Langmuir 2005, 21, 4801-4804
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Deposition of Amorphous Calcium Carbonate Hemispheres on Substrates Xurong Xu, Joong Tark Han, and Kilwon Cho* Department of Chemical Engineering, Polymer Research Institute, Pohang University of Science and Technology, Pohang 790-784, Korea Received November 30, 2004. In Final Form: March 31, 2005 The amorphous calcium carbonate (ACC) hemispheres were deposited on the mica and poly(diallyldimethylammonium chloride) modified surface. The form of the ACC deposit on the substrates can be controlled by modifying the substrate surface, the introduction of additives, or both. It demonstrated that substrates (insoluble matrix) and additives (soluble macromolecules) have significant influence on the crystallization of CaCO3.
Introduction In biomineralization, organisms control the nucleation of inorganic crystals at a specific site and their growth into complex and intricate hierarchical structures. Structures that have a structural function, such as bones, teeth, and shells, usually have superior properties compared to those of conventional man-made materials with similar components.1 Calcium carbonate (CaCO3) is one of the most abundant biominerals, and extensive research into the biomimetic synthesis of CaCO3-based biominerals has been carried out. CaCO3 crystals with a variety of complex shapes have been prepared by using many different additives.2-6 Various functional templates, including langmuir monolayers7 and self-assembled monolayers,8 have been used to induce crystallization of CaCO3. CaCO3 thin films have also been produced using the cooperation between insoluble matrixes and soluble macromolecules.9 It is thought, however, that amorphous calcium carbonate (ACC), a metastable form of calcium carbonate, plays an important role in the biomineralization and crystallization of CaCO3.8e,10-15 The rich variety of CaCO3 structures in * To whom correspondence should be addressed. E-mail: kwcho@ postech.ac.kr. Fax: (+82)54-279-8269. (1) Currey, J. D. Proc. R. Soc. London, Ser. B 1977, 196, 443. (2) Walsh, D.; Mann, S. Nature 1995, 377, 320. (3) Walsh, D.; Lebeau, B.; Mann, S. Adv. Mater. 1999, 11, 324. (4) Co¨lfen, H.; Qi, L. M. Chem.sEur. J. 2001, 7, 106. (5) Co¨lfen, H. Curr. Opin. Colloid Interface Sci. 2003, 8, 23. (6) Meldrum, F. C. Int. Mater. Rev. 2003, 48, 187. (7) (a) Mann, S.; Heywood, B. R.; Rajam, S.; Birchall, J. D. Nature 1988, 334, 692. (b) Heywood, B. R.; Mann, S. Adv. Mater. 1994, 6, 9. (c) Heywood, B. R.; Rajam, S.; Mann, S. J. Chem. Soc., Faraday Trans. 1991, 87, 735. (d) Xu, G. F.; Yao, N.; Aksay, I. A.; Groves, J. T. J. Am. Chem. Soc. 1998, 120, 11977. (8) (a) Ku¨ther, J.; Seshadri, R.; Knoll, W.; Tremel, W. J. Mater. Chem. 1998, 8, 641. (b) Aizenberg, J.; Black, A. J.; Whitesides, G. M. Nature 1999, 398, 495. (c) Aizenberg, J.; Black, A. J.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 4500. (d) Han, Y. J.; Aizenberg, J. Angew. Chem., Int. Ed. 2003, 42, 3668. (e) Aizenberg, J.; Muller, D. A.; Grazul, J. L.; Hamann, D. R. Science 2003, 299, 1205. (9) (a) Zhang, S. K.; Gonsalves, K. E. Langmuir 1998, 14, 6761. (b) Kato, T.; Suzuki, T.; Amamiya, T.; Irie, T.; Komiyama, M.; Yui, H. Supramol. Sci., 1998, 5, 411. (c) Kato, T.; Sugawara, A.; Hosoda, N. Adv. Mater. 2002, 14, 869. (d) Hosoda, N.; Kato, T. Chem. Mater. 2001, 13, 688. (e) Sugawara, A.; Ishii, T.; Kato, T. Angew. Chem., Int. Ed. 2003, 42, 5299. (10) Addadi, L.; Raz, S.; Weiner, S. Adv. Mater. 2003, 15, 959. (11) Becker, A.; Bismayer, U.; Epple, M.; Fabritius, H.; Hasse, B.; Shi, J. M.; Ziegler, A. Dalton Trans. 2003, 551. (12) Weiner, S.; Levi-Kalisman, Y.; Raz, S. Connect. Tissue Res. 2003, 44 (Suppl. 1), 214. (13) Xu, X. R.; Han, J. T.; Cho, K. Chem. Mater. 2004, 16, 1740. (14) (a) Gower, L. B.; Odom, D. J. J. Cryst. Growth 2000, 210, 719. (b) Olszta, M. J.; Odom, D. J.; Douglas, E. P.; Gower, L. B. Connect. Tissue Res. 2003, 44 (Suppl. 1), 326.
