Facile Preparation of Raspberry-Like Superhydrophobic Polystyrene

Aug 14, 2013 - fabricate raspberry-like or snowman-like particles via seeded dispersion ... confirmed by field-emission scanning electron microscopy a...
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Facile Preparation of Raspberry-Like Superhydrophobic Polystyrene Particles via Seeded Dispersion Polymerization Rui-kun Wang, Hua-rong Liu,* and Feng-wei Wang CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China S Supporting Information *

ABSTRACT: A simple and facile approach was developed to fabricate raspberry-like or snowman-like particles via seeded dispersion polymerization by just changing the ratio of second monomer styrene (St) to seeds in which poly(styrene-cohydrolyzed-methacryloxypropyltrimethoxysilane) [P(St-coMPS)] latex was used as seeds with hydrolyzed-MPS as a cross-linking agent. The morphologies of final products were confirmed by field-emission scanning electron microscopy and transmission electron microscopy. Interestingly, the seed part of snowman-like particles showed raspberry-like with adsorbing quantities of PS particles while the other part smooth. The formation mechanism of the raspberry-like particles was also discussed. The superhydrophobic surface with both the static contact angle of 158° and high adhesion to water could be achieved by the hydrophobization of the particulate film with octadecyltrimethoxysilane that was formed from the raspberry-like particles decorated by a thin layer of silica nanoparticles. Further, through encapsulating Ag nanoparticles within the surface, the obtained raspberry-like PS/Ag/SiO2 nanocomposite particles exhibited excellent antibacterial property simultaneously.

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applications in biomedical implications,16 optical devices,17 solid surfactants18 and so on. Various methods have been developed to prepare anisotropic particles, including microfluidic method,19,20 toposelective surface modification method,21,22 and different template methods.23,24 Among these methods, seeded polymerization as one of template methods has been proved an efficient and flexible method to prepare snowman-, dumbbell-, raspberry-like particles in high yields with tunable size, shape, and composition.25−27 Generally, cross-linking of the seed particles using a cross-linking agent such as DVB is a prerequisite in this method for the formation of nonspherical shapes. For example, Sheu et al.28 successfully prepared various uniform nonspherical particles by seeded emulsion polymerization. Okubo et al.29 produced snowman/ confetti-shaped monodisperse polymer particles by seeded dispersion polymerization. Weitz et al.30 introduced a facile approach to synthesize uniform amphiphilic dimer particles based on the seeded polymerization technique in which the seed particles were cross-linked with DVB and modified with 3trimethoxysilylpropylacrylate (TMSPA). During latest several years, silane agents like 3-methacryloxypropyltrimethoxysilane (MPS) have been applied in the fabrication of anisotropic particles. For instance, Nagao et al.31 successfully synthesized snowman-shaped polymer particles only using hydrolyzed MPS

INTRODUCTION Over the past few decades, colloidal particles have been widely investigated both theoretically and technologically.1,2 Recently, more and more attention has been focused on the multifunctionalization of colloidal particles, because the topology and surface properties of colloidal particles are highly important for various applications. Multilayer core−shell microspheres, the most common multifunctional particles, can easily tailor their physical properties,3 catalytic function,4 solvent compatibility5 and anticancer drug release and delivery6,7 through the formation of multilayer coatings on the core. For example, microspheres encapsulating iron oxide nanoparticles and fluorescent quantum dots have been widely used as a multiple-mode imaging contrast agent8 and magnetic separation of proteins.9 Schneider et al.10 reported a highly versatile nanoparticle composite system with multilayer shells that combine several properties. Yin and co-workers11 demonstrated a general “encapsulation and etching” strategy using porous shells to protect catalyst nanoparticles against aggregation. Further tuning this microspheres system, the performance of multilayer microspheres may be optimized through adding temperature- or photoresponsive material into the shells.12 Microspheres containing Ag nanoparticles as a core or loaded on the surface have found applications in optical fields and antibacterial performance.13−15 Anisotropic particles that vary in shape and surface chemistry are another type of multifunctional particles and have attracted increasing attention because of their unique properties for © XXXX American Chemical Society

