Stratified PEI-(PSS-PDADMAC)20-PSS-(PDADMAC-TiO2)n multilayer films produced by spray deposition

Colloids and Surfaces A: Physicochem. Eng. Aspects 322 (2008) 142–147

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Stratified PEI-(PSS-PDADMAC)20 -PSS-(PDADMAC-TiO2 )n multilayer films produced by spray deposition Nadia Ladhari a,b , Joseph Hemmerle´ a,b , Christian Ringwald b , Youssef Haikel a,b , Jean-Claude Voegel a,b , Pierre Schaaf c , Vincent Ball a,b,∗ a b c

Universit´e Louis Pasteur, Facult´e de Chirurgie Dentaire, 1 Place de l’Hˆ opital, 67000 Strasbourg, France Institut National de la Sant´e et de la Recherche M´edicale, Unit´e mixte de recherche 595, 11 rue Humann, 67085 Strasbourg Cedex, France Centre National de la Recherche Scientifique, Institut Charles Sadron, 6 rue Boussingault, 67083 Strasbourg Cedex, France

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Article history: Received 19 January 2008 Received in revised form 20 February 2008 Accepted 1 March 2008 Available online 15 March 2008 Keywords: Polyelectrolyte multilayer films Spray deposition Titanium dioxyde nanoparticles Photoinduced superhydrophilicity

a b s t r a c t Polyelectrolyte multilayer films (PEMs) offer the opportunity to functionalize surfaces of different topologies, either planar surfaces or colloids, in a very convenient manner. Two kinds of growth regimes have been discovered: either linear or exponential growth. The later types of PEMs are highly hydrated and can be used as drug reservoirs. To reduce the drug release rate or to confine the exponentially growing PEMs, they have been capped by linearly growing PEMS, hence allowing to build-up reservoirs capped by barriers. The aim of this study is to demonstrate that spray deposition can also be used to build up stratified films comprising a lower exponentially growing compartment, made from poly(diallyldimethylammonium chloride) (PDADMAC) and poly-4-styrene sulfonate (PSS) deposited in the presence of 1 M NaCl, and an upper linearly growing PEM made by reactive layer-by-layer deposition of PDADMAC and titanium (IV) bis (ammonium lactato) dihydroxyde (TiBisLac). Hence, the upper layer is a composite film made from an organic polymer and inorganic nanoparticles, namely TiO2 . This upper layer is not penetrating in the underlying PDADMAC-PSS stratum and changes the surfaces properties of the whole film. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The modification of surfaces to control their interaction with fluids or gas phases is a major concern in modern technology. This goal can be achieved by either covalent grafting of molecules to active surface groups of the metal or oxide surfaces, for instance the deposition of self assembled monolayers on the surfaces of metals like gold, silver or platinum [1,2]. Various deposition methods like spin coating of polymer solutions or Langmuir–Blodgett (LB) deposition of amphiphilic molecules [3] also allows to deposit active molecules on surfaces. However, all the well established deposition methods, with the exception of painting, are pretty cumbersome and need to work either in presence of very pure organic solvents (in the case of the deposition of SAMs) or with special equipment (in the case of the deposition of LB films). These complications are even more pronounced in the case where one wishes to deposit multilayers of the molecules of interest. In the mid of the sixtees, Iler proposed the layer-by-layer deposition of charged colloids to build-up multilayer films [4] from aqueous solutions and in a very convenient

∗ Corresponding author at: Institut National de la Sante´ et de la Recherche ´ Medicale, Unite´ mixte de recherche 595, 11 rue Humann, 67085 Strasbourg Cedex, France. Tel.: +33 3 90 24 32 58. E-mail address: [email protected] (V. Ball). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.03.011

manner. However, the lack of sensitive surface characterization techniques precluded the detailed study of such assemblies and the pioneering work of Iler was totally neglected until the beginning of the nineteens when Decher et al. demonstrated the feasability of the layer-by-layer deposition of charged amphiphiles and polyelectrolytes, leading to polyelectrolyte multilayer films (PEMs) [5,6]. It was rapidly demonstrated, by means of small angle X-ray and small angle neutron reflectivity measurements that such films are pretty well stratified [7], each deposited polyelectrolyte interacting only with the very few layers being deposited before itself or with the layers deposited after [8]. In addition, the thickness as well as the deposited mass per unit area increase linearly with the number of deposited pairs of layers [9], where one layer pair consist of one polycation and one polyanion deposited in an alternated manner, both deposition steps being separated by buffer rinse to eliminate the weakly anchored polymers. It has been demonstrated that the driving force for the deposition of PEMs is the reversal of surface charge upon the deposition of each polyelectrolyte [10–13]. It is however possible to build up polymer multilayer films using polymers that interact through hydrogen bonding [14–16] or interactions between specific host and guest molecules [17]. These linearly growing PEMs are not only fascinating coatings from a fundamental point of view [18] but are also interesting for a plethora of applications; to cite only a few: as coatings affording protection against corrosion [19], as light emitting diodes [20], for non linear

