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Photoreactive Azido-Containing Silica Nanoparticle/Polycation Multilayers: Durable Superhydrophobic Coating on Cotton Fabrics Yan Zhao, Zhiguang Xu, Xungai Wang, and Tong Lin* Australian Future Fibres Research and Innovation Centre, Deakin University, Geelong VIC 3217, Australia S Supporting Information *

ABSTRACT: In this study, we report the functionalization of silica nanoparticles with highly photoreactive phenyl azido groups and their utility as a negatively charged building block for layer-by-layer (LbL) electrostatic assembly to produce a stable silica nanoparticle coating. Azido-terminated silica nanoparticles were prepared by the functionalization of bare silica nanoparticles with 3-aminopropyltrimethoxysilane followed by the reaction with 4-azidobenzoic acid. The azido functionalization was confirmed by FTIR and XPS. Poly(allylamine hydrochloride) was also grafted with phenyl azido groups and used as photoreactive polycations for LbL assembly. For the photoreactive silica nanoparticle/polycation multilayers, UV irradiation can induce the covalent cross-linking within the multilayers as well as the anchoring of the multilayer film onto the organic substrate, through azido photochemical reactions including C−H insertion/abstraction reactions with surrounding molecules and dimerization of azido groups. Our results show that the stability of the silica nanoparticle/polycation multilayer film was greatly improved after UV irradiation. Combined with a fluoroalkylsilane post-treatment, the photoreactive LbL multilayers were used as a coating for superhydrophobic modification of cotton fabrics. Herein the LbL assembly method enables us to tailor the number of the coated silica nanoparticles through the assembly cycles. The superhydrophobicity of cotton fabrics was durable against acids, bases, and organic solvents, as well as repeated machine wash. Because of the unique azido photochemistry, the approach used here to anchor silica nanoparticles is applicable to almost any organic substrate.

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and/or CC addition reaction.15 In past decades, phenyl azido chemistry has commonly been employed in biochemistry and molecular biology to photochemically immobilize various biological components including proteins, antibodies, polysaccharides, and thermoresponsive polymers.15−18 In these cases, the phenyl azido groups were usually incorporated into the biocomponents to be immobilized. To extend this covalent immobilization method to a wide range of materials that have no reactive groups for azido functionalization, Yan’s group introduced phenyl azido groups onto a silicon wafer/glass substrate by treatment with an azido-derivated silane. By using this photoreactive substrate, they have demonstrated the immobilization of biomolecules,19 polymers,20−23 and even graphene sheets.24 However, they found that it was difficult to immobilize spherical nanoparticles on such rigid substrate due to the small contact area. So, they recently designed a relatively soft polymer-based substrate that bears azido groups to enhance the contact area.25 However, the formation of a conformal contact between the substrate and the nanoparticles has not yet been established.

INTRODUCTION Functional multilayer films prepared by layer-by-layer (LbL) self-assembly technique have attracted great attention recently because of their wide applications in areas including sensors, nanoreactors, drug delivery, nonlinear optics, and antireflective films.1−3 The LbL technique is known to be advantageous in the easy preparation of versatile multilayer films with tailored composition and functionality through noncovalent interactions (e.g., electrostatic,1,4 hydrogen-bonding,5−7 or chargetransfer interaction8,9). However, one problem associated with LbL multilayer films is the low stability, especially when they are in an environment of high ionic strength, or an acidic or alkaline condition.2,6 To improve the film robustness, covalent bonding between the neighboring layers has been employed, such as through a step-by-step reaction using click chemistry,10 or by post-thermal/photo treatment to convert the ionic interaction into covalent bonding.11−14 Among these methods, the most convenient way is to graft photoreactive groups (e.g., azido group14) onto the polyelectrolyte chains to fabricate photoreactive building blocks, which then can be used to crosslink the LbL multilayers upon post photoirradiation. Phenyl azido group is known to easily photolyze upon UV irradiation to generate a highly reactive intermediate, phenyl nitrene, which can react with neighboring organic matter to form a covalent bond through C−H insertion or abstraction © 2012 American Chemical Society

