pubs.acs.org/Langmuir © 2009 American Chemical Society
Fabrication of Hierarchical CaCO3 Mesoporous Spheres: Particle-Mediated Self-Organization Induced by Biphase Interfaces and SAMs Xiaohong Liang,† Junhui Xiang,*,† Fushi Zhang,‡ Li Xing,† Bo Song,† and Shiwei Chen† †
College of Chemistry and Chemical Engineering, Graduate University of the Chinese Academy of Sciences, Beijing, 100049 China and ‡Department of Chemistry, Tsinghua University, Beijing, 100084 China Received October 8, 2009. Revised Manuscript Received November 27, 2009
Highly ordered hierarchical calcium carbonate is an important phase involved in calcification by a wide variety of invertebrate organisms, and its formation is of technological interest in the development of functional materials. In this article, porous CaCO3 hierarchical microspheres with a hedgehoglike appearance have been fabricated on the flexible substrate under mild conditions. There are two points that play important roles in the regular organization of the terminal products: one is the biphase interfaces, which are generated by organic solvent n-hexane and an aqueous saturated solution of Ca(OH)2, and the other is hydroxyl-terminated monolayers assembled on the flexible PET (poly(ethylene terephthalate)) substrate. The SEM images show that novel CaCO3 hierarchical microspheres consist of densely stacked “shuttles” by the oriented self-organization of CaCO3 nanoparticles. The IR and XRD spectra indicate that the as-synthesized products are composed of a calcite phase obtained by an ACC (amorphous calcium carbonate)to-calcite transformation. In view of the results, a nanoparticle-mediated self-organization process induced by biphase interfaces and SAMs template is proposed for the integration of functional materials and nanodevices.
Introduction Calcium carbonate has important applications in various fields, especially in biomedical applications such as drug delivery systems1,2 and bone implants.3,4 Great achievements and progress have been made in controllable synthesis, but the fabrication of oriented crystal units with regular microscale and nanoscale features still presents a challenge. Therefore, the synthesis and morphological control of CaCO3 crystals with complex structures have attracted extensive interest.5-8 Many organisms, including mollusks, echinoderms, calcisponges, corals, certain algae, and others, form their hierarchical mineral skeletons out of calcium carbonate minerals through the organization of nanoscale units under mild condition.9 In these processes, two main types of organic components are utilized: soluble biomolecules and insoluble organic matrixes. These organic components are thought to regulate crystal nucleation and growth, modulate crystal shape and size, and control the organization of nanoscale building blocks into complex structures through molecular recognition as well as influence the final mechanical properties.10-12 To mimic *Corresponding author. Tel: þ86(10)8825-6532. Fax: þ86(10)8825-6092. E-mail:
[email protected]. (1) Wei, W.; Ma, G. H.; Hu, G.; Yu, D.; Mcleish, T.; Su, Z. G.; Shen, Z. Y. J. Am. Chem. Soc. 2008, 130, 15808. (2) Butler, M. F.; Frith, W. J.; Rawlins, C.; Weaver, A. C.; Heppenstall-Butler, M. Cryst. Growth Des. 2009, 9, 534. (3) Schillera, C.; Rascheb, C.; Ollerb, M. W.; Beckmannc, F.; Eufingerb, H.; Epplea, M.; Weiheb, S. Biomaterials 2004, 25, 1239. (4) Piattelli, A.; Podda, G.; Scarano, A. Biomaterials 1997, 18, 623. (5) Chen, S. F.; Yu, S. H.; Wang, T. X.; Jiang, J.; C€olfen, H.; Hu, B.; Yu, B. Adv. Mater. 2005, 17, 1461. (6) Chen, S. F.; Yu, S. H.; Jiang, J.; Li, F. Q.; Liu, Y. K. Chem. Mater. 2006, 18, 115. (7) Ahmad; Sastry, M. J. Am. Chem. Soc. 2003, 125, 14656. (8) Aizenberg, J. Adv. Mater. 2004, 16, 1295. (9) Politi, Y.; Arad, T.; Klein, E.; Weiner, S.; Addadi, L. Science 2004, 306, 1161. (10) Pokroy, B.; Fitch, A. N.; Zolotoyabko, E. Cryst. Growth Des. 2007, 7, 1580. (11) Wang, R. Z.; Addadi, L.; Weiner, S. Phil. Trans. R. Soc. Lond. B 1997, 352, 469. (12) Marı´ n-Garcı´ a, L.; Frontana-Uribe, B. A.; Reyes-Grajeda, J. P.; Stojanoff, V.; Serrano-Posada, H. J.; Moreno, A. Cryst. Growth Des. 2008, 8, 1340.
