Study of Polyelectrolyte Complex Nanoparticles as Novel Templates

Apr 9, 2012 - Zhi-Wei Sun, Quan-Fu An,* Qiang Zhao, Yong-Gang Shangguan, and Qiang Zheng. MOE Key Laboratory of Macromolecular Synthesis and ...
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Study of Polyelectrolyte Complex Nanoparticles as Novel Templates for Biomimetic Mineralization Zhi-Wei Sun, Quan-Fu An,* Qiang Zhao, Yong-Gang Shangguan, and Qiang Zheng MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Needle-like water-soluble polyelectrolyte complex nanoparticles (PEC NPs), consisting of sodium carboxylmethyl cellulose (CMCNa) and poly(methacryloxyethyl trimethyl ammonium chloride) (PDMC), were studied as novel templates for biomimetic mineralization. Barium acetate and sodium sulfate solutions were added simultaneously into CMCNa/PDMC polyelectrolyte complex (PEC) solutions as BaSO4 precursors. Spherical BaSO4 crystals with unique annual ring cross section were synthesized in different concentrations of PEC solution. Energy dispersive X-ray spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and thermogravimetric analysis (TGA) showed that these crystals were composed of ca. 90−95 wt % BaSO4 and 5−10 wt % PEC NPs. Study of ionic strength effect on PEC mineralization revealed that the rigid needle-like structure of PEC is responsible for BaSO4 morphology. In addition, the mineralization differences between CMCNa and CMCNa/PDMC PEC are discussed.

1. INTRODUCTION Biomimetic mineralization is the process of preparing inorganic crystals in the presence of organic molecules as additives. Biomimetic mineralization attracts vigorous interest for it usually leads to crystals with multiscale ordered structures, which are rare in natural minerals. Besides, it is also an easy way to synthesize novel organic/inorganic hybrids.1 CaCO3, BaSO4, BaCO3, and hydroxyapatite (HAP) are the mostly investigated minerals synthesized via biomimetic mineralization.2 Organic additives, sometimes named as templates, are crucial in biomimetic mineralization because they determine morphologies and properties of mineral crystals. Single molecular polyelectrolytes are the most commonly used additives for biomimetic mineralization because they are water-soluble and would interact with mineral ions such as Ca2+ and Ba2+. Polyanions, polycations, and double-hydrophilic block copolymers all have been well studied in biomimetic mineralization.3 Many multicomponent systems have also been used as mineralization templates like self-assembled films,4 peptide/ surfactant complexes,5 and polymer nanoparticles.6 Multicomponent templates may lead to crystals with attractive structures and properties compared with single-component additives; for example, Tang et al.5 synthesized self-assembled organic−hydroxyapatite hybrid elastic crystals with ultra strong mechanical properties via biomimetic mineralization with bovine serum albumin/sodium bis(2-ethylhexyl) sulfosuccinate complex as templates. However, as an important type of multicomponent polymers, polyelectrolyte complexes are least used in biomimetic mineralization for their insolubility and lack of suitable mineralization sites. Though polyelectrolyte complex © 2012 American Chemical Society

structure (layer-by-layer self-assembled PSS/PAH capsules) was reported to be used in the synthesis of BaCrO4 nanofibers, it only works in the heterogeneous nucleation step instead of throughout the whole mineralization process.7 Biomimetic mineralization of polyelectrolyte complexes is still waiting to be explored. Polyelectrolyte complexes (PECs) are mixtures of positively and negatively charged polyelectrolytes blended at the molecular level. Since first investigated by A. S. Michaels8 in the 1960s, PECs have been widely used in separation membranes and biomedical materials. Stoichiometrically combined PECs are insoluble and infusible solids, which are limited in practical applications. However, PECs would be water-soluble if stabilized by excessive surface hydrophilic groups such as PEO chains,9 carboxyl10 and amide groups.11 We recently reported a novel type of water-soluble PECs, which are high performance membrane materials for pervaporation and nanofiltration.12 Moreover, it was found that their aqueous dispersion is composed of needle-like PEC nanoparticles (NPs), which are stabilized by excessive carboxyl groups in an aqueous environment.13 Excessive free carboxyl groups may act as binding sites for mineral ions such as Ca2+ and Ba2+, so these new types of water-soluble PECs could break limitations of traditional PECs and be potential multicomponent templates for biomimetic mineralization. Received: January 12, 2012 Revised: March 25, 2012 Published: April 9, 2012 2382

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without stirring in a 1000 mL beaker. The concentration of CMCNa/ PDMC PEC solution was varied from 0.05 g/L to 2.0 g/L, and the dropping rate was controlled at about 20 mL/h. With the addition of Na2SO4 and Ba(OAc)2, PECs solution became opaque and BaSO4 precipitates were observed. BaSO4 precipitate was collected with a piece of 6 × 6 mm silicon wafer placed at the bottom of the beaker previously. When the reaction was finished, the wafer was deposited with a layer of barium sulfate crystals, and the obtained crystals were kept in their mother solution for 72 h for full equilibrium, washed several times, and finally dried at 35 °C for 24 h. The reaction was described with the following formula.

