Langmuir 2007, 23, 9737-9744
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Biomimetic Deposition of Silica Templated by a Cationic Polyamine-Containing Microgel Fen Zhou,† Shuhong Li,‡ Cong Duan Vo,§ Jian-Jun Yuan,*,† Shigan Chai,† Qing Gao,† Steven P. Armes,§ Chaojing Lu,† and Shiyuan Cheng*,† Faculty of Materials Sciences and Engineering, Hubei UniVersity, Wuhan 430062, China, School of Chemical and EnVironmental Engineering, Beijing Technology and Business UniVersity, Beijing 100037, China, and Department of Chemistry, Dainton Building, The UniVersity of Sheffield, Brook Hill, Sheffield S3 7HF, United Kingdom ReceiVed March 12, 2007. In Final Form: June 18, 2007 We report using poly(acrylamide-co-2-(dimethylamino)ethyl methacrylate, methyl chloride quaternized) cationic microgels as a porous colloidal template for biomimetic in situ silica mineralization, allowing the well-controlled synthesis of submicrometer-sized hybrid microgel-silica particles and porous silica particles by subsequent calcination. The microgels were prepared by inverse emulsion polymerization in the presence of a bisacrylamide cross-linker. Silica deposition was achieved by simply stirring an aqueous mixture of the microgel particles and tetramethyl orthosilicate (TMOS) at 20 °C for 30 min. No experimental evidence was found for nontemplated silica, which indicated that silica deposition occurred exclusively within the cationic microgel template particles. The resulting microgel-silica hybrid particles were characterized by electron microscopy, dynamic light scattering, FT-IR spectroscopy, 1H NMR and solid-state 29Si magic angle spinning NMR spectroscopy, thermogravimetry, aqueous electrophoresis, and surface area measurements. Aqueous electrophoresis studies confirmed that the hybrid microgel-silica particles had positive zeta potentials over a wide pH range and isoelectric points could be tuned by varying the synthesis conditions. This suggests that these particles could form complexes with DNA for improved gene delivery. The porosity of the calcined silica particles could be controlled by varying the amount of TMOS, suggesting potential encapsulation/controlled release applications.
Introduction Nanoscale silicas (i.e., nanoparticles, hollow spheres, and rods or fibers or tubes) and organic-inorganic hybrid silica nanostructures are widely applicable for bioimaging, gene and drug delivery, enzyme immobilization, photoelectronics, catalysis, separation, and so forth.1 Silica deposition often involves nonideal and environmentally unfriendly conditions, such as high or low pH, high temperature and/or pressure, use of toxic and/or expensive organic solvents, as well as multiple steps and complex protocols.2 Moreover, precise control over the silica nanostructure and morphology still remains a major technical challenge, despite recent advances.2b,d,e In contrast, biosilicification occurs in water under ambient conditions for various biological systems such as diatoms and sponges.3 Moreover, this natural process leads to exquisite hierarchical structures and multiple morphologies with precise nanoscale control. Inspired by this natural example of * To whom correspondence should be addressed. E-mail: yuanjj1999@ yahoo.com.cn (J.J.Y.);
[email protected] (S.C). † Hubei University. ‡ The University of Business and Technology of Beijing. § University of Sheffield. (1) (a) Slowing, I. I.; Trewyn, B. G.; Giri, S.; Lin, V. S. Y. AdV. Funct. Mater. 2007, 17, 1225-1236. (b) Barbe, C.; Bartlett, J.; Kong, L.; Finnie, K.; Lin, H. Q.; Larkin, M.; Calleja, S.; Bush, A.; Calleja, G. AdV. Mater. 2004, 16, 19591965. (c) Tan, W.; Wang, K.; He, X.; Zhao, X. J.; Drake, T.; Wang, L.; Bagwe, R. P. Med. Res. ReV. 2004, 24, 621-638. (d) Mulvaney, P.; Liz-Marzan, L. M.; Giersig, M.; Ung, T. J. Mater. Chem. 2000, 10, 1259-1270. (2) (a) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: Boston, 1990. (b) Ozin, G. A. Acc. Chem. Res. 1997, 30, 17-27. (c) Davis, D. E. Nature (London) 2002, 417, 813-821. (d) Hayward, R. C.; Alberius-Henning, P.; Chmelka, B. F.; Stucky, G. D. Microporous Mesoporous Mater. 2001, 44-45, 619-624. (e) van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 980-999. (3) Mu¨ller, W. E. G. Silicon Biomineralization: Biology-BiochemistryMolecular Biology-Biotechnology; Springer: Berlin, 2003.