Received: January 19, 2012 Revised: March 27, 2012 Published: April 1, 2012 6328

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additional 2.0 mL of TEOS was added and the reaction was allowed to continue for another 12 h with stirring. Preparation of Amino-Terminated Silica (Silica-NH2) Nanoparticles. A 300 μL portion of APTMS in 5 mL of ethanol was added into the suspension of silica nanoparticles, and the mixture was stirred at 90 °C for 24 h. The resulting amino-terminated silica nanoparticles were purified by centrifugation and redispersion in water three times. Preparation of Azido-Terminated Silica (Silica-N3) Nanoparticles. A dispersion of silica-NH2 nanoparticles was added with 10 mL (2.0 mmol) of 4-azidobenzoic acid solution with pH of 8 followed by the addition of 0.38 g of EDC. The solution was stirred and kept at 4 °C for 48 h. The obtained azido-functionalized nanoparticles were recovered and washed by centrifugation. Synthesis of Azido-Grafted PAH (PAH-N3). PAH (0.187 g, 2.0 mmol on monomer unit) was dissolved in 30 mL of water, and the solution was neutralized with aqueous solution of sodium hydroxide to convert part of the amine hydrochloride to free amine. 4-Azidobenzoic acid (0.033 g, 0.2 mmol) was dissolved in 20 mL of water by adjusting the pH to be about 8. This solution was mixed with EDC (0.115 g, 0.6 mmol) and the PAH solution with stirring. After incubation at 4 °C for 48 h, the solution was dialyzed against deionized water. The powder of PAH-N3 was obtained by lyophilization. Preparation of PAH-N3/Silica-N3 Multilayers. The multilayer films were prepared by sequentially immersing the substrate, a flat O2 plasma treated cellulose acetate butyrate film, into 10 mg/mL aqueous solutions of PAH-N3 or silica-N3. For each layer, the immersion time was 5 min, followed by washing three times with water. By repeating this process in a cyclic manner, a multilayer of (PAH-N3/silica-N3)n was obtained, where n represents the cycle number. For instance, (PAH-N3/silica-N3)1.5 consists of 3 layers of PAH-N3/silica-N3/PAHN3. Preparation of Superhydrophobic Coating on Cotton Fabric. The cotton fabric was first treated with poly(acrylic acid) according to our previous report.40 The PAH-N3/silica-N3 multilayers were prepared using the same procedure as above. Then the coated fabric was dipped in 2.0 wt % FAS in hexane for 1 h, and subsequently dried at 100 °C for 30 min. Characterization. Fourier transform infrared (FTIR) spectra were recorded on a Bruker VERTEX 70 instrument using attenuated total reflectance (ATR) mode with a resolution of 4 cm−1 accumulating 32 scans. Thermogravimetric (TG) analysis was conducted on a Netzsch STA 409 PC thermal analyzer at a heating rate of 10 °C/min in nitrogen atmosphere. X-ray photoelectron spectroscopy (XPS) measurements were made using a Kratos Axis Ultra system with a monochromatized Al Kα radiation at 1486.6 eV as the X-ray source. UV−vis absorption spectra were recorded using a Varian Cary 3 spectrophotometer. The zeta potential of silica nanoparticles was measured using a Malvern Zetasizer Nano ZS instrument. The surface morphologies of the fibers were examined using a Zeiss Supra 55VP scanning electron microscope (SEM) operated at an acceleration voltage of 10.0 kV. Contact angles were measured using a CAM101 video camera based contact angle measurement system (KSV Instruments Ltd., Finland). Contact angle hysteresis was taken as the difference between the advancing and receding contact angles. To measure the advancing and receding contact angles, a perpendicular syringe needle was maintained in contact with the water drop and the substrate was slowly moved horizontally. The change of droplet shape due to the shift of the substrate was recorded by the video camera at a speed of 30 frames per second. At the instant when the drop was about to move, the contact angles at the advancing and receding contact lines were taken as the advancing and receding contact angles, respectively. Wash durability test was conducted according to the standard method for fabric coating (AATCC Test Method 61−2006 test no. 2A). This accelerated wash procedure is equivalent to five cycles of home machine washing. For convenience, we use the equivalent number of home machine washing in this paper. The UV irradiation light source was a portable 6 W UV lamp (UVP, model UVSL-26P) operated with 254 nm shortwave.

