Double Hydrophilic Block Copolymer Controlled Growth and Self

In contrast, higher density of PEG blocks will stabilize the growing crystals more efficiently and inhibit subnucleation on the polymer−CaCO3 interf...
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Langmuir 2006, 22, 6125-6129

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Double Hydrophilic Block Copolymer Controlled Growth and Self-Assembly of CaCO3 Multilayered Structures at the Air/Water Interface Yun-Xiang Gao, Shu-Hong Yu,* and Xiao-Hui Guo DiVision of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, The School of Chemistry & Materials, UniVersity of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ReceiVed January 2, 2006. In Final Form: May 2, 2006 Double hydrophilic block copolymers PEG-b-PEI-linear with different PEI block lengths have been examined for CaCO3 mineralization at the air/water interface. The results demonstrated that either PEI length or the solution acidity had a significant influence on the morphogenesis of vaterite crystals at the air/water interface. A possible mechanism for the stratification of CaCO3 vaterite crystals has been proposed. Increasing either PEI length or the initial pH value of the solution will decrease the density of the PEG block anchored on the binding interface and result in exposing more space as binding interface to solution and favoring the subnucleation and stratification growth on the polymerCaCO3 interface. In contrast, higher density of PEG blocks will stabilize the growing crystals more efficiently and inhibit subnucleation on the polymer-CaCO3 interface, and thus prevent the formation of stratified structures. This study provides an example that it is possible to access morphogenesis of calcium carbonate structures by a combination of a block copolymer with the air/water interface.

Introduction In nature, biomineralization applies natural proteins1 to control and direct the crystallization of biominerals such as calcium carbonate with special orientation, texture, and morphology at ambient conditions.2 Inspired by the formation of biominerals, a variety of mimetic methodologies have been designed and carried out to investigate the possible mechanism of the formation of natural biominerals.3 Also, following the idea of proteindirected biomineralization, many other soluble organic templates such as amino acids,4 polycarboxylic acids,5 synthetic peptides,6 and dendrimers7 are applied to control the mineralization process in vivo. In recent years, based on the fact that the biomineralized materials often contain proteins that are rich in glutamic or aspartic acid residues, double hydrophilic block copolymers (DHBCs) containing anionic groups have been successfully developed to control the mineralization process of calcium carbonate in solution and have helped to produce a variety of materials with novel microstructures.8-12 * Corresponding author. Fax: +86 551 3603040. E-mail: shyu@ ustc.edu.cn. (1) Co¨lfen, H. Curr. Opin. Colloid Interface Sci. 2003, 8, 23. (b) Falini, G.; Albeck, S.; Weiner, S.; Addadi, L. Science 1996, 271, 67. (c) Naka, K.; Chujo, Y. Chem. Mater. 2001, 13, 3245. (d) Addadi, L.; Weiner, S. Proc. Natl. Acad. Sci. U.S.A. 1985, 82, 4110. (2) Belcher, A. M.; Wu, X. H.; Christensen, R. J.; Hansma, P. K.; Stucky, G. D.; Morse, D. E. Nature 1996, 381, 56. (b) Mann, S. J. Chem. Soc., Dalton Trans. 1997, 3953. (c) Choi, C. S.; Kim, Y. W. Biomaterials 2000, 21, 213. (3) Co¨lfen, H.; Mann, S. Angew. Chem., Int. Ed. 2003, 42, 2350. (b) Kato, T.; Sugawara, A.; Hosoda, N. AdV. Mater. 2002, 14, 869. (c) Estroff, L. A.; Hamilton, A. D. Chem. Mater. 2001, 13, 3227. (d) Yu, S. H.; Co¨lfen, H.; Tauer, K.; Antonietti, M. Nat. Mater. 2005, 5, 51. (4) Kai, A.; Fujikawa, K.; Miki, T. Jpn. J. Appl. Phys., Part 1 2002, 41, 439. (b) Vdovi, N.; Kralj, D. Colloids Surf., A 2000, 161, 499. (5) Marentette, J. M.; Norwig, J.; Stockelmann, E.; Meyer, W. H.; Wegner, G. AdV. Mater. 1997, 9, 647. (b) Kato, T.; Amamiya, T. Chem. Lett. 1999, 199. (c) Dalas, E.; Klepetsanis, P.; Koutsoukos, P. G. Langmuir 1999, 15, 8322. (d) Sedlak, M.; Antonietti, N.; Co¨lfen, H. Macromol. Chem. Phys. 1998, 199, 247. (6) DeOliveira, D. B.; Lauren, R. A. J. Am. Chem. Soc. 1997, 119, 10627. Zhang, Z. P.; Gao, D. M.; Zhao, H.; Xie, C. G.; Guan, G. J.; Wang, D. P.; Yu, S. H. J. Phys. Chem. B 2006, 110, 8613. (7) Naka, K.; Tanaka, Y.; Chujo, Y.; Ito, Y. Chem. Commun. 1999, 1931. (b) See a review and references therein: Naka, K. Top. Curr. Chem. 2003, 228, 141.

