Polyamine Films Prepared by a Reactive Layer

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Langmuir 2007, 23, 3706-3711

Nanocomposite Silica/Polyamine Films Prepared by a Reactive Layer-by-Layer Deposition Nicolas Laugel,† Joseph Hemmerle´,‡ Claudine Porcel,† Jean-Claude Voegel,‡ Pierre Schaaf,† and Vincent Ball*,‡,§ Centre National de la Recherche Scientifique, Unite´ Propre de recherche 22, Institut Charles Sadron, 6 rue Boussingault, 67085 Strasbourg Cedex, France, Institut National de la Sante´ et de la Recherche Me´ dicale, Unite´ Mixte de Recherche 595, 11 rue Humann, 67083 Strasbourg Cedex, France, and Faculte´ de Chirurgie Dentaire, 1 Place de l’Hoˆ pital, 67000 Strasbourg, France ReceiVed October 17, 2006 Composite material nanofilms of controlled thickness constituted by ceramics and polymers find more and more applications to improve the properties and functionalities of material surfaces. In this paper we present a new way to deposit such composite coatings by alternated contacts of a surface with a polyamine solution (either poly(allylamine), poly(ethyleneimine), poly-L-lysine, or poly(diallydimethylammonium)chloride and silicic acid. The experiments are mainly realized by spraying of solutions onto the surface. The polyamines deposited in the first spraying step catalyze silica formation upon further spraying of a silicic acid-containing solution. The film thickness increases linearly with the number of deposition steps, the thickness increment being of the order of a few nanometers per silicic acid/ polyamine layer. Infrared spectroscopy in the total attenuated reflection mode reveals spectra that are close to those of pure silica particles. The film morphology is further investigated by means of atomic force microscopy and environmental scanning electron microscopy. This reactive layer-by-layer deposition constitutes a new way to build, in an easy way, nanocomposite coatings with precise control of their thickness.

Introduction The fabrication of well-controlled composite coatings on solid surfaces constitutes a promising new tool to tune the mechanical, optical, and permeability properties of interfaces between solids and their surroundings. Many strategies have been employed to this aim by using the layer-by-layer self-assembly1-4 of charged polymers and oppositely charged colloidal particles5-8 or nanotubes.9 The layer-by-layer embedding of inorganic particles with polymers is a means to strongly improve the mechanical properties of these thin films,10-12 a way to modify their optical properties13 and wettability,8 to use them as chemical sensors,14 as well as to create microelectronic devices.15-16 It has also been * Corresponding author. Telephone: 0033 3 90 24 32 58. E-mail: [email protected]. † Centre National de la Recherche Scientifique. ‡ Institut National de la Sante ´ et de la Recherche Me´dicale. § Faculte ´ de Chirurgie Dentaire. (1) Decher, G.; Hong, J. D. Macromol. Chem. Macromol. Symp. 1991, 46, 321. (2) Decher, G.; Hong, J. D.; Schmitt J. Thin Solid Films 1992, 831, 210. (3) Decher, G. Science 1997, 277, 1232. (4) Hammond, P. T. Curr Opin. Colloid Interface Sci. 1999, 4, 430. (5) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065. (6) Caruso, F.; Lichtenfeld, H.; Giersig, M.; Mo¨hwald, H. J. Am. Chem. Soc. 1998, 120, 8523. (7) Voigt, A.; Buske, N.; Sukhorukov, G. B.; Antipov, A. A.; Leporatti, S.; Lichtenfeld, H.; Bau¨mler, H.; Donath, E.; Mo¨hwald, H. J. Magn. Magn. Mater. 2001, 225, 59. (8) Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349. (9) Paloniemi, M.; Lukkarinen, T.; A ¨ a¨ritalo, S.; Areva, J.; Leiro, M.; Heinonen, K.; Haapakka, J.; Lukkari, J. Langmuir 2006, 22, 74. (10) Tang, Z.; Kotov, N. A.; Magonov, S.; Ozturk, B. Nat. Mater. 2003, 2, 413. (11) Shim, B. S.; Kotov, N. A. Langmuir 2005, 21, 9381. (12) . Markutsya, S.; Jiang, C.; Pikus, Y.; Tsukruk V. V. AdV. Funct. Mater. 2005, 15, 771. (13) Nolte, A. J.; Rubner, M. F.; Cohen, R. E. Langmuir 2004, 20, 3304. (14) Koktysh, D. S.; Liang, X.; Yun, B. G.; Pastoriza-Santos, I.; Matts, R. L.; Giersig, M.; Serra-Rodriguez, C.; Liz-Marzan, L. M.; Kotov, N. A. AdV. Funct. Mater. 2002, 12, 255. (15) Eckle, M.; Decher, G. Nano Lett. 2001, 1, 45.

