Orientation-Selective Alignments of Hydroxyapatite Nanoblocks

Article pubs.acs.org/Langmuir

Orientation-Selective Alignments of Hydroxyapatite Nanoblocks through Epitaxial Attachment in a and c Directions Kazuki Nakamura, Yoshitaka Nakagawa, Hiroyuki Kageyama, Yuya Oaki, and Hiroaki Imai* Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan ABSTRACT: Nanometric rods of hydroxyapatite (HA) were aligned in selective crystallographic directions by the alternation of adsorbing molecules. The side and end faces of HA nanorods elongated in the c direction were covered with oleic acid (OA) and tetraoctylammonium (TOA) ions, respectively. Alignment in the c direction of the OA-modified nanorods was produced through epitaxial attachment of the bare end faces in toluene because the side faces were hydrophobized with the negatively charged modifier. Another alignmentin the a direction of the TOA-modified HA nanorodswas obtained through the epitaxial attachment of the bare side faces in ethanol due to stabilization of the end faces with the positively charged modifier. Controlled alignments of the nanorods in the a and c directions were achieved through oriented attachment with the selective coverage of the c and a faces with the specific modifiers.

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INTRODUCTION Hydroxyapatite (HA, Ca10(PO4)6(OH)2), the main component of human bones and teeth, is an important biomedical material. Biogenic hierarchical structures, consisting of c-axis-oriented nanometric HA and collagen fibrils, show excellent mechanical properties.1 Various artificial composites based on HA nanocrystals have been suggested as possible bone substitutes.2,3 Several kinds of HA nanocrystals as building blocks of the hierarchical structures were prepared by hydrothermal methods with the addition of several kinds of inorganic and organic controlling agents.4−6 On the other hand, the assembling technique of the nanometric HA blocks in forming sophisticated architectures similar to those of biogenic products has not been sufficiently investigated. In recent years, anisotropic nanocrystals have attracted much attention as useful building blocks for the fabrication of various ordered arrays. Several arrangements of nanorods were achieved through dimension-controlled assembly by means of their shape anisotropy.7−10 Two types of one-dimensional (1D) arrays were obtained through the side-by-side and end-to-end attachment of anisotropic nanoblocks, such as gold nanorods.11 However, the crystallographic directions of the building blocks in these arrays were not strictly aligned because the assembly was mediated by molecules adsorbing to the crystal faces of the nanometric building blocks. Since Penn and Banfield presented a nonclassical crystallization mechanism involving oriented attachment between two or more nanocrystals,12 various experimental results have demonstrated crystal growth by the assembly of fine particles. One-dimensional nanostructures were synthesized by the oriented attachment of PbSe,13 CuO,14 TiO2,15 and ZnO nanocrystals.16 Especially, monocrystalline nanorods of PbSe and ZnO were clearly formed through 1D alignment of single © 2016 American Chemical Society

particles. Organic molecules that are adsorbed to specific surfaces of nanocrystals promote their oriented attachment. However, the selective oriented attachment of nanocrystals in a and c directions has hardly been achieved. Here, we controlled the direction of the alignments of HA nanocrystals by the specific adsorption of two kinds of organic molecules. Skillful mesoscopic manipulation of nanometric building blocks including control of the crystallographic directions is very important for designing their chemical and physical properties. Single-crystalline architectures would be obtained by the epitaxial or oriented attachment of nanocrystals that are oriented in the same direction.17−19 This is an interesting approach to micrometric architectures with specifically designed shapes and crystallographic directions using nanometric blocks. Especially, rectangular nanoblocks tend to assemble in the same crystallographic direction as a single crystal by facing well-defined facets. The crystallographic orientation of ordered arrays that consist of manganese oxide nanocuboids has been controlled for the fabrication of diversely shaped assemblies by means of the shape anisotropy of building blocks. Recently, direction-selective alignment of the anisotropic nanoblocks has been achieved by using systems composed of single building blocks. Manganese oxide nanocuboids were selectively aligned in the a or c direction through the hydrophobic interaction of the attached molecules.20 In this case, however, direct contact with the crystalline lattices was not achieved because hydrophobic interactions of the attached molecules regulated the assembly. After removal of the organic mediators, single-crystalline 1D chains and 2D panels were Received: February 25, 2016 Revised: March 27, 2016 Published: April 14, 2016 4066

