Calcite Crystallization on Polarized Single Calcite Crystal Substrates

Article Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Calcite Crystallization on Polarized Single Calcite Crystal Substrates in the Presence of Poly-Lysine Norio Wada,* Naohiro Horiuchi, Miho Nakamura, Kosuke Nozaki, Akiko Nagai, and Kimihiro Yamashita Department of Inorganic Biomaterials, Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University, 2-3-10, Kanda-Surugadai, Chiyoda, Tokyo 101-0062, Japan ABSTRACT: We demonstrated the crystallization of calcite on nonpolarized and polarized (10.4)-oriented calcite single crystal substrates in the presence of poly-lysine (PLys) as a poly amino acid. The PLys and the electric field due to polarized calcite substrates showed a significant effect on the morphologies of calcite precipitates. At a small amount of PLys, rhombohedral calcite crystals, with notches in the edges of the rhombohedra, formed through homoepitaxy and constructed aggregates with an island structure. At a large amount of PLys, the calcite precipitates had overgrown calcite crystals, which were formed by epitaxial growth on each of the rhombohedral surfaces of already formed calcite rhombohedra, and the calcite aggregates became elongated in the direction perpendicular to the substrate. As the amount of PLys increased, the overgrown calcite crystals tapered toward their ends, and some of the overgrown calcite crystals exhibited a cone-shaped appearance. These cone-shaped crystals consisted of aggregates with a needle-like morphology. The PLys addition and the electric fields due to the polarized calcite substrates on calcite crystallization were apparent most remarkably on a negatively charged surface of these substrates. We proposed the formation mechanism of calcite precipitates on the oriented calcite substrates in the presence of PLys.

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INTRODUCTION Biomineralization is an important common phenomenon in nature. Biominerals are the biomineralization products, such as bones, teeth, and shells, and they exhibit elaborate architectures and remarkable physical properties because of their complex hierarchical structures. These biomaterials are organized by combinations of inorganic and organic components. Biomineralization processes involve specific proteins that are released at different times during the mineralized tissue formation. The proteins play an important role in the control of the nucleation, polymorphism, morphology, and orientation of the formed crystals.1−5 It has been suggested that insoluble and soluble organic additives have the following two possible functions in the processes: (a) they provide a heterogeneous nucleation site, and (b) they regulate crystal growth by their adsorption.6 In general, it is accepted that anionic, soluble biomacromolecules have an important role in biomineralization. Among them, it is well-known that the negatively charged poly amino acids, polyaspartic acid and poly-glutamic acid, which contain carboxyl groups, play a critical role in the controlled nucleation and growth of the mineral phase because they can interact with calcium ions in solution.7−12 On the other hand, positively charged poly amino acids are generally considered to be less active in controlling biominerals. As a result, the study of the effects of a soluble basic, positively charged poly amino acid, such as poly-lysine (PLys), on biomineralization is poor.13−16 We consider that cationic poly amino acids have a potential role in the crystallization of biomaterials. The study of the role of © XXXX American Chemical Society

negatively and positively charged poly amino acids in biomineralization and insight into the elementary step of polyamide-mediated calcium carbonate (CaCO3) mineralization are of importance for a better understanding of the nucleation and growth processes of minerals in biological systems and for providing important information for promoting biomimetics. Mechanisms of biomineralization have been proposed to explain the interactions between the poly amino acids and formed biominerals through a combination of electrostatic and stereochemical interactions, as well as geometrical matching.1,4,5 Considering the organic−inorganic interaction in biomineralization, it has been shown that the electric field due to the charged group of an organic molecule plays an important role in the biomineralization process.17 The electric field is considered to regulate the reorganization and condensation of clusters at the interface and adsorption of organic additives. CaCO3 has been widely used as an attractive model mineral in the laboratory because its crystals are easily characterized, and its morphology and polymorphism are subject to control during the biomineralization process. CaCO3 has three anhydrous crystalline polymorphs: calcite, aragonite, and vaterite. Calcite is thermodynamically more stable at ambient pressure and temperature than the other anhydrous CaCO3 polymorphs. Received: September 26, 2017 Revised: December 21, 2017 Published: December 22, 2017 A