silicification,4 numerous studies have successfully demonstrated silica formation under ambient conditions by using either synthetic or biologically derived amine-containing (macro)molecules.5 Deming and co-workers6 reported that self-assembled block copolypeptides can facilitate silica condensation under ambient conditions and simultaneously direct silica morphologies on micrometer-length scales. By screening series of long-chain amines or polypeptides or by applying external fields, complex silica structures such as fibers, hexagonal/sheet-like plates, aligned platelets, or dendrites have been also reported.7 Recently, templated deposition of silica under ambient conditions has been exploited to achieve more precise control over the nanoscale morphology and structure of silicas. Shantz and co-workers8 described using polypeptide-based vesicles as templates for the formation of hollow silica spheres. Alternatively, (4) (a) Harrison, C. C. Phytochemistry 1996, 41, 37-42. (b) Shimizu, K.; Cha, J. N.; Stucky, G. D.; Morse, D. E. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 62346238. (c) Kro¨ger, N.; Deutzmann, R.; Sumper, M. Science 1999, 286, 11291132. (d) Cha, J. N.; Shimizu, K.; Zhou, Y.; Christiansen, S. C.; Chmelka, B. F.; Stucky, G. D.; Morse, D. E. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 361-365. (e) Perry, C. C.; Keeling-Tucker, T. J. Biol. Inorg. Chem. 2000, 5, 537-50. (f) Poulsen, N.; Sumper, M.; Kro¨ger, N. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 12075-12080. (g) Hildebrand, M.; York, E.; Kelz, J. I.; Davis, A. K.; Frigeri, L. G.; Allison, D. P.; Doktycz, M. J. J. Mater. Res. 2006, 21, 2689-2698. (h) Frigeri, L. G.; Radabaugh, T. R.; Haynes, P. A.; Hildebrand, M. Mol. Cell. Proteomics 2006, 5, 182-193. (5) For a review, see Patwardhan, S. V.; Clarson, S. J.; Perry, C. C. Chem. Commun. 2005, 1113-1121. (6) Cha, J. N.; Stucky, G. D.; Morse, D. E.; Deming, T. J. Nature (London) 2000, 403, 289-292. (7) (a) Bellomo, E. G.; Deming, T. J. J. Am. Chem. Soc. 2006, 128, 22762279. (b) Tomczak, M. M.; Glawe, D. D.; Drummy, L. F.; Lawrence, C. G.; Stone, M. O.; Perry, C. C.; Pochan, D. J.; Deming, T. J.; Naik, R. R. J. Am. Chem. Soc. 2005, 127, 12577-12582. (c) Rodriguez, F.; Glawe, D. D.; Naik, R. R.; Hallinan, K. P.; Stone, M. O. Biomacromolecules 2004, 5, 261-265. (d) Sumper, M. Angew. Chem., Int. Ed. 2004, 43, 2251-2254. (e) Naik, R. R.; Whitlock, P. W.; Rodriguez, F.; Brott, L. L.; Glawe, D. D.; Clarson, S. J.; Stone, M. O. Chem. Commun. 2003, 238-239. (f) Patwardhan, S. V.; Mukherjee, N.; Steinitz-Kannan, M.; Clarson, S. J. Chem. Commun. 2003, 1122-1123.
10.1021/la700715t CCC: $37.00 © 2007 American Chemical Society Published on Web 08/08/2007
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Coradin and co-workers9 reported the biomimetic growth of silica tubes in poly(L-lysine)-filled channels of polycarbonate membranes. A biomimetic silica coating on single-wall carbon nanotubes (SWNTs) was also achieved by templating R5 peptidemodified SWNTs, where the R5 peptide repeat unit, silaffin, is capable of precipitating silica from a hydrolyzed alkoxide precursor (silicic acid) at room temperature.10 Silica micropatterns have been obtained using amine-containing templates by lithography and microcontact printing technologies.11 Very recently, Xu et al.12 demonstrated an approach for creating synthetic diatom frustules via the biomimetic silicification of polyamine-rich scaffolds assembled by direct ink writing. A concentrated poly(allylamine hydrochloride)-rich ink was robotically deposited in a complex 3D pattern that mimics the shape of naturally occurring diatom frustules. Moreover, Cha et al.13 reported the biomimetic fabrication of two-dimensional silica nanopatterned arrays by using self-assembling thin films of poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) as nanopatterned catalytic templates to deposit silica from tetraethyl orthosilicate, both at neutral pH and also selectively within the P4VP domains. We are interested in the templated synthesis of nanostructured silicas using self-assembling polyamine aggregates under ambient conditions.14-17 Polyethyleneimine (PEI) nanofibers can act as a biomimetic template for silica deposition, which leads to a PEI@silica hybrid nanofiber and also silica nanotubes after removing the PEI cores.14 Multiple morphologies and hierarchical structures based on such PEI nanofibers were achieved by simply adjusting either the solution conditions15 or polymer architectures.16 Furthermore, cationic block copolymer micelles were used as colloidal scaffolds to template the synthesis of hybrid core-shell particles comprising pH-responsive copolymer cores and silica-rich shells.17 In this case, in situ silica deposition effectively cross-links the copolymer micelles, suggesting a convenient route to silica-stabilized shell cross-linked micelles under mild conditions.17 The functional polymer cores of PEI@silica nanofibers and hybrid copolymer-silica nanoparticles can be converted into metal nanowires14 and nanoparticles17 coated with silica-rich shells by reacting with suitable metal precursors. Polymer microgels are cross-linked colloidal particles with a network structure that are swollen in suitable solvents.18 Recently, such microgels have been used as templates or microreactors for the in situ synthesis of inorganic nanoparticles, leading to the organic-inorganic hybrid microspheres.19-26 For example, Kumacheva and co-workers26 reported using polymer microgel (8) Jan, J.-S.; Lee, S.; Carr, C. S.; Shantz, D. F. Chem. Mater. 2005, 17, 4310-4317. (9) Gautier, C.; Lopez, P. J.; Hemadi, M.; Livage, J.; Coradin, T. Langmuir 2006, 22, 9092-9095. (10) Pender, M. J.; Sowards, L. A.; Hartgerink, J. D.; Stone, M. O.; Naik, R. R. Nano Lett. 2006, 6, 40-44. (11) (a) Butler, R. T.; Ferrell, N. J.; Hansford, J. Appl. Surf. Sci. 2006, 252, 7337-7342. (b) Kim, D. J.; Lee, K.-B.; Lee, T. G.; Shon, H. K.; Kim, W.-J.; Park, H.; Choi, I. S. Small 2005, 1, 1-6. (c) Tahir, M. N.; Theato, P.; Muller, W. E. G.; Schroder, H. C.; Borejko, A.; Faiss, S.; Janshoff, A.; Huth, J.; Tremel, W. Chem. Commun. 2005, 5533-5535. (d) Coffman, E. A.; Melechko, A. V.; Allison, D. P.; Simpson, M. L.; Doktycz, M. J. Langmuir 2004, 20, 8431-8436. (12) Xu, M.; Gratson, G. M.; Duoss, E. B.; Shepherd, R. F.; Lewis, J. A. Soft Matter 2006, 2, 205-209. (13) Cha, J. N.; Zhang, Y.; Wong, H.-S. P.; Raoux, S.; Rettner, C.; Krupp, L.; Deline, V. Chem. Mater. 2007, 19, 839-843. (14) Yuan, J. J.; Zhu, P. X.; Fukasawa, N.; Jin, R. H. AdV. Funct. Mater. 2006, 16, 2202-2212. (15) (a) Yuan, J. J.; Jin, R. H. AdV. Mater. 2005, 17, 885-888. (b) Jin, R. H.; Yuan, J. J. Macromol. Chem. Phys. 2005, 206, 2160-2170. (16) (a) Jin, R. H.; Yuan, J. J. Chem. Commun. 2005, 1399-1410. (b) Jin, R. H.; Yuan, J. J. Chem. Mater. 2006, 18, 3390-3396. (17) Yuan, J. J.; Mykhaylyk, O. O.; Ryan, A. J.; Armes, S. P. J. Am. Chem. Soc. 2007, 129, 1717-1723. (18) Saunders, B. R.; Vincent, B. AdV. Colloid Interface Sci. 1999, 80, 1-25. (19) Pich, A.; Adler, H.-J. P. Polym. Int. 2007, 56, 291-307.