Superhydrophobicity is known to originate from a combination of rough surface structure with a low surface free energy.26 Superhydrophobic surfaces have shown wide applications including water repellency, self-cleaning, anticontamination, and corrosion protection. For fabrics, superhydrophobicity is usually generated by forming a nanostructure on fiber surface and meanwhile reducing the surface free energy. So far, various nanostructured fiber surfaces have been prepared by introducing carbon nanotubes,27,28 gold particles,29 Ag/Pt aggregates,30,31 silica particles,32,33 ZnO nanorods,34 or copper nanofibers/crystallites35,36 onto the fibers. For these nanoparticle coatings, however, a major limitation of their uses is the poor stability. Recently, to make the nanoparticles covalently attach to the fiber surface, Luzinov et al.37 used an epoxy-containing polymer, poly(glycidyl methacrylate) (PGMA), to modify silica nanoparticles. The epoxy groups then reacted with the carboxyl or hydroxyl groups generated by NaOH treatment on the polyester fiber surface to form covalent bonding. Similarly, Synytska et al.38 coated a thin PGMA layer on the polyester fiber first, and then covered the fiber with amphiphilic Janus silica particles, which had amino groups on the hydrophilic side to react with the epoxy groups of PGMA. In these cases, both the substrate and the nanoparticles need to be functionalized with suitable reactive groups, and the contact area between them is also limited. In our case, to make a covalently attached silica nanoparticle coating, phenyl azido groups were introduced to the surface of silica nanoparticles. The as-prepared azido-terminated silica nanoparticles were negatively charged and therefore can be used as a building block for LbL assembly. Herein, the employment of LbL assembly technique improves the contact between the silica nanoparticles and the polyelectrolyte chains so that they can be effectively bonded together through the reaction between the azido groups and the C−H bonds. In addition, the polyelectrolyte was also functionalized with phenyl azido groups, and thus can form covalent bonds with both the silica nanoparticles and the substrate. So our approach described here provides a way to form a covalently linked network among nanoparticles, polyelectrolyte chains, and substrate, considerably improving the stability and adhesion. Moreover, another advantage of using LbL assembly method is that the number of the immobilized silica nanoparticles can be easily tailored by controlling the assembly cycles. When this photoreactive silica nanoparticle/polyelectrolyte multilayer film was applied onto cotton fabric, a durable superhydrophobic coating was obtained.

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EXPERIMENTAL SECTION

Materials. Tetraethoxysilane (TEOS), N-ethyl-N′-(3dimethylaminopropyl)carbodiimide hydrochloride (EDC), and poly(allylamine hydrochloride) (PAH, Mw ∼ 56 000) were purchased from Sigma-Aldrich. 4-Azidobenzoic acid was provided by Tokyo Chemical Industry (TCI). Poly(acrylic acid) (PAA, Mw ∼ 50 000, 25% aqueous solution) was obtained from Polysciences. 3-Aminopropyltrimethoxysilane (APTMS) and fluoroalkylsilane (FAS, tridecafluorooctyltriethoxysilane, CF3(CF2)5(CH2)2Si(OCH2CH3)3, Dynasylan F8261) were kindly supplied by Degussa. All chemicals were used as received without further purification. Synthetic Methods. Preparation of Silica Nanoparticles. Silica nanoparticles were prepared according to the Stöber method.39 Briefly, 3 mL of TEOS was added dropwise to a solution containing 100 mL of ethanol, 2.0 mL of deionized water, and 4.5 mL of ammonium hydroxide (25%) under magnetic stirring. After the mixture was stirred at 40 °C for 4 h, an 6329

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Figure 1. Schematics for the synthesis of (a) silica-N3 nanoparticles and (b) PAH-N3, (c) the UV cross-linking of the PAH-N3/silica-N3 multilayers, and (d) the possible photochemical reactions of phenyl azido groups.

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RESULTS AND DISCUSSION

acid. Due to the different surface functionality, the silica nanoparticles showed difference in the zeta potential. The zeta potential of the bare silica nanoparticles was −31 mV (pH 7). The presence of amino groups on nanoparticle surface rendered the nanoparticles positively charged with a zeta potential of 2.9 mV under the same pH condition. The zeta potential of the silica-N3 particles was changed to −19 mV (pH 7). It was also noted that the change in surface functionality and zeta potential also altered the suspension stability of the nanoparticles in water. In contrast to the stable aqueous suspension of bare silica particles, the suspension of silica-NH2

Figure 1a shows the reaction route to prepare azidofunctionalized silica nanoparticles. Bare silica nanoparticles were synthesized by the base-catalyzed hydrolysis and condensation of TEOS. The average size of silica particles calculated based on TEM images was 68 ± 5 nm. Before azido functionalization, amino groups were first introduced to the surface of silica nanoparticles by treatment with APTMS. The obtained silica-NH2 particles were then converted into azidoterminated nanoparticles by the reaction with 4-azidobenzoic 6330

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Figure 2. FTIR spectra of silica, silica-NH2, and silica-N3 nanoparticles.