DOI: 10.1021/acs.cgd.7b01364 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

The electrical polarization and an experimental system adapted to slow diffusion were the same as those used in previous studies.33,35 A brief overview of the experimental procedure is as follows. The polarization was performed under a DC electric field of 5 kV·cm−1 at 400 °C for 1 h in air. The surface potentials of the polarized calcite substrates were measured by using a potentiometer. The polarized surfaces with negative and positive charges and nonpolarized surface with heat treatment alone were labeled as the N-, P-, and H-surface, respectively. CaCO3 precipitation experiments were performed in the absence of PLys using a 5 mM calcium chloride solution (10 mL) in the presence of differing amounts of PLys (0.5, 1.0, 2.0, and 3.0 mg/10 mL) through slow diffusion of carbon dioxide and ammonia into calcium chloride solutions, based on the gas diffusion method by thermal decomposition of ammonium carbonate.30−35,37 PLys was added to the calcium chloride solution before vapor diffusion. Initially, the pH values of the calcium chloride solutions with and without PLys were adjusted to ca. 8.2 using NaOH. The oriented calcite substrates were suspended by threads in the calcium chloride solutions and were immersed in the calcium chloride solution for 48 h. The system temperature was maintained at 28 °C. The crystal phases and orientation of the CaCO3 precipitates were examined by XRD using Ni-filtered CuKα radiation (λ = 1.5406 Å) at room temperature. Fourier-transform infrared (FTIR) analysis using KBr pellets was also performed to examine the crystal phases and the existence of the PLys contained in the precipitates. The resolution of the FTIR spectrometer was 2 cm−1. The morphologies of the precipitates were observed by scanning electron microscopy (SEM) and polarized optical microscopy (POM). The roughness of the calcite substrates was measured by atomic force microscopy (AFM).

Several studies have been conducted on the formation of CaCO3 crystals mediated by poly amino acids in an attempt to mimic biomineralization.7−10,18−26 However, the mechanisms of interaction between the electric field and organic additive during the biomineralization process are still unclear. Yamashita et al. made the discovery that hydroxyapatite (HAp) material can be polarized by applying an external electric field at elevated temperature, and the resulting polarized state can be maintained at room temperature, namely, an electret, and indicated that the growth of HAp was accelerated at the negatively charged surface, but not at the positively charged surface.27−29 We reported that the surface electric fields due to HAp, calcite, and yttria-stabilized zirconia electrets regulated not only the nucleation of the precipitate and but also the subsequent growth on calcite crystallization in the presence of poly acrylic acid (PAA) and poly aspartic acid (PAsp).30−35 We also suggested that the cooperation of PAA or PAsp and the local electric field due to the electrets promoted the formation of calcite thin films with c-axis orientation and acted remarkably on the negatively charged surfaces of these ceramics. On the crystallization of CaCO3 on polarized calcite single crystal substrates, the presence of PAA favored the formation of calcite thin films that were composed of a large number of prismatic calcite crystals with c-axis orientation, and the presence of PAsp led to the formation of CaCO3 films with a multiple phase structure with thin-film-like and hemispherical forms which consisted of calcite as the main polymorphic form and aragonite, respectively.33−35 The obtained results indicated that the negatively charged poly amino acids, which contain carboxyl groups that can interact with calcium ions in solution, played an important role in CaCO3 crystallization. However, the effects of positively charged poly amino acids on CaCO3 crystallization have not fully been understood. An understanding of how the surface electric field and a poly amino acid affect crystallization is of importance for promoting the synthesis of organic and inorganic composites by biomimetics. Therefore, to provide useful knowledge about this issue, we have investigated the effects of a soluble basic, positively charged poly amino acid, PLys, on the nucleation, growth, morphology, and polymorphic phases of CaCO3 in the presence and absence of an electric field due to polarized single calcite crystal substrates, using a gas diffusion technique by thermal decomposition of ammonium carbonate (slow diffusion method).

