Article pubs.acs.org/Langmuir

Size-Controlled Vaterite Composite Particles with a POSS-Core Dendrimer for the Fabrication of Calcite Thin Films by Phase Transition Shiho Nakamura and Kensuke Naka* Department of Chemistry and Materials Technology, Graduate School of Science and Technology, Kyoto Institute of Technology, Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan S Supporting Information *

ABSTRACT: Vaterite composite particles with a sizecontrolled sphere were obtained by a carbonate controlledaddition method by using a carboxylate-terminated poly(amidoamine) (PAMAM)-type polyhedral oligomeric silsesquioxane (POSS)-core dendrimer. An aqueous ammonium carbonate solution was added to an aqueous solution of the dendrimer and CaCl2 at different times (3 min, 30 min, and 1 h) and stirred for 1 h at 30 °C. When the complexation time of the POSS-core dendrimer-CaCl2 solution was increased from 3 min to 1 h, the average particle sizes of the spheres increased from 0.71 ± 0.08 to 1.86 ± 0.22 μm, respectively. However, the average particle sizes decreased with decreasing temperature. Particles with minimum sizes of 70 ± 6 nm were obtained when COONa to calcium ion molar ratio was 16 and the complexation time was 3 min at 20 °C. Incubation of the vaterite composite particles in distilled water for 3 days led to complete phase transition to calcite. Negative zeta potential values, ranging from −30 to −10 mV, were detected for the vaterite particles, indicating that the POSS-core dendrimers were exposed on the CaCO3 particles. The CaCO3 particle surfaces were successfully coated with poly(diallyldimethylammonium chloride) (PDDA) in aqueous dispersions by adding a controlled concentration of the polymer. Alternate vaterite composite particles and polyelectroyte multilayer films were prepared by a layer-by-layer method. The obtained (PDDA/vaterite)10(PDDA) multilayer films were incubated in distilled water at 30 °C. Incubation for 5 days led to complete phase transition to calcite, as estimated by Fourier transform infrared (FTIR) spectroscopic and XRD analyses. The SEM observation of the sample after 5 days of incubation showed a granular network structure of irregularly shaped calcite particles. Although some patches and pores were present in the films, the SEM image clearly demonstrated that large-area and continuous CaCO3 films were formed.

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INTRODUCTION At present, the fabrication of organic−inorganic hybrid materials with controlled mineralization analogous to that of materials produced by nature is of interest to both organic and inorganic materials scientists in order to understand the mechanisms of natural biomineralization and to seek their industrial and technological applications.1−4 Calcium carbonate (CaCO3) is one of the most studied systems because it is a major inorganic substance produced in biological organisms as well as an important precursor for developing new materials in many fields.5−7 CaCO3 particles with controlled sizes should be important for application as pigments, fillers, and medical applications. CaCO3 exists as three anhydrous crystalline polymorphs (calcite, aragonite, and vaterite) and as amorphous CaCO3 (ACC). It was reported that organisms may use metastable forms of CaCO3 such as ACC or vaterite as transient precursors for the production of calcite and aragonite with hierarchically complicated structures.8 Living systems use transiently stabilized ACC as a precursor for calcification. The final crystalline phase, calcite or aragonite, with complex and © 2013 American Chemical Society

desired shapes can form through a series of steps initiated by the formation of an amorphous phase that undergoes subsequent phase transition via a metastable crystal phase, vaterite.8 Recently, the ACC precursor pathway was proposed as an attractive biomimetic synthesis strategy for the preparation of a wide range of crystalline structures such as CaCO3 fibers and thin films.9,10 This approach may allow the fabrication of fibers with complex curved shapes and patterned thin films via the self-assembly of the ACC precursors. Layer-by-layer deposition is a versatile route to preparing multilayered functional polymer thin films employing adsorption driving forces such as electrostatic interaction, hydrogen bonding, and many other chemical/physical interactions for adsorbing pairs by alternately immersing a charged substrate in two solutions of oppositely charged polyelectrolytes.11 Among the various functional multilayered assemblies, much attention Received: August 27, 2013 Revised: November 24, 2013 Published: December 5, 2013 15888

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Figure 1. Structures of the G0.5 PAMAM dendrimer and POSS-COOH.

functionalization, size control, and isolation can more easily be achieved in comparison with those for ACC precursors. Recently, a simple new method called carbonate controlled addition was developed to control the particle size. In this method, carbonate ions were added to an aqueous polymerCaCl2 solution at different times.19 Herein, the nucleation and mineralization of CaCO3 were mediated by the coordination structures of the polymer and Ca2+ complexes. We applied the carbonate controlled-addition method using PAMAM dendrimers with carboxylate groups on the external surfaces.20 Dendrimers are monodisperse macromolecules with a regular and highly branched 3D architecture. Because of their unique and well-defined secondary structures, they should be good candidates for controlling inorganic crystallization.21−23 Crystallization of CaCO3 using the G0.5 and G1.5 PAMAM dendrimers with carboxylate groups at the external surfaces resulted in the formation of stable spherical vaterite crystals.22,23 The particle sizes of the vaterite spheres can be controlled by changing the incubation time of a PAMAM dendrimer-CaCl2 solution before the addition of carbonate ions. The vaterite spheres using the PAMAM dendrimer are, however, not suitable for vaterite precursors used to fabricate calcite thin films because no phase transition to calcite was observed when the isolated and dried vaterite particles were incubated in fresh water at room temperature for 7 days. In the present study, we used size-controlled vaterite particles obtained with a carboxylate-terminated PAMAM-type polyhedral oligomeric silsesquioxane (POSS)-core dendrimer (POSS-COOH) (Figure 1) by the carbonate controlledaddition method and found that the vaterite composite particles fabricated in this study exhibited a phase transition to calcite in water. We used the vaterite particles obtained in the present study to fabricate a calcite thin film via the layer-by-layer deposition approach. The use of a POSS core for dendrimer synthesis is particularly attractive because not only are minimal synthesis steps required but also their polyhedral structures are expected to form spherically symmetric dendrimers even with earlier generations than for conventional cores.24 The cubic silica core is rigid and completely defined, and the eight organic functional groups are appended to the vertexes of the cube via spacer linkages. Not all of the functional terminal groups are able to attach to the same surfaces because of the steric hindrance provided by the cubic structure. The POSS-core dendrimers have relatively globular conformations and few