Figure 3. XPS (a) survey spectra and high-resolution (b) C 1s and (c) N 1s spectra of silica, silica-NH2, and silica-N3 nanoparticles.

Compared with the spectrum of pure silica, the silica-NH2 nanoparticles displayed the characteristic absorption of amino groups at 1400 cm−1 .42 The spectrum of the silica-N3 nanoparticles showed the typical absorption of the azido group at 2130 cm−1 and the absorption at 1500 cm−1 due to aromatic ring (ϕ).43 The nanoparticles were further analyzed by XPS (Figure 3). The survey spectrum of the bare silica nanoparticles shows Si 2s, Si 2p, C 1s, and O 1s peaks. For silica-NH2 and silica-N3 particles, the N 1s peak at ∼400 eV appeared in the spectra along with other peaks present on pure silica. Compared with the survey spectrum of pure silica particles, there was a substantial increase in the carbon peak after modification with APTMS, and the peak had an even greater increase after the azido functionalization (Table 1). The high-resolution C 1s

particles was not stable and sedimentation occurred in 1 h (see Supporting Information, Figure S1), which can be ascribed to the relatively low electrostatic repulsive force between nanoparticles. When azido groups were introduced to the nanoparticle surface, a relatively stable suspension was formed, where the flocculation was not obvious until after 2 days. The surface coverage of the amino or azido group was roughly estimated by TG analysis (see Supporting Information, Figure S2). The result is 3.5 amino groups per nm2 for silicaNH2 nanoparticles, and 2.0 azido groups per nm2 for silica-N3 nanoparticles. It is known that the silica structure has 4.6 Si− OH groups per nm2 of its surface.41 This suggested that the silica surface was not fully covered with amino groups, and part of the positive charge of the amino groups was balanced by the native negative surface charge of the silica, which may contribute to the relatively low zeta potential of the silicaNH2 particles. Once the amino groups were amidized with 4azidobenzoic acid (grafting rate ∼57.1%), the formation of amide neutralized part of the positive charge brought by amino groups. As a result, the negative charge due to Si−OH groups prevailed over the positive charge due to amino groups, which rendered the silica-N3 particles negatively charged. The formation of azido-functionalized nanoparticles was also characterized by UV−vis absorption and FTIR. The UV−vis spectrum of the silica-N3 nanoparticles showed a typical absorption at 269 nm, which originated from the π−π* transition of the azido groups (see Supporting Information, Figure S3).14 Figure 2 shows the FTIR spectra of the silica particles. The peak at 1100 cm−1 was due to Si−O−Si vibrations, while the peak at 3400 cm−1 was due to the O−H vibration of Si−OH group. The vibration peak at 1630 cm−1 indicated the presence of physically adsorbed water molecules.

Table 1. Apparent Surface Atomic Concentration (%) Determined by XPS for the Silica, Silica-NH2, and Silica-N3 Nanoparticles atomic concentration (%) sample

C

O

Si

N

silica silica-NH2 silica-N3

4.2 11.9 49.3

57.2 50.2 31.7

38.6 35.7 16.8

0 2.2 2.2

spectrum is presented in Figure 3b. The major peak at 284.8 eV corresponded to C−C bond, and the peak around 286 eV corresponded to C−N bond. In the case of silica-N3 particles, a peak at 288.5 eV appeared and was attributed to the CO bond from amide group. The successful azido functionalization was also confirmed by the high-resolution N 1s spectrum 6331

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Figure 4. (a) UV−vis absorption spectra of PAH-N3/silica-N3 multilayers with different layers. The inset shows the linear increase of absorption at 269 nm as a function of layer number. (b) UV−vis absorption spectra of (PAH-N3/silica-N3)6.5 film under different UV irradiation periods. The inset shows the decrease of the azido absorption at 269 nm with increasing the irradiation time.