DOI: 10.1021/acs.langmuir.6b00732 Langmuir 2016, 32, 4066−4070

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Langmuir

Figure 1. TEM and HRTEM images (a−c), a schematic illustration (e), XRD pattern (f), and size distribution (g) of resultant nanocrystals. (d) FFT pattern of HRTEM image in (c).

achieved through the epitaxial attachment of crystallographically aligned nanocrystals.21 In the present work, 1D arrays elongated along the a- and c-axes were selectively fabricated using hexagonal HA nanorods having crystallographic anisotropy by means of molecularly mediated manipulation. Face-selective adsorption of organic modifiers on the nanocuboids is essential for nanometric manipulation. The oriented attachment of bare a and c faces produced selective alignments in specific directions. Our findings would be useful for understanding biomineralization and applying nonclassical crystallization to biomedical materials.

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EXPERIMENTAL SECTION

In accordance with process of the previous report,22 HA nanocrystals were prepared by a hydrothermal method. We added 50 cm3 of 0.5 mol dm−3 Ca(NO3)2,aq (Wako Chemicals) into 50 cm3 of 0.3 mol dm−3 Na2HPO4,aq (Junsei Chemical) containing 0.3 mol dm−3 of NaHCO3 (Wako Chemicals). After adjusting pH to 12 with adding NaOHaq (Junsei Chemical), the mixture was stirred for 2 h at 60 °C and then treated at 200 °C for 72 h in a Teflon-lined autoclave. Nanocrystals were obtained by centrifugation, washing with purified water, and drying. The resultant powder was characterized by conventional X-ray diffraction (XRD, Rigaku MiniFlex II) using Cu Kα radiation. The resultant nanocrystals were dispersed in 10 cm3 of toluene that contained a controlling agent, 150 mmol dm−3 of oleic acid (OA) (Wako Chemicals), or 50 mmol dm−3 of tetraoctylammonium (TOA) bromide (Wako Chemicals) to produce OA- or TOA-modified nanocrystals. The dispersion was cooled by water under sonication for 1 h, and we then recovered precipitates after centrifugation. The wet products were dispersed again in pure toluene and then recovered by centrifugation to remove excess amounts of organic molecules. The resultant powder was characterized by the KBr method using Fourier transform infrared absorption (FTIR, Jasco FT-IR 4000). Nanocrystals partially covered with organic molecules were added to pure toluene or ethanol to produce particular alignments.

Figure 2. TEM images (a−c), HRTEM images (e−h, k) and their FFT patterns (i, j), and a schematic illustration (d) of the c-axis-oriented 1D alignment of HA nanocrystals; (l) FTIR spectra of the HA nanorods before and after OA modification. A copper grid covered with a collodion film was placed on a piece of filter paper. A drop of the resultant dispersion was placed on the grid. The products deposited on the grid were characterized by a 4067

DOI: 10.1021/acs.langmuir.6b00732 Langmuir 2016, 32, 4066−4070

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Figure 3. TEM images (a, b) and a schematic illustration (c) of the long alignment of OA-modified HA nanocrystals and an TEM image taken at low magnification (d). transmission electron microscope (TEM, FEI Tecnai F20). The crystallographic direction was estimated from fast Fourier transform (FFT) patterns of the lattice fringe in high-resolution (HR) TEM images.