has been paid to the fabrication of inorganic−organic hybrid films with various inorganic particles and polyelectrolytes.12 Although the layer-by-layer assembly of suitable CaCO3 precursor particles with polyelectrolytes would provide access to new fabrication techniques to fabricate CaCO3-polymer composite films in a controlled manner, ACC precursor nanoparticles are generally very unstable and rapidly aggregate to large particles. Therefore, the conventional layer-by-layer method is not feasible for fabricating CaCO3 nanoparticle/ polyelectrolyte hybrid multilayered films. Therefore, CaCO3 layers have been prepared by conventional solution mineralization techniques on polyelectrolyte films.13−15 Simple layer-bylayer deposition of CaCO3 particles and polyelectrolytes to fabricate CaCO3-organic hybrid films has not been reported. From a synthetic chemical viewpoint of fabricating CaCO3organic hybrid films with CaCO3 particles and polyelectrolytes, CaCO3 precursors must be functionalized with a wide variety of organic functional moieties by simple chemical processes to control the particle size. The most important requirement is that CaCO3 precursors should be repeatedly isolated and redispersed in a solvent. Another important requirement is to control the rates of phase transition after the self-assembly of the CaCO3 precursors on the substrates. In this study, we propose that calcite-organic composite thin films can be fabricated by the layer-by-layer deposition of metastable CaCO3 particles and subsequent phase transition. Vaterite is the most unstable phase among the three different types of crystalline polymorphs of CaCO3 that are crystallized from ACC. The inherent energetic instability of vaterite results in its phase transition to more stable crystalline phases in aqueous solution, which occurs via a dissolution and recrystallization process.16,17 Stabilizing the vaterite phase has been a major problem in biomineralization studies not only because of its rarity owing to its intrinsic instability but also because of its potential in biomedical and industrial applications because vaterite exhibits unique properties such as a high surface area, solubility, dispersion, and a smaller specific gravity than calcite or aragonite. Size-controlled CaCO3 can be used in various applications such as fillers and medical applications because of its biodegradable properties. However, few studies were reported on the use of vaterite particles as precursors to forming more stable crystalline phases.18 Here, we propose to develop a vaterite precursor method for fabricating calcite thin films. The advantage of using vaterite precursors is that surface 15889

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Table 1. Fabrication of CaCO3 in the Presence of POSS-COOH at Different Complexation Times of Dendrimer-CaCl2 Solution and Different COONa to Calcium Ion Molar Ratios at 30 °C

a

run

[COONa]/[Ca2+]

stirring time, min

1 2 3 4 5 6 7 8 9 10 11 12

1

3 30 60 3 30 60 3 30 60 3 30 60

2

8

16

polymorphisma vaterite vaterite vaterite vaterite vaterite vaterite vaterite vaterite vaterite vaterite vaterite vaterite

+ + + + + +

particle size, μmb

yield, %c

POSS-COOH, wt %d

water, wt %d

± ± ± ± ± ± ± ± ± ± ± ±

79 80 78 48 64 70 89 54 71 59 81 71

17.4 6.9 5.1 18.3 8.5 6.6 19.2 14.8 10.8 22.9 17.1 11.9

7.3 4.6 3.7 8.5 5.8 3.6 9.9 7.8 5.0 11.0 8.5 7.4

calcite calcite calcite calcite calcite calcite

1.01 2.86 2.93 0.85 2.81 2.92 0.71 1.05 1.86 0.64 0.81 1.56

0.19 0.56 0.42 0.08 0.46 0.47 0.08 0.11 0.22 0.06 0.12 0.18

Determined by FTIR. bParticle size was measured by SEM. cCalculated on the basis of CaCO3. dCalculated by TGA. poly(diallyldimethyl ammonium chloride) (PDDA, Mw = 100 000− 200 000, 1 mg/mL) was added. After the dispersion was stored for 3 h at room temperature, precipitates were collected by filtration. Layer-by-Layer Assembly of Vaterite Particles. Prior to film assembly, a glass slide was cleaned by ultrasonicating in acetone for 30 min followed by rinsing with water, ultrasonicating in a wash solution (1% KOH + 39% water + 60% ethanol) for another 30 min, rinsing with water, and drying with a stream of nitrogen. Multilayered (PDDA/PSS)3/PDDA was precoated on both sides of the cleaned glass substrate by alternatively immersing for 15 min in aqueous PDDA (5 mg/mL, containing 0.1 M NaCl) and in aqueous PSS (5 mg/mL, containing 0.1 M NaCl) at 30 °C and then rinsing with distilled water in three separate vials (1 min each), followed by drying with a gentle stream of air. The glass substrate was alternately immersed for 15 min in an ethanol dispersion of the vatertite particles (4 mg/mL, pH 11) and in an ethanol solution of PDDA (1 mg/mL) at 30 °C and then rinsed three times with ethanol (1 min each), followed by drying with a gentle stream of air. Multilayer films were then formed by continuing this sequential adsorption process. The final deposition was denoted as (PDDA/PSS)3/(PDDA/vaterite)n(PDDA), where n equals the deposition layers of the vaterite particles.

entanglements of their branches with a high proportion of terminal functional groups positioned on the external surfaces of the dendrimers even in early generations.24 These characteristic features are suitable for preparing the vaterite precursors for fabricating a calcite thin film via a layer-by-layer approach.