5882 DOI: 10.1021/la9037815
these processes, many organic additives such as surfactants,13 biomolecules,14,15 double hydrophilic block copolymers (DHBCs),16,17 homopolymers,18 and so on have been implemented into in vitro syntheses. However, such efforts focus on the influence of organic functional groups;the hydroxyl group, carboxyl group, amino group, phosphate, sulfates, and so forth. It is generally believed that the regulation of organic components on the crystals is due to their adsorptions onto crystals by the functional groups, but in real biosystems, these processes occur within specific environments, usually between the interfaces of the insoluble organic matrix and aqueous solution. In the insoluble organic matrixes, there are many alkyl chains, except the functional groups. However, very little effort19,20 has been made to investigate the roles that such interface environments and nonfunctional groups;alkyl chains;play in the development of highly ordered hierarchical materials. In this research, we construct an organic-aqueous biphase interface system to mimic the living biomineralization environment. In this system, the static biphase interface acts as the organic-inorganic coexisting reaction regions and a simple chained alkane is selected as the organic phase for investigating the influence of nonfunctional group sections in the organic matrix. Although they limit the function of the organic matrix, such in vitro systems provide much necessary information on the role of the organic matrix within mineralizing systems. (13) Walsh, D.; Lebeau, B.; Mann, S. Adv. Mater. 1999, 11, 324. (14) Voinescu, A. E.; Touraud, D.; Lecker, A.; Pfitzner, A.; Kunz, W.; Ninham, B. W. Langmuir 2007, 23, 12269. (15) Lukeman, P. S.; Stevenson, M. L.; Seeman, N. C. Cryst. Growth Des. 2008, 8, 1200. (16) Guillemet, B.; Faatz, M.; Gr€ohn, F.; Wegner, G.; Gnanou, Y. Langmuir 2006, 22, 1875. (17) Kulak, A. N.; Iddon, P.; Li, Y. T.; Armes, S. P.; C€olfen, H.; Paris, O.; Wilson, R. M.; Meldrum, F. C. J. Am. Chem. Soc. 2007, 12, 3729. (18) Donnet, M.; Bowen, P.; Jongen, N.; Lema^itre, J.; Hofmann, H. Langmuir 2005, 21, 100. (19) Maas, M.; Rehage, H.; Nebel, H.; Epple, M. Langmuir 2009, 25, 2258. (20) Maas, M.; Rehage, H.; Nebel, H.; Epple, M. Prog. Colloid Polym. Sci. 2008, 134, 11.
Published on Web 12/18/2009
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To prepare materials with highly ordered architectures, the emergence of self-organization with a matrix and control of the organization with a template are necessary.21 The constructed organic surfaces, such as self-assembled monolayers (SAMs),22,23 Langmuir monolayers,24,25 and polymers,26 are widely used as templates for directional crystallization and assembly. In this study, the authors choose the hydroxyl-terminated SAMs as constructed organic surfaces to induce the nucleation and modulate the organization of crystals. The SAMs are modified on a flexible PET (poly(ethylene terephthalate)) substrate surface via layer-by-layer electrostatic self-assembly so that the substrate can just float between the biphasic interfaces. Maas et al. described the growth of the coherent calcium carbonate thin film under Langmuir monolayers of stearic acid at the liquid-liquid interfaces.19,20 Under the associated action of liquid-liquid interfaces and Langmuir monolayers, the film consisted of aggregated stearic acid-stabilized CaCO3 nanoparticles However, until now, there has been no research to directly identify the respective rules of the biphase interfaces (namely, nonfunctional groups) and monolayers (namely, functional groups). In this research, SAMs on the PET substrate can not only provide a constructed organic surface but also separate the functional groups and the nonfunctional sections of the organic matrix in the space. Mineralization can be performed first between the organic-aqueous biphasic interfaces without the influence of hydroxyl-terminated SAMs. Under this premise, nonfunctional sections can play their full roles in the crystallized direction and rate, which is helpful in investigating their influence on the details. Therefore, a model system that combines the organic-inorganic biphase interfaces and the SAMs is established to mimic a similar process in nature. A detailed investigation of their influence on mineralization and in synthesizing high-performance materials was performed by the room-temperature mineralization of calcium carbonate.