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RESULTS AND DISCUSSION Anisotropic nanoblocks were synthesized using the hydrothermal method (Figure 1a,b). A typical XRD pattern indicates that the products were HA (Figure 1f). From TEM observation, the average length and width of the HA rods were estimated to be ∼75 nm and ∼38 nm, respectively (Figure 1e,g). From the FFT pattern (Figure 1d) of the HRTEM image (Figure 1c), the HA rods were deduced to be elongated in the c direction. Bare HA nanorods were dispersed in toluene containing OA. Although most of the bare nanocrystals were precipitated in toluene, Tyndall scattering was observed in the supernatant fluid. The scattering indicated the presence of hydrophobized HA nanocrystals in the supernatant fluid. The dispersion was dropped to a collodion film of a copper grid to observe the assembly of the nanorods. As shown in Figure 2, we found 1D alignments of several HA nanorods on the collodion film. HRTEM images show continuous lattices at the interfaces of three adjacent nanorods (Figure 2e−h). Their FFT patterns indicate that the HA nanorods were aligned in the c direction through the epitaxial attachment (Figure 2i,j). In HRTEM image (Figure 2k), the side faces were found to be covered with an amorphous layer ∼1.7 nm thick. Specific absorption bands in the FTIR spectra indicated the presence of OA molecules covering the HA nanorods (Figure 2l). Therefore, the selective adsorption of OA on the side faces of the HA nanorods induced alignment in the c direction. The negatively charged

Figure 4. TEM images (a−c), HRTEM images (e−h) and their FFT patterns (i, j), and a schematic illustration (d) of TOA-modified HA nanorods; (k) FTIR spectra of HA nanorods before and after TOA modification.

carboxy groups of OA are attached to the Ca2+-rich a faces of HA rather than the PO43−-rich c faces.23 Thus, the side faces were selectively hydrophobized with the specific adsorption, 4068

DOI: 10.1021/acs.langmuir.6b00732 Langmuir 2016, 32, 4066−4070

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Figure 5. Schematic illustration of the orientation-selective alignments of OA- and TOA-modified HA nanorods in c and a directions.

of the crystalline lattice. Alignment of HA rods in the c direction is produced by epitaxial attachment of the bare end faces because the side faces are stabilized with negatively charged OA. Another alignmentin the a direction of the nanorodsis achieved through the attachment of the bare side faces due to stabilization of the end faces with positively charged TOA. Positively charged TOA would attach to the relatively small end faces of the nanorods. Thus, the hydrophobization of the TOA-modified HA nanorods would be insufficient for dispersion in toluene. Moreover, longer or wider alignments of TOAmodified HA nanorods have not been obtained even in ethanol. Dispersivity is important for the epitaxial attachment of bare surfaces because the frequency of random collision should be sufficient in the medium. When the frequency of the collision of the HA nanograins is insufficient, isolated single particles remain in the dispersion medium. Random aggregates are formed by nanograins that are insufficiently covered with the modifiers. Sophisticated modification of the HA nanorods including their surface properties, shapes, and sizes is required to achieve a large-scale ordered array of the nanometric building blocks.

and the bare end faces remained hydrophilic. The c-axis-oriented 1D alignment of the OA-modified HA nanorods was formed through end-to-end attachment through hydrophilic interaction in the hydrophobic medium (Figure 2d). A single chain contained three or four crystals of HA on average. Occasionally, we observed long chains consisting of more than five grains (Figure 3a−c). In these cases, the c-axis orientation was segmentalized by a wrongly attached grain. The hydrophobic nature of the surface resulted in the stable dispersion of the nanorod alignments in toluene. On the other hand, isolated single particles were more than 50% dispersed with the 1D alignments in toluene (Figure 3d). The frequency of HA nanograins colliding would be insufficient for the alignment of all particles. The HA nanorods were added to toluene containing TOA to hydrophobize their surfaces. The TOA-modified HA nanorods were recovered from the toluene phase by centrifugation. We redispersed the TOA-modified HA nanorods in ethanol because its dispersivity in toluene was very low. Tyndall scattering was observed in the supernatant fluid, whereas most of the nanorods were precipitated in ethanol. This means that some of the HA nanorods were successfully modified with TOA. We observed the alignments of TOA-modified HA nanorods in a drop of the ethanol dispersion on a collodion film (Figure 4a−c). HRTEM images show continuous lattices at the boundary of three adjacent nanorods (Figure 4e−h). Their FFT patterns indicate that the HA nanorods were aligned in the a direction through side-by-side attachment (Figure 4i,j). Specific absorption bands in the FTIR spectra indicated the presence of TOA molecules covering the HA nanorods (Figure 4k). Therefore, the selective adsorption of TOA on the a faces of the HA nanoblocks induced alignment in the c direction. The positively charged amine groups of TOA attached to the PO43−-rich c faces of HA rather than the Ca2+-rich a faces.23 Thus, the end faces were selectively hydrophobized by the specific adsorption, and the bare side faces remained hydrophilic. The a-axis-oriented alignment was formed through the side-by-side epitaxial attachment of bare a faces of the TOA-modified HA nanorods (Figure 4d). The average number of the bundles was roughly estimated to be 2−3. On the other hand, isolated single particles and random aggregates coexisted with the specific alignments in ethanol. The frequency of HA nanograins colliding is deduced to be insufficient for the alignment of all particles. Moreover, misalignment of nanograins would occur easily with relatively large bare a faces. Figure 5 shows a schematic illustration of the orientationselective alignments of HA nanorods with the modification of OA and TOA molecules. Here, two kinds of alignments of the HA nanorods in c and a directions are achieved by the adsorption of specific negatively and positively charged modifiers, respectively. Ca2+-rich side faces of the HA nanorods are selectively covered with OA, while the PO43−-rich end faces are hydrophobized by TOA. In the dispersion medium, the collision of nanoblocks occurs with random migration. The bare faces of the nanoblocks would be connected each other through the oriented attachment

