17156
J. Phys. Chem. C 2008, 112, 17156–17160
A Facile Method for the Fabrication of Thiol-Functionalized Hollow Silica Spheres Junjie Yuan,* Decheng Wan, and Zhenglong Yang Institute of Functional Polymers, School of Materials Science and Engineering, Tongji UniVersity, Shanghai, 200092, People’s Republic of China ReceiVed: July 7, 2008; ReVised Manuscript ReceiVed: August 27, 2008
In this paper, we adopt a facile method to prepare a new type of hollow silica spheres with thiol functional groups. Thiol-functionalized silica shells were coated on positively charged polystyrene particles by hydrolysis and condensation of 3-mercaptopropyltriethoxysilane (MPTS); the polystyrene cores were dissolved in the same medium to form monodispersed thiol-functionalized hollow silica spheres. Neither additional dissolution nor a calcination process was needed to remove the polystyrene cores. Transmission electron microscopy, scanning electron microscopy, and Fourier transform infrared analysis and porosity measurements were used to characterize the monodispersed thiol-functionalized hollow silica spheres. TABLE 1: Recipe of the Samples
Introduction In recent years, hollow silica spheres have found wide variety of applications in chromatography, shield for enzymes or proteins, delivery vehicles of drugs, dyes, inks, photonic crystals, artificial cells, waste removal, and large biomolecular release systems.1-7 The reasons behind are their high chemical and thermal stability, large surface areas, low density, low toxicity, and good compatibilities with other materials. Many research groups8-22 have reported the methods for the fabrication of hollow silica spheres of various sizes. Organic or inorganic templates were usually employed in the processes for preparing hollow silica spheres. The hollow silica spheres were formed after these templates were removed by either calcinations or chemical etching. In addition, hollow silica spheres were also synthesized through self-templating method.23 Although quite a lot of preparation methods were developed up to now, the obtained hollow silica were pretty monotonous, i.e., the functional groups on the hollow silica spheres surface was almost hydroxyl only. It would no doubt restrict the applications of hollow silica spheres in specific fields such as catalysis, removal of heavy metals ions in waters and so on. On the basis of our best knowledge, there are very few publications reporting the preparation of nonhydroxyl organic group functionalization of hollow silica spheres. Hah et al.23 and Wang et al.24 adopted a two-step method without templates to synthesize monodispersed phenyl-functionalized hollow silica spheres, and the obtained hollow silica spheres were able to dissolve in organic solvents including acetone, chloroform, tetrahydrofuran, etc. This advantage of the phenyl-functionalized hollow silica spheres is that they can be used in fields including delivery systems, nanoreactors, coating technologoy, catalysis, and so forth. Regardless of the hollow silica spheres, the various terminally bound organic groups (e.g., vinyl, mercaptopropyl, aminopropyl) functionalized silica materials such as silica particles or silica mesoporous materials have been studied for quite a long time and applied in many areas. For instance, mesoporous vinyl silica was used for the immobilization of penicillin acylase, which showed good initial enzymatic activity for the hydrolysis of * To whom correspondence should be addressed. E-mail: yuanjunjie@ mail.tongji.edu.cn. Phone/Fax: +86 021 65982461.
samples no.
