Biomimetic Synthesis of Calcium Carbonate Thin Films Using

Dec 5, 2006 - the transformation of amorphous calcium carbonate (ACC) into the crystalline ... calcium carbonate and biomacromolecules.1,4 Mimicking t...
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Biomimetic Synthesis of Calcium Carbonate Thin Films Using Hydroxylated Poly(methyl methacrylate) (PMMA) Template Jayaraman,‡

Subramanyam,‡

Akhila Gayathri Swaminathan Parayil Kumaran Ajikumar,† and Suresh Valiyaveettil*,†

Sindhu,#

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 1 142-146

Department of Chemistry, NUS-Nanoscience and Nanotechnology InitiatiVe (NUSNNI), Singapore-MIT Alliance, National UniVersity of Singapore, 3 Science DriVe 3, Singapore 117543 ReceiVed March 29, 2006; ReVised Manuscript ReceiVed September 6, 2006

ABSTRACT: Morphosynthesis of calcium-rich materials by tuning the chemical structure of organic matrices has tremendous potential in the preparation of functional mineralized materials. In this paper, we have demonstrated the deposition of thin films of CaCO3 by subtle modifications of the backbone of poly(methyl methacrylate) (PMMA) by the incorporation of many hydroxyl groups. The water-insoluble hydroxylated PMMA (HyPMMA) was used as a template along with poly(acrylic acid) (PA) as an additive for CaCO3 mineralization. Thin film deposition was controlled by the addition of an appropriate amount of PA to the crystallization medium. At lower concentrations (PA ) 50 and 100 µg/mL), irregular aggregates of calcite crystallites were formed. As the concentration of PA was increased (500 µg/mL and 1 mg/mL), calcite thin films were deposited. Time-dependent crystallization showed that the precipitates obtained after 3 h were biphasic in structure, consisting of both amorphous and crystalline domains. Observations suggest that crystal aggregates and thin films were formed through a multistep mechanism in which an amorphous phase was deposited initially and was subsequently transformed into stable crystalline form. In contrast, mineralization in the presence of HyPMMA or PA alone yielded only calcite or calcite aggregates. Our results indicate that a concerted interplay of interactions between the insoluble polymer matrix (HyPMMA) and soluble PA determines the growth and morphology of CaCO3 by influencing the transformation of amorphous calcium carbonate (ACC) into the crystalline phase. More specifically, we have investigated the interplay of the role of acid groups on PA and accelerating alcohol groups on HyPMMA in calcium carbonate crystallization. 1. Introduction Morphosynthesis or the control of architecture and morphologies ranging from the nanoscale to the macroscopic scale elicits inspiration from organic/inorganic hybrid structures formed by biological systems.1-3 For example, nacre of the mollusk shell is a laminated composite of high mechanical strength built from calcium carbonate and biomacromolecules.1,4 Mimicking the natural mineralization processes stems from the understanding that functionalized organic templates and/or additives exert tremendous control over the nucleation, growth, and morphology of inorganic crystals.5 Such a bioinspired approach has afforded high performance and ecofriendly functional materials.5,6 Calcium carbonate (CaCO3), due to its natural abundance, low cost, and industrial applications has been used as a prototype system for biomimetic studies.7 Recently, it has been demonstrated that the biomimetic synthesis of calcium carbonate can be manipulated through the use of organic templates or crystal modifiers and additives.8 Biomacromolecules are known to template the formation of biominerals into intriguing shapes and properties.1,9 However, when synthetic polymers are used as templates, it is often difficult to predict the structure-property relationships unambiguously in terms of their role in crystallization as they predominantly adopt random coil conformation. Templatedirected crystallization of CaCO3 has been achieved using ordered supramolecular assemblies such as self-assembled films,10-12 Langmuir monolayers,13-16 dendrimers,17 and selfassembled thiol on gold (111) surface,18 to yield well-defined crystal morphologies. Low molecular weight and polymeric additives19,20 strongly influence the nucleation and growth of CaCO3 crystals, thus leading to the formation of complex21,22 * To whom correspondence should be addressed. Tel: +65 65164327. Fax: +65 67791691. E-mail: [email protected]. ‡ Department of Chemistry. # NUS-Nanoscience and Nanotechnology Initiative (NUSNNI). † Singapore-MIT Alliance.

