Formation of Cagelike Sulfonated Polystyrene Microspheres via

Nov 22, 2013 - Lei Tian , Xiangjie Li , Panpan Zhao , Xin Chen , Zafar Ali , Nisar Ali , Baoliang Zhang , Hepeng Zhang , and Qiuyu Zhang. Macromolecul...
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Formation of Cagelike Sulfonated Polystyrene Microspheres via Swelling-Osmosis Process and Loading of CdS Nanoparticles Hanqin Weng, Xuefeng Huang, Mozhen Wang,* Xiang Ji, and Xuewu Ge 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: In this report, we studied the formation mechanism of cagelike polymer microspheres fabricated conveniently and efficiently through a swelling-osmosis process of sulfonated polystyrene (SPS) microspheres in a ternary mixed solvent (water/ethanol/heptane). The scanning electron microscopy and transmission electron microscopy observations indicated that the morphology of the final cagelike SPS microspheres is mainly controlled by the composition of the mixed solvent and the swelling temperature. Considering the solubility parameters of related reagents and the low interface tension of heptane and the aqueous solution of ethanol (only 6.9 mN/m), we confirm that the porogen procedure starts from the swelling of SPS microspheres by heptane, followed by the osmosis process of water molecules into the swollen SPS microspheres forced by the strong hydrophilicity of −SO3H group. The water molecules permeated into SPS microspheres will aggregate into water pools, which form the pores after the microspheres are dried. These prepared cagelike SPS microspheres are further served as the scaffold for the in situ generated CdS nanoparticles under γ-ray radiation. The CdS/SPS composite microspheres show good fluorescence performance. This work shows that the cagelike SPS microspheres have a wide industrial application prospect due to their economical and efficient preparation and loading nanoparticles.

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INTRODUCTION Multihollow microspheres possess competitive advantages such as high specific surface and low density, which can lead to a variety of surface-related applications including chromatography,1 scavenging,2 catalysis,3 life science,4 and so on. In recent years, it has been proved that emulsion polymerization is a kind of efficient approach to produce multihollow microspheres. As early as 1990s, Okubo et al. prepared styrene-methacrylic acid copolymer microspheres by emulsion copolymerization. The original microspheres were then treated by the stepwise alkali/ acid method and alkali/cooling method respectively to obtain the final multihollow microspheres.5,6 Later on, they synthesized submicrometer-sized multihollow polymer particles through seeded emulsion polymerization of styrene in the presence of nonionic emulsifier, poly(oxyethylene)nonylphenylether, instead of alkali (or acid) swelling method.7 Suh et al. produced multihollow poly(methyl methacrylate) by employing the multiple-emulsion (W/O/W) polymerization technique. In the multiple-emulsion, sorbitan monooleate was the primary surfactant. Sodium laurylsulfate and glucopen, a polypeptide derivative, were both used as secondary surfactants.8 Recently, Tong and Wang et al. synthesized multihollow dual nanocomposite polymer microspheres with anchored closed pores by suspension polymerization in which different nanoparticle stabilizers were needed to form multiple Pickering emulsion.9 However, there are multisteps in the above-mentioned methods, which is inconvenient to control the final morphology of the microspheres. © 2013 American Chemical Society

In our previous work, polymer microspheres with a hollow core and porous shell, identified as cagelike polymer microspheres, were synthesized through a Pickering emulsion polymerization.10,11 The formation mechanism of such cagelike microspheres was also demonstrated.11 When sulfonated polystyrene (SPS) microspheres were dispersed in water, a Pickering emulsion was formed after methyl methacrylate (MMA) monomers were added into the system. The SPS microspheres were swollen by MMA and finally became the multihollow microspheres after the polymerization of monomer. Then a new trial had been developed for the fabrication of cagelike polymer microspheres by swelling SPS microspheres in an ethanol/water medium with heptane as the phase separation agent.12 However, during the further investigations we cannot get any evidence on the existence of a Pickering emulsion, which implies that the formation mechanism of cagelike microspheres has no relationship with a Pickering emulsion system. Therefore, it is necessary to restudy the effect of the heptane, water, and ethanol on the formation of the holes and propose a new and reliable formation mechanism of cagelike structure in this mixed solvent system. This research will not only contribute to the control of the morphology but also be valuable for the industrial applications of the microspheres. Received: July 10, 2012 Revised: October 21, 2013 Published: November 22, 2013 15367