positively charged PS (g)
ethanol (ml)
ammonia (ml)
MPTS (g)
1 2 3 4 5 6
5.0 5.0 5.0 5.0 5.0 5.0
40.0 40.0 40.0 40.0 40.0 40.0
1.0 3.0 5.0 5.0 5.0 7.0
2.0 2.0 2.0 1.0 3.0 2.0
penicillin G.25,26 Self-assembled monolayers of 3-mercaptopropyltriethoxysilane (MPTS) on oxidized silicon had been the subject for metal corrosion protection.27 Thiol-functionalized ordered mesoporous silica exhibited a highly selective adsorption capability for noble metal ions such as Pd2+ and Pt2+.28 For example, Molna´r et al.29,30 first synthesized mesoporous silica host (MCM-41, HMS, and SBS-15) and then grafted sulfonic groups on the mesoporous silica using chlorosulfonic acid, and then obtained functionalized materials exhibited higher acid catalysis activation and selectivity. Generally, incorporation of organic functional groups into these silica materials is prepared by postgrafting or co-condensation route;30,31 however, the amount of the introduced functionalized groups on the surfaces of these materials is very limited by the above-mentioned methods, the silica constituents the main body of the obtained functionalized silica materials only with small quantity of functional groups in these materials. As a matter of fact, the application properties of functionalized silica materials largely depends on the density of functional groups. For example, the capacity of heavy metal removal in wastewater treatment is proportional stochemically to the amount of the specific functional groups of the silica materials.32 In the same way, the catalytic efficiency is also dependent on the number of catalytic site, i.e., the number of functional groups.33 In this paper, we adopt a facile method to synthesize thiolfunctionalized hollow silica spheres via a “one-step” process, the obtained thiol-functionalized hollow silica spheres obtained have large numbers of thiol groups. Theoretically, the content of the thiol group is about 7.87mmol/g. The thiol-functionalized hollow silica spheres or derivatives can be used in extensive areas, such as removal of heavy metals ions,34-36 catalysis,37-42 proton conductivity membranes,43,44 templates for the preparation of hollow metal oxide spheres,45 and so on.
10.1021/jp805954r CCC: $40.75 2008 American Chemical Society Published on Web 10/11/2008
Thiol-Functionalized Hollow Silica Spheres
J. Phys. Chem. C, Vol. 112, No. 44, 2008 17157
Figure 2. SEM images of the PS spheres (a); thiol-functionalized hollow silica spheres with different additional amounts of ammonia of 1.0 (b), 3.0 (c), and 5.0 mL (d).
Figure 1. TEM images of the PS spheres (a); thiol-functionalized hollow silica spheres with different additional amounts of ammonia of 1.0 (b), 3.0 (c), and 5.0 mL (d).
2. Experimental Section 2.1. Materials. Styrene (St) was purchased from Shanghai Chemical Reagent Co. (China). It was distilled in a vacuum to remove the inhibitor and stored at 4 °C until use. R,R′Azodiisobutyramidine dihydrochloride (AIBA) was purchased from Sigma-Aldrich. MPTS, absolute ethanol, polyvinylpyrrolidone (PVP, Mw ) 40000) and aqueous ammonia solution (28 wt %) were purchased from Shanghai Chemical Reagent Co. (China) and used as received. Deionized water was prepared in our laboratory. 2.2. Synthesis of Polystyrene (PS) Template Particles. The monodispersed PS particles were prepared by emulsifier-free emulsion polymerization as follows: 3.0 g of St, 1.5 g of PVP, 0.39 g of AIBA, and 100.0 g of H2O were charged into a 250mL three-neck flask equipped with a mechanical stirrer, a thermometer with a temperature controller, a N2 inlet, a Graham condenser, and a heating mantle. The reaction solution was deoxygenated at the begin by bubbling nitrogen gas at room temperature for 60 min. Then, the reaction was carried out at 70 °C for 24 h under a constant stirring at 100 rpm. 2.3. Synthesis of Thiol-Functionalized Hollow Silica Spheres. The obtained PS suspension was first dialyzed in ethanol using a cellulose membrane in order to remove the undesirable water. After 40.0 mL of ethanol was added into 5.0 g of PS suspension, a certain aqueous ammonia solution was dropped into the mixture and stirred at 100 rpm for 5 min. MPTS was added quickly and the mixture was reacted at 50 °C for around 3 h with constant stirring. The thiol-functionalized hollow silica spheres were prepared. The recipe of the samples was listed in Table 1. 2.4. Characterization. 2.4.1. Transmission Elecron Microscopy (TEM) ObserWation. A transmission electron microscope (TEM; Hitachi H-600, Hitachi Corp.) was used to observe the morphologies of the obtained spheres. The dispersions were diluted with ethanol and ultrasonicated at 25 °C for 5 min and then dried onto carbon-coated copper grids before examination.
Figure 3. TEM images of thiol-functionalized hollow silica spheres with different additional amounts of MPTS of 1.0 g (a) and 3.0 g (b).