nature might be due to the amorphous character of ACC, which is easily molded into many different shapes. Here we describe the deposition of ACC hemispheres on mica and the surfaces modified with a positively charged polyelectrolyte, poly(diallyldimethylammonium chloride) (PDADMAC) and how control of the deposited ACC forms can be obtained by modifying the surface characteristics of these substrates. Experimental Section The silicon wafers were cleaned by immersion in freshly prepared piranha solution (concentrated H2SO4/H2O2 ) 7:3, w/w) and heated for 1 h at 100 °C, then rinsed thoroughly with distilled water. Newly cleaved mica was produced by peel-off using adhesive tape. Silicon wafers and mica modified with PDADMAC (Mw ∼1 × 105 to 1 × 106, from Aldrich) and poly(allylamine hydrochloride) (PAH; Mw 70 000, from Aldrich) were obtained by immersing clean silicon wafers and mica in 1 wt % solutions of either PDADMAC and PAH for 30 min and subsequently rinsing thoroughly with distilled water. A vial containing a 50 mM CaCl2 solution was placed in a desiccator along with a dish containing ammonium carbonate powder. The substrate was inverted and placed on top of the CaCl2 solution.13 ACC was deposited onto the substrate by slow diffusion of the CO2 produced by decomposition of the ammonium carbonate at room temperature. The deposition time was 50 min. After deposition, the substrate was rinsed with ethanol and dried using nitrogen gas. The substrate was immediately placed under a vacuum until ready for observation with a scanning electron microscope (Hitachi S-4200 field emission scanning electron microscope). The electron diffraction pattern was obtained by using a transmission electron microscope (JEOL1200EX). When poly(acrylic acid) (PAA, Mw 2000, from Aldrich) was used as an additive, PAA was mixed with a 50 mM CaCl2 solution. The other steps for the use of this additive were the same as described above.
Results and Discussion In a previous paper,13 we have already reported the preparation of large-area and continuous ACC films on clean silicon wafers in both the presence and the absence of PAA inhibitor under mild conditions. Amorphous character of the as-deposited film has been confirmed by X-ray, IR, and optical microscope. Here we used several different substrates such as mica and polyelectrolyte modified silicon wafer and mica. When mica is used as the substrate in the absence of PAA, the properties of the resulting deposits are dramatically different. As shown in Figure 1a, many homogeneous “spheres” appear to have (15) Faatz, M.; GrO ¨ hn, F.; Wegner, G. Adv. Mater. 2004, 16, 996.
