Langmuir 2009, 25, 1923-1926
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Degradable Nanogels as a Nanoreactor for Growing Silica Colloids Yong-Yong Li, Jie Yang, Wei-Bing Wu, Xian-Zheng Zhang,* and Ren-Xi Zhuo Key Laboratory of Biomedical Polymers of Ministry of Education and Department of Chemistry, Wuhan UniVersity, Wuhan 430072, PR China ReceiVed NoVember 25, 2008. ReVised Manuscript ReceiVed January 5, 2009 Degradable nanogels with cleavable disulfide bonds were designed and used as a catalytic template, providing an alkali microenvironment. Well-defined hybrid silica colloids could be obtained by hydrolyzing tetraethyl orthosilicate (TEOS) in the nanogels. The size of silica colloids was found to be dependent on the size of the nanogels. After the removal of nanogels through reduction with 1,4-dithiothreitol (DTT), mesoporous silica colloids with a rough surface were obtained. The mesoporous structure of the colloids after reduction was characterized by transmission electron microscopy (TEM), surface area analysis, and X-ray diffraction (XRD). This work also provides an effective route for the preparation of mesoporous silica nanostructures, which may find wide applications as catalyst templates and drug carriers.
1. Introduction Biological systems employ molecular self-assembly as a ubiquitous method of creating very complex and functionally efficient architectures for establishing biochemical networks.1 In these networks, chemical reactions usually occur in a strictly confined space with high efficiency.2 These reactions are usually realized by using well-defined reaction environments that vary from relatively simple nanometer-sized systems, such as enzymes,3 to micrometer-sized complex assemblies, such as cells.4 Inspired by biological counterparts, there is growing interest in designing various nanoreactors from the notion of “green chemistry” as well as the expectation of increasing the efficiency of chemical conversions from the laboratory bench to industrial scale.5 Much work has been reported recently in these fields, and various types of nanoreactors have been proposed, such as nanocapsules based on micellar6 and vesicular7 assemblies that are built from low-molecular-weight molecules and macromolecular blocks as well as the use of biomacromolecules (i.e., viruses).8 Nanogels with internally cross-linked polymer chains have attracted much attention because of their potential application * Corresponding author. Tel and Fax: + 86 27 6875 4509. E-mail address:
[email protected]. (1) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (2) Vriezema, D. M.; Aragones, M. C.; Elemans, J. A. A. W.; Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M. Chem. ReV. 2005, 105, 1445. (3) (a) Gorenstein, D. G. Chem. ReV. 1987, 87, 1047. (b) Kirby, A. J. Angew. Chem., Int. Ed. 1996, 35, 706. (4) Bolinger, P. Y.; Stamou, D.; Vogel, H. Angew. Chem., Int. Ed. 2008, 47, 5544. (5) Dwars, T.; Paetzold, E.; Oehme, G. Angew. Chem., Int. Ed. 2005, 44, 7174. (6) (a) Wen, F.; Zhang, W. Q.; Wei, G. W.; Wang, Y.; Zhang, J. Z.; Zhang, M. C.; Shi, L. Q. Chem. Mater. 2008, 20, 2144. (b) Park, C.; Rhue, M.; Lim, J.; Kim, C. Macromol. Res. 2007, 15, 39. (c) Kim, D.; Kim, J. Y.; Kim, E. R.; Sohn, D. Mol. Crst. Liq. Crst. 2002, 377, 345. (d) Oehme, G.; Grassert, I.; Paetzold, E.; Fuhrmann, H.; Dwars, T.; Schmidt, U.; Iovel, I. Kinet. Catal. 2003, 44, 766. (e) Fuji, K.; Morimoto, T.; Tsutsumi, K.; Kakiuchi, K. Angew. Chem., Int. Ed. 2003, 42, 2409. (f) Manabe, K.; Mori, Y.; Wakabyashi, T.; Nagayama, S.; Kobayashi, S. J. Am. Chem. Soc. 2000, 122, 7202. (7) (a) Gebicki, J. M.; Hicks, M. Nature 1973, 243, 232. (b) Klijn, J. E.; Engberts, J. B. F. N. J. Am. Chem. Soc. 2003, 125, 1825. (c) Schenning, A. P. H. J.; Spelberg, J. H. L.; Hubert, D. H. W.; Feiters, M. C.; Nolte, R. J. M. Chem.sEur. J. 1998, 4, 871. (d) Goedheijt, M. S.; Hansen, B. E.; Reek, J. N. H.; Kamer, P. C. J.; Van Leeuwen, P. W. N. M. J. Am. Chem. Soc. 2000, 122, 1650. (e) Treyer, M.; Walde, P.; Oberholzer, T. Langmuir 2002, 18, 1043.
