Biomimetic fabrication of pseudohexagonal aragonite

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Biomimetic fabrication of pseudohexagonal aragonite tablets through a temperature-varying approachw

Published on 16 April 2010. Downloaded by SHANDONG UNIVERSITY on 11/03/2015 08:31:21.

Fenglin Liu,a Yanyan Gao,a Shiqiang Zhao,a Qiang Shen,*a Yunlan Sub and Dujin Wangb Received 8th December 2009, Accepted 24th March 2010 First published as an Advance Article on the web 16th April 2010 DOI: 10.1039/b925593a Pseudohexagonal and single-crystal-like aragonite tablets, found in nacre, could be uniformly fabricated through a temperaturevarying approach for the first time, indicating the triplet twinning nature and implying a potential significance in biomineralization. Aragonite mineral is usually colorless and brittle, with a small, elongated, and prismatic form, so it is not usually faceted for jewellery. If the triplet twinning occurred, the resulting crystallite becomes pseudohexagonal, referred to as the flos-ferri and found near hot springs.1 Resembling the high magnesiumcontaining calcite, aragonite is also one kind of biogenic crystals in invertebrate carbonate skeletons. For example, abalone shells have a mother-of-pearl luster and a brick-andmortar structure. The bricks are flat, porous, and polygonal crystals of aragonite, and the mortar is the polysaccharide and protein bonding and aligning with the brick axes.2,3 These cause nacreous aragonite to be twice harder and B1000 times tougher than those of natural products.4,5 Nevertheless, highperformance mechanical biomaterials need a long time to be produced in vivo, so does the artificial culture of pearls. These processes correlate well with seasonally survival environments (e.g., variable temperatures and different Ca/Mg ratios) and should be controlled by the structural properties of organism secretion under thermoregulatory conditions.6,7 In this communication, a temperature-varying approach was used for the fabrication of pseudohexagonal aragonite tablets found in nacre, in which the surfactant mixture of divalent metal dodecyl sulfate M(DS)2 (M = Ca, Mg) was used as both the metal-ion source and the structure-directing agent. On the uncovering of the structural properties of skeletal aragonite, the lamellar arrangement and amorphous nature of tabular building blocks, as well as the functionalization of interlamellar organic sheets, have been emphasized.8–10 Furthermore, the polysynthetic twinning nature and pseudohexagonal shape feature of an aragonite tablet, together with the assembling of tiny nanoparticles into an individual tablet with a single-crystal-like nature, have also been recognized.11–15 In the biomimetic synthesis aspects of aragonite, organic additives (e.g., extracted biomolecules, synthesized polymers, a

Key Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of Education, Jinan 250100, P. R. China. E-mail: [email protected]; Fax: +86-531-88564750; Tel: +86-531-88361387 b Beijing National Laboratory for Molecular Sciences, Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China w Electronic supplementary information (ESI) available: Experimental Section; Fabrication of Aragonite (Fig. S1, S2); Properties of Tabular Aragonite (Fig. S3–S6); Formation Mechanism (Fig. S7–S10). See DOI: 10.1039/b925593a

