FULL PAPER DOI: 10.1002/asia.201402636

Synthesis of a Large-Sized Mesoporous Phosphosilicate Thin Film through Evaporation-Induced Polymeric Micelle Assembly Yunqi Li,[a, b] Bishnu Prasad Bastakoti,*[a] Masataka Imura,[c] Norihiro Suzuki,[d] Xiangfen Jiang,[a] Shinobu Ohki,[e] Kenzo Deguchi,[e] Madoka Suzuki,[f, g] Satoshi Arai,[f] and Yusuke Yamauchi*[a, b] Abstract: A triblock copolymer, poly (styrene-b-2-vinyl pyridine-b-ethylene oxide) (PS-b-P2VP-b-PEO) was used as a soft template to synthesize largesized mesoporous phosphosilicate thin films. The kinetically frozen PS core stabilizes the micelles. The strong interaction of the inorganic precursors with the P2VP shell enables the fabrication of highly robust walls of phosphosili-

cate and the PEO helps orderly packing of the micelles during solvent evaporation. The molar ratio of phosphoric acid and tetraethyl orthosilicate is crucial to achieve the final mesostructure. Keywords: block copolymers · mesoporous materials · micelles · phosphosilicates · self-assembly

The insertion of phosphorus species into the siloxane network is studied by 29 Si and 31P MAS NMR spectra. The mesoporous phosphosilicate films exhibit steady cell adhesion properties and show great promise as excellent materials in bone-growth engineering applications.

1. Introduction Mesoporous materials have been widely used in several research fields because of their favorable structural advantages, such as high surface area, large pore volume, and uniformly sized pores. The design of mesoporous materials with desired shapes and compositions is of significant importance. Of several available strategies, the soft-templating method is one of the most studied methods for synthesizing mesoporous structures.[1–3] Doping of different elements at the atomic level has also been reported by using self-assembly of surfactants or block copolymers. The selection of structure-directing agents (i.e., templates) plays a vital role to determine the final mesoporous architecture. The physical and chemical properties of silica can be modified by integration of different elements into the siloxane network.[4–6] Phosphosilicate is a silicate-based material that has high biocompatibility and nontoxic character compared with pure silica. This makes phosphosilicate an excellent biomaterial for drug delivery, cell culture, and bonegrowth engineering.[7, 8] Sol-gel precursors prepared from silicon alkoxides and phosphoric acid/salts have been often used to prepare mesoporous phosphosilicate with the assistance of structure-directing agents.[9, 10] Bhaumik et al. reported the synthesis of hollow mesoporous phosphosilicate nanoparticles with dimensions of 20 to 25 nm and a thin pore wall through a dual-template system that consisted of pluronic F127 block copolymer and trivinyl cyclohexane.[11] Nogami et al. used a sol-gel method with pluronic P123 block copolymer as a template to synthesize mesoporous phosphosilicate monoliths with a pore size of 7 nm.[12] It is obvious that larger diameter of mesopores is more suitable

[a] Y. Li, Dr. B. P. Bastakoti, Dr. X. Jiang, Prof. Y. Yamauchi World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA) National Institute for Materials Science (NIMS) 1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan) E-mail: [email protected] [email protected] Homepage: http://www.yamauchi-labo.com [b] Y. Li, Prof. Y. Yamauchi Faculty of Science and Engineering Waseda University 3-4-1 Okubo, Shinjuku, Tokyo 169-8555 (Japan) [c] Dr. M. Imura Optical and Electronic Materials Unit, Environment and Energy Materials Division National Institute for Materials Science (NIMS) 1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan) [d] Dr. N. Suzuki International Center for Young Scientists (ICYS) National Institute for Materials Science (NIMS) 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 (Japan) [e] Dr. S. Ohki, Dr. K. Deguchi High Magnetic Field Station National Institute for Materials Science (NIMS) 3-13 Sakura, Tsukuba, Ibaraki, 305-0003 (Japan) [f] Prof. M. Suzuki, Dr. S. Arai Waseda Bioscience Research Institute in Singapore (WABIOS) 11 Biopolis Way, Singapore, 138667 (Republic of Singapore) [g] Prof. M. Suzuki Organization for University Research Initiatives Waseda University 513 Wasedatsurumaki-cho, Shinjuku, Tokyo 162-0041 (Japan) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402636.

