Covalent Layer-by-Layer Assembled Superhydrophobic Organic

pubs.acs.org/Langmuir © 2009 American Chemical Society

Covalent Layer-by-Layer Assembled Superhydrophobic Organic-Inorganic Hybrid Films Sonia Amigoni, Elisabeth Taffin de Givenchy, Mickael Dufay, and Fr
ed
eric Guittard* Universit
e de Nice-Sophia Antipolis, Laboratoire de Chimie des Mat
eriaux Organiques et M
etalliques (CMOM), Equipe Chimie Organique aux Interfaces, Parc Valrose, 06108 Nice Cedex 2, France Received April 19, 2009. Revised Manuscript Received May 28, 2009 Using the concept of covalent layer-by-layer assembly (covalent LbL), used until now for the elaboration of films from polymers or dendrimers, we have constructed hybrid organic/inorganic surfaces by alternating different layers of amino-functionalized silica nanoparticles (295 nm diameter) and epoxy-functionalized smaller silica nanoparticles (20 nm diameter). The so-realized macromolecular edifice leads to a hierarchical integration of nanoscale textures. Then hydrophobization of the last layer of amino-functionalized silica particles was carried out by grafting a new designed highly fluorinated aldehyde, creating a monomolecular layer via the formation of an imine function. Five highly fluorinated surfaces were built, and their water-repellent abilities were directly correlated to the surface topologies (i.e., the number of layers of silica nanoparticles and their organization on the glass support). The hydrophobicity increased with the number of layers and stable highly water-repellent surfaces (static contact angle with water of 150 ( 3° and a contact angle hysteresis of 12°) were obtained with the alternation of nine layers. This result demonstrates the possibility to construct covalent LbL edifices with functionalized silica nanoparticles of different sizes and open this field for the elaboration of responsive, sensing, and therapeutic surfaces with improved film stability.

Introduction Controlling and modifying the surface roughness of materials is one of the most studied prospects of the past decade.1 Indeed, the appropriate texturing/roughening of surfaces in the nano- and microscale is known to impart unusual optical, wetting, and mechanical characteristics and, in particular, can amplify hydrophobicity which plays an increasing role in numerous industrial applications. Two factors have to be controlled in order to produce highly hydrophobic surfaces: their topography and their chemical composition.2-7 Regularly structured surfaces provide useful models for quantitatively evaluating relationship between wettability and surface structure, whereas for practical applications, irregularly structured surfaces provide the advantage of simplicity. Among all the procedures known in the literature (lithographic patterning,8 plasma or laser etching,9 acidic corrosion,10 sol-gel synthesis, etc.),11 the layer-by-layer thin film technique (LbL) is a method of choice to realize nano- or *To whom correspondence should be addressed: Tel þ33 (0)4 92 07 61 59, fax þ33 (0)4 92 07 6156, e-mail [email protected].

(1) Feng, X.; Jiang, L. Adv. Mater. 2006, 18, 3063. (2) Darmanin, T.; Guittard, F. J. Am. Chem. Soc. 2009, in press (DOI: 10.1021/ ja901392s). (3) Darmanin, T.; Guittard, F. Langmuir 2009, 25, 5463. (4) Campbell, J. L.; Breedon, M.; Latham, K.; Kalantar-zadeh, K. Langmuir 2008, 24, 5091. (5) Zheng, Y.; Wynne, K. J. Langmuir 2007, 23, 11964. (6) Sun, C.; Zhao, X.-W.; Han, Y.-H.; Gu, Z.-Z. Thin Solid Films 2008, 516, 4059. (7) J€arn, M.; Brieler, F. J.; Kuemmel, M.; Grosso, D.; Lind
en, M. Chem. Mater. 2008, 20, 1476. (8) Oner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777. (9) Baldacchini, T.; Carey, J. E.; Zhou, M.; Mazur, E. Langmuir 2006, 22, 4917. (10) Taffin de Givenchy, E.; Amigoni, S.; Martin, C.; Andrada, G.; Caillier, L.; G
eribaldi, S.; Guittard, F. Langmuir 2009, 25, 6448. (11) Zhang, G.; Wang, D.; Gu, Z. Z.; M€ohwald, H. Langmuir 2005, 21, 9143. (12) Pavan, K. A.; Jian, L. Surf. Coat. Technol. 2008, 202, 2690. (13) Crespo-Biel, O.; Dordi, B.; Maury, P.; Reinhoudt, D. N.; Huskens, J. Chem. Mater. 2006, 18, 2545. (14) Isaacs, S. R.; Choo, H.; Ko, W.-B.; Shon, Y.-S. Chem. Mater. 2006, 18, 107. (15) Clark, S. L.; Hammond, P. T. Adv. Mater. 1998, 10, 1515.