2.4.2. Scanning Electron Microscopy (SEM) ObserWation. The morphologies of thiol-functionalized hollow silica spheres were further characterized by using a scanning electron microscope (SEM, Quanta 200 FEG). The dispersions of the obtained thiol-functionalized hollow silica spheres were diluted using ethanol and dried on a cover glass slide and sputter-coated with gold prior to examination. 2.4.3. FT-IR Analysis. FT-IR analyses of the thiol-functionalized hollow silica spheres were carried out by a Magna-IRTM 550 spectrometer (NEQUINOXSS/HYPERION2000). The scan wavenumber was in the range of 400-4000 cm-1. The samples were prepared by the usual KBr pellet method. 2.4.4. Porosity Measurements. Nitrogen sorption curves, pore size distributions, and surface areas were obtained using a Micromeritics Tristar 3000 surface area and porosity analyzer. Before measurement, the samples were degassed at 90 °C under vacuum for 8 h. The pore sizes were calculated from desorption isotherm curves using the Brunauer-Joyner-Halenda (BJH) method. 3. Results and Discussion 3.1. Effect of the Amount of Ammonia. The monodispersed positively charged PS particles were first prepared by emulsifierfree emulsion polymerization on the basis of the procedures described in the experimental section. The TEM image of the obtained PS particles was shown in Figure 1a, it can be seen
17158 J. Phys. Chem. C, Vol. 112, No. 44, 2008
Yuan et al.
Figure 4. The FT-IR spectrum of thiol-functionalized hollow silica spheres (sample 2).
Figure 5. Nitrogen sorption isotherm of the samples synthesized with different amounts of NH3 · H2O and 2.0 g of MPTS. The inset showed the pore size distribution of the corresponding sample from desorption branch. (a) 30.0 mL of NH3 · H2O (sample 2); (b) 5.0 mL of NH3 · H2O (sample 3); (c) 7.0 mL of NH3 · H2O (sample 6).
that uniform spherical PS particles with about 120nm average diameter were obtained. Subsequently, 2.0 g of MPTS and variable amount of ammonia were added into 5.0 g of PS suspension for the sol-gel reaction. The core-shell composite particles with thiol-functionalized silica shell were formed when 1.0 mL of ammonia was added into the system, as shown in Figure 1b. To our surprise, thiol-functionalized hollow silica spheres were formed when 3.0 mL of ammonia were added into the PS suspension, although there still had some incompletely dissolved PS on the shell of thiol-functionalized hollow silica sphere, as indicated by Figure 1c. The results indicated the PS particle core was dissolved in the system when the amounts of
ammonia increased a certain concentration.17,18 Figure 1d showed that the better morphological thiol-functionalized hollow silica spheres were obtained when the amounts of ammonia increased to 5.0 mL, it can be inferred that PS particles cores were removed more completely at higher ammonia concentration. Figure 2 further displayed the SEM images of the PS particles and thiol-functionalized hollow silica spheres prepared with the ammonia amounts of 1.0, 3.0, and 5.0 mL, respectively. It was found that the PS particles had good spherical shape and polydispersity as shown in Figure 2a. However, the obtained core-shell composite particles (Figure 2c) and thiol-functionalized hollow silica spheres (parts c and d of Figure 2) exhibited
Thiol-Functionalized Hollow Silica Spheres
J. Phys. Chem. C, Vol. 112, No. 44, 2008 17159
TABLE 2: Surface Areas of the Thiol-Functionalized Hollow Silica Spheres with Different Amounts of NH3 · H2O sample no.