shown that the incorporation of silver nanoparticles in gels made of biocompatible polymers has the potential to provide antimicrobial properties to these composite materials.17 Another way to prepare such composite materials is to use self-assembled polyelectrolyte films that allow the permeation of small reducible cations and to reduce them afterward to finally obtain a film containing inorganic loads.18 Finally, it is also possible to coat surfaces with polymers and inorganic particles, using the ability of the chosen adsorbed19 or grafted polymer,20 or with amphiphilic molecules21 to induce a polycondensation reaction of the inorganic precursor. The method, as well as the applications of these surface sol gel processes, has been recently reviewed.22 Understanding the mechanisms by which organisms such as diatoms produce such organic-inorganic protective shells can help the material scientist to develop new coatings by the synthesis of polymers with structures similar to those used by living organisms.23 In general, such hybrid organicinorganic materials display very interesting mechanical properties24 and structures such as a hierarchy of porous structures ranging from nanopores to macropores. Such materials constitute also very interesting reservoirs for bioencapsulation.25,26 (16) Fou, A. C.; Onitsuka, O.; Ferreira, M.; Rubner, M. F.; Hsieh, B. R. J. Appl. Phys. 1996, 79, 7501. (17) Ho, C. H.; Tobis, J.; Sprich, C.; Thomann, R.; Tiller, J. C. AdV. Mater. 2004, 12, 957. (18) Wang, T. C.; Chen, B.; Rubner, M. F.; Cohen, R. E. Langmuir 2001, 17, 6610. (19) Coradin, T.; Mercey, E.; Lisnard, L.; Livage J. Chem. Comm. 2001, 2496. (20) Kim, D. J.; Lee, K. B.; Chi, Y. S.; Kim, W. J.; Paik, H. J.; Choi, I. S. Langmuir 2004, 20, 7904. (21) Hubert, D. H. W.; Jung, M.; Frederik, P. M.; Bomans, P. H. H.; Meuldijk, J.; German, A. L. AdV. Mater. 2000, 12, 1286. (22) Ichinose, I.; Kuroiwa, K.; Lvov, Y.; Kunitake T. In Multilayer Thin Films; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: New York, Weinheim, 2002; p 155. (23) Sumper, M.; Brunner, E. AdV. Funct. Mater. 2006, 16, 17. (24) Mammeri, F.; Le Bourhis, E.; Rozes, L.; Sanchez, C. J. Mater. Chem. 2005, 15, 3787. (25) Nassif, N.; Bouvet, O.; Rager, M. N.; Roux, C.; Coradin, T.; Livage, J. Nat. Mater. 2002, 1, 42.

10.1021/la063052w CCC: $37.00 © 2007 American Chemical Society Published on Web 02/27/2007

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Scheme 1. Illustration of the Growth of a Silica-Polycation Hybrid Film by Using the Ability of the Polycation to Catalyze the Polycondensation of Silica from the Sprayed Precursor, Silicic Acid

a Step a: spray deposition of the polycation. Step b: spray deposition of a silicic acid solution. (Inset) Silicic acid monomers around the amino groups of the adsorbed polycation. Upon polycondensation and release of water, these precursors build up an inorganic framework whose solution-exposed groups are negatively charged at pH 7.5. These charged silanol groups allow further spray deposition of polycations (step c). Hence, the reactive spray deposition can be repeated (step d) by a new spray deposition of silicic acid, as in step b.