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an oil bath at 70 °C for a certain time. Finally, the system was cooled down to room temperature. The product was collected by centrifugation, washed with ethanol for three times, and dried in a vacuum oven at 40 °C for 48 h. The detailed recipes were listed in Table 1.

Multihollow polymer materials are suitable candidates for the matrix of inorganic nanoparticles. Compared with the traditional inorganic scaffolds, such as clay13 and silica,14,15 polymeric matrix affords much more advantages since their chemical composition, size, and accessibility are tunable, and their good stability under reaction conditions can lead to excellent recovery and recycling for catalytic activity.16,17 Up to now, many inorganic nanoparticles have been immobilized onto the skeleton of various kinds of porous polymer.18−22 In a latest work, Zhang et al. synthesized a kind of cagelike porous polymeric microspheres used as the catalyst scaffold of Pd nanoparticles with a size ranging from 2.1 to 5.7 nm.23 The result of the catalytic hydrogenation of nitrobenzene by H2 demonstrated that the heterogeneous Pd catalyst is efficient and reusable. This article aims to study the formation mechanism of cagelike polymer microspheres fabricated through a swellingosmosis process of SPS microspheres in a ternary mixed solvent (water/ethanol/heptane). On the basis of a series of analysis on the morphological changes of final cagelike polymer microspheres under different conditions and the discussion on the thermodynamic parameters of the related reagents, it proves that heptane, water and ethanol are necessary component for the formation of the holes, and a new formation mechanism of the cagelike microspheres is proposed. Furthermore, the primary application of the cagelike polystyrene microspheres as the scaffold for the in situ generated CdS nanoparticles is also investigated.

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Table 1. Recipes for the Preparation of Cagelike SPS Microspheresa

a

sample identification

SPS (g)

water/ethanol/heptane (g/g/g)

swelling time (h)

S1 S2 S3 S4 S5 S6 S7

0.02 0.02 0.02 0.02 0.02 0.02 0.02

0/10/1 10/0/1 5/5/0.1 5/5/0.5 5/5/1 5/5/0.5 5/5/0.5

5 5 5 5 5 1 20

The swelling temperature is 70 °C.

Loading of CdS Nanoparticles onto cagelike SPS Microspheres. Cagelike SPS microspheres (Sample S5, 0.003 g) were first ultrasonic dispersed in ethanol (4 g) or the ethanol solution of triethylolamine (1%, 4 g). The aqueous solution of CdCl2 (2 mL, 0.1 M) was subsequently introduced under magnetic stirring at room temperature. Afterward, the aqueous solution of Na2S2O3 (2 mL, 0.1 M) and isopropanol (0.6 mL) were added. The system was sealed and irradiated in a field of 60Co γ-ray source (located in the University of Science and Technology of China) at a dose rate of 40 Gy/min and a total absorbed dose of 36 kGy. The resultant yellow dispersion was centrifuged, and the product, identified as cagelike CdS/SPS microspheres, was washed with the water/ethanol mixture (1:1, v/v) for three times. Characterization. The morphologies of PS particles, SPS microspheres, cagelike SPS microspheres, and cagelike CdS/SPS microspheres were observed with scanning electron microscope (SEM) (JEOL JSM6700, 5 kV), and transmission electron microscopy (TEM) (Hitachi H7650, 100 kV). Samples were prepared at room temperature by dispersing a small drop of the ethanol dispersion of the sample onto a piece of copper grids and then drying in air. The average diameter (Dn), weight average diameter (Dw), and particle distribution index (PDI) of the microspheres were calculated by the following equations with the diameters of at least 100 particles measured in the SEM and TEM images

