Biomimetic Synthesis of Calcium Carbonate Polymorphs Using the

Sep 12, 2008 - solid-liquid-gas reaction resulted only in rhombohedral calcite particles at the beginning, then ultimately in layer-cake aggregates of...
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Biomimetic Synthesis of Calcium Carbonate Polymorphs Using the Lamellar Lyotropic Liquid Crystalline Systems of Calcium Dodecyl Sulfate Qiang Shen,* Liancheng Wang, Xinping Li, and Fenglin Liu

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 10 3560–3565

Key Laboratory for Colloid & Interface Chemistry of Education Ministry, School of Chemistry & Chemical Engineering, Shandong UniVersity, Jínan 250100, China ReceiVed December 10, 2007; ReVised Manuscript ReceiVed July 3, 2008

ABSTRACT: Lyotropic liquid crystalline phases have immediate relevance in biology because of the prevalence of organized lipid structures in living systems. The incorporation of hydrophilic reagents in water domains with well-defined nanoscale geometry favors the construction of nanostructured materials of inorganics, and/or the inorganic-organic hybrids. In this paper, the lamellar mesophases composed of calcium dodecyl sulfate (CDS), n-pentanol, and ammonia (5 wt% NH3 · H2O in water) were constructed and used as the precipitation media of calcium carbonate (CaCO3). Under the atmosphere of carbon dioxide gas, the occurring solid-liquid-gas reaction resulted only in rhombohedral calcite particles at the beginning, then ultimately in layer-cake aggregates of calcite and stick-bundle aggregates of aragonite. Furthermore, the aragonite content increased with the proceeding time interval of the three-phase reaction, indicating a special crystallization habit of CaCO3 in the lamellar mesophases of CDS. Aside from these, the influence of the structure of CDS lamellar mesophases on the formation of CaCO3 crystals is also discussed. These imply an effective biomimetic approach for the simultaneous fabrication of metastable aragonite and thermodynamically stable calcite under mild conditions.

1. Introduction Lyotropic liquid crystal as one of important organized molecular assemblies is a thermodynamically stable system, which can be formed by dispersing surfactant molecules in a polar solvent at given temperature and relatively high concentration. Generally, the hexagonal, cubic, and lamellar structures are the most common ones for the mixture of a single-chain surfactant of soaps and water.1 Among these gel-like phases, the lamellar is a very interesting and important one, because its structure, consisting of parallel bimolecular layers of surfactant and the swollen water layers, is related to the basic structure of biological cell membranes.2 Aside from the template effectiveness of lamellar structures in material synthesis,3 liquid crystals have received considerable attention lately, because of their potential application in biological studies, such as the amplification of the receptor-ligand binding of proteins, the capture and the replication of viruses, and so on.4 Two-tail surfactants of calcium dodecyl sulfate (CDS) can also form lamellar mesophases in water when n-pentanol is present. 5 This resembles the phase behavior of double hydrocarbon-chain phospholipids which has potential applications in drug-delivery, catalysis, and material sciences.6 When CDS was used both as the organic additive and as the source of calcium ions in the precipitation process of CaCO3, the bound calcium ions on the surface of CDS micelles and the slowly dissolved CO2 gas could simulate the selective enrichment of mineral elements and carbonate ions in the biomineralization process. And this consequently induced the formation of aragonite at room temperature. 7 As a less stable polymorph of CaCO3 crystals, aragonite is very difficult to be homogeneously fabricated under ambient conditions without magnesium ions. However, it can be stabilized in the majority of invertebrate skeletal tissues and can be successfully fabricated by biomimetic crystallization pathways.8 * Corresponding author. Tel: +86-531-88361387; fax: +86-531-88564750; e-mail: [email protected].