10.1021/la047069v CCC: $30.25 © 2005 American Chemical Society Published on Web 04/23/2005
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Figure 1. SEM microphotographs of hemispheres deposited on mica before crystallization: (a) low magnification; (b) higher magnification with the stage tilted at 45°, the inset shows the electron diffraction pattern of the hemisphere by TEM; (c) after crystallization; and (d) side view for the measurement of the “contact angle”.
been deposited on the mica substrate. It was found from microphotographs taken at high magnification with the stage tilted at 45° (Figure 1b) that these “spheres” are in fact hemispheres. The amorphous character of the hemispheres is confirmed from the electron diffraction pattern (Figure 1b inset). ACC is unstable in humid air, in which it transforms into anhydrous crystal phases. When using crossed-polarized optical microscopy (see Supporting Information), the hemispheres are dark before crystallization but bright after crystallization (ACC will transform into crystalline forms in humid air). Scanning electron microscopy (SEM) microphotographs were used to investigate the apparent differences between the surfaces of the hemispheres before and after crystallization (kept in humid air to crystallize), which also indicate the amorphous character of the hemispheres. Prior to crystallization, the surfaces of the hemispheres have a smooth, glassy appearance (Figure 1b), but the surfaces of the crystallized hemispheres are heterogeneous and rough. Many nanoparticles can also clearly be seen on the surfaces of the hemispheres (Figure 1c). Transformation of ACC into a crystalline form involves the release of water molecules included in ACC.8e The pores and rough surface observed in the present work are possibly related to the release of water during the transformation of ACC. These images of hemispheres on mica are very similar to those of the classic contact angle phenomena that occur when a liquid droplet contacts with a solid in air. If the liquid molecules at a liquid-solid interface are more strongly attracted to each other than to the molecules of the solid surface (i.e., the cohesive forces are stronger than the adhesive forces), then the liquid beads up and does not wet the solid surface. On the other hand, if the molecules of the liquid have a stronger attraction to the molecules of the solid surface than to each other (i.e., the adhesive forces are stronger than the cohesive forces), then wetting the surface occurs. In practice, a contact angle usually forms on the solid/liquid interface according to the balance between the three interface tensions involved, those at the solid-gas interface, the liquid-gas interface, and the solid-liquid interface. The appearance of the ACC hemispheres on mica is very similar to the phenomena often found in static contact angle measurements. The “contact angle” of ACC on mica is about 90°,
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which was estimated from the SEM image in Figure 1d by Adobe Photoshop 6.0. However, continuous ACC films are formed on clean silicon wafers under the same experimental conditions. These ACC films have a smooth, homogeneous appearance in the SEM image (not shown here), which is similar to the images of ACC films on silicon wafer surfaces in the presence of PAA (Figure 3a,d). The surfaces of mica and clean silicon wafers are negatively charged and hydrophilic in water. However, the reasons for this charge on the two substrates are different. The surface of a clean silicon wafer (coated with a silicon dioxide layer) is negatively charged in water due to the presence of deprotonated silanol groups. The surface of newly cleaved mica is inherently negatively charged in water due to the dissociation of the potassium cations from the mica surface. It is well-known that potassium ions in mica are easily replaced with other ions such as H+, Mg2+, Ca2+, and so forth when mica is treated with different solutions.16,17 The ion exchange process is quick and efficient. This ion exchange can modify the friction and adhesion properties of mica surfaces.16 A small amount of net positive charge is accumulated on mica surfaces in calcium dichloride (CaCl2) solution (concentration > 20 mM), and its surfaces are slightly more hydrophobic.17 It has been shown that the contact angle of ultrapure water on mica changes from 7 ( 1° to 16 ( 2° after contact of mica with a 50 mM CaCl2 solution for 10 min. In contrast, for silicon wafers there is no difference in the contact angle before and after contact with CaCl2 solution. Therefore, we speculate that ion exchange between potassium ions and calcium ions modifies the