EXPERIMENTAL SECTION

Materials. Styrene (St, > 98%, Shanghai Chemical Reagents Corporation, China) was distilled under vacuum before used. 2,2′azobis(isobutyronitrile) (AIBN) was purified by recrystallization in methanol. The reagents including concentrated sulfuric acid (98%), polyvinylpyrrolidone (PVP, K30, 97%), CdCl2·2.5H2O (99%), Na2S2O3·5H2O (99%), isopropyl alcohol (99.7%), n-heptane (97%), triethylolamine (78%), methanol (99.5%), and ethanol (99.5%) were all purchased from Shanghai Chemical Reagents Corporation and used without further purification. Deionized water was used for all experiments. Preparation of Sulfonated Polystyrene (SPS) Microspheres. Synthesis of Polystyrene (PS) Particles through Dispersion Polymerization. St (23 g) and AIBN (0.2 g) were added under ultrasonically stirring in the mixture of ethanol (76 g) and deionized water (3 g) in which PVP (1.5 g) dissolved. The dispersion was degassed by purging nitrogen and then placed in an oil bath at 70 °C. The polymerization was performed under nitrogen atmosphere with vigorous stirring for 20 h. After the polymerization completed, the PS particles were collected via centrifugation at 6000 rpm for 5 min. The sediments were then washed with ethanol. The centrifugation-washing cycle was repeated for three times. The obtained PS particles were dried in vacuum oven for 48 h at 40 °C. Sulfonation of PS Particles. In a typical run for the preparation of sulfonated polystyrene (SPS) microspheres, the powder of the above prepared PS particles (1.5 g) was dispersed ultrasonically in concentrated sulfuric acid (60 mL). The sulfonation reaction was carried out at 40 °C under magnetic stirring for 20 h. Then the system was diluted by deionized water. The products were collected via centrifugation at 6000 rpm, dispersed in ethanol, and then subjected to further centrifugation. This purification cycle was repeated for three times. The final SPS microspheres were obtained after being dried in vacuum oven for 48 h at 40 °C. Preparation of Cagelike SPS Microspheres. The as-prepared SPS microspheres (0.02 g) were dispersed in a mixture of ethanol and water (1:1) under magnetic stirring in a flask. Subsequently, a certain amount of heptane was added. The system was ultrasonic dispersed for 15 min. The flask was equipped with a condenser and immersed into

Dn =

∑ niDi ; ∑ ni

Dw =

∑ niDi4 ∑ niDi3

;

PDI =

Dw Dn

Where ni is the number of particles with a diameter of Di. The specific surface area was analyzed by the nitrogen adsorption at 77.3 K on a Micromeritics Tristar II 3020 M V1.03 analyzer. Samples were degassed at 40 °C overnight under vacuum prior to data collection. Surface area data were collected over 0.005−0.994 p/p0 via the BET (Brunauer−Emmett−Teller) method. The morphology and the size of emulsion droplets were examined by Leica DM1000 optical microscope (OM) equipped with a Lecia EC3 high-resolution digital charge-coupled device (CCD) camera. The interfacial tension between the heptane and water or aqueous solution of ethanol was obtained through Tensiometer K9 Automatic Surface Tensiometer at 70 °C. The atom ratio (atom %) of the element C and S on the surfaces of the SPS microspheres was measured by X-ray photoelectron spectroscopy (XPS, ESCALAB250) with Al Kα excitation (1486.6 eV) in high vacuum (5 × 10−9 Pa). The degree of sulfonation (DS) of SPS microspheres, defined as the mole fraction of sulfonic groups, is calculated by the following eq 1

⎛ atom%S ⎞ ⎟ × 100% DSXPS = 8⎜ ⎝ atom%C ⎠ 15368

(1)

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Figure 1. TEM images of PS (A), SPS (B), and cagelike SPS (C) (Sample 5) microspheres. (Inset: the TEM image of ultrathin cross section of cagelike SPS microspheres). Elemental analysis (EA) performed at VarioELIII Elemental Analysis provided the weight ratio (wt %) of carbon (% C) and sulfur (% S) in 2 mg samples of SPS microspheres. Therefore, DS should be calculated using the following eq 2

DSEA =

⎛ 96 ⎞⎛ wt%S ⎞ ⎟ × 100% ⎜ ⎟⎜ ⎝ 32 ⎠⎝ wt%C ⎠

concluded that SPS microspheres and the used mixed solvent cannot form a Pickering emulsion.

