Asymmetric Crystal Morphology of Apatite Induced by the Chirality of

Communication pubs.acs.org/crystal

Asymmetric Crystal Morphology of Apatite Induced by the Chirality of Dicarboxylate Additives Yu-Ju Wu, Tim W. T. Tsai, and Jerry C. C. Chan* Department of Chemistry, National Taiwan University, No. 1, Section 4, Roosevelt Road, Taipei 106, Taiwan S Supporting Information *

ABSTRACT: Apatite crystallites with asymmetric morphology are successfully prepared by using the L- or D-form of glutamic acid and aspartic acid as crystal modifiers. Our results suggest that the chirality of the additives plays an important role in the asymmetric crystal growth. Imperfect oriented attachment and the stereospecific interaction between the additives and the crystal surface steps are used to rationalize the observed morphologies of the apatite crystallites.

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the predefined amounts of additive were mixed in 30 mL of doubly distilled (DD) water. The molar ratio of additive/Ca2+ was adjusted to 6. The solution was stirred continuously, and the pH was adjusted to 6.0 using HNO3(aq) and NaOH(aq). Then, the solution mixture was sealed in a Teflon-lined autoclave and aged at 120 °C for 24 h. After washing with doubly distilled water three times, the samples were dried at 60 °C for 1 day. The details of the other characterization techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), Fourier-transform infrared spectroscopy (FT-IR), elemental analysis (EA), and thermogravimetric analysis (TGA) are given in the Supporting Information. Figure 1 shows the SEM images of the FHAp crystallites prepared with and without L-Glu at 120 °C for 24 h. In the

he biological selection of L-amino acids in nature is an intriguing issue whose origin and molecular mechanism remain largely unknown.1 An interesting manifestation of this distinctive chirality signature has been observed in the surface recognition of biomolecules such as amino acids,1,2 oligopeptides,3 and cells.4 Organic−inorganic interactions of this kind have played an important role in biomineralization processes. Accordingly, many chiral morphologies are observed for natural biominerals, such as gastropod shell and foraminifer shell.5,6 To have a deeper understanding of these interesting phenomena, it is desirable to study how chiral biomolecules affect the crystal growth of synthetic minerals. Although the discipline of biomimetics has been developed for many years,7,8 stereochemical effects between chiral molecules and crystal morphology are observed only in relatively few in vitro systems, as demonstrated in the studies of gypsum,9 potassium dichromate,10 calcium carbonates,11,12 and calcium oxalate.13 Hydroxyapatite (HAp, Ca10(PO4)6(OH)2) and its fluorinesubstituted form (FHAp, Ca10(PO4)6Fx(OH)2−x, x ≤ 2) are of great biological importance because they are the main constituents of the hard tissues in vertebrates.14 Although the morphologies of HAp and FHAp crystallites could be strongly influenced by glutamic acid (Glu),15,16 aspartic acid (Asp),17 and citric acid,18 to the best of our knowledge, the stereochemical effect on crystal morphology is not yet reported for apatite crystallites. In this communication, FHAp crystallites with asymmetric morphology are obtained when the L- or D-form of Glu and Asp are used as the crystal modifiers. It suggests that the asymmetric crystal growth is due to the stereospecific interactions between the amino acids and the crystallite surface. Our experimental results may provide valuable insights into the adsorption−nucleation process in biomineralization. All the chemicals were obtained from Acros and used without purification. In a typical sample preparation, 2.81 g of EDTA−2Na−Ca, 1.71 g of Na3PO4·12H2O, 0.063 g of NaF, and © 2012 American Chemical Society

Figure 1. SEM images of the samples prepared by hydrothermal treatment at 120 °C: (a) without additive; (b) with L-Glu. Received: September 22, 2011 Revised: November 23, 2011 Published: January 11, 2012 547

dx.doi.org/10.1021/cg201246m | Cryst. Growth Des. 2012, 12, 547−549

Crystal Growth & Design

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absence of Glu, typically rodlike FHAp crystallites (1 μm × 5 μm) were obtained (Figure 1a). On the other hand, an asymmetric morphology is observed for the samples prepared in the presence of L- or D-Glu (see also Figure S1 of the Supporting Information). As shown in Figure 1b, each crystallite (3 μm × 15 μm) comprises a bundle of tiny rods of diameter 0.3 μm. In particular, at one end along the long axis, the rods are fused to form a single crystal, whereas the bundled rods splay outward in the other end. This kind of asymmetric crystal growth must be induced by the interaction between Glu and the crystal surface because the intrinsic hexagonal symmetry of HAp or FHAp cannot produce any asymmetric crystal morphology. In order to monitor the crystal growth process, the SEM images of the samples collected at different reaction times were obtained (Figure S2). The results show that the crystallites have developed the asymmetric morphology within 45 min, which became more apparent as the reaction time increased to 2 h. Furthermore, the crystal morphologies of the 24-h and 72-h samples are very similar, indicating that the SEM images shown in Figure 1 have captured the crystal morphology at the equilibrium state. More interestingly, the FHAp sample prepared with the racemic mixture of Glu shows two distinctively symmetric morphologies, viz. sheaf-like and spindle-like (Figure 2). This

the surface steps of the FHAp crystallites. As shown by the SAED pattern in Figure 3a, the long axis of FHAp crystallites is