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templates for the controlled synthesis of semiconductor, metal and magnetic nanoparticles. Li et al.27 demonstrated that preformed CdTe nanocrystals can be incorporated into microgel templates to produce pH-sensitive hybrid particles. Alternatively, inorganic nanoparticles can be incorporated into microgels by conducting an in situ emulsion polymerization using a suitable cross-linker.28,29 Such hybrid organic-inorganic particles have been used as pH-responsive particulate emulsifiers28 and highly efficient catalysts,30 and also have potential applications in advanced drug/gene delivery,31 chemical and biosensing,32 photonic crystals,33 and so forth. In contrast, there are very few studies describing microgel-templated biomimetic mineralization.34-37 In nature, biomineralization can be mediated by biological hydrogel spheres, directing a range of biominerals into diverse forms and with specific biological functions.38 For example, each cell of the compound eyes of insects has a CaCO3 crystal embedded in its organic gel.39 Recently, inspired by biomineralization embodied in single-cell organisms, Wang and co-workers34,35 reported using poly(N-isopropylacrylamide)based hydrogels as templates for the in situ biomineralization of calcium carbonate to produce submicrometer-sized particles with unusual shapes that allowed further encapsulation of Au nanoparticles. In addition, the mineralization of calcium phosphate in the nanosized gel structures has been also achieved by templating either the physical nanogels from cholesterol-bearing pulluan40 or the shell cross-linked polymer micelles and nanocages from a poly(acrylic acid-b-isoprene) block copolymer.41 These new organic-inorganic nanostructures with structural robustness and biocompatibility may have potential applications in drug delivery, bioimaging, and therapeutics.40,41 Herein, we report using a polyamine-based microgel as a colloidal template for biomimetic silica deposition, leading to (20) Antonietti, M.; Gro¨hn, F.; Hartmann, J.; Bronstein, L. Angew. Chem., Int. Ed. 1999, 36, 2080-2083. (21) Bradley, M.; Bruno, N.; Vincent, B. Langmuir 2005, 21, 2750-2753. (22) (a) Pich, A.; Karak, A.; Lu, Y.; Ghosh, A. K.; Adler, H.-J. P. Macromol. Rapid Commun. 2006, 27, 344-350. (b) Pich, A.; Hain, J.; Lu, Y.; Boyko, V.; Prots, Y.; Adler, H.-J. Macromolecules 2005, 38, 6610-6619. (c) Pich, A.; Bhattacharya, S.; Lu, Y.; Boyko, V.; Adler, H.-J. P. Langmuir 2004, 20, 1070610711. (23) Biffis, A.; Orlandi, N.; Corain, B. AdV. Mater. 2003, 15, 1551-1555. (24) (a) Bai, C.; Fang, Y.; Zhang, Y.; Chen, B. Langmuir 2004, 20, 263-265. (b) Zhang, Y.; Fang, Y.; Wang S.; Lin, S. J. Colloid Interface Sci. 2004, 272, 321-325. (25) Chen, Q.; Shen, X.; Gao, H. Colloid Surf., A 2006, 275, 45-49. (26) Zhang, J.; Xu, S.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126, 79087914. (27) Li, J.; Liu, B.; Li, J. Langmuir 2006, 22, 528-531. (28) (a) Fujii, S.; Read, E. S.; Binks, B. P.; Armes, S. P. AdV. Mater. 2005, 17, 1014- 1018. (b) Fujii, S.; Armes, S. P.; Binks, B. P.; Murakami, R. Langmuir 2006, 22, 6818-6825. (29) Menager, C.; Sandre, O.; Mangili, J.; Cabuil, V. Polymer 2004, 45, 24752481. (30) Biffis, A. J. Mol. Catal. A: Chem. 2001, 165, 303-307. (31) Gorelikov, I.; Field, L. M.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126, 15938-15939. (32) Das, M.; Zhang, H.; Kumacheva, E. Annu. ReV. Mater. Res. 2006, 36, 117-144. (33) (a) Xu, S.; Zhang, J.; Paquet, C.; Lin, Y.; Kumacheva, E. AdV. Funct. Mater. 2003, 13, 468-472. (b) Suzuki, D.; Kawaguchi, H. Langmuir 2006, 22, 3818-3822. (34) Kuang, M.; Wang, D.; Gao, M.; Hartmann, J.; Mo¨hwald, H. Chem. Mater. 2005, 17, 656-660. (35) Kuang, M.; Wang, D.; Mo¨hwald, H. Chem. Mater. 2006, 18, 1073-1075. (36) Nassif, N.; Gehrke, N.; Pinna, N.; Shirshova, N.; Tauer, K.; Antonietti, M.; Co¨lfen, H. Angew. Chem., Int. Ed. 2005, 44, 6004-6009. (37) Wehnert, F.; Pich, A. Macromol. Rapid Commun. 2006, 27, 1865-1872. (38) Mann, S. Biomineralization: Principles and Concepts in Bioinorganic Materials Chemisty; Oxford University Press: Oxford, UK, 2001. (39) Land, M. F.; Nilsson, D. E. Animal Eyes; Oxford University Press: Oxford, 2001. (40) Sugawara, A.; Yamane, S.; Akiyoshi, K. Macromol. Rapid Commun. 2006, 27, 441-446. (41) Perkin, K. K.; Turner, J. L.; Wooley, K. L.; Mann, S. Nano Lett. 2005, 5, 1457-1461.