Interestingly, cationic proteins, although very few, have also been found as the structure-control molecules in biomineralization in the diatom cell wall.13 They contain abundant long polyamines and can induce rapid precipitation of silica spheres with characteristic diameters in vitro.14 Recently, it has been proved that the basic polymers, which will be positively charged under acidic conditions, are also useful templates for the growth of calcium carbonate.15 Hemispherical vaterite and needlelike aragonite can be selectively synthesized at the air/water interface by the mediation of poly(ethylene imines) dissolved in supersaturated calcium bicarbonate solution with different molecular weights of PEI blocks,15 suggesting that cationic polychains are also versatile templates for artificial material synthesis. In this paper, we present the influence of poly(ethylene glycol)block-poly(ethylene imines) (PEG-b-PEI-linear), which is a family of cationic double hydrophilic block copolymers, on the crystallization of calcium carbonate at the air/water interface. The results demonstrated that the crystal morphology of calcium carbonate with layered structures formed at the air/water surface could be well controlled with different PEI block lengths and pH values of the initial solution. Experimental Section Materials. PEG-b-PEI-linear (PEG, Mn ) 5000; PEI, Mn ca. ) 1200) and PEG-b-PEI-linear (PEG, Mn ca. ) 5000; PEI, Mn ca. ) 400) were synthesized as reported previously.16 All other chemicals (8) Co¨lfen, H. Macromol. Rapid Commun. 2001, 22, 219. (b) Co¨lfen, H.; Antonietti, M. Langmuir 1998, 14, 582. (9) Yu, S. H.; Co¨lfen, H. J. Mater. Chem. 2004, 14, 2123. (b) Co¨lfen, H.; Yu, S. H. MRS Bull. 2005, 30, 727. (c) Yu, S. H.; Chen, S. F. Curr. Nanosci. 2006, 2, 81. (10) Yu, S. H.; Co¨lfen, H.; Antonietti, M. J. Phys. Chem. B 2003, 107, 7396. (11) Chen, S. F.; Yu, S. H.; Wang, T. X.; Jiang, J.; Co¨lfen, H.; Hu, B.; Yu, B. AdV. Mater. 2005, 17, 1461. (12) Gao, Y. X.; Yu, S. H.; Cong, H. P.; Jiang, J.; Xu, A. W.; Dong, W. F.; Co¨lfen, H. J. Phys. Chem. B 2006, 110, 6432. (13) Kro¨ger, N.; Deutzmann, R.; Sumper, M. Science 1999, 286, 1129. (14) Kro¨ger, N.; Deutzmann, R.; Bergsdorf, C.; Sumper, M. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 14133. (15) Park, H. K.; Lee, I.; Kim, K. Chem. Commun. 2004, 24.

10.1021/la060005v CCC: $33.50 © 2006 American Chemical Society Published on Web 06/03/2006