Received: May 4, 2013 Revised: August 13, 2013

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Scheme 1. The Procedure for Preparation of Anisotropic Particles

adsorbing quantities of PS particles while the other part smooth. The overall synthetic procedure and the treatment of raspberry particles are illustrated in Scheme 1. A particulate film is formed by deposition of the raspberry-like particles decorated with a thin layer of silica nanoparticles on a glass substrate. After surface treatment with octadecyltrimethoxysilane, a superhydrophobic film with strong adhesion to water is obtained. We further load Ag nanoparticles onto the raspberry-like particles by the reduction of Ag+ absorbed on the sulfonated raspberry particles. After the same modification procedure, the obtained raspberry-like PS/Ag/SiO2 nanocomposite particles exhibit excellent antibacterial and superhydrophobic properties with strong adhesion to water, which has not been reported for raspberry-like particles as we know.

as the cross-linking agent via seeded emulsion polymerization. This method is applicable to the preparation of anisotropic composite particles containing inorganic cores. Dufresne et al.32 presented a method for producing highly monodisperse dumbbell-shaped polymer nanoparticles based on noncrosslinking core−shell particles which originated from seeded emulsion polymerization of St and TMSPA in the presence of PS spheres. Among various anisotropic particles, raspberry-like particles have witnessed their potential applications in the fabrication of superhydrophobic surfaces due to their dual-size hierarchical structures and rough surface morphology.33,34 Normally, there are two models for explaining superhydrophobicity on a rough surface, namely Wenzel’s state and Cassie’s state.35 The former describes a wet-contact mode in which liquid fully penetrates into the rough surface, while the latter respresents a nonwetcontact mode with trapped air underneath liquid droplets. For example, superhydrophobic surface of a lotus leaf with a static contact angle larger than 150° is a special case of the Cassie’s state, and water droplets can roll off easily owing to the low contact angle hysteresis (less than 10°),36 which has found important applications in self-cleaning,37 transparent and antireflective coatings,38 and enhanced corrosion resistance.39 Thus many efforts have been devoted to the fabrication of lotus-inspired superhydrophobic surfaces.40−42 Recently, another novel superhydrophobic surface derived from rose petal obeying Wenzel’s state has begun to attract considerable attention due to high adhesive force coming of the high contact angle hysteresis36 and may have potential applications in the transport of liquid microdroplets over a surface without sliding or rolling, microsample analysis and cell diagnosis.36,43,44 Although there are a series of researches on the fabrication of rose petal-like superhydrophobic surface, such as aligned polystyrene nanotubes layer45 and micropillar arrays fabricated from photoresist and silicon,46 there are few reports on the fabrication of rose petal-inspired surfaces from raspberry-like particles up to now. Recently, Xu et al.47 prepared raspberrylike SiO2/polystyrene particulate films exhibiting a large contact angle hysteresis (116°) and strong adhesion to water which can be used as a “mechanical hand”. In this work, we present a simple and facile approach to fabricate raspberry-like or snowman-like particles via seeded dispersion polymerization with hydrolyzed-MPS as the crosslinking agent and only St as a monomer by just changing the ratio of monomer St to seeds. Interestingly, the seed part of snowman-like particles showed raspberry-like morphology with

