pubs.acs.org/Langmuir © 2009 American Chemical Society
Characteristics of Model Polyelectrolyte Multilayer Films Containing Laponite Clay Nanoparticles M. Elzbieciak,*,† D. Wodka,† S. Zapotoczny,‡ P. Nowak,† and P. Warszynski† †
Institute of Catalysis and Surface Chemistry, PAS, 30-239 Krakow, Poland, and ‡Jagiellonian University, Faculty of Chemistry, 30-060 Krakow, Poland Received June 10, 2009. Revised Manuscript Received July 7, 2009
Polyelectrolyte films structure formed by the “layer-by-layer” (LbL) technique can be enriched by addition of charged nanoparticles like carbon nanotubes and silver or hydroxyapatite nanoparticles, which can improve properties of the polyelectrolyte films or modify their functionality. In our paper we examined the formation and properties of model polyelectrolyte multilayers containing a synthetic layered silicate, Laponite. The Laponite nanoparticles were incorporated into multilayer films, which were formed from weak, branched polycation PEI and strong polyanion PSS. Since charge of PEI is pH-dependent, we build up multilayer films in two deposition conditions: pH=6 when PEI was strongly charged and pH=10.5 when charge density of PEI was low. Thicknesses of the films constructed with various numbers of Laponite layers were measured by single wavelength ellipsometry. We also determined the differences in permeability for selected electroactive molecules using cyclic voltamperometry. Properties of the films containing clay nanoparticles were compared with model polyelectrolyte multilayer films PEI/PSS formed at the same conditions. We found that Laponite nanoparticles strongly influenced PEI/PSS multilayer film properties. Replacement of PSS by Laponite eliminated the oscillations of the film thickness in the case when PEI was weakly charged. PSS layer adsorbed on top of PEI/Laponite bilayers increased the thickness of multilayer films and improved their barrier properties so synergistic effects between these properties for polyelectrolytes and Laponite nanoparticles could be observed.
1. Introduction Polyelectrolyte films obtained via sequential adsorption of oppositely charged polyions from their solutions have been widely studied in recent years. The “layer-by-layer” (LbL) technique, introduced for polyelectrolytes by Decher and co-workers,1-3 can provide materials with broad potential applications in the fields of surface modification, sensors, separation membranes, or microencapsulation.1-11 Formation of multilayer films is driven mainly by electrostatic interactions between oppositely charged polyelectrolytes; therefore, any charged objects of the size in the nanometer range can be used, together with polyelectrolytes for the structure build-up. One may incorporate into the multilayer films metal nanoparticles, like silver or gold,12,13 oxide nanoparticles,14-18 *To whom correspondence should be addressed. (1) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210, 831. (2) Lvov, Y.; Decher, G.; M€ohwald, H. Langmuir 1993, 9, 481. (3) Decher, Y.; Lvov, J.; Schmitt, J. Thin Solid Films 1994, 244, 772. (4) Castelnovo, M.; Joanny, J.-F. Langmuir 2000, 16, 7524. (5) B€uscher, K.; Graf, K.; As, H.; Helm, C. A. Langmuir 2002, 18, 3585. (6) Riegler, H.; Essler, F. Langmuir 2002, 18, 6694. (7) Schwarz, A. A. B.; Sch€onhoff, M. Langmuir 2002, 18, 2964. (8) Antipov, A. A.; Sukhorukov, G. B.; Leporatti, S.; Radtchenko, I. L.; Donath, E.; M€ohwald, H. Colloids Surf., A 2002, 198, 535. (9) Sch€onhoff, M. Curr. Opin. Colloid Interface Sci. 2003, 8, 86. (10) Sukhorukov, G. B.; Donath, E.; Davis, S.; Lichtenfeld, H.; Caruso, F.; Popov, V. I.; M€ohwald, H. Polym. Adv. Technol. 1998, 9, 759. (11) Ai, H.; Jones, S.; de Villiers, M.; Lvov, Y. J. Controlled Release 2002, 84, 122. (12) Schmitt, J.; Decher, G.; Dressik, W.; Shashidhar, R.; Calvert, J. Adv. Mater. 1997, 9, 61. (13) Bukreeva, T. V.; Parakhonsky, B. V.; Skirtach, A. G.; Susha, A. S.; Sukhorukov, G. B. Crystallogr. Rep. 2006, 51, 863–869. (14) Kleinfeld, E.; Ferguson, G. Science 1994, 265, 370. (15) Lvov, Y.; Ariga, K.; Kunitake, T. Langmuir 1997, 13, 6195. (16) Ichinose, I.; Tagawa, H.; Lvov, Y.; Kunitake, T. Langmuir 1998, 14, 187. (17) Bogdanovic, G.; Sennerfors, T.; Zhmud, B.; Tiberg, F. J. Colloid Interface Sci. 2002, 255, 44. (18) Cassagneau, T.; Fendler, J. Adv. Mater. 1998, 10, 877.