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optical properties [21] or for the optimization of the proton conductivity in fuel cells [22,23] and as permselective membranes [24]. The range of applications of PEM films, mainly in the biological field [25], has been increased with the discovery of films whose thickness increases exponentially with the number of deposited pairs of layers [26,27]. Contrarily to linearly growing PEMs, such films are not stratified and highly hydrated [28]. Their growth is related to the diffusion of at least one of the participating polyelectrolytes through the whole film [29] as long as its thickness is smaller than a critical value. This critical value is related to the distance the diffusing polyelectrolyte can diffuse during the deposition time which is constant whatever the film thickness [30]. These exponentially growing PEMs can be loaded with DNA for gene transfection [31,32] and with a variety of functional molecules [33,34] affording the film with specific biological properties. From this point of view, it is interesting to note that most of the described combinations of polyelectrolytes leading to exponentially growing PEMs are made from biodegradable polyelectrolytes. It seemed also interesting to render these films multifunctional and to control the time schedule of the active molecule release. It is thus interesting to cover an exponentially growing PEM with a linearly growing PEM [35] which is not permeable to the film encapsulated drug or active molecules as long as no active stimulus has been applied. Such a stimulus can be the application of a lateral stress on a (PLL-HA)30 -PLL-(PDADMAC-PSS)5 stratified film [36]: upon its application nanopores are appearing in the upper stratum of the multi-compartment film allowing for the diffusion of labeled PLL out of the film. As soon as the strain is released, the nanopores are closing, thus impeding the further diffusion of PLL out of the multi-compartment film. An other strategy to isolate the different compartments is to separate them by a slowly hydrolysable film made from a degradable polyester, for instance poly-l-lactic-coglycolic acid [37]. Up to now, multi-compartment films have never been produced by spray deposition, which allows to considerably speed up the film deposition [38,39]. In all the described examples, the different compartments were made from polymers only. It is also the aim of the present paper to present the build up of a twocompartment film made exclusively by spray deposition and in which the second, linearly growing stratum, is a composite film made from PDADMAC and TiO2 nanoparticles. In addition, using the concept of reactive layer-by-layer deposition, these nanoparticles are obtained by condensation of a water-soluble precursor upon contact with the polycation [40,41]. We will demonstrate herein that the growth regime of the two strata, namely the underlying PEI(PSS-PDADMAC)20 -PSS film (exhibiting exponential growth at 1 M NaCl [42]) and the upper (PDADMAC-TiO2 )n film obtained by reactive layer-by-layer deposition, are markedly different. We will check by means of infrared spectroscopy that the film with two strata is indeed stratified. The stratification is also indirectly suggested by means of ellipsometry and UV–vis spectroscopy experiments as well as by the wetability properties of the whole architecture that are those of the (PDADMAC-TiO2 )n film which exhibits photoinduced superhydrophilicity [41]. Titanium dioxyde is one of the most used biomaterials and is also a semi conductor displaying photocatalytic activity [43,44] upon illumination, antibacterial activity in presence of UV light [45].

2. Materials and methods 2.1. Polyelectrolyte solutions and adsorption substrates All solutions were made from Milli Q water ( = 18.2 M cm). The polyelectrolytes used to build up the PEMs were PDAD-