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microparticulate assembled structures on solid substrate.12-16 Indeed, the successive deposition of silica particles of different sizes via the LbL process lead easily to the building of rough surfaces17,18 and has been widely used to enhance the hydrophobic properties of low surface energy material such as fluorinated coatings.19-25 However, the conventional use of highly charged materials to build the LbL assemblies by means of electrostatic interactions presents some key limitations as a possible disassembly with changes of pH or ionic strength.26 In this paper we report the synthesis of original designed surfaces that explores the recent concept of covalent layer-bylayer assembly.27 This process, already applied to polymer or dendrimer LbL assemblies,28 allows the construction of films with high stability. Unlike the majority of LbL reports that use preformed polymers to “stick” the layers together (charged polymers17,29 or other more sophisticated ionic systems30), our concept is based on the successive covalent grafting of functionalized silica nanoparticules onto a previously silanized glass (16) Ariga, K.; Hill, J. P.; Ji, Q. Phys. Chem. Chem. Phys. 2007, 9, 2319. (17) Bravo, J.; Zhai, L.; Wu, Z.; Cohen, R. E.; Rubner, M. F. Langmuir 2007, 23, 7293. (18) Soeno, T.; Inokuchi, K.; Shiratori, S. Appl. Surf. Sci. 2004, 237, 543. (19) Gu, G.; Dang, H.; Zhang, Z.; Wu, Z. Appl. Phys. A: Mater. Sci. Process. 2006, 83, 131. (20) Tsai, P.-S.; Yang, Y.-M.; Lee, Y.-L. Langmuir 2006, 22, 5660. (21) Su, C.; Li, J.; Geng, H.; Wang, Q.; Chen, Q. Appl. Surf. Sci. 2006, 253, 2633. (22) Ji, J.; Fu, J.; Shen, J. Adv. Mater. 2006, 18, 1441. (23) Soeno, T.; Inokuchi, K.; Shiratori, S. Trans. Mater. Res. Soc. Jpn. 2003, 28, 1207. (24) Jisr, R. M.; Rmaile, H. H.; Schlenoff, J. B. Angew. Chem., Int. Ed. 2005, 44, 782. (25) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z.; Jiang, L.; Li, X. J. Am. Chem. Soc. 2004, 126, 3064. (26) Quinn, J. F.; Johnston, A. P. R.; Such, G. K.; Zelikin, A. N.; Caruso, F. Chem. Soc. Rev. 2007, 36, 707. (27) Bergbreiter, D. E.; Liao, K.-S. Soft Matter 2009, 5, 23. (28) Liu, Y. L.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 2114. (29) Yancey, S. E.; Zhong, W.; Heflin, J. R.; Ritter, A. L. J. Appl. Phys. 2006, 99, 034313. (30) Soeno, T.; Inokuchi, K.; Shiratori, S. Appl. Surf. Sci. 2004, 237, 543.

Published on Web 07/14/2009

DOI: 10.1021/la901369f

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Scheme 1. Construction of the Covalent LbL Building (i) Amine-Functionalized Particles (AP), EtOH, Sonication; (ii) Epoxy-Functionalized Particles (EP), EtOH, Sonication; (iii) the Same as (i); and (iv) 2-Perfluorohexylethyl-4-formylthiobenzoate (Aldehyde F), Na2SO4, EtOH

support to lead to a highly stable hybrid organic/inorganic film (as illustrated in Scheme 1). The covalent connection of the silica particles is constituted by a hydrocarbon flexible linker authorizing their self-assembly to form a nanorough surface. Then, as superhydrophobicity depends not only on the topology but also on the surface chemistry, a monomolecular layer of highly fluorinated molecules is directly grafted on the last layer of the hybrid edifice. The wettability, the surface properties, and the morphologies of the resulting hybrid materials are also reported and discussed.