Biomimetic Deposition of Templated Silica
o the facile synthesis of submicrometer-sized microgel-silica hybrid particles. Compared to polypeptide vesicle templates,8 PEI nanofibers,14 and cationic polyamine micelles,17 these microgel templates can be readily synthesized on a large scale, with reasonable size control and relatively facile functionalization. Hu and co-workers42 recently described using poly(N-isopropylacrylamide-co-acrylic acid) microgels to template silica deposition to produce hybrid core-shell particles and also hollow silica particles with diameters of about 40 µm. In this case, silica deposition was conducted under basic conditions (using ammonia) by a hydrothermal process. In addition, Molvinger et al.43 reported the synthesis of porous chitosan-silica hybrid microspheres with sizes of around several hundred micrometers by templating chitosan beads using either NaF as a catalyst or urea as a porogen for silica deposition. In comparison, our biomimetic synthesis was performed under ambient conditions (aqueous media and room temperature) and leads to much smaller, submicrometersized hybrid particles. Moreover, subsequent calcination produces nanostructured particles with porosities that can be tuned by adjusting the synthesis conditions. Experimental Section 1. Materials. Acrylamide (AM) was supplied by Hubei University Chemicals Co. Ltd. (China), and was recrystallized from acetone. The methyl chloride quaternized form of 2-(dimethylamino)ethyl methacrylate (DMC) and tetramethyl orthosilicate (TMOS) were purchased from Xinyu Chemicals Co. Ltd. (China) and Wuhan University Silicone New Material Co. Ltd, respectively, and were used without further purification. White mineral oil was washed with concentrated H2SO4 and NaOH before use. Other chemicals were used as received. Deionized water was used in all experiments. 2. Preparation of Microgel by Inverse Emulsion Polymerization. The poly(AM-co-PDMC) microgel was prepared by inverse emulsion polymerization using the methods reported by Inchausti et al.44 and Ge et al.45 with suitable modifications. The copolymerization was carried out in a 250 mL round-bottomed glass reactor equipped with a stirrer, a reflux condenser, a sampling device, and an inlet system for temperature control. The inverse emulsion was prepared by adding a mixture of water (10.0 g), monomers (AM, 2.05 g; DMC, 9.0 g), cross-linker (N,N′-methylene bisacrylamide, DMAM, 1.49 g), and initiator (ammonium persulphate, APS, 0.05 g) to an oil phase containing 58 g white mineral oil, 3.0 g Tween, and 9.0 g Span 80 as emulsifiers. The mixture was pre-emulsified by stirring at 2000 rpm for 10 min, and purified nitrogen was bubbled at room temperature through the emulsion for about 30 min to eliminate oxygen. The polymerization was conducted at 75 °C for 2 h. After polymerization, the stable emulsion was precipitated into a large excess of methanol, followed by acetone or methanol washing for several times. The poly(AM-co-PDMC) microgel was obtained as an off-white powder by drying under vacuum for 12 h at 20 °C. 3. Synthesis of Microgel-Silica Hybrid Spherical Particles. A 0.25 wt % aqueous poly(AM-co-PDMC) microgel dispersion was prepared by dispersing dried microgel powder in water. Silica deposition was achieved by adding desired amounts of TMOS to 2.0 mL of this aqueous microgel solution. The initially heterogeneous solution was stirred under ambient conditions for typically 30 min. Spherical hybrid microgel-silica particles were obtained by washing with ethanol, followed by three centrifugation/redispersion cycles at 5400 rpm for 10 min. Redispersal of the sedimented particles was achieved with the aid of an ultrasonic bath. The hybrid particles were also calcined by heating samples to 600 °C at a heating rate of 20 °C per min, and maintaining this temperature for 4 h in order (42) Yang, J.; Hu, D.; Fang, Y.; Bai, C.; Wang, H. Chem. Mater. 2006, 18, 4902-4907. (43) Molvinger, K.; Quignard, F.; Brunel, D.; Boissiere, M.; Devoisselle, J.M. Chem. Mater. 2004, 16, 3367-3372. (44) Inchausti, I.; Sasia, P. M.; Katime, I. J. Mater. Sci. 2005, 40, 4833-4838. (45) Ge, X.; Ye, Q.; Zhang, Z.; Chu, G. J. Appl. Polym. Sci. 1998, 67, 10051010.
Langmuir, Vol. 23, No. 19, 2007 9739 to completely pyrolyze the organic microgel and hence obtain porous silica particles. 4. Characterization of Microgel, Hybrid Microgel-Silica Particles, and Calcined Silica Particles. Dynamic Light Scattering (DLS). A Malvern Zetasizer instrument (Nano S, UK) operating at a laser wavelength of 633 nm and a fixed detector angle of 173° was used for DLS measurements on highly dilute aqueous particles dispersions. ThermograVimetry. A Perkin-Elmer Diamond TG/DTA instrument was used at a heating rate of 20 °C per min. Dried samples were heated in air up to 800 °C, and the observed mass loss was attributed to the quantitative pyrolysis of the copolymer, with the remaining incombustible residues assumed to be pure silica (SiO2). FT-IR Spectroscopy. FT-IR spectra were recorded in KBr disks using a Perkin-Elmer Spectrum One instrument. The average number of scans per spectrum was 128 and the spectral resolution was 4 cm-1. NMR Spectroscopy. The degree of polycondensation of the resulting silica framework (Q4, Q3, and Q2) was assessed by solidstate 29Si NMR spectroscopy using magic angle spinning, with spectra being recorded on a Varian-INOVA 600 MHz NMR spectrometer. The relaxation delay for these measurements was 30 s. The 1H NMR spectrum of the hybrid microgel-silica sample was also recorded using the