One phenomenon concerning biomineralization should be pointed out: calcareous structures of organisms are not made of single crystals. The size, crystallization, and aggregation habits of polymorphic species of CaCO3 vary by the organism, and each organism maintains specific characteristics of these crystalline patterns. For example, the intrinsically unstable amorphous can coexist with the thermodynamically stable calcite in ascidian skeletons and in sea urchin spine.9 The coexistence of aragonite and calcite were also observed in the shells of Haliotis lamellosa, Xeropicta Vesalis, and Eopolita protensa.10 Another phenomenon should also be emphasized: gathering the components is only a small part of the biologically controlled mineralization process. Although calcium and carbonate ions are abundant in the world’s oceans, they need to be gathered, enriched, and induced to crystallize by a controlled procedure. Then, the growing crystallites need to be properly manipulated and confined to the desired size, shape, and crystallographic orientation. Even though the templates of micelles, monolayers, polymers could successfully be applied to fabricate aragonite under ambient conditions,7,8 these biomimetic pathways also exhibit some defects in imitating the in vivo biomineralization process. Aside from the bulk phase of aqueous solutions, the hydrophilic domains of lyotropic liquid crystalline systems can also be used as the crystallization media of CaCO3. Kjellin et al. reported the CaCO3 synthesis in the hexagonal and reverse hexagonal mesophases of different Pluronic surfactants, and demonstrated the template growth of vaterite in the water domains throughout experimental courses.11 Nevertheless, this generic three-phase route cannot explain the extraction of mineral ions at the organic-inorganic interface. In this study, we developed a biomimetic strategy to prepare CaCO3 polymorphs in the lamellar mesophases of CDS under the atmosphere of CO2 gas. These mineralization systems could be used to imitate the “adsorption” of calcium ions at the surface of

10.1021/cg7012107 CCC: $40.75  2008 American Chemical Society Published on Web 09/12/2008

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Figure 1. Polarizing optical micrograph (A) and SAXS pattern (B) of the lamellar mesophase of 35 wt% CDS at room temperature.

surfactant bimolecular layers and to induce the crystallization and aggregation of CaCO3 polymorphs in confined water domains.

2. Experimental Section 2.1. Materials. All chemicals, sodium dodecyl sulfate (SDS), calcium chloride, and ammonia (NH3 · H2O), are of A. R. grade and were used without further purification. Carbon dioxide (CO2, 99.99%) was purchased from Ji’nan Deyang Special Gas Co. Ltd. (Ji’nan, China). Deionized water was used throughout the sample preparations. Calcium dodecyl sulfate (CDS) was synthesized by reacting 100 g of SDS with 34 g of CaCl2 in 1 L of deionized water.5,12 The CDS precipitate formed was filtered through a cellulose nitrated/acetate membrane (0.22 µm) and fully rinsed with deionized water. After three-time recrystallization from water, the obtained CDS was dried in a vacuous desiccator at room temperature. The purity of the CDS sample was ascertained by the absence of the minimum in the surface tension curve measured at 55 °C, showing a critical micellization concentration (CMC) of ∼1.0 × 10-3 M.7,12 2.2. CaCO3 Synthesis. CDS lamellar lyotropic liquid crystals were the pseudoternary-component systems of CDS, n-pentanol, and ammonia (5 wt% NH3 · H2O in water, ibid). Three lyotropic liquid crystalline systems were selected to conduct CaCO3 syntheses: 25 wt% n-pentanol + 25 wt% CDS + 50 wt% ammonia, 25 wt% n-pentanol + 35 wt% CDS + 40 wt% ammonia, 25 wt% n-pentanol + 45 wt% CDS + 30 wt% ammonia. In the context, these elements were abbreviated, referred to as 25, 35, and 45 wt% CDS, respectively. In a typical procedure, the lamellar sample (0.5 g) was prepared by weighing the appropriate amounts of components into a 12-mL glass bottle, which was sealed and then sonicated for ca. 5 min. Then, the sample was left to equilibrate at 25 °C for one week, ensuring complete homogenization. After being aerated with CO2 gas, the glass vial was sealed again and was allowed to stand for a period of time depending upon the measurement intervals. A few minutes later, the lyotropic aqueous mesophase became opaque from the top, indicating the successful diffusion of CO2 gas and the consequent formation of CaCO3 in the hydrophilic domains. Unless otherwise stated, the resulting CaCO3-mesophase composite was washed extensively to remove the unbound amphiphiles. Each washing involved dispersing the composites in a 50/50 (vol/vol) solution of water/ethanol via sonication, followed by centrifugation and removal of the clear supernatant. 2.3. Liquid Crystal Phase Determination. An Olympus BX51 polarizing optical microscope (POM) equipped with a Linkam THMSE600 (Linkam, England) hot stage was used to observe the optical texture of lyotropic liquid crystal. POM samples were made by sandwiching the gel-like phase between two glass slides, and the temperature was kept at 25 ( 0.1 °C. To further determine the structure information of lyotropic liquid crystals, all the viscous phases were identified by using a small-angle X-ray scattering (SAXS) (HMBG-SAX, Austria) system with a nickel-filtered Cu KR radiation (0.154 nm), operating at 50 kV and 40 mA. 2.4. Crystal Characterization. At different reaction intervals, the CaCO3-mesophase composites were directly collected as previously described for the in situ POM observation. Aside from this, other characterization was conducted using the final powder of CaCO3. The