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RESULTS AND DISCUSSION Characteristics of Calcite Single Crystal Plates To Be Used As Growth Substrates. It was confirmed, through analysis by SEM, XRD, and FTIR, that there were no significant differences in the surface structures of the (10.4)-oriented calcite substrates before and after the polarization and heat treatments. Potentiometer measurements showed that the surface electric potentials of the polarized calcite substrates were in the range of ca. 5.0 V, either positive or negative. The potential of the H-surface was zero. The roughness of the (10.4)-oriented calcite substrates was determined by AFM to be ca. 0.03 μm. Effects of PLys and Surface Electric Fields on CaCO3 Crystallization. The pH of the solution changed from 8.2 to 9.4 after approximately 0.5 h of vapor diffusion and remained at ca. 9.4 throughout the rest of the crystallization process because of the buffer effect of the ammonium/ammonium-hydroxide solution due to the adsorption of ammonia. We consider that, in the closed crystallization system using the slow diffusion method, the CaCO3 crystallization did not occur immediately after the experiment onset because the CaCO3 supersaturation at the initial stage of the crystallization was zero, while the PLys adsorption could occur on the calcite substrates at the moment that the experimental test started. Hence, we believe that PLys adsorbed onto the calcite substrate in the solution with pH 8.2, and CaCO3 crystallization occurred in the solution with pH 9.4, while the degree of protonated amino groups held constant during the crystallization process. The dissociation constant for the NH4 groups of PLys was ca. 9.0.36 Hence, under our experimental conditions, with the pH ranging from 8.2 to 9.4, the degree of protonated amino groups of the PLys varied from ca. 69% (pH = 8.2) to ca. 41% (pH = 9.4). PLys can adopt three different secondary conformations, a random coil conformation, an α-helical conformation, and a β-sheet structure, depending on the pH and temperature of the

EXPERIMENTAL SECTION

CaCO3 crystallization was conducted on a substrate with a (10.4) orientation obtained by cleaving crystals of high-quality natural calcite in the presence and absence of PLys (trifluoroacetate, MW = 2400) that had molecules with approximately 10 equiv of basic groups per molecule. PLys is a biocompatible poly amino acid whose amino group side chains are positively charged depending on the pH of the reactant solution and has a dissociation constant (pKa) of ca. 9.0 and becomes progressively more protonated as the pH is decreased.36 Calcite substrates with and without electrical polarization treatment were prepared. The crystallographic indices refer to the structural unit cell of calcite in the hexagonal system. The four-symbol Miller−Bravais indices (hkil) are abbreviated (hk.l). The substrate with the (10.4) planes of calcite inclined by 44.7° to the crystallographic c-axis of calcite. The atomic modeling of the surface structure showed that the (10.4) plane, in which the number of Ca2+ ions equals that of CO32−, was neutral. The orientation was checked using X-ray diffraction (XRD). Oriented calcite substrates had, in their final form, a typical dimension of ca. 5 mm × 3 mm × 1 mm. B



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solution. According to the literature,38 the shorter-chain PLys, with approximately 20 equiv basic groups per molecule, adopts a random coil conformation at both pH 7.5 and pH 11. Therefore, we believe that the PLys provided for the study was positively charged and that the most of the chains had the random coil conformation under our experimental conditions. The reaction was stopped after 12 h to study the influence of the electric field upon the polarized calcite substrate without PLys at the initial stage of the crystallization. Only oriented calcite crystals, the most thermodynamically stable polymorph, were observed, regardless of the polarization treatment, based on direct observation of the crystal morphology (Figure 1a,c,e).