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quantum dots,18,19 carbon nanotubes,20 or clays. In particular, Kleinfeld and Ferguson14 applied for the first time the electrostatic layer-by-layer adsorption technique to produce multilayers of anionic synthetic silicate-hectorite and cationic PDADMAC. Lvov et al. formed multilayers with linear polycations (PEI and PDADMAC) and montmorillonite21 while Glinel et al. used Laponite platelets22 with the same polycations. Andreeva and Shchukin formed PEI/PSS multilayers, which contained halloysite filled with corrosion inhibitor (benzotriazole).23 This type of coating can be used for active corrosion protection since the inhibitor can be released from clay tubules by pH trigger. Caruso et al. formed polyelectrolyte multilayers containing titania, silica, or Laponite nanoparticles on polystyrene colloidal particles.24 Then, after dissolution of polystyrene core, they obtained hollow spheres with composite shells. Clays have been also exploited for the design of biosensors, as they possess many advantages as enzyme immobilization matrix. The porosity of clays allows using the swelling properties, for example, in enzyme immobilization without covalent bonding. Therefore, entrapment of biomolecules in clays can effectively constitute a cheap and easy method for the elaboration of enzyme electrodes.25 In this paper we concentrated on the influence of clay nanoparticles on thickness and barrier properties of polyelectrolyte multilayer films formed at various conditions. For our research we have chosen synthetic layered silicate Laponite RD, which has (19) Cassagneau, T.; Mallouk, T.; Fendler, J. J. Am. Chem. Soc. 1998, 120, 7848. (20) Shim, B. S.; Starkovich, J.; Kotov, N. Compos. Sci. Technol. 2006, 66, 1174. (21) Lvov, Y.; Ariga, K.; Kunitake, T. Langmuir 1996, 12, 3038. (22) Glinel, K.; Laschewsky and, A.; Jonas, A. M. Macromolecules 2001, 34, 5267–5274. (23) Andreeva, D. V.; Shchukin, D. G. Mater. Today 2008, 11, 24–30. (24) Caruso, R. A.; Susha and, A.; Caruso, F. Chem. Mater. 2001, 13, 400–409. (25) Fan, Q.; Shan, D.; Xue, H.; He, Y.; Cosnier, S. Biosens. Bioelectron. 2007, 22, 816.
Published on Web 07/17/2009
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an advantage over natural clays of being chemically pure and free from crystalline silica impurities.26 It is insoluble in water but hydrates and swells to give clear and colorless dispersions. At concentrations of 2% or greater in water, highly thixotropic gels can be produced. There are two key areas of functional use for Laponite: as an inert modifier of rheological properties it may be added to formulations of waterborne products such as surface coatings, household cleaners, and personal care products; as a film forming agent which is used to produce electrically conductive, antistatic, and barrier coatings and also in optical waveguide applications.27,28 Since Laponite in aqueous environment is strongly hydrated, it can be also used as water trap in hydrophobic coatings. A single Laponite nanoparticle has a geometrical form of disklike platelet with the average diameter of 25 nm and thickness of about 1 nm; therefore, it seems to be an ideal model nanoparticle to study formation of nanostructured multilayer films. For formation of multilayer films we chose two model polyelectrolytes: branched weak polycation polyethylenimine (PEI) and linear strong polyanion poly(4-styrenesulfonate) (PSS). PEI is often used as a precursor layer for the formation of multilayer films consisting of various polyelectrolytes.29-32 In our recent paper33 we determined the dependence of thickness of PEI/PSS multilayers, mass of adsorbed polymer, their topology, and permeability on the conditions of film formation. We used these results as a reference for current study, where we examined properties of films containing Laponite RD nanoparticles. Thicknesses of films formed at Si/SiO2 surface of silicon wafers were measured by ellipsometry, and permeabilities of polyelectrolyte films for electroactive molecules were determined by using the electrochemical technique cyclic voltamperometry (CV).
