO3 solution (0.010 M) for 5 min, and the films were then rinsed with water thoroughly and dried in a N2 stream. These films are denoted (PDDA/PSS)n/Ag(I), where n is the number of bilayers in the film. The PDDA/PSS films loaded with silver ions were immersed in a freshly prepared aqueous solution of NaBH4 (0.010 M) for 5 min to convert the silver ions to silver nanoparticles, removed and rinsed with water for 1 min, and then dried in a stream of N2. These films are denoted (PDDA/PSS)n/Ag(0).

Preparation of PDDA/PSS Films with Sandwiched Structure. An example is described here. A (PDDA/PSS)6 film was assembled at 3.0 M NaCl, dipped into a AgNO3 solution (0.010 M) for 5 min, and then treated with a freshly prepared aqueous solution of NaBH4 (0.010 M) for 5 min. Ten PDDA/PSS bilayers were then deposited at 0.75 M NaCl, followed by 4 bilayers assembled at 3.0 M NaCl, and the film was dipped into a AgNO3 solution (0.010 M) for 5 min and then a freshly prepared aqueous solution of NaBH4 (0.010 M) for 5 min. Each of the above steps was followed with a through rinsing with water. Characterization. UV-vis spectra of the multilayers deposited on quartz slides were collected on a Shimadzu UV-2450 spectrophotometer. Contact angle analyses were carried out using the static sessile drop method on a KRUSS DSA1 version 1.80 drop shape analyzer with water as the probe liquid. Each contact angle value reported was an average of at least five measurements. X-ray photoelectron spectra (XPS) were obtained on a ThermoElectron ESCALAB 250 spectrometer equipped with a monochromatic Al X-ray source (1486.6 eV). The spectra were recorded at 90° takeoff angle with 20 eV pass energy. The high takeoff angle (23) (24) (25) (26)

Salomaki, M.; Kankare, J. Macromolecules 2008, 41, 4423. Wang, L.; Lin, Y.; Peng, B.; Su, Z. Chem. Commum. 2008, 5972. Wang, L.; Lin, Y.; Su, Z. Soft Matter 2009, 5, 2072. Wu, G.; Su, Z. Chem. Mater. 2006, 18, 3726.

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Figure 1. XPS spectra of (a) (PDDA/PSS)3 and (b) (PDDA/ PSS)3/Ag(I). The inset is the N1s region. was used in order to assess the bulk film composition instead of the film surface only, which was confirmed by the Si peaks observed for the wafer substrate. Transmission electron microscopy (TEM) observations were carried out on a JEM-2010 microscope operating at 100.0 kV. A small piece of the PEM loaded with silver nanoparticles was peeled off from the substrate in hydrofluoric acid, floated in water, and transferred to carboncoated copper grids for TEM observation. To examine the cross section, the PEM film floating in water was collected on a small piece of hardened epoxy, and the epoxy piece with the film was embedded in liquid epoxy that was then cured in three steps, lasting 12 h each, at progressively higher temperatures of 35, 45, and 55 °C. The ultrathin sections, ca. 70 nm thick, were microtomed at room temperature using a LEICA Ultracut R microtome and a glass knife. Slices were floated on a water surface and retrieved with carbon-coated copper grids.

Results and Discussion PDDA and PSS are common polyelectrolyte building blocks for fabrication of LbL films. Using 1.0 mg/mL PDDA and PSS aqueous solutions with 3.0 M NaCl present, a multilayer thin film, (PDDA/PSS)3, was assembled on a cleaned silicon wafer. The XPS spectra of the (PDDA/PSS)3 film as assembled and after immersed in an AgNO3 solution for 5 min are shown in Figure 1. First it can be seen that Na is present in the pristine film, and Cl is absent. This is a good indication that the Na signals arise from the counterions for the excess sulfonate groups of the PSS, and no absorption of NaCl salt by the multilayer is detected. For (PDDA/PSS)3/Ag(I), the Na peaks disappear and Ag peaks are clearly observed, and in the N1s region (Figure 1 inset), only one peak is present at ∼402 eV, identical with that for the pristine film, due to the nitrogen in the PDDA; and no extra N1s peak at higher binding energy of ∼405 eV is observed for NO3-, the ion one would expect to find if absorption of AgNO3 by the multilayer film had occurred. Therefore the XPS spectra indicate that the Na+ counterions for the PSS units have been exchanged by Ag+. The atomic compositions of these two films by XPS are listed in Table 1. It can be seen that the atomic percentage of Ag+ in the (PDDA/PSS)3/Ag(I) is roughly the same as that of Na+ in the pristine film. In the films there are two kinds of positively charged species, the quaternary ammonium ions (each containing one N atom) and the salt ions (Na+ or Ag+), and one kind of negatively charged species, the sulfonates (each containing one S atom). From the atomic composition data in Table 1, it is obvious that the amount of the positive charges ((N + Na) for the pristine film and (N + Ag) for the treated ones) roughly equaled that of the negative charges (S) in both cases. In addition, the Ag content in the treated film remained about the same after the film was immersed in pure water for 5 h. These semiquantitative results Langmuir 2009, 25(20), 12355–12360