NH3 · H2O (mL)
Brunauer-Emmett-Teller area (m2/g)
Langmuir area (m2/g)
2 3 6
3.0 5.0 7.0
19.8 23.3 23.6
28.7 32.3 34.4
irregular spherical shape and rough surface, it might be caused by the nonuniformity of reaction. Furthermore, the particle sizes of these samples were bigger than that of PS template particles distinctively; it also suggested MPTS encapsulated onto the surface of PS particles successfully. 3.2. Effect of the Addition of MPTS. Figure 3 and Figure 1d showed the TEM images of the obtained thiol-functionalized hollow silica spheres with 1.0, 3.0, and 2.0 g of MPTS, respectively, with ammonia amount fixed at 5.0 mL. It can be seen that the shell thickness of thiol-functionalized hollow silica spheres increased as the amounts of MPTS increased. There appeared many nanoparticles in the TEM images when the amount of MPTS was 3.0 g as shown in Figure 3b; it was caused by the self-condensation of MPTS. 3.3. FT-IR Spectra Analysis. To confirm the presence of thiol groups on the functionalized hollow silica spheres, FT-IR spectrum was obtained from the obtained hollow spheres. The results were shown in Figure 4. The weakly basic characteristic absorption peak at 2553 cm-1 can be assigned to S-H stretching.46 There exhibited a peak at 3436 cm-1, which was attributed to the -OH stretching vibration from hollow silica spheres, ethanol, and water. The adsorption peaks at around 1129 and 1029 cm-1 were due to the -SisOsSi stretching from the hydrolysis and condensation of -SisOC2H5 of MPTS. These characteristic absorption peaks indicated thiol-functionalized hollow silica spheres were successfully prepared by this facile method. At the same time, a distinctive CdC stretching peak was displayed at about 1603 cm-1; the peaks at 758 and 698 cm-1 attributed to -CsH bending vibration of monosubstituted phenyl; these characteristic peaks belonged to PS. It illustrated a small quantity of PS remained in thiol-functionalized hollow silica spheres.47 3.4. Porosity and Pore Size. Figure 5 gave a sorption isotherm of thiol-functionalized hollow silica spheres with different amounts of ammonia. When relative pressure P/P0 was less than 0.8, the slope of the curves was very small; this illustrated very little amount of small size pores existed on the surface of thiol-functionalized hollow silica spheres. When relative pressure P/P0 was more than 0.8, the slope of the curves increased sharply. A slight separation of the adsorption isotherms and desorption isotherms of all the samples was also observed, which should be the evidence of a small quantity of micropores existed on shell of the thiol-functionalized hollow silica spheres.48 The pore size distributions of the obtained thiolfunctionalized hollow silica spheres were measured by the nitrogen sorption method and calculated by the BJH method from the desorption curves. It can be seen that the pore size distributions of all samples were very broad in the range from 2 to 40 nm as shown in Figure 5. The proportion of the large pore sizes increased with the amounts of ammonia increased. The surface areas of the thiol-functionalized hollow silica spheres were listed in Table 2. It was found that the surface areas of thiol-functionalized hollow silica spheres slightly increased as the amounts of ammonia increased, which should be a result of the porous structure of the silica shells. This could be the reason why the specific surface area of the thiol-
Figure 6. The schematic diagram of the formation mechanism of the thiol-functionalized core-shell and hollow silica spheres at different concentration of ammonia.
functionalized hollow silica spheres did not change distinctively with the increasing ammonia amounts. 3.5. Formation Mechanism. Figure 6 displayed the possible formation mechanism of the thiol-functionalized hollow silica spheres. On the basis of analysis of the experimental results, MPTS coated on the positive charged PS particles or formed thiol-functionalized hollow silica spheres both attributed to the combination with the interaction of the positive and negative charge and the hydrolysis and condensation of MPTS. The amount of ammonia is the key factor for the formation of thiolfunctionalized hollow silica spheres. When the ammonia concentration was low in the system, PS could not be “dissolved”, so core-shell composite particles formed; when the ammonia concentration increased to a certain threshold, thiolfunctionalized hollow silica spheres were formed being of PS “dissolving” and permeation out from the core of the composite particles. Conclusions In this study, a facile method for the fabrication of monodispersed thiol-functionalized hollow silica spheres based on the positive charged PS template particles and the sol-gel process has been proposed, in which monodispersed, small PS particles were first prepared by emulsifier free emulsion polymerization using PVP as the stabilizer, and then the thiolfunctionalized silica were coated on the PS particles via ammonia catalyzed, hydrolysis, and condensation of MPTS. The PS particles were dissolved during the hollow spheres formation process. The obtained thiol-functionalized hollow silica spheres or it is derivative (sulfonic groups functionalized hollow silica spheres by oxidization with hydrogen peroxide), for instance, can find application in removal of heavy metals ions, catalysis, proton exchanged membranes, and so on. This facile method also could be used to prepare other functionalized hollow silica spheres bearing various organic groups (e.g., -CN, -CHdCH2, -SCN, -NH2, etc.) or different particle sizes (e.g., from ca. 20 nm to micrometers) can also be prepared. Acknowledgment. We were grateful to Program for Young Excellent Talents in Tongji University (2006KJ049) for financial support for this research. The project is also sponsored by National Natural Science Foundation of China (50803046) and International Cooperation Foundation for Science & Technology of Shanghai (065207064). References and Notes (1) Li, Z. Z.; Wen, L. X.; Shao, L.; Chen, J. F. J. J. Controlled Release 2004, 98, 245–254. (2) Chen, J. F.; Ding, H. M.; Wang, J. X.; Shao, L. Biomaterials 2004, 25, 723–727.