It has been demonstrated that polycations27 and, in particular, polyamines28,29 are able to promote the condensation of silicic acid into silica nanoparticles that can further coalesce to produce sol-gel materials. The polyamines not only play a catalytic role by enhancing the rate of silica polycondensation but they are also an active component of the obtained composite material: (i) the infrared spectra of the obtained composite material show that the Si-O stretching band is significantly displaced from 1085 cm-1 in the absence of polycation to 1039 cm-1 for the polycation-containing material.28 (ii) The silica-polycation composite displays selective permeability toward small anions, whereas the pure silica gel is permeable toward small cations. This shows that the sign of the fixed charges in the composite material has been reversed from negative to positive upon polycation incorporation.28 Even if the precise mechanism has yet not been established, it remains clear that the presence of polyamines catalyzes the formation of silica from silicic acid (see Scheme 1), and we will make use of this property to propose a new preparation method of nanocomposite films. This method will be called “reactive layer-by-layer deposition”. It consists of an alternated contact of a surface with polyamines [poly(allylamine) which carries primary amino groups (PAH); poly(diallyldimethylammonium) chloride which carries quaternary ammonium groups (PDADMAC); polyL-lysine which also carries primary amines (PLL); or polyethylene imine which contains simultaneously primary, secondary, and tertiary amino groups (PEI)] and silicic acid at a controlled pH value. We will mainly, but not exclusively, use the spraying method to bring the solutions in contact with the surface. This deposition method combines both the advantages of the layerby-layer deposition of oppositely charged polyelectrolytes30-32 (26) Avnir, D.; Coradin, T.; Lev, O.; Livage, J. J. Mater. Chem. 2006, 16, 1013. (27) Iler, R. K. J. Colloid Interface Sci. 1971, 37, 364. (28) Mizutani, T.; Nagase, H.; Fujiwara, N.; Ogoshi, H. Bull. Chem. Soc. Jpn. 1998, 71, 2017. (29) Coradin, T.; Durupthy, O.; Livage, J. Langmuir 2002, 18, 2331. (30) Schlenoff, J. B.; Dubas, S. T.; Fahrat, T. R. Langmuir 2000, 16, 9968.

and of the reported catalytic effect of polyamines in the formation of silica from silicic acid.27-29 It should lead to silica/ nanoinorganic composites. The preparation is schematically represented in Scheme 1. The originality of the proposed deposition method lies in the fact that the oppositely charged species are not only deposited in an alternated way but also that the contact between the deposited polyamines and the incoming silicic acid monomers or small oligomers induces the polycondensation of silicic acid into silica. Silica particles being negatively charged allow a new deposition step of polyamines, and the buildup process can go on indefinitely. Experimental Section Solutions. All solutions were prepared from milli-Q ultrapure water (F )18.2 MΩ‚cm). All polyelectrolytes and silicic acid solutions were freshly prepared before each deposition experiment. Poly(ethylene imine) (PEI, Sigma, P3143, 093K0098, Mw ) 750000 g/mol as a 50% w/w solution), poly-4-styrene sulfonate (PSS, cat. number 24,305-1, Mw ≈ 70000 g/mol), and poly(allylamine hydrochloride) (PAH, Aldrich, cat. number 28,322-3, Mw ≈ 70000 g/mol), poly-L-lysine (PLL, Fluka Bio Chemika, ref 81339, Mw ) 70000-150000 g/mol) and poly(diallyldimethylammonium) chloride (PDADMAC, Mw between 100000 and 200000 g/mol) were used without further purification. Silicic acid was purchased from Riedel De Hae¨n (ref 13729, lot 1219A) in the form of water glass. All films were made with polyelectrolytes and chemical species dissolved in 0.05 mol‚L-1 Tris whose pH was adjusted to 7.5. The pH of the silicic acid solution had to be adjusted again to 7.5 with concentrated hydrochloric acid after the dissolution of silicic acid whose mother solution contained 11% NaOH. This high pH of the mother solution prevents spontaneous polycondensation to produce silica. UV-Vis Spectroscopy. UV-vis experiments were performed in the kinetic mode to check the absence of spontaneous polymerization of the silicic acid solutions used for the spray deposition (31) Izquierdo, A.; Ono, S. S.; Voegel, J.-C.; Schaaf, P.; Decher, G. Langmuir 2005, 21, 7558. (32) Porcel, C.; Lavalle, Ph.; Ball, V.; Decher, G.; Senger, B.; Voegel, J.-C.; Schaaf, P. Langmuir 2006, 22, 4376.