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MAC as the polycation (Aldrich, solution at 20% w/w, Mw 100,000–200,000 g mol−1 ) and PSS as the polyanion (Aldrich, 24,305-1, Mw = 70,000 g mol−1 ). Both polyelectrolytes have a charge density that is independent of the solution pH in a wide pH range (between 1 and 14) therefore no attempt was made to adjust the pH of the polyelectrolyte solutions for the deposition of the first (PDADMAC-PSS)20 stratum. PSS and PDADMAC were used as received and dissolved at a concentration of 1 mg mL−1 , hence at 4.5 and 6.2 mM with respect to their monomeric units, respectively. The ionic strength of the solutions used for the film deposition was fixed to 1 M with NaCl as the supporting electrolyte. For the build up of the second stratum, a (PDADMAC-TiO2 )n film, the polycation as well as the water-soluble precursor of TiO2 , namely titanium (IV) bis (ammoniumlactato) dihydroxyde (named TiBisLac in the following) were dissolved in 50 mM Tris buffer at pH 7.5. When TiBisLac is brought in contact with a polycation, it undergoes hydrolysis and TiO2 is produced [46]. This concept can be used to build up composite thin films containing a polycation and TiO2 , mostly in the form of nanoparticles, by a reactive layer-by-layer deposition [47]. We demonstrated recently that this reactive layer-by-layer deposition can be preformed by alternated spray deposition [41] of the polycation and TiBisLac. In addition, the obtained nanoparticles are pretty monodisperse with an average size of (4.9 ± 1.2) nm and are made of crystalline anatase [41]. It is noteworthy that the obtained nanoparticles are crystalline even in the absence of any heat treatment. The obtained films are smooth and display photoinduced superhydrophilicity [41]. In the present investigation, the concentration of TiBisLac was hold constant at 10 mM. The PEMs were deposited on oxidized silicon wafers (Wafernet, San Jose, CA) that were cut in the form of rectangular slides of 4 cm × 1 cm with a diamond knife and cleaned with ethanol, a 2% ¨ (w/v) Hellmanex solution (Hellma, GmbH, Mullheim, Germany) at 70 ◦ C during half an hour, extensive rinse with Milli Q water, and with a 1 M hydrochloric acid solution at 70 ◦ C during half an our. The silicon chips were then rinsed with Milli Q water and blown dry under a stream of nitrogen. A new silicon chip was used for each new experiment. The thickness of the silicon oxide layer covering the silicon substrate was measured by means of ellipsometry (Jobin Yvon, model PZ 2000, France) at a wavelength of 632.8 nm and an incidence angle of 70◦ . The film thickness was calculated from the measured and  ellipsometric angles assuming the film to be uniform and isotropic and using a refractive index value of 1.465. The thickness values given are the average (±one standard deviation) over 5 independent measurements taken at regular intervals along the major axis of the rectangular silicon slide. The optical thickness of the whole film was determined in the same manner and the thickness of the PEM was determined by substracting the thickness of the SiO2 layer from the whole measured thickness. For characterization of the film build up by UV–vis spectroscopy, the films were sprayed on one face of rectangular quartz slides that were cleaned in the same manner as the silicon slides used for the ellipsometry experiments. The UV–vis spectra were acquired with a Safas UV-mc2 double beam spectrophotometer (Monaco, France). 2.2. Film build up by spray deposition The PEMs were deposited by means of spray deposition as described previously in detail [39]. The primer layer of the PEM was PEI which was allowed to adsorb onto the surface during 5 min before extensive rinsing with the 1 M NaCl solution. Briefly, the freshly cleaned and PEI coated silicon slide was vertically oriented with tweezers in order to allow solution drainage upon spray deposition. The polyelectrolyte solutions were put in spray cans which