(2)

The UV−visible absorption spectra were recorded on a Shimadzu spectrophotometer (UV-2600). The photoluminescence spectra were collected on Perkin-Elmer fluorescence spectrophotometer (LS55), where an excitation wavelength of 300 nm was used for all the measurements. Single-cycle loading nanoindentation tests were carried out with a Fisher NanoIndentor HM2000S to characterize the nanomechanical properties of the prepared microspheres. A force-controlled trapezoidal waveform was used (indents had a 0.25 mN/s loading rate, a 5 s hold at a maximum force of 5 mN, and a 0.25 mN/s unloading rate).

Figure 2. Optical microscope image of the suspension taken after 5 min of the mixing of SPS microspheres, water, ethanol and heptane ultrasonically. (The recipe of the suspension is the same with that of Sample 5).

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RESULTS AND DISCUSSION Synthesis of the Cagelike SPS Microspheres. To fabricate the cagelike SPS microspheres, seed PS particles with an average diameter of 2.04 μm (PDI = 1.01) were first prepared (Figure 1A). They were sulfonated in concentrated sulfuric acid to be SPS microspheres with a little smaller diameter (1.94 μm, PDI = 1.01) as shown in Figure 1B. The element analysis is utilized to characterize the degree of sulfonation (DS) of these SPS microspheres. The result shows that the weight ratio of C/S is 88.71:1.64, which implies the DSEA is 5.55%. However, the XPS analysis shows the DSXPS on the surface layer of SPS microspheres is up to 23.38% (Supporting information, Figure S1). It can be concluded that the sulfonic groups are mainly distributed on the surface layer of the microspheres. After these SPS microspheres were swollen into a mixed solvent (water/ethanol/heptane) at 70 °C for 5 h, cagelike SPS microspheres with an expanded average diameter of 3.6 μm were obtained (Figure 1C, Sample S5). Figure 1C shows that each single cagelike microsphere contains many pores inside. The BET specific surface area of SPS microspheres with a size of 4 μm is 1.83 m2/g, while that of the SPS cagelike microsphere with the same size is as high as 38.06 m2/g. These results further confirm the multihollow character of the cagelike SPS microspheres. The Formation Mechanism of the Cagelike SPS Microspheres. It is ever reported that heptane could form droplet in the water/ethanol medium and SPS microspheres disperse to the interface between heptane and water/ethanol medium to form a stable Pickering emulsion.12 However, only particles with a similar diameter to the SPS microspheres could be observed in the used ternary solvent (water/ethanol/ heptane =5 g/5 g/1g), as typically shown in Figure 2. No big droplets stabilized by the SPS microspheres are found. It can be

In order to figure out the contribution of each component in the mixed solvent to the formation of the cagelike microsphere, we have carried out a series of analysis in the morphology of final cagelike SPS microspheres under different experiment conditions. The swelling or dissolution behavior of polymer in solvent can be expected by the solubility parameters of the polymer and the solvent. According to the solubility parameter listed in Table 2, ethanol cannot dissolve PS due to their great difference Table 2. The Thermodynamic Parameters of the Reagents reagent Hildebrand δ (cal/cm ) polarity