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EXPERIMENTAL SECTION

Materials. The carboxylate-terminated poly(amidoamine)-type POSS-core dendrimer (POSS-COOH) was prepared according to the reported procedure.24 Poly(sodium 4-styrenesulfonate) (Mw = 70 000), and poly(diallyldimethylammonium chloride) (Mw = 100 000− 200 000) were purchased from Sigma-Aldrich Chemical Co. Calcium chloride and ammonium carbonate were purchased from Wako Pure Chemical Industries, Ltd. Measurements. The X-ray diffraction (XRD) was recorded on a Smart Lab with Cu Kα radiation (λ = 1.5406 Å) in θ/2θ mode at room temperature. The 2θ scan data were collected at a 0.01° interval, and the scan speed was 1° (2θ)/min. Fourier transform infrared (FTIR) spectra were recorded with a Jasco FT/IR-4100 spectrometer with the KBr pellet method. The morphologies of CaCO3 particles were observed using an EV-8800 scanning electron microscope (SEM, Keyence, Osaka, Japan) and a JSM-7600F field emission SEM (FESEM, Jeol Ltd., Tokyo, Japan) with an X-max energy-dispersive X-ray spectrometer (EDX, Oxford Instruments, Oxfordshire, U.K.). Thermogravimetric analysis (TGA) was carried out on a hi-res modulated TGA 2950 (TA Instruments, Inc.) to a temperature of 700 °C at a heating rate of 10 °C/min under an air atmosphere. The zeta potential was measured using an ELSZ-1000ZS (Otsuka Electronics Co. Ltd., Osaka, Japan), which is based on laser Doppler velocimetry in an electric field. The zeta potential was calculated from the electrophoretic mobility using the Smoluchowski equation. Precipitation of CaCO3. Standard precipitation of CaCO3 was carried out as follows. First, a stock aqueous solution of POSS-COOH (2.5 × 10−2 mmol) was prepared in distilled water, and the pH value was adjusted to 11 with a dilute aqueous solution of NaOH. Then, 0.5 mL of a 0.1 M CaCl2 aqueous solution (adjusted to pH 8.5 with aqueous NH3) was added to a 4.5 mL aqueous solution of the dendrimer under gentle stirring at 30 °C. After mixing the reaction solution with a different time period from 3 min to 1 h, we added 0.5 mL of a 0.1 M (NH4)2CO3 aqueous solution (adjusted to pH 10.0 with aqueous NH3) to the reaction solution. This solution was kept at 30 °C for 1 h of incubation with stirring. The precipitated CaCO3 product was collected by using a 0.1 μm pore-size membrane filter and washed with water several times and then dried at room temperature under reduced pressure. Complexation of the CaCO3 Particles with Polymer. The CaCO3 particles (2 mg) obtained by 3 min of complexation were dispersed in 2 mL of an aqueous solution containing poly(sodium 4stylenesulfonate) (PSS, Mw = 70 000, adjusted to pH 10), or

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RESULTS

Effect of Complexation Time of the POSS-Core Dendrimer-CaCl2 Solution upon Formation of CaCO3 Particles. To an aqueous solution of POSS-COOH (adjusted to pH 11 with aqueous NaOH), a CaCl2 aqueous solution (adjusted to pH 8.5 with aqueous NH3) was added and stirred at 30 °C. The molar ratios of COONa in POSS-COOH against Ca2+ were varied from 1 to 16. The concentration of CaCl2 was constant in all experiments. The experimental conditions and the results are summarized in Table 1. No turbidity of the solution was observed before the addition of ammonium carbonate, even after incubation for 1 h. After an aqueous ammonium carbonate solution was added to the reaction mixture at different times (3 min to 1 h), the solution became turbid. The reaction mixtures were kept at 30 °C for 1 h, and then the products were collected.25 The CaCO3 crystal phases of the obtained products were characterized by using FTIR spectroscopy. For the molar ratio [−COONa]/[Ca2+] = 8, all of the products showed two bands at 877 and 745 cm−1, indicating vaterite formation (Figure S1, Supporting Information). A broad band at around 1100 cm−1 can be assigned to the Si−O vibration derived from the POSS unit. The crystal phases of the obtained CaCO3 products were further confirmed by XRD analysis (Figure S2, Supporting Information). All of the precipitates consisted entirely of 15890

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Figure 2. SEM images of vaterite composite particles by adding (NH4)2CO3 after stirring POSS-core dendrimer-CaCl2 solution with COONa to a calcium ion molar ratio of 8 at 30 °C for (a) 3 min, (b) 30 min, and (c) 1 h corresponding to runs 7−9 in Table 1, respectively.

vaterite. The 1H NMR spectrum of the product after the decomposition of CaCO3 by an aqueous HCl solution was the same as that of POSS-COOH. This result suggests that the decomposition of POSS-COOH did not occur during crystallization. SEM observations show that all of the obtained products are spherical particles (Figure 2). The average particle sizes of the spheres increased from 0.71 ± 0.08 to 1.86 ± 0.22 μm on increasing the complexation time of the POSS-core dendrimer-CaCl2 solution from 3 min to 1 h, respectively. These results indicate that the particle sizes of the spheres were controlled simply by changing the complexation time; the shorter the complexation time, the smaller the particle size. The average particle sizes of the spheres obtained from the longer complexation time of the POSS-core dendrimer-CaCl2 solution (i.e., 24 h was 1.85 ± 0.09 μm). No increase in the particle size was observed. The sphere compositions were estimated by TGA (Figure S3, Supporting Information). The results are summarized in Table 1. The samples were dried under reduced pressure at room temperature for more than 1 day to remove water physically adsorbed on the particle surfaces. The TGA analysis showed that the absorbed amount of H2O in the particles decreased from 9.9 to 5.0 wt % on increasing the incubation times of the POSS-core dendrimer-CaCl2 solution. The weight losses at 250 to 560 °C are due to the decomposition of the organic component of POSS-COOH. The absorbed amount of the POSS-core dendrimer calculated from these data decreased

from 19 to 11 wt % upon increasing the complexation time of the POSS-core dendrimer-CaCl2 solution. The observed weight loss in the temperature range between 560 and 640 °C was 27 to 35 wt %. From these data, we calculated that the relative weight loss of the inorganic component excluding the weight of the decomposed products of POSS-COOH above 560 °C varied from 38 to 41 wt %. These values are in reasonable agreement with the theoretical loss of 44 wt % attributed to the decomposition of pure CaCO3 to CaO. Thus, the amount of the residual observed at 560 °C was estimated to be the contents of CaCO3 in the obtained samples. At higher concentrations of POSS-COOH ([COONa]/ [Ca2+] = 16), all products also showed two bands at 877 and 745 cm−1, indicating vaterite formation (Figure S4, Supporting Information). The average particle sizes of the spheres increased from 0.64 ± 0.06 to 1.56 ± 0.18 μm on increasing the complexation time of the POSS-core dendrimer-CaCl2 solution from 3 min to 1 h, respectively (Figure S5, Supporting Information); increasing the POSS-COOH concentration decreased the particle sizes. At lower concentrations of POSS-COOH ([COONa]/[Ca2+] = 1 and 2), all samples showed the bands at 874 and 712 cm−1 assignable to calcite coexisting with vaterite (Figures S6 and S7, Supporting Information). The relative intensity of the bands corresponding to calcite increased with increasing complexation times. The average particle sizes increased on increasing the complexation 15891

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Table 2. Fabrication of CaCO3 in the Presence of POSS-COOH at Different Complexation Times for Dendrimer-CaCl2 Solution and Different COONa to Calcium Ion Molar Ratios at 20 °C

a

run

[COONa]/[Ca2+]

stirring time, min

polymorphisma

1 2 3 4 5 6

8

3 30 60 3 30 60

vaterite vaterite vaterite vaterite vaterite vaterite

16

particle size, μmb 102 0.56 0.93 70 0.44 0.58

± ± ± ± ± ±

4 (nm) 0.06 0.11 6 (nm) 0.05 0.06

yield, %c

POSS-COOH, wt %d

water, wt %d

62 63 73 24 80 82

22.6 16.9 11.4 24.0 19.8 12.2

9.9 8.4 7.2 10.4 8.4 7.3

Determined by FTIR. bParticle size was measured by SEM. cCalculated on the basis of CaCO3. dCalculated by TGA.

