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Synthesis of supermicro-macroporous silica with polypeptide-based triblock copolymer
Recent Progress in Mesostructured Materials D. Zhao, S. Qiu, Y. Tang and C. Yu (Editors) © 2007 Elsevier B.V. All rights reserved.
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Synthesis of supermicro-macroporous silica with polypeptide-based triblock copolymer Yuping Liu, Liying Li, Zhurui Shen, Pingchuan Sun, Zhongyong Yuan and Tiehong Chen College of Chemistry, Department of Materials Chemistry, Key Laboratory of Functional Polymer Materials ofMOE, Nankai University, Tianjin, 300071, P.R. China
With anilino-methyl triethoxy silane (AMTS) as an intermedium, supermicro-macroporous silica was synthesized through 71-71 interaction under ambient conditions, utilizing synthetic polypeptide-based triblock copolymer poly(L-phenylalanine)-b-poly(ethylene glycol )-b-poly(L-phenylalanine) (Phe7PEGi35-Phe7) as a template . The prepared silica has mesoscale short-rangeorder and hierarchical structure with both supermicropores and interconnected macropores. It is proposed that the supermicropores are templated by the polypeptide segments, while the open 3D interconnected macroporous networks are presumably attributed to both PEG segments and organic solvent. Both polypeptide-based block copolymer and AMTS play important roles in the formation of mesoscale short-range-order and hierarchical structure. 1. Introduction Exquisite silica structures existing in the diatoms and sponges are generally controlled by specific interactions between peptides (and/or polyamines) and silicic acid derivatives under mild physiological conditions [1]. Peptides such as silaffin [2] and silicateins [3] (isolated from diatom biosilica and sponges, respectively) can induce the formation of silica from silicic acid solution at neural pH. Stucky group for the first time used self-assembly structured aggregates of synthetic poly (Cysteine)-b-poly(Lysine) block copolypeptide to synthesize transparent mesoporous silica microspheres [4]. Shantz et al. reported the fabrication of polypeptide-templated nanoporous materials by using commercially available PLL [5, 6], however, the synthesized silica did not exhibit well-defined morphology and multi-scale architecture.
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Inspired by the works mentioned above, here we describe an approach to prepare hierarchically structured silica templated from synthetic polypeptidebased triblock copolymer poly(L-phenylalanine)-b-poly(ethylene glycol)i35-bpoly(L-phenylalanine) (Phe7-PEGi35-Phe7, synthesized by our lab). It is firstly reported that hierarchical silica material is successfully prepared by utilizing polypeptide-based triblock copolymer as a template, through it—% interaction under ambient conditions. In addition, control experiments without AMTS or without the triblock polymer have been performed for comparison. The mechanism on the formation of the hierarchically structured silica in the presence of polypeptide-based triblock copolymer and AMTS is discussed. 2. Experimental Section Phe7-PEGi35-Phe7 was synthesized by our lab according to our previous works[7]. Anilino-methyl triethoxy silicane (denoted as AMTS, 95%, whose chemical formula is shown in Scheme 1) was obtained from Wuhan Tianmu Co. Ltd. (China). Tetratetraethoxysilane (TEOS, Aldrich) and dioxane (Aldrich) were used without further purification. Preparation of Hierarchical silica: In a typical experiment, 40 ml of Phe7-PEGi35-Phe7 solution in dioxane organic solvent (2.5 mg/ml) was gently mixed with 120 ml benzyl alcohol under stir for 20 min. Then 5ml of AMTS was added to the solution. The mixture was heated at 80°C for 10 min. Subsequently, 3 ml of benzyl ammine (as catalyst), 3 ml of deionized water and 5 ml of TEOS were added to the mixture, respectively. The final mixture was stirred for 10 min and was sonicated (with 20 kHz frequency, 300 w power) for 20 min to obtain a clear solution. The solution was kept static in a sealed flask at room temperature for two weeks. The product (denoted as SI) was obtained by washing with methanol and deionized water for several times, and dried at 60 °C for 24 hrs. The white as-synthesized product was calcined at 550°C for 5 hrs to remove organic solvents and the copolymer. In addition, white product was also obtained under the same reaction conditions except either without AMTS (denoted as S2) or without the triblock copolymer (no product was obtained). The obtained product was charaterization by means of TEM,SEM,XRD and N2 adsorption and desorption measurement. 