6126 Langmuir, Vol. 22, No. 14, 2006 were of analytical grade and used as received. All glassware and small pieces of glass substrates were soaked with a H2SO4 (98%)H2O2 (7:3, v/v) solution at 90 °C for 4 h, cleaned and sonicated in ethanol, rinsed with distilled water, and finally dried with acetone. Mineralization Procedures. The mineralization of CaCO3 was carried out by a gas diffusion technique as described by Addadi et al.17 The reaction was conducted in glass bottles with a volume of 10 mL, which were put in a closed desiccator at room temperature (ca. 20 °C). In a typical procedure, a 5 mL mixed solution of CaCl2 and PEG-b-PEI-linear was prepared in the glass bottle under mild stirring, in which the concentration of CaCl2 remains consistent (20 mM), while polymer concentration varies. The pH was adjusted to a fixed value using a diluted 1 M HCl and NaOH solution. Next, four pieces of glass substrates were carefully put at the bottom of the bottle for collecting the crystals. Each bottle was then covered with Parafilm, which was punched with three needle holes, and placed in a larger desiccator. Two small glass bottles (10 mL) of crushed ammonium carbonate were also covered with Parafilm, each of which was punched with three needle holes and placed at the bottom of the desiccator. Characterization. After different mineralization times, the Parafilm was removed, and the precipitate on the glass substrates laid on the bottom was rinsed with distilled water and ethanol, and allowed to dry at room temperature. Crystals floating on the airwater interface were transferred onto a clean glass substrate. The time-dependent crystallization experiments were done by taking out samples from the bottles to stop the mineralization reaction for examination after different mineralization times. Crystals on small glass substrates were examined by optical microscopy, and then gold coated for scanning electron microscopy (SEM) characterization on a DSM 940 A (Carl Zeiss, Jena) microscope. Powder X-ray diffraction (XRD) patterns were recorded on a PDS 120 diffractometer (Nonius GmbH, Solingen) with Cu KR radiation. Thermogravimetric (TG) analysis was carried out on a Diamond TG/DTA thermal analyzer (Perkin-Elmer Corp.) with a heating rate of 10 °C/min-1 in nitrogen atmosphere.

Results The crystallization experiments were performed using the ammonium carbonate diffusion technique, which endows CO2 gas an easy access to the air/water interface. The X-ray diffraction patterns show that all mineralized products obtained are mixtures of vaterite and calcite (Figure 1). The formation of mixed phases of vaterite and calcite in the present system is similar to that reported by Kim et al. using only branched PEI700.15 The results demonstrated that the use of PEG-b-PEI tends to induce the CaCO3 mineralization at the air/water interface. The intensity of (001) faces is very strong, implying that these faces are mostly exposed faces for the vaterite crystals obtained in the present case. TG-DTA analysis suggested that the polymer occluded in the samples obtained in PEG5000-b- PEI1200 and PEG5000-b-PEI400 is about 1 and 0.5 wt %, respectively. The scanning electron microscopy (SEM) images of the CaCO3 structures collected from the bottom of the solution after crystallizing for 5 days (Figure 2a) indicated that disklike crystals with a diameter of about 30-45 µm were obtained. The XRD results indicate that these disks are composed of vaterite, and the exposed disk surface is its (001) face. Figure 2b is a magnified image of the side view of the disklike crystals, showing the typical multilayered structures. Figure 3 presents the typical scanning electron microscopy (SEM) images of the CaCO3 particles collected at the air/water interface after crystallization for 5 days. Different from the particles obtained in the bulk solution, multilayered disks (16) Rudloff, J. Ph.D. Thesis, Potsdam, 2001. (b) Sedla´k, M.; Co¨lfen, H. Macromol. Chem. Phys. 2001, 202, 587. (17) Weiner, S.; Albeck, S.; Addadi, L. Chem.-Eur. J. 1996, 2, 278.

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Figure 1. The X-ray diffraction patterns of CaCO3 crystals formed in a solution with an initial pH 4 in the presence of (a) PEG5000b-PEI1200, 10 days at air/water interface, (b) PEG5000-b-PEI1200, 10 days in solution, (c) PEG5000-b-PEI1200, 2 days in solution, and (d) PEG5000-b-PEI400, 2 days in solution. Polymer concentration: 2 g L-1. Vaterite peaks are marked with “*”.

assembled together at the air/water interface and formed big dendritic superstructures. The structures of the surface facing on the water side are highly rough and stratified (Figure 3a-c), while the side facing the air side is quite flat and smooth (Figure 3d). Figure 3c is a magnified typical image of such multilayered structure, which is composed of layers less than 1 µm in thickness. To investigate the influence of the length of the PEI block on the growth of CaCO3 crystals at the air/water interface, PEG5000b-PEI400-linear with a shorter PEI block but the same length of PEG block was applied also. Keeping all other conditions the same, neither CaCO3 assemblies at the air/water interface (Figure 4a) nor particles produced in the bulk solution (Figure 4c) could stratify to form multilayered structures as in the case of long block length of PEI, indicating that the length of PEI block plays a critical role in controlling CaCO3 crystal morphologies selfassembled at the air/water interface. The results demonstrated that longer PEI blocks tend to result in more stratified structures, while shorter PEI blocks favor the formation of monolayered vaterite. Surprisingly, if the initial pH value decreased to 3.0, despite the disadvantage of short PEI blocks, the stratification effect is partially restored either at the air/water interface (Figure 4b) or in bulk solution (Figure 4d). This underlies that the pH value can also influence the crystal morphology in this polymercontrolled mineralization process.

