Transformation of amorphous calcium carbonate into aragonite

Journal of Crystal Growth 343 (2012) 62–67

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Transformation of amorphous calcium carbonate into aragonite Zhuona Zhang, Yidong Xie, Xurong Xu n, Haihua Pan, Ruikang Tang Department of Chemistry and Centre for Biomaterials and Biopathways, Zhejiang University, Hangzhou 310027, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 October 2011 Received in revised form 6 January 2012 Accepted 13 January 2012 Communicated by S. Veesler Available online 24 January 2012

Amorphous calcium carbonate (ACC) is one of the important precursors of calcium carbonate crystal phases and its transformation plays key roles in biomineralization. In this paper, the transformation of ACC is studied in the presence of magnesium ion, which often appears in the crystallization of calcium carbonate. We use powder X-ray diffraction, Fourier transformed infrared (FTIR) spectroscopy and scanning electron microscopy to monitor this transformation process and find that in high Mg2 þ environment, there appears a transformation process with multiple-step self-assembly from ACC into aragonite. To the best of our knowledge, it is the first time the growth of aragonite via the nano-crystal particles assembly and transformation has been discovered, which may have a great significance in understanding the mystery of aragonite formation in an ocean. & 2012 Elsevier B.V. All rights reserved.

Keywords: A1. Crystallites A1. Recrystallization B1. Calcium compound B1. Minerals

1. Introduction Calcium carbonate is paid great attention for its scientific value and wide range of industrial applications [1], which is used as an additive or modifier in paper, paints, plastics, inks, adhesives, pharmaceuticals and so on. Besides, calcium carbonate is one of the important biominerals in biominerliazation, where it is the main constituent of mollusk shells, crustacean cuticles, corals, skeletons of protozoa, etc [2]. To date, studies have shown that calcium carbonate consists of six different phases under normal temperature and pressure, which are amorphous calcium carbonate (ACC), calcium carbonate hexahydrate, calcium carbonate monohydrate (monohydrocalcite), vaterite, aragonite and calcite.Recently, ACC has been extensively found as an important precursor of crystal phases in biological organisms [3–8], where it is used as temporary storage deposits. It is known that ACC with high unstability and solubility easily transforms into vaterite, aragonite or calcite depending on the conditions, so that it is important to control the transformation of ACC into desired crystal phase. Though aragonite is metastable in aqueous solution, aragonite can nucleate and grow stably, and widely exist in the biological system [9–11]. For example, aragonite exists in the nacre of the shells of bivalve mollusks, which provide a protection for these animals. Weiner et al. [12] have shown that ACC is a precursor phase of aragonite from the study of the marine bivalves Mercenaria mercenaria and Crassostreagigas. However, it is not an easy job to understand how

n

Corresponding author. Tel./fax: þ 86 571 87953736. E-mail address: [email protected] (X. Xu).

0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2012.01.025

to transform from ACC into aragonite in vivo due to the extremely complex environment in the animal body. The formation and insitu transformation of ACC have been studied in solution [13–15]. In our previous work [16], ACC has been synthesized and dispersed into inert ethanol solution and then its transformation process has been independently studied in the presence of polyelectrolytes. It is known that magnesium ion Mg2 þ can inhibit the calcite growth efficiently and aragonite is preferred in the presence of high concentration of magnesium ion. In this work, ACC is synthesized, Mg2 þ ion is added to the dispersion solution of ACC and then the transformation process of ACC in the presence of Mg is studied.

2. Experimental section 2.1. Materials Dimethyl carbonate (DMC) was purchased from Aldrich. Sodium hydroxide (NaOH, AR), anhydrous calcium chloride (CaCl2, AR) and magnesium chloride hexahydrate (MgCl2  6H2O, AR) were purchased from Sino Chemical Company. The solution was filtered through a 220 nm membrane before being used. Double-distilled water was used in all experiments. 2.2. Synthesis of ACC ACC was synthesized according to the literature [17]. Briefly, 0.4 g of NaOH was dissolved in 20 mL water to form a 0.5 M solution. This solution was quickly mixed with 80 mL of an aqueous solution containing 0.45 g of DMC and 0.111 g of