3 1/2

PS

water

ethanol

heptane

MMA

9.1 nonpolar

23.4 10.2

12.9 4.3

7.4 0.2

8.7

in solubility parameter. Heptane can dissolve PS microspheres at 70 °C because of the similarity in the solubility parameter (Supporting Information, Figure S2). However, we find that the SPS microspheres dispersed in an ethanol/heptane (10 g/1 g) mixture (Sample S1) have no morphological change (Figure 3A1, 3B1), which indicates that the ethanol/heptane mixture cannot dissolve even swell the SPS microspheres at 70 °C. It can be explained with the related solubility parameter. Heptane is miscible with ethanol at temperatures above 30 °C.24 Thus the solubility parameter of ethanol/heptane mixture can be calculated according to the following eq 3 δmix = δ1ϕ1 + δ2ϕ2

(3)

Where δmix is the solubility parameter of mixed solvent and δ1 and δ2 are the solubility parameters of the component 1 and 2 15369

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has been reported in our previous work in which multihollow polystyrene microspheres were prepared via W/O/W emulsion with the help of surfactant OP-10.26 The amphiphilic surfactant OP-10 was not only stabilizing monomer droplets in water but also helping water permeate into droplets due to its hydrophilicity to fabricate the multihollow microspheres after polymerization. We know that in our previous work, cagelike polymer microspheres can be synthesized through a Pickering emulsion polymerization when SPS microspheres were dispersed in water/MMA binary system.10,11 However, we found no evidence of the formation of a Pickering emulsion system in water/ethanol/heptane ternary system. This can be explained when we investigate the solubility parameters listed in Table 2 and the solubility of PS particles in the related solvents shown in Supporting Information, Figure S2. In water/MMA binary system, both MMA and PS particles cannot dissolve in water so that a typical Pickering emulsion forms. In water/ethanol/ heptane ternary system, the interface tension between heptane and the water/ethanol mixture (1/1, w/w) drops rapidly due to the presence of ethanol. Thus, it is difficult to form separated heptane phase in water/ethanol/heptane mixture. On the basis of the above analysis, we can propose a new swelling-osmosis mechanism on the formation of cagelike SPS microspheres. It involves three steps, as illustrated in Scheme 1.

Figure 3. TEM (A) and SEM (B) photographs of microspheres prepared via swelling-osmosis method. A1, B1: Sample S1 (water/ ethanol/heptane = 0 g/10 g/1g). A2, B2: Sample S2 (water/ethanol/ heptane = 10 g/0 g/1 g).

in the mixed solvent, respectively. φ1 and φ2 are volume fractions (or mole fractions, supposedly there is no volume change during the mixing) of the component 1 and 2 in the mixed solvent, respectively. Substituting the data in Table 2 in the eq 3, the solubility parameter of ethanol/heptane mixture is 12.66 (cal/cm3)1/2, much bigger than that of PS (9.1). Therefore, the ethanol/heptane mixture should be the poor solvent for SPS microspheres, just like ethanol. This result implies that water plays an important role in the formation of the multihollow structure. On the other hand, if the SPS microspheres are mixed with water (10 g) and heptane (1 g) (Sample S2), we can observe a strange morphological change of SPS microspheres, that is, a spherical cap of the microspheres is dissolved (Figure 3A2, 3B2). We know that water and heptane are immiscible. They will separate into two phases in the absence of ethanol. However, the SPS microspheres possess certain hydrophilicity since the hydrophilic −SO3H groups are mainly distributed on the surface. Therefore, SPS microspheres readily locate at the interface between water and heptane. As a result, the part in the heptane phase will dissolve gradually, and the other side remaining in water has no change, just like the morphology shown in Figure 3A2 and 3B2. This result gives an obvious evidence that ethanol is also necessary for the formation of multihollow structure of SPS microspheres. Only in the water/ethanol/heptane ternary mixture, cagelike SPS microspheres could be successfully fabricated (Sample S5, Figure 1C). With the help of the surface tensiometer, we find that the interface tension between water and heptane is 34.5 mN/m, while the interface tension between heptane and the water/ethanol mixture (1/1, w/w) rapidly decline to 6.9 mN/ m. It means that the miscibility between heptane and the aqueous solution of ethanol is so good that heptane will not separate from water easily. As a result, SPS microspheres dispersed in aqueous solution of ethanol could be swollen by heptane homogeneously, rather than the case of Sample S2 (Figure 3A2 and 3B2). Once the SPS microspheres are swollen by heptane, water molecules will penetrate into the swollen SPS microspheres under the osmotic pressure produced by the strong hydrophilicity of sulfonic acid groups (−SO3H), as in the case that water molecules can permeate into sulfonated copolymer membranes.25 The similar water osmosis process