Discussion Polymorph Selectivity. It is well known that CaCO3 has three polymorphs, that is, calcite, aragonite, and vaterite, and two hydrated ones including mono- and hexahydrate. Calcite and aragonite, which are both common in biological and geological samples, have similar thermodynamic stabilities under standard

Double Hydrophilic Block Copolymers

Figure 2. The SEM images of CaCO3 particles formed at the bottom of the solution in the presence of PEG5000-b-PEI1200. Polymer concentration is 2 g L-1, and the initial pH value is 4. The mineralization time is 5 days.

Figure 3. The SEM images of CaCO3 superstructures formed at the air/water interface in the presence of PEG5000-b-PEI1200. The initial pH value is 4.0. (a-c) Water side; (d) air side. The mineralization time is 5 days.

conditions. Vaterite is metastable and is extremely rare in nature;18 it is detected as a minor component of only a few biomineralized structures but does not exist in geological samples. In fact, as early as in 1996, Sujihara et al.19 had reported that straight polyamines can help to produce disklike vaterite. In the present study, the XRD results confirmed that the DHBCs containing PEI blocks used here could stabilize vaterite phase. In addition, the influence of ammonium ions on calcium carbonate precipitation has been found in the mineralization under simulated gas diffusion conditions via the Kitano method with added ammonia20 (18) Brown, R.; Severin, K. P. Can. J. Fish. Aquat. Sci. 1999, 56, 1898. (19) Sugihara, H.; Ono, K. I.; Adachi, K.; Setoguchi, Y.; Ishihara, T.; Takita, Y. J. Ceram. Soc. Jpn. 1996, 104, 832.

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or the gas diffusion method in the absence of any additives.21 The hexagonal-shaped vaterite superstructures assembled by a three-dimensional oriented attachment of vaterite nanoparticles can be obtained after mineralization for 18 h.21 The (001) vaterite face can be stabilized by the weakly adsorbed ammonium ions; however, this stabilization will get completely lost upon the transformation into calcite after prolonging the mineralization time even though the isolated vaterite superstructures are stable in the absence of water.21 The default experiments show that the products are calcite rhombohedrons without adding polymer into solution after mineralization for 1 week or 10 days, indicating that ammonia is not the critical molecule for the formation of stratified CaCO3 in the present system. The vaterite multilayered structures obtained in the present system are stable at least half a month. Even though the ammonia molecules definitely have influences on CaCO3 mineralization, PEG-b-PEI-linear polymer obviously plays a crucial role in the formation of unusual stratified structures. A slow transition of vaterite to calcite occurred after mineralization for 10 days in the present case. The pH value of the sample solution changed from the initial pH 3 or 4 to over pH 10, due to the dissolution of ammonia in the solution. The results showed that if the initial pH value was adjusted to 10 (the pKa of PEI is about 10-11), only irregularly modified calcite particles were produced, indicating that PEI blocks can only stabilize vaterite when their N-sites are protonated. Here, the polymorph selectivity and the stereo requirements on stretch directions of the bonds that provide atoms to form hydrogen bonds between polymer and binding crystal face were discussed. Under acidic conditions, PEI will be protonated and positively charged. The hydrogen atom in the N-H bond can form a hydrogen bond with the oxygen atom in CO32- groups. For this reason, the PEI block will be preferentially absorbed onto the mostly negative crystal face and inhibit its growth. For the vaterite phase, the (001) face is energy-preferred because Ca2+ and CO32groups are organized in alternating layers, which are vertical to the c axes and have the most negative layers that are composed of unique CO32- groups (Figure 5). Similarly, poly(styrene sulfonate) polyanion adsorption on the calcite (001) face has been reported recently by the gas diffusion process.22 However, how does the PEI block select CaCO3 polymorphs such as calcite or vaterite in a controlled mineralization process? We assume that if the N-H bond approaches the C-O bond parallel to the CO32- triangular plane, formation of the hydrogen bond is favorable because the N-H bond and the C-O bond match in their stretch directions. However, if the N-H bond approaches vertically to the CO32- triangular plane, it would not favor the formation of the hydrogen bond because of the mismatch between N-H and C-O in their stretch directions. Therefore, PEI adsorption on the (001) crystal face is permitted and is more favorable, while its adsorption on the vaterite crystal face (100) and (010) is not favorable for that N-H bond approaches vertically to the CO32- triangular plane. This could explain the reason disklike CaCO3 particles form. In the same way, PEI adsorption on the (001) face of calcite is inhibited for the mismatch of bond stretch directions. Although it is also possible for PEI to adsorb on the (100) and (010) faces of calcite for that N-H bond and C-O bond match in their stretch directions, each pair of N-H/ CO32- attracting each other alternates with a pair of N-H/Ca2+ repelling each other, leading to an unstable adsorption. Therefore, under acidic conditions, PEI blocks could prefer to stabilize (20) Kojima, Y.; Sadomoto, A.; Yasue T.; Arai, Y. J. Ceram. Soc. Jpn. 1992, 100, 1128. (21) Gehrke, N.; Colfen, H.; Pinna, N.; Antonietti, M.; Nassif, N. Cryst. Growth Des. 2005, 5, 1317. (22) Wang, T.; Co¨lfen, H.; Antonietti, M. J. Am. Chem. Soc. 2005, 127, 3246.