Experimental Section Materials. Calcium hydroxide, ammonium bicarbonate, and n-hexane were purchased from Beijing Chemical Reagents Company. Aminopropyl triethoxysilane (APTES) was purchased from Alfa Aesar. Octadecyltrichlorosilane (OTS) was purchased from Acros Organics. All chemicals were of analytical grade and used as received without further purification. All of the glassware was cleaned in water for 5 min by ultrasonication, rinsed with distilled water, and finally dried at 105 °C.
Fabrication of a Buffer Layer on a PET Substrate. 27
APTES was dissolved in acetone (1 vol %) and aged for 240 h. PET substrate was ultrasonically cleaned for 5 min successively in distilled water, ethanol, and acetone and then dried at 105 °C for 5 min. The dried substrate was immersed in an acetone solution of APTES for 5 min and baked at 105 °C for 5 min to form a buffer layer.
Fabrication of SAMs on an APTES-Buffered PET Substrate.28 OTS was dissolved in anhydrous toluene (1 vol %). The substrate grafted by buffer layers was immersed in an anhydrous toluene solution under a nitrogen atmosphere for 2 min and then (21) Imai, H.; Oaki, Y.; Kotachi, A. Bull. Chem. Soc. Jpn. 2006, 79, 1834. (22) Han, Y. J.; Aizenberg, J. J. Am. Chem. Soc. 2003, 125, 4032. (23) Travaille, A. M.; Kaptijn, L.; Verwer, P.; Hulsken, B.; Elemans, J. A. A. W.; Nolte, R. J. M.; Kempen, H. J. Am. Chem. Soc. 2003, 125, 11571. (24) Chen, Y. J.; Xiao, J. W.; Wang, Z. N.; Yang, S. H. Langmuir 2009, 25, 1054. (25) Popescu, D. C.; Smulders, M. M. J.; Pichon, B. P.; Chebotareva, N.; Kwak, S.-Y.; Asselen, O. L. J.; Sijbesma, R. P.; DiMasi, E.; Sommerdijk, N. A. J. M. J. Am. Chem. Soc. 2007, 129, 14058. (26) Murphy, W. L.; Mooney, D. J. J. Am. Chem. Soc. 2002, 124, 1910. (27) Zhu, P. X.; Teranishi, M.; Xiang, J. H.; Masuda, Y.; Seo, W. S.; Koumoto, K. Thin Solid Film 2005, 473, 351. (28) Xiang, J. H.; Zhu, P. X.; Masuda, Y.; Koumoto, K. Langmuir 2004, 20, 3278.
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rinsed with anhydrous toluene and baked at 105 °C for 5 min to remove residual solvent and promote the chemisorption of SAMs. Finally, OTS-SAM-modified PET was treated by 254 nm ultraviolet irradiation, leading to the hydroxyl-terminated substrate.
Deposition of CaCO3 on the SAM-Modified PET Substrate. In this research, n-hexane was chosen as the organic phase and the aqueous phase was a saturated solution of calcium hydroxide. Room-temperature mineralization of CaCO3 was performed by a slow gas-diffusion procedure. A 100 mL beaker was covered with Parafilm (with four pinholes), and CO2 was removed from it with a stream of nitrogen lasting for 10 min. A 50 mL aqueous solution of saturated Ca(OH)2 and 10 mL of n-hexane were injected into the beaker. The modified PET substrate was dipped upside down in the organic-aqueous biphase boundary. In this experiment, the upside of the substrate was the side modified with OTS-SAMs. At room temperature, the beaker was put into a closed desiccator containing a vial of ammonium bicarbonate. The nucleation and growth of CaCO3 were induced with carbon dioxide from the decomposition of NH4HCO3 diffusing into the system. After a period of time, the PET substrate with CaCO3 was removed and then the overgrown specimens were slightly rinsed with deionized water. Finally, the PET substrate on which CaCO3 has grown was dried in air for further characterization. Scanning Electron Microscopy (SEM) Observation. SEM observations were conducted by using a JMS-6301F scanning electron microscope. The surfaces of blank and OTS-SAMmodified PET substrates as well as the as-deposited CaCO3 crystals after different mineralization times on the PET substrates were observed by SEM. For better conductivity, the samples were sputtered with nanogold before observations. Fourier Transform Infrared Spectroscopy. To identify the crystallization procedure in this study, the structures of initial products between the biphase interfaces were investigated by Fourier transform infrared (FTIR) spectrometry (AVATAR 360). Products after 1 h of mineralization were collected on cover glasses and dried with a stream of nitrogen for the analysis of FTIR spectrometry. In a contrast experiment, products after 48 h of mineralization were also characterized by FTIR. X-ray Powder Diffractometry (XRD). A CaCO3 sample on the PET substrate (15 mm 25 mm) was obtained after 48 h of CO2 diffusion. The XRD analyzer (MSAL XD-2 powder X-ray diffractometer) was operated at 30 kV with a current of 30 mA with Cu KR radiation (λ = 1 054 056 A˚).