Figure 2. Schematic presentations of the structure of CDS lamellar mesophases (A) and the plausible formation process for the lamellar array of dimension-anisotropic CaCO3 particles (B-D). collected solids were Au-coated prior to examination by a JEOL JSM7600F scanning electron microscope (SEM), fitted with a field emission source and operating at an accelerating voltage of 15 kV. While for the transmission electron microscopy (TEM) measurements, the CaCO3 crystals dispersed in ethanol were directly deposited on a carbon film supported by a copper grid, recorded by a Hitachi H-800 TEM operating at 200 kV. The Fourier transform infrared red (FTIR) spectrum measurements were performed on a Bruker IFS 100 FT spectrometer with the resolution of 4 cm-1.

3. Results and Discussion 3.1. Characterization of CDS Lamellar Mesophases. CDS can be dissolved in the mixture of water and n-pentanol to form lamellar mesophases at room temperature. 5,7 Herein ammonia (5.0 wt% NH3 · H2O in water) was used to substitute for the deionized water, and the resulting gels displayed the same POM pattern as the previous one (Figure 1A).13 It is well-known that the structural unit for lamellar structures is simple and doublelayer (Figure 2A). Under CO2 atmosphere, the added 5.0 wt% NH3 · H2O ammonia in the hydrophilic domain between CDS bilayer could supply ammonium ions to exchange with calcium ions (Figure 2B). Therefore, the structural parameters could be kept still during the consequent crystallization of CaCO3. For this assay, the thickness of water domains (dw, shown in Figure 2A) should be evaluated for the confined crystallization. To determine the repeat spacing (d) of these lamellar stacking form, the SAXS method was used to measure the relative positions of scattering peaks along the scattering vector (q) axis

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Figure 3. Polarizing optical micrographs (left-hand side) and corresponding transmission optical micrographs (right-hand side) of the CaCO3mesophase composites sampled from the lamellar mesophases of 35 wt% CDS at the reaction intervals of 20 min (A), 12 h (B), 24 h (C), and 4 months (D), respectively. Insets in panel C show the TEM image and the corresponding selected area diffraction pattern (SAED) of an individual aragonite stick.

at room temperature. All the SAXS results presented two Bragg peaks as shown in Figure 1B, and the ratio of scattering vector obeyed the relation 1:2. 13a,14 For these lamellar mesophases of 25, 35, and 45 wt% CDS, the calculated d values were 3.9, 3.3, and 3.0 nm, respectively. After subtracting the double length of a surfactant molecule, which can be calculated from the empirical expression l ) 0.15 + 0.127nc (l is given in nanometers, and nc is the number of carbon in main methylene chain),15 the evaluated dw values were confined within the range of 1.0 nm.

By comparing the thickness of water layer with any dimensions in the plane of surfactant lamellae, it was deduced that the resulting CaCO3 should be highly dimension-anisotropic (e.g., rod-, wire-, ribbon-, and sheet-like) to keep the template persistence (Figure 2C). When crystallization of CaCO3 continued in different water layers, the development of anisotropic particles into arrays was expected (Figure 2D). 3.2. Formation Process of CaCO3 Polymorphs. A major barrier to technological application of these CDS lyotropic

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Figure 4. FT IR spectra of the CaCO3 particles sampled from the lamellar mesophases of 35 wt% CDS at different reaction intervals. The rectangular block of these FTIR curves was magnified and shown as an inset.