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were manually pressurized, and after each spray deposition or rinsing with NaCl (or Tris buffer) solution, the cans were pressurized again to reduce the pressure fluctuations during the spraying process [39]. Each spray deposition was performed by spraying the polyelectrolyte solution during 5 s, then 15 s were allowed to complete the adsorption and to allow for drainage of the solution (this time was insufficient to achieve drying of the film) before spraying of the 1 M NaCl solution (or Tris buffer) and 15 s of resting time. The successive deposition of one PSS and one PDADMAC layer (or one PDADMAC and one TiBisLac) constitutes one “layer pair”. After deposition of a given number of layer pairs, the surface was rinsed with Milli Q water and dried with a stream of nitrogen to avoid NaCl (or Tris) crystal formation on top of the substrate, before characterization by ellipsometry (or by UV–vis spectroscopy). Before deposition of additional PDADMAC/PSS (or PDADMAC/TiO2 ) layer pairs, the film was rehydrated by NaCl (or Tris) solutions spraying. The final PEM architectures corresponded to PEI-(PSSPDAMAC)20 -PSS or PEI-(PSS-PDADMAC)20 -PSS-(PDADMAC-TiO2 )n (With 3 ≤ n ≤ 20). The spray depositions were performed at a controlled temperature of (25 ± 3) ◦ C. Indeed the thickness evolution of the PSS/PDADMAC films is strongly temperature dependent [48]. The thickness of films made from 20 (PDADMAC-PSS) layer pairs with additional 20 layer pairs of PDADMAC-TiO2 was also characterized by means of atomic force microscopy (Nanoscope IV from Veeco) after needle scratching. The images were acquired at a scanning frequency of 2 Hz in the contact mode and in the dry state in a direction perpendicular to the scratched line. 2.3. ATR-FTIR experiments These experiments were performed to demonstrate that the TiO2 nanoparticles formed upon the reactive spray deposition of PDADMAC and TiBisLac [41,47] are not able to penetrate in the initially deposited PEI-(PSS-PDADMAC)n –PSS layer. Indeed, ATR-FTIR is an evanescent wave technique able to detect molecules present in a thin film. If the film thickness becomes significantly thicker than the penetration depth of the evanescent wave, the additional deposition of molecules on top of the film can only be detected if these molecules are able to diffuse in the film in order to interact with the evanescent wave. For additional details of this kind of experiments, see reference [49]. PEI-(PSS-PDADMAC)n –PSS films were built up from D2 0 solutions (Aldrich) containing 1 M NaCl and polyelectrolytes at 1 mg mL−1 . After each polyelectrolyte adsorption, during 5 min, from flowing solutions atop the ZnSe substrate, the polyelectrolyte solution was replaced by NaCl (or Tris) solution at 1 M (50 mM) and the infrared spectrum of the film was acquired by accumulating 512 scans at 2 cm−1 spectral resolution on an IFS 55 spectrometer (Bruker, Wissembourg, France). The detector was a liquid nitrogen cooled MCT detector. The transmitted intensity was compared to that transmitted by the naked ZnSe crystal to calculate the absorption spectrum of each layer. The film was build up to the level where the additional deposition of PSS, characterized by the stretching bands attributed to its sulfonate group (at 1007 and 1035 cm−1 ), could not be detected anymore. Then, three layer pairs of PDADMAC and TiBisLac were deposited on top of the film during 5 min each, as for the PSS-PDADMAC layer pairs. An independent experiment was performed in which the three (PDADMAC-TiO2 ) layer pairs were deposited directly on the cleaned ZnSe crystal in order to identify the spectral features of the as grown TiO2 nanoparticles. 2.4. Contact angle measurements 6 ␮L water droplets were deposited on the PEI-(PSSPDADMAC)20 -PSS or on the PEI-(PSS-PDADMAC)20 -(PDAMAC-

Fig. 1. Evolution of the film thickness with the number of layer pairs for a PEI(PSS-PDADMAC)20 -PSS film (䊉) followed by a (PDADMAC-TiO2 )14 film (). The film deposition was performed by spraying each polyelectrolyte or titanium dioxide precursor solution during 5 s, followed by a waiting time of 15 s. The full line and the dotted line correspond to the linear regression and the 95% confidence interval respectively. Each point corresponds to the average over 5 thickness measurements on the same surface. The error bars correspond to ±one standard deviation.

TiO2 )n films as well as on (PDADMAC-TiO2 )n deposits in a temperature regulated room and the advancing contact angles were automatically acquired by means of a Digidrop system. The data are average values over 3 different droplets. Before contact angle measurement, the films were exposed during 5 min under a UV-lamp, the UV light being filtered around 254 nm. 3. Results and discussion In presence of 1 M NaCl, the thickness of the PEI-(PSSPDADMAC)n films increases, in a supralinear manner as a function of the number of the deposited layer pairs n (Fig. 1). This in agreement with published data [42]. It has to be noted that a PEI(PSS-PDAMAC)20 PEM film deposited by spray reaches a thickness of around 350 ± 30 nm and exhibits a wormlike surface morphology (data not shown). We then investigated the possibility to deposit a (PDADMACTiO2 )n strata on top of the first PEM film made of a PEI-(PSSPDADMAC)20 -PSS film. The characterization of the film growth (Fig. 1) demonstrates that the thickness increment per layer pair of the (PDADMAC-TiO2 )n strata is equal to 9.1 ± 0.5 nm. The growth regime of the (PDADMAC-TiO2 )n compartment is linear with n, the number of deposited layer pairs, whereas the growth of the underlying (PDADMAC-PSS)m compartment is supralinear with the number of deposited layer pairs. When a (PDADMAC-TiO2 )n film is deposited by reactive spray deposition on a silicon plate (covered by a 3 nm thick spontaneously grown oxide layer), the thickness increment is equal to (8.0 ± 0.5) nm (data not shown). This demonstrates that the underlying PEI-(PSS-PDADMAC)20 -PSS film does not significantly modify the build up of the (PDADMAC-TiO2 )n film deposited on top of it with respect to the same build up made on the SiO2 covered silicon plate. The evolution of the absorbance at 226 nm, characteristic of PSS and at 260 nm (characteristic of TiO2 ) demonstrate that the build up of the second (PDADMAC-TiO2 )n compartment corresponds to the deposition of different molecules than in the first stratum (Fig. 2). In addition the evolution of the absorbance at 260 nm shows that the