3. Results and Discussion From XRD pattern of calcined sample SI (unshown), one broad diffraction peak with d spacing of 3.1 nm appears, indicating the presence of disordered short-range ordering structure. No such diffraction peak appears in the lowangle region of XRD pattern for sample S2(unshown), so it is believed that anilino-methyl triethoxy silicane (AMTS) plays a significant role in the formation of the disordered mesostructure of sample SI. If the triblock copolymer was not used under same synthesis condition of sample SI, silica precipitation did not occur and the system remained an optically clear solution
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without formation of gel. Evidently, the triblock copolymer effectively controls the nucleation and growth of silica. From the SEM images (Fig. 1 a, b), the calcined sample SI displays interesting macroporous morphology, with the macropore size of about 200-700 nm. The macroporous network is composed of layers interconnected by struts-like pillars. For sample S2, the SEM images only display irregular particles (unshown), indicating that without the addition of AMTS, the triblock copolymers were not involved in the silica precipitation to give rise to formation of the specific morphology. The results from the nitrogen adsorption-desorption test of the calcined sample SI (unshown) indicate the presence of micropores (<2 nm) and its BET surface area of calcined sample is 439 m2/g (micropore area is 389 m2/g). The Barrett-Joynes-Halenda (BJH) average pore size of sample SI is estimated to be about 1.6 nm. This supermicroporous character is also confirmed by the TEM images (Fig.lc). The N2 adsorption line of the calcined sample S2 (unshown) is a type II isotherm, indicating the presence of mesovoids resulted from the aggregation of silica particles.
Fig.l. SEM images (a, b, with different magnifications)and TEM image (c) of calcined samples SI
In this experiment, the triblock copolymer, as a template, is indispensable to the formation of silica through controlling the nucleation and growth of silicate precursors. And anilino-methyl triethoxy silane (AMTS) which contains phenyl and amine groups was used as a bridge, i.e., on the one hand it interacts with polypeptide-based triblock copolymer through template-bridge interactions (hydrogen bonding and n-n interactions) between copolymer and AMTS; on the other hand, AMTS can co-condense with tetraethoxylsilane (TEOS) through hydrolysis process. 4. Conclusion The presented results suggest a bio-inspired approach for the preparation of hierarchical silica material with both supermicropores and macropores utilizing polypeptide-based triblock copolymer Phe7-PEGi35-Phe7 as a template through n —% interaction under ambient conditions. The formation of supermicropores
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4. Conclusion Under acidic condition, using the co-assembly aggregate, i.e., supermicelles, driven by hydrogen bonding between hydrophilic polymer poly(vinyl alcohol) (PVA) and hydrophobic polymer hydroxyl-terminated polybutadiene (HTPB) as a template, mesoporous silica was synthesized by sol-gel reaction. The calcined silica had higher BET surface area (S = 500-600m2/g). The pore size on nanoscale can be easily adjusted through changing the ratio of PVA/HTPB. 5. Acknowledgement This work was supported by National Science Foundation of China (Grants No. 20373029, 20233030), and joint-research fund of Nankai University and Tianjin University on Nano-sciences. 6. References [1] C.T. Kresge, C.Z. Leonow, W.J. Roth et al.Nature, 359 (1992)710. [2] D. Zhao, J. feng, Q. Huo, N. Melosh, G.H. Fredericson, B.F. Hmelkaand G.D. Stucky, Science, 279(1998)548. [3] S. M. Yang, I. Sokolov, N. Coombs, C.T. Kresge and G.A. Ozin, Adv. Mater. 11 (1999)1427. [4] B. C. Chen, H.P. Lin, M.C. Chao et al., Adv. Mater. 16 (2004)1657. [5] F. Iskandar, Mikrajuddin and K. Okuyama, Nano Lett. 1 (2001)231.