2. Materials and Methods 2.1. Materials. In our studies we used polycation-branched
polyethylenimine (MW ∼ 70 kDa) and the polyanion poly(4styrenesulfonate) (MW ∼ 70 kDa). PEI was purchased from Polysciences and PSS from Aldrich. Laponite was obtained from Rockwood Additives Ltd. Illustration of single Laponite is depicted in Figure 1. Silicon wafers (orientation 100) were purchased from On Semiconductor, Czech Republic. Before experiments silicon wafers were cleaned with piranha solution (composed of H2O2 and H2SO4 in 1:1 ratio) and rinsed with hot water (70 °C). Adsorption of polyelectrolytes was performed by the LbL method from NaCl (99.5% Fluka) solution of constant ionic strength (0.15 M) with the concentration of polyelectrolytes (PE) 0.5 g/L. This concentration of polyelectrolyte was high enough to establish a saturated adsorbed layer during the 10 min adsorption step. The pH of the solution was adjusted by addition of HCl (obtained from Chempur, Poland) or NaOH (Aldrich). The pH regulation by adding of HCl/NaOH did not influence the ionic strength of the solution. Adsorption of the first layer of polyelectrolyte was performed in the same conditions as for the rest of the film as in all experimental conditions (26) Negrete-Herrera, N.; Putaux, J.-L.; Bourgeat-Lami, E. Prog. Solid State Chem. 2006, 34, 121. (27) Rockwood Additives - Laponite RD product information brochure. (28) Luyer, C.; Lou, L.; Bovier, C.; Plenet, J.; Mugnier, D. G. J. Opt. Mater. 2001, 18, 211. (29) Kolasinska, M.; Krastev, R.; Warszynski, P. J. Colloid Interface Sci. 2007, 305, 46. (30) Decher, G. In Multilayer Thin Films; Decher, G., Schlenhoff, J. B., Eds.; Wiley-VCH: Weinheim, 2003; p 23. (31) Brunot, C.; Ponsonnet, L.; Lagneau, Ch.; Farge, P.; Picart, C.; Grosgogeat, B. Biomaterials 2007, 28, 632. (32) Kolasinska, M.; Trybalea, A.; Warszynski, P. In Surfactants and Dispersed Systems in Theory and Practice; Wilk, K. A., Ed.; EDU-SA: Wrocleaw, 2005; p 543. (33) Elzbieciak, M.; Zapotoczny, S.; Nowak, P.; Krastev, R.; Nowakowska, M.; Warszynski, P. Langmuir 2009, 25, 3255–3259.