17160 J. Phys. Chem. C, Vol. 112, No. 44, 2008 (3) Sharma, R. K.; Das, S.; Maitra, A. J. Colloid Interf. Sci. 2005, 284, 358–361. (4) Wang, J.; Ding, H.; Tao, X.; Chen, J. Nanotechnology 2007, 18, 245705. (5) Zhu, Y. F.; Shi, J. L.; Chen, H. R.; Shen, W. H.; Dong, X. P. Microporous Mesoporous Mater. 2005, 84 (1-3), 218–222. (6) Li, Z. Z.; Xu, S. A.; Wen, L. X.; Liu, F.; Liu, A. Q.; Wang, Q.; Sun, H. Y.; Yu, W.; Chen, J. F. J. Controlled Release 2006, 111 (1-2), 81–88. (7) Zhou, J.; Wu, W.; Caruntu, D.; Yu, M. H.; Martin, A.; Chen, J. F.; O’Connor, C. J.; Zhou, W. L. J. Phys. Chem. C 2007, 111 (47), 17473– 17477. (8) Van Bommel, K. J. C.; Jung, J. H.; Shinkai, S. AdV. Mater. 2001, 13 (19), 1472–1476. (9) Fan, W. G.; Gao, L. J. Colloid Interf. Sci. 2006, 297 (1), 157–160. (10) Yeh, Y. Q.; Chen, B. C.; Lin, H. P.; Tang, C. Y. Langmuir 2006, 22 (1), 6–9. (11) Zhu, G. S.; Qiu, S. L.; Terasaki, O.; Wei, Y. J. Am. Chem. Soc. 2001, 123 (31), 7723–7724. (12) Caruso, F. AdV. Mater. 2001, 13, 11–22. (13) Tissot, I.; Reymond, J. P.; Lefebvre, F.; Bourgeat-Lami, E. Chem. Mater. 2002, 14, 1325–1331. (14) Liu, S. Q.; Lu, L. C.; Sui, X. Y.; Vera, M.; Pegie, C.; Etienne, F. V. Microporous Mesoporous Mater. 2007, 98, 41–46. (15) Chen, Y. W.; Kang, E. T.; Neoh, K. G.; Grener, A. AdV. Funct. Mater. 2005, 15 (1), 113–117. (16) Darbandi, M.; Thomann, R.; Nann, T. Chem. Mater. 2007, 19 (7), 1700–1703. (17) Deng, Z. W.; Chen, M.; Zhou, S.; You, B.; Wu, L. M. Langmuir 2006, 22, 6403–6407. (18) Chen, M.; Wu, L.; Zhou, S.; You, B. AdV. Mater. 2006, 18, 801– 805. (19) Tsai, M. S.; Li, M. J. J. Non-Cryst. Solids. 2006, 352 (26-27), 2829–2833. (20) Botterhuis, N. E.; Sun, Q. Y.; Magusin, P. C. M. M.; Van Santen, R. A.; Sommerdijk, N. A. Chem-Eur. J. 2006, 12 (5), 1448–1456. (21) Han, Y. S.; Jeong, G. Y.; Lee, S. Y.; Kim, H. K. J. Solid State Chem. 2007, 180 (10), 2978–2985. (22) Wan, Y.; Yu, S. H. J. Phys. Chem. C. 2008, 112 (10), 3641–3647. (23) Hah, J. S.; Kim, B. J.; Jeon, S. M.; Koo, Y. E. Chem. Commun. 2003, 14, 1712–1713. (24) Wang, Q. B.; Liu, Y.; Yan, H. Chem. Commun. 2007, (23), 2339– 2341. (25) Chong, A. S. M.; Zhao, X. S. Appl. Surf. Sci. 2004, 237, 398.