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Table 1. XPS Atomic Concentrations of the PEMs atomic concentration (%) sample

C

O

N

S

Cl

Na

Ag

(PDDA/PSS)3 70.56 16.80 3.11 5.96 0 (PDDA/PSS)3/Ag(I)a 70.80 16.85 3.11 5.79 0 (PDDA/PSS)3/Ag(I)b 70.46 17.19 3.07 6.18 0 a As prepared. b After immersed in pure water for 5 h.

3.57 0 0

0 3.45 3.11

Figure 2. Ag atomic concentration in the (PDDA/PSS)3 film obtained by XPS as a function of the AgNO3 solution concentration.

indicate that significant amount of Na+ existed in the multilayer film as the counterions for the free sulfonates, and they were exchanged by Ag+ when the film was treated with AgNO3 solution; and no detectable amounts of salts, either NaCl or AgNO3, were absorbed by the film, which is consistent with the qualitative analysis of the XPS spectra discussed above. Next, we treated the (PDDA/PSS)3 films with AgNO3 solutions of different concentrations for 10 min, which was sufficient to complete the reaction (see kinetics discussed below), and then analyzed the Ag content in each film by XPS. In Figure 2 it can be seen that the Ag content in the PEM film is independent of the concentration of the AgNO3 solution used to treat the PEMs, which further supports our argument above that the Ag+ ions were introduced via ion exchange rather than absorption. In the latter case one would expect to observe an increase of Ag content in the film with the solution concentration. It has been found that surface wettability of PEMs terminated with a polycation depends on the counteranion,24,25,27-29 and the same effect is expected for PEMs terminated with a polyanion with different counteractions. Therefore, contact angle analysis was applied to monitor the ion exchange kinetics, and the contact angles of a (PDDA/PSS)3 film treated with AgNO3 for different times are plotted in Figure 3a. It can be seen that the water contact angle of the multilayer surface increases by about 20° after the Na+ counterion is exchanged by Ag+, in line with the hydration characteristics of these two cations, and reaches a plateau in less than 3 min, indicating fast exchange kinetics. The kinetics was also investigated by XPS by following the surface Ag content of the (PDDA/PSS)3 film as a function of time the film was treated with AgNO3, and the data are plotted in Figure 3b, which shows that the Ag atomic concentration at the surface grows quickly from 0 to 3.31 in several minutes and then plateaus, in good (27) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153. (28) Kolbeck, C.; Killian, M.; Maier, F.; Paape, N.; Wasserscheid, P.; Steinr€uck, H.-P. Langmuir 2008, 24, 9500. (29) Lee, B. S.; Chi, Y. S.; Lee, J. K.; Choi, I. S.; Song, C. E.; Namgoong, S. K.; Lee, S. G. J. Am. Chem. Soc. 2004, 126, 480.