Results and Discussion As-prepared CaCO3 Crystals on PET Substrates. Figure 1 shows representative SEM images of as-deposited CaCO3 crystals on flexible PET substrates. After 20 h of CO2 gas diffusion, a novel CaCO3 “shuttle” structure was obtained (Figure 1a). Furthermore, it was found that these isolated shuttles formed some angle to the substrate. The SEM image (Figure 1a) shows that the shuttle-shaped CaCO3 crystals are rather regular in shape. According to a shuttle nearly parallel to the PET surface, its length was about 7 to 8 μm and its maximum width was about 2.5 μm in the middle, from which the width of the shuttle decreased in a streamlined manner. In the terminal region, the width became close to 500 nm. It was worthy noting that the surface of the shuttle was porous. The magnified SEM image in the inset of Figure 1a shows the detailed surface structures in which each shuttle consists of many small protuberances with a mean size of 60 nm and many small holes. These protuberances are small nanoparticles, and the pore diameters range from 50 to 150 nm as expressed in the magnified image (inset of Figure 1a). According to the SEM images, it can be presumed that the formation procedure is a nanoparticle-mediated self-organization process. The shuttles were formed by the self-organization of the DOI: 10.1021/la9037815
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Figure 1. Typical SEM images and corresponding XRD patterns of self-organized CaCO3 grown on hydroxyl-terminated OTS-modified PET substrates obtained by the gas-diffusion method at ambient temperature: (a, b) after 20 h of the gas-diffusion reaction, with the inset showing a high-magnification image of the specimen presented in image a; (c, d) after 48 h of the gas-diffusion reaction, with the inset showing a low-magnification image of the specimen presented in image c; (e) XRD spectra of CaCO3 after 48 h of mineralization; the inset is the XRD pattern of a blank PET substrate.
small nanoparticle protuberances. In the same sample, some CaCO3 crystals tend to have flowerlike structures as presented in Figure 1b. It is interesting that the flower “petals” preferred to grow out of the opposite side to show a decussating feature. With the increment of the petals, deviation from normal positions took place mainly as a result of a steric hindrance effect. When the reaction time was prolonged from 20 to 48 h, spherical particles with a hedgehoglike appearance of 25 μm diameter (Figure 1c) were formed. The inset in Figure 1c is the lower-magnification image with large-scale spherical products. In the mineralization sample after 48 h, it was found that there were still some flowerlike products as presented in Figure 1d that evidently contrast with the products shown in Figure 1b. The corresponding XRD spectrum (Figure 1e) clearly indicates that the as-synthesized products are composed of a calcite phase with the characteristic (104), (110), (113), (202), and (116) planes. The two broad peaks observed at 47.2° and 54.1° are consistent with the XRD pattern of blank PET shown in the inset of Figure 1e. Emergence of Amorphous Calcium Carbonates (ACCs). To elucidate the mechanisms involved in the formation of spherical products, the initial mineralization products were analyzed. 5884 DOI: 10.1021/la9037815
In the early stage of the mineralization reaction (about 1 h), many snow-shaped flakelets were formed between the organic-aqueous biphase interfaces, but the solution remained clear at the same time. FTIR was used to investigate the nature of the products further after 1 h of mineralization (shown in Figure 2a). As a comparison, the products after 48 h of mineralization were also investigated by FTIR (Figure 2b). The characteristic sharp peak of in-plane carbonate bending (ν4) at 713 cm-1 and a single absorption at 1425 cm-1 (Figure 2b) show that the product obtained after 48 h of mineralization is calcite. Figure 2a shows that there are two main carbonate absorptions around 1450 cm-1 as compared to a single absorption for calcite, which is the asymmetric stretch (ν3) of the carbonate ion.29 This is indicative of a lack of symmetry in the environment of the carbonate ions. The lack of symmetry is also expressed by the presence of a broad peak at around 1080 cm-1 attributed to the symmetric stretch (ν1) in noncentrosymmertric structures.30 These feature peaks show (29) Addadi, L.; Raz, S.; Weiner, S. Adv. Mater. 2003, 15, 959. (30) Aizenberg, J.; Muller, D. A.; Grazul, J. L.; Hamann, D. R. Science 2003, 299, 1205.
