Insights into the Confined Crystallization in Microfluidics of Amorphous

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

pubs.acs.org/crystal

Insights into the Confined Crystallization in Microfluidics of Amorphous Calcium Carbonate Youpeng Zeng,†,‡ Jianwei Cao,*,† Zhi Wang,*,† Jianwei Guo,† Qiqi Zhou,† and Jinshan Lu‡ †

National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, P. R. China

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S Supporting Information *

ABSTRACT: As a precursor phase, amorphous calcium carbonate (ACC) plays a key role in the formation of CaCO3 biominerals, but the detailed information about the structure evolution during the crystallization process of ACC is limited. Herein, based on the confined environment in microfluidics, we have demonstrated a strategy to investigate the crystallization processes of ACC. The characteristics of a confined environment in microfluidics were analyzed through COMSOL Multiphysics simulations. By mixing equimolar CaCl2 and Na2CO3 aqueous solutions directly, the reactive crystallization of CaCO3 was monitored on chip by online microscope observation and micro Raman spectroscopy scanning. Combined with offline scanning electron microscopy characterization, we showed that branched aggregate ACC(I) precipitated first once mixing reactant solutions, then the more ordered and whiskerlike ACC(II) was an unexpected result. These whiskerlike ACC(II) either gradually transformed to spherical structure nanocrystalline, vaterite was then formed from it through spherulitic growth mechanism, or the initial rhombohedral crystallographic calcite was formed from whiskerlike ACC(II), then the complete rhombohedral calcite crystals formed through the dissolution/recrystallization of ACC(II). Our results showed an intuition-based way to directly observe the structure evolution of ACC crystallization in the confined environment; it could be of inspiration for the understanding of biomineralization processes. ment under laboratory conditions18 because experiments under laboratory conditions are usually performed in the bulk system. Many crystallization phenomena cannot obtain an adequate description in accordance with the crystallization from the bulk system.18,19 Moreover, the nucleation events naturally occur within a time scale of seconds in bulk solution,20 so the detailed investigation of ACC crystallization is more difficult. Therefore, effects have been made to mimic the well-confined environment to investigate it by new methods.12,13,18 The emergence of microfluidics provides a confined environment, permitting one to insights into the crystallization mechanism in solution.21 Because the nucleation events are confined in a particular environment, which the space is scaled down to submillimeter dimensions, the crystallization in microfluidics are therefore slowed down to some degree, especially in the microdroplet.13,22 Li et al.’s research22 revealed that crystallization obviously becomes slower, and metastable phases (ACC) are longer lived in a droplet microfluidic, but the origins of that effect on the crystallization

1. INTRODUCTION CaCO3 is one of the most abundant inorganic minerals in nature. The remarkable structures and morphologies that CaCO3 biominerals exhibited have long attracted researchers’ widespread interest.1−3 In order to replicate or design advanced materials, which possess extraordinary properties closed to biominerals,4−7 many effects have been made to investigate the enigmatic biomineralization processes. Generally, preceding the formation of crystalline CaCO 3 biominerals, the amorphous precursor phase often appears. This is deemed to be a strategy in biomineralization.8 Therefore, the amorphous precursor phase is the key to reveal the fundamental mechanisms of biomineralization. Despite typically studies about the crystallization of amorphous calcium carbonate (ACC) in solution,9−11 our understanding about its crystallization mechanisms is remarkably limited. The environment, in which amorphous precursor occurs, turned out to be a critical factor for crystallization of amorphous precursor.12 Biomineralization provides us a typical example for the study of crystallization in restricted environment.13,14 Due to the confined environment, it plays an important role in the formation of well-recognized composite structures such as shell,15 teeth,16 and bone.17 However, it is often difficult to replicate and imitate that restricted environ© XXXX American Chemical Society

Received: May 2, 2018 Revised: September 11, 2018 Published: September 20, 2018 A

DOI: 10.1021/acs.cgd.8b00675 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

microscope (Imager.A1m), and micro Raman spectrometer (Renishaw inVia) was used to confirm CaCO3 crystal structure and polymorphism using a 532 nm laser beam as the excitation light source. The morphology of calcium carbonate crystals were imaged using off-line scanning electron microscopy (SEM) (JSM-7610F) with an acceleration voltage of 10 kV. 2.3. COMSOL Multiphysics Computational Details. COMSOL Multiphysics 5.3 was used to complete the fluid dynamics calculation and simulation. In details, the model was a space dimension of 2D axisymmetric, two physics interfaces, Creeping Flow (CF) and Transport of Diluted Species (TDS), selected for coupled computational fluid dynamics (CFD) simulation. CF physics interface was used to simulate fluid flows, especially the flow of fluid, which stayed at very low Reynolds numbers, and TDS physics interface was responsible for the computation of the concentration field of dilute solute. Stationary study was selected for preset studies at a selected physics interface. All calculations were performed on the microfluidic module of COMSOL Multiphysics 5.3.