plane remained straight, while the other two edges were inhibited and became rounded, and new faces, along with a terraced morphology with macro-steps, appeared. Inhibition started at the corners where two of these edges met. The symmetry of the newly expressed planes could correspond to the {11.0} plane of calcite.39 We consider that the different morphologies appearing on the nonpolarized and polarized calcite substrates were caused by the effect of the polarization treatment, but we cannot explain the appearance of these surface morphologies at present. After the experiment period of 48 h, calcite aggregates with an island structure developed into a partial film structure on both the nonpolarized and polarized substrates. We have discussed, in our previous papers,32,35 a theory regarding nucleation in the presence of an electric field, based on classical nucleation theory and electromagnetic theory and have indicated that the activation energy for nucleation, ΔG*, decreases with an electric field generated by the polarized substrate and the supersaturation of the reactant solution. In addition, discussion on the effect of the electrostatic force density, given by ρE, where ρ is the charge density of the ion (including its sign, either positive or negative) and E is the electric field due to the polarized substrate, suggests that the surface electric field at the N-surface induced a high concentration of Ca2+ ions around this surface, while the Psurface exhibited a low concentration; the electrostatic interactions between the CO32− ions and the N- and P-surfaces were neglected because of the relatively low charge density of the CO32− ions. These speculations would result in a high local supersaturation of CaCO3 around the N-surface during the CO2 diffusion into the CaCl2 solution, while a low supersaturation would be found around the P-surface. The degree of supersaturation near the H-surface without the surface electric potential may be somewhere between that of the P-surface and that of the N-surface. The rate of nucleation is directly proportional to the term of exp(−ΔG*) from the fluctuation theory, indicating that the decrease in ΔG* given by the previously described nucleation theory would lead to an increase in the rate of nucleation. This discussion explains why the density of calcite precipitates formed after 12 h increased in the order of N-, H-, and P-surfaces. Moreover, the surface electric field may also regulate the local concentration of PLys in the calcite substrate vicinity. With PLys, CaCO3 precipitates exhibited a change in their morphologies, depending on the amount of PLys and regardless of polarization treatment (Figure 2). With a small amount of PLys, regardless of polarization treatment, CaCO3 precipitates exhibited a morphology with notches in the edges of the rhombohedra and built aggregates with an island structure. With a large amount of PLys, the CaCO3 precipitates had overgrown crystals formed by epitaxial growth on each of the rhombohedral surfaces of the crystals formed on the calcite substrates, and the aggregates became elongated in the longitudinal direction.40−42 A detailed discussion of these phenomena is provided later in this section. The polymorphism and orientation of the as-precipitated CaCO3 crystals were performed by XRD analysis in the θ/2θ mode. The obtained XRD profiles indicated that the precipitates were calcite crystals with a (10.4) orientation (Figure 3), revealing that the calcite crystals formed through homoepitaxy. The peaks attributed to a gold coating were observed because the XRD analysis was performed after the SEM measurements. The (11.6) peak may reflect the

Figure 1. Scanning electron micrographs of calcite precipitates formed on the H-, P-, and N-surfaces of the oriented calcite substrates without PLys. (a) H-surface, (c) N-surface, and (e) P-surface. (b) and (d) are the high magnification images of (a) and (c), respectively. The scale bars are 100 μm for panels (a), (c), (e), and 50 μm for panels (b) and (d).

The calcite crystals were rhombohedra with the {10.4} plane parallel, as judged from the fact that a parallelogram of the calcite crystals had angles of 105°, corresponding to the shape of the (10.4) plane (Figure 1b,d). The results reveal that the calcite crystals were formed by epitaxy, as shown by their textual and azimuthal orientation. The areal density of the calcite crystals formed on the calcite substrates increased in the following order: N- > H- > P-surfaces. The individual rhombohedral calcite crystals developed into flat island-shaped aggregates by their coalescence. The calcite crystals formed on the nonpolarized calcite substrates were observed to have a rough surface structure and notches in the edges between the {10.4} planes. These morphologies were different from the morphologies of a typical pure rhombohedral calcite formed through layer-by-layer growth. On the other hand, the calcite crystals formed on the polarized calcite substrates demonstrated a porous surface structure, and two edges of each (10.4) C



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Figure 4. Fourier-transform infrared spectra of as-precipitated aggregates formed on the P-surface of the oriented calcite substrate in the presence of 3.0 mg PLys (solid blue line), of PLys only (solid green line), and of pure calcite only (solid red line).