33 33
Synthesis of supermicro-macroporous silica with polypeptide-based triblock copolymer Yuping Liu, Liying Li, Zhurui Shen, Pingchuan Sun, Zhongyong Yuan and Tiehong Chen College of Chemistry, Department of Materials Chemistry, Key Laboratory of Functional Polymer Materials ofMOE, Nankai University, Tianjin, 300071, P.R. China
With anilino-methyl triethoxy silane (AMTS) as an intermedium, supermicro-macroporous silica was synthesized through 71-71 interaction under ambient conditions, utilizing synthetic polypeptide-based triblock copolymer poly(L-phenylalanine)-b-poly(ethylene glycol )-b-poly(L-phenylalanine) (Phe7PEGi35-Phe7) as a template . The prepared silica has mesoscale short-rangeorder and hierarchical structure with both supermicropores and interconnected macropores. It is proposed that the supermicropores are templated by the polypeptide segments, while the open 3D interconnected macroporous networks are presumably attributed to both PEG segments and organic solvent. Both polypeptide-based block copolymer and AMTS play important roles in the formation of mesoscale short-range-order and hierarchical structure. 1. Introduction Exquisite silica structures existing in the diatoms and sponges are generally controlled by specific interactions between peptides (and/or polyamines) and silicic acid derivatives under mild physiological conditions [1]. Peptides such as silaffin [2] and silicateins [3] (isolated from diatom biosilica and sponges, respectively) can induce the formation of silica from silicic acid solution at neural pH. Stucky group for the first time used self-assembly structured aggregates of synthetic poly (Cysteine)-b-poly(Lysine) block copolypeptide to synthesize transparent mesoporous silica microspheres [4]. Shantz et al. reported the fabrication of polypeptide-templated nanoporous materials by using commercially available PLL [5, 6], however, the synthesized silica did not exhibit well-defined morphology and multi-scale architecture.
34 34
Inspired by the works mentioned above, here we describe an approach to prepare hierarchically structured silica templated from synthetic polypeptidebased triblock copolymer poly(L-phenylalanine)-b-poly(ethylene glycol)i35-bpoly(L-phenylalanine) (Phe7-PEGi35-Phe7, synthesized by our lab). It is firstly reported that hierarchical silica material is successfully prepared by utilizing polypeptide-based triblock copolymer as a template, through it—% interaction under ambient conditions. In addition, control experiments without AMTS or without the triblock polymer have been performed for comparison. The mechanism on the formation of the hierarchically structured silica in the presence of polypeptide-based triblock copolymer and AMTS is discussed. 2. Experimental Section Phe7-PEGi35-Phe7 was synthesized by our lab according to our previous works[7]. Anilino-methyl triethoxy silicane (denoted as AMTS, 95%, whose chemical formula is shown in Scheme 1) was obtained from Wuhan Tianmu Co. Ltd. (China). Tetratetraethoxysilane (TEOS, Aldrich) and dioxane (Aldrich) were used without further purification. Preparation of Hierarchical silica: In a typical experiment, 40 ml of Phe7-PEGi35-Phe7 solution in dioxane organic solvent (2.5 mg/ml) was gently mixed with 120 ml benzyl alcohol under stir for 20 min. Then 5ml of AMTS was added to the solution. The mixture was heated at 80°C for 10 min. Subsequently, 3 ml of benzyl ammine (as catalyst), 3 ml of deionized water and 5 ml of TEOS were added to the mixture, respectively. The final mixture was stirred for 10 min and was sonicated (with 20 kHz frequency, 300 w power) for 20 min to obtain a clear solution. The solution was kept static in a sealed flask at room temperature for two weeks. The product (denoted as SI) was obtained by washing with methanol and deionized water for several times, and dried at 60 °C for 24 hrs. The white as-synthesized product was calcined at 550°C for 5 hrs to remove organic solvents and the copolymer. In addition, white product was also obtained under the same reaction conditions except either without AMTS (denoted as S2) or without the triblock copolymer (no product was obtained). The obtained product was charaterization by means of TEM,SEM,XRD and N2 adsorption and desorption measurement. 