of ACC are still not entirely clear. Still, microfluidics has became a new tool for the investigation of crystallization processes, making the direct observation and monitoring of ACC crystallization possible. As to the crystallization of ACC in solution, some details have been uncovered by the combination of some powerful instruments, such as cryo-transmission electron microscopy (TEM_,23−25 in situ small- and wide-angle X-ray scattering (SAXS/WAXS),26,27 time-resolved energy dispersive X-ray diffraction (ED-XRD),20,28 and so on. Previous researches have demonstrated that disordered hydrated ACC first undergoes the ordering stage to form a more ordered and anhydrous ACC through dehydration,20,26 then it crystallizes to form a nanocrystalline vaterite through spherulitic growth of vaterite,26,29 aggregation of nanosized primary particles,30,31 or solid-state transformation.32,33 We have to admit that our understanding on ACC crystallization has achieved little progress with the help of these powerful instruments. However, they mostly infer the evolution course of ACC by detecting the time-resolved diffraction patterns or molecular spectrometry of ACC, rather than through an intuition-based way to directly observe the crystallization process of ACC.30 Besides, a clear understanding and a detailed structure evolution information on the crystallization process of ACC is still limited and lacking. In our previous works,34 we have observed the dissolution/ recrystallization behavior of ACC, and much of our attention was paid to the crystallization pathways and crystallization behaviors of calcium carbonate in microfluidics. Herein, we present a simple microfluidics approach that enables us to focus our attentions on the investigation of crystallization mechanisms of ACC to vaterite and calcite. The characteristics of confined environment in microfluidics were analyzed through COMSOL Multiphysics simulations. Based on the confined environment that microfluidics provided, the crystallization process of ACC was monitored on-chip by online microscope observation and micro Raman spectroscopy scanning. Combined with offline scanning election microscopy (SEM) characterization, we captured a more detailed structure evolution information on ACC crystallization to vaterite and calcite. Our work provides direct experimental evidence about the structure evolution of calcium carbonate during its crystallization in an intuition-based way, which may exist in biomineralization processes.

3. RESULTS AND DISCUSSION 3.1. Characteristics Analysis of the Confined Environment in Microfluidics. When the dimensions of environment in which crystal occurs is scaled down to submillimeter, the physics of fluid will be changed dramatically,35 such as mixing, surface effects, and electrokinetic phenomena.36 The crystallization of CaCO3 in microfluidics begins with the mixing of two miscible fluids using two dimensionless numbers, Reynolds number (Re), which represents the ratio of inertial to viscous forces, and Peclet number (Pe), which expresses the relative importance of convection and diffusion, and can quantitatively describe the characteristics of mixing. They were defined by formulas 1 and 2,37 respectively Re =

ρvL η

(1)

Pe =

U0L D

(2)

where ρ is the density of the fluid, ν and U0 stands for the velocity and the average velocity of the fluid, η is viscosity, L represents characteristic length of the microchannel, and D stands for the diffusion coefficient of ions. Figure 1a gave the result of computational unit Reynolds number in microchannel. It exhibited that Re was far less than 1 when the flow rate was 0.1 μL/min, indicating the fluid was creeping flow (when Re ≪ 1). In such a low-Re and laminar flow, viscosity overwhelms inertial force; therefore, viscosity was mainly responsible for the motion of liquid in microfluidics.36 Resulting in the mixing between streams of two miscible liquids driven by diffusion. Another dimensionless number, Peclet number, can also be used to describe the mixing of fluids. When the flow rate was 0.1 μL/min, the Peclet number of Ca2+ and CO32− in microchannel was shown in Figure 1b. It indicated that the fluid carried with Ca2+ will achieve homogenization between streams of two miscible liquids in more short time and diffusion distance. Because the channel widths Z, the distance when fluids achieved complete mixing, can be approximately estimated by Pe as the formula 337 shown, the diffusion time t can be described by the formula 4