3708 Langmuir, Vol. 23, No. 7, 2007 experiments. To this aim a silicic acid solution was prepared in the 0.05 M Tris buffer, its pH was adjusted to 7.5, and it was put in a 1-cm path quartz cuvette. Immediately after that, its absorbance was followed as a function of time at a wavelength of 350 nm with a time resolution of 1 min during at least 3 h. This time duration corresponded to the longest spray experiments. Since neither silicic acid nor silica absorbs light at 350 nm, any increase in absorbance should correspond to light lost by scattering and hence reflects the spontaneous appearance of silica particles. In these experiments the silicic acid concentration was varied between 0 and 0.04 M. At higher concentrations, the solutions appeared turbid just after pH adjustment. Hence, the silicic acid solutions used for the spray deposition-reaction experiments never exceeded this critical concentration value of 0.04 M. Spray Deposition of the Silica/Polyelectrolyte Nanocomposite Films. The films were obtained by means of spray deposition as was previously described in detail.30-32 For the characterization of the film growth by means of ellipsometry, we used freshly cleaned (100) silicon wafers (Wafernet, INC., San Jose, CA) as substrates. The cleaning method consisted in an ethanol-washing step, water rinse, immersion in a hot (∼70 °C) Hellmanex solution (2% v/v, Hellma, Gmbh, Mu¨llheim, Germany) during half an hour, followed by intensive MilliQ water rinse, immersion in a hot (∼70 °C) 1 M HCl solution during half an hour, and a final MilliQ water rinse. The thickness of the silicon oxide film grown atop the silicon substrate was then measured by means of ellipsometry (Jobin Yvon, model PZ 2000, France) at a constant angle of incidence (70°) after nitrogen drying. At the working wavelength of 632.8 nm, the refractive index of silicon oxide was taken equal to 1.465. A single “one layer Fresnel model” was used to evaluate the data. The silicon wafer was then again hydrated and immersed in a 0.5 mg/mL positively charged PEI solution during 5 min to allow for the deposition of a primer polyelectrolyte layer. The substrate was then immobilized in a close to vertical orientation and held with tweezers at its upper part to allow for optimal liquid drainage upon spraying. We then deposited a (PSS-PAH)5 polyelectrolyte multilayer film by spraying each polyelectrolyte solution (at 0.5 mg/mL in 0.05 M Tris buffer) during 5 s and allowing for further polyelectrolyte solution drainage during an additional duration of 15 s. The buffer solution was then sprayed for 5 s and followed by a rest time of 15 additional seconds before starting the next polyelectrolyte deposition step. The thickness of the PEI-(PSSPAH)5 multilayer was then determined by means of ellipsometry after a water rinse (to avoid the appearance of crystals from the buffer molecules) and drying under a nitrogen flow. For all the evaluations of the ellipsometry measurements, we assumed the deposited film to have a refractive index of 1.465, which in the case of PEI-(PSS-PAH)5 is pretty close to the values obtained in situ by means of Brewster angle reflectometry and optical waveguide lightmode spectroscopy. However, it should be kept in mind that ellipsometry allows for the accurate calculation of nfilm‚d where nfilm and d are the refractive index and the geometrical thickness of the adlayer. Hence, any overestimation in nfilm results in an underestimation in d. For this reason the film thickness will also be estimated by atomic force microscopy in the dry state after needle scratching, as will be described later. The composite silica-polycation film was then deposited by alternately spraying a silicic acid solution at x mM (with x < 20 mM as obtained from the UV-vis experiments in order to avoid spontaneous polycondensation of silica and a polycation solution PDADMAC, PAH, PEI, PLL which was always kept at 0.5 mg/ mL. These experiments will be called “reactive spray deposition”, and each species was sprayed during 5 s followed by an additional 15-s rest time for drainage. Between each deposition, Tris buffer was sprayed during 5 s to wash away all nonreacted and deposited species. Buffer drainage was also allowed during 15 s. In most experiments, the silicon chip with the deposited material was water rinsed, dried with a nitrogen flow, and characterized by ellipsometry after every two deposition-reaction cycles, where a depositionreaction cycle corresponds to the spraying of silicic acid and of the polycation. Before continuing the reactive spray deposition, the film