Z. Zhang et al. / Journal of Crystal Growth 343 (2012) 62–67

anhydrous CaCl2. The reaction solution was stirred for 1 min at room temperature and pH of the solution was about 12. The resulting precipitate was separated rapidly by centrifugation (4000 rpm) and was washed with ethanol and acetone once separately. The final powder was dried under vacuum at 35 1C for 48 h. 2.3. Phase transformation In a typical experiment, 15 mg ACC was added into a 50 mL vial with 10 mL ethanol and then dispersed by ultrasonication for 1 h. Another 10 mL MgCl2 solution with different concentration was added to reach all kinds of Mg/Ca ratios. In a control experiment, 10 mL double-distilled water was added instead. The vials were shaken in a thermostatically controlled oscillator (120 rpm and 30 1C). Periodically, 1 mL aliquots of solution were withdrawn. The solid obtained by centrifugation (4000 rpm) was washed using anhydrous ethanol and dried in vacuum at 35 1C. The concentrations of calcium and carbonate ion calculated from the solubility of amorphous calcium carbonate (ACC) at 30 1C [18] are both 5.91  10  3 mol/L when the solubility of ACC in ethanol is neglected; pH of the solution is 7.4. 2.4. Characterization Fourier transformed infrared (FTIR) measurements were performed on a IRAffinity-1 FTIR spectrophotometer (SHIMADZU, Japan) with a resolution of 2 cm  1, using solid pellets prepared by grinding the sample with potassium bromide (KBr). Samples were uniformly spread on a monocrystalline silicon wafer and Pt-coated prior to microscopy experiments. Scanning electron micrographs (SEM) of the powder samples were taken on a S-4800 scanning electron microscope (Hitachi, Japan) fitted with a field emission source and with an accelerating voltage of 20 kV. A D/max-rA ˚ Rigaku, Japan) was used in the diffractometer (Cu Ka, l ¼1.5405 A, X-ray diffraction (XRD) studies.

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seen that the size of ACC synthesized is around 400 nm and the surface of particles looks smooth.

3.2. Effect of Mg/Ca ratio We used FTIR to study the effect of Mg ion on the transformation process of amorphous calcium carbonate. Different calcium carbonate phases show distinct, different absorption positions in the IR spectrum (Fig. 2). The absorption bands of carbonate are divided into four areas: u1  1080 cm  1; u2  870 cm  1; u3  1400 cm  1 and u4  700 cm  1. ACC has a characteristic broad u2 absorption band at  866 cm  1, and a split peak at  1418 and 1475 cm  1, except that there is a broad adsorption peak of water at  3400 cm  1 [3]. Vaterite’s characteristic absorption bands are u2  875 cm  1 and u4 745 cm  1 and a split peak of u3 at  1440 and 1490 cm  1. Aragonite shows two characteristic absorption bands at u2 856 cm  1 and u4 713 cm  1 along with a weak  700 cm  1 absorption peak, and also a u3 absorption band at  1490 cm  1. For calcite, there are two absorption bands at u2  875 cm  1 and u4  713 cm  1 and an absorption peak at  1420 cm  1. Monohydrocalcite shows weak absorption bands at u4 700 and 727 cm  1, at u2  873 cm  1, at u1  1068 cm  1 and a split peak of u3 at 1418 and 1488 cm  1 [19,20]. The effect of the Mg/Ca ratio on the transformation of ACC is shown in Fig. 3. In the control experiment, when Mg2 þ was not added, ACC began to crystallize instantly (Fig. 3a). Just in one day, the broad u2 absorption peak at  866 cm  1 of ACC disappears;

1490

856 713 700 aragonite

aragonite 1490

875

1440

745

1487

vaterite

1408

875

monohydrocalcite

3. Results and discussion

713

calcite 1475

1418

866

00

00

15

Wavenumber (cm-1)

Fig. 2. FT-IR spectra of pure calcium carbonate phase.