in various fields, including drug delivery9 and encapsulation of metals10 and enzymes.11 Nanogels have been prepared by traditional methods via microemulsion,12 inverse miniemulsion,13 and self-assembly of biopolymers.14 Although some effort has been expended on the exploration of other applications of hydrogels (e.g., some colloidal nanocomposites or nanoporous silica was obtained by using templates of nanogels or bulk hydrogels, respectively15-17), the use of nanogels as nanoreactors has been relatively undeveloped. In this study, we describe a novel method of synthesizing degradable nanogels via the Michael addition reaction of triethylenetetramine (TETA) or tetraethylenepentamine (TEPA) to N,N′-bis(acryloyl)cystamine (BAC). Well-defined nanogels that comprise cleavable S-S bonds and amine groups in their network were fabricated. The disulfide bonds could be degraded rapidly by reducing with DTT, and the amine groups would offer a basic microenvironment in the confined space of the nanogels. Then, the amine-catalyzed hydrolysis of tetraethyl orthosilicate (TEOS) would happen in the confined space of the nanogels; subsequently, monodisperse silica particles could be prepared. Because of the inherent cleavable property of disulfide bonds (8) (a) De La Escosura, A.; Verwegen, M.; Sikkema, F. D.; Comellas-Aragones, M.; Kirilyuk, A.; Rasing, T.; Nolte, R. J. M.; Cornelissen, J. J. L. M. Chem. Commun. 2008, 1542. (b) Fujikawa, S.; Kunitake, T. Langmuir 2003, 19, 6545. (b) Fowler, C. E.; Shenton, W.; Stubbs, G.; Mann, S. AdV. Mater. 2001, 13, 1266. (c) Whaley, S. R.; English, D. S.; Hu, E. L.; Barbara, P. F.; Belcher, A. M. Nature 2000, 405, 665. (d) Lee, S. W.; Mao, C. B.; Flynn, C. E.; Belcher, A. M. Science 2002, 296, 892. (e) Mao, C. B.; Flynn, C. E.; Hayhurst, A.; Sweeney, R.; Qi, J. F.; Georgiou, G.; Iverson, B.; Belcher, A. M. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 6946. (f) Wang, Q.; Chan, T. R.; Hilgraf, R.; Fokin, V. V.; Sharpless, K. B.; Finn, M. G. J. Am. Chem. Soc. 2003, 125, 3192. (9) (a) Van Thienen, T. G.; Raemdonck, K.; Demeester, J.; De Smedt, S. C. Langmuir 2007, 23, 9794. (b) Shin, Y. S.; Chang, J. H.; Liu, J.; Williford, R.; Shin, Y. K.; Exarhos, G. J. J. Controlled Release 2001, 73, 1. (c) Kohli, E.; Han, H. Y.; Zeman, A. D.; Vinogradov, S. V. J. Controlled Release 2007, 121, 19. (10) Terashima, T.; Kamigaito, M.; Baek, K. Y.; Ando, T.; Sawamoto, M. J. Am. Chem. Soc. 2003, 125, 5288. (11) Yan, M.; Ge, J.; Liu, Z.; Ouyang, P. J. Am. Chem. Soc. 2006, 128, 11008. (12) Quan, C. Y.; Sun, Y. X.; Cheng, H.; Cheng, S. X.; Zhang, X. Z.; Zhuo, R. X. Nanotechnology 2008, 19, 275102. (13) Oh, J. K.; Tang, C.; Gao, H.; Tsarevsky, N. V.; Matyjaszewski, K. J. Am. Chem. Soc. 2006, 128, 5578. (14) (a) Yu, S. Y.; Hu, J. H.; Pan, X. Y.; Yao, P.; Jiang, M. Langmuir 2006, 22, 2754. (b) Li, J.; Yu, S. Y.; Yao, P.; Jiang, M. Langmuir 2008, 24, 3486. (15) Sahiner, N. Colloid Polym. Sci. 2007, 285, 413. (16) Otsuka, E.; Kurumada, K.; Suzuki, A.; Matsuzawa, S.; Takeuchi, K. J. Sol-Gel Sci. Technol. 2008, 46, 71. (17) Kurumada, K.; Nakamura, T.; Suzuki, A.; Umeda, N. AdV. Powder Technol. 2007, 18, 763.