Experimental Methods Materials. 3-Glycidoxypropyltrimethoxysilane (GPS), (3-aminopropyl)trimethoxysilane (AMS), fumed silica (14 nm), tetraethyl orthosilicate (TEOS), ammonium hydroxide (30% in water), 4-carboxybenzaldehyde, dicyclocarbodiimide (DCC), and all other chemicals were obtained from Aldrich. Glass wafers used were microscope slides (25 mm  75 mm) cut into 12.5 mm  25 mm pieces. Procedures. 1. Silanization of Glass Surfaces. Glass slides were washed with a 50/50 solution of HCl (36 wt %) in methanol for 40 min followed by thorough rinsing in methanol and dried. The slides were then activated in H2SO4 (97 wt %) for 20 min and rinsed with water. GPS (0.8 mL) or AMS (0.8 mL) and 3 drops of acetic acid were added in 8 mL of ethanol. The solution was stirred at room temperature for 30 min. A glass slide previously washed was then dipped into this solution and finally cured at 120 °C for 3 h. The sample was washed three times with ethanol to eliminate excess of silane. 2. Epoxy Silica Particles (20 nm) (EP). Fumed silica (2 g), GPS (7 mL), and anhydrous toluene (75 mL) were charged in a 250 mL round-bottom flask equipped with a mechanical stirrer. The mixture was heated at 126 °C for 3 h under N2. The reaction was followed by cooling at room temperature. The particles 11074 DOI: 10.1021/la901369f

were washed three times for 1 h with anhydrous toluene before centrifugation (1600 rpm). Finally, the substrate was dried at 53 °C for 12 h and at 100 °C for 2 h. The resulting particles have diameters of 20 ( 2 nm with a polydispersity of 0.05. Elementary analysis of the purified silica particles afforded: C: 17.4%, H: 3.0%, which corresponds to ∼2.4 mequiv/g of epoxy groups. 3. Amine Silica Particles (∼295 nm) (AP). TEOS (13.2 mL), deionized water (1.08 mL), and ethanol (5.24 mL) were mixed with an ammonium hydroxide solution (30% in water, 3.56 mL), heated at 50 °C for 8 h, and then cooled at room temperature to afford colloidal silica. This colloidal silica and the appropriate amount of AMS (molar ratio of TEOS/AMS=10/1) were heated at 70 °C for 6 h. The final solution was ultrasonicated for 30 min. The suspension was used readily after for the grafting onto glass support. The particles have a diameter of about 295 nm with a polydispersity of 0.8. Elementary analysis of the purified silica particles afforded: C: 12.1%, H: 3.2%, N: 3.6%, which corresponds to ∼2.5 mequiv/g of amine functions.

4. Covalent Layer-by-Layer Assemblies (Surfaces 1 to 5).

Glass slides silanized with GPS were ultrasonicated in an AP solution (2 wt % in 8 mL of ethanol) for 3 h at room temperature. The glass slides were then washed with ethanol before drying at 120 °C for 1 h to afford surface 1. The reaction could then be repeated with EP (1 wt % of epoxy silica particles in 8 mL of ethanol) to afford surface 10 . Several cycles could be repeated to afford surfaces 2 to n.

5. Synthesis of 2-Perfluorohexylethyl-4-formylthiobenzoate (Aldehyde F). Methylene chloride (20 mL), 4-carboxybenzaldehyde (2 g, 13.3 mmol), and DCC (13.3 mmol) were stirred at room temperature for 1 h. 2-Perfluorohexylethanethiol (13.3 mmol) was then added dropwise. After 24 h the bulk was filtered and the solvent evaporated under vacuum. The crude product was purified by column chromatography with methylene chloride as eluent. 5.23 g (77% yield) of 2-perfluorohexylethyl-4-formylthiobenzoate was obtained as a white solid. 1 H NMR (CDCl3/TMS δ ppm J Hz): 10.11 (1H, CHO, s); Langmuir 2009, 25(18), 11073–11077