mesophases in CaCO3 mineralization is the lack of a controlled method for the homogeneous generation of carbonate ions in the confined water domains. In other words, there is a possibility that CO2 gas might react with the primarily formed CaCO3 to result in water-soluble Ca(HCO3)2, and then to generate CaCO3 crystals again. Therefore, the mass ratio of liquid crystal to CO2 gas in the sealed reaction system, as well as the reaction time and the diffusion rate of CO2 gas in the viscous medium, determines the final phase structure of CaCO3. Shortly after the exposure in CO2 gas atmosphere, the lamellar mesophase became opaque from the top, due to the doping of CaCO3 particles. The corresponding POM texture changed from the original Maltese cross (Figure 1A) to the oily streak pattern (the left-hand side of Figure 3A) between crossed polarizers. However, no crystalline particles were observed in the brightfield micrograph (the right-hand side of Figure 3A). It was observed that the oily streak texture was dilute and bundle-like aggregates of crystalline particles were formed at 12 h (Figure 3B). As the solid-liquid-gas reaction proceeded, the lyotropic liquid crystalline texture gradually diminished and more and more stick-bundle of crystals were formed (Figure 3C,D). TEM image and the corresponding SAED pattern of an individual stick (insets in Figure 3C) indicate that these stick-bundles are aragonite superstructures. Statistic analyses (data omitted) showed that these bundles were approximately 10.0 ( 3.0 µm in length. FTIR characteristics of the collected CaCO3 sample at different reaction times were shown in Figure 4. The CaCO3 crystals sampled at 1 h only showed the calcite characteristic absorption peaks at 1441, 877, and 712 cm-1, respectively. According to the previous results,7,16 the absorption peak at 1419 cm-1 with a shoulder at 1498 cm-1 should be assigned to ν3 characteristics of amorphous calcium carbonate and the peak at 856 and the twin at 712 and 700 cm-1 confirmed the existence of aragonite.8 The intensity ratio of aragonite to calcite (I856/ I877) increased with reaction time, qualitatively indicating that these stick-bundles shown in Figure 3 are the aggregates of aragonite. In Figure 4 the appearance of the characteristic absorption at 745 cm-1 indicated the formation of intermediate vaterite (i.e., another metastable polymorph of crystalline CaCO3) (Figure S1, Supporting Information). Interestingly, there is an obvious blue-shift of calcite ν3 absorption peak at 877 cm-1 shown in the FTIR spectra marked at 20 days. This phenomenon has never

Figure 5. SEM images of the CaCO3 particles sampled from the lamellar mesophases of 25 wt% CDS at the reaction intervals of 1 h (A), 12 h (B), and 10 days (C-G), respectively. Panels D and E show the top-viewed and the side-viewed pictures of aragonite aggregates, respectively, whereas panels F and G represent the top-viewed and the side-viewed pictures of calcite aggregates, respectively.

been reported previously, which should relate with the strong interaction between surfactants and growing calcites.17 Perhaps the blue-shift of calcite ν3 adsorption peak could be observed only after the sufficiently stacking of calcite particles with the adsorbed surfactants completely embedded between them. It should be emphasized that these results were reproducible, and similar results were also obtained in the reaction systems of 25 (Figure S2, Supporting Information) and 45 wt% CDS lamellar mesophases (Figure S3, Supporting Information), respectively. SEM image of the CaCO3 sampled at a reaction time of 1 h demonstrated only the rhombohedral morphology of calcite particles in a nanosized scale (Figure 5A), while that of the CaCO3 sampled at a reaction time of 12 h exhibited irregular morphologies of plates and stick-bundles (Figure 5B). At a reaction time of 10 days, the collected CaCO3 particles show two populations of aggregates (Figure 5C), namely, the major bundle-like aggregates

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Figure 6. SEM images of CaCO3 microarrays of highly dimension-anisotropic particles for the visually determined polymorphs of calcite (A) and aragonite (B).