2. EXPERIMENTAL SECTION 2.1. Materials and Microfluidic Device. Calcium chloride anhydrous and sodium carbonate anhydrous were purchased from Guangzhou Xilong Chemicals Incorporated Company and were of analytical grade. All reagents were used as received. Syringe (5 mL) and disposable needle filter with 0.45 μm membrane filters were obtained from Beijing Kepujia Laboratory Instrument Company. The microfluidic device used in this work was described in our previously published paper.34 In summary, the channel structure of microfluidic chips is a simple Y-shape, and the microchannels with typical dimension of 200 μm wide and 100 μm high are fixed to 4.5 cm in length. For more details about the microfluidic chips, refer to ref 34. 2.2. Characterization of Crystallization Processes of Amorphous Calcium Carbonate. CaCl2 and Na2CO3 aqueous solutions were filtered by using a disposable needle filter with 0.45 μm membrane filters and delivered using syringe pumps (LSP01−1A). The two equimolar aqueous solutions were injected into microchannel, respectively, through the two inlets of the chip with the same flow rate, then CaCO3 precipitates were produced after mixing. The crystallization behavior of ACC was in situ recorded by Zeiss optical

UL Z ≈ 0 ≡ Pe L D B

(3) DOI: 10.1021/acs.cgd.8b00675 Cryst. Growth Des. XXXX, XXX, XXX−XXX


Article

S=

L2 D

3

K sp(CaCO3)

(5)

where Ksp(CaCO3) stands for the solubility constant of CaCO3, γCa2+ is the activity of Ca2+, and γCO32+ is the activity of CO32−; to such a situation, S is far higher on the one side (Figure 2b, the side of CO32− stream), especially the position close to the channel outlet, where the values of the largest supersaturation ratio (S) are clearly shifted to the left of channel center. Therefore, CaCO3 crystals formed predominately on the left side of the microchannel (Figure 2a), while this crystallization phenomenon was not obvious when the flow rate increased because the mixing from one stream to the other through diffusion will not be perfect for both Ca2+ and CO32− at the higher flow rate (Figure 2c). The difference in terms of supersaturation ratio (S) between the right and left sides of channel was decreased (Figure 2d). Comsol Multiphysics is a powerful analytical tool to comprehensively analyze the characteristics of flow field in this confined environment, including the distribution of velocity field, pressure field, and concentration field. By applying Comsol Multiphysics software, we can obtain well understanding about the novel crystallization behavior of CaCO3 in microfluidics. The analysis results were shown in Supporting Information (section 1.2). In summary, the mixing of fluids driven by diffusion alone would enhance the time for ions or molecules to mix well. Without the disturbance of turbulent mixing, the physical gradients thus become more significant in this confined environment (Figures 3 and S4), while this mixing characteristic is desirable for the investigation of ACC crystallization. Because the nucleation events will thus be slowed down, then more detailed crystallization information would be captured and observed. Over bulk systems, this is one significant advantage offered by microfluidic crystallization in the investigation of crystallization processes. 3.2. Effects of Confined Environment on Crystallization of ACC. 3.2.1. Capture of Different Amorphous Phases. ACC would be precipitated instantaneously in bulk solution once higher supersaturated aqueous solution of Ca2+ and CO32− were rapidly mixed, then it tends to transform rapidly into one of the crystalline CaCO3 phases in bulk solution (within seconds to minutes).20 Similarly, the branched aggregate ACC could be observed under the optical microscope when directly mixing equimolar CaCl2 and Na2CO3 aqueous solutions in microfluidics (Figure 4). However, in contrast, the transformation process of ACC to crystalline CaCO3 phase became slower in confined environment (Figure S5). In other words, confined microenvironment increased the lifetimes of ACC, making ACC seem to become more stable. When these initial branched ACC were formed in microchannel (Figure 5a), we observed the transformation process of ACC under the optical microscope in real time. The crystalline CaCO3 was gradually formed (Figure 5b), and they grew in number and size with reaction time at the expense of the initial amorphous calcium carbonate (Figure 5c). When the branched ACC almost could not be observed under the optical microscope (Figure 5d), we took the CaCO3 products for further off-line analysis by SEM imaging. As shown in Figure 6 (Figure S6), the whiskerlike CaCO3 precipitates around rhombohedral (calcite) and spherical (vaterite) CaCO3 particles were discovered in our research

Figure 1. Computational unit Reynolds number (a) and Peclet number (b) profiles of Ca2+ and CO32− in microchannel with the distance away from the Y-junction when the flow rate was 0.1 μL/ min.

t≈

γCa 2+·γCO 2−

(4)