Laugel et al. was rehydrated again with a short pulse (about 2 s) of Tris buffer spray. However, in order to ensure that these drying-rehydration cycles did not disturb the deposition of the investigated composite, some experiments were also performed by depositing material during 10 deposition-reaction cycles without intermediate drying. It is mandatory to check if the water rinse (to avoid crystalization of ions from the Tris buffer), nitrogen drying, and further rehydration by Tris buffer does not modify the buildup regime of the films. Indeed, it has been shown that intermediate drying steps can modify the film buildup and the surface roughness of the obtained films.33,34 The total thickness of the obtained film was then compared to the thickness of a PEI-(PSS-PAH)5 film onto which two deposition-reaction cycles were performed five times with intermediate drying steps. Each film thickness value corresponds to an average of over 5 measurements performed on regularly spaced points along the main axis of the rectangular silicon chip along which solution drainage occurred. Infrared Spectroscopy in the ATR Mode. Fourier transformed infrared spectra in the attenuated total reflection mode (FTIR-ATR spectra) were acquired on a trapezoidal ZnSe crystal before and after the reactive deposition, realized by flowing solutions above the ZnSe crystal during 5 min, by adding 512 interferograms at 2 cm-1 spectral resolution. To this aim we used a liquid nitrogen-cooled mercury cadmium detector on a Bruker Equinox 55 spectrometer (Bruker, Wissembourg, France). Before film deposition the spectrum of a freshly cleaned ZnSe crystal was taken as the reference spectrum, and the absorbance was calculated as the logarithm of the ratio between the transmitted intensity after film deposition and the transmitted intensity before film deposition. Atomic Force Microscopy. In order to check the uniformity of the surface coverage due to the polyelectrolyte film deposition, we performed atomic force microscopy experiments in the dry state for films deposited at silicic acid concentrations of 10.8, 20, and 50 mM and a PAH concentration of 0.5 mg/mL. These films were prepared as previously described on silicon wafers and were constituted by either 6 or 15 pairs of (silicic acid-PAH) depositions. The surfaces were imaged in the contact mode with a Nanoscope IV microscope (Veeco) using silicon nitride cantilevers and tips with a nominal spring constant of 0.05 N‚m-1. Just before imaging, the film was rapidly rinsed with distilled water, blown dry with a stream of nitrogen, and scratched with an ethanol-cleaned syringe needle. The film was then imaged in the dry state in the contact mode in a scan direction perpendicular to the scratched line. Before image acquisition, the stability of the visualized structures was checked by allowing several scans over the investigated area. Scanning was always performed at a frequency of 2 Hz. Environmental Scanning Electron Microscopy. Multilayer films were sprayed as previously described onto cleaned glass slides. Secondary electron imaging was performed by using scanning electron microscopy (FEI Quanta 400, FEI Company TM, Hillsboro, OR) operating in the low vacuum mode, at a water pressure of 1 Torr and with an acceleration voltage of the electrons of 30 kV.

Results and Discussion The concentration range of silicic acid that allows spraying monomers or small oligomers of silicic acid alternately with a polycation was determined by following the turbidity changes of the silicic acid solutions at pH 7.5 as a function of time at a wavelength of 350 nm. At this wavelength neither silicic acid nor silica absorbs light. An increase in turbidity indicates that colloidal silica particles are formed in solution. Such colloidal particles would then also form in the bottles in which silicic acid is stored before spraying and would have to be avoided. Thus, control experiments had to be performed in order to ensure that the deposited particles are not the result of an artifact. The (33) Lvov, Y.; Ariga, K.; Onda, M.; Ichinose, I.; Kunitake, T. Colloids Surf., A 1999, 146, 337. (34) de Souza, N. C.; Silva, J. R.; Di Thommazo, R.; Raposo, M.; Balogh, D. T.; Giacometti, J. A.; Oliveira, O. N., Jr. J. Phys. Chem. B 2004, 108, 13599.