same spectrometer. The 29Si and 1H chemical shifts are reported using the δ scale and are referenced to tetramethylsilane (TMS) at 0 ppm. Transmission Electron Microscopy (TEM). TEM studies were conducted on a Tecnai G20 microscope (FEI Corp. USA) operating at 200 kV. Dilute dispersions of the microgel and hybrid microgelsilica particles were allowed to dry onto a carbon-coated copper grid under ambient conditions prior to examination. To examine the cross sections of hybrid particles, samples were microtomed prior to TEM studies. The elemental compositions of hybrid particles were also analyzed by energy-dispersive X-ray spectrometry (EDX, GEN-ESIS2000 XMS 30T, EDAX Corp. USA). Typically, electron beams of around 50 nm diameter were applied to illuminate a randomly selected particle for EDX analysis. In addition, the chemical compositions of these hybrid particles were assessed by CHN microanalyses using a Perkin-Elmer 2400 automatic analyzer. Field-Emission Scanning Electron Microscopy (FE-SEM). The surface morphology of hybrid microgel-silica and calcined pure silica particles was imaged using a FE-SEM instrument (JEOL JSM6700F) working at 3 kV. Individual drops of the diluted dispersions were allowed to dry onto silicon wafers under ambient conditions, prior to sputter-coating with a thin overlayer of Pt to avoid sample charging. Aqueous Electrophoresis. Measurements were performed in 1 mM NaCl solution using a Malvern Zetasizer NanoZS instrument. The zeta potential (ζ) was calculated from the electrophoretic mobility (u) using the Smoluchowsky relationship, ζ ) ηu/, assuming that κa . 1 (where η is the solution viscosity, is the dielectric constant of the medium, and κ and a are the Debye-Hu¨ckel parameters and the particle radius, respectively). The solution pH was adjusted by the addition of HCl or NaOH. Brunauer-Emmett-Teller (BET) Measurements. The BET surface areas of the microgel-silica and calcined silica samples were estimated from N2 adsorption-desorption experiments conducted at 77 K using an Autosorb-1-MP surface area analyzer (Quantachrome Company, USA).
Results and Discussion We have previously demonstrated that partially quaternized poly(2-(dimethylamino)ethyl methacrylate) chains are suitable polyamines for catalyzing silica deposition under ambient conditions using TMOS as a silica precursor.17 In this work, we incorporated a closely related cationic comonomer, methyl chloride-quaternized 2-(dimethylamino)ethyl methacrylate (DMC), into polyacrylamide-based microgels for the biomimetic synthesis of microgel-silica hybrid particles. Our synthetic strategy is shown in Figure 1. The approimately 10% cross-
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Figure 1. Schematic formation of hybrid microgel-silica particles synthesized by biomimetic deposition of silica using tetramethyl orthosilicate (TMOS) as a silica precursor and a poly(AM-co-PDMC) microgel template under ambient conditions. Porous silica particles were obtained by calcining the hybrid particles at 600 °C for 4 h.
Figure 2. (A) TEM image of the poly(AM-co-DMC) microgel synthesized by inverse emulsion copolymerization with 60 mol % DMC; (B) SEM image of hybrid microgel-silica particles synthesized by mixing 2.0 mL of a 0.25 wt % microgel solution with 0.10 mL TMOS for 30 min; (C) a higher magnification image from B; (D) SEM image of porous silica particles obtained by calcining the hybrid particles (as shown in B) at 600 °C for 4 h.
linked poly(AM-co-DMC) cationic microgel obtained from inverse emulsion polymerization contains approximately 60 mol % DMC and acts as a colloidal template to ensure that a spherical morphology is obtained. DLS studies on dilute aqueous microgel dispersions indicated an intensity-average diameter of around 598 nm, whereas TEM particle diameters ranged from 200 to 600 nm, with a number-average diameter of 413 ( 63 nm (see Figure 2A). These TEM observations are in good agreement with SEM studies of the microgels (see Supporting Information Figure S1). The discrepancy between the DLS and electron microscopy diameters is partly due to polydispersity effects (since DLS is more biased toward larger particles than TEM and SEM)
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and also reflects the swollen dimensions of these microgels in aqueous solution (compared to the high-vacuum conditions required for electron microscopy). The microgel-silica hybrid particles were synthesized by stirring a mixture of 0.1 mL TMOS and 2.0 mL of 0.25 wt % aqueous microgel dispersion at room temperature for 30 min. After purification, the dried microgel-silica particles can be easily redispersed in either water or alcohols with the aid of an ultrasonic bath, indicating that good colloidal stability was maintained even after silica deposition. The size and surface morphology of the hybrid microgel-silica particles were directly visualized by SEM (see Figure 2B). Particle diameters ranged from 300 to 700 nm, with a number-average diameter of 459 ( 78 nm estimated by analyzing more than 100 particles. Thus, the hybrid microgel-silica particles are slightly larger than the copolymer microgel precursor particles imaged by TEM (see Figure 2A). This is presumably because silica deposition occurs within the swollen microgels in aqueous solution and fixes their somewhat larger dimensions under these conditions. A typical high-magnification SEM image of hybrid microgel-silica particles is shown in Figure 2C and indicates a rough surface morphology. This is interesting, since silica particles formed