Within the time t, when the fluids achieved complete mixing, it has moved a distance Z ≈ U0L2/D along the channel. For example, when the fluids mixed in a 200 μm microchannel at the flow rate of 1.0 mm/s, the Pe is about 25 for the fluid carried with Ca2+, requiring approximately 5.0 mm and 5.1 s to completely mix. However, for the fluid carried with CO32−, the Pe is about 217, requiring approximately 43.4 mm and 43.3 s to completely mix. Therefore, based on the differences in diffusion coefficients (D) between Ca2+ and CO32− (that the diffusion coefficient of Ca2+ is one magnitude greater than CO32−),38 the mixing between two miscible liquids can be controlled by the adjustment of flow rate (U0). Figure 2 showed the result of CaCO3 crystals formation in microchannel by the adjustment of flow rate (U0). Because of the larger diffusion coefficient for Ca2+, when mixing between streams of two miscible liquids, the amount of species (Ca2+) diffused into the CO32− stream is more than the amount of species (CO32−) diffused into the Ca2+ stream at lower flow rate (1.0 μL/min), making the supersaturation ratio (S), which is defined by eq 539 C



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Figure 2. CaCO3 crystals precipitated on-chip and corresponding supersaturation distribution profiles at different distances of channel in the direction of flow. The flow rates were (a,b) 1 μL/min and (c,d) 5 μL/min.

morph of products and even the original position where crystals formed in microchannel were unchanged. We found that the distribution characteristics of ACC(II) at different positions of microchannel, that is 1, 2, and 3 marked in Figure 8a, were obviously different. Just as demonstrated in Figure 8, whiskerlike ACC(II) can be observed at the position 2 of the microchannel (Figure 8d), and most of the CaCO3 particles are spherical vaterite and whiskerlike ACC(II), especially at position 1 of the microchannel, where it is close to the Y-junction. Vaterite and calcite crystals were almost surrounded by the whiskerlike ACC(II) (Figure 8(c)), but a small amount of whiskerlike ACC(II) could be observed at position 3 of the microchannel (Figure 8e), where it is close to the channel outlet. These distribution characteristics of CaCO3 products actually demonstrated the diffusion-controlled crystallization process of CaCO3.34 To our best knowledge, the stability of anhydrous crystalline CaCO3 decreased as calcite, aragonite, and vaterite in sequence,41,42 and the metastable phases will naturally transform into the more thermodynamically stable phases. However, when this transformation process took place in confined environment, as laminar microfluidics, it would become a diffusion-controlled process, then the degree of transformation is particularly different at different spaces of the channel even if at the same time somewhere has not started the crystallization process of ACC but somewhere else the crystallization process of ACC is almost completed. According

(the polymorph of spherical and rhombohedral CaCO3 particles was vaterite and calcite, respectively, which was confirmed by Raman spectra in Figure S7). These whiskerlike CaCO3 were amorphous phase, which was confirmed by Raman spectra in Figure 7, rather than a crystalline CaCO3.40 However, different from the initial branched ACC, the characteristic peak (around the 1086 cm−1) of these whiskerlike ACC was not a broad peak as the branched ACC shown in Raman spectra (Figure 7), but a sharp peak, as it indicated that these whiskerlike ACC were more ordered than the initial branched ACC.11 For the convenience of discussion, we defined the initial branched ACC as ACC(I) and the whiskerlike ACC as ACC(II). 3.2.2. Distribution Characteristics of ACC in Confined Environment. After discovering the whiskerlike ACC(II), we have carefully analyzed their distribution characteristics in the confined environment. In order to retain the important information about the crystallization process of ACC, we used a reversibly sealed chip, which enabled the separation of coverslip (PDMS) from substrate (glass) and avoided the retrieval of crystals from chip, and then we could directly put the chip on the sample stage of SEM. Therefore, once the experiment finished, we first washed the CaCO3 products by introducing ethanol to the microchannel and then dried products on-chip for subsequent off-line SEM imaging. This treatment process ensured that the morphology and polyD



Article

Figure 4. Initial branched ACC formed at the center of the Y-junction when directly mixing equimolar CaCl2 and Na2CO3 aqueous solutions (20 mM) in microfluidics.