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EXPERIMENTAL SECTION

Materials. Styrene (St, Sinopharm Chemical Regent Co., Ltd.) was purified by passing through a basic alumina column to remove the inhibitor before use. The 3-methacryloxypropyltrimethoxy-silane (MPS, 97%, Alfa Aesar) and octadecyl trimethoxysilane (OTS, ≥90%, Aldrich) were used as received. The 2,2′-Azobis(2-methylpropionitrile) (AIBN) and potassium persulfate (KPS) were recrystallized before use. Ethanol (99.7%), hexane, tetraethoxysilane (TEOS), aqueous ammonia (25 wt %), polyvinylpyrrolidone (PVP, K30), concentrated sulfuric acid (98%), silver nitrate (≥99.8%), and polyoxyethylene (20) sorbitan monolaurate (Tween 20) were all purchased from Shanghai Chemical Reagents Co., China and used without further purification. Double distilled water was used in all experiments. Preparation of P(St-co-MPS) Seed Particles. The seed particles of MPS-copolymerized with St (P(St-co-MPS)) were synthesized via dispersion polymerization according to a modified method reported previously.29 Fifty microliters of MPS was dissolved in water (10 g) for hydrolysis of 30 min. Then 20 mL of ethanol and 0.3 g of PVP were added under stirring, followed by adding 2.5 mL of St dissolved with 0.02 g of AIBN into the above solution. At last, all ingredients were moved into a 100 mL three-neck flask equipped with a mechanical stirrer, a condenser, and a gas inlet. After the mixture was degassed with nitrogen at room temperature for 20 min, the flask was placed in a 70 °C water bath and stirred at 300 rpm. The reaction was continued for 20 h to ensure its complete polymerization. The obtained P(St-coMPS) latex was directly used as seed latex in the next step without post treatment. Synthesis of Anisotropic Particles via Seeded Polymerization. The standard recipes for the synthesis of anisotropic particles are listed in Table 1. P(St-co-MPS) latex (0.3 g) and a certain amount of monomer St were added into 20 g of water. After the mixture was sonicated for 20 min, 0.043 g KPS was added. The prepared seeded dispersion was removed into 50 mL flask and degassed with nitrogen for 20 min. Then the flask was placed in a 70 °C water bath and reacted for 7 h. B

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Table 1. Recipes for the Synthesis of Anisotropic Particles samples

seed latex (g)

water (g)

monomer st (ml)

KPS (g)

1 2 3

0.3 0.3 0.3

20 20 20

0.5 0.7 0.8

0.043 0.043 0.043

Dn =

∑ niDi ∑ ni

(1)

where ni is the number of the particle with a diameter of Di. 13C solid-state CP-MAS nuclear magnetic resonance (NMR) spectrum was recorded with Bruker AV400 NMR spectroscope. Fourier transform infrared (FTIR) spectra were carried out on a Vector-22 FTIR spectrometer using the KBr pellet method. X-ray diffraction (XRD) was recorded on a Philips X’Pert PRO diffractometer equipped with graphite monochromatized Cu Kα irradiation (λ = 0.1541841 nm). The surface wettability of raspberry-like particles was characterized by the water contact angle. The static contact angle of water on the particulate film was measured on a Contact Angle Meter SL200B (Solon Tech. Co., Ltd.). A droplet of water (2 mL) was injected onto the film. The water contact angle was recorded by special software (CAST 2.0) in the computer. For each set of experimental conditions, three specimens were analyzed and the mean value was taken as the final result. The deviation of the measured contact angles between various specimens was less than 2°. The antibacterial activity of the samples against E. coli (DH5α) was performed by the plate counting method.15,50 The density of bacterial suspensions employed for the tests was 106 colony forming units (CFU)/ml. The following mediums were used: agar medium, fluid nutrient medium, and phosphate buffer (PBS). All the mediums and samples were sterilized at 121 °C for 15 min in advance. The test process was described as follows: 0.1g of PS/Ag/SiO2 particles was added into PBS solution, then 800 μL of bacterial suspensions was added. The mixture was then incubated at 37 °C for 6 h after vibrating for 12 h by an orbital shaker. Following that, the suspension was gradient diluted with PBS solution according to the ratio of 1:10 for several multiples. The above suspension (0.1 mL) at proper dilution degree was extracted and quickly spread on an agar plate, then cultured at 37 °C for 24 h. The average number of discrete colonies was counted as the number of the remaining bacteria by repeating the above procedure three times with error bars. Parallel tests of PS/SiO2 particles without any loaded Ag nanoparticles were also conducted for the comparison. In this work, the degree of antibacterial or bacteriostatic effect was presented as the percent reduction of the bacteria. The equation for quantitative antibacterial evaluation is given as follows50