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Fig. 2. (A) UV–vis spectral characterization of the PEI-(PSS-PDADMAC)m film deposition on a quartz slide ((—) m = 5 layer pairs, (+) m = 10 layer pairs, (䊉) m = 20 layer pairs) followed by the deposition of a PSS-(PDADMAC-TiO2 )n film ((—) n = 2, (—) n = 4, (—) n = 6, (—) n = 8). (B) Evolution of the absorbance at 226 nm as a function of the number of deposited layer pairs. (C) Evolution of the absorbance at 260 nm as a function of the number of deposited layer pairs. In parts (B and C), the lines correspond to a linear regression from the 26th layer pair on “compartment 1” and “compartment 2” correspond to the deposition of a PEI-(PSS-PDADAMAC)20 and a PSS-(PDADAMAC-TiO2 )8 film respectively

thickness increase observed upon the deposition of the (PDADMACTiO2 )n film corresponds indeed to additional material deposition and not to an effect that could be attributed to swelling of the film (the (PDADMAC-TiO2 )n film has been deposited from 50 mM Tris buffer whereas the PEI-(PSS-PDADMAC)20 -PSS film has been deposited from a 1 M NaCl solution). The films made of two compartments were imaged in the dry state by means of AFM and their surface morphology appears wormlike (Fig. 3) which is reminiscent of the surface morphology of PDADMAC-PSS films deposited in the presence of 1 M NaCl [42]. This means that the thin (PDADMAC-TiO2 )20 film (about 180 nm in thickness) is not able to smooth out the roughness of the underlying PEI-(PSS-PDADMAC)20 stratum. It has to be noted that the thickness of the two-compartment film determined by AFM in the dry state (500 ± 20) nm (Fig. 3) is very close to the value obtained by ellipsometry. The ellipsometry and UV–vis characterization however does not allow to demonstrate that the final film is stratified, hence that the TiO2 nanoparticles [41] produced upon alternated spraying of PDADMAC and TiBisLac do not diffuse in the PEI-(PSSPDADMAC)20 -PSS architecture. To have a qualitative information concerning this question, we performed an ATR-FTIR experiment on a PEI-(PSS-PDADLMAC)n film significantly thicker than the wavelength dependent penetration depth of the evanescent wave originating from the ZnSe crystal. When integrating the area under the infrared spectra between 970 and 1050 cm−1 , it appears that the build up of the PEI-(PSS-PDADMAC)n film can not be followed any more when n > 15 (Fig. 4a). This does not mean that the film build

Fig. 3. (A) AFM topography of a two-compartment PEI-(PSS-PDADMAC)20 -PSS(PDADMAC-TiO2 )20 film after needle scratching. (B) Line scan along the two lines shown in part (A).

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Fig. 5. Advancing contact angles (average and standard deviation on 5 independent surfaces) measured on 1: PEI-(PSS-PDADMAC)20 -PSS, 2: PEI-(PSS-PDADMAC)20 PSS-(PDADMAC-TiO2 )3 , and 3: (PDADMAC-TiO2 )3 films immediately after 5 min of exposure to a UV-lamp (at  = 254 nm).