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Figure 1. Illustration of single Laponite nanoparticle.27 the substrates: silicon wafers and gold electrodes were strongly charged. Si/SiO2 substrates was first immersed in polycation (PEI) at pH=6 and pH=10.5 and then rinsed three times for 2 min in distilled water. Then the sample with a monolayer adsorbed was dipped in polyanion solution and rinsed. The procedure was repeated until the required number of layers was obtained. After formation of the film, the wafers were carefully dried in a stream of nitrogen at room temperature to remove the water layer from the surface and avoid contamination with dust. Suspensions of Laponite RD were prepared by dispersing of dry powder in water to obtain 5000 ppm (5 g/L) concentration of nanoparticles. That suspension was further used to form multilayer films without pH adjustment and addition of NaCl. To verify the degree of dispersion of Laponite, we measured its effective hydrodynamic size by DLS (Malvern Zeta-Sizer). The measured hydrodynamic diameter DH =10 ( 2 nm. The hydrodynamic diameter of platelets can be evaluated assuming the oblate spheroid shape34 DH ¼ a
ð1 -λ2 Þ1=2 cos -1 λ
ð1Þ
where λ = b/a, a and b are longer and shorter spheroid axis length, respectively, or disklike shape (b f 0):34 DH ¼
2a π
ð2Þ
For the Laponite nanoparticle with the dimensions as shown in Figure 1, i.e., a =25 nm, b = 1 nm, we obtained DH=16.3 nm for oblate spheroid or DH=16.0 nm for the disk shape. It means that average size of the suspened objects is dominated by large fraction of well-dispersed small nanoparticles. For voltammetric studies quinone derivatives were chosen as electroactive molecules, namely, 1,2-naphthoquinone-4-sulfonic acid, sodium salt, and 9,10-anthraquinone-2,6-disulfonic acid, disodium salt (98%), from Sigma. The concentration of electroactive compounds was 10-3 g/L. 2.2. Methods. 2.2.1. Ellipsometry. Optical parameters of dried polyelectrolyte films;refractive index, absorption coefficient, and their thickness;were determined using EP3 single-wavelength multiangle imaging ellipsometer (Nanofilm). Measurements were carried out in autonulling mode in PCSA configuration (polarizer-compensator-sample analyzer). The refractive index of the purely PE film was found using multiple angle of incidence analysis35 by fitting elliptical parameters obtained during the measurements to the two-layer (SiO2/film) constant n, k model. So the determined value of the refractive index was equal to 1.55 while the absorption coefficient was negligibly small. Since the value of the refractive index of Laponite RD is equal to 1.5,36 we also used constant n, k model for the ellipsometric analysis of films containing clay nanoparticles. The ellipsometric thickness of multilayers was determined at the (34) Adamczyk, Z. Particles at Interfaces: Interactions, Deposition, Structure; Elsevier: Amsterdam, 2006; p 433. (35) Tompkins, H. G., Irene, E. A., Eds. Handbook of Ellipsometry; Springer: Berlin, 1999. (36) Rockwood Additives: http://www.laponite.com.
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Figure 2. Dependence of ellipsometric thickness of PEI/PSS films and films containing Laponite RD layers formed at various pH of PEI solution: left, pH = 6; right, pH = 10.5. 75° angle of the incident beam, near the Brewster angle for support material (silicon-SiO2 wafer),37 in order to obtain results with the highest possible sensitivity. 2.2.2. Cyclic Voltammetry. The CV measurements were carried out using Autolab rotating disk electrode and potentiostat/galvanostat (PGSTAT302N). Autolab glass cell with Pt-foil counter electrode and Ag/AgCl/sat. KCl electrode as the reference electrode was used in the measurements. Solutions were deoxidized before measurements by bubbling with laboratory grade argon (Linde, Poland). The diameter of the rotating disk electrode was 3 mm, and the diameter of the whole disk was 10 mm. Disks were first polished using aluminum oxide (Al2O3, j = particle diameter 0.3 μm) and then cleaned with isopropyl alcohol (P.O. Ch, Poland). The polyelectrolyte multilayers were build on surfaces of electrodes using the LbL technique. All conditions for the PE films formation were identical as for ellipsometry. The sweep rate for CV measurements was 100 mV/s. The measurements were performed in an indifferent electrolyte;0.15 M NaCl;to increase conductivity of the solution. 2.2.3. Atomic Force Microscopy. Atomic force microscopy (AFM) images of dried polyelectrolyte multilayers deposited on silicon wafers, previously used for ellipsometry experiments, were obtained with a NanoSope IV multimode atomic force microscope (Veeco, Santa Barbara, CA) working in the tapping mode with silicon cantilevers of nominal spring constant of 40 N/m (Veeco).
3. Results and Discussion In our previous work33 we investigated thickness and barrier properties of PEI/PSS polyelectrolyte multilayers. Films were formed under two conditions: when PEI was strongly charged, i.e., at pH=6 and pH=10.5, and when its charge density was low. We observed that films formed when PEI was strongly charged, i.e., at pH=6 exhibited the linear growth of thickness with the number of layers, while for films built at pH=10.5, i.e., when PEI was weakly charged, the nonmonotonous increase or their thickness was observed. Simultaneously films formed from the PEI solution with pH=10.5 were more permeable for 1,2-naphthoquinone-4-sulfonic acid, sodium salt, and 9,10-anthraquinone-2,6-disulfonic acid. We attributed those differences of properties of the films to the formation of weakly bound individual PEI/PSS complexes at the surface of films under the conditions of weakly charged polycation. These complexes could be partially removed in the consecutive adsorption steps, which (37) Tompkins, H. G.; McGahan, W. A. Spectroscopic Ellipsometry and Reflectometry; Wiley & Sons: New York, 1999.