The resulted particles were repeatedly centrifugated at 4000 rpm for 5 min and washed with ethanol, and repeated several times, and then dried in an oven at 40 °C for 24 h. Surface Modification of Raspberry-Like Particles by Coating Silica. The surface of raspberry-like P(St-co-MPS)/PS particles was modified by coating of silica particles according to the literature.48 Raspberry-like particle (0.1 g) and ethanol (20 g) were added into 10 g of 0.5 wt % Tween 20 aqueous solution. After the mixture was stirred for 10 min, 0.5 mL of ammonium hydroxide and 500 μL of TEOS were consecutively added. The hydrolytic condensation reaction for coating was continued for 5 h. Then resulted particles were collected by centrifugating at 4000 rpm, washed with ethanol several times, and then dried in an oven at 40 °C for 24 h. Preparation of Superhydrophobic Particulate Film from Raspberry-Like P(St-co-MPS)/PS/SiO2 particles. A glass substrate presoaked in a piranha solution (18 mol/L H2SO4 and 30 wt % H2O2 solutions in a 2:1 v/v mixture) was rinsed with water several times. The raspberry-like P(St-co-MPS)/PS/SiO2 particles were ultrasonically dispersed in ethanol for a few minutes to ensure uniform dispersion. Then the particle dispersion was dipped onto the glass substrate, followed by evaporating in an oven at 60 °C. This procedure was repeated several times until a thin particulate film was formed. As a control, different particulate films were prepared by seed, silica-coated seed, and raspberry-like P(St-co-MPS)/PS particles according to the same procedure as above. Finally, the particulate films from raspberrylike P(St-co-MPS)/PS/SiO2 and silica-coated seed particles were chemically modified by being immersed in 1 wt % OTS in hexane for 30 min. Preparation of Raspberry-Like PS/Ag Nanocomposite Spheres and Particulate Film. Raspberry-like PS/Ag nanocomposite particles were prepared according to a modified literature procedure.49 Raspberry-like P(St-co-MPS)/PS particles (0.12 g) were added into 5 mL of concentrated sulfuric acid (98%) at 40 °C under constant magnetic stirring. After 4 h, the sulfonated raspberry-like P(St-coMPS)/PS particles were obtained by centrifugation, dispersed in a large quantity of ethanol repeatedly, then redispersed in 10 mL of water. Then 10 mL of a freshly prepared aqueous solution of [Ag(NH3)2]+ (0.35 mol/L) was quickly added into the above dispersion. Under stirring at room temperature for 1 h, 20 mL of the aqueous solution containing 1 g of PVP was added. Subsequently, the mixture was kept at 70 °C and stirred for 7 h. The final product was collected by centrifugation, washed with ethanol several times, and dried in an oven at 40 °C for 24 h. A particulate film was then constructed using raspberry-like PS/Ag nanocomposite particles that were surface-modified by coating of silica particles in advance, followed by the hydrophobization with OTS according to the same procedure as above.

R(%) =

(A − B ) A × 100%

(2)

where R is the percentage reduction; A represents the number of bacterial colonies from the untreated bacteria suspension (without PS/Ag/SiO2 particles), while B is that from the bacteria suspension treated by PS/Ag/SiO2 particles for 24 h.