Figure 2. Scanning electron micrographs of the surface morphologies of calcite precipitates formed on the H-, P-, and N-surfaces of the oriented calcite substrates with different amounts of PLys. H-surface (a, b, c), P-surface (d, e, f), N-surface (g, h, i). The scale bars are 10 μm.

The SEM image in Figure 5 shows that the structure of the overgrown calcite crystals had an oblique prismatic layer

Figure 3. X-ray diffraction patterns of as-precipitated aggregates formed on the P-surface of the oriented calcite substrate in the presence of 1.0 mg PLys (solid blue line) and of the oriented calcite substrate alone (solid red line).

appearance of new faces in the notches or overgrown layers. An FTIR analysis was performed to examine the existence of PLys in the precipitates and the polymorphism of the precipitates. KBr bulk crystals were soaked in a small quantity of PLys solution that was adjusted to a pH 8.2 with NaOH and were dried. The KBr bulk crystals with adsorbed PLys were ground. Hence, the KBr powder was used to prepare a KBr pellet for FTIR. FTIR analysis using KBr pellets was performed to obtain the FTIR spectrum of PLys, and revealed the amide I band region centered at ca. 1650 cm−1, as shown in Figure 4. The FTIR spectrum of the CaCO3 precipitates formed in the presence of PLys showed absorption bands at 713 and 876 cm−1, characteristic of the calcite phase (the solid red line in Figure 4) and an adsorption band at ca. 1652 cm−1 assigned to amide I of PLys (Figure 4). These results indicate that the precipitates consisted of calcite crystals with PLys and had the {10.4} plane parallel to the calcite substrate.

Figure 5. Scanning electron micrographs of the calcite precipitates with overgrowth layers formed on the oriented calcite substrates with PLys. (a) H-surface in the presence of 0.5 mg PLys. (b) N-surface in the presence of 2.0 mg PLys. The scale bars are 3 μm.

consisting of fine prisms and truncated corners. The truncated faces may be assigned to the (11.0) and (00.1) planes. It is conceivable that the fine calcite prisms in the oblique prismatic layer were stacked with a (10.4) plane parallel to each surface of the already formed rhombohedral calcite crystals. This morphology could be attributed to the fact that the prismatic layer consisted of interconnected micrometer-size crystallites, namely, mesocrystals, that were an inorganic−organic hybrid D



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material.43 The individual calcite prisms may have been bound to each other by PLys acting as a binder, which was supported by the results of the FTIR analysis. As the amount of PLys added was increased, the calcite crystals formed with and without polarization treatment became polycrystalline aggregates with complex morphologies, and these aggregates grew in a direction perpendicular to the substrate. In addition, the overgrown calcite crystals tapered toward the ends (Figure 2). This phenomenon suggests that the reduced expression of the {10.4} planes, reflecting the slowest growth direction, could be interpreted as selective PLys adsorption that predominated the expression of new faces. It can be speculated that the increase in the amount of PLys added led to its higher adsorption and resulted in the unique morphology of the calcite aggregates. Hence, to further study the influence of PLys added in the presence of an electric field, we performed another experiment with 3 mg of PLys that indicated that the fractional area of the (10.4) planes of the overgrown calcite crystals was decreased much more than with 2 mg of PLys (Figure 6) and that some of the overgrown calcite

Figure 7. Scanning electron micrographs of the calcite precipitates with unique morphologies formed on the oriented calcite substrates with polarization treatment in the presence of 3.0 mg of PLys. (a) The aggregate with a cone-shaped morphology, (b) the cone-shaped crystals assembled of oriented needle-like crystals, and (c) the growing state before reaching the cone-shaped morphology. The scale bars are 10 μm for panels (a), (c), and 5 μm for panel (b).