Yuan et al. (26) Chong, A. S. M.; Zhao, X. S. Catal. Today 2004, 93-95, 293. (27) Li, Y. S.; Wang, Y.; Tran, T.; Perkins, A. Spectrochim. Acta A 2005, 61, 3032. (28) Kang, T.; Park, Y. G.; Yi, J. H. Ind. Eng. Chem. Res. 2004, 43, 1478–1484. ´ .; Forgo, P.; Mohai, M.; Berto´ti, I. J. Mol. Catal. (29) Ra´c, B.; Molna´r, A A: Chem. 2005, 244, 46–57. ´ . Appl. Catal. A 2006, (30) Ra´c, B.; Hegyes, P.; Forgo, P.; Molna´r, A 299, 193–201. (31) Liao, J. F.; Wu, Q. Z.; Y., Q.; Wang, C. T.; Li, H. Y.; Li, Y. G. Acta Chim. 2006, 64 (24), 2419–2424. (32) Bibby, A.; Mercler, L. Chem. Mater. 2002, 14, 1591–1597. (33) Schroden, R. C.; Al-Daous, M.; Sokolov, S.; Meld, B. J.; Lytle, J. C. J. Mater. Chem. 2002, 12, 3261–3267. (34) Shin, S.; Jang, J. Chem. Commun. 2007, 41, 4230–4232. (35) Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Liu, J.; Kemner, K. M. Science. 1997, 276, 923. (36) Nam, K. H.; Gomez-Salazar, S.; Talavrides, L. L. Ind. Eng. Chem. Res. 2003, 42, 1955–1964. (37) Das, D.; Lee, J.; Cheng, S. J. Catal. 2004, 223 (1), 152–160. (38) Cano-Serrano, E.; Campos-Martin, J. M.; Fierro, J. L. G. Chem. Commun. 2003, 2, 246–247. (39) Dı´az, I.; Ma´rquez-Alvarez, C.; Mohino, F.; Pe´rez-Pariente, J.; Sastre, E. J. Catal. 2000, 193, 283–294. (40) Dı´az, I.; Ma´rquez-Alvarez, C.; Mohino, F.; Pe´rez-Pariente, J.; Sastre, E. J. Catal. 2000, 193, 295–302. (41) Yang, Q.; Kapoor, M. P.; Shirokura, N.; Ohashi, M.; Inagaki, S.; Kondo, J. N.; Domen, K. J. Mater. Chem. 2005, 15, 666–673. (42) Bossaert, W. D.; De Vos, D. E.; Van Rhijn, W. M.; Bullen, J.; Grobet, P. J.; Jacobs, P. A. J. J. Catal. 1999, 182, 156–164. (43) Liu, Y. L.; Hsu, C. Y.; Su, Y. H.; Lai, J. Y. Biomacromolecules 2005, 6, 368–373. (44) Sua, Y. H.; Wei, T. Y.; Hsu, C. H.; Liu, Y. L.; Sun, Y. M.; Lai, J. Y. Desalination 2006, 200, 656–657. (45) Deng, Z. Y.; Chen, M.; Gu, G. X.; Wu, L. M. J. Phys. Chem. B 2008, 112, 16–22. (46) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy III; Academic Press: San Diego, 1990; pp 360375. (47) Tang, J. C.; Li, G. W.; Zhang, R. F.; Shen, J. C. J. Mater. Chem. 2003, 13, 232–234. (48) Fan, Y. G.; Li, Y. H.; Ma, J. B. Chem. J. Chin. 2002, 8, 1622–1626.
JP805954R