10.1021/la803902r CCC: $40.75 2009 American Chemical Society Published on Web 01/22/2009
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contained in the nanogels, mesoporous silica spheres could be obtained by removing the nanogels.
2. Experimental Section Materials and Methods. Cystamine dihydrochloride was purchased from Acros and used as received. TETA, TEPA, sodium hydroxide, TEOS, CH3OH, CH2Cl2, 1,4-dithiothreitol (DTT), and acryloyl chloride were purchased from Shanghai Chemical Reagent Co. and used as received. Deionized water was used in this work. FT-IR spectra were recorded on an Avatar 360 spectrometer. Samples were pressed into potassium bromide (KBr) pellets. 1H NMR spectra were recorded on a Varian Unity 300 MHz spectrometer, and TMS was used as an internal standard. The size distribution of nanogels and silica spheres was determined by a particle size analyzer (Nano-ZS ZEN3600). The morphologies of hybrid silica spheres and mesoporous silica spheres were observed on a transmission electron microscope (JEM-100CXa) at an acceleration voltage of 100 kV and a transmission electron microscope (Hitachi JEM-2010) at an acceleration voltage of 200 kV, respectively. X-ray diffraction patterns were measured using Cu Ks radiation (λ )1.5406 Å) on a D8 advance X-ray diffractometer (Bruker). TGA was performed on a TG209 instrument (NETZSCH Co., Germany) under a nitrogen atmosphere from 25 to 600 °C at a heating rate of 10 °C min-1. The specific surface area of the samples was measured on a surface area analyzer (ST-3000, China) with N2 gas as an adsorbate. The specific surface area of the sample was calculated by using a standard reference material supplied by the manufacturer. Synthesis of N,N′-Bis(acryloyl)cystamine. In a 250 mL threenecked flask equipped with a thermometer and two 50 mL dropping funnels, cystamine dihydrochloride (17.4 g, 0.075 mmol) dissolved in 60 mL of water was added. After the mixture was cooled to 0-5 °C in an ice bath, a solution of acryloyl chloride (15 mL, 0.225 mmol) dissolved in dichloromethane (30 mL) and a solution of sodium hydroxide (12 g, 0.3 mmol) in 30 mL water were simultaneously added dropwise to the flask under stirring over a total time span of 1 h. The temperature was kept at 0-5 °C. After the completion of the addition, the reaction mixture was stirred at room temperature for 2 h. The raw product was recovered according to the following procedure. The organic phase was separated from the aqueous layer, and the water was extracted with dichloromethane. The collected organic phases were dried over Na2SO4. The suspension was finally filtered off, and the solid was discarded. The solvent was removed in vacuum, and the raw product was washed with water and dichloromethane several times. Finally, N,N′-bis(acryloyl)cystamine was purified by crystallization from ethyl acetate. Preparation of Nanogels. The nanogels were prepared via the polyaddition of amines to N,N′-bis(acryloyl)cystamine. A typical procedure was as follows. In a two-necked 50 mL flask equipped with a thermometer and two 50 mL dropping funnels, N,N′bis(acryloyl)cystamine (100 mg, 0.38 mmol) was dissolved in 8 mL of methanol. The solution was degassed by bubbling with N2. The temperature was controlled at 50 °C. Then triethylenetetramine (28 µL, 0.19 mmol) dissolved in 24 mL of water was added dropwise to the flask with a 50 mL dropping funnels. The reaction was carried out under vigorous stirring for 24 h. Afterwards, the obtained nanogels were purified by dialysis (molecular weight cut off of 8000-12 000 g mol-1) against distilled water for 2 days, and the water was refreshed twice daily during this period. Amine-Catalyzed Hydrolysis of Tetraethyl Orthosilicate in the Nanogels. The purified nanogels were subsequently used for TEOS hydrolysis without centrifugation. The above nanogels were transferred to a two-necked 50 mL flask equipped with a thermometer and a 50 mL dropping funnel. Tetraethyl orthosilicate (500 mg, 2.4 mmol) dissolved in 10 mL of methanol was added dropwise to the flask. The reaction was stirred vigorously and carried out for 24 h at 50 °C. Finally, the obtained silica spheres were purified by dialysis against distilled water/methanol for 2 days, and the solvent was refreshed twice daily during this period. For the degradation of the nanogels, excess DTT was added, and stirring was continued at room temperature, followed by dialysis to remove excess DTT.