20

30

calcite

ACC

90 0 85 0 80 0 75 0 70 0 65 0 60 0 55 0 50 0

ACC

20

Calcium carbonate synthesized was characterized by FT-IR, X-ray diffraction and SEM. There appears only a broad bump on the X-ray diffraction pattern (Fig. 1b) and no sharp diffraction peaks can be found, which demonstrates that the particles are amorphous. Furthermore, there are broad and strong absorption peaks at 866 cm  1 and no absorption at 713 cm  1 or 745 cm  1 in FT-IR spectra (Fig. 2), which also shows that the calcium carbonate particles are amorphous. From SEM image (Fig. 1a), it can be

monohydrocalcite 700

875

1420

3.1. Characterization of ACC particles

vaterite

40 2θ

Fig. 1. (a) SEM image of ACC prepared and (b) XRD pattern of ACC.

50

60

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0:1

1:2 856 875

5 days

856

712 875

5 days

712

4 days 4 days 3 days

3 days

2 days

2 days

1 day 1 day

1600

1400 1200 1000 wavenumbers (cm-1)

2:1

800

1400 1200 1000 wavenumbers (cm-1)

800

4:1

856

856 712

8 days 712

8 days 7 days

1600

877856

877 856

7 days

875 700

6 days 875

875

700

5 days

700

6 days 5 days

875 3 days

3 days

1600

1400 1200 1000 wavenumbers (cm-1)

800

1600

1400 1200 1000 wavenumbers (cm-1)

856 866

800

Fig. 3. FT-IR spectra for the transformation process of ACC with different Mg/Ca ratios. (a) Mg/Ca ¼ 0; (b) Mg/Ca ¼ 1:2; (c) Mg/Ca ¼ 2:1 and (d) Mg/Ca ¼4:1.

the sharp peaks at 856 cm  1, 875 cm  1, 712 cm  1 and 700 cm  1 appear instead, which suggest that the product is a mixture of calcite and aragonite. There appears no great change even after longer transformation time. When Mg2 þ was added, the results changed dramatically. In Fig. 3b, the ratio of Mg/Ca was 1:2, ACC crystallized after one day. A strong absorption at 856 cm  1 along with a weak absorption at 872 cm  1 can be clearly seen, which suggests that the main transformation product is aragonite and a trace of calcite is in existence either. On prolonging the transformation time, the intensity ratio of 856 cm  1/872 cm  1 increases, which shows more aragonite content in the product [21]. The result is similar when the Mg/Ca ration is 1:1 (the data is not shown). When Mg/Ca is increased to 2:1, the crystallization of ACC is significantly inhibited. Even after 3 days, there appears no apparent IR absorption of crystal phase and only a slightly split peak at  866 cm  1 can be seen, which indicates the sample is still amorphous (Fig. 3c). However, after 5 days, the broad u2 absorption band of ACC at 866 cm  1 becomes sharper and shifts to 875 cm  1, but no u4 absorption at 745 cm  1 or at 712 cm  1 can be found. Simultaneously, an apparent split peak of u3 at  1408 and 1487 cm  1 along with a weak absorption at 700 cm  1 appears, which shows the formation of monohydrocalcite. When the transformation time is increased, the intensity of peak at  1408 cm  1 and 873 cm  1 becomes weaker and the peaks at 856 cm  1 and 712 cm  1 appear, which suggests that monohydrocalcite gradually transform into aragonite. It is clear that ACC transformed to monohydrocalcite and then into aragonite at the high Mg/Ca ratio 4:1. It is found that aragonite is formed directly in the transformation process of ACC when the Mg2 þ concentration is low and the intermediate CaCO3  H2O is, firstly, obtained in the presence of high Mg2 þ before the transformation into aragonite.