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agreement with the contact angle data. These results indicate that the PEMs can be loaded with metal ions quickly for further reactions, which is advantageous compared to other modification methods such as postdiffusion, which can take hours to finish.30-33 The contact angle of the surface of the PEMs with silver counterions remained unchanged after they were immersed in a NaNO3 solution, indicating the particular exchange discussed above (exchange of Na+ by Ag+) is not reversible. This was also verified by XPS analyses. This probably is due to the covalent characteristics of the silver-sulfonate bond. Ag(I) can be reduced to Ag(0) by UV radiation, H2 plasma, and NaBH4.34 Figure 4 shows the UV-vis spectra of a (PDDA/PSS)5 film with silver counterions and the film after the Ag+ counterions were converted into silver nanoparticles in situ with NaBH4. The peak at 225 nm is attributed to the absorbance of the PSS and can be used for monitoring the multilayer growth.35 A new peak is found at 414 nm for the film after the reduction, a typical surface plasma resonance band of silver nanoaparticles,11,34,36,37 which suggests the formation of silver nanoparticles in the film. This is further confirmed by TEM and selected-area electron diffraction (SAED), shown in Figure 5, where nanoparticles are clearly observed, and the (100), (200), (220), (311), (331), and (422) diffractions of Ag are identified. It is generally known that salt has strong influence on assembled PEMs.35,38 This may be used to control the content and morphology of the silver nanoparticles incorporated in the PEMs. Figure 6 displays the Ag contents measured by XPS in (PDDA/PSS)4/Ag(I) films where the PEMs were assembled under different NaCl concentrations. It can be seen that the silver content in the PEM first increases with the salt concentration, reaches a maximum at ∼3 M, and then drop sharply when the salt concentration is further increased. It has been well accepted that when salts of high concentrations are present in the solution, the charges on the polyelectrolytes are screened by the salt ions; the electrostatic repulsion is largely diminished, and polyelectrolyte chains would adopt an entropically more favorable coiled conformation, resulting in thicker and less interpenetrating layers.19 Therefore, at higher salt concentrations much more charged groups are not compensated by the oppositely charged polyelectrolyte and are available for binding Ag+ ions, resulting in higher silver contents in the PEMs. However, when the salt concentration is higher than a critical point, where the few remaining ion pairs are no longer able to bind the polyelectrolytes together, the amount of polyelectrolyte depositing diminishes quickly.39 The critical NaCl concentration for the decomposition of PDDA/PSS multilayers was reported to be 3.6 M.27 Figure 7 displays the UVvis spectra of the PEMs assembled at different salt concentrations and loaded with silver nanoparticles. The silver content represented by the peak intensity at 414 nm as a function of the salt concentration is plotted in the inset, and the same trend as that revealed by XPS is evident. From the spectra it is also clear that the absorbance for PSS at 225 nm drops dramatically at 4 and 5 M compared to that at 3 M, indicating that much less PSS were (30) (31) (32) (33) (34) (35) 6655. (36) (37) (38) (39)

Burke, S. E.; Barrett, C. J. Macromolecules 2004, 37, 5375. Chung, A. J.; Rubner, M. F. Langmuir 2002, 18, 1176. Kharlampieva, E.; Sukhishvili, S. A. Langmuir 2004, 20, 9677. Nicol, E.; Habib-Jiwan, J. L.; Jonas, A. M. Langmuir 2003, 19, 6178. He, J.; Ichinose, I.; Kunitake, T.; Nakao, A. Langmuir 2002, 18, 10005. McAloney, R. A.; Sinyor, M.; Dudnik, V.; Goh, M. C. Langmuir 2001, 17, Stathatos, E.; Lianos, P.; Falaras, P.; Siokou, A. Langmuir 2000, 16, 2398. Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. Langmuir 1998, 14, 3740. Dubas, S. T.; Schlenoff, J. B. Langmuir 2001, 17, 7725. Dubas, S. T.; Schlenoff, J. B. Macromolecules 2001, 34, 3736.

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Figure 3. Dependence of (a) the water contact angle and (b) the Ag atomic concentration obtained by XPS of the (PDDA/PSS)3 surface on the time the film is immersed in the AgNO3 solution.

Figure 6. Ag content measured by XPS in the (PDDA/PSS)4/Ag Figure 4. UV-vis spectra of a (PDDA/PSS)5/Ag(I) film (;) and

the film after the reduction of its Ag+ counterions to silver nanoparticles by NaBH4 for 5 min (---).

(I) film as a function of the NaCl concentration in the polyelectrolyte solutions used in the assembly.