Therefore, the confined environment makes it possible to gain more insight into the crystallization process. Furthermore, the flow of fluids in microfluidics is just like the continuous plug-flow, without the presence of backmixing,43 and fluids entering the microchannel have the same residence time at the same distances away from the junction in the direction of flow, without the disturbance of turbulent mixing and high shear rates caused by stirring.44 When the nucleation events of CaCO3 took place at low supersaturations in laminar microfluidics, the nucleation events occurring upstream have little disturbances compared to those occurring downstream. Due to the differences of supersaturation level from the center to the two sides of channel, resulting in the different crystallization kinetics of ACC, different stages during the reactive crystallization of ACC into vaterite and calcite could take place at the same time; that is, the transformation of ACC to crystalline CaCO3 might not have started in some positions, while the crystallization of ACC might be finished somewhere else in channel. What’s more, this important information could be retained in a reversibly sealed microfluidic chip through washing and then immediately drying the CaCO3 products after the experiments, as illustrated in Figure 9. Based on this information captured by off-line SEM imaging and the previous reports on the crystallization of ACC,11,20,26 we proposed the following multistage crystallization pathway of ACC to crystalline CaCO3 in the confined environment, as shown in Figure 10. First, when the meeting of two miscible fluids was carried with Ca2+ and CO32−, respectively, the initial branched aggregate ACC(I) precipitated once the localized supersaturation of solution reached the critical supersaturation level of ACC. The initial formed ACC(I) was normally hydrated and disordered,11 then more ordered and whiskerlike ACC(II) appeared through an orderly internal structural adjustment of ACC(I), such as dehydration and aggregation.20 The whiskerlike ACC(II) would either gradually transform to spherical nanocrystalline, then vaterite formed through spherulitic growth of nanocrystalline; thus, vaterite inlaid with whiskers could be observed, or the initial rhombohedral

Figure 3. (a) Concentration evolution profiles of Ca2+ (solid) and CO32− (dash) at different distances of channel in the direction of flow. (b) Concentration gradient profiles at different distances of channel in the direction of flow. The flow rate was 1.0 μL/min, and the initial concentration of solution was 20 mM.

to the characteristics analysis of confined environment in microfluidics, laminar flow and low-Re made the reactive crystallization of calcium carbonate controlled by diffusion, leading to most of the initial formed ACC(I), especially near the channel inlet, have not enough time to complete the crystallization process, while most of them, especially near the channel outlet, have transformed completely to crystalline CaCO3, resulting in calcite crystals that account for the majority of products. 3.3. Crystallization Mechanisms of ACC to Vaterite and Calcite in the Confined Environment. The mixing between two miscible liquids in the low-Re world was almost driven by diffusion alone, which might be unacceptable for chemical reactions because it would take a few minutes or more for complete mixing.37 The mixing of two aqueous solutions in laminar microfluidics at low flow rate is the case. However, it is an ideal platform into the crystallization of ACC in solution because the crystallization process of ACC to vaterite and calcite in that confined environment will thus be slowed down, then more detailed crystallization information would be captured and observed (Figure 9, Figure S6). E



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Figure 5. Micrographs about the crystallization process of ACC to crystalline CaCO3 under optical microscope: (a) the initial branched ACC formed in microchannel, (b) the crystalline CaCO3 was gradually formed and (c) grew in number and size with further transformation of the initial ACC, and (d) the initial ACC almost could not be observed under the optical microscope.

Figure 6. SEM image of CaCO3 particles formed on-chip; whiskerlike CaCO3 precipitates can be observed around calcite and vaterite particles.

crystallographic calcite was formed from these whiskerlike ACC(II), then the complete rhombohedral calcite crystals were formed through the dissolution/recrystallization of ACC(II). Due to this transformation process, it became slower in confined environment; thus, numerous intergrowths of calcite attached with whiskers on their surfaces could be observed (Figure 9, Figure S8). Overall, the mixing characteristics in this confined environment offered an opportunity to shine a new light on the crystallization mechanisms of ACC. Finally, in regards to in what conditions ACC would transform directly into vaterite or calcite, Rodriguez-Blanco et al.’s research45 suggested that the crystallization pathways of ACC were pH dependent. When the starting pH was a neutral value (∼7) during mixing, the initial ACC tended to transform directly to calcite by the way of dissolution/reprecipitation, but if mixing started from an alkaline pH (∼11.5), the initial ACC would transform to metastable vaterite first. Then metastable vaterite transformed to calcite through dissolution−recrystallization. In our experiments, equimolar aqueous solutions of 20 mM Na2CO3 (pH ≈ 11.5) and CaCl2 (pH ≈ 6.7) were delivered to the Y-shaped microchannel under the same flow rate (1.0 μL/min). The two fluids (Na2CO3 and CaCl2) operated at a low Re and demonstrated laminar flow in the microchannel. Ca2+ and CO32− diffused into the other side of channel, leading to the crystalline CaCO3 forming in alkaline pH environment on the side of Na2CO3 stream but in neutral

Figure 7. Raman spectra of (a) the initial branched ACC(I) and (b) the whiskerlike ACC(II).

pH environment on the side of CaCl2 stream; thus, the direct crystallization of ACC to calcite or metastable vaterite could occur and then been captured in this confined environment. Microfluidics offers a new tool for one to gain insight into the crystallization of ACC. Besides, a droplet-based confinement system has been developed in recent years and attracted F



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Figure 9. SEM image of CaCO3 formed on-chip; products at the different crystallization stages of ACC to vaterite and calcite can be captured and observed.