The products after 4 days and 10 days were characterized using powder X-ray diffraction and the data are plotted in Fig. 4. It is clearly shown that ACC transforms into monohydrocalcite firstly, and then transforms into aragonite from XRD data when the ratio of Mg2 þ /Ca2 þ is above 2. Additionally, the ratio of calcite to aragonite can be calculated from the Rietveld analysis of powder XRD patterns. The results calculated are listed in Table 1. Calcite is the main crystal phase in the absence of Mg2 þ . With the addition of small amount of Mg2 þ , the transformation product is totally different and aragonite becomes the main crystal. Increasing the concentration of Mg2 þ , the intermediate CaCO3  H2O is formed firstly and then transformed into aragonite. Monohydrocalcite (CaCO3  H2O) is a rare mineral in geological settings and metastable with respect to calcite and aragonite, produced in the presence of additives such as Mg and phosphate ions [22]. 3.3. Process of the transformation of ACC into aragonite ACC is not a stable phase compared with calcite and aragonite. It is reported to change into aragonite or calcite in different cases [23–25]. We study the process of the transformation of ACC into aragonite in the presence of Mg ion. The ACC particles synthesized are spherical and their surface looks smooth, whose size is about 400 nm in diameter (Fig. 1a). After 3 days of keeping in the reaction solution, particles aggregate and their size becomes smaller than before. The amorphous character is still kept from the IR spectra but the broad absorption band 866 cm  1 appears sharper and shifts to higher wavenumber. It can be seen that these nanoparticles aggregate organizedly and form lots of spindle after 4-day transformation (Fig. 5a).

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aragonite

monohydrocalcite M

M M

M M

M M

M

M

20

f

30

40

50

60 f

A A

e

C

A

A AAA

A A

A

A A AA

d A A

A

A

C

AC

AA

d

A

C AA

C

A

C AA

C

A

C C

C A CA

C

e

A

A AA

A A A A

c c b b a

a

aragonite calcite

calcite 20

30

40

50

60 20

30

40

50

60

Fig. 4. Powder X-ray diffraction data of the samples: (1) for 4 days, and (2) for 10 days. Mg/Ca ratio: a, 0; b, 0.5; c, 1; d, 3; e, 4 and f, 5. Symbol C denotes calcite, A is for aragonite and M for monohydrocalcite. Table 1 Percentage of aragonite for the products was calculated from XRD data. Mg/Ca

0 0.5 1 3 4 5

Day 4

Day 10

Calcite

Aragonite

Monohydrocalcite

Calcite

Aragonite

83 18 11 15 11 10

17 82 89 7 3 –

– – – 78 86 90

88 15 14 8 5 6

12 85 86 92 95 94

From Fig. 5b, regular aligned nanoparticles are clearly seen on one spindle. A new phase, monohydrocalcite, is obtained, which is confirmed from IR spectra. It can be inferred that random ACC nanoparticles become organized monohydrocalcite aggregates in the presence of Mg ion after 4-day transformation. Monohydrocalcite is not stable thermodynamically and will transform into other crystal phases. The absorption peaks at 856 cm  1, 712 cm  1 and 700 cm  1 from aragonite can be clearly seen from IR spectra (Fig. 5e) of the product after 5-day transformation. It suggests the phase transformation starts from monohydrocalcite to aragonite phase. As the time prolongs, the characteristic absorption of monohydrocalcite disappears in IR spectra after 7 days and only the absorptions of aragonite and trace calcite can be seen, which indicate that monohydrocalcite finally transforms into aragonite. Meanwhile, those nanoparticles disappear and only aragonite with smooth surface is left. The final product shows typical aragonite morphology, which the shape of spindle is kept (Fig. 5c, d). Comparing the SEM of monohydrocalcite (Fig. 5b) to that of aragonite (Fig. 5d), it can be found that their shapes are very similar but aragonite is composed of needles with smooth surface (Fig. 5d). When monohydrocalcite transforms into aragonite, the well-ordered nanoparticles of monohydrocalcite change into aragonite needles with the smooth surface and trace particles also can be found in the middle of the broken spindle.

4. Discussion Magnesium and calcium belong to the same main group in the periodic table of elements. Magnesium ion has smaller ion