Figure 7. UV-vis spectra of (PDDA/PSS)4/Ag(0) films with the multilayers prepared under different NaCl concentrations. The inset plots the absorbance at 414 nm vs the salt concentration. Figure 5. TEM image of a (PDDA/PSS)5 film containing silver nanoparticles. The inset is the electron diffraction pattern.

deposited at such high salt concentrations. Thus, the UV data are consistent with the XPS results and support the above argument on the effect of salt concentration. On the basis of these results, in this work most of the PDDA/PSS multilayers were assembled at 3 M NaCl concentration. Figure 8 shows the morphology and size distribution of silver nanoparticles in the PEMs prepared under different salt concentrations. It is obvious that both the average particle size and the 12358 DOI: 10.1021/la901655m

number of the particles increase with the salt concentration, because more Ag+ ions can be introduced to the PEM via ion exchange as we discussed above. The size distribution becomes broader as well. Next, PDDA/PSS films with different number of bilayers were immersed in AgNO3 solution to incorporate silver ions that were then converted to silver nanoparticles. Their UV-vis spectra are shown in Figure 9, indicating that the silver content in the PEM increases with the number of bilayers in the PEM. The inset is the UV absorbances at 225 and 414 nm as functions of the number of the bilayers, both being exponential. This is in contrast with what Langmuir 2009, 25(20), 12355–12360

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Figure 8. TEM images of (PDDA/PSS)4/Ag(0) films with the multilayers assembled under salt concentrations of (a) 0.5, (b) 1.0, (c) 2.0, and (d) 3.0 M. The insets are the corresponding nanoparticle diameter histograms.

Figure 9. UV-vis spectra of (PDDA/PSS)n/Ag(0) films with n = 1-6. The inset plots the absorbances at 225 nm (3) and 414 nm (9) vs the number of bilayers in the film, and the lines are exponential curves fitted to the data points.

we found for PDDA/PSS multilayers assembled at low salt concentrations, where with the number of deposition cycle increasing, the amount of the polyelectrolyte assembled increases linearly, and the silver content in the PEM remains constant.40 It has been well established that, depending on the concentration of added salt in the polyelectrolyte solutions, there are two growth modes of PEMs.35 At salt-free or low salt concentrations, the thickness and mass of the film linearly increase with the number of layers deposited, and the average layer thickness is governed by chain conformation; the ionic groups are paired within the multilayer, and excess ionic groups with salt counterions are only present at the surface of the PEM, the content of which should be independent of the number of bilayers. Such was the case in our previous study.40 At high salt concentrations, the thickness and mass of the film increase exponentially with the number of deposition cycles, and the phenomenon has been attributed to the diffusion of the polyelectrolyte chains in the multilayer films. 41-48 Obviously, in addition to the excess charges at the film (40) Zan, X.; Su, Z. Thin Solid Films submitted. (41) Podsiadlo, P.; Michel, M.; Lee, J.; Verploegen, E.; Kam, N. W. S.; Ball, V.; Lee, J.; Qi, Y.; Hart, A. J.; Hammond, P. T.; Kotov, N. A. Nano Lett. 2008, 8, 1762. (42) Lavalle, P.; Vivet, V.; Jessel, N.; Decher, G.; Voegel, J. C.; Mesini, P. J.; Schaaf, P. Macromolecules 2004, 37, 1159. (43) Porcel, C.; Lavalle, P.; Ball, V.; Decher, G.; Senger, B.; Voegel, J. C.; Schaaf, P. Langmuir 2006, 22, 4376. (44) Porcel, C.; Lavalle, P.; Decher, G.; Senger, B.; Voegel, J. C.; Schaaf, P. Langmuir 2007, 23, 1898. (45) Garza, J. M.; Schaaf, P.; Muller, S.; Ball, V.; Stoltz, J. F.; Voegel, J. C.; Lavalle, P. Langmuir 2004, 20, 7298. (46) Jourdainne, L.; Arntz, Y.; Senger, B.; Debry, C.; Voegel, J. C.; Schaaf, P.; Lavalle, P. Macromolecules 2007, 40, 316. (47) Lavalle, P.; Picart, C.; Mutterer, J.; Gergely, C.; Reiss, H.; Voegel, J. C.; Senger, B.; Schaaf, P. J. Phys. Chem. B 2004, 108, 635. (48) Liu, G. M.; Zou, S. R.; Fu, L.; Zhang, G. Z. J. Phys. Chem. B 2008, 112, 4167.