Scheme 1. Schematic Illustration of the Formation of Cagelike SPS Microspheres

First, the SPS microspheres are swollen by heptane with the help of ethanol. Meanwhile, water molecules penetrate into the microspheres forced by the strong hydrophilicity of −SO3H groups and aggregate gradually to form aqueous phase inside the SPS microspheres stabilized by the hydrophilic sulfonic acid groups. At last, the volume of SPS microspheres expands, and the cagelike structure is achieved after the solvent is evaporated during the following drying process. On the basis of this mechanism, water, ethanol and heptane are all necessary components for the formation of cagelike structure. Water becomes the real porogen agent with the help of ethanol and heptane. The Effect of Heptane Content, Swelling, and Sulfonation Time on the Morphology of Cagelike SPS Microspheres. Figure 4 shows the TEM and SEM images of cagelike SPS microspheres fabricated at different content of heptane. When the composition of the solvent is 5 g/5 g/0.1 g (water/ethanol/heptane, Sample S3), the average diameter of the cagelike microspheres is 2.7 μm (Figure 4A1), which is a little larger than that of the original SPS microspheres (1.94 μm, Figure 1B). The inner pores are very tiny, and there are no pores located on the surface of the SPS microspheres (Figure 4B1). When the amount of heptane rises to 0.5 g (Sample S4), the diameter of SPS microspheres expands to 3.5 μ, with many large inner pores appearing (Figure 4-A2). It is observed from the SEM image (Figure 4B2) that some peripheral pores of the 15370

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Figure 4. TEM (A) and SEM (B) images and the load−displacement curves (C) of cagelike SPS microspheres fabricated at a composition of water/ ethanol/heptane of 5 g/5 g/0.1 g (A1, B1, C1), 5 g/5 g/0.5 g (A2, B2, C2), 5 g/5 g/1 g (A3, B3, C3).

microspheres readily collapsed under the load so that little recovery could be observed from the unloading curves. Thus, we can only analyze the harness of the microspheres qualitatively from the slope of the loading curve. The wavy loading curve in Figure 4 (C1) indicates the existence of a large amount of fine pores because the load is constant at the site of pore, but the indention depth shows an obvious increase. As a result, a lot of platforms will occur on the loading curve. On the other hand, if the pore is big enough the indenter can penetrate the microspheres easily and contact the support of samples, resulting in an abnormal increase in the slope of the loading curve. Figure 4C2,C3 shows just the cases. It is also noted that the maximum indentation depth decreases with the pore size. In a word, the results of nanomechanical properties also reflect the pore size and structure of the cagelike SPS microspheres. The swelling time of SPS microspheres can affect the swelling degree of the SPS microspheres too. In the case of Sample S6, a small quantity of pores inside the microspheres appears when the swelling time is only 1 h (Figure 5A1,B1). If the swelling time extends to 5 h (Sample S4), we can see the diameter of the microspheres increases a lot from 1.96 to 3.5 μm, the multihollow morphology of the microsphere shows up and there are many peripheral macropores located on the surface of the microspheres (Figure 5A2,B2). When the time is further prolonged to 20 h (Sample S7), the diameter of the microsphere remains but the pore diameter increases while the number of the pores decreases (Figure 5A3,B3). Evidently, with the swelling time prolonging, there is much more time for the water molecules to permeate into the swollen microspheres and

microspheres turn out. If the amount of heptane further increases to 1 g (Sample S5), the diameter of SPS cagelike microspheres does not change apparently, as shown in Figure 4A3,B3. However, the pore size increases and the pore number decreases. The BET analysis (Table 3) shows that the specific Table 3. The Specific Surface Area of Cagelike Microspheres Fabricated in Different Mixed Solvents

a

sample

water/ethanol/heptane (g/g/g)

specific surface area (m2/g)a

S3 S4 S5

5/5/0.1 5/5/0.5 5/5/1

14.06 34.59 38.06

Nitrogen adsorption by BET method.