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CHARACTERIZATION The morphology of anisotropic particles was observed by fieldemission scanning electron microscope (FESEM) and transmission electron microscope (TEM). TEM images were taken on a Japan JEOL H7650 operated at an accelerating voltage of 100 kV. FESEM images and energy-dispersive X-ray spectroscopy (EDX) of PS/Ag nanocomposite particles were conducted on a FEI JEOL JSM6700 field-emission scanning electron microscope. Samples for TEM and SEM measurements were ultrasonically dispersed in ethanol and then dipped onto copper grid coated with carbon, successively dried in air. The average size of the seed particles (Dn) is calculated by the eq 1

RESULTS AND DISCUSSIONS The prehydrolysis of silane coupling agent MPS is critically important for the formation of raspberry-like and snowman-like particles. MPS contains not only an acrylate group that can copolymerize with styrene, but also trialkoxysilane, which can be hydrolyzed for coupling with each other. When MPS is dissolved in water (pH ∼ 7), two or more MPS molecules can be coupled to form a polyfunctional molecule with two or more acrylate groups (see Scheme 2). Therefore, the hydrolyzed MPS can be used as a cross-linking agent which may copolymerize with St. The structure of the copolymer P(Stco-MPS) was confirmed by 13C NMR and FTIR spectra as C

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formation of cross-linked network of the seed particles. Moreover, the unpolymerized MPS on the surface of seed particles might copolymerize with St that was conducive to the adsorption of PS particles during seeded dispersion polymerization. These two reasons may be responsible for the formation of raspberry-like and snowman-like particles. If the hydrolytic reaction of MPS was carried out at pH = 9, a few raspberry-like particles accompanied with a great number of irregular particles instead of well-defined raspberry-like particles were achieved (Supporting Information Figure S2c), which may be caused by the formation of some silica particles with double bonds (a white dispersion appeared) prior to the copolymerization with St due to the base-catalyzed hydrolysis and condensation reaction of MPS. Therefore, only a very little hydrolyzed-MPS really played a role of the cross-linking agent. The formation of irregular particles may be caused by the copolymerization of double bonds in silica particles with St and the hydrolytic condensation of MPS that can form the siloxane linkages between MPS on the surface of seed particles. The Formation Mechanism of Raspberry-Like Particles. To further explain the growth mechanism of raspberrylike particles, we investigated the formation process at different reaction time. Keeping other conditions the same, the polymerization reaction was quenched by adding a certain amount of hydroquinone solution at expected reaction time. The morphology evolution at different reaction time is shown in Figure 3. At the reaction of 30 min, the deflated balls were formed accompanying with few semiraspberry-like particles. After the reaction reached 1 h, the raspberry-like particles were obtained (Figure 3b−d), which is closer to that of final particles as shown in Figure 2b. This means that raspberry-like particles are almost achieved in about 1 h. From the morphology evolution above, we propose a possible formation mechanism of raspberry-like particles as follows. During ultrasonic dispersion, a part of monomers was still dispersed in water, while the rest would swollen the seed particles. Therefore, the elastic-retractile force in the limited swollen seed particles was not sufficient to extrude monomer, namely no phase separation occurred. The St-swollen seed particles became deflated due to the incomplete polymerization of St swollen in seed particles. At the same time, they also adsorbed polystyrene particles self-nucleated in the dispersion medium during seeded polymerization,53 which has been proved in Figure 3e. The appearance of pits in the surface of individual particles may be attributed to the shedding of polystyrene particles during post treatment of sonication. The growth mechanism of snowman-like particles (Supporting