Table 1. Average Lengths of the Calcite Aggregates Formed with Differing Amounts of PLys average lengths and standard deviation values (μm) added amount of PLys (mg/ 10 mL) 0 0.5 1.0 2.0 3.0

H-surface 18.1 7.8 21.2 27.9

± ± ± ±

5.5 2.4 6.5 7.1

N-surface 24.1 9.5 27.5 32.9 57.7

± ± ± ± ±

7.7 2.7 5.3 3.8 10.8

P-surface 17.6 7.0 20.2 26.2 50.6

± ± ± ± ±

5.9 2.3 5.1 6.2 9.8

for the length and precipitated regions of the aggregates with PLys may be explained by the regulation of the CaCO3 supersaturation by the electric field, due to the polarized calcite substrate, as previously described, and the following speculation. In the closed crystallization system, at the onset of the experiment, the concentration of PLys in the solution was maximum, and the degree of CaCO3 supersaturation was zero. At the start of the experiment, the PLys alone adsorbed immediately onto the N-surface, in which the PLys adsorption onto the N-surface occurred mainly through an electrostatic interaction with the electric field, due to the polarized calcite substrates, and the PLys adsorption onto the P- and H-surfaces mainly occurred due to van-der-Waals forces. Hence, it is expected that, considering the dissociated state of the PLys, the fractional coverage of the PLys adsorbed onto each surface of the calcite substrates increased in the order, P-, H-, and Nsurfaces and also increased with the amount of PLys. The crystallization on the regions of the calcite substrate covered with PLys was interrupted by the inhibitory effect of additives, while crystallization occurred on the regions without PLys. On the other hand, it is expected that the PLys amounts remaining in solution decreased in the order, P-, H-, and N-surfaces. PLys, remaining as mobile cations in solution, controlled the calcite crystallization onto the calcite substrates through its adsorption. Possible Formation Mechanism of Calcite Precipitates with PLys. In the closed crystallization system, the evolution of the CaCO3 supersaturation during the crystallization is complex

Figure 6. Scanning electron (a, c) and optical (b, d) micrographs of the calcite precipitates formed on the oriented calcite substrates with polarization treatment in the presence of 3.0 mg of PLys. (a) and (b) P-surface, (c) and (d) N-surface. Note that the bright regions in optical micrographs correspond the (10.4) planes of calcite. The scale bars are 3 μm.

crystals exhibited a cone-shaped appearance (Figure 7a,b). The cone-shaped crystals consisted of crystalline assemblies of oriented needle-like crystals (Figure 7c). To obtain the average lengths of aggregates and standard deviation of the length distribution of calcite aggregate samples, we prepared eight calcite aggregate samples at each experimental arrangement, that is, n number = 8. The length was calculated from the difference between the focus positions of the top and bottom surfaces of the aggregates through polarized optical microscopy. The average lengths of the aggregates and the standard deviation are summarized in Table 1, showing that the length increased with increasing amounts of PLys, regardless of polarization treatment; moreover, the length of the aggregates increased in the following sequence: N- > H> P-surfaces. It was concluded that the degree of the region where the aggregates formed decreased as the amount of PLys increased and decreased in the order: N-, H-, and P-surfaces. Hence, we determined that the degree of the region where the aggregates formed reflected the degree of the coverage of the PLys adsorbed onto the calcite substrates. The results obtained E



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stage, the crystallization takes place on five {10.4} planes of the rhombohedral calcite crystals already formed on the calcite substrates, namely, the formation of the overgrown calcite crystals (Figure 8c). The oblique prismatic layer of the overgrown crystals consists of needle-like calcite microcrystals that are stacked with their (10.4) planes parallel to the surface of the already formed calcite crystals. The growth of the overgrown calcite crystals proceeds, and the crystals taper toward the ends because of the selective adsorption of PLys (Figure 8d,e). Various models for the proper interpretation of the inhibition effect of additives have been discussed.44,45 These models propose that the adsorption of additives inhibits the crystallization, and below a critical supersaturation that is dependent on the amount of the additives adsorbed on the crystal surfaces, no crystallization occurs. The crystallization temporarily stops because of a decrease in the supersaturation associated with the crystallization. When the supersaturation in the solution containing PLys becomes more than the critical value for the crystallization, the calcite crystallization starts again, and new calcite crystals form on the already formed calcite crystals (Figure 8f). Finally, alternately repeating the starting and stoppage of crystallization leads to the formation of calcite aggregates with a complex morphology.