mica surface and lowers the interaction between ACC and the mica surface, which results in the formation of ACC hemispheres on the mica surface. To study the effect of surface characteristics on the deposition of ACC in more detail, two different positively charged polyelectrolytes, PDADMAC and PAH, which are often used in the preparation of layer-by-layer polyelectrolyte mutilayers, were employed to modify the surface of mica and silicon wafers by electrostatic assembly. The structures of the polyelectrolytes are shown in Figure 2. The pH of the solution changes from ∼7 to ∼9 during the deposition of ACC. The surfaces modified by the polyelectrolytes are both positively charged in solution. The PDADMAC modified surface with its numerous quaternary ammonium cations is highly positively charged in solution. The PAH modified surface has only a few positively charged amino groups in this pH range. It was found that deposited ACC takes on the form of hemispheres on PDADMAC modified surfaces (Figure 2a,b), which is similar to those found on mica. In contrast, an ACC film with dense and partial coalescence hemispheres forms on the PAH modified surface. This result shows that the interaction between ACC and the PAH modified surface is somewhat stronger than that between ACC and the PDADMAC modified surface. The same behavior is found on the polyelectrolyte modified mica surfaces. This demonstrates that the positive charge of the surface is responsible for the formation of the ACC hemispheres. We can find that ACC hemispheres are formed on the mica surface, but on the PAH modified mica surface an ACC film will be obtained. Accordingly, on the silicon wafer surface an ACC film is formed, but ACC hemispheres will be obtained on PDADMAC modified silicon wafer. Thus, we can conclude that the surface characteristics of substrates exert important effects on the deposition of CaCO3, especially the shape of the deposits. (16) Xu, L.; Salmeron, M. Langmuir 1998, 14, 2187. (17) Dunstan, D. E. Langmuir 1992, 8, 740.
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Figure 3. SEM microphotographs of ACC deposited on all kinds of substrates in the presence of PAA: (a) bare silicon wafer; (b) mica; (c) PDADMAC modified silicon wafer; (d) PAH modified silicon wafer.
Figure 2. SEM microphotographs of ACC deposited on the surfaces of silicon wafer modified with polyelectrolytes, deposition time 50 min: (a) the PDADMAC modified surface; (b) part a at higher magnification; (c) the PAH modified surface; (d) part c at higher magnification.
The structure of ACC has previously been determined using several kinds of analysis methods.10-15,18-21 Thermogravimetric analysis has been used to show that ACC contains some water.15,18,19 The short-range order of biogenic ACC has been determined using extended X-ray absorption fine structure.10-12 Electron microscope images of ACC generally show that ACC is composed of spheres.11,18 ACC prefers a spherical shape in solution to minimize its contact area with solution.19 The initial composition of ACC in solution has not been determined due to the lack of available characterization techniques.20 However, Gower et al.14 has found that a polymer-induced liquid-precursor process occurs and suggested that polymer induces the liquid-liquid phase separation of droplets of a mineral precursor. A recent paper postulated a liquidliquid phase segregation, and ACC particles are formed from a loss of water of the highly concentrated solution by the gelation process. 15 It can be speculated that ACC prefers a spherical droplet shape and contains a lot of water in solution. Our results can be rationalized by considering ACC as liquidlike colloids, which contain many water molecules. When excess calcium chloride is mixed with sodium carbonate, the zeta potential of CaCO3 precipitates is positive because calcium cations are adsorbed on the surface of precipitates.22 Even for equimolar solutions, the zeta potential of CaCO3 is still small positive during first 30 min.22 Here a method of slow diffusion of carbon dioxide (CO2) into CaCl2 solution is used, and the excess calcium ions exist in CaCl2 solution. On one hand, ACC, which spontaneously formed first in CaCl2 solution, preferentially adsorbs calcium ions to form stable posi(18) Brecˇevic´, Lj.; Nielsen, A. E. J. Cryst. Growth 1989, 98, 504. (19) Raz, S.; Hamilton, P. C.; Wilt, F. H.; Weiner, S. Addadi, L. Adv. Funct. Mater. 2003, 13, 480. (20) Rieger, J.; Thieme, J.; Schmidt, C. Langmuir 2000, 16, 8300. (21) Pontoni, D.; Bolze, J.; Dingenouts, N.; Narayanan, T.; Ballauff, M. J. Phys. Chem. 2003, 107, 5123. (22) Chibowski, E.; Hotysz, L.; Szczes´, A. Colloids Surf., A 2003, 222, 41.