stabilized amorphous calcium carbonate (ACC) precursors.17 Acidic macromolecules are known to adsorb onto the surface of insoluble matrices and influence the mineralization process.23,24 Recently, our attention has been given to develop strategies for synthesizing CaCO3/organic composite materials through synergistic interactions associated with organic components.25-28 In a biomimetic approach, calcium carbonate thin films on biocompatible matrices were deposited through co-operative interactions between natural/synthetic substrates (polyamides/ collagenous eggshell membrane) and soluble polymeric additives.25 Calcium carbonate thin films have been previously fabricated in vitro by combined effects of additives such as poly(acrylic acid) (PA) and templates such as Langmuir films,16b poly(vinyl alcohol),29 and polysaccharides.30 In all these methods, the number, orientation, and nature of the functional groups are important in controlling the nucleation of calcium carbonate crystallites. To devise simple methods to achieve controlled deposition of CaCO3 thin films, a water-insoluble, amphiphilic polymer, hydroxylated poly(methyl methacrylate) (HyPMMA), was synthesized by modifying the backbone of poly(methyl methacrylate) (PMMA) to incorporate polar hydroxyl groups. In this paper, the formation of calcium carbonate thin films over the HyPMMA film in the presence of an acidic macromolecule such as PA is discussed in detail. The underlying motivation to use HyPMMA as the polymer matrix evolved from the fact that the hydroxyl groups of HyPMMA are similar to water-soluble poly(vinyl alcohol) (PVA), which has been previously used for selective nucleation of aragonite in solution.31 Moreover, the polymer backbone of HyPMMA is incorporated with ester and hydroxyl functionalities, both of which are known to influence the morphogenesis of calcium carbonate independently or in the presence of other functional groups.29,32,33 Thus, it is anticipated that the cooperative effects between the various functional groups (-OH and -COOR) could play a key role

10.1021/cg060172t CCC: $37.00 © 2007 American Chemical Society Published on Web 12/05/2006

Biomimetic Synthesis of CaCO3 Thin Films

Figure 1. Molecular structure of polymer HyPMMA and poly(acrylic acid) (PA).

in the formation and stabilization of calcium carbonate thin films. In addition, the effect of concentration of PA on the formation of CaCO3 thin films is also examined. 2. Experimental Section The synthesis of polymer HyPMMA is provided in Supporting Information.34 Preparation of CaCO3 Crystals. Poly(acrylic acid) (PA, MW ca. 5.0 × 103) was purchased from Aldrich. AR-grade CaCl2·2H2O was obtained from commercial sources. The crystallization method involved the slow infusion of CO2 (generated by the sublimation of ammonium carbonate) into an aqueous solution of calcium chloride.35 A 7.5 mM solution of CaCl2 was prepared in Millipore-Q water (18 MΩ cm). PA was dissolved in 7.5 mM CaCl2 solution to give a series of solutions containing 50 µg/mL, 100 µg/mL, 500 µg/mL, and 1 mg/mL of the water-soluble additive. Films of HyPMMA on circular cover slips were obtained by spin coating from a solution of the polymer in methanol (30 mg/mL). The spin-coated cover slips were placed in a Nunc dish with 12 wells, and 2 mL of CaCl2/PA solutions with different concentrations was introduced into each well. The crystallization wells were covered with aluminum foil, and a few pin holes were made to allow gas diffusion. The whole assembly was placed inside a desiccator with ammonium carbonate powder placed at the bottom and left undisturbed. After the stipulated time period, the cover slips were carefully removed, rinsed with water, and air-dried for characterization. Powder X-ray diffraction patterns were performed using a D5005 Siemens X-ray diffractometer with Cu-KR radiation at 40 kV and 40 mA. The samples were scanned over a 2θ range of 20-70° at a step size of 0.02°. Crystal habit and polymorph type were identified by comparing peak positions in the X-ray diffractograms with standard data available from the Joint Committee on Powder Diffraction Standards. Scanned electron microscopy (SEM) measurements for the crystals grown on the cover slips were performed on a JEOL JSM 5600LV scanning electron microscope. Before measurement, the samples were mounted on copper stubs using double-sided carbon tapes and sputter coated with gold. The ATR-FTIR spectra were collected on a Shimadzu ATR 8800M spectrophotometer equipped with an argon gas generator and a DLATGS detector. The samples were pressed against a Ge prism tip at the focal position to obtain the spectrum, and 64 scans were collected at a resolution of 4 cm-1.