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CONCLUSION We fabricated direction-selective architectures that consisted of anisotropic nanoblocks through molecularly mediated manipulation. Hydroxyapatite (HA) nanorods were selectively aligned by modifying their end and side faces with specific organic modifiers. One-dimensional arrays elongated in the c direction of the HA nanorods were produced through end-to-end attachment of nanorods whose a faces were stabilized with oleic acid. Another alignmentin the a direction of the HA nanorodswas obtained by the side-by-side attachment of the bare a faces. Faceted nanorods having crystallographic anisotropy are useful building blocks for fabricating various elaborate architectures with uniform crystallographic directions. Molecularly mediated manipulation of the building blocks is regarded as a novel direction-controlled accumulation technique for designing a wide variety of functional nanomaterials.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (H.I.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for Challenging Exploratory Research (Grant 15K14129), and a Grant-in-Aid for Scientific Research (Grant 22107010) on Innovative Areas of “Fusion Materials: Creative Development of Materials and Exploration of Their Function through Molecular Control” (Area no. 2206) from the Ministry of Education, Culture, Sports, Science and Technology. 4069

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Bottom-Up Routes through Surfactant-Mediated Arrays of Oriented Nanocrystals. Langmuir 2015, 31, 6197−6201. (22) Ren, F.; Leng, Y.; Ding, Y.; Wang, K. Hydrothermal growth of biomimetic carbonated apatite nanoparticles with tunable size, morphology and ultrastructure. CrystEngComm 2013, 15, 2137−2146. (23) Kawasaki, T. Hydroxyapatite as a liquid chromatographic packing. J. Chromatogr. 1991, 544, 147−184.