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Figure 10. Cross-sectional TEM images of (PDDA/PSS)n/Ag(0) films with the multilayers fabricated (a) at 3.0 M NaCl concentration and n = 10 and (b) at 0.75 M NaCl concentration and n = 15.

surface, there exist ionic groups within the multilayer due to the presence of the free polyelectrolyte chains (PSS in our case). These ionic groups within the multilayer can bind silver ions in the ion exchange process, therefore we observed the exponential growth of the silver content with the number of layers. Figure 10a shows the TEM cross sectional image of a (PDDA/ PSS)10 film containing silver nanoparticles with an average bilayer thickness of ∼40 nm and a total thickness of approximately 400 nm. The large layer thickness is attributed to the high concentration of the added salt and the exponential growth mode under such conditions.41-48 It can be seen that silver nanoparticles distribute in the whole thickness homogeneously instead of at the surface only, which also confirms the presence of free PSS chains in the whole film. In comparison, the TEM image for a (PDDA/PSS)15 film assembled at 0.75 M NaCl (Figure 10b) shows that the silver nanoparticles are confined in the top layers of the PEM.40 It has been demonstrated that by using PSS in addition to PAA to assemble with PAH, heterostructures composed of alternating (PAH/PSS)n and (PAH/PAA)m blocks can be prepared; and then nanoparticles can be selectively incorporated into the (PAH/ PAA)m blocks, where ionic binding sites exist, and the (PAH/ PSS)n blocks do not bind metal ions and thus act as spacers.1,2,14 This aqueous-based process can be used to fabricate one-dimensional photonic structures of large areas with precise control under mild conditions.1,2 The same kind of control over the spatial distribution of the nanoparticles can be achieved using only two polyelectrolytes based on the principle discussed above, by tuning the salt concentration in the polyelectrolyte solutions for PEM preparation. The (PDDA/PSS)n blocks assembled at high salt concentrations contain free PSS chains and are capable of binding metal ions in the entire block, whereas the (PDDA/ PSS)m blocks assembled at low salt concentrations or salt free contain no free ionic groups within the block that can bind metal DOI: 10.1021/la901655m

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easily fabricated. It is conceivable that the structure can be conveniently tailored to target, an advantage afforded by the LbL technique.2,14,17 The thickness of each block can be precisely controlled by adjusting the number of layers in the block and the salt concentration, and the silver content can be manipulated by varying the salt concentration and the number of the exchange/ reduction cycle.17 These films with precisely controlled heterostructure may find applications in optical and photonic devices.

Conclusions

Figure 11. Cross-sectional TEM image of a (PDDA/PSS)6/Ag(0)/ (PDDA/PSS)10/(PDDA/PSS)4/Ag(0) sandwiched film. The blocks containing silver and the spacer block were assembled at 3.0 and 0.75 M NaCl concentration, respectively.

ions, and thus can act as the spacers. A sandwiched structure was fabricated by stacking (PDDA/PSS)n blocks assembled under high and low NaCl concentrations alternately with ion exchange and in situ reduction in between. Figure 11 shows the crosssectional TEM image of a structure of this kind, composing of two (PDDA/PSS)n blocks (n = 4 and 6, respectively) assembled at 3.0 M NaCl concentration and loaded with silver nanoparticles separated by a (PDDA/PSS)10 block assembled at 0.75 M NaCl concentration that contains no silver particles. The purpose of this example is to demonstrate that this kind of heterostructure can be

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We have demonstrated a facile and general approach to the fabrication of nanoparticle-loaded multilayer films with controllable content and size of the nanoparticles, as well as onedimensional heterostructure. Capitalizing on the excess charges and associated counterions that are present in all multilayer films, metal ions are introduced into the multilayer via ion exchange, which can then be converted into metal nanoparticles via in situ reduction to create a composite thin film. The methodology is generally applicable to multilayer films assembled from strong or weak polyelectrolytes and does not require pH manipulation in the assembly process or after. The aqueous ion exchange process is rapid and facile, and the content, size, and the spatial distribution of the nanoparticles can be conveniently manipulated by controlling the salt concentration in the polyelectrolyte solutions in the assembly step. This approach, in principle, can also be applied to incorporate any (positively or negatively) charged species, and the metal ions introduced can also be converted into other products such as semiconductor nanoparticles via in situ chemical reactions. This provides a versatile and facile route to the fabrication of a wide variety of complex thin film nanocomposites with molecularly tunable structure and properties. Acknowledgment. This work was supported by National Natural Science Foundation of China (20423003, 20774097). Z. S. thanks the NSFC Fund for Creative Research Groups (50621302) for support.

Langmuir 2009, 25(20), 12355–12360