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8.11 (2HAr, d, JHH =8.2 Hz); 7.98 (2HAr, d, JHH =8.2 Hz); 3.33 (2H, COSCH2, t, JHH =8.0 Hz); 2.51 (2H, CH2C6F13, tt, JHF = 18.0 Hz, JHH =8.0 Hz). 19F NMR (CDCl3, δ ppm): -81.25; 114.93; -122.35; -123.32; -123.80; -126.59. IR: 1658 cm-1, 1702 cm-1. Mp=70 °C. 6. Imine Formation (Surfaces 0F to 5F). Glass slides silanized with AMS and glass surfaces 1 to 5 were immersed into absolute ethanol (8 mL) with small amounts of Na2SO4. Aldehyde F (10 wt % in ethanol, 5 mL) was added to the solution. The mixture was stirred for 3 h. The modified glass slides were finally washed with absolute ethanol and dried at 100 °C for 1 h to afford highly fluorinated surfaces 0F to 5F. Characterization. 1. Molecular Structure. NMR experiments were carried out using a Br€ uker AC 200 MHz spectrometer. The spectra were recorded using deuterated solvents. The mass spectra were carried out using a Thermo Finnigan spectrometer and electronic ionization at a 70 eV by direct introduction. IR spectra were performed with a Perkin-Elmer Paragon 1000 FI-IP spectrometer. Elemental analysis was performed using a Thermo Electron Eager 300 elemental analyzer. 2. Nanoparticle Sizes (AP and EP). The average particle size, size distributions, and surface charges were determined by Zeta Sizer (Malvern Instruments, Model 3000 HSA, France).

3. Contact Angles and Surface Energy Determination. Static contact angles, dynamic contact angles measurements, and the deducted surface free energy of polymer samples were performed using the sessile drop method on a motorized syringe mechanism equipped Kr€ uss DSA 10 contact angles goniometer interfaced to image-capture software. They were recorded at 25 ( 1 °C. Measurements of static contact angle were made with deionized water, diiodomethane, and hexadecane taking an average of five 1 μL drops with each type of reference liquid. Reproducibility was within 3°. Dynamic contact angles were performed with a motorized syringe at velocities of the threephase contact line in the range from 0.1 to 1.5 mm min-1. The advancing and receding contact angles for water were evaluated by axisymmetric drop shape analysis profile.31 4. Surface Roughness. A VEECO noncontacting laser profilometer (WYKO NT1100) was used to assess the three-dimensional surface roughness of the ground surfaces at nanometer resolution in terms of arithmetic mean roughness (Ra). Scanning electron microscopy (SEM) observations were carried out with a JEOL 6700F microscope.

Results and Discussion Construction of LbL Films. Silica nanoparticles with different sizes and different surface functionalities are consecutively grafted in a covalent manner onto glass supports previously chemically modified by silanization with GPS. The nanoparticles utilized are two sized functionalized silica nanoparticles: epoxyfunctionalized particles (EP) are 20 nm diameter commercially available nanoparticles silanized with glycidoxypropyltrimethoxysilane, and amine-functionalized particles (AP) are 295 nm diameter prepared nanoparticles, silanized with (3-aminopropyl)trimethoxysilane. As illustrated in Scheme 1, the first layer of silica nanoparticles (surface 1) is achieved by dipping the silanized glass substrate in a colloidal suspension of 295 nm diameter amine-functionalized silica (AP), previously prepared according to the Str€ober process32 and silanized with AMS, to afford reactive NH2 groups at the surface of the nanoparticles. The amine functions can then react with 20 nm diameter epoxy-functionalized silica (EP) by ring-opening of the epoxy groups induced by a nucleophilic attack to afford surface 10 . To successively bind the layers together, we were inspired by Ming (31) Hoorfar, M.; Neumann, A. W. J. Adhes. 2004, 80, 727. (32) St€ober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62.

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Scheme 2. Chemical Reactions Involved in the Fluorination of the LbL Last Layer