Fig. 4. (A) Evolution of the area under the infrared spectrum between 970 and 1050 cm−1 , corresponding to the elongation bands of the sulfonate groups, as a function of the number of deposited layer pairs during the deposition of a PEI-(PSS-PDADMAC)25 PEM (compartment 1) followed by the deposition of a (PDADMAC-TIO2 )3 film (compartment 2). (B) Change in absorbance, between 1300 and 800 cm−1 , during the deposition of: 1 (—), 2 (- - -) and 3 (+) PDADMAC-TiO2 layer pairs on top of the PEI-(PSS-PDADMAC)25 film. The absorbance was calculated by using the spectrum of the 25th PSS layer as the reference. Indeed, even when 7 additional layer pairs of PDADMAC-TiO2 are deposited, no additional increase in absorbance is found between 1300 and 800 cm−1 (data not shown). (C) Increase in absorbance, between 1300 and 800 cm−1 , during the deposition of: 1 (—), 2 (- - -) and 3 (+) PDADMAC-TiO2 layer pairs on the bare ZnSe crystal. The peaks at 1058, 1123 cm−1 as well as the broad peak between 1000 and 800 cm−1 are attributed to the PDADMAC-TiO2 composite material.

up stops after n = 15, as is demonstrated in Fig. 1, but that the film thickness exceeds significantly the penetration depth of the evanescent wave in the film (the penetration depth is around 600 nm at 1000 cm−1 ). This is not in contradiction with Fig. 1, which shows that the thickness of a PEI-(PSS-PDADMAC)15 film is around 250 nm in the dry state. This thickness in the dry state is certainly lower than the thickness in the wet state, as has been shown for other combinations of polyelectrolytes leading to an exponential growth of the film thickness [30]. We hence deposited 25 layer pairs of PSSPDADMAC before the deposition of the compartment containing TiO2 nanoparticles. When a (PDADMAC-TiO2 )3 film is deposited atop the PEI(PSS-PDADMAC)25 –PSS film no signal due to the TiO2 vibrations, between 800 and 1000 cm−1 as well as at 1058 and 1123 cm−1 is found in the infrared spectrum (Fig. 4b). Such broad bands are found for a PDADMAC-TiO2 film directly deposited on the ZnSe substrate

(Fig. 4c), as well as for a solution containing a mixture of PDADAMAC and TiBisLac (and hence TiO2 which is produced upon condensation of TiBisLac in presence of polycations [46]). This demonstrates clearly that TiO2 nanoparticles are not diffusing in the part of the PEI-(PSS-PDADMAC)25 –PSS film that is sensed by the evanescent wave. This is not a proof of the stratification of the whole architecture in two distinct compartments, but a proof that the intermixing of the TiO2 nanoparticles with the PEI-(PSS-PDAMAC)25 –PSS film is not complete, and hence that most of the TiO2 material is located closer to the film aqueous solution interface than to the ZnSe–film interface. The contact angle measurements described in the next section will provide additional evidence that the top of the twocompartment film is rich in PDADMAC-TiO2 material. Nevertheless, it appears clearly from Fig. 4b, that the deposition of the (PDADMAC-TiO2 )3 compartment leads to a change in the underlying PSS-PDADMAC film. Indeed, upon deposition of PDADMAC-TiO2 layer pairs atop the PEI-(PSS-PDAMAC)25 –PSS film, the characteristic IR bands attributed to the sulfonate groups of PSS increase in intensity. The increase in the intensity of the PSS band occurs however only during the first PDADMAC-TiO2 layer deposition, as appears also in Fig. 4a. This observation is of course not due to additional PSS deposition but has to be attributed to a compaction of the underlying PEI-(PSS-PDAMAC)24 -PSS film. Such a compaction leads to an increase in the PSS concentration in the region of the film that is probed by the evanescent wave. The interaction between the first PDADMAC-TiO2 layer pair and the underlying PEI-(PSS-PDADMAC)24 -PSS film could also explain the somewhat discontinuous transition between the build up of the first and the second compartment of the film, as appears in Figs. 1 and 2. To further demonstrate that the deposition of the (PDADMACTiO2 )n compartment leads to an enrichment of the film–solution interface in TiO2 , and hence a change in the chemical composition of the top of the film, we performed some contact angle measurements after irradiation of the films with UV light (at  = 254 nm). Indeed, we [41] and others demonstrated that the films rich in TiO2 become superhydrophilic upon exposure to UV light. The advancing contact angles for a PEI-(PSS-PDADMAC)20 -PSS-(PDADMAC-TiO2 )3 and a (PDADMAC-TiO2 )3 film directly deposited on a silicon wafer are displayed in Fig. 5. It appears clearly that the deposition of only 3 (PDADMAC-TiO2 ) layer pairs (about 27 nm in thickness, Fig. 1) on top of a PEI-(PSSPDADAMAC)20 -PSS film is sufficient to obtain the photoinduced superhydrophilicity of a (PDADMAC-TiO2 )3 film. This is additional evidence that the two-compartment film displays some stratifica-

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