Figure 5. UV−vis absorption spectra of (PAH-N3/silica-N3)6.5 film (a) before and (b) after UV irradiation showing the absorption change after treatment with 1 M NaOH under ultrasonication.

the surrounding molecules that causes further radical reactions. For those adjacent azido moieties, they may undergo dimerization to form azobenzene dimers.47 These possible photochemical reactions are shown in Figure 1d. Herein, the multilayer film was exposed to UV light of 254 nm, and the photodissociation of azido groups was monitored by the change in the UV−vis spectrum. As shown in Figure 4b, with increasing the UV irradiation time, the absorption of azido groups at 269 nm was gradually decreased and disappeared completely after irradiation of 360 s. To ensure the azido groups reacted completely, the UV irradiation was performed for 30 min. In this system, it is likely that the azido groups mainly react with C−H bonds, compared with the dimerization reaction, because of the relatively low azido density. To further prove the photoreaction between the azido groups and the C− H bonds, a mixture of 4-azidobenzoic acid and polyvinyl chloride was used as a model system and characterized by XPS before and after UV irradiation (see Supporting Information, Figure S6). The effect of the photoinduced azido reactions on the stability of the multilayers was studied by treating the multilayer film with 1 M NaOH under ultrasonication for 5 min. In such a high-ionic strength environment, the attractive electrostatic interaction between each layer was greatly weakened, which could result in the dissociation of the electrostatic assembled multilayer film.14 As shown in Figure 5a, before UV irradiation, the UV−vis absorption of the (PAH-N3/silica-N3)6.5 film decreased dramatically after NaOH treatment, which indicates

(Figure 3c), where the characteristic double peak structure of azido groups at 400 and 403.9 eV was observed for the silica-N3 particles.44 Azido-grafted polycations (PAH-N3) were synthesized by the condensation of PAH and 4-azidobenzoic acid (Figure 1b). In the UV−vis spectrum of PAH-N3, the typical absorption peak of azido groups was slightly red-shifted from 266 nm for 4azidobenzoic acid to 269 nm (see Supporting Information, Figure S4). Such a small shift may be due to electron delocalization of the azidophenyl group caused by the formation of amide bond.45 On the basis of the 1H NMR spectrum of PAH-N3, the grafting rate of azido groups was calculated to be ∼7% (see Supporting Information, Figure S5). To demonstrate the feasibility of using PAH-N3 and silica-N3 for LbL electrostatic self-assembly, we monitored the UV−vis absorption of the multilayers assembled on a flat cellulose acetate butyrate film. With increasing the layer number, the linear growth of the azido absorption peak at 269 nm suggests that the LbL assembly was successfully conducted (Figure 4a). The PAH-N3/silica-N3 multilayers were cross-linked by irradiating with UV light. The schematic for the multilayer film and its UV cross-linking is shown in Figure 1c. It is known that UV irradiation of the azido groups leads to the formation of highly reactive singlet or triplet nitrene radicals. These reactive species can readily react with the surrounding C−H bonds through insertion (singlet) or abstraction (triplet) reactions.46 The insertion reaction is a termination step, but the abstraction reaction leads to the formation of radicals on 6332

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the pristine cotton fiber, the native striations along the fiber can be clearly observed (Figure 6a). When the surface was assembled with 1.5 or 3.5 layers of PAH-N3/silica-N3, silica nanoparticles were found to partially cover the fiber surface at random (Figure 6b,c). For the case of 5.5 layers of PAH-N3/ silica-N3, a uniform layer of silica nanoparticles was observed on the cotton fibers (Figure 6d). It is known that the pristine cotton fabrics are hydrophilic and can be wetted completely by water because of the abundant surface hydroxyl groups. After being coated with (PAH-N3/ silica-N3)n multilayers and further treated with FAS, the fabrics became superhydrophobic. The water contact angles for fabrics coated with 1.5, 3.5, and 5.5 layers of PAH-N3/silica-N3 were 152 ± 4°, 157 ± 2°, and 158 ± 4°, respectively. Figure 7a shows the bouncing behavior of a water droplet on a cotton fabric coated with (PAH-N3/silica-N3)5.5 multilayers and FAS. Upon impacting on the surface, the water droplet deformed and spread before retracting, and eventually bounced completely off the surface, which demonstrates the excellent water repellency. The chemical durability of the UV cross-linked superhydrophobic coating was studied by immersing the fabrics in various organic solvents and aqueous solutions at different pHs. The changes in water contact angles as a function of immersion time were recorded. For all the five organic solvents tested, i.e., ethanol, acetone, ethyl acetate, toluene, and dimethyl formamide (DMF), the contact angles showed no change, within experimental error, over 95 h of immersion (Figure 7b). When the superhydrophobic fabric was immersed in the aqueous solution at different pH values, the trapped air between the fabric and the water forms a mirror-like surface,40

the dissociation of the multilayers. However, for the multilayer film irradiated with UV light, there was almost no change in the UV−vis spectra after NaOH treatment (Figure 5b). Therefore, the stability of the PAH-N3/silica-N3 multilayers was greatly enhanced by the cross-linking using photoreactive azido groups. We then applied the photoreactive PAH-N 3 /silica-N3 multilayers onto cotton fabric to render the fabric superhydrophobic. The surface morphologies of the uncoated and coated cotton fibers were observed with SEM (Figure 6). For