and relates to the rate of the precipitation and the rate of CO2 diffusion from the (NH4)2CO3. The supersaturation decreases after the start of crystallization, and the crystallization temporarily stops; after that, the supersaturation increases due to the continued supply of CO32− reactants from the system, and the crystallization restarts. This repetition is the phenomenon of essence by crystallization. Additionally, the supersaturation decreases with the repetition because no additional Ca2+ ions are supplied. We consider that the PLys adsorbs onto the surfaces of the overgrowth calcite crystals by electrostatic interaction between the NH3+ groups and carbonate anions, by chelation of the Ca2+ ions to the neutral NH2 groups, or by van-der-Waals forces. We also propose a possible mechanism for the formation of calcite precipitates on the oriented calcite substrates in the presence of PLys, as shown in Figure 8.

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CONCLUSIONS A basic, positively charged poly amino acid, PLys, and the electric field due to polarized calcite substrates showed a significant effect on the morphologies of calcite precipitates. At a small amount of PLys, rhombohedral calcite crystals, with notches in the edges of the rhombohedra, formed through homoepitaxy, regardless of the polarization treatment, and built aggregates with an island structure. At a large amount of PLys, overgrown calcite crystals were formed by epitaxial growth on each of the rhombohedral surfaces of the already formed calcite rhombohedra, and the calcite precipitates became aggregates elongated in the direction perpendicular to the substrate. In addition, the overgrown calcite crystals tapered toward the ends because the reduced expression of the {10.4} planes was interpreted as selective PLys adsorption. In another experiment, with further added amounts of PLys in the presence of the electric field, it was indicated that the fractional area of the (10.4) planes of overgrown calcite crystals was decreased much more, and some of the overgrown calcite crystals exhibited a cone-shaped appearance. The cone-shaped crystals consisted of crystalline assemblies with oriented needle-like crystals. The length of the elongated aggregates increased with the addition of PLys, and the length increased in the following sequence: N> H- > P-surfaces. PLys with a random coil conformation inhibits calcite crystallization through its adsorption and does not act as a template for nucleation. The results probably contribute to a better understanding and promote the synthesis of organic and inorganic composites by biomimetics.

Figure 8. Possible growth sequence of the calcite precipitates on the oriented calcite substrates. At first, PLys with a random coil conformation in the CaCl2 solution, adsorbs onto the surface of the oriented calcite substrate before starting the calcite crystallization, and then the calcite crystals formed on the regions without PLys adsorption through homoepitaxy; (a) the case of a small additional amount of PLys, (b) the case of a large additional amount of PLys. (c) The formation of overgrown calcite crystals takes place on the {10.4} faces of the rhombohedral calcite crystals that already formed on the calcite substrates. This nucleation process is homoepitaxial. (d) The growth of the overgrown calcite crystals proceeds, and the crystals taper toward the ends because of selective adsorption of PLys. (e) and (f) Alternately repeating the starting and stoppage of crystallization occurs in the closed crystallization system. Finally, the repetition leads to the formation of calcite aggregates with a complex morphology. (g) The scanning electron micrograph shows the calcite aggregates formed through the sequential growth in the presence of 2 mg of PLys, corresponding to the stage (f). The scale bar is 10 μm for panel (g).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-03-5280-8015. Fax: +81-03-5280-8015.

At the initial stage, the PLys with the random coil conformation adsorbs onto the H-, N-, and P-surfaces of the calcite substrates. The PLys adsorbs in a random way on the calcite substrates, and the coverage of PLys adsorption increases with an amount of PLys added. After that, the rhombohedral calcite crystals precipitate through homoepitaxy on the regions without PLys adsorption (Figure 8a,b). At next

ORCID

Norio Wada: 0000-0003-3883-197X Notes

The authors declare no competing financial interest. F



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