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Figure 1. Evolution of the turbidity of silicic acid solutions, measured by UV -vis spectroscopy at a wavelength of 350 nm, as a function of the silicic acid concentration : (_ _ _) 5.4 mM, (O) 20 mM and (____) 40 mM and after addition of PAH (under agitation to reach a final concentration of 0.04 mg/mL) to a solution containing 10 mM of silicic acid (b).

evolution of turbidity with time is represented for different concentrations of silicic acid in Figure 1.The pH of the silicic acid solution was adjusted to 7.5 after 40 min. and the PAH was also added to the silicic acid solution after 40 min. It appears that colloidal particles are not formed for at least 2 h as long as the silicic acid concentration is below 20 mM. This time duration is sufficient to deposit more than 40 pairs of silicic acid-PAH layers. However, as soon as PAH is added under agitation to the 10 mM silicic acid solution so as to reach a final PAH concentration of 0.04 mg/mL, a burst in turbidity is observed. This indicates the formation of silica particles. It is followed by a slow decrease in turbidity which is probably due to the sedimentation of the formed colloidal particles. This shows, in accordance with published data, that the presence of polyamines induces a rapid polycondensation of silicic acid with the formation of silica particles and eventually a silica gel.27-29 Next, we sprayed alternately PAH and silicic acid solutions onto a vertically maintained silicon wafer. The concentration of silicic acid was always less than 20 mM. These alternated sprays were performed (see Experimental Section) on silicon slides already coated with a PEI-(PSS-PAH)5 primer film. We first verified that, if silicic acid is sprayed in alternation with Tris buffer, without PAH, the thickness of the film measured by ellipsometry remains constant and equal to the value found at the end of the first silica deposition (Figure 2a). On the other hand, if silicic acid at the same concentration as in the control experiment (10.8 mM) is sprayed alternately with a PAH solution (at 0.5 mg/mL), the optical thickness of the deposited material increases linearly with the number of spraying steps (see Figure 2a). The thickness increment was of the order of 10 nm per silica/PAH layer pair. This proves material deposition in each cycle and the validity of our concept, namely that PAH deposited on a surface induces the formation of a film of nanometric size when brought in contact with silicic acid solutions and that the process can be pursued by alternated depositions (Scheme 1). We then investigated the effect of the concentration of silicic acid on the thickness increment per (silica-PAH) layer pair (see Figure 2b). It appears that the thickness increment increases with the concentration and does not seem to level off for concentrations higher than 20 mM. Unfortunately, it was not possible to perform experiments at higher silicic acid concentrations due to spontaneous condensation in the spraying bottles. Spraying is a very rapid and convenient way to bring solutions in contact with a surface. However, one can also dip the surface

Figure 2. (a) Evolution of the optical thickness of deposits obtained by alternated spray depositions of silicic acid at 10.8 mM and of PAH (at 0.5 mg/mL) onto a PEI-(PSS-PAH)5 primer layer (see Experimental Section) as a function of the number of deposited layer pairs, n. The optical thickness of the PEI-(PSS-PAH)5 primer film has been subtracted from every further measured value on each corresponding sample. The experiments labeled with (O, 9, 4) are aimed to illustrate the reproducibility of the deposition method, whereas the experiments labeled with (b, red; [, blue) were performed without intermediate drying steps and are aimed to illustrate that the drying-rehydration cycles necessary for the ellipsometry measurements do not drastically modify the reactive layer-by-layer deposition. The control experiment consisting of spraying silicic acid and buffer without polycation steps (as described in text) is labeled with (b). (b) Evolution of the thickness increment per deposited layer pair for reactive layer-by-layer spray depositions of silicic acid and PAH as a function of the silicic acid concentration (PAH is kept constant at 0.5 mg/mL). Each point corresponds to an individual experiment. The line does not correspond to a fit but is aimed to guide the eye.

alternately into solutions of PAH, buffer, silicic acid, and again buffer, as it is usually done for the layer-by-layer deposition of polyelectrolyte multilayers. This also leads to a continuous formation of a (silica-PAH)n film whose optical thickness grows linearly with the number of deposition steps. However, the thickness increment per layer is smaller than that of the film obtained by the spray deposition method (5.7 nm compared to 10.8-12 nm per layer pair at 10.8 mM in silicic acid). A possible reason for such a difference may well be related to the hydrodynamic conditions under which the deposition is performed. Next, we have investigated if the reactive buildup concept can be extended to other polyamines. We have investigated three