Figure 6. SEM images of (a) pristine cotton fiber and cotton fibers coated with (PAH-N3/silica-N3)n multilayers: (b) n = 1.5, (c) n = 3.5, and (d) n = 5.5.

Figure 7. (a) Series of frames obtained using a high-speed digital camera that shows the bouncing of a water droplet falling from 5 cm height on a cotton fabric coated with (PAH-N3/silica-N3)5.5 multilayers and FAS (the interval between two successive frames is 4 ms). (b−d) Changes in the water contact angles of the superhydrophobic cotton fabric as a function of immersion time (b) in various organic solvents, (c) in HCl or NaOH aqueous solutions at different pH values, and (d) after different wash cycles. 6333

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and the fabric tends to float. To make the fabric contact well with the aqueous solution, the air in the mirror-like layer was removed by vacuuming with a bulb pipet. As shown in Figure 7c, for acidic and neutral solutions the contact angle was almost not changed after 95 h of immersion. For the case of strong basic solutions, the superhydrophobicity (>150°) was maintained for up to 44 h of immersion. In addition, the superhydrophobic fabric showed reasonable wash durability with contact angle hysteresis remaining lower than 10° after 25 cycles of home machine washing, and static contact angle above 150° after 50 cycles (Figure 7d). It should be noted that the top layer of FAS molecules could also be covalently immobilized on the multilayer film through the photochemical reactions with azido groups, despite their ability to hydrolyze and react with the hydroxide groups of silica particles. The enhancement in the stability of the FAS layer should also contribute to the good durability of the superhydrophobic coating.