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resulted in the nonmonotonic increase of film thickness and they higher permeability. The hypothesis of formation of surface complexes was supported by QCM (quartz crystal microbalance) and AFM measurements.33 In this paper we used the results obtained for thickness and permeability of the PEI/PSS films as the reference for investigation of Laponite-containing multilayers. In the first set of experiments we investigated thickness of the films, in which some or all PSS layers were replaced by layers of Laponite RD. As for pure PEI/PSS structures, films were formed under following conditions: when PEI was strongly charged at pH=6 and when charge density of polycation was low, i.e., at pH=10.5. The results are presented in Figure 2. One can see that when films were formed at the conditions of strongly charged PEI, replacement of some or all polyanion layers by Laponite RD led to small decrease of multilayer film thickness. It can suggest planar orientation of clay platelets when they are adsorbed at positively charged polyelectrolyte layers as thickness of the Laponite platelets is comparable to the diameter of PSS chain, but in contrast to polyelectrolyte, they cannot form loop type conformations at the interface. A much more pronounced effect of addition of Laponite could be observed for the films formed at the conditions of weakly charged PEI. Here, replacement of PSS by Laponite eliminates the oscillations of film thickness with the number of adsorption cycles observed for PEI/PSS multilayers. It can be seen that films formed at the conditions of weakly charged PEI, in which all PSS layers were replaced by Laponite ones (i.e., PEI/Laponite), are 2 times thicker than the ones created with strongly charged PEI (left picture). It suggests more random orientation of clay platelets (see Figure 3). Figure 4 show the SEM picture (JEOL JSM-7500F, field emission scanning electron microscope) of the PEI/Laponite multilayer containing seven bilayers deposited at the conditions of strong charged PEI. The single Laponite platelets can be clearly visible, but the whole structure is rather hollow. Unfortunately, it was impossible, using SEM, to find distinguishable differences in the multilayer structure between the films deposited at the two described conditions. We also determined surface topography of PEI/PSS multilayer films with Laponite and compared it with polyelectrolyte films without nanoparticles. The examples of AFM images are depicted in Figure 5. One can see that the features of topography of the multilayer films with the Laponite top layer are similar for both conditions of the film formation (cf. Figure 5b). However, the surface of the DOI: 10.1021/la902077j
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Figure 3. Scheme of formation of PEI/PSS multilayer films with Laponite: (a) one adsorbed bilayer, (b) polyelectrolyte multilayer films, (c, d) multilayer films with Laponite.
Figure 4. SEM picture of (PEI/Laponite)7 multilayer.
film deposited at pH 10.5 with outer Laponite layer seems to be smoother than the one produced without addition of the nanoparticles. That can be explained by the lack of polyelectrolyte complexes that are formed during deposition of consecutive PSS layer, on top of weakly bound PEI.33 That is supported by the observation illustrated in Figure 2b that the replacement of PSS by Laponite in the 10th layer suppresses next oscillation of the film thickness when weakly charged PEI is adsorbed. Successive adsorption of PEI layer on a top of (PEI/PSS)4PEI/Laponite film results in significant differences in the topography of the film surface. In the case of weakly charged PEI the surface is rougher due to more spatial adsorption of polycation. Addition of PEI layer at pH = 6 does not affect the film surface features, as planar conformations of adsorbed polycation predominate. Figure 6 illustrates the results obtained by cyclic voltamperometry for two kinds of electroactive compounds: 1,2-naphthoquinone-4-sulfonic acid sodium salt and 9,10-anthraquinone-2,6disulfonic acid, disodium salt, for PEI/Laponite films deposited at 280 DOI: 10.1021/la902077j