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tween adjacent P2VP + units. The hydrodynamic diameter of fully extended polymeric micelles was found to be 110 nm by using dynamic light scattering (DLS) analysis. The Tyndall effect was observed clearly, which indicates the formation of micelles after the addition of HCl (Figure S1 in the Supporting Information). The concrete evidence for the micelle formation was observed by using scanning electron microscope (SEM). Uniformly distributed spherical micelles were clearly seen (Figure 2a). The zeta-potential measure-

for the application with enhanced performance because larger biomolecules can be adsorbed with high rate.[13] Pluronic-type triblock block copolymer (P123, F127) is one of the most commonly used templates, but the pore size is always limited to less than 10 nm due to the smaller molecular weight and shorter chain length of core-forming block.[14] The use of various additives, such as swelling agents, cosurfactants, and salts, is effective for expansion of the pore size, but it is still hard to obtain larger mesopores (> 20 nm).[15, 16] Herein, we report an additive-free approach to synthesize mesoporous phosphosilicate films with a pore size of approximately 50 nm by using a polymeric micelle assembly (Figure 1). Very stable polymeric micelles of core-shell-

Figure 1. Formation mechanism of mesoporous phosphosilicate thin film through evaporation-induced polymeric micelle assembly.

Figure 2. a) SEM image of PS-b-P2VP-b-PEO micelles. b, c) SEM and d) TEM images of mesoporous phosphosilicate films calcined at 500 8C with a H3PO4/TEOS molar ratio of 2:1.

corona-type block copolymer, poly(styrene-b-2-vinyl pyridine-b-ethylene oxide) (PS-b-P2VP-b-PEO), can serve as a template to construct mesopores with larger diameters. It has several advantages in the synthesis of mesoporous phosphosilicate with larger pores. The highly stable polymeric micelles possess a PS core, a reactive quarternized P2VP shell in acidic conditions, and a neutral PEO corona. The strong interaction of micelles with inorganic precursors and the high carbon content of the block copolymer enable us to fabricate thermally stable mesoporous phosphosilicate thin films. Such mesoporous bioglasses with high biocompatibility, including phosphosilicate composites, are attractive as a promising material in the field of bone-tissue engineering.[17]

ment was carried out to determine the surface charge of the micelles, which provides useful information about the interaction of inorganic precursors with polymeric micelles.[18, 19] For neat polymeric micelles, the zeta-potential value was approximately 30 mV. After addition of the inorganic species (TEOS and H3PO4), the zeta-potential value was sharply reduced to approximately 15 mV due to masking of the charged micelle surface by the inorganic species. The negative zeta-potential value suggests that more PO43 interacts with micelles, as expected from the stoichiometry. The composite micelles are still well dispersed in the solution, as shown in Figure S1c in the Supporting Information. Through evaporation-induced assembly of the formed composite micelles, mesoporous phosphosilicate films were prepared, as shown in Figure 1. The composite micelles deposited on the substrate formed the shell cross-linked micelles during solvent evaporation. After complete evaporation of the solvent, the as-prepared films were calcined at different temperatures to remove the polymeric template. The uniform film of phosphosilicate on the entire area of substrate was observed under SEM (Figure 2b and 2c). Figure 2c shows an enlargement of Figure 2b. The pore size and the wall thickness were found to be approximately 50 and 20 nm, respectively. TEM measurements further proved the mesoporous architecture. In the phosphosilicate film calcined at 500 8C, the mesopores with a pore diameter of approximately 50 nm were distributed over the entire area (Figure 2d).

2. Results and Discussion PS-b-P2VP-b-PEO block copolymer was dissolved in tetrahydrofuran (THF) to form a clear polymer solution. THF is a good solvent to dissolve each unit of the triblock copolymer. An optimum amount of hydrochloric acid (HCl) was added dropwise to stimulate micellization of the triblock copolymer. The addition of HCl initiates self-assembly of the triblock copolymer by lowering the free energy in the system because it is a poor solvent for the hydrophobic PS unit, and the H + ions help the protonation of P2VP unit and this results in a change in micellar morphology from shrunken to extended due to the electrostatic repulsion be-

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The beauty of this approach is that the pore size and wall thickness can be easily tuned by changing the molecular weight of the block copolymer and precursor/polymer ratio, respectively.[20–22] Sol-gel precursors of phosphosilicate comprise different molar ratios of inorganic precursors (H3PO4 and TEOS). It is necessary to find the optimized molar ratio of inorganic precursors to control the mesostructure. We investigated the mesostructures of phosphosilicate films prepared with different molar ratios of H3PO4 and TEOS, with the other parameters unchanged (Figure S2 in the Supporting Information). As the molar ratios of H3PO4 and TEOS were varied from 2:1 (Sample A) to 3:1 (Sample B) and 5:1 (Sample C), the local ordering of mesopores was destroyed and, therefore, the optimized molar ratio of H3PO4 and TEOS in the present study was 2:1 (Sample A). It has been already reported that the water-soluble PEO unit becomes insoluble at higher PO43 concentrations.[23] Thus, the insolubility of PEO unit cannot work well to preserve the micelle structure and finally causes the deformation of the mesoporous structure of phosphosilicate films, as seen in SEM images (Figure S2 in the Supporting Information). To confirm the crystallinity of the mesoporous phosphosilicate, wide-angle XRD analysis was performed for sample A calcined at 500 8C. All the peaks were perfectly matched with 2 SiO2·P2O5 (JCPDS card 00-016-0582), as shown in Figure 3. Aronne et al. reported amorphous phos-