3. Results and Discussion Spin-coated films of the water-insoluble polymer, HyPMMA, were used as templates for the mineralization experiments. The CaCO3 precipitation experiments were performed at 25 ( 3 °C via slow diffusion of CO2 obtained from the decomposition of ammonium carbonate into 7.5 mM CaCl2 solution. Rhombohedral calcite crystals consisting of six {104} faces were deposited over the spin-coated polymer film when no PA was added as additive to the crystallization solution (Figure 2A). In the absence of insoluble matrix, the calcium carbonate precipitation experiments yielded spherical calcite aggregates for all concentrations of PA (Figure 2B). Morphological examination of the precipitates obtained at different intervals of time showed progressive changes in morphology with increase in PA concentration (Figure 3 A-D). The Fourier transform infrared (FTIR) spectra and X-ray

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Figure 2. SEM images of rhombohedral calcite crystals deposited on the HyPMMA polymer film (A) and spherical calcite aggregates (B) precipitated in the presence of poly(acrylic acid) (100 µg/mL) on a glass cover slip.

diffraction patterns (Figure 3E,F) confirmed the formation of calcite phase after 72 h of crystallization in the presence of PA. However, the presence of diffraction peaks from all the crystal planes of the calcite lattice indicated the lack of any preferential orientation in the morphologies formed on the substrate. From the SEM images for low concentrations of PA (50 and 100 µg/ mL) (Figure 3), it can be inferred that the slow evaporation process leads to the formation of large calcite aggregates, which consist of intermediate shapes such as small spheres, peanut and dumb-bell structures, and well-grown large spheres. Similar CaCO3 morphologies in the presence of PEG-b-PEIPA have been observed by Colfen et al.37 On increasing the additive concentration (PA ) 500 µg/mL), thin films of calcite with radial outgrowths were observed. The films are not flat but consist of aggregates that assembled into large spheres (∼50 µm) and subsequently grew radially outward from the center as shown in Figure 3C. Flat thin films of 100 µm in diameter consisting of small rudimentary calcite crystallites of ∼0.1-2 µm sizes were formed at PA concentrations of 1 mg/mL. Each crystallite in the aggregate has a morphology similar to the calcite structures formed at [PA] ) 50 and 100 µg/mL, i.e., there appears to be a preferential elongation along the c-axis. To understand the mechanism involved in the formation of aggregates and thin films, the morphology of precipitates obtained after 3 h were examined (Figure 4). In all cases, an amorphous calcium carbonate (ACC) phase was identified, which subsequently transformed into crystalline CaCO3. For PA concentrations of 50 and 100 µg/mL, the incipient calcite crystals amidst the regions of ACC were observed from the SEM micrographs, while for PA ) 500 µg/mL and 1 mg/mL, ACC films predominantly covered the substrate. The polymorphic nature of the precipitates was characterized by a FTIR technique (Figure 4A). The strong absorption in the FTIR spectra at 1435 cm-1, with simultaneous absorption peaks occurring at 715 and 875 cm-1, indicated the presence of calcite phase.37,38 The peaks at 715 and 875 cm-1 were, respectively, assigned to the υ4 and υ2 absorption bands of the carbonate groups of the calcite. The intensity ratio (Imaxυ2/Imaxυ4) of the absorption bands was used to identify the coexistence of both the crystalline and amorphous phases. A high (Imaxυ2/Imaxυ4) value represents a higher content of ACC. The (Imaxυ2/Imaxυ4) ratio for pure calcite is c.a. 3.0,37 and in our experiments the ratio was increased from 4.2 to 6.3, when the PA concentration was changed from 50 µg/mL to 1 mg/mL. Additionally, the occurrence of a slightly broad absorption band at 1086 cm-1 indicated the existence of ACC.39 At the early stages of crystallization showing the coexistence of amorphous, partially transformed, and completely transformed crystalline phases, it was difficult to detect the characteristic band for ACC in the FTIR spectra of precipitates formed in the presence of the soluble additive. Recently, the coexistence of biogenic ACC and calcite phase in sea-urchin larval spicules was detected in a similar manner as X-ray diffraction patterns

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Jayaraman et al.