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Figure 2. (a) Infrared spectrum of ACC obtained from the biphase interfaces after 1 h of CO2 diffusion at ambient temperature. (b) Control spectrum of a calcite crystal obtained from the biphase interfaces after 48 h of mineralization. (c) Characteristic SEM micrograph of the ACC aggregates prepared on the OTS-modified PET substrates, which were obtained after 1.5 h of the mineralization reaction. (Inset) Low-magnification SEM image of the ACC aggregates.
that the products after 1 h of mineralization are amorphous calcium carbonates (ACC). The peak at 713 cm-1 comprises a peak superimposed on a very broad peak instead of a characteristic sharp peak of calcite, which indicates the formation of calcite by the transformation of a transient ACC. Furthermore, the ratio between the maximum intensity of the peaks at 875 cm-1, which is a sharp out-of-plane bending peak, and 713 cm-1 has been demonstrated to be quantitatively related to the percentage of ACC in the sample.9,29 The above data indicate that the flakelets are a mixture of amorphous calcium carbonates and calcite. The SEM image (Figure 2c) of the products on the modified PET substrate after 1.5 h of CO2 diffusion reveals that spherical particles are ∼50 nm ACC.31 The ACC particles tend to aggregate on the substrate as shown in the inset of Figure 2c. The nanoparticle-mediated self-organization process of the mineralized products could be further inferred with the aid of the magnified SEM images (shown in Figure 3). According to Figure 3, it does not take much to see that all of the products, including the shuttle, flower, and microsphere, are composed of the subunit nanoparticles. The results of nanoparticle self-attachment show that they are porous on the surface. On the modified PET surface, the nanoparticles preferred to form the shuttleshaped structure via particle-mediated self-organization, whether in the flower or in the microsphere (Figure 3b,c). When two nanoparticles approached each other closely enough, they were mutually attracted by van der Waals forces. However, because of their thermal energy they could still rearrange to find the lowenergy configuration represented by a coherent particle-particle interfaces.32 Stereochemical and epitaxial relationships between crystalline surfaces could drive the oriented self-organization. The interface free energies of the liquid-crystal, liquid-substrate, and substrate-crystal interfaces played important roles in the process. All of these factors ensured the directional crystallization and organization so that the morphology of the products was unified. (31) Wei, H.; Ma, N.; Song, B.; Yin, S. C.; Wang, Z. Q. J. Phys. Chem. C 2007, 15, 5628. (32) Banfield, J. F.; Welch, S. A.; Zhang, H. Z.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751.
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With passing time, the nanoparticles generated between the biphase interfaces diffused to the original nanoparticles on the substrate. Then the CaCO3 nanoparticles assembled to the initial shuttles to form new shuttles, and the flowerlike products were obtained gradually. The self-organization remained as long as there was enough space. Finally, hierarchical spherical particles were developed. Both the flowerlike products and spherical particles were composed of the self-stacked shuttles. In particular, the microspheres had hedgehoglike morphology that increased the surface area. There were some slight differences among the different products: the sizes of the nanoparticles and the pores were different (Figure 3). It is more obvious between the middle part of the shuttle and the top of the microsphere (shown in the Figure 3a,c). In the shuttle, the nanoparticles were primarily 60 nm in diameter, and they became 30 nm in diameter in the flower. In the microsphere, the diameter of the nanoparticles changed to