diameter than the calcium ion so that magnesium ion shows stronger hydrate ability. When magnesium ion is included in the lattice of calcite, the thermodynamic stability of calcite decreases and aragonite becomes preferable in solution. Therefore, magnesium ion is one kind of additives often used in the crystallization and biomineralization of calcium carbonate. It is known that the preferable crystal phase of calcium carbonate is aragonite in the presence of high magnesium environment. Additionally, magnesium ion has been found to inhibit the transformation of ACC. It is clearly shown here that high concentration magnesium ion inhibits the transformation of ACC and the transformation product preferred is aragonite phase in the presence of high magnesium ion. Furthermore, there appears a metastable monohydrocalcite phase during the transformation from ACC into aragonite in high magnesium ion environment. It is known that monohydrocalcite is metastable and will transform into aragonite and calcite. Extended X-ray fine structure (EXAFS) has been used to characterize the structure of ACC, which provides some information on the local short-order around the selected atom [26–29]. ACC synthesized generally contains about 1 mole water molecules in its molecular structure, which has the same stoichiometry as that of monohydrocalcite. Furthermore, ACC is usually most similar to monohydrocalcite based on the EXAFS results [28,29]. Meldrum et al. have studied the structure of ACC containing Mg ion (ACC–Mg) and found that ACC–Mg has a short-range order structure most similar to that monohydrocalcite or aragonite [29]. Here ACC in high Mg environment first transforms into intermediate, monohydrocalcite, then crystallizes to aragonite, which confirm the structure similarity of ACC–Mg with monohydrocalcite and aragonite. Munemoto and Fukushi [22] have studied the transformation kinetics of monohydrocaclite into aragonite in aqueous solution and suggested that the transformation of monohydrocalcite into aragonite is through a dissolution and recrystallization process. However, another image appears in our experiments. The classical crystallization model starts from primary building blocks (atom, ions, molecules, etc.). Some clusters reach a critical crystal nucleus size, and these primary building blocks grow further via ion-by-ion attachment and unit cell replication [30–32]. Living organisms may make use of proteins and peptides to deterministically modify nucleation, growth and facet stability in biomineralization [33]. However, the classical crystallization

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7 days 6 days 5 days 4 days 3 days 2 days 1 day

1600

1400 1200 1000 wavembers (cm-1)

800

Fig. 5. Scanning electron microscope images (a, b 4 days; c, d 7 days) and IR spectra (e) of phase transformation of ACC at Mg/Ca ¼ 3 for different transformation times.

mechanism has been already challenged by a model involving aggregation-based growth. It has been demonstrated [34,35] that inorganic nanocrystals can aggregate into ordered solid phases via oriented attachment to control the reactivity of nanophase materials in nature. Besides, mesocrystals have been suggested, which consist of crystallographic oriented crystallites connected by polymers or surfactants [36,37]. The interaction between crystallites and organic molecules is a crucial condition in the architecture of mesocrystal. Furthermore, we have demonstrated the potential role of amorphous calcium phosphate (ACP) in facilitating the assembly of hydroxyapatite (HAP) nanoparticles into highly ordered structures through the aggregation of nanocrystals by oriented attachment, and the assembly of inorganic nanoparticles mediated by organic molecules [38]. In our present results, ACC particle firstly dissolves partly due to high solubility and then transforms into monohydrocalcite nanocrystals with defined structure in high Mg ion environment. These nanocrystals aggregate and form spindle-like structure by oriented attachment. Due to metastability of monohydrocalcite, these nanoparticles continuously fuse and grow to finally form a

smooth needle-like aragonite. It is well known that the aragonite will be the preferred phase in high magnesium ion environment. However, are seldom reported transformations of ACC into aragonite in high magnesium ion environment. As there is no such process happening at low Mg2 þ concentration or in the absence of Mg2 þ , this arrangement must has a relationship with Mg2 þ , though the relationship is not clear yet.

5. Conclusions Though aragonite is less stable than calcite thermodynamically, Mg2 þ can promote the formation of aragonite effectively. The aragonite is formed directly through a reaction of Ca2 þ with CO23  ion when the Mg2 þ concentration is low, while at a high concentration, a process via intermediate CaCO3  H2O is preferred. This is a multi-step process, where ACC particles dissolve and rearrange into a high order particles aggregation and finally transform to needle like aragonite, which can be interpreted by the non-classic crystallization theory. We show the details of the

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aragonite formation in high Mg2 þ environment for the first time, which has a potential significance in understanding the aragonite formation in vivo.

Acknowledgments This study was supported by the National Natural Science Foundation of China (20601023, 20871102 and 91127003), Zhejiang Provincial Natural Science Foundation (R407087), the Fundamental Research Funds for the Central Universities and Daming Biomineralization Foundation. References [1] [2] [3] [4]

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