Figure 3. Scanning electron micrographs of calcite aggregates formed in the presence of PA ) 50 µg/mL (A), irregular aggregates formed on addition of PA ) 100 µg/mL (B), calcite thin films obtained at PA ) 500 µg/mL (C) consisting of spherical aggregates that grow radially outward and flat crystalline CaCO3 thin films grown in the presence of PA ) 1 mg/mL (D) after 72 h crystallization. The scale bar is 50 µm in the SEM. The higher magnification images are shown in the inset. (E) FTIR spectra of the precipitates obtained in the presence of PA. (F) X-ray diffraction of calcite aggregates and thin films formed on the insoluble polymer matrix in the presence of PA.

Figure 4. FTIR (A) and (B) XRD spectra of the precipitates obtained after 3 h in the presence of various concentrations of PA. SEM micrographs of CaCO3 in the presence of 50 µg/mL of PA (C), concomitant ACC spheres and calcite aggregates at PA ) 100 µg/mL (D), ACC thin films at PA ) 500 µg/mL (E), and formation of flat amorphous thin films at [PA] ) 1 mg/mL (F). The scale bar is 10 µm in all the SEM images.

failed to give conclusive results.40 The diffuse patterns of the X-ray diffractograms and the presence of a broadband at 1082 cm-1 in the Raman spectra of the precipitates obtained after 1 h further confirmed the formation of ACC at the early stages of crystallization (Supporting Information and Figure 4B). Apparently, the stability of ACC increased with a decrease in the HyPMMA/PA ratio. The porous structure of ACC formed at room temperature (Figure 4C-F) is attributed to the release of water and impurities during crystallization. The ACC films with several micro- to nanometer-sized particles, undergo dissolution and recrystallization at ambient conditions and high humidity to form flat films consisting of micro- and nanopores (Figure 4E,F).16b Thus, with an increase in time 13 h, to 72 h, bigger calcite aggregates, spheres, and twinned superstructures were obtained at PA 50 and 100 µg/mL. For higher concentrations of PA, the process of Ostwald’s ripening led to the formation of larger films. To confirm the adsorption of PA on the polymer surface, the polymer-coated substrates were soaked in PA and characterized using ATR FTIR spectroscopy. PA is adsorbed on the HyPMMA matrix through interactions between the hydroxyl groups and the carboxylic acid functionality. The increase in the intensity of the carbonyl band at 1727 cm-1 and broadening of the -OH band at 3350 cm-1 as compared to that of HyPMMA can be attributed to the adsorption of the COO- moieties on the polymer matrix (Figure 5). The ability of structured organic surfaces to control nucleation and crystal growth in biological/synthetic environments has

Figure 5. ATR-FTIR spectra of the spin-coated film of polymer HyPMMA (A) and polymer HyPMMA film soaked in PA (B).

prompted a number of model studies on oriented crystallization of inorganic materials.41 Using the synthesized HyPMMA template, controlled formation of stable ACC films and their transformation into crystalline phase at room temperature was observed. The formation of the amorphous phase is particularly interesting and beneficial in composite material design. It is wellknown that ACC is a kinetic product that results from the inhibition of the nucleation of crystalline forms.42,43 It has been patented that the presence of hydroxyl-containing molecules in the crystallization solution promotes the stabilization of ACC at low temperatures.44 The deposition of ACC as a transient precursor phase45 and the growth of calcium carbonate thin films on solid substrates in the presence of soluble additives has been previously investigated.16b,31,32 The inhibitory role of additives on the transformation of ACC provides possible routes to understand the role of ACC and derive molecular mechanisms