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Figure 3. Comparison of the optical thickness evolution for reactive layer-by-layer spray deposits of (O) PDADMAC-silicic acid (yielding a thickness increment of 5.4 nm/layer pair) , (9) PEIsilicic acid (yielding a thickness increment of 10.8 nm/layer pair) and ([) PLL-silicic acid (yielding a thickness increment of 6.0 nm/layer pair) as a function of the number of deposition steps. All these experiments were performed at a polycation concentration of 0.5 mg/mL and with silicic acid at 10.8 mM. Even if the reactive spray deposition process starts at layer 1 [at the top of the PEI(PSS-PAH)5 layer], the thickness evolution has been represented only after 5 (silicic acid-PAH) layer pairs have been deposited. The black lines correspond to the linear regression curves of the thickness versus n.

other polyamines, namely poly(ethyleneimine) (PEI), poly-Llysine (PLL), and poly(diallyldimethylammonium) chloride (PDADMAC). For all three systems, the alternate spraying of the polyamine and the silicic acid solution at 10.8 mM led to a linear increase of the film thickness measured by ellipsometry. It appears, however, from Figure 3 that the alternated reactive deposition is the most efficient when the used polycation is PAH; hence, we will characterize the deposits with this particular polycation. The reasons for the differences in the thickness increments when the different polycations were used are out of the scope of this article and will be investigated in a future publication. Until now we have shown that the alternate contact of a surface with a polyamine and a silicic acid solution leads to the buildup of a film whose thickness increases linearly with the number of buildup steps. We will now try to characterize the film that is formed by means of FTIR-ATR spectroscopy. Figure 4 shows the evolution of the IR spectrum during the buildup of a PEI(PSS-PAH)5-(silicic acid-PAH)3 film. We take as reference the spectrum of the precursor PEI-(PSS-PAH)5 film which is systematically subtracted. These spectra are compared to the IR spectrum of silica particles directly deposited on a bare ZnSe crystal. One observes that the shape of the (silicic acid-PAH)n spectra are very close to those of the silica particles which were deposited directly on the surface of the cleaned ZnSe crystal. However, the bands attributed to the silica stretching modes are shifted to higher wavenumbers, by about 30 cm-1, with respect to those of pure silica particles, whose main band is located at 1085 cm-1. This is in agreement with published data.28 This points to the fact that the interaction of silica with PAH molecules slightly modifies the Si-O stretching vibrations. These observations have also been described in the literature.28,35 The mechanism by which polyamines catalyze the polycondensation of silicic acid is, however, still not clearly established. According to Coradin and Livage the catalytic effect of polyamines is mainly of electrostatic nature. They propose that the polyamines may serve as substrates for silica formation, the (35) Gendron-Badou, A.; Coradin, T.; Maquet, J.; Fro¨hlich, F.; Livage, J. J. Non-Cryst. Solids 2003, 316, 331.

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Figure 4. Infrared spectra in the ATR mode of PEI-(PSS-PAH)5(silicic acid-PAH)n deposits produced by the alternate dipping of polyelectrolytes and inorganic precursors. The concentration of silicic acid was equal to 10.8 mM. The spectra of PEI-(PSS-PAH)5 were subtracted from the spectra acquired after the deposition of each (silicic acid-PAH) layer pair. The numbers above the spectra indicate the number of pairs of layers of (silicic acid-PAH) deposited on the top of the PEI-(PSS-PAH)5 precursor layer. The spectra of the silica particles, deposited by solvent casting from an ethanolic solution, was shifted by 0.12 absorbance units (a.u.) for the sake of clarity.