Scheme 2. Hydrolytic Reaction of MPS

shown in Figure 1. In Figure 1a, the peak at 175 ppm is attributed to the carbonyl group of the methacrylate at MPS, while the signals around 35−50 ppm are assigned to CH− and CH2− that are formed by the polymerization of CC double bonds, indicating that the MPS molecules have polymerized or copolymerized with St.51 In FTIR spectrum (Figure 1b), the band at 1637 cm−1 assigned to the CC bond had disappeared, also indicating that the compounds with CC bonds had already been copolymerized.51,52 The experiment in coating silica on the surface of seeds (Figure S1 in Supporting Information) demonstrated the formation of core−shell structure, which further confirmed that the P(St-co-MPS) particles were successfully prepared by the copolymerization of MPS with St. We investigated the influence of the ratio of monomer to seed latex on the morphologies of resulted particles with the mass of seed latex fixed at 0.3 g. The TEM image of seed particles is shown in Figure 2a with the average diameter of 667 nm, and SEM image in the inset shows the smooth surface of seed particles. When the volume of monomer St is 0.5 mL, large quantities of raspberry-like particles were obtained with the seed core particles absorbing abundant tens of nanometers sized PS particles at during seeded polymerization, as shown in Figure 2b. When the volume of St was increased to 0.7 or 0.8 mL, snowman-like polymer particles with one rugged part covered with many small nanoparticles and the other smooth were obtained (Supporting Information Figure S2a and Figure 2c). These results are different from other previous work in which only snowman-like polymer particles were obtained,31 which may stem from different synthetic methods of seed latex and different ratios of monomer to seed. To bolster our argument that MPS plays an essential role, we demonstrated that spherical particles instead of raspberry-like particles were obtained if the seed latex is not modified by MPS (Supporting Information Figure S2b), which is owing to the noncross-linked PS seeds and the compatibility of PS with St. In the case of P(St-co-MPS) at pH 7 as seeds, the hydrolyzed-MPS acted as a cross-linking agent to copolymerize with St, leading to the

Figure 1. 13C NMR and FTIR spectra of the P(St-co-MPS) seed particles. D

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Figure 2. (a) TEM image of seed particles (SEM image of seed particles shown in the inset, the scale bar is 500 nm; SEM images of (b) raspberrylike and (c) snowman-like PS particles.

Figure 3. SEM image of morphology development during seeded polymerization with 0.3 g of seed latex and 0.5 mL of St at different reaction time of (a) 30 min; (b) 1 h; (c) 3 h; (d) 5 h; (e) incomplete raspberry-like particles with pits in the surface; and (f) schematic illustration of the formation of raspberry-like particles.

Information, Figure S3) was explained in Supporting Information. The Wettability of Raspberry-Like Particles. It is wellknown that the roughness of a hydrophobic surface will enhance its hydrophobicity. In our work, because the seeded polymerization was initiated by KPS, the outer surface of raspberry-like particles was anchored by sulfate ions that may improve the surface hydrophilicity. It can be deduced that the decrease of the mass of KPS will increase the hydrophobic property of raspberry-like particle film (Figure S4 in Supporting Information). So the surface hydrophobization of the raspberrylike particles is necessary for fabricating superhydrophobic particulate film. We modified the surface of raspberry-like PS particles by coating of a thin layer of silica particles according to the previous literature.48 The FT-IR spectrum of resulted particles in Figure 4 shows that the typical adsorption peaks of the Si−O−Si bonds at 1101 and 460 cm−1 are strong,54 indicating the presence of silica in the raspberry-like particles. It is noteworthy that the raspberry-like morphology was maintained after modification, which can be judged from the TEM and SEM images in Figure 5(a,b,d,e). Because the coating of a silica layer on the surface will have effect on the surface roughness of raspberry-like particles, we investigate the influence of the volume of TEOS on the contact angle of particulate films, and the detailed experimental conditions are shown in Table 2. The silica-modified raspberry-like particles were deposited on a clean glass substrate to form a particulate film, followed by the treatment with octadecyltrimethoxysilane.54 The SEM image of the particulate film shows that it is multilayers (Figure S5a in Supporting Information), which may

Figure 4. FT-IR spectrum of the silica-coated raspberry particles.