tively charged ACC colloids with lots of water. On the other hand, ACC colloids have a tendency to aggregate, coalesce, dehydrate, and solidify. The nature of the ACC deposit on the substrate depends on the relative strengths of the cohesive force of ACC on each other and the adhesive force between ACC and the substrate. The adhesive forces between mica surfaces and PDADMAC modified surfaces with positively charged ACC colloids are weakened because of the electrostatic repulsive interaction, so ACC hemispheres are deposited on these surfaces. There is a stronger interaction between the negatively charged clean silicon wafer and the positively charged ACC colloids, which results in the formation of a continuous ACC film. The PAH modified surface has only a few positively charged and many free amino groups. Thus, because there is not much repulsion between ACC and the substrate, an ACC film also is obtained on it. When PAA is employed as an additive in the deposition process, the situation changes dramatically: continuous ACC films are deposited on all the substrates despite the variations in the surfaces’ charges (Figure 3). PAA is wellknown as an anti-scaling additive, so it has the effects of inhibiting and dispersing CaCO3 precipitates. When PAA is introduced into the solution, it is easily adsorbed onto the surfaces of the ACC colloids and changes the zeta potential of CaCO3,23 which results in inhibition of the crystallization and aggregation of ACC and a decrease in the cohesive forces of ACC. The adhesive force between ACC and the substrates is then stronger than the cohesive force, so ACC tends to spread over the surface, which results in a continuous ACC film on the substrate. This confirms again that the form of the ACC deposit on a substrate is dependent on the balance between the cohesive and the adhesive forces. From the above results, we conclude that the surface characteristics significantly influence the deposition of ACC on the substrate. A schematic diagram of the deposition of ACC on various substrates is shown in Figure 4. ACC hemispheres form on positively charged mica surfaces and PDADMAC modified surfaces. A continuous ACC film is deposited on negatively charged clean silicon wafers. On the weakly positively charged PAH modified surface, a film is also produced. The variety of deposited ACC forms reflects the differences in the adhesive forces (23) Jada, A.; Verraes, A. Colloids Surf., A 2003, 219, 7.
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Figure 4. Schematic diagram showing the deposition of ACC on all kinds of substrates in the absence or presence of PAA in solution.
between ACC and the various substrates, which derive from the different surface characteristics of the substrates. Our study has shown that ACC hemispheres are deposited on both mica and PDADMAC modified surfaces. Further, the above discussion indicates that ACC has some liquid character and exhibits behavior similar to that of liquid droplets. Therefore, we suggest that ACC in highly supersaturated solutions spontaneously forms liquidlike colloids with an open structure in solution, tends to coalesce, and deforms into hemispheres or forms continuous films when deposited on substrates. Many different structures of CaCO3 biominerals in organisms may derive from the character of liquidlike ACC colloids. Also insoluble matrix and soluble macromolecules exert important effects on the formation of CaCO3 biominerals. In summary, we have demonstrated that substrates (insoluble matrix) and additives (soluble macromolecules) have significant influence on the crystallization of CaCO3 and suggest that ACC plays an important role in the crystallization and biominerlization of CaCO3. The form
of the ACC deposit on substrates can be controlled by modifying the substrate surface, the introduction of additives, or both. Acknowledgment. This work was supported by Grant 04K1501-01310 from “Center for Nanostructured Materials Technology” under “the 21st Century Frontier R&D Programs” of the Ministry of Science and Technology, the National Research Laboratory Program; R&D Program for Fusion Strategy of Advanced Technologies of the Ministry of Science and Technology of Korea, Advance Environmental Biotechnology Research Center; and the Brain Korea 21 Program of the Ministry of Education of Korea. Supporting Information Available: Optical microphotographs of partial crystallized hemispheres on mica under crossed-polarized light in reflective mode. This material is available free of charge via the Internet at http://pubs.acs.org. LA047069V