(3) Wang, Y.; Angelatos, A. S.; Caruso, F. Template Synthesis of Nanostructured Materials Via Layer-by-Layer Assembly. Chem. Mater. 2008, 20, 848. (4) Caruso, F.; Caruso, R. A.; Mohwald, H. Nanoengineering of Inorganic and Hybrid Hollow Spheres by Colloidal Templating. Science 1998, 282, 1111. (5) Podsiadlo, P.; Kaushik, A. K.; Arruda, E. M.; Waas, A. M.; Shim, B. S.; Xu, J.; Nandivada, H.; Pumplin, B. G.; Lahann, J.; Ramamoorthy, A.; Kotov, N. A. Ultrastrong and Stiff Layered Polymer Nanocomposites. Science 2007, 318, 80. (6) Cho, J.; Caruso, F. Polymeric Multilayer Films Comprising Deconstructible Hydrogen-Bonded Stacks Confined between Electrostatically Assembled Layers. Macromolecules 2003, 36, 2845. (7) Wang, L. Y.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Chi, L. F.; Fuchs, H. A New Approach for the Fabrication of an Alternating Multilayer Film of Poly (4-Vinylpyridine) and Poly (Acrylic Acid) Based on Hydrogen Bonding. Macromol. Rapid Commun. 1997, 18, 509. (8) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Preparation of the Layer-by-Layer Deposited Ultrathin Film Based on the ChargeTransfer Interaction. Langmuir 1997, 13, 1385. (9) Wang, X.; Naka, K.; Itoh, H.; Uemura, T.; Chujo, Y. Preparation of Oriented Ultrathin Films Via Self-Assembly Based on Charge Transfer Interaction between π-Conjugated Poly (Dithiafulvene) and Acceptor Polymer. Macromolecules 2003, 36, 533. (10) Such, G. K.; Quinn, J. F.; Quinn, A.; Tjipto, E.; Caruso, F. Assembly of Ultrathin Polymer Multilayer Films by Click Chemistry. J. Am. Chem. Soc. 2006, 128, 9318. (11) Harris, J. J.; DeRose, P. M.; Bruening, M. L. Synthesis of Passivating, Nylon-Like Coatings through Cross-Linking of Ultrathin Polyelectrolyte Films. J. Am. Chem. Soc. 1999, 121, 1978. (12) Sun, J.; Wu, T.; Sun, Y.; Wang, Z.; Zhang, X.; Shen, J.; Cao, W. Fabrication of a Covalently Attached Multilayer Via Photolysis of Layer-by-Layer Self-Assembled Films Containing Diazo-Resins. Chem. Commun. 1998, 1853. (13) Zhang, X.; Wu, T.; Sun, J.; Shen, J. Ways for Fabricating Stable Layer-by-Layer Self-Assemblies: Combined Ionic Self-Assembly and Post Chemical Reaction. Colloids Surf., A 2002, 198−200, 439. (14) Wu, G. L.; Shi, F.; Wang, Z. Q.; Liu, Z.; Zhang, X. Poly(Acrylic Acid)-Bearing Photoreactive Azido Groups for Stabilizing Multilayer Films. Langmuir 2009, 25, 2949. (15) Matsuda, T.; Sugawara, T. Photochemical Protein Fixation on Polymer Surfaces Via Derivatized Phenyl Azido Group. Langmuir 1995, 11, 2272. (16) Coll Ferrer, M. C.; Yang, S.; Eckmann, D. M.; Composto, R. J. Creating Biomimetic Polymeric Surfaces by Photochemical Attachment and Patterning of Dextran. Langmuir 2010, 26, 14126. (17) Liu, H.; Ito, Y. Gradient Micropattern Immobilization of a Thermo-Responsive Polymer to Investigate Its Effect on Cell Behavior. J. Biomed. Mater. Res., Part A 2003, 67, 1424. (18) Ito, Y. Photoimmobilization for Microarrays. Biotechnol. Prog. 2006, 22, 924. (19) Al-Bataineh, S. A.; Luginbuehl, R.; Textor, M.; Yan, M. Covalent Immobilization of Antibacterial Furanones Via Photochemical Activation of Perfluorophenylazide. Langmuir 2009, 25, 7432. (20) Yan, M.; Bartlett, M. A. Micro/Nanowell Arrays Fabricated from Covalently Immobilized Polymer Thin Films on a Flat Substrate. Nano Lett. 2002, 2, 275. (21) Bartlett, M.; Yan, M. Fabrication of Polymer Thin Films and Arrays with Spatial and Topographical Controls. Adv. Mater. 2001, 13, 1449. (22) Liu, L.; Engelhard, M. H.; Yan, M. Surface and Interface Control on Photochemically Initiated Immobilization. J. Am. Chem. Soc. 2006, 128, 14067. (23) Yan, M.; Ren, J. Covalent Immobilization of Polypropylene Thin Films. J. Mater. Chem. 2005, 15, 523. (24) Liu, L. H.; Yan, M. Simple Method for the Covalent Immobilization of Graphene. Nano Lett. 2009, 9, 3375.

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CONCLUSIONS We have demonstrated the fabrication of a stable silica nanoparticle coating by introducing photoreactive phenyl azido groups onto the surface of silica nanoparticles. To overcome the problem of small contact area when immobilizing spherical nanoparticles on solid substrate, poly(allylamine hydrochloride) was also functionalized with phenyl azido groups and used as polycations for LbL assembly, together with azido-terminated silica nanoparticles. Thus, a covalently linked network can be established among the silica nanoparticles, the surrounding polycation chains, and the substrate under UV irradiation. Herein another advantage of using LbL assembly method is that the amount of coated silica nanoparticles can be facilely tailored by controlling the assembly cycles. When the photoreactive LbL multilayers were used to form a superhydrophobic coating on cotton fabrics, the superhydrophobicity showed good chemical stability against acids, bases, and organic solvents, as well as reasonable wash durability. Because of the unique photoreactivity of phenyl azido groups, this strategy may be applicable to form various stable nanoparticle coatings on almost any organic substrate.

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ASSOCIATED CONTENT

* Supporting Information S

Photograph of aqueous suspensions, zeta potentials, and UV− vis spectra of silica, silica-NH2, and silica-N3 nanoparticles; UV−vis and 1H NMR spectrum of PAH-N3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +61 3 5227 1245. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Decher, G. Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites. Science 1997, 277, 1232. (2) Zhang, X.; Chen, H.; Zhang, H. Layer-by-Layer Assembly: From Conventional to Unconventional Methods. Chem. Commun. 2007, 1395. 6334

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dx.doi.org/10.1021/la300281q | Langmuir 2012, 28, 6328−6335