surface area of Samples S3, S4, and S5 is 14.06 m2/g, 34.58 m2/ g, and 38.06 m2/g, respectively. Obviously, the specific surface area of the cagelike microspheres increases with the content of heptane. According to the proposed swelling-osmosis mechanism, the SPS microspheres can be swollen more rapidly to a greater degree with the increase of heptane. Thus, much more water molecules can permeate into the SPS microspheres to form aqueous phase with larger size, resulting in the enlarged size of both final cagelike SPS microspheres and the pores. The nanomechanical properties of the cagelike SPS microspheres have been measured as shown by the load−displacement curves in Figure 4C1−C3. Generally, the hardness can be obtained from the common Oliver-Pharr method from the unloading curves.27 However, it can be clearly seen that the 15371

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Figure 5. TEM (A) and SEM (B) images of cagelike SPS microspheres synthesized in the mixed solvent with composition of 5 g/5 g/0.5 g (water/ ethanol/heptane) for 1 h (A1, B1), 5 h (A2, B2), and 20 h (A3, B3) at 70 °C.

microspheres through two steps (Scheme 2). First, the nucleation of Cd2+ ions within the microspheres takes place

aggregate into bigger water pools, resulting in the increase of the pore diameter. The effect of the sulfonation time of PS microspheres to the final cagelike microspheres is shown in Figure 6. It is easily

Scheme 2. Schematic Illustration of the Preparation of Cagelike CdS/SPS Composite Microspheres

in solution through ion-exchange with the H+ ions of the sulfonic groups. Cd2+ ions aggregate at the sites of sulfonic acid group that distributes on the skeleton of the cagelike microspheres. The CdS nanoparticles are in situ formed when S2− ions are released through 60Co γ-ray radiation reaction of eaq− and Na2S2O3 in aqueous solution. In this method, CdS nanoparticles are immobilized on the scaffold surface of the cagelike microspheres. Figure 6. TEM (A) and SEM (B) images of cagelike SPS microspheres obtained by swelling SPS seed microspheres in water/ ethanol/heptane mixture (5 g/5 g/0.5 g) for 5 h at 70 °C. The sulfonation time of the original PS microspheres are 4 h for (A1, B1) and 20 h for (A2, B2).

H 2O ∼ ∼ → eaq − , •H, •OH, H3O+ , etc

(4)

2eaq − + S2 O32 − → S2 −

(5)

S2 − + Cd2 + → CdS

(6)

Figure 7A1,B1 shows clearly that CdS particles with a size of 150 nm are loaded onto the skeleton of the cagelike microspheres. The load capacity is 18.1 wt %, as calculated from the TGA curve listed in Supporting Information, Figure S3. When triethylolamine is added to protect CdS nanoparticles from aggregation, cagelike SPS microspheres are relatively uniformly decorated with CdS nanoparticles as shown in Figure 7A2,B2. At the same time, the average diameter of CdS nanoparticles decrease to 35 nm, and the load capacity is 12.0 wt % (see Supporting Information, Figure S3). It can be explained that when triethylolamine is added, the pH of the system will increase from 5−6 to 7−8. −SO3H groups will become −SO3−, which improves the absorption efficiency of Cd2+ due to the strong electrostatic interaction. As a consequence, the size of CdS nanoparticles decreases at the same feed ratio. The optical properties of the prepared cagelike CdS/SPS microspheres were characterized. The UV absorption of pure

