DOI: 10.1021/cg900418w
Direct Growth of Highly Crystalline, Idiomorphic Fluorapatite Crystals on a Polymer Substrate
2009, Vol. 9 3832–3834
Katsuya Teshima,*,† SunHyung Lee,‡ Kunio Yubuta,|| Yoshitaka Kameno,† Takaomi Suzuki,†,§ Toetsu Shishido,|| Morinobu Endo,§,^ and Shuji Oishi†,§ †
Department of Environmental Science and Technology, Faculty of Engineering and ‡Faculty of Engineering and §Institute of Carbon Science and Technology and ^Department of Electrical and Electronic Engineering, Faculty of Engineering, Shinshu University, Nagano 380-8553, Japan, ||Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Received April 13, 2009; Revised Manuscript Received June 30, 2009
ABSTRACT: Well-developed, idiomorphic one-dimensional (1D) crystals of fluorapatite (FAp) were successfully grown for the first time on a poly(ethylene terephthalate) (PET) substrate at 150 C by the cooling of the KNO3-LiNO3 flux. The grown 1D FAp crystals were transparent-colorless, hexagonal prisms bounded by well-developed six-sided {1010} and flat {0001} end faces, and elongated in the Æ0001æ directions. Their average size was relatively large, reaching about 5.2 0.5 μm. In addtion, the 1D FAp crystal was of very good crystallinity because no defects were observed in its lattice image by transmission electron microscopy. Furthermore, highquality, idiomorphic 1D crystals of FAp were tried to be grown directly on a PET substrate by the flux method. The PET surface was completely covered with aggregated FAp crystals, which were flowerlike in shape (hemisphere) with FAp rods radiating from a central nucleus. Apatite is the principal phosphate ore and occurs as an accessory mineral in a wide range of rocks. It includes three mineral species, that is, fluorapatite (FAp, Ca5F(PO4)3) (the most common), hydroxyapatite (HAp, Ca5(OH)(PO4)3), and chlorapatite (CAp, Ca5Cl(PO4)3).1-3 It is well-known as the main constituent of bones and teeth in the human body. Bones and teeth consist of fine whiskers or platelike crystals of a mineral closely related to carbonated HAp. These whiskers and crystals were complicatedly mixed with organic collagen, that is called nano- or microcomposites. A variety of apatites have been widely used for industrial applications (e.g., fertilizer, fluorescent lamp, phosphors, and laser hosts), and their composites with proteins, polymers, metals, or ceramics have also attracted particular attention as highly biocompatible materials.4-6 There have been many studies on the preparation of apatite crystals and powders by various techniques, such as solid-state reaction, melt growth, solution (including flux) growth and biomimetic process, and the preparation of apatite films by laser deposition, rf-sputtering, electron deposition, electron beam deposition, sol-gel preparation, and so on.1,4,5,7-11 Among apatite compounds, the excellent biocompatibility of HAp has resulted in many studies focusing on its synthesis. In particular, biomimetic processes using simulated body fluid have been widely used for preparation of HAp related composites. In these biomimetic processes, well-developed idiomorphic crystals of HAp are hardly obtained. On the other hand, no study has been reported on the preparation of high-quality, idiomorphic 1D FAp crystals and their layers on polymeric substrates under an atmosphere and such a low-temperature. In the previous studies, high-quality FAp crystals were successfully grown at the holding temperature higher than 500 C by the cooling of a CaF2 or KF flux.1,7 These growth techniques (or these growth conditions), however, cannot be applied to growing FAp crystals on a polymer surface. In this communication, 1D FAp crystals were directly grown on polymeric substrates by a cooling method of the KNO3LiNO3 flux. The crystals of FAp belong to the hexagonal system (space group P63/m (No. 176)) with the highest symmetry among the apatite minerals (lattice constants a = 0.938 and c = 0.688 nm).12 Compared with HAp, the incorporation of fluorine atoms *Corresponding author. E-mail:
[email protected]. pubs.acs.org/crystal
Published on Web 08/04/2009
Figure 1. SEM images showing well-developed, idiomorphic FAp crystals grown at 150 C for 20 h by the cooling of the KNO3LiNO3 mixed flux. (a) Low and (b) high magnification.
Figure 2. XRD patterns (Cu KR) of FAp crystals. (a) Pulverized crystallites and (b) Ca5F(PO4)3 ICDD PDF.12.