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Figure 4. SEM images of CaCO3 structures formed in the presence of PEG5000-b-PEI400. (a,c) pH 4, at the air/water interface and in solution, respectively; (b,d) pH 3, at the air/water interface and in solution, respectively. The mineralization time is 5 days.

vaterite rather than the calcite phase. In a word, stereo requirement on the matching of bond stretch directions of N-H and C-O induces CaCO3 to crystallize into vaterite. Stratification Control at the Air/Water Interface and the Possible Formation Mechanism. Table 1 summarizes the results of the influence of PEI length and pH value on CaCO3 morphogenesis, showing that the longer PEI block or higher acidity favors the formation of multilayered structures. The mechanistic understanding of additive controlled mineralization is still lacking. Weiner and Traub have proposed an epitaxial model for aragonite formation in nacre with distinct 001 orientation.23,24 Volkmer et al. demonstrated that it is not the epitaxial match but the charge density that plays a key role in the oriented growth of CaCO3 crystals under a monolayer of amphiphilic octaacids.25 Our recent results have shown that the shape of the surface structures of calcite formed using a macrocycle-coupled block copolymer poly(ethylene glycol)-b1,4,7,10,13,16-hexaazacycloocatadecan (hexacyclen) ethylene imine macrocycle (PEG-b-Hexacyclen) as a crystal modifier can be controlled by the polymer concentration, the initial pH value, as well as the concentration of calcium ions, demonstrating that it is not only epitaxial match that drives selective polymer adsorption but furthermore charge/ion surface density, particle stabilization, and time for polymer rearrangement from initially occupied less favorable faces to more preferred faces.11 According to the above experimental results and analysis, a mechanism for PEG-b-PEI-linear controlled CaCO3 stratification could be proposed as illustrated in Figure 6. At a fixed pH value, a longer PEI block (PEG5000-b-PEI1200) has more positively (23) Weiner, S.; Traub, W. FEBS Lett. 1980, 111, 311. (24) Weiner, S. Philos. Trans. R. Soc. London, Ser. B 1984, 304, 425. (25) Volkmer, D.; Fricke, M.; Agena, C.; Mattay, J. J. Mater. Chem. 2004, 14, 2249.

charged sites, and accordingly each PEI block occupies a larger adsorption area (Figure 6a), leading to PEI adsorption with a lower density on the specific crystal face and thus lower density of PEG blocks accordingly. Either PEI block or the (001) crystal face of vaterite has a bigger space exposed to the solution. CO32or Ca2+ in solution can approach the PEI-CaCO3 binding interface through electrostatic adsorption and form a new crystal nucleus, which could continue the growth of new CaCO3 layers. Otherwise, if using a shorter PEI block (PEG5000-b-PEI400) that has less positively charged sites than does PEG5000-b-PEI1200, each polymer molecule will occupy a relatively smaller adsorption area (Figure 6b), leading to PEI adsorption on the PEI-CaCO3 binding interface with a higher density. Higher density of PEG blocks can stabilize more efficiently for the formed vaterite disks, and also separate the PEI-CaCO3 binding interface from the solution like a shelter protecting the binding surface from further nucleation. Thus, subnucleation on the polymer-CaCO3 interface can be inhibited, leading to thinner and bigger disks without stratification feature. In fact, previous research on the DHBCcontrolled mineralization mainly focused on functions of binding blocks, while solvating blocks are almost neglected. Here, the higher density of the relatively long solvating PEG blocks with respect to PEI blocks mainly distributed on the outside of the aggregates can stabilize the forming vaterite microparticles more efficiently and prevent the microparticles from further aggregation, which is similar to that previously demonstrated in a comprehensive study using poly(ethylene oxide)-block-poly(methacrylic acid) (PEO-b-PMAA) and PMAA as additives for the mineralization of CaCO3.26 In the PEG5000-b-PEI400 controlled crystallization system, if the pH value was decreased from 4.0 to 3.0, the stratification of (26) Co¨lfen, H.; Qi, L. M. Chem.-Eur. J. 2001, 7, 106.