Stephens et al.46 studied the effects of confinement on the crystallization of calcium carbonate within picoliter droplets, and they observed early stages in the crystallization process of ACC. Compared with those reports, what they have in common is that the crystallization of CaCO3 was confined in a particular space, resulting in the slower crystallization rate; thus, some early stages preceding CaCO3 crystallization could be observed, including some intermediate phases. However, in contrast with droplet-based confinement, the reaction crystallization of CaCO3 is not intermittent in continuous flow microfluidics, and due to the residence time distributions, the upstream products have different residence time with downstream products in microchannel; thus, intermediates that stayed at different crystallization stages could be captured during the crystallization process of CaCO3 in continuous flow microfluidics, including amorphous phase and metastable crystalline phases. While these characteristics are not available in microdroplets, because the reaction crystallization of CaCO3 would be terminated once the reactants in droplet were consumed, normally reagents will be rapidly consumed due to the rapid mixing of fluids in the droplet. What’s more, intermediate growth forms of CaCO3 could be effectively frozen in microchannels, and we need not remove products from the reversibly sealed chip in our research. This helps to retain the original morphologies of products in situ, and thus, we can reveal the structure evolution of ACC in the transformation process of amorphous phases to crystalline phases in an intuition-based way.

4. CONCLUSIONS In this work, based on the confined environment in microfluidics, we have shown an intuition-based way to study the crystallization mechanisms of ACC. Unlike the bulk systems, the mixing of fluids was derived by diffusion alone in microfluidics, leading to the diffusion-controlled crystallization process of ACC, and the crystallization process of ACC in that confined environment would thus be slowed down; thus, the confined environment in microfluidics made it possible to gain insight into the crystallization process of ACC. The finding of more ordered and whiskerlike ACC was an unexpected result. The amount of this whiskerlike ACC decreased from the near-inlet to the outlet of channel, which actually demonstrated the diffusion-controlled crystallization process of ACC in this confined environment. Using online microscope observation and micro Raman spectroscopy

Figure 8. (a) Geometric channel structure; (b) global distribution of CaCO3 particles formed in microchannel at low magnification times of SEM; (c−e) representative SEM image of CaCO3 particles formed at the 1, 2, and 3 positions marked in picture (a) in the microchannel. Arrow in picture (b) represents the direction of flow.

researchers’ increased interest. For example, Isaac et al.13 stabilized transient ikaite in nanoliter and picoliter droplets. Controllable crystallization of calcium carbonate was achieved by Li et al.22 with the aid of a droplet microfluidic device. G



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Figure 10. Schematic representation of the proposed multistage crystallization pathway of ACC to crystalline CaCO3.

51374192) and the free exploration project of Key Laboratory of Green Process and Engineering, Chinese Academy of Sciences (LGPE2016FE01).

characterization, combined with offline SEM imaging, our results revealed that the initial branched aggregate ACC(I) precipitated first after mixing reactant solutions, and then more ordered and whiskerlike ACC(II) occurred through an orderly internal structural adjustment of the initial ACC(I). These whiskerlike ACC(II) either gradually transformed to spherical structure nanocrystalline, then vaterite formed through spherulitic growth of nanocrystalline, or the initial rhombohedral calcite was formed from these whiskerlike ACC(II), then the complete rhombohedral calcite crystals formed through the dissolution/recrystallization of whiskerlike ACC(II). Our work showed the structural change of ACC crystallization in an intuition-based way, which may exist in biomineralization processes.