makes the FAp structure more stable. The FAp crystal growth is, therefore, expected to progress more readily under relatively simple and mild reaction conditions. In particular, we paid attention to the KNO3-LiNO3 flux with relatively low eutectic temperature (about 125 C).13 This flux is thought to be effective for the low-temperature growth of idiomorphic FAp crystals. A mixture of Ca(NO3)2 3 4H2O (2.556 g), (NH4)2HPO4 (0.858 g), and KF (0.126 g) powders were used as a solute. In addition, a mixture of KNO3 (12.999 g) and LiNO3 (5.910 g) was chosen as the flux. The eutectic temperature of the mixed flux was approximately 125 C. The solute and flux powders were weighed out, r 2009 American Chemical Society
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mixed together, and put into a platinum crucible (capacity, 30 cm3; height, 40 mm). Additionally, a poly(ethylene terephthalate) (PET) substrate, which was parallel to the crucible wall, was hung in the crucibles including the mixture. The crucibles were heated to 150 C at a rate of 15 C min-1 and held at this temperature for 20 h. After that, they were cooled to 100 C at a rate of 3.3 C min-1. The crystal products were separated from the remaining flux in warm water. First, idiomorphic 1D FAp single nano/microcrystals were successfully grown for the first time at 150 C by the cooling of the KNO3-LiNO3 flux (Figure 1). The grown 1D FAp crystals were transparent-colorless and hexagonal needle/rodlike in shape. Their average size was relatively large, reaching about 5.2 0.5 μm. The grown 1D crystals were identified as FAp by their powder X-ray diffraction (XRD) patterns (Figure 2), using literature data.12 Four characteristic diffraction lines corresponding to Ca5F(PO4)3 between 30 and 35 (2θ/θ) were clearly observed, and the XRD pattern of the grown crystals in this
communication indicated good crystallinity. Furthermore, the variations in the concentration of the major constituents in the grown crystals were investigated by means of an energy-dispersive X-ray spectrometry (EDS, Figure 3). Calcium, phosphorus, and oxygen atoms were homogeneously distributed in the crystals, whereas potassium and nitrogen from the flux were not detected.
Figure 3. Energy-dispersive X-ray spectrometry data showing the distribution of Ca, P, O, and F in the idiomorphic 1D FAp crystal.
Figure 4. (a) Bright-field TEM micrograph, (b) selected area diffraction pattern, and (c) lattice image of a typical 1D FAp crystal.
Figure 5. SEM images showing well-developed, idiomorphic FAp crystals grown directly on a PET substrate. (a, b) Surface and (c) crosssectional images.
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Figure 4 shows (a) a bright-field transmission electron micrography (TEM) image, (b) a selected area electron diffraction pattern (SAED), and (c) a lattice image obtained from a 1D FAp single crystal. FAp easily produces anisotropic growth for its unique crystal structure, that is, columnar structure. From this SAED pattern, the elongated direction and well-developed faces were found to be clearly corresponded to the Æ0001æ directions and {1010} faces, respectively. This result is quite reasonable, considering the lattice parameters and its crystal structure. From the results of SEM and TEM observation, we conclude that the 1D crystals were hexagonal prisms bounded by well-developed six-sided {1010} and flat {0001} end faces, and they elongated in the Æ0001æ directions. Moreover, the 1D FAp crystal was of very good crystallinity because no defects were observed in this lattice image (Figure 4c). The SAED indicated that the lattice parameters of the 1D FAp are a = 0.944 and c = 0.688 nm, which are in good agreement with those found in the previous study (a= 0.938 and c=0.688 nm).12 This morphology was similar to those of the FAp crystals grown from a KF flux,7 the CAp crystals from a NaCl flux,10 and the HAp crystals from a KNO3-LiNO3 flux at a relatively high-temperature (>500 C). As mentioned above, we can recognize that the KNO3-LiNO3 mixed flux is adequate for a very low-temperature growth of FAp single crystals. Next, high-quality, idiomorphic 1D crystals of FAp were, therefore, tried to be grown directly on a polymeric substrate. No study has been reported on the direct formation of such a high-quality FAp crystals on a polymer substrate. Figure 5 shows idiomorphic 1D FAp crystals grown on the PET substrate. The PET surface was completely covered with the densely packed, idiomorphic 1D crystals of FAp (Figrue 5a). The aggregated FAp crystals (hemispheres) were flowerlike in shape with FAp rods radiating from a central nucleus. The enlarged image clearly indicated that each FAp crystal was an idiomorphic, angular hexagonal prism (Figure 5b). From the results of XRD and TEM, the unique 1D crystals were found to be highly crystalline FAp. The cross-sectional image showed that the hemispherical FAp flowers were grown directly on the PET surface (Figure 5c). Hereafter, surface finishing techniques for a variety of substrates are expected to become considerably important in researching the direct growth/patterning of biocrystals.
Teshima et al. In conclusion, the KNO3-LiNO3 mixed flux, which has a relatively low eutectic temperature, was successfully used as a new flux to grow the idiomorphic 1D FAp crystals directly on polymer substrates. Because the grown FAp crystals in this communication have a high crystallinity and characteristic morphology, they should be favorable materials for various technological applications, such as biomaterials and bioMEMS. Finally, our fabrication techniques of biocrystals on polymer surfaces are quite unique and an environmentally friendly, that is low environmental damage and low product cost. In the future, it will become more and more important to develop various functional biomaterials with the use of environmentally friendly (low-temperature) processes. Acknowledgment. This research was partially supported by a Grant-in-Aid (20350093) and CLUSTER Project (the second stage) of Ministry of Education, Culture, Sport, Science and Technology. This research was partially performed under the interuniversity cooperative research program of Advanced Research Center of Metallic Glasses, Institute for Materials Research, Tohoku University.
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