an aqueous solution containing 1 mg/mL poly(sodium 4styrenesulfonate) (PSS) (Mw = 70 000) (adjusted to pH 10) or poly(diallyldimethyl ammonium chloride) (PDDA) (Mw = 100 000−200 000) was added to the dispersion. The precipitates were collected by filtration after keeping the dispersion at room temperature for 3 h. Adding an aqueous solution of PSS after adjusting its pH to 10 as an anionic polymer to the dispersion of the CaCO3 particles did not lead to any change. However, the addition of an aqueous solution of PDDA as a cationic polymer led to immediate aggregation to form precipitates. The polymorphs of the resulting precipitates were still those of vaterite, as measured by FTIR analysis. The SEM analysis of the precipitates showed aggregates of spherical vaterite particles. We studied the effect of the addition of PDDA aqueous solutions with different concentrations of the vaterite particles in order to coat the particles with PDDA. An ethanol solution (5 mL) containing 2.5, 5.0, or 10 mg/mL PDDA was added to 5 mg of the vaterite composite particles. The particles were collected by filtration after storing the dispersion overnight. The SEM analysis showed that the aggregation of the vaterite particles was observed when PDDA (2.5 mg/mL) was used (Figure S15a, Supporting Information). The SEM analysis showed that the addition of 10 mg/mL PDDA, however, did not result in any particle aggregation (Figure S15c, Supporting Information). The FTIR analysis showed that polymorphs of all of the particles were composed of vaterite (Figure S16, Supporting Information). The TGA analysis of the particles obtained by the addition of PDDA showed two clear weight losses at 250 and 560 °C (Figure S17, Supporting Information). The weight losses at 250 to 560 °C were due to the decomposition of the organic component of the products. The absorbed amounts of PDDA in the composite particles obtained by adding 2.5, 5.0, and 10 mg/mL PDDA were 2, 3, and 5 wt %, respectively. A positive zeta potential value of ∼1.5 mV was detected for the vaterite particles obtained from 3 min of incubation after the addition of 10 mg/mL PDDA. The results of the particle zeta potential can be explained by the fact that PDDA was exposed on the CaCO3 particles. The PDDA-coated vaterite composite particles that formed by using the aqueous solution containing 10 mg/mL PDDA were added to distilled water to incubate the particles further at room temperature. The products obtained after incubation for 1 day showed that the bands at 874 and 712 cm−1 assignable to calcite coexisted with vaterite (Figure S18, Supporting Information). Further incubation for 2 days led to a complete phase transition to calcite, as estimated by FTIR. The weight loss at 250 to 560 °C was 2.5 wt %, as determined by TGA, indicating that the organic moieties remained on the calcite particles. The SEM observation shows irregularly shaped calcite particles (Figure S19, Supporting Information). As described before, a negative zeta potential value of −19 mV was observed

times and decreased on increasing the COONa to the calcium ion molar ratios (Figures S8 and S9, Supporting Information). A lower temperature of 20 °C was employed to form composite particles. The experimental conditions and results are summarized in Table 2. The average particle sizes decreased on decreasing the temperature. A minimum particle size of 70 ± 6 nm was obtained for the COONa to calcium ion molar ratio of 16 and a complexation time of 3 min (Figures S10 and S11, Supporting Information). Stability of Vaterite Composite Particles in Water. It is well known that vaterite transforms into the thermodynamically most stable calcite via a solvent-mediated process.16,26−28 We studied the phase transition of the vaterite particles in aqueous solution for a long incubation period at 30 °C. The vaterite composite particles obtained for the COONa to calcium ion molar ratio of 8 and the complexation time of 3 min at 30 °C were added to distilled water for further incubation at room temperature. The products obtained after incubation for 1 day in distilled water showed that the bands at 874 and 712 cm−1 assignable to calcite coexisted with vaterite (Figure S12b, Supporting Information). The SEM observation shows two different crystal modifications: spherical vaterite and irregularly shaped calcite particles (Figure S13b, Supporting Information). Further incubation for 3 days in distilled water led to a complete phase transition to calcite, as estimated by the FTIR analysis (Figure S12c, Supporting Information). The POSScore dendrimer content in the composite particles after the complete phase transition was 13.6 wt %, as determined by TGA. The SEM observation shows irregularly shaped particles (Figure S13c, Supporting Information). The vaterite composite particles obtained from the complexation time of 30 min and 1 h at 30 °C were also added to distilled water for further incubation at room temperature. Both products obtained after incubation for 3 days also led to the complete phase transition to calcite. No substantial differences in the complexation time against stability were observed (Figure S14, Supporting Information). Surface Properties of Vaterite Composite Particles. The zeta potential of a colloidal particle is mainly determined by its surface charge. Although the calcite surface is negatively charged, vaterite surfaces are positive.29 Indeed, vaterite particles prepared without using the POSS-core dendrimer showed a zeta potential value of +10 mV.30 However, negative zeta potential values ranging from −30 to −10 mV were detected for the vaterite particles obtained from the complexation time of 3 min at 30 °C. These zeta potential results can be explained only by the fact that POSS-COOH was exposed on the CaCO3 particles. The POSS-core dendrimer-CaCO3 particles (2 mg) with an average diameter of 0.71 ± 0.08 μm obtained from a complexation time of 3 min were dispersed in water. Then, 15892

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Figure 3. Schematic illustration of the layer-by-layer assembly process of (PDDA/PSS)3/(PDDA/vaterite)n(PDDA) multilayer films.

Figure 4. SEM images of the surface and cross-section of a (PDDA/vaterite)10(PDDA) multilayer film (a) before and (b) after immersion in distilled water for 5 days. POSS-core dendrimer-CaCO3 particles with an average diameter 0.71 ± 0.08 μm were used.