silicic acid monomers being brought close enough by electrostatic interactions with amino groups to favor oligomerization.29 Mizutani et al. suggest that the catalytic effect of polyamines is due to the cooperative action of amino groups that would be arranged in appropriate positions.28 This would allow the stabilization of the transition state of the condensation reaction by aiding the proton transfer. On the other hand, Corriu suggested that in the presence of amines, where nitrogen has a free lone pair of electrons, Si expands its coordination sphere to a pentacoordinate intermediate.36 Hydrolysis and condensation are enhanced in a mechanism that includes such intermediates. However, Delak and Sahai have shown that, in the case of the condensation of trimethylethoxysilane under mild acidic conditions in the presence of amines, such a mechanism seems not to take place.37 In their case they conclude that acid catalysis remains the only plausible mechanism for the condensation reaction. However, it is interesting to notice that PDADMAC is a quaternary polyamine and thus does not possess a lone pair of electrons on the nitrogen atoms. Nevertheless, it catalyzes the silica formation from silicic acid. This implies that the presence of such an electron pair is not required to get the catalytic effect. This points, in our case, toward an electrostatic origin of the catalytic effect of polyamines as proposed by Coradin and Livage29 and not to a chemical effect as first suggested by Corriu.36 In order to verify that the formed films are continuous, we imaged a PEI-(PSS-PAH)5-(silicic acid-PAH)15 film by AFM. We then scratched the surface with a needle and determined the surface profile. As shown in Figure 5, the film covers entirely the substrate. It presents a granular morphology with emerging particles ranging between about 50 and 300 nm in diameter. This granular morphology is also apparent when films are built up at other concentrations in silicic acid, namely at 5 and 20 mM and also when only six pairs of (silicic acid-PAH) have (36) Corriu, R. J. P.; Guerin, C.; Henner, B. J. L.; Wong Chi Man, W. W. C. Organometallics 1988, 7, 237. (37) Delak, K. M.; Sahai, N. Chem. Mater. 2005, 17, 3221.

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Figure 5. (Left-hand panel) AFM topographical image, obtained in contact mode and in the dry state of a PEI-(PSS-PAH)5-(silicic acid-PAH)15 film prepared by the reactive spray deposition method. Silicic acid was sprayed at a concentration of 10.8 mM. The image size is of 5 × 5 µm2. (Right-hand panel) Height evolution of the same film along a line perpendicular to the scratched line of the left-hand panel image.

been deposited on top of the PEI-(PSS-PAH)5 precursor film (see Supporting Information). ESEM observations confirm the uniformity and the slightly grainy aspect of the spray deposits (data not shown). This holds even if only five layer pairs have been deposited by the reactive spray deposition method. Indeed, the surface morphology of the films prepared by the reactive spray deposition is hardly affected by the number of reactive deposition cycles between 5 and 30 pairs (data not shown) of reactive spray depositions. This shows the possibility to obtain very uniform deposits on macroscopic surfaces by this new reactive spray deposition method. Indeed, even a visual observation of the spray deposits shows a color evolution from transparent to blue and finally yellow-orange when 5, 15, and 30 pairs of layers are deposited by the reactive spray method.

Conclusion We demonstrated the feasibility to construct silica-polycation composite films by alternatively bringing a surface in contact with silicic acid and different polycations. The polycations act as catalysts for the condensation of silicic acid into silica. This

buildup method allows building nanometer-sized films of wellcharacterized thickness. FTIR-ATR spectroscopy indicates that their structures must be close to that of silica. We are currently studying the extension of this new method of reactive layerby-layer coating to different metal alkoxides and in particular titanium alkoxides. The applications of these coatings are numerous. They range from antibacterial, antifogging coatings to photovoltaic applications. They are also used to create bioactive coatings, and the present method offers a new way to realize easily such coatings. Such hybrid films may even constitute model systems to understand the formation of biological systems such as the silica nanoparticles in demosponge biosilica which seem to be ordered in a layer-by-layer manner.38 Supporting Information Available: Topographical AFM pictures of PEI-(PSS-PAH)5-(silicic acid-PAH)6 multilayer films prepared from silicic acid solutions at 5 and 20 mM. This material is available free of charge via the Internet at http://pubs.acs.org. LA063052W (38) Weaver, J. C.; Pietrasanta, L. I.; Hedin, N.; Chmelka, B. F.; Hansma, P. K.; Morse, D. E. J. Struct. Biol. 2003, 144, 271.