3. Results and Discussion From XRD pattern of calcined sample SI (unshown), one broad diffraction peak with d spacing of 3.1 nm appears, indicating the presence of disordered short-range ordering structure. No such diffraction peak appears in the lowangle region of XRD pattern for sample S2(unshown), so it is believed that anilino-methyl triethoxy silicane (AMTS) plays a significant role in the formation of the disordered mesostructure of sample SI. If the triblock copolymer was not used under same synthesis condition of sample SI, silica precipitation did not occur and the system remained an optically clear solution
35
without formation of gel. Evidently, the triblock copolymer effectively controls the nucleation and growth of silica. From the SEM images (Fig. 1 a, b), the calcined sample SI displays interesting macroporous morphology, with the macropore size of about 200-700 nm. The macroporous network is composed of layers interconnected by struts-like pillars. For sample S2, the SEM images only display irregular particles (unshown), indicating that without the addition of AMTS, the triblock copolymers were not involved in the silica precipitation to give rise to formation of the specific morphology. The results from the nitrogen adsorption-desorption test of the calcined sample SI (unshown) indicate the presence of micropores (<2 nm) and its BET surface area of calcined sample is 439 m2/g (micropore area is 389 m2/g). The Barrett-Joynes-Halenda (BJH) average pore size of sample SI is estimated to be about 1.6 nm. This supermicroporous character is also confirmed by the TEM images (Fig.lc). The N2 adsorption line of the calcined sample S2 (unshown) is a type II isotherm, indicating the presence of mesovoids resulted from the aggregation of silica particles.
Fig.l. SEM images (a, b, with different magnifications)and TEM image (c) of calcined samples SI
In this experiment, the triblock copolymer, as a template, is indispensable to the formation of silica through controlling the nucleation and growth of silicate precursors. And anilino-methyl triethoxy silane (AMTS) which contains phenyl and amine groups was used as a bridge, i.e., on the one hand it interacts with polypeptide-based triblock copolymer through template-bridge interactions (hydrogen bonding and n-n interactions) between copolymer and AMTS; on the other hand, AMTS can co-condense with tetraethoxylsilane (TEOS) through hydrolysis process. 4. Conclusion The presented results suggest a bio-inspired approach for the preparation of hierarchical silica material with both supermicropores and macropores utilizing polypeptide-based triblock copolymer Phe7-PEGi35-Phe7 as a template through n —% interaction under ambient conditions. The formation of supermicropores
36
4. Conclusion Under acidic condition, using the co-assembly aggregate, i.e., supermicelles, driven by hydrogen bonding between hydrophilic polymer poly(vinyl alcohol) (PVA) and hydrophobic polymer hydroxyl-terminated polybutadiene (HTPB) as a template, mesoporous silica was synthesized by sol-gel reaction. The calcined silica had higher BET surface area (S = 500-600m2/g). The pore size on nanoscale can be easily adjusted through changing the ratio of PVA/HTPB. 5. Acknowledgement This work was supported by National Science Foundation of China (Grants No. 20373029, 20233030), and joint-research fund of Nankai University and Tianjin University on Nano-sciences. 6. References [1] C.T. Kresge, C.Z. Leonow, W.J. Roth et al.Nature, 359 (1992)710. [2] D. Zhao, J. feng, Q. Huo, N. Melosh, G.H. Fredericson, B.F. Hmelkaand G.D. Stucky, Science, 279(1998)548. [3] S. M. Yang, I. Sokolov, N. Coombs, C.T. Kresge and G.A. Ozin, Adv. Mater. 11 (1999)1427. [4] B. C. Chen, H.P. Lin, M.C. Chao et al., Adv. Mater. 16 (2004)1657. [5] F. Iskandar, Mikrajuddin and K. Okuyama, Nano Lett. 1 (2001)231.