CaCl2 aqueous solution ([−COONa]/[Ca2+] = 8) and a complexation time of 3 min at 30 °C. Although only partial surface coverage of the vaterite particles was observed on the anionic substrate (i.e., the (PDDA/PSS)4 layer) by SEM analysis, high-density adsorption of the particles on the cationic substrates (i.e., the (PDDA/PSS)3/PDDA layer) was observed (Figure S20, Supporting Information). The glass substrate of (PDDA/PSS)3/PDDA was alternately immersed for 15 min in an ethanol dispersion of the vatertite particles (4 mg/mL, pH 10) and in an ethanol solution of PDDA (1 mg/mL) at 30 °C and then rinsed with ethanol three times (1 min each). Multilayer films denoted as (PDDA/PSS)3/(PDDA/vaterite)n(PDDA), where n denotes the deposition layers of the vaterite particles, were prepared by continuing this sequential

for the POSS-COOH stabilized particle after its phase transition to calcite. In the case of the PDDA-coated vaterite composite particles, the zeta potential value decreased to −4 mV after the phase transition to calcite. These observations suggest that no inhibition of the phase transition occurred after the PDDA coating, and even after the phase transition, PDDA was still present on the surface. Vaterite Precursor Method for Fabricating Calcite Thin Films. Prior to the first deposition of the vaterite composite particles, a precoated (PDDA/PSS)3/PDDA or (PDDA/PSS)4 layer was deposited on a clean glass substrate. Both precoated layers were immersed for 15 min at 30 °C in an ethanol dispersion of vaterite particles with an average diameter of 0.71 ± 0.08 μm, obtained from the POSS-core dendrimer15893

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of the sample after incubation for 5 days shows a granular network structure of irregularly shaped calcite particles (Figure 4b). Although some patches and pores were present in the film, the image clearly demonstrates that a large-area, continuous CaCO3 film was formed. After the phase transition, the initial particle-deposited surface structure of vaterite was converted to a porous structure. The cross-section of the calcite film in the SEM images showed that the average film thickness was 10 μm, which was the same as that of the (PDDA/vaterite)10(PDDA) multilayer film (Figure 3b). The composition of the calcite film was further probed by EDX analysis (Figure S23, Supporting Information). From the distribution of Si and Ca in the film measured by the EDX analysis at an electron accelerating voltage of 15 kV, it was found that the distribution of Si decreased more than that of the vaterite multilayer film, although both Si and Ca were uniformly distributed on the film. These results indicate that the Si elements in the film were partially desorbed during the phase-transition process. A multilayer film without any PDDA coating on the top surface denoted as (PDDA/vaterite)10 was also incubated in distilled water at 30 °C. Incubation for 5 days led to complete phase transition to calcite, as estimated by FTIR and XRD analyses. A broad band at ∼1100 cm−1 assignable to the Si−O vibration derived from the POSS unit was also observed. The cross section of the calcite film in the SEM images showed that the average film thickness decreased from 10 to 7.5 μm after the phase transition to calcite (Figure S24, Supporting Information). Compared to the results obtained with the (PDDA/vaterite)10(PDDA) multilayer film, immersing the (PDDA/vaterite)10 multilayer film resulted in the formation of larger cubic crystals. These observations suggest that the PDDA coating on the top surface inhibits the dissolution of the vaterite composite particles in the aqueous phase. We studied the effect of the particles sizes on the fabrication of calcite films. The vaterite composite particles with an average diameter of 0.30 ± 0.05 μm, obtained from the POSS-core dendrimer-CaCl2 aqueous solution ([−COONa]/[Ca2+] = 8) and a complexation time of 3 min at 22 °C, were used for the layer-by-layer method of preparing the (PDDA/vaterite)10(PDDA) multilayer. The deposition of the (PDDA/ vaterite)n(PDDA) multilayer was confirmed by the SEM analysis. The obtained thin layers of the vaterite precursors with an average size 0.30 μm on the cationic substrates were incubated in fresh water at 30 °C. Although the phase transition from vaterite to calcite was observed after the substrates were incubated in distilled water at 30 °C, desorption of some parts of the particles was observed. Flat calcite surfaces were observed by SEM analysis (Figure S25, Supporting Information). The morphologies of the calcite films are highly dependent on the vaterite particle sizes. The present study suggests that the particle sizes are the chief factors responsible for the formation of calcite films from vaterite particles.

adsorption process (Figure 3). The deposition of the (PDDA/ vaterite)n(PDDA) multilayer films was confirmed by the SEM images (Figure 4a). The top surface of the (PDDA/vaterite)10 multilayer film was coated with PDDA. On the basis of a crosssection of the SEM image of the (PDDA/vaterite)10(PDDA) multilayer film, it was found that the average film thickness was ∼10 μm, which was reasonable for n = 10 because the average particle size was observed to be 0.71 μm. The XRD analysis of the film shows predominant peaks corresponding to vaterite and minor peaks assignable to calcite (Figure 5a). The vaterite

Figure 5. X-ray diffraction (XRD) patterns of (PDDA/vaterite)10(PDDA) multilayer films (a) before and (b) after immersion in distilled water for 5 days. POSS-core dendrimer-CaCO3 particles with an average diameter of 0.71 ± 0.08 μm were used. Characteristic reflections of XRD patterns for calcite (d spacing/2θ peak = 3.04 Å/ 29.4°, corresponding to hkl = 104) and vaterite (d spacing/2θ peak = 3.58 Å/24.9°, 3.3 Å/27°, and 2.73 Å/32.8° corresponding to hkl = 110, 111, and 112, respectively).

content of the film was 94%, as determined by Rao’s equation. An FTIR analysis of the film showed two bands at 877 and 745 cm−1, indicating vaterite (Figure S21a, Supporting Information). A broad band at ∼1100 cm−1 can be assignable to the Si−O vibration derived from the POSS unit. The composition of the (PDDA/vaterite)10(PDDA) multilayer film was probed by energy-dispersive X-ray (EDX) analysis (Figure S22, Supporting Information). On the basis of the distribution of Si and Ca in the films measured by EDX analysis at an electron acceleration voltage of 15 kV, it was found that Si and Ca were uniformly distributed on the film. The (PDDA/vaterite)10(PDDA) multilayer film was incubated in distilled water at 30 °C. In the FTIR analysis, the products obtained after incubation for 1 day showed the bands at 874 and 712 cm−1 assignable to calcite coexisting with vaterite (Figure S21b, Supporting Information). Further incubation for 5 days led to the complete phase transition to calcite, as estimated by FTIR analysis (Figure S21c, Supporting Information). A broad band at ∼1100 cm−1 assignable to the Si−O vibration derived from the POSS unit was also observed. The crystal phases of the obtained films were further confirmed by XRD, suggesting that the films after incubation for 5 days consisted entirely of calcite (Figure 5b). The SEM observation