Figure 1. Size distribution and TEM image of the nanogels.
3. Results and Discussion Synthesis of BAC Monomers and Nanogels. BAC was synthesized on the basis of a classical reaction involving the N-acrylation of cystamine dihydrochloride with acryloyl chloride. Because of the poor solubility of cystamine dihydrochloride in organic solvent, the reaction was carried out in the mixture of water/dichloromethane. The neutralization of cystamine dihydrochloride proceeded simultaneously with N-acrylation of cystamine dihydrochloride in the water/dichloromethane interface. A molar ratio 3/1 acryloyl chloride/cystamine dihydrochloride was used to ensure complete reaction. The structure of BAC was characterized by 1H NMR. The spectrum is in good agreement with the proposed structure, and the detailed peaks assignment may be found in Figure S1 in Supporting Information. Very recently, poly(β-aminoester) with pendant primary amine was synthesized via the polyaddition of N-Boc-protected diamine to 1,4-butanediol diacrylate, following by the removal of the N-Boc protective group under anhydrous acidic conditions.18 Herein, the application of this reaction is extended to the fabrication of novel degradable nanogels by the polyaddition of TETA (or TEPA) to BAC. Because 1 mol of amines could react with 2 mols of the acrylate group in the Michael addition procedure, the molar ratio of amine to acrylate must be strictly kept at 1:2 to obtain the nanogels. The concentration of the reactant is another important factor in preparing the nanogels. High concentration would lead to bulk hydrogel, whereas too low a concentration might lead to un-cross-linked polymers. Size and Morphology of the Nanogels. The average size of the nanogels at 25 °C is around 190 nm with a low PDI of 0.15 obtained from the particle-size analyzer (Figure 1). TEM observation shows that the nanogels had a regular spherical shape with a size of around 50 nm. The average size from TEM observation is smaller than that obtained from the particle analyzer. Nanogels absorb large amounts of water and exhibit a swollen state in aqueous solution as a result of their hydrophilic network. The shrinkage of the nanogels under TEM observation is mainly ascribed to the collapse of the network accompanied by the evaporation of the absorbed water. Hybrid Silica Colloids and Their Morphologies. Stober et al.,19 in 1968, reported pioneering work in the synthesis of spherical and monodisperse silica colloids from aqueous alcohol solutions based on the hydrolysis of silicon alkoxides in the presence of ammonia as a catalyst. In the current work, silica colloids were prepared by using the nanogels as a catalytic template. Here, how organosilicate hydrolyze in the microenvironment of the nanogels and what would be fabricated is an important issue. In the course of the reaction, the solution became turbid, indicating the formation of silica colloids. The TEM image (Figure S2-a in Supporting Information) shows that the size of (18) Chen, J.; Huang, S. W.; Liu, M.; Zhuo, R. X. Polymer 2007, 48, 675. (19) Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62.
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Figure 2. Schematic illustration of the formation of hybrid silica colloids and the removal of the nanogels with DTT.