Figure 4. a) HAADF-STEM image and nanoscale elemental mapping of b) P and c) Si, and d) overlapping image of P and Si in mesoporous phosphosilicate. This mesoporous phosphosilicate film with a H3PO4/TEOS molar ratio of 2:1, was calcined at 500 8C.

prepared films were calcined at different temperatures to check the thermal stability of phosphosilicate (Figure S3 in the Supporting Information). The well-connected mesoporous structures of phosphosilicate films were well preserved, with thin pore walls even when the calcination temperature reached 900 8C. When the calcination temperature increased to 1100 8C, the mesoporous structure was completely melted. Thermogravimetric analysis demonstrates that triblock copolymer PS-b-P2VP-b-PEO has a higher thermal decomposition temperature (up to 500 8C) than pluronictype copolymer (up to 250 8C).[26–28] The pluronic-type block copolymer is completely burnt out before the crystallization of phosphosilicate, which makes the mesostructure weak and unstable. However, in the present study, crystallization of phosphosilicate occurs before the complete combustion of PS-b-P2VP-b-PEO block copolymer. This provides good thermal stability of the crystalline framework even at high temperature without any deformation of the mesostructure. Inductively coupled plasma-optical emission spectrometry (ICP-OES) was applied to determine the exact content of P and Si. The molar ratios of P and Si in the final products are shown in Table S1 in the Supporting Information. 29Si and 31 P MAS NMR spectroscopy measurements were carried out for mesoporous phosphosilicate powder calcined at 500 8C (Sample A). In the 29Si NMR spectra, the main signal for this phosphosilicate sample was observed with a chemical shift of d = 113.11 ppm (Figure 5). The decrease in the chemical shift of the siloxane network from d = 110 to 113.11 ppm indicates some interaction has occurred between the siloxane network and the phosphorus species. The fully substituted Si(OP)4 structure has a chemical shift of d = 119 ppm.[29] The 29Si NMR spectroscopy results reveal that the partial insertion of phosphorus species into the siloxane network has taken place with the formation of a P O Si bond. In the 31P NMR spectra (Figure 5), resonances at d = 10.6 and 23.5 ppm were consistent with phosphosi-

Figure 3. Wide-angle XRD patterns of mesoporous phosphosilicate with a H3PO4/TEOS molar ratio of 2:1, calcined at 500 8C.

phosilicate up to 700 8C.[24] Unlike the previous study, the crystallized framework of our mesoporous phosphosilicate powder was formed at 500 8C. The homogeneous distribution of a high number of phosphorus atoms into the siloxane network induces a lower crystallization temperature of the phosphosilicate material.[25] According to elemental mapping (Figure 4), both P and Si were homogeneously distributed throughout the entire film. The calcination temperature influences the final crystallinity and morphology of the mesoporous structure. The as-

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proteins, DNA, and small chemical compounds effective for new bone growth can be loaded and released gradually. Our methods will open up the possibilities to fabricate improved functionalized bioglasses compared with previous ones.

3. Conclusion A facile one-step method based on polymeric micelle assembly was adopted to synthesize large-sized mesoporous phosphosilicate films. The triblock copolymer PS-b-P2VP-b-PEO proved to be an excellent template to construct the mesoporous structure through the micelles assembly. The as-obtained robust mesoporous architectures were preserved even at high calcination temperatures (900 8C) without collapse of the mesoporosity. The insertion of phosphorus on the siloxane network was further confirmed by 29Si and 31 P MAS NMR spectroscopy. As supported by a cell-adhesion test, we believe that the highly biocompatible phosphosilicate would be an excellent material for future application in bone-growth engineering.