Figure 7. Schematic illustration of the plausible formation process for the buckling deformation of the microarray of distorted CaCO3 particles (A). SEM images of the occasionally observed CaCO3 superstructures sampled from the lamellar mesophases of 35 wt% CDS at the reaction times of 10 days: a calcite sphere (B) and an aragonite rod (C). Arrows in panel B and C indicate the direction of buckling deformation for the aggregates of anisotropic crystals.

of aragonite sticks (Figure 5D,E) and the minor layer-cake aggregates of calcite particles (Figure 5F,G). These imply a special crystallization habit of CaCO3 in the lamellar mesophase of CDS, described as follows. (1) First, the solid-liquid-gas reaction resulted only in the rhombohedral calcite particles. (2) If the viscous medium restrained the continuous diffusion of CO2 gas, it should cause the dissolving of rhombohedrons, and then induce the formations of amorphous CaCO3. (3) The phase transformation of the amorphous to the crystalline caused the formation of irregular plates of calcite, suggesting the formation mechanism of calcite layer-cakes. (4) The presence of n-pentanol could induce the phase transformation of vaterite to aragonite, suggesting the formation mechanism of aragonite bundles.7

3.3. Template Effect of Lyotropic Lamellar Structures. It is well-known that the structural properties for lamellar mesophases are both the long-range positional order and the short-range positional disorder of surfactant bilayers. The bidimensional infinite bilayers of CDS molecules are disposed one under another through the third dimension, periodically alternating with the water layer. If inorganic crystals were formed inside the confined water layers, the positional order of the interbedded anisotropic particles could copy the geometrical symmetry of the lamellar templates.18 As it was described above, anisotropic crystals of plate-like calcite and stick-like aragonite were obtained in the reaction systems of CDS lamellar mesophase. Although the observed lyotropic lamellar textures diminished with the depletion of

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calcium ions (as shown in Figure 3), the resulting CaCO3 particles in water domains could be delimited by the adsorbed surfactants. Then, the planar calcite sheets or the aragonite needles could align with each other to constitute the microarray of anisotropic particles, shown in Figure 6, panels A and B, respectively. If these dimension-anisotropic particles (i.e., the planar calcite sheets and the aragonite needles) grew up along the direction perpendicular to the original surfactant bilayer, the appropriate alignment of mature crystals could result in the aggregate of calcite plates (Figure 5F,G) and the aggregate of aragonite sticks (Figure 5D,E). If these dimension-anisotropic particles grew up along the direction parallel to the original surfactant bilayer, then, the short-range disorder of surfactant bilayers and the oriented attachment of adjacent particles (Figure 7A-1) could determine the formation of anisotropic particles with distorted morphology (Figure 7A-2). As it is schematically illustrated in Figure 2, these anisotropic particles could also develop into microarrays with buckling deformation (Figure 7A-3). Occasionally, the buckling deformations for the spherical structure of calcite lamellae (Figure 7B) and for the rod-like superstructure of aragonite needles (Figure 7C) were observed. The formation mechanisms for these superstructures were so complicated that it involved the template effectiveness of organic precursors, the oriented attachment of inorganic particles, steric, van der Waals, and hydrophobic interactions of pendent surfactants, and so on. Aside from the shear stress imposed during experimental processing, the formation of buckling deformations along the arrows (Figure 7B,C) could further support the template effectiveness of CDS lamellar mesophases.

4. Conclusions The lyotropic liquid crystalline systems of functionalized CDS molecules were constructed and used as effective templates for the special crystallization and aggregation processes of CaCO3 in this study. The diffusion of CO2 gas in the periodically hydrophilic domains favored the gradual fabrication of the stickbundle aggregates of aragonite. At the meantime, the depletion of calcium ions played an important role in the diminishing of surfactant plane-bilayer structures and in the stacking of calcite plates through the third dimension. Interestingly, the blue-shift of the calcite ν3 absorption peak at 877 cm-1 was observed for the first time following the sufficient stacking of calcite particles. These results suggest a special crystallization habit of CaCO3 in the lyotropic lamellar mesophases of CDS and a potential application of the functionalized surfactant self-assembly in the structural control procedure for closely related biominerals. Acknowledgment. The financial support from the National Natural Science Foundation of China (20773079) and from the Science and Technology Development Plan of Shandong Province (2007GG10003004) is gratefully acknowledged. Supporting Information Available: SEM and TEM images, POM images, FTIR spectra, and polarizing and corresponding transmission

Crystal Growth & Design, Vol. 8, No. 10, 2008 3565 optical micrographs. This material is available free of charge via the Internet at http://pubs.acs.org.

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