the resulting silica colloids is around 190 nm with a very narrow distribution, which is in good agreement with the size of nanogels used as the nanoreactor. In addition, the TEM image shows that the shells of the silica colloids are relatively smooth. A highmagnification image (Figure S2-b in Supporting Information) also shows that the silica colloids have a core-shell structure, and the shell layer exhibits reduced contrast, suggesting a lower density of the material. Interestingly, a small quantity of the silica colloids with a small core and a collapsed shell were also found. As seen from TEM images (Figure S2-c,d in Supporting Information), it seems that their shells collapsed onto the carbon grid. Such a deformation of silica colloids might be attributed to the drying process before the TEM observation. However, the shells of most silica colloids are rigid enough to maintain the 3D spherical structure after drying. The above TEM observations suggest that the hydrolysis of tetraethyl orthosilicate occurs preferably inside the nanogels. A similar finding was also reported by Pastoriza-Santos et al.20 for the fabrication of gold nanoparticles in thermoresponsive microgels with a different strategy. The core-shell structure of the silica particle might be ascribed to the solubility discrepancy between the components (i.e., BAC and TETA) of the nanogels. Because the solubility of TETA is much higher than that of BAC, more BAC would be inclined to exist inside the nanogels, which is favorable to the residence of hydrophobic TEOS. This difference might make more TEOS hydrolyze inside the nanogels, subsequently leading to the core-shell structure of the silica colloids in the TEM observation. It was also found that the size of silica colloids is dependent on the size of the nanogels. For example, smaller silica colloids (Figure S2-e in Supporting Information) were obtained when the concentrations of TETA and BAC were reduced by half. A similar reduced size (Figure S2-f in Supporting Information) was also found if TETA was
replaced with TEPA while keeping the other condition. Thusprepared silica colloids are termed hybrid silica colloids, and a schematic illustration of the formation of hybrid silica colloids is shown in Figure 2. Structure Characterizations of the Hybrid Silica Colloids. The structure of the hybrid silica colloids was characterized by FT-IR spectroscopy. As shown in Figure S3 in Supporting Information, intense absorption bands attributed to the silicate (Si-O) asymmetric stretching vibration of hybrid silica colloids is observed between 1000 and 1300 cm-1. In addition, a broad band centered at 955 cm-1 from the Si-O stretching of silanol (Si-OH) groups confirms unpolymerized silanols in the particles. In addition, the band centered around 800 cm-1 is associated with the symmetrical Si-O stretching vibration. However, the band centered at 1633 cm-1 is assigned to the water bending commonly found in this region.21 This band is overlapped with the absorbance of CdC groups in BAC. However, absorption bands centered at 1550 and 1440 cm-1 are observed and are attributed to the bending mode of N-H and the asymmetric bending vibration of C-H in nanogels, respectively, indicating the existence of nanogels in silica colloids. After calcination at 400 °C for 24 h, these absorptions disappear but other peaks do not change. Thermogravimetric analysis (TGA), which characterizes the decomposition behavior of the samples during heating, was used to analyze the content of the organic component in the hybrid silica colloids. Figure S4-a (Supporting Information) shows the decomposition behavior of hybrid silica colloids. The TGA result shows that the weight loss of these colloids is divided into three phases. The initial 6.8% weight loss at 0-100 °C, which corresponds to the first peak in the differential thermogravimetry (DTG) curve, is ascribed to the evaporation of the water absorbed in the colloids, further confirming the existence of water, which
(20) Contreras-Caceres, R.; Sanchez-Iglesias, A.; Karg, M.; Pastoriza-Santos, I.; Perez-Juste, J.; Pacifico, J.; Hellweg, T.; Fernandez-Barbero, A.; Liz-Marzan, L. M. AdV. Mater. 2008, 20, 1666.
(21) Alcantara, E. F.C.; Faria, E. A.; Rodrigues, D. V.; Evangelista, S. M.; Deoliveira, E.; Zarac, L. F.; Rabelo, D.; Prado, A. G. S. J. Colloid Interface Sci. 2007, 311, 1.
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Figure 3. TEM images of silica colloids obtained after DTT reduction. b, c, and d are enlargements of images shown in a, b, and c, respectively.
is in agreement with FT-IR characterization. A further 24% weight loss at 100 °C-400 °C, corresponding to the second peak in the DTG curve, is attributed to the decomposition of the nanogels. In addition, a little weight loss (about 2%) is observed at 400-600 °C and is accompanied by the condensation of silanol groups. Thus, the content of nanogels is calculated to be around 25.7 wt % in hybrid silica colloids. Thermogravimetric analysis of the silica colloids after treatment with DTT was further characterized (Figure S4-b in Supporting Information), and it was found that very low weight loss at 100-400 °C (