Biomimetic Synthesis of CaCO3 Thin Films

for the biomineralization process. Mg2+ ions,42,46 triphosphate ions,43and polymers42 have been utilized to transiently stabilize the ACC for a considerable period of time. According to Aizenberg et. al.,41c ACC film crystallization occurs through mass transport between the amorphous and the crystalline states and is not limited to solid-solid transformation. Gower et. al.46 proposed the concept of a polymer induced liquid precursor (PILP) mechanism to demonstrate the role of polymers in the formation of the mineral precursor. In this multistep process, the addition of small amounts of the polymer to the crystallization medium results in liquid-liquid-phase separation, generating liquid droplets that adapt complex shapes before transforming into the crystalline phase. In the present study, an amorphous precursor was formed on the insoluble polymer matrix that subsequently transformed into the crystalline calcite phase through a dissolution/reprecipitation mechanism. Thus, for all concentrations of PA, small ACC particles that grow into bigger spheres are formed at the initial stages. The particles gradually dissolve, while the spheres mature into larger ones by the process of Oswald’s ripening. The ACC particles are encapsulated by the growing crystallites leading to the formation of crystalline rods, spheres, and peanut or dumb-bell shaped aggregates. There is subsequent overgrowth of the spheres once they are formed. It can be inferred from the results that a suppression of crystal growth occurs for all concentrations of PA, and the resultant film formation can be observed only for PA ) 500 µg/mL and 1 mg/mL. Such concentration-dependent formation of thin films has also been observed by Kato et al.30d and shows that PA, both adsorbed on the surface and in solution, plays a dual role toward the formation of thin films. High concentrations of PA (500 µg/ mL and 1 mg/mL) create a local high concentration of Ca2+ ions as a result of interactions between the carboxylic acid functionality and the calcium ions. This increase in the concentration of Ca2+ ions on the large surface area leads to the nucleation and growth of CaCO3 crystals. Simultaneously, the excess PA remaining in the solution inhibits the crystallization of calcium carbonate. The addition of PA prevents the growth of calcite crystals normal to the template and with an increase in concentration confines the space available for mineral growth to a thin layer on the matrix. Thus, a delicate balance between crystallization and inhibition is established in certain concentration ranges that lead to the formation of thin films. The resultant morphologies of CaCO3 can be attributed to the cooperative interactions operating between the acidic PA and the insoluble matrix. Similar synergism has been observed between PA and functionalized natural polymers such as cellulose and chitin.30d 4. Conclusion Investigations on the deposition of thin films of calcium carbonate using a tailored functional polymer, HyPMMA, as a template is reported. PA was used as an additive to control the deposition of CaCO3. PA is adsorbed onto the surface of the polymer matrix through electrostatic interactions with the polymer matrix. The observed crystal growth mechanism showed the formation of an ACC precursor phase and its subsequent transformation into calcite films. By fine-tuning the PA concentrations, thin films of ACC and calcite crystal were obtained at high PA concentrations of 500 µg/mL and 1 mg/ mL. Blending of the amorphous and crystalline phase in organic-inorganic hybrid materials is an easy and efficient method of tailoring desired and optimum properties. The ambient temperature biomimetic synthesis offers a facile strategy

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for fabricating such functional materials. Also, the present study directs the design of similar macromolecules, which can provide new insights into biosynthesis of functional materials. Indeed, there are numerous examples about the controlled synthesis of calcium carbonate films, but our strategy offers yet another simple biomimetic approach to synthesize macroscopic-scale inorganic thin films through the synergistic interplay between the insoluble matrix and the soluble macromolecules. Acknowledgment. The authors thank the Agency for Science, Technology and Research (ASTAR) for funding support and National University of Singapore (NUS) for research scholarship. All technical support from various laboratories at the Faculty of Science is acknowledged. Supporting Information Available: Powder pattern X-ray diffractograms, Raman spectra of the precipitates obtained after 1 h, and synthesis of polymer HyPMM. This material is available free of charge via the Internet at http://pubs.acs.org.

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