Figure 5. 68.2 MHz 29Si and 202.4 kHz 31P MAS NMR spectra of mesoporous phosphosilicate powder with a H3PO4/TEOS molar ratio of 2:1, calcined at 500 8C.

licate material.[24, 29] Uncertain resonances between d = 40.0 and 60.0 ppm are probably assigned to unreacted phosphorous on the surface of the phosphosilicate. For a preliminary test for biomedical applications, we examined the cell adhesion properties of the mesoporous phosphosilicate film. HeLa cells were cultured on a mesoporous phosphosilicate film (calcined at 500 8C, Sample A) for a couple of days. The microscopic image in Figure 6a indi-

Experimental Section Materials Triblock copolymer PS (14 500)-b-P2VP (20 000)-b-PEO (33 000) with a polydispersity index of 1.15 (numbers in parentheses indicate the molecular weight of each block) was used as a template. Phosphoric acid (H3PO4 ; TCI), tetraethyl orthosilicate (TEOS; Wako), tetrahydrofuran (THF; Nacalai), and hydrochloric acid (HCl; Nacalai) were used without further purification. Preparation of Mesoporous Phosphosilicate Film The mesoporous phosphosilicate films were synthesized through our previous micelle assembly approach.[20–22] PS-b-P2VP-b-PEO block copolymer (20 mg) was dissolved in THF (4 mL). HCl (80 mL, 35 %) was added to the polymer solution with vigorous stirring at RT. Inorganic sources (TEOS and H3PO4) were successively added dropwise to the micelle solution and then the mixture was stirred continuously for 4 h. The mixture (200 mL) was spin-coated onto clean silicon substrate (typical size 1.5  1.5 cm) and glass substrate (typical size 2.0  2.0 cm) to form mesoporous thin films (the remaining solution was left to evaporate on glass dishes at RT for further characterizations, such as wide-angle XRD and NMR spectroscopy measurements). The speed of the spin coater was fixed at 3000 rpm for 30 seconds. After complete evaporation of solvent, the thin films and powders were transferred to a furnace and calcined at various temperatures for 4 h at a ramping rate of 1 8C min 1.

Figure 6. a) The adhesion of HeLa cells on the surface of a mesoporous phosphosilicate films on glass substrate. The image was taken by using a confocal microscope (FV 1000, Olympus, objective lens  20). The cells were stained with a cell tracker (left: fluorescence image (FI); right: bright field image (BF)). b) The merged FI and BF images of cells grown on mesoporous phosphosilicate films on a glass substrate (left) and on an unmodified glass substrate (right). In terms of cellular morphology, there was little difference between these two surfaces. Scale bar: 50 mm.

Characterization An Otsuka ELSZ particle and zeta-potential analyzer was used to measure the hydrodynamic diameter and zeta-potential of micelles and composite micelles. All the measurements were carried out at 25 8C. A scanning electron microscope (HITACHI SU-8000) and transmission electron microscope (JEOL JEM-2100F) were used to observe the morphology of the mesoporous thin films. A wide-angle powder X-ray diffraction instrument (XRD, Rigaku RINT 2500X) was used to check the crystal structure by using monochromatic CuKa radiation (40 kV, 40 mA) at a scanning rate of 2.08 min 1. Inductively coupled plasma atomic emission spectroscope (ICP-AES; SII Nano Technology Inc.) was used to determine the concentration of Si and P of the powders. Solid-state NMR spectroscopy experiments were performed by using a JNM ECA500 spectrometer. NMR spectra were measured by using MAS (magic angle spinning) with

cates that HeLa cells are adhered to the surface and proliferated well. For comparison, the adherence of HeLa cells on the surface of a glass substrate without modification is also shown in Figure 6b. How the mesoporous phosphosilicate material affects the cellular activities at a molecular level still remains elusive. Therefore, this should be addressed in future studies through assessment of the adhesion properties of various cell types. Surface modification by using polymeric micelles will also provide mesoporous spaces in which

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H dipolar decoupling. The MAS speed was at 10 kHz for 29Si and 15 kHz for 31P, and pulse recycle delay time was set at 100 s for 29Si and 300 s for 31P.

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FULL PAPER Polymeric micelle assembly: Evaporation-induced assembly of spherical micelles composed of an asymmetric poly(styrene-b-2-vinyl pyridine-b-ethylene oxide) triblock copolymer (see figure) enables the fabrication of a mesoporous phosphosilicate thin film with a uniform pore size of 50 nm. The mesoporous phosphosilicate films exhibit steady cell adhesion properties and show great promise as excellent materials in bone-growth engineering.

Thin Films Yunqi Li, Bishnu Prasad Bastakoti,* Masataka Imura, Norihiro Suzuki, Xiangfen Jiang, Shinobu Ohki, Kenzo Deguchi, Madoka Suzuki, Satoshi Arai, &&&&—&&&& Yusuke Yamauchi* Synthesis of a Large-Sized Mesoporous Phosphosilicate Thin Film through Evaporation-Induced Polymeric Micelle Assembly

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Synthesis of a large-sized mesoporous phosphosilicate thin film through evaporation-induced polymeric micelle assembly.

A triblock copolymer, poly(styrene-b-2-vinyl pyridine-b-ethylene oxide) (PS-b-P2VP-b-PEO) was used as a soft template to synthesize large-sized mesopo...
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