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(1) Gower, L. B. Biomimetic model systems for investigating the amorphous precursor pathway and its role in biomineralization. Chem. Rev. 2008, 108, 4551−4627. (2) Berman, A.; Hanson, J.; Leiserowitz, L.; et al. Biological control of crystal texture: a widespread strategy for adapting crystal properties to function. Science 1993, 259, 776−779. (3) Cartwright, J. H.; Checa, A. G.; Gale, J. D.; et al. Calcium carbonate polyamorphism and its role in biomineralization: how many amorphous calcium carbonates are there? Angew. Chem., Int. Ed. 2012, 51, 11960−11970. (4) 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. (5) Aizenberg, J.; Livage, J.; Mann, S. New developments in biorelated materials. J. Mater. Chem. 2004, 14, E5−E6. (6) Dujardin, E.; Mann, S. Bio-inspired materials chemistry. Adv. Eng. Mater. 2002, 4, 461−474. (7) Xu, A. W.; Ma, Y.; Cölfen, H. Biomimetic mineralization. J. Mater. Chem. 2007, 17, 415−449. (8) Weiner, S.; Sagi, I.; Addadi, L. Choosing the crystallization path less traveled. Science 2005, 309, 1027−1028. (9) Politi, Y.; Metzler, R. A.; Abrecht, M.; et al. Transformation mechanism of amorphous calcium carbonate into calcite in the sea urchin larval spicule. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 17362− 17366. (10) Koga, N.; Nakagoe, Y.; Tanaka, H. Crystallization of amorphous calcium carbonate. Thermochim. Acta 1998, 318, 239− 244. (11) Radha, A. V.; Forbes, T. Z.; Killian, C. E.; et al. Transformation and crystallization energetics of synthetic and biogenic amorphous calcium carbonate. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 16438− 16443. (12) Wang, Y. W.; Christenson, H. K.; Meldrum, F. C. Confinement increases the lifetimes of hydroxyapatite precursors. Chem. Mater. 2014, 26, 5830−5838. (13) Rodríguez-Ruiz, I.; Veesler, S.; Gómez-Morales, J.; et al. Transient calcium carbonate hexahydrate (Ikaite) nucleated and stabilized in confined nano- and picovolumes. Cryst. Growth Des. 2014, 14, 792−802.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.8b00675. Additional figures depict the simulated results of the distribution of velocity field, pressure, and concentration field in microchannel; crystallization phenomena of CaCO3 in microfluidics, additional off-chip SEM images, and Raman spectra of CaCO3 during the crystallization process of ACC (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-010-82544818. Fax: +86-010-82544818. *E-mail: [email protected]. ORCID

Youpeng Zeng: 0000-0002-9314-0808 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (51404225, H



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(14) Meldrum, F. C.; Cölfen, H. Controlling mineral morphologies and structures in biological and synthetic systems. Chem. Rev. 2008, 108, 4332−4432. (15) Falini, G.; Albeck, S.; Weiner, S.; et al. Control of aragonite or calcite polymorphism by mollusk shell macromolecules. Science 1996, 271, 67−69. (16) Ma, Y. R.; Weiner, S.; Addadi, L. Mineral deposition and crystal growth in the continuously forming teeth of sea urchins. Adv. Funct. Mater. 2010, 17, 2693−2700. (17) Mahamid, J.; Sharir, A.; Addadi, L.; et al. Amorphous calcium phosphate is a major component of the forming fin bones of zebrafish: Indications for an amorphous precursor phase. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 12748−12753. (18) Stephens, C. J.; Ladden, S. F.; Meldrum, F. C.; et al. Amorphous calcium carbonate is stabilized in confinement. Adv. Funct. Mater. 2010, 20, 2108−2115. (19) Olszta, M. J.; Cheng, X.; Sang, S. J.; et al. Bone structure and formation: A new perspective. Mater. Sci. Eng., R 2008, 58, 77−116. (20) Rodriguezblanco, J. D.; Shaw, S.; Benning, L. G. The kinetics and mechanisms of amorphous calcium carbonate (ACC) crystallization to calcite, viavaterite. Nanoscale 2011, 3, 265−271. (21) Leng, J.; Salmon, J. B. Microfluidic crystallization. Lab Chip 2009, 9 (1), 24−34. (22) Li, S.; Zeng, M.; Gaule, T.; et al. Passive picoinjection enables controlled crystallization in a droplet microfluidic device. Small 2017, 13, 1702154−1702163. (23) Pouget, E. M. The initial stages of template-controlled CaCO3 formation revealed by cryo-TEM. Science 2009, 323, 1455−1458. (24) Nielsen, M. H.; Aloni, S.; De Yoreo, J. J. In situ TEM imaging of CaCO3 nucleation reveals coexistence of direct and indirect pathways. Science 2014, 345, 1158−1162. (25) Tester, C. C.; Wu, C. H.; Weigand, S.; et al. Precipitation of ACC in liposomes-a model for biomineralization in confined volumes. Faraday Discuss. 2012, 159, 345−356. (26) Bots, P.; Benning, L. G.; Rodriguezblanco, J. D.; et al. Mechanistic insights into the crystallization of amorphous calcium carbonate (ACC). Cryst. Growth Des. 2012, 12, 3806−3814. (27) Pontoni, D.; Bolze, J.; Dingenouts, N.; et al. Crystallization of calcium carbonate observed in-situ by combined small- and wideangle X-ray scattering. J. Phys. Chem. B 2003, 107, 5123−5125. (28) Kellermeier, M.; Glaab, F.; Klein, R.; et al. The effect of silica on polymorphic precipitation of calcium carbonate: an on-line energydispersive X-ray diffraction (EDXRD) study. Nanoscale 2013, 5, 7054−7065. (29) Andreassen, J. P. Formation mechanism and morphology in precipitation of vaterite-nano-aggregation or crystal growth? J. Cryst. Growth 2005, 274, 256−264. (30) Shen, Q.; Wei, H.; Zhou, Y.; et al. Properties of amorphous calcium carbonate and the template action of vaterite spheres. J. Phys. Chem. B 2006, 110, 2994−3000. (31) Vacassy, R.; Lemaître, J.; Hofmann, H.; et al. Calcium carbonate precipitation using new segmented flow tubular reactor. AIChE J. 2000, 46, 1241−1252. (32) Pouget, E. M.; Bomans, P. H. H.; Dey, A.; et al. The development of morphology and structure in hexagonal vaterite. J. Am. Chem. Soc. 2010, 132, 11560−11565. (33) Xu, X.; Han, J. T.; Kim, D. H.; et al. Two modes of transformation of amorphous calcium carbonate films in air. J. Phys. Chem. B 2006, 110, 2764−2770. (34) Zeng, Y.; Cao, J.; Wang, Z.; et al. Formation of amorphous calcium carbonate and its transformation mechanism to crystalline CaCO3 in laminar microf luidics. Cryst. Growth Des. 2018, 18, 1710− 1721. (35) Tabeling, P.. Introduction to Microfluidics, translated by Suelin and Chen; Oxford University Press, 2005; Chapter 1; pp 24−69. (36) Novotný, J.; Foret, F. Fluid manipulation on the micro-scale: Basics of fluid behavior in microfluidics. J. Sep. Sci. 2017, 40, 383− 394.