be beneficial to improving the surface roughness. The corresponding contact angle (CA) is 148 and 158° when the volume of TEOS is 0.5 and 0.3 mL, respectively, as shown in Figure 5(c,f). The latter contact angle is more than 150° which indicates the formation of superhydrophobic surface. These results show that the less the amount of TEOS was, the more the roughness of modified raspberry-like particles would be due to a thinner silica coating, and thus the more hydrophobic the formed particulate film is after the hydrophobization with OTS. Therefore, the surface roughness of raspberry-like particles plays an important role in the superhydrophobic property of the formed particulate film. In order to further illustrate this opinion, we used the PS seed microspheres as a control. It was E

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Figure 5. TEM, SEM and contact angle images of raspberry particles coated with a layer of silica particles. The volume of TEOS is 0.5 mL for (a−c) and 0.3 mL for (d−f).

angles were also measured on a clean glass substrate and various particulate films comprised of seed PS particles, raspberry-like PS particles and silica-coated raspberry-like particles, corresponding to 48, 89, and 10°, respectively. Schematic illustration of different surfaces together with the corresponding contact angles are summarized in Figure 7. It

Table 2. Recipes for Hydrolysis of TEOS sample

raspberry particles (ml)

5 wt % Tween 20 (g)

ethanol (g)

ammonia (ml)

TEOS (ml)

1 2

4 4

10 10

20 20

0.5 0.5

0.5 0.3

demonstrated that the contact angle of the particulate film formed from the PS seed microspheres was 122° (Supporting Information Figure S5b) after the same modification, which is less than those from raspberry-like particles. This result is consistent with the reported literature.54 In order to judge which model the superhydrophobic surface made from sample 2 listed in Table 2 belongs to, we measured the dynamic advanced and receding water contact angles on the as-prepared superhydrophobic particulate film. It is found that the contact angle hysteresis was 43° (as the advancing and receding contact angles were 156 and 113°, respectively), which did not meet the requirement of “lotus effect” with contact angle hysteresis less than 10°.36 Moreover, the water droplet on the superhydrophobic surface does not roll off even if the film turns upside down, as depicted in Figure 6a. When the volume

Figure 7. Schematic illustration of different surfaces and contact angle of corresponding surface.

can be seen that the water contact angle on the clean glass substrate is only 28° due to hydrophilic −OH groups on the surface. Seed PS particles surface absorbing abundant hydropholic PVP led to the corresponding water contact angle of 48° for the as-prepared film. Along with the increased roughness of the particles, such as the raspberry-like PS particles with submicrostructure, the water contact angle is increased to 89°. After the surface of raspberry-like particles was coated with a layer of silica, the surface property changed into hydrophilic, so the water contact angle is only 10° owing to the increasing surface roughness.35 After hydrophobic modification of raspberry-like particles with OTS, superhydrophobic surface with the water contact angle of 158° was achieved.

Figure 6. Water droplet of (a) 4 μL and (b) 10 μL adheres to the glass slide covered by a raspberry-like particulate film when the glass slide turns upside down.

of water droplet was increased to the 10 μL, this adhesive state was still maintained (Figure 6b). Therefore, the superhydrophobic surface prepared from raspberry-like particles obeys the Wenzel’s state, which was demonstrated by both high adhesive force between water droplets and the raspberry-like particulate film and high contact angle hysteresis of the raspberry-like particulate film.47,55 To study the effect of microstructures and surface chemistry on superhydrophobic property, different static water contact F

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Antibacterial and Superhydrophobic Property of Raspberry-Like PS/Ag Nanocomposite Particles. For the encapsulation of Ag nanoparticles, [Ag(NH3)2]+ ions absorbed on the surface of raspberry-like PS microspheres via the electrostatic attraction of −SO3− were reduced to Ag nanoparticles by PVP. The Ag element can also be detected in EDX pattern shown in the inset of Figure 8. Moreover, a TEM image in the inset of

Figure 10. (a) Contact angle of raspberry-like PS/Ag nanocomposite film; Water droplet of (b) 2 μL and (c) 10 μL adheres to the film when the film turns upside down. Photos of a water droplet adhering to the film when the film was placed at different positions: (d) the film flatly on the table; (e) the film perpendicularly to the table; (f) the film turned upside down. Figure 8. XRD and EDX patterns of PS/Ag nanocomposite.