either by the conventional Sto¨ber method46 or by biomimetic approaches using micelle,17 vesicle,8 or nanofiber14 templates usually have relatively smooth, featureless surfaces. We assume that the porous nature of swollen poly(AM-co-DMC) microgels contributes to the formation of nanostructured surfaces for these hybrid particles. Furthermore, we found that there was no SEM evidence for nontemplated silica, suggesting that silica deposition occurred exclusively within the microgel network. Templated silicification was further supported by a simple control experiment: 0.1 mL TMOS was mixed with 2.0 mL water. This mixture was stirred at room temperature for 30 min in the absence of microgel. No silica was obtained after centrifugation at 12 000 rpm for 10 min, indicating that silicification is negligible under these conditions in the absence of polyamine catalysis. Therefore, silicification is likely to be confined within the polymeric microgel, where the polyamine chains, gel network, and spherical particles serve as the catalyst, scaffold, and template for silica deposition, respectively. The microgel-silica hybrid particles were calcined at 600 °C at a heating rate of 20 °C min-1 in order to pyrolyze the copolymer and obtain pure silica spheres. As shown in Figure 2D, the spherical particle morphology was retained, indicating that the particles survive this high-temperature processing. However, the surface of the silica particles became much rougher compared to that observed before calcination (Figure 2C). Interconnected networks of nanoscale valleys were observed on the surface of the calcined particles. This increased surface roughness caused by calcination presumably arises from the relatively low-density silica produced within the hybrid particles. Calcination converts this initial silica framework into higher-density silica, leading to the formation of nanostructured features. Furthermore, both the hybrid microgel-silica and the pure silica particles were studied by TEM. Figure 3A shows a representative TEM image of microgel-silica particles prepared using identical conditions to those used for the sample shown in Figure 2. A rough surface was still evident, which is in good agreement with the SEM observations (see Figure 2C). Intriguingly, we found that there is an interface within each particle: dense cores were surrounded by a lower-density shell of around (46) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62-69. (47) Bertermann, R.; Kro¨ger, N.; Tacke, R. Anal. Bioanal. Chem. 2003, 375, 630-634.
Biomimetic Deposition of Templated Silica
Figure 3. TEM images of (A) hybrid microgel-silica particles (the synthesis conditions are identical to those shown in Figure 2); (B) cross section of the hybrid particles shown in A; (C) porous silica particles obtained by calcining the hybrid particles (as shown in A) at 600 °C; (D) cross section of the porous silica particles shown in C.
50 nm thickness (as shown by the white arrow in Figure 3A). This possibly arises from the inhomogeneous microstructure of the copolymer microgel precursor, which may well have a nonuniform cross-link density. To examine the internal nanostructure of the hybrid microgel-silica particles, TEM analysis was performed on selected cross-sectioned samples. As shown in Figure 3B, irregular nanosized channels are distributed within the polymer-silica hybrid matrix, suggesting significant porosity. Figure 3C shows a TEM image of the silica particles obtained after calcination at 600 °C. In contrast to the hybrid microgelsilica particles shown in Figure 3A, the interface separating the compact core from the low-density shell can no longer be resolved. This suggests that a high degree of additional condensation occurred within the low-density shell during calcination, which is consistent with the SEM observations shown in Figure 2C,D. Cross-sectional analysis was also conducted on the calcined silica particles to examine their internal nanostructure. As shown in Figure 3D, nanochannels are clearly observed. However, compared to the hybrid microgel-silica particles prior to calcination (Figure 3B), these nanochannels appeared to be more prevalent, and the characteristic grain size of these features is around 30 nm. The hybrid microgel-silica and calcined silica particles shown in Figure 2 were further studied by FT-IR spectroscopy, EDX, elemental microanalyses, thermogravimetry, surface area analysis, solid-state 23Si MAS NMR and 1H NMR spectroscopy, and aqueous electrophoresis. FT-IR studies confirmed silica formation, since additional bands were observed at 1080, 950, 800, and 470 cm-1 for the hybrid microgel-silica particles due to the inorganic component; these new bands were absent in the spectrum obtained for the poly(AM-co-DMC) microgel prior to silica deposition (see Supporting Information Figure S2). After calcination at 600 °C, the characteristic band at 1726 cm-1 due to the copolymer microgel completely disappeared, while all the bands assigned to the thermally stable silica were still observed (see Supporting Information Figure S2). EDX was employed to verify the coexistence of copolymer (microgel) and silica within individual hybrid particles. As shown in Supporting Information
Langmuir, Vol. 23, No. 19, 2007 9741