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gold electrode surface at the conditions of strongly and weakly charged PEI. As a reference CV curves for bare gold electrode and PEI/PSS multilayer film with the same number of adsorbed layers were used. One can see that PEI/Laponite films formed at both conditions are fully permeable for both electroactive molecules as the shape of voltamperometric loops for the electrode with adsorbed films is almost the same as ones for bare gold electrode. Therefore, one can see that even though thickness of the PEI/ Laponite films is either comparable to (for PEI adsorbed at pH = 6) or higher than PEI/PSS films (for PEI adsorbed at pH=10.5), they are much more permeable. These results indicate that the multilayer PEI/Laponite films have very hollow structure and consequently exhibit no barrier properties. Additionally, the results of the voltammetric experiments, presented in Figure 6, indicate that the current in the voltammetric maximum in the case of Laponite-containing multilayer is higher than in the case of bare gold electrode. The most probable explanation of that phenomenon is that due to the steric hindrance of voluminous and stiff Laponite particles and/or their Coulombic repulsion, the negative charge accumulated in the Laponite layer is insufficient to compensate the positive charge of the PEI layer. So, that charge is partly compensated by the anions of the base electrolyte (Cl- ions). When the electrode is dipped in the solution, the concentration of the electroactive anions of the 1,2-naphthoquinone-4-sulfonic acid or anthraquinone-2,6-disulfonic acid becomes also higher inside the layer in comparison to the bulk of the solution. Probably both electroactive anions can better compensate the excess charge than chloride anions. So, a significant part of chloride anions may be replaced by sulfonate anions, leading to substantially increased concentration of these anions in the layer. An additional argument supporting this hypothesis is the observed shift of the potential of the voltammetric maxima (cf. Figure 6). In the case of Laponite-containing layer the potential difference between cathodic and anodic maxima becomes smaller than in the case of the PEI/PSS layer. The explanation of this phenomenon is as follows. Voltammetry with Laponite-containing layer resembles to some extent the thin-layer cell voltammetry (diffusion from a limited space). In the case of the voltammetry on a planar electrode from the infinite space, when the electrode reaction is fast (diffusion control), the distance between the anodic and cathodic maxima is equal to 0.059 V/n,38 where n is the number of electrons exchanged in the reaction. However, in the case of a thin-layer cell voltammetry (diffusion from a limited space) the distance between the cathodic and anodic maxima is zero,39 provided that the electrode reaction is fast and the process is diffusion-controlled. Therefore, the decrease in the distance between the anodic and cathodic maxima in the case of Laponite-containing layer may be a consequence of the fact that the electrode process is partially controlled by the diffusion from the limited space (layer of the thickness equal to the thickness of the multilayer). In the second set of experiments the procedure of PEI/Laponite multilayer film formation was modified. After deposition of Laponite and rinsing the surface with distilled water, the intermediate step, immersion in the solution of PSS followed by rinsing in distilled water, was applied. Then the next layer of PEI was deposited. The results for ellipsometric thickness of so-prepared PEI/(Laponite þ PSS) films are presented in Figure 7 and compared with the ones for PEI/PSS and PEI/ Laponite films. (38) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706. (39) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 1980; Chapter 10.
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Figure 5. AFM images of PE films formed under various pH of PEI solutions obtained for the following multilayers: (a) (PEI/PSS)5, (b) (PEI/PSS)4PEI/Laponite, and (c) (PEI/PSS)4PEI/Laponite/PEI.
One can see that the applied procedure of additional adsorption of PSS on top of Laponite layer produces thicker films in comparison with both PEI/PSS and PEI/Laponite ones. These differences are visible, especially in the case of weakly charged PEI (right picture). That can be explained as follows. Because of longrange electrostatic interactions between the clay nanoparticles during their deposition on top of polycation layer, the final coverage of that layer by Laponite never reaches fully saturated uniform monolayer40 (cf. Figure 8a). During adsorption of PSS, voids between the clay platelets in the layer are filled by polyanions, which results in more uniformly, negatively charged film surface (Figure 8b). The consecutive PEI layer can be more effectively adsorbed on top of such uniform layer, and that contributes to more effective film growth. Otherwise, when the layer of PEI is formed on Laponite layer without PSS, polycations can adsorb only at clay platelets which induces nonuniformity of the film. As can be seen in Figure 7, at conditions of weakly (40) Adamczyk, Z.; Zembala, M.; Siwek, B.; Warszynski, P. J. Colloid Interface Sci. 1990, 140, 123.