(37) Squires, T. Microfluidics: Fluid physics at the nanoliter scale. Rev. Mod. Phys. 2005, 77, 977−1026. (38) Handbook of Chemistry and Physics, 95th ed.; Lide, D. R., Ed.; CRC Press, 1988; Chapter 5, pp 955−957. (39) Verdoes, D.; Kashchiev, D.; van Rosmalen, G. M. Determination of nucleation and growth rates from induction times in seeded and unseeded precipitation of calcium carbonate. J. Cryst. Growth 1992, 118, 401−413. (40) Huang, J.; Liu, C.; Xie, L.; et al. Amorphous calcium carbonate: A precursor phase for aragonite in shell disease of the pearl oyster. Biochem. Biophys. Res. Commun. 2018, 497, 102−107. (41) Hassani, L. N.; Hindré, F.; Beuvier, T.; et al. Lysozyme encapsulation into nanostructured CaCO3 microparticles using a supercritical CO2 process and comparison with the normal route. J. Mater. Chem. B 2013, 1, 4011−4019. (42) Ahmed, J.; Menaka; Ganguli, A. K. Controlled growth of nanocrystalline rods, hexagonal plates and spherical particles of the vaterite form of calcium carbonate. CrystEngComm 2009, 11, 927− 932. (43) Jiang, M.; Zhu, Z.; Jimenez, E.; et al. Continuous-flow tubular crystallization in slugs spontaneously induced by hydrodynamics. Cryst. Growth Des. 2012, 14, 851−860. (44) Besenhard, M. O.; Neugebauer, P.; Cheng, D. H.; et al. Crystal size control in a continuous tubular crystallizer. Cryst. Growth Des. 2015, 15, 1683−1691. (45) Rodriguez-Blanco, J. D.; Shaw, S.; Bots, P.; et al. The role of pH and Mg on the stability and crystallization of amorphous calcium carbonate. J. Alloys Compd. 2012, 536, S477−S479. (46) Stephens, C. J.; Kim, Y. Y.; Evans, S. D.; et al. Early Stages of Crystallization of Calcium Carbonate Revealed in Picoliter Droplets. J. Am. Chem. Soc. 2011, 133, 5210−5213.

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Insights into the Confined Crystallization in Microfluidics of Amorphous

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