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Figure 6. A proposed model for DHBC adsorptions on the (001) crystal face of vaterite in solution in the presence of PEG-b-PEI polymer with different PEI block lengths under different acidities. (a,c) Favorable for the formation of multilayered vaterite crystals. (b) Unfavorable for the formation of multilayered vaterite crystals.

previous result has demonstrated that the PEG5000-b-PEI400-linear was less effective than the branched PEG5000-b-PEI700-branched in controlling the CdS particle size.27 Further investigation on the influence of linear PEI block and branched PEI block on the mineralization of CaCO3 at the air/water interface also will be interesting.

Conclusion

Figure 5. PEI adsorption on vaterite crystals. Red line, PEI block; blue line, PEG block; red ball, calcium atom; green ball, oxygen atom; blue ball, carbon atom. Table 1. Summary of the Experimental Conditions and the Morphologies of the Product

sample

polymer

1 2 3

PEG5000-b-PEI1200 PEG5000-b-PEI400 PEG5000-b-PEI400

polymer concentration (g L-1) pH 2 2 2

morphology interface bottom

4 stratified stratified 4 nonstratified nonstratified 3 stratified stratified

disks was partially restored. This could be explained in that increasing the acidity of the solution promoted the protonation degree of PEI blocks, leading to higher positive charge density on the block and a better ability of providing cationic charges, which is equal to the effect of using longer PEI blocks. As compared to the case of pH 4, although still shorter in length, each PEI block will occupy a relatively larger adsorption area (Figure 6c), the PEG block density on the outer surface will decrease and thus result in a decrease in the stabilization ability, and subnucleation and growth of new layers on the exposed PEI-CaCO3 interface will be favored. In fact, for controlling the stratification of CaCO3 particles, regulating the solution acidity and controlling the length of linear PEI block can be combined to adjust the density of positive charges and the density of PEG blocks distributed on the polymer-CaCO3 interface. In fact, a

In conclusion, the influences of two cationic double hydrophilic block copolymers, PEG5000-b-PEI1200-linear and PEG5000-bPEI400-linear, on CaCO3 mineralization at the air/water interface have been studied. The mineralization experiments showed that these DHBCs containing linear PEI blocks could stabilize the vaterite phase. It has been proposed that regulating the linear PEI block length and the pH value of the initial solution can control the density of the PEG block anchored on the binding interface, the stabilization ability of the PEG blocks, and the subsequent subnucleation and stratification growth on the polymer-CaCO3 interface. The detailed mechanism for the stratification that occurred at the air/water interface under control of PEG-b-PEI is still not so clear and needs more depth work in the future. This study provides an additional example that it is possible to access morphogenesis of calcium carbonate structures by a combination of a DHBC with the air/water interface, besides a welldemonstrated example based on the templating effect of a DHBC with CO2 gas bubbles at the air/water interface.28 Acknowledgment. S.-H.Y. acknowledges the special funding support from the Centurial Program of the Chinese Academy of Sciences, the National Natural Science Foundation of China (NSFC, Nos. 20325104, 20321101, 50372065) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, the Specialized Research Fund for the Doctoral Program (SRFDP) of Higher Education State Education Ministry, and the PartnerGroup of the Chinese Academy of Sciences-the Max Planck Society. We genuinely thank PD Dr. Habil H. Co¨lfen and Prof. Dr. M. Antonietti for donating the block copolymers and for the long-standing and enjoyable collaboration in the area of biomimetic mineralization. LA060005V (27) Qi, L. M.; Co¨lfen, H.; Antonietti, M. Nano Lett. 2001, 1, 61. (28) Rudloff, J.; Colfen, H. Langmuir 2004, 20, 991.