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and surfactants), inorganic magnesium, and a fixed environmental temperature have been considered.16–20 However, triplet twinning aragonite tablets with a pseudohexagonal appearance have not been uniformly obtained through a facile solutionthermal approach so far. Here is to demonstrate the biomimetic fabrication of pseudohexagonal and porous aragonite tablets with a triplet twinning and single-crystal-like nature. This implies a great significance to get to the bottom of biomineralization and to fabricate composite materials with high mechanical performance. The mixed aqueous solutions of Ca(DS)2 and Mg(DS)2 at different Ca/Mg molar ratios were prepared at 60 1C because of the relatively high Krafft point of Ca(DS)2 at B55 1C.21,22 After the complete addition of the equal molarity of (NH4)2CO3 solution dropwise, referred to as the initial aging time, the reaction system was then allowed to stand still in a 30 1C thermostatic chamber for the biomimetic fabrication of pure aragonite tablets found in nacre (Fig. S1, S2, ESI).w In the first instance, the structural properties of a skeletal aragonite tablet should be recalled, especially for the interpenetration twinning and single-crystal-like traits.11–13 As schematically shown in Fig. 1a and b, the triplet twinning planes of {110} can divide a two-dimensional tablet into three pairs of sectors. Also, the interfacial angle (i.e., 63.81) between the twinning planes of (110) and ( 110) can be accurately calculated according to the lattice parameters of orthorhombic aragonite. Therefore, the product of 63.8 and 6, which is not the 360 required by hexagonal symmetry, clearly demonstrates the pseudohexagonal form of biogenic aragonite tablets. At microscopic level, a biogenic building block of aragonite has been proved to be assembled by tiny crystallites through interparticle connection of amorphous calcium carbonate, showing the manifest (001) and side {110} crystal faces (Fig. 1a) and referred to as the single-crystal-like tablet.14,15 TEM images of the synthesized tabular aragonite intermediates showed outwardly the pseudohexagonal shape with six sectors (Fig. 1c, d). It was the central cavity and radial interspaces that solved the distinct conflict of a perigon (3601) with the summation of six interfacial angles (382.81). Also, the unequal size for different diagonal sector-pairs (Fig. 1d) answered for the interpenetrating twinning (i.e., the triplet twinning) nature of tabular aragonite. A mature aragonite particle showed the pseudohexagonal characteristic of synthetic tablets (Fig. 1e, see also Fig. S3, ESI),w while another gave out also the seemingly porous property (Fig. 1f, see also Fig. S4, ESI).w Aragonite intermediates sampled at the aging time of 0 (Fig. 2a) and 12 h (Fig. 2c) showed the presence and absence of accumulation feature of tiny particles, respectively. The corresponding selected area electron diffraction (SAED) Chem. Commun., 2010, 46, 4607–4609 | 4607


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Fig. 1 (a, b) Schematic drawings of biogenic aragonite tablets displaying the pseudohexagonal appearance and triplet twinning nature. (c, d) TEM and (e, f) SEM images of representative aragonite particles sampled from the mixed surfactant systems of M(DS)2 at a Ca/Mg molar ratio of 8 : 2 or 5 : 5.

patterns further indicated the time-dependent transformation from the polycrystalline (Fig. 2b) to the single-crystal-like nature (Fig. 2d, see also Fig. S5, ESI).w By focusing vertically on the tabular intermediate shown in Fig. 2a, the high-resolution TEM observation further demonstrated the assembling of tiny aragonite particles (Fig. 2e, see also Fig. S6, ESI).wAlthough the numbered crystalline regions in the tablet did not hold the same crystal orientation (inset in Fig. 2e), these nanoparticles with the same (200) crystal faces could effectively assemble into a pseudo-single-crystalline plane of (001). This further supported the screw dislocation of tiny particles for the generation of single-crystal-like aragonite tablets in vivo.15 It is well-known that the doping magnesium salts in a precipitation system of CaCO3 could control the crystallization of metastable aragonite. In order to demonstrate the novel formation mechanism of pseudohexagonal aragonite tablets, the mineralization reactions of M(DS)2 at different Ca/Mg molar ratios were performed. In the ‘‘pure’’ reaction system of Ca(DS)2 unstable vaterite (B94 wt.%) and thermodynamically stable calcite (B6 wt.%) was simultaneously obtained (Fig. S7, ESI),w while in the mixed M(DS)2 system at the Ca/Mg molar ratio of 9 : 1 only the vaterite was observed (Fig. S8-a, ESI).w The latter also indicates the formation of vaterite twins (white arrows, Fig. S8-a, ESI)w and the initiation of vaterite dissolution (inset, Fig. S8-a, ESI)w under the temperature-varying conditions. In fact, the residual of these dissolving vaterite particles could still be observed at the Ca/Mg molar ratio of 8 : 2 (black arrows, Fig. S8-b, ESI).w Then, pure aragonite tablets sampled at the Ca/Mg molar ratio of 5 : 5 (Fig. S8-c, ESI),w as well as the increase of aragonite weight percentage with the increasing Mg(DS)2 content (Fig. 3), imply the Mg2+-induced phase transformation of vaterite to aragonite. However, these do not suggest the transformation twinning nature of tabular aragonite, because no vaterite was detected in the mineralization process at the Ca/Mg molar ratio of 5 : 5 (Fig. S9, ESI).w 4608 | Chem. Commun., 2010, 46, 4607–4609

Fig. 2 (a, c) Low-resolution TEM images of tabular aragonite intermediates sampled at the reaction times of 0 and 12 h, respectively. (b, d) The corresponding SAED patterns of panels (a) and (c), respectively. (e) High-resolution TEM image of an aragonite tablet, and the inset in top left corner is the Fourier transform pattern of the relatively big rectangular region selected. Each of the five crystalline regions numbered (Scale bar = 1 nm) corresponds to the aragonite lattice planes of (200) with an interplanar spacing of B2.48 A˚ (JCPDS 41-1475).