and co-workers’ work33 where raspberry-like particles are fabricated by covalently bond epoxy- and amine-based silica particles. The reaction of the latest layer composed of EP allows adding a second stratum of AP (surface 2). This cycle of reactions can be repeated as many times as the number of layers is needed. N cycles lead to n different surfaces: surface 3 corresponds to 3 layers of AP (surface 4: 4 layers; surface 5: 5 layers of AP; ...). The covalent LbL buildings with controlled number of layers are, in all cases, ended by silica amino particles AP that can easily react with a lot of other molecules. Surface 5 is used to investigate the stability of the edifices. The film was immersed in different aqueous media for 4 h: 0.9% normal saline or buffered solutions at pH 4.5, 7, and 8.5. Laser profilometry of the surface after immersion shows no difference from that of the freshly prepared one. The films were also found to be stable in a range of organic solvents (ethanol, acetone, ethyl acetate, and cyclohexane). Surface Chemical Hydrophobization. The formation of an imine function by the reaction with an aldehyde moieties with the AP layer seemed to us a rapid and efficient way (Scheme 1) to introduce the hydrophobic moiety. So, we have designed a highly fluorinated aldehyde (coded aldehyde F) able to quantitatively react with the amino groups of the surface (Scheme 2). In this aldehyde, the perfluorinated tail is associated with a phenyl unit to achieve a stable low energy surface. Indeed, in a previous study devoted to the creation of nonreconstructing water-repellent surfaces, we have demonstrated that the rigidity induced by the introduction of a mesogenic core (phenyl unit in this case) associated with the “fluorophobic effect”34 of a perfluorinated tail is able to stimulate the auto-organization of the monolayer35 and then could contribute to reach high water contact angles along with weak value of hysteresis. The structure of the fluorinated agent chosen for this study is derived from the monomers skeleton, previously described in the literature,36-38 comprising one phenyl unit as the mesogenic core linked to a fluorinated tail via a thioester spacer. The synthesis of aldehyde F is realized by reacting perfluorohexylethanethiol and 4-carboxybenzaldehyde (33) Ming, W.; Wu, D.; van Benthem, R.; de With, G. Nano Lett. 2005, 5, 2298. (34) Wang, J.; Mao, G.; Ober, C. K.; Kramer, E. J. Macromolecules 1997, 30, 1906. (35) Cailler, L.; Taffin de Givenchy, E.; Geribaldi, S.; Guittard, F. J. Mater. Chem. 2008, 18, 5382. (36) Taffin de Givenchy, E.; Guittard, F.; Bracon, F.; Cambon, A. Liq. Cryst. 1999, 26, 1163. (37) Taffin de Givenchy, E.; Guittard, F.; Bracon, F.; Cambon, A. Liq. Cryst. 1999, 26, 1371. (38) Fornasieri, G.; Guittard, F.; Geribaldi, S. Liq. Cryst. 2003, 30, 251.

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Figure 1. SEM images of (a) one layer of AP particles: surface 1; (b) two layers of AP (and one layer of EP): surface 2; (c, d) five layers of AP (and four layers of EP): surface 5. The insets represent contact angle with water of the surface once reacted with the fluorinated molecules (surfaces 1F (a), 2F (b), and 5F (d), respectively). Scale bar corresponds to 10 μm for (a), (b) and (c) and to 1 μm for (d).

Figure 2. Morphology of surfaces 1 and 5 obtained from laser noncontacting profilometer.

using DCC as coupling agent. Its reaction with the LbL assemblies, via the formation an imine linker, leads to surfaces 1F to 5F. A reference surface (surface 0F) is also prepared from a glass wafer silanized with AMS and functionalized with the aldehyde F (Scheme 1). Topology. SEM is used to examine the effect of the number of layers increment on the film morphologies. Figure 1 shows SEM analysis of surfaces 1, 2, and 5, indicating different degrees of surface roughness and porosity of the samples. It reveals that morphology of the edifice changes considerably all along the layer additions. For surface 1 (Figure 1a), the particles do not homogeneously cover the surface, and some lines are designed on the support during the first deposition of the AP layer. The distribution is then more covering for surface 2 (Figure 1b). Figure 1c,d shows the erratic particle distribution in surface 5, leading to a (pseudo-) random surface. This “natural” 11076 DOI: 10.1021/la901369f

tendency to aggregate into porous structure gives quite different textures than those attained from on-demand topographies obtained with Langmuir-Blodgett depositions,20,39 but as shown by the insets in Figure 1, the so-created highly rough surface 5F is able to reach superhydrophobic properties as the water partially sit on the air pockets trapped between the voids randomly created. In Figure 2, the topographic images obtained by 3-dimensional laser noncontacting profilometry for surfaces 1 and 5 show the increase of arithmetic mean roughness (Ra =114 nm for surface 1 to Ra =382 nm for surface 5). The data collected for all the surfaces are summarized in Table 1 and show a gradual increase of the measured roughness after each bilayer addition. (39) Synytska, A.; Ionov, L.; Dutschk, V.; Stamm, M.; Grundke, K. Langmuir 2008, 24, 11895.