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DISCUSSION Size-Controlled Crystal Formation Mechanism. The particle sizes of the vaterite composite particles were simply controlled by changing the addition time of the aqueous carbonate reagent to the POSS-core dendrimer-CaCl2 solution, concentrations of POSS-COOH, and reaction temperatures. Shorter complexation times of the POSS-core dendrimer-CaCl2 solution gave smaller average particle sizes. Higher concentrations of POSS-COOH and lower reaction temperatures also decrease the average particle sizes. The absorbed amounts of 15894

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dissociate from CaCO3 particles and promotes the phase transition of vaterite to thermodynamically most stable phase calcite via a solvent-mediated process on increasing the complexaton times. The smallest absorbed amount of POSSCOOH of 5.1 wt % was observed in the composite particles obtained from the POSS-core dendrimer-CaCl2 aqueous solution ([−COONa]/[Ca2+] = 1) and the complexation time for 1 h at 30 °C. An SEM image of the interior of the fractured vaterite composite particles obtained using the POSS-core dendrimer with an incubation time of 1 h is shown in Figure S26 (Supporting Information). The crystals were formed by a spherulitic growth mechanism, showing the characteristic radiating features.31 In this growth process, the mixing of Ca2+ and carbonate ions results in the formation of ACC nanoparticles. The ACC nanoparticles grow and transform within minutes to form crystalline vaterite particles. A stronger stabilization of amorphous CaCO3 particles by POSS-COOH might inhibit further growth to larger particles. It has been proposed that the so-called Eigen mechanism of a metal ion complex formation reaction involves a ratedetermining step in which water loss occurs from the primary coordination sphere of the metal ion after the metal ion has formed an outer-sphere complex (or solvent-separated ion pair) with an incoming ligand.32 The complexation of poly(amino carboxylate) ligands such as EDTA, EGTA, and CDTA with Ca2+ ions has been found to involve rapidly formed intermediates that transform slowly to the final products.33 On conversion of the intermediates to the final complexes, the process involves the simultaneous stripping of several water molecules from the first-coordination sphere of the Ca2+ ions. In the early stage of POSS-core dendrimer-Ca2+ complexation, the POSS-core dendrimer-Ca2+ complex in the intermediate state might form predominately. The POSS-core dendrimerCa2+ complex in the intermediate form may convert to the final complex with an increase in the incubation time. The intermediate form might tend to absorb on the CaCO3 particles because of its hydrophilic character. On the contrary, the final complex might dissociate from the CaCO3 particles because of its hydrophobic character. Surface Properties and Phase Transition. The incubation of vaterite composite particles prepared using POSSCOOH in distilled water for 3 days led to the complete phase transition to calcite. In the case of the carboxylate-terminated PAMAM dendrimer, however, no phase transition to calcite was observed when the obtained vaterite composite particles were added to distilled water for further incubation for 7 days at room temperature.20 The results indicate that the isolated and dried vaterite particles modified with the PAMAM dendrimer exhibit a strong stabilizing effect that prevents phase transformation in comparison to that modified with the POSS-core dendrimer. It is well known that vaterite transforms into the most thermodynamically stable form, calcite, via a solventmediated process. Although the organic content of the products formed using the POSS-core dendrimer was greater than that of the vaterite particles using the PAMAM dendrimer, the stabilizing effect of POSS-COOH seems to be weaker. As mentioned before, the intermediate form obtained by using the PAMAM dendrimer would tend to dissociate from the CaCO3 particles during crystallization. The binding strength of the PAMAM dendrimer should be stronger than that of POSSCOOH in the final vaterite composite particles because of the flexible property of the PAMAM dendrimer.

POSS-COOH in the composite particles in Tables 1 and 2 are plotted as a function of the average particle size (Figure 6a).

Figure 6. Plots of the absorbed amounts of POSS-COOH in the CaCO3 composite particles as a function of (a) the average particles and (b) the absorbed amounts of water in the CaCO3 composite particles as a function of the absorbed amounts of POSS-COOH. Data were used from Tables 1 and 2.

The plot clearly shows that the absorbed amounts of POSSCOOH proportionally decrease from 24 to 5.1 wt % on increasing the average particle size. We also found a good linear relationship between the absorbed amounts of water and POSS-COOH in the composite particles (Figure 6b). This means that the absorbed amounts of water in the composite particles decrease proportionally on increasing the average particle size. These results show that the absorbed amounts of POSS-COOH and water determine the average particle size. The shorter the complexation time, the greater the concentrations of POSS-COOH. Lower temperatures increase the absorbed amounts of POSS-COOH and decrease the average particle size. Longer complexation times decrease the content of POSSCOOH. At lower concentrations of POSS-COOH ([COONa]/[Ca2+] = 1 and 2), FTIR analysis indicates that calcite coexists with vaterite, and the relative intensity of the bands corresponding to calcite increases on increasing the complexaton time (Figures S6 and S7, Supporting Information). These observations suggest that POSS-COOH tends to 15895

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tion and higher isolation stability in comparison to those for ACC). The self-assembly of the CaCO3 precursors in a controlled manner further requires new fabrication techniques to develop suitable CaCO3 precursor particles. From a synthesis viewpoint, these CaCO3 precursors must be functionalized with a wide variety of organic functional moieties using a simple chemical process to control the size of the particles. The most important requirement for this purpose is repeated isolation and redispersion in a solvent. The second most important requirement is to control the rates of phase transition after self-assembly on the substrates. The present POSS-COOH-stabilized vaterite particles satisfy these requirements.