Figure 9a clearly illustrated the successful loading of Ag in raspberry-like PS particles, and the raspberry-like morphology has not been destroyed in nanocomposite particles as well as shown in the SEM image of Figure 9a. As aforementioned reason, a thin layer of silica was coated on the surface of the raspberry-like PS/Ag particles. SEM and TEM images of the resulted PS/Ag/SiO2 in Figure 9b demonstrate that a layer of silica was successfully coated on the surface and the raspberrylike morphology remained. A thin film from raspberry-like PS/Ag/SiO2 particles was prepared on the glass substrate. After hydrophobization with OTS, the brown film showed superhydrophobic property with the water contact angle of 154° as well as high adhesion to water droplet as shown in Figure 10a−c. Furthermore, the photos in Figure 10d−f show the water droplet firmly adhered to the film no matter what position the film was placed. Therefore, the encapsulating of Ag nanoparticles did not change the superhydrophobic property of raspberry-like particulate film together with high adhesion to water droplet. Moreover, the as-prepared PS/Ag/SiO2 particles may possess antibacterial property. Figure 11 shows the antibacterial effect against E. coli of PS/ SiO2 and PS/Ag/SiO2 particles after culturing for 24 h in the same environment, respectively. It can be seen that raspberrylike PS/Ag/SiO2 particles have a strong antibacterial effect

Figure 11. Antibacterial activity of different samples against E. coli: (a) PS/SiO2 particles; (b) PS/Ag/SiO2 particles.

against E. coli. The percent reduction of bacteria is 98.6% according to eq 2. The superhydrophobic PS/Ag/SiO2 particulate film has not been reported yet, and it may have potential applications in antibiofouling. Because bacteria are mostly hydrophilic, so the film can block the bacterial reproduction owing to its superhydrophobicity. In addition, its antibacterial property can kill a few bacteria absorbed on the surface of the film.

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CONCLUSIONS This paper has presented a simple method to prepare raspberry-like PS particles by using hydrolyzed-MPS as the cross-linking agent. Through increasing the volume of monomer of the seeded polymerization process, snowmanlike particles with one part smooth and the other rugged adsorbing abundant small PS particles were also obtained. The

Figure 9. (a) SEM and TEM of raspberry PS/Ag nanoparticles; (b) SEM and TEM of silica coated nanoparticles. G

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superhydrophobic film prepared from raspberry-like particles can be obtained with a high water contact angle of 158° and shows high adhesion to water droplet. After loading Ag nanoparticles onto the raspberry-like PS particles, the asprepared particles not only maintained the superhydrophobic property but also showed excellent antibacterial property of 98.6% percent reduction, which can be applied in antibiofouling.

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ASSOCIATED CONTENT

* Supporting Information S

The TEM images of silica coated MPS-copolymerized PS latex particles. SEM images of particles obtained from (a) 0.7 mL of St and 0.3 g of P(St-co-MPS) latex synthesized by dispersion polymerization at pH ∼ 7; (b) 0.7 mL of St and 0.3 g of PS latex without MPS; and (c) 0.5 mL of St and 0.3 g of P(St-coMPS) latex synthesized by dispersion polymerization at pH ∼ 9. SEM images of the formation mechanism of snowman-like particles. Photographs of water contact angles of particulate films made from raspberry-like PS particles via the polymerization initiated by different amount of KPS: (a) 0.043 g; (b) 0.03 g; (c) 0.02 g. SEM image of a superhydrophobic particulate film. Photograph of water contact angle of the particulate film from silica-coated seed particles modified by hydrophobization of OTS. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the funding support from the National Natural Science Foundation of China (Project No. 21074122 and 50873096), and Ministry of Science and Technology of China (Project No. 2007CB936401).

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