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Table 1. Surface Characterisation of the Samples after Reaction with the Aldehyde F surfaces

θaa

θrb

Hc

f2d

Ra (nm)e

113 71 42 0 50 120 92 28 0.34 114 137 111 26 0.55 119 154 129 25 0.78 243 152 131 21 0.81 359 151 139 12 0.84 382 a Advancing contact angle with water ((3°). b Receding contact angle with water ((3°). c Hysteresis. d Fraction of the water-air contact area. e Arithmetic mean roughness. 0F 1F 2F 3F 4F 5F

Figure 4. Advancing contact angle with water (]) and hysteresis () for the fluorinated surfaces 1F (one layer of AP) to 5F (five layers of AP).

Figure 3. Evolution of static contact angle with water (9) and roughness (0) for the fluorinated grafted surfaces 1F to 5F (one AP layer to five AP layers).

Influence of the Roughness on the Hydrophobicity of the Surfaces. The surface modifications are classically revealed by the evolution of the static and dynamic contact angles with water. As suggested in the literature,40 the wettability of such close packed sphere arrays may be well evaluated by the Cassie-Baxter equation (1)41

where θ0 is the apparent water contact angle on the rough surface, θ is the intrinsic water contact angle on the corresponding flat surface, f1 is the fraction of water-solid contact area, and f2 is the fraction of the water-air contact area (f1 þ f2=1). As the water contact angle on the reference flat surface 0F is 110°, f2 values can be calculated from eq 1 for the colloidal silica assemblies; the results are provided in Table 1. f2 values increased gradually with increasing the number of layers to reach, for surface 5F, a value as high as f2 =0.84. Such elevated values of f2 are usually achieved by introducing air cavities or channels on the surface mainly via more sophisticated processes such as lithography, etching, or phase separation.8,39,42-44 (40) Shiu, J.; Kuo, C.; Chen, P.; Mou, C. Chem. Mater. 2004, 16, 561. (41) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 3, 16. (42) Li, H.; Wang, X.; Song, Y.; Liu, Y.; Li, Q.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2001, 40, 1743. (43) Yabu, H.; Takebayashi, M.; Tanake, M.; Shimomura, M. Langmuir 2005, 21, 3235. (44) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2003, 19, 5626.

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Figure 3 represents the evolution of static contact angles with water in parallel with the increase of roughness induced by the successive addition of layers. We can notice a rapid evolution of the wettability as a function of the number of layers and the roughness. A plateau is reached for surfaces 4F and 5F, attesting a good compromise between the number of layers and the macroscopic properties desired. Furthermore, it is apparent that the higher the number of layers the lower the hysteresis (Table 1 and Figure 4). It can be correlated to the increase of the roughness all along the layer additions to aim toward a nonadhesive Cassie-Baxter state.41 The low values of hysteresis reached (3F: 25°; 4F: 21°; and 5F: 12°) are a sign of the stability of the F monomolecular layer at the surface and reflect a good homogeneity of the coating.

Conclusions Five surfaces were constructed by the successive covalent grafting of functional nanoparticles (AP: 295 nm diameter; EP: 20 nm diameter), commercially available or readily synthesized by the Str€ober process, onto a glass support previously chemically modified by the reaction of 3-glycidoxypropyltrimethoxysilane. The natural erratic distribution of the nanoparticles onto the support induced a nanoroughness and porosity known to be able to present superhydrohobic properties after a further step of hydrophobization. This step was provided by the reaction between the amino groups of the last layer of AP and a novel designed highly fluorinated aldehyde. It has been demonstrated that the water repellence properties increase with the number of layers to reach superhydrophobic surfaces (static contact angle with water >150° and a hysteresis of 12°). Moreover, this covalent layer by layer assemblies of silica nanoparticles lead to stable high surface area materials that can be easily functionalized by a large number of other active molecules than the hydrophobic precursor used in the present study. Therefore, it is believed that this work offers new short-term prospects for the fabrication of highly active materials in technologies such as catalysis, sensing, or antibacterial surfaces. Acknowledgment. This work was supported by the “Region PACA” Council, and we acknowledge support from the society “Bou
ee L
eo” for its contribution to the grant. Supporting Information Available: ATR-IR spectrum of surface 5F and SEM images of all surfaces. This material is available free of charge via the Internet at http://pubs.acs.org. DOI: 10.1021/la901369f

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