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charged PEI, some oscillations in the film thickness can still be observed, whereas for PEI/Laponite the film grows monotonically. It indicates that, similarly to the case of purely polyelectrolyte multilayers at these conditions, PEI/PSS complexes are formed despite Laponite presence. The AFM images of PEI/ Laponite multilayers with and without deposited PSS layer did not show any significant differences in surface topography. That indicates that thin PSS layer covers uniformly the voids between platelets without formation of any additional structures. In Figure 9 the cyclic voltamperograms for 1,2-naphthoquinone-4-sulfonic acid, sodium salt, and 9,10-anthraquinone-2,6disulfonic acid, disodium salt, at gold electrode with modified PEI/(Laponite þ PSS) are compared with the ones for PEI/PSS films. It can be seen that the phenomenon observed previously with the Laponite-containing multilayers, i.e., the increase in the peak current in comparison to the bare electrode, may not be observed; on the contrary, the CV current is substantially reduced in comparison to either bare gold electrode or gold electrode covered with PEI/PSS film with the same number of layers. That means DOI: 10.1021/la902077j
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Figure 6. Comparison of voltamperometric curves for bare electrode PEI/PSS and PEI/Laponite multilayers deposited in pH 6 (left) and 10.5 (right) (PEI solution): (1, 2) 1,2-naphthoquinone-4-sulfonic acid, sodium salt; (3, 4) 9,10-anthraquinone-2,6-disulfonic acid, disodium salt.
Figure 7. Dependence of ellipsometric thickness of PEI/(Laponite þ PSS) films and PEI/PSS films formed at various pH of PEI solution on the number of deposited layers.
Figure 8. Scheme of PEI/Laponite/PSS multilayer films: (a) PEI/Laponite bilayer; (b) PEI/Laponite multilayer with PSS on the top.
that films PEI/Laponite þ PSS are less permeable than multilayers formed from polyelectrolytes alone or from polycation with the clay nanoparticles. As in the case of purely polyelectrolyte multilayers, PEI/(Laponite þ PSS) films formed at the conditions 282 DOI: 10.1021/la902077j
of strongly charged PEI have better barrier properties then the one formed with weakly charged PEI, despite their smaller thickness. Similar results were obtained for both electroactive compounds. They indicate the possibility of tuning of film Langmuir 2010, 26(1), 277–283
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Figure 9. Comparison of voltamperometric curves for bare electrode and PEI/PSS and PEI/Laponite þ PSS multilayers deposited (PEI solution) in pH 6 (left) and 10.5 (right) obtained for (1, 2) 1,2-naphthoquinone-4-sulfonic acid, sodium salt, and (3, 4) 9,10-anthraquinone-2,6disulfonic acid, disodium salt.
permeability by coadsorption of PSS and the nanoparticles. One can claim that synergistic effect between barrier properties of polyelectrolytes and Laponite nanoparticles can be observed. It is possible that a similar type of synergy can be observed when clay particles are embedded in other types of model films.
4. Conclusions The results of our experiments indicate that thickness and barrier properties of polyelectrolyte PEI/PSS films containing Laponite clay nanoparticles are strongly dependent on the conditions of their formations and the total composition of the film. Replacement of polyanion, PSS, layers by negatively charged Laponite in the films deposited with weakly charged PEI stabilizes the growth of the films but deteriorates their barrier properties. A similar effect is observed for the film formed with
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strongly charged PEI. It indicates that multilayer PEI/Laponite films have very hollow structure. These holes can be filled when deposition of Laponite is followed by an additional step; adsorption of PSS. The resulting composite films have larger thickness in comparison with PEI/PSS and PEI/Laponite ones. Moreover, films containing a combination of clay particles, polyanion, and polycation have improved barrier properties (over purely polyelectrolyte or polycation/clay films); therefore, a synergistic barrier effect between polyelectrolytes and clay platelets is observed. Acknowledgment. The authors thank El_zbieta Bielanska for providing the scanning electron microscope pictures. This work was partially supported by MNiSW project N204 166 31/3734 and EC FP7 project NMP3-LA-2008-214261 - “MUST”.
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