Figure S3, analysis of a randomly selected particle produced a Si signal due to the silica component and Cl signal due to the copolymer. CHN elemental microanalyses of the same sample confirmed the presence of 2.8% nitrogen due to the copolymer within the hybrid particles. This corresponded to a copolymer content of 24% by mass. Thermogravimetric analyses suggested that the hybrid microgel-silica particles (as shown in Figure 2) were silica-rich. The mean silica content is about 62% by mass, indicating a silica deposition efficiency of approximately 29% (see Table 1). The surface areas of the hybrid microgel-silica and pure silica particles after calcination were determined by N2 adsorptiondesorption experiments. The hybrid particles have a BET surface area of 54 m2 g-1. In contrast, the BET surface area of the calcined silica particles dramatically increased to 294 m2 g-1 (see Table 1). Copolymer pyrolysis and additional condensation of the silica framework both contributed to this much higher specific surface area. This result is consistent with our SEM and TEM observations. To evaluate the degree of polycondensation of the deposited silica, solid-state 29Si MAS NMR measurements were performed47 on two hybrid microgel-silica samples and their corresponding calcined silicas. As revealed in Figure 4A, the hybrid microgelsilica particles show two strong signals at approximately -110 and -100 ppm assigned to Q4 (Si(OSi-)4) and Q3 (HOSi(OSi-)3), respectively, with the former feature being more intense, whereas the weak feature at about -90 ppm attributed to Q2 ((HO)2Si(OSi-)2) was barely detectable. This suggested that the silica framework has a relatively high degree of polycondensation, as expected. After calcination, the Q3 peak was reduced to a very small shoulder on the Q4 signal, indicating a significantly higher degree of polycondensation due to calcination. This is consistent with the expected evolution of the silica framework during calcination, since additional dehydration and polycondensation occurs at higher temperatures.2a In contrast, calcination decreases the relative abundance of Q4 species for PEI nanofibertemplated silica.14 This difference is attributed to the quaternized nature of the cationic DMC units, whereas the linear PEI comprises secondary amines.48 Zeta potential measurements also supported the deposition of silica within the microgel template particles (see Figure 5). The precursor microgel exhibited positive zeta potentials over the whole pH range investigated due to the cationic character of the quaternized DMC units. However, the hybrid microgel-silica particles exhibited negative zeta potentials at high pH, with an isoelectric point at around pH 9.1. This latter behavior is clearly due to the deposition of silica onto the cationic network, since aqueous colloidal silica sols typically exhibit negative zeta potentials over a wide pH range (i.e., from pH 3 to pH 10). The difference between the electrophoretic curves obtained for the hybrid microgel-silica particles and the aqueous silica sol most likely reflects the cationic nature of the PDMC-containing chains, which may well protrude beyond the silica framework. 1H NMR studies supported this hypothesis. As shown in Supporting Information Figure S4, proton signals attributed to the CH3and -CH2-CH2- groups of PDMC were observed for the hybrid microgel-silica particles, indicating that a significant proportion of the cationic poly(AM-co-DMC) chains were not part of the silica framework. Such behavior is very different from that observed for cationic copolymer micelle-silica nanoparticles, which have an isoelectric point at around pH 3.3.17 This is much lower than that found for the hybrid microgel-silica particles, indicating that the latter have more cationic surface character. (48) Yuan, J. J.; Jin, R. H. Langmuir 2005, 21, 336-3145.
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Table 1. Synthesis Conditions, Compositions, and Properties of Hybrid Microgel-Silica and Calcined Silica Particles
runsa
TMOS (mL)
SiO2 content of hybrid particles (wt %)b
deposition efficiency of SiO2 (%)
1 2 3 4
0.025 0.05 0.1 0.4
41.7 44.3 61.7 72.4
46.1 27.1 29.2 12.1
isoelectric points of hybrid particles (pH) c
BET surface area of particles (m2 g-1) before calcination
after calcination
54
348 294 224
10.2 9.1
a 2.0 mL of a 0.25 wt % aqueous microgel dispersion was used for all runs. The silica depositions were performed in aqueous media at 20 °C for 30 min. b As estimated by TGA analysis. c As determined from the zeta potential vs pH curves (see Figure 5).
Figure 4. Solid-state 29Si MAS NMR spectra of (A) microgelsilica hybrid particles and (B) pure silica particles obtained by calcining the hybrid particles at 600 °C at a heating rate of 20 °C per min and maintaining this temperature for 4 h.
Figure 5. Zeta potential vs pH curves obtained for the original poly(AM-co-DMC) microgel prepared by inverse emulsion polymerization with a DMC content of 60 mol % (4), the hybrid microgelsilica particles (with a mean silica content of 61.7% in mass) obtained by stirring a mixture of 2.0 mL of a 0.25 wt % microgel solution and 0.10 mL TMOS for 30 min (O) and the hybrid microgel-silica spheres (with a mean silica content of 41.7% in mass) prepared by mixing 2.0 mL of a 0.25 wt % microgel solution with 0.025 mL TMOS for 30 min (0).
Prior to silica deposition, the cationic copolymer micelles have a dense brush-like polyamine shell, which produces a relatively compact polymer-silica hybrid shell. In contrast, silica deposition
templated by the poly(AM-co-DMC) microgel occurs within a more open porous polymeric network, which produces a hybrid matrix with protruding cationic chains. In addition, we also found that the hybrid polymer-silica particles exhibited higher zeta potentials than the polymer microgel at low pH (