REFERENCES

(1) Weiner, S.; Wagner, H. D. THE MATERIAL BONE: StructureMechanical Function Relations. Annu. Rev. Mater. Sci. 1998, 28, 271− 298. (2) Yokoyama, Y.; Oyane, A.; Ito, A. Preparation of a Bonelike Apatite−Polymer Fiber Composite Using a Simple Biomimetic Process. J. Biomed. Mater. Res., Part B 2008, 86B (2), 341−352. (3) Oyane, A.; Kawashita, M.; Nakanishi, K.; Kokubo, T.; Minoda, M.; Miyamoto, T.; Nakamura, T. Bonelike apatite formation on ethylene-vinyl alcohol copolymer modified with silane coupling agent and calcium silicate solutions. Biomaterials 2003, 24, 1729−1735. (4) Chen, M.; Jiang, D.; Zhu, D. L. J.; Li, G.; Xie, J. Controllable synthesis of fluorapatite nanocrystals with various morphologies: Effects of pH value and chelating reagent. J. Alloys Compd. 2009, 485, 396−401. (5) Coleman, R. J.; Jack, K. S.; Perrier, S. B.; Grøndah, L. Hydroxyapatite Mineralization in the Presence of Anionic Polymers. Cryst. Growth Des. 2013, 13, 4252−4259. (6) Parthiban, S. P.; Kim, I. Y.; Kikuta, K.; Ohtsuki, C. Formation of serrated nanorods of hydroxyapatite through organic modification under hydrothermal processing. J. Nanopart. Res. 2013, 15, 1657− 1666. (7) Martín, A.; Schopf, C.; Pescaglini, A.; Wang, J. J.; Iacopino, D. Facile Formation of Ordered Vertical Arrays by Droplet Evaporation of Au Nanorod Organic Solutions. Langmuir 2014, 30, 10206−10212. (8) Kang, C. C.; Lai, C. W.; Peng, H. C.; Shyue, J. J.; Chou, P. T. 2D Self-Bundled CdS Nanorods with Micrometer Dimension in the Absence of an External Directing Process. ACS Nano 2008, 2 (4), 750−756. (9) Baker, J. L.; Cooper, A. W.; Toney, M. F.; Geissler, P. L.; Alivisatos, A. P. Device-Scale Perpendicular Alignment of Colloidal Nanorods. Nano Lett. 2010, 10, 195−201. (10) Graña, S. G.; Juste, J. P.; Puebla, R. A. A.; Martínez, A. G.; Marzán, L. M. L. self-Assembly of Au@Ag Nanorods Mediated by Gemini Surfactants for Highly Efficient SERS-Active Supercrystals. Adv. Opt. Mater. 2013, 1, 477−481. (11) Kim, D.; Kim, W. D.; Kang, M. S.; Kim, S. H.; Lee, D. C. SelfOrganization of Nanorods into Ultra-Long Range Two-Dimensional Monolayer End-to-End Network. Nano Lett. 2015, 15, 714−720. (12) Penn, R. L.; Banfield, J. F. Oriented attachment and growth, twinning, polytypism, and formation of metastable phases: Insights from nanocrystalline TiO2. Am. Mineral. 1998, 83, 1077−1082. (13) Cho, K. S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. Designing PbSe Nanowires and Nanorings through Oriented Attachment of Nanoparticles. J. Am. Chem. Soc. 2005, 127, 7140−7147. (14) Zhang, Z.; Sun, H.; Shao, X.; Li, D.; Yu, H.; Han, M. ThreeDimensionally Oriented Aggregation of a Few Hundred Nanoparticles into Monocrystalline Architectures. Adv. Mater. 2005, 17 (1), 42−47. (15) Dai, Y.; Cobley, C. M.; Zeng, J.; Sun, Y.; Xia, Y. Synthesis of Anatase TiO2 Nanocrystals with Exposed {001} Facets. Nano Lett. 2009, 9 (6), 2455−2459. (16) Pacholski, C.; Kornowski, A.; Weller, H. Self-Assembly of ZnO: From Nanodots to Nanorods. Angew. Chem., Int. Ed. 2002, 41 (7), 1188−1191. (17) Kato, K.; Mimura, K.; Dang, F. BaTiO3 nanocube and assembly to ferroelectric supracrystals. J. Mater. Res. 2013, 28 (21), 2932−2945. (18) Yang, H. J.; He, S. Y.; Chen, H. L.; Tuan, H. Y. Monodisperse Copper Nanocubes: Synthesis, Self-Assembly, and Large-Area DensePacked Films. Chem. Mater. 2014, 26, 1785−1793. (19) Fang, C.; Brodoceanu, D.; Kraus, T.; Voelcker, N. H. Templated silver nanocube arrays for single-molecule SERS detection. RSC Adv. 2013, 3, 4288−4293. (20) Nakagawa, Y.; Kageyama, H.; Oaki, Y.; Imai, H. Direction Control of Oriented Self-Assembly for 1D, 2D, and 3D Microarrays of Anisotropic Rectangular Nanoblocks. J. Am. Chem. Soc. 2014, 136, 3716−3719. (21) Nakagawa, Y.; Kageyama, H.; Oaki, Y.; Imai, H. Formation of Monocrystalline 1D and 2D Architectures via Epitaxial Attachment: 4070

DOI: 10.1021/acs.langmuir.6b00732 Langmuir 2016, 32, 4066−4070