Figure 3. (a) TEM image measured for the converging end of FHAp crystallites prepared with L-Glu. The inset shows the SAED pattern corresponding to the area marked by the circle. The SAED pattern is assigned to the reflections along the FHAp [13̅0] zone axis. (b) SEM image measured for the FHAp crystallite prepared with L-Glu. A typically hexagonal cross section is observed.

along the [001] direction. Together with the fact that the cross sections of the FHAp crystallites are in a hexagonal shape (Figure 3b), we conclude that the asymmetric growth may be attributed to the stereospecific interaction between Glu and the surface steps of the {001} planes of FHAp. Similar effects on the crystal morphology of FHAp have been observed for L- or D-Asp (Figure S8). As control experiments, we found that the morphologies of FHAp crystallites were invariably symmetrical when achiral molecules such as glycine, glutaric acid, succinic acid, and malonic acid were used as the organic additives (Figure S9). The preparation conditions and the crystal morphologies of the aforementioned samples are summarized in Table S1 of the Supporting Information. Both the formation of spindle- and sheaf-like morphologies can be characterized by the imperfect oriented attachment or crystal splitting, as commonly observed in semiconductor nanocrystals such as TiO2,21 Sb2S3,22 Bi2S3,23 and PbS.24 The molecular interaction leading to the asymmetric or symmetric crystal growth of FHAp is suggested as shown in Figure 4. Upon the binding of the L- or D-form of the dicarboxylate additives at the step sites on the crystal surface, the atomic arrangement of the subsequent layers will be distorted. According to the mechanism of the imperfect oriented attachment,21 the lattice distortion will occur for a few atomic layers only and the crystallinity will be resumed in the subsequent layers. As a result, there will be a rotational misorientation between the two neighboring crystalline phases (Figure 4), which could rationalize the observation of the spindle-like and sheaf-like morphologies for FHAp. In the presence of enantiopure isomer, however, splaying of rodlike bundles was observed for one end of the crystallites (Figure 1b) because chiral molecules would bind to one of the stereospecific steps only. In conclusion, the effects of the chirality of organic additives on the crystal morphology of apatite are illustrated for the first time. Our experimental results suggest that this phenomenon is a consequence of the stereospecific interaction between the dicarboxylate ions and the surface steps of the {001} planes of FHAp.

Figure 2. SEM images of the racemic Glu FHAp samples with different morphologies: (a) sheaf-like shape; (b) spindle-like shape. Symmetric morphologies were found in the sample prepared in the presence of both L- and D-Glu.

observation clearly demonstrates that L-Glu and D-Glu interact with FHAp at stereospecific sites and their simultaneous cooperative bindings with the FHAp surface would render the morphology symmetrical. This interpretation is consistent with the observation that asymmetric FHAp crystallites are obtained in the presence of a pure enantiomer of Glu. Similar results were obtained in the experiments with different molar ratios of L-Glu/D-Glu (2 and 0.5), where the total concentration of the additives was kept constant (Figure S3). As documented in the literature, the binding of biomolecules onto the step edges of the crystal surface could be stereospecific, and it might have dramatic effects on the crystal morphology or phase.11,12,19,20 The XRD patterns obtained for the samples prepared with and without the addition of Glu are very similar (Figure S4). That is, the stereospecific interaction between Glu and FHAp can hardly affect the coherence length of the crystallites. Furthermore, the experimental results of FT-IR, EA, and TGA (Figures S5−S7) show that the loading of Glu is remarkably low for the FHAp samples (≤1 wt %). These data are in line with the notion that Glu molecules prefer to bind on 548

dx.doi.org/10.1021/cg201246m | Cryst. Growth Des. 2012, 12, 547−549

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Figure 4. Schematic diagram of a plausible formation mechanism for the asymmetric and symmetric FHAp crystallites. Glu1 and Glu2 denote the two enanotiomers of Glu. The formation of the sheaf-like (top) and spindle-like (bottom) morphologies of FHAp crystallites can be rationalized by the binding of Glu molecules onto the crystal step sites as shown in the regions marked by the dotted lines. The two magnified regions illustrate the asymmetric bindings of Glu1 (red box) and Glu2 (green box) on the step edges of the {001} planes of FHAp. The central dashed line denotes the mirror plane indicating the chiral relationship between the binding configurations of Glu1 and Glu2.

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ASSOCIATED CONTENT

S Supporting Information *

Details of sample characterization, XRD patterns, FT-IR spectra, EA results, TGA data, and SEM images of FHAp samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 886-2-33662994. Fax: 886-2-23636359. E-mail: [email protected].

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ACKNOWLEDGMENTS This work was financially supported by the National Science Council. REFERENCES

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