In our previous report, we ran experiments using the carboxylate-terminated hyperbranched PAMAM. In these studies, the addition of an aqueous solution of PDDA to the vaterite composite particles in aqueous solution transformed the polymorph of the CaCO3 to thermodynamically stable calcite.34 In the present study conducted using the POSS-core dendrimer, no phase transition was detected during complexation with PDDA. The stability of the PDDA ionic complex with the hyperbranched polymer might be stronger than that formed with the POSS-core dendrimer because of the flexible properties of the hyperbranched polymer. In the present study conducted using the POSS-core dendrimer, complete phase transition of the PDDA-coated vaterite particles to calcite occurred within 2 days in an aqueous solution at room temperature, indicating that no substantial stability change was observed after the PDDA coating. A positive zeta potential value of ∼1.5 mV was detected for the vaterite particles obtained from 3 min of incubation after the addition of 10 mg/mL PDDA. Because the vaterite particles prepared without POSS-COOH showed a zeta potential value of +10 mV, the positive zeta potential value is not direct evidence of the formation of PDDA-coated vaterite particles. A negative zeta potential value of −4 mV was detected after the phase transition to calcite in the PDDA-coated vaterite particles. Because the negative zeta potential value of −19 mV was observed for the POSS-COOH-stabilized particles after the phase transition to calcite, decreasing the negative zeta potential value to −4 mV indicates that some part of the PDDA was exposed on the CaCO3 particles, even when the phase transition from vaterite to calcite was performed in fresh water. Vaterite Precursor Method for Fabricating Calcite Thin Films. The transformation of vaterite to calcite has been well studied. The inherent energetic instability of vaterite results in the phase transition to other stable crystalline phases in aqueous solution, which occurs via a dissolution and recrystallization process. The vaterite dissolves and releases calcium and carbonate ions into the solution, which then reprecipitate on the surface of the growing calcite crystals. In supplying a dense precursor phase to the reprecipitation sites, slow ion diffusion is avoided and high rates of mineralization can be achieved. Because of the unique characteristics of the POSS-core dendrimer, the present vaterite composite particles are excellent candidates as precursors for fabricating calcite films via phase transition. When complexation was performed using PDDA as a cationic polymer, no substantial stability change was observed in the phase transition to calcite. The PDDA coating on the top surface of the present (PDDA/ vaterite)10 multilayer film avoided the dissolution of calcium and carbonate ions in the aqueous phase. This was supported by the observation that the film thickness on the surface decreases when the phase transition occurs without the PDDA coating on the surface. Contact of the vaterite particles with water is a critical factor in determining its rate of phase transition to calcite. The cross-section of the (PDDA/ vaterite)10(PDDA) multilayer film in the SEM images shows a continuous micropore in which water can penetrate even the first layer of the vaterite particles. The ACC precursor pathway has been studied as an attractive biomimetic synthesis strategy for preparing thin films of CaCO3. We propose here that the vaterite precursor pathway is a superior alternative candidate for fabricating calcite thin films because it offers many advantages in comparison with the ACC precursor (e.g., easily achievable surface functionaliza-

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CONCLUSIONS

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ASSOCIATED CONTENT

Vaterite composite particles with a size-controlled sphere were obtained by the carbonate controlled-addition method using the carboxylate-terminated POSS-core dendrimer. A minimum particle size of 70 ± 6 nm was obtained when the COONa to calcium ion molar ratio was 16 and the complexation time was 3 min at 20 °C. However, a maximum particle size of 2.93 ± 0.42 μm was obtained when the COONa to calcium ion molar ratio was 1 and the complexation time was 1 h at 30 °C. Incubation of the vaterite composite particles in distilled water for 3 days led to complete phase transition to calcite. Negative zeta potential values, ranging from −30 to −10 mV, were detected for the vaterite particles, indicating that the POSS-core dendrimers were exposed on the CaCO3 particles. Surface coating of the CaCO3 particles with PDDA in an aqueous dispersion was successfully achieved by the addition of a controlled concentration of the polymer. Alternate vaterite composite particles with average diameters of 0.71 ± 0.08 μm and polyelectroyte multilayer films were prepared by a layer-bylayer method. The obtained (PDDA/vaterite)10(PDDA) multilayer film was incubated in distilled water at 30 °C. Incubation for 5 days led to its complete phase transition to calcite, as estimated by FTIR and XRD analysis. The SEM observation of the sample after incubation for 5 days shows a granular network structure of irregularly shaped calcite particles. Although some patches and pores were present in the films, the image clearly demonstrates that large-area and continuous CaCO3 films were formed. After the phase transition, the initial particle-deposited surface structure of vaterite was converted to a porous structure. We also demonstrated that the morphologies of the calcite films are highly dependent on the vaterite particle sizes. Using the vaterite composite particles with an average diameter of 0.30 ± 0.05 μm formed flat calcite films. The vaterite precursor pathway is a superior alternative candidate for fabricating calcite thin films. A study of the size effect of the vaterite precursors is underway.

S Supporting Information *

FTIR spectra, X-ray diffraction patterns, TGA, and SEM images of CaCO3 particles and (PDDA/PSS)3/(PDDA/vaterite)10 multilayer films. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 15896

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Notes

Phase Transition from Vaterite to Calcite. Cryst. Growth Des. 2010, 10, 4030−4037. (19) Huang, S.-C.; Naka, K.; Chujo, Y. A Carbonate ControlledAddition Method for Amorphous Calcium Carbonate Spheres Stabilized by Poly(acrylic acid)s. Langmuir 2007, 23, 12086−12095. (20) Tanaka, Y.; Naka, K. A Carbonate Controlled-Addition Method for Size-Controlled Calcium Carbonate Spheres by Carboxylic Acid Terminated Poly(amidoamine) Dendrimers. Polym. J. 2010, 42, 676− 683. (21) Donners, J. J. J. M.; Heywood, B. R.; Meijer, E. W.; Nolte, R. J. M.; Sommerdijk, N. A. J. M. Control over Calcium Carbonate Phase Formation by Dendrimer/Surfactant Templates. Chem.Eur. J. 2002, 8, 2561−2567. (22) Naka, K.; Tanaka, Y.; Chujo, Y.; Ito, Y. The Effect of an Anionic Starburst Dendrimer on the Crystallization of CaCO3 in Aqueous Solution. Chem. Commun. 1999, 1931−1932. (23) Naka, K.; Tanaka, Y.; Chujo, Y. The Effect of Anionic Starburst Dendrimers on the Crystallization of CaCO3 in Aqueous Solution: Size Control of Spherical Vaterite Particles. Langmuir 2002, 18, 3655− 3658. (24) Naka, K.; Fujita, M.; Tanaka, K.; Chujo, Y. Water-Soluble Anionic POSS-Core Dendrimer: Synthesis and Copper(II) Complexes in Aqueous Solution. Langmuir 2007, 23, 9057−9063. (25) Even the reaction mixtures were kept for 24 h, and the particle sizes and CaCO3 crystal phase of the products were the same as those corrected after incubation for 1 h. (26) Perić, J.; Vučak, M.; Krstulović, R.; Brečević, Lj.; Kralj, D. Phase Transformation of Calcium Carbonate Polymorphs. Thermochim. Acta 1996, 277, 175−186. (27) Jiménez-López, C.; Caballero, E.; Huertas, F. J.; Romanek, C. S. Chemical, Mineralogical and Isotope Behavior, And Phase Transformation during the Precipitation of Calcium Carbonate Minerals from Intermediate Ionic Solution at 25°C. Geochim. Cosmochim. Acta 2001, 65, 3219−3231. (28) Wolf, G.; Günther, C. Thermophysical Investigations of the Polymorphous Phases of Calcium Carbonate. J. Therm. Anal. Calorim. 2001, 65, 687−698. (29) Sawada, K. The mechanisms of crystallization and transformation of calcium carbonates. Pure Appl. Chem. 1997, 69, 921−928. (30) Vaterite particles were prepared by the standard precipitation of CaCO3 as described in the Experimental Section, in which no dendrimer was added and the precipitates were immediately isolated after the addition of a 0.1 M (NH4)3CO3 aqueous solution. XRD analysis of the particles shows predominately peaks corresponding to vaterite with minor peaks assignable to calcite. The vaterite content was 60% as determined by Rao’s equation. The average particle size was 280 nm by SEM analysis. (31) Andreassen, J.-P. Formation Mechanism and Morphology in Precipitation of Vateritenano-Aggregation or Crystal Growth? J. Cryst. Growth 2005, 274, 256−264. (32) Caw, J. D.; Swartzfager, D. G. Kinetics of the Ligand Exchange and Dissociation Reactions of Calcium-Aminocarboxylate Complexes. J. Am. Chem. Soc. 1975, 97, 315−321. (33) Wu, S. L.; Johnson, K. A.; Horrocks, W. D., Jr. Kinetics of Formation of Ca2+ Complexes of Acyclic and Macrocyclic Poly(amino carboxylate) Ligands: Bimolecular Rate Constants for the FullyDeprotonated Ligands Reveal the Effect of Macrocyclic Ligand Constraints on the Rate-Determining Conversions of Rapidly-Formed Intermediates to the Final Complexes. Inorg. Chem. 1997, 36, 1884− 1889. (34) Tanaka, Y.; Naka, K. Synthesis of Calcium Carbonate Particles with Carboxylic Terminated Hyperbranched Poly(amidoamine) And Their Surface Modification. Polym. J. 2012, 44, 586−593.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study is part of the Kyoto City Collaboration of Regional Entities for the Advancement of Technology Excellence of JST and a Grant-in-Aid for Scientific Research on Innovative Area “New Polymeric Materials Based on Element-Blocks (no. 2401)” (24102003) of The Ministry of Education, Culture, Sports, Science, and Technology, Japan.