understood that the amount of sulfonic acid groups will increase with the sulfonation time. According to the formation mechanism illustrated in Scheme 1, more sulfonic acid groups will be helpful to stabilize the inner water phase. The results shown in Figure 6 are in accord with the mechanism. The longer the sulfonation time, the more and the bigger pores formed in the SPS microspheres. Loading of CdS Nanoparticles on the Cagelike SPS Microspheres. Usually, immobilization of noble metal nanoparticles on the polymeric substrate can be achieved either with ion-exchange followed by reduction or coordination between the pendent ligands with the metal precursor. In addition, it is demonstrated that many kinds of metal ions can be reduced by γ-ray radiation in water and even S2O32− can be reduced into S2− by eaq− in this way.28 Because the surface of cagelike SPS microspheres contains the −SO3H group, it is expected that CdS nanoparticles can be loaded on the skeleton of the 15372

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CONCLUSION Cagelike sulfonated polystyrene microspheres with diameter of several micrometers are fabricated efficiently via a swellingosmosis process of the sulfonated polystyrene microspheres in the ternary mix solvent of water/ethanol/heptane. On the basis of analysis on the thermodynamic parameters of the related reagents and the morphological change of final cagelike polymer microspheres with different experiment conditions, the formation mechanism of cagelike SPS microspheres is proposed. The whole porogen procedure involves three steps: swelling by heptane, osmosis process of water molecules, the formation of aqueous phase inside the swollen SPS microspheres. Heptane plays the role of swelling agent with the help of ethanol to create the access for water to permeate into the SPS microspheres. Water is the real porogen agent. These prepared cagelike SPS microspheres are further served as scaffold for the in situ generated CdS nanoparticles. The cagelike CdS/SPS microspheres have good fluorescence performance. These controllable cagelike CdS/SPS microspheres are expected to have potential applications in controlled-release nanocarriers, smart optic, and even its stereoscopic morphology could render them be used as biomaterials for tissue engineering.

Figure 7. TEM images of cagelike CdS/SPS composite microspheres prepared (A) at the absence of triethylolamine, and (B) at the presence of triethylolamine. (A1, A2 at low magnification (×20 k) and B1, B2 at high magnifications (×50 k). The scaffold cagelike SPS microspheres were Sample S5).

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SPS microspheres is below 300 nm as displayed in Figure 8A, which is in accord with the previous report.29 When the SPS microspheres have been loaded with CdS particles with a size of 35 nm, a maximum UV absorbance appears at 412 nm. As the size of CdS particles increases to 150 nm, the maximum UV absorbance shifts to 440 nm. It is known that the absorption peak for bulk CdS is 515 nm.30 The blue shift of the absorption peak should be the effect induced by the decrease of the size of the loaded CdS. Obviously, the SPS microspheres will not affect the UV absorption behavior of the loaded CdS particles. The photoluminescence spectrum of CdS-loaded SPS microspheres is shown in Figure 8B. After the samples were excited by the light of 300 nm, a broad emission band is observed around 350 nm for SPS cagelike microspheres. After the SPS cagelike microspheres are loaded with CdS particles with a size of 150 nm, a new maximum emission peak occurs at 465 nm and shifts to 445 nm when the size of CdS particles decrease to 35 nm. At the same time, the intensity of the emission of the SPS microspheres becomes much weaker in the CdS-loaded SPS microspheres. All the above results show that the cagelike CdS/ SPS microspheres can remain the good fluorescence performance of the loaded CdS particles.

ASSOCIATED CONTENT

S Supporting Information *

The element distribution on the surface of SPS microspheres, the solubility of PS microspheres in different solvents, and the TGA measurement of cagelike CdS-loaded SPS microspheres. 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]. Tel: +86-551-63600843. Fax: +86-551-63601592. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 51073146, 51103143, and 51173175), the Fundamental Research Funds for the Central Universities (WK2060200005, 2010) and Specialized Research

Figure 8. UV−vis absorption spectrum of cagelike CdS/SPS microspheres (A) and fluorescence spectrum of with an excitation wavelength of 300 nm (B). 15373

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Fund for the Doctoral Program of Higher Education (No. 20093402110021).

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