Herein, we would like to cite Volkmer’s results that the formation of rhombohedral calcite and tabular aragonite could be accomplished through an amorphous precursor under a Langmuir monolayer.12 To our best knowledge, it should be the one and only case to fabricate pseudohexagonal aragonite tablets in laboratory. In this solution-thermal approach, three aspects should be highlighted. First, the negatively charged surfactant-ion aggregates with bounded counterions (i.e., the mixed M(DS)2 micelles) simulate to a great extent the enrichment of Ca2+ and Mg2+ ions onto biological liposome surfaces, owing to the dissociation of ionic surfactants and the phase separation model of surfactant micelles. The presence of dodecyl

Fig. 3 XRD profiles of the CaCO3 crystals sampled from the reaction systems of M(DS)2 at the Ca/Mg molar ratios of 9 : 1, 8.75 : 1.25, 8 : 2, and 5 : 5, the calculated weight percentages of aragonite (abbr. to A) and vaterite (abbr. to V) are also marked with the Ca/Mg molar ratios.

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Notes and references

Fig. 4 Electron microscopy pictures and the corresponding sketches of various aragonite particles: (a) an intergrowth of rods, (b) a tabular intermediate with six petals, (c) a single-crystal-like tablet, and (d) a nanolamellated aggregate with the featured pseudohexagonal form.

sulfate ions (DS ions) could induce the oriented attachment of tabular aragonite intermediates along the c axis; while in the absence of DS ions no pseudohexagonal tablets were obtained (Fig. S2, ESI).w The second aspect is the seemingly Mg2+-induced phase transformation from unstable vaterite to metastable aragonite. The vaterite-to-aragonite phase transformation was indeed intriguing,22 however, herein it was the Ca/Mg molar ratio that acted as a ‘‘driving force’’ for the selective nucleation of aragonite. The third is the crystal growth under the temperature-varying conditions, which control the formation of crystal twins. Aside from the two or more intergrown vaterite (Fig. S8-a, ESI),w individual aragonite rods and their intergrowths could be occasionally observed at an initial mineralization stage (Fig. S10-a, ESI).w Also, the frequently observed pseudohexagonal intermediates with six petals proved the triplet twinning nature of pseudo-singlecrystalline aragonite tablets (Fig. S10-b, ESI).w By comparison, the morphological change from an intergrowth of rods (Fig. 4a) to a tabular intermediate with six sectors (Fig. 4b) then to a tablet with a single-crystal-like nature (Fig. 4c) and then to the pseudohexagonal prim (i.e., the thick tablet, Fig. 4d) suggest the formation process of a triplet twinning aragonite tablet (see also Fig. S10-c, ESI).w In summary, the mixed surfactant solution of Ca(DS)2 and Mg(DS)2 could be used for the biomimetic fabrication of pseudohexagonal and porous aragonite tablets with a triplet twinning and single-crystal-like nature. In some sense, the symmetrical intergrowth of a crystal twin can be referred to as a special lattice defect in the single-crystalline structure and a diagnostic feature in macroscopic appearance. Through the biomimetic and temperature-varying approach, the defective aragonite intermediate with six sectors and the featured aragonite tablet with a pseudohexagonal form could be easily produced. Also, the oriented attachment of single-crystal-like tablets along the c axes could form a nanolamellated aragonite aggregate. These resemble to some extent the nanostructured aragonite tablets found in nacre, however, to screen out a mortar-like organic and then to construct the brick-and-mortar architectures of nacreous aragonite still challenge our attention. The financial support from the National Natural Science Foundation of China (20773079 and 20833010), from the National Basic Research Program of China (2009CB930802), and from the NCET Program in University is gratefully acknowledged.

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