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REFERENCES

(1) Estroff, L. A.; Hamilton, A. D. At the Interface of Organic and Inorganic Chemistry: Bioinspired Synthesis of Composite Materials. Chem. Mater. 2001, 13, 3227−3235. (2) Meldrum, F. C.; Cölfen, H. Controlling Mineral Morphologies and Structures in Biological and Synthetic Systems. Chem. Rev. 2008, 108, 4332−4432. (3) Imai, H.; Oaki, Y.; Kotachi, A. A Biomimetic Approach for Hierarchically Structured Inorganic Crystals through Self-Organization. Bull. Chem. Soc. Jpn. 2006, 79, 1834−1851. (4) Yao, H.-B.; Fang, H.-Y.; Wang, X.-H.; Yu, S.-H. Hierarchical Assembly of Micro-/Nano-Building Blocks: Bio-Inspired Rigid Structural Functional Materials. Chem Soc. Rev. 2011, 40, 3764−3785. (5) Kato, T.; Sugawara, A.; Hosoda, N. Calcium Carbonate Organic Hybrid Materials. Adv. Mater. 2002, 14, 869−877. (6) Naka, K.; Chujo, Y. Control of Crystal Nucleation and Growth of Calcium Carbonate by Synthetic Substrates. Chem. Mater. 2001, 13, 3245−3259. (7) Sommerdijk, N. A. J. M.; de With, G. Biomimetic CaCO3 Mineralization Using Designer Molecules and Interfaces. Chem. Rev. 2008, 108, 4499−4550. (8) Cölfen, H.; Mann, S. Higher-Order Organization by Mesoscale Self-Assembly and Transformation of Hybrid Nanostructures. Angew. Chem., Int. Ed. 2003, 42, 2350−2365. (9) Addadi, L.; Moradiam, J.; Shay, E.; Maroudas, N. G.; Weiner, S. A Chemical Model for the Cooperation of Sulfates and Carboxylates in Calcite Crystal Nucleation: Relevance to Biomineralization. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 2732−2736. (10) Gower, L. B. Biomimetic Model Systems for Investigating the Amorphous Precursor Pathway and Its Role in Biomineralization. Chem. Rev. 2008, 108, 4551−4627. (11) Lvov, Y.; Decher, G.; Möhwald, H. Assembly, Structural Characterization and Thermal Behavior of Layer-by-Later Deposited Ultrathin Films of Poly(vinyl sulfate) and Poly(allylamine). Langmuir 1993, 9, 481−486. (12) Pichon, B. P.; Louet, P.; Felix, O.; Drillon, M.; Begin-Colin, S.; Decher, G. Magnetotunable Hybrid Films of Stratified Iron Oxide Nanoparticles Assembled by the Layer-by-Layer Technique. Chem. Mater. 2011, 23, 3668−3675. (13) Serizawa, T.; Tateishi, T.; Ogomi, D.; Akashi, M. Deposition of Calcium Carbonate Disks on Polyelectrolyte Multilayer Matrices by the Alternate Soaking Process. J. Cryst. Growth 2006, 292, 67−73. (14) Wei, H.; Ma, N.; Shi, F.; Wang, Z.; Zhang, X. Artificial Nacre by Alternating Preparation of Layer-by-Layer Polymer Films and CaCO3 Strata. Chem. Mater. 2007, 19, 1974−1978. (15) Finnemore, A.; Cunha, P.; Shean, T.; Vignolini, S.; Guldin, S.; Oyen, M.; Steinertru, U. Biomimetic Layer-by-Layer Assembly of Artificial Nacre. Nat. Comm. 2012, 3, 966. (16) Ogino, T.; Suzuki, T.; Sawada, K. The Rate and Mechanism of Polymorphic Transformation of Calcium Carbonate in Water. J. Cryst. Growth 1990, 100, 159−167. (17) Rodriguez-Blanco, J. D.; Shaw, S.; Benning, L. G. The Kinetics and Mechanisms of Amorphous Calcium Carbonate (ACC) Crystallization to Calcite, via Vaterite. Nanoscale 2011, 3, 265−271. (18) Fujiwara, M.; Shiokawa, K.; Araki, M.; Ashitaka, N.; Morigaki, K.; Kubota, T.; Nakahara, Y. Encapsulation of Proteins into CaCO3 by 15897

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