In this study, the water-soluble PEC sodium carboxylmethyl cellulose (CMCNa)/poly(methacryloxyethyl trimethyl ammonium chloride) (PDMC) was proven to be an effective template for biomimetic mineralization synthesis of BaSO4. Spherical BaSO4 with annual ring-like cross section was synthesized with CMCNa/PDMC PEC NPs as templates. BaSO4 microspheres14 and CaCO3 spherulites15 with radiating cross sections have been reported elsewhere, but annual ring like structure is first reported here.

2. EXPERIMENTAL SECTION

Ba(OAc)2 + Na 2SO4 + PECs → BaSO4 /PECs ↓ + 2NaOAc

2.1. Materials. CMCNa (sodium carboxymethylcellulose 300− 800) was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai. Sodium sulfate, barium acetate, hydrochloric acid, sodium hydroxide, and sodium chloride were all of A.R. grade and were used as received without further purification. Deionized water (resistance was 18 MΩ·cm) was used in experiments. PDMC was polymerized from monomer methacryloxyethyl trimethylammonium chloride (Alfa Aesar) via free radical polymerization in aqueous solution. 2.2. Synthesis of CMCNa/PDMC Polyelectrolyte Complex. Water-soluble CMCNa/PDMC polyelectrolyte complex was synthesized via an “acid-protection and alkali-deprotection” route, as we reported previously.13a,b First, CMCNa and PDMC were dissolved in 0.005 M HCl solution, respectively. The concentrations of CMCNa and PDMC were both 0.01 M. Then PDMC solution was added into CMCNa solution dropwise with vigorous stirring. Precipitates of CMCNa/PDMC complex were obtained at the end point (mole ratio of PDMC to CMCNa is about 0.46 as reported previously13a). Wet and gel-like CMCNa/PDMC PEC was soaked and rinsed in deionized water several times. Firm solid PEC was obtained after drying at 60 °C to a constant weight. Finally, PEC solid was dissolved in 0.10 M NaOH solution. The volume of NaOH solution was controlled so that the final pH of PEC solution was approximately 7−8. Stock solution of PEC with a concentration of 20.0 g/L was prepared and then diluted to low concentration for future use.

For the sake of convenience, BaSO4 synthesized in x g/L CMCNa/ PDMC PEC solution was denoted as BaSO4/PEC-x; for example, BaSO4 synthesized in 0.50 g/L CMCNa/PDMC PEC solution was named as BaSO4/PEC-0.50. 2.4. Characterization. Morphologies of BaSO4 particles on the silica wafer were observed with field emission scanning electronic microscopy (FESEM, Hitachi S4800), operating at an accelerating voltage of 3.0 kV. Energy dispersive X-ray spectrum (EDX) was also collected on Hitachi S4800. All samples were Pt-coated prior to FESEM observation. The cross-sectional morphology of BaSO4 particles was also observed with FESEM, and the sample was prepared as follows. First was dispersion of BaSO4 crystals in 2.0 wt % CMCNa/ PDMC solution with ultrasonication, and then the mixture was cast on glass substrate. After drying in a 60 °C oven, the thin polymer film was peeled away from the glass and was fractured in liquid nitrogen. Crosssectional morphology of BaSO4 crystals could be observed on the broken cross section. X-ray diffraction (XRD) patterns of BaSO4 crystals were collected on a Rigaku D/max-2550pc instrument with monochromatized Cu Kα radiation (λ = 0.154 nm). The scanning steps of 0.02°/s and 2θ range from 10 o to 50° were selected to analyze the crystal structure. Fourier transform infrared (FT-IR) spectroscopy spectra were collected with a BRUKER VECTOR 22 FT-IR spectrometer by dispersing BaSO4 crystal powders in KBr slides after grinding. Particle sizes of polyelectrolyte and PEC NPs in various NaCl concentrations were measured in an aqueous environment with dynamic light scattering (DLS, 90Plus/BI-MAS) at 25 °C. Concentration of DLS samples was all 0.005 wt %, and all solutions were filtered through a 2-μm microfiltration membrane before testing. Thermogravimetric analysis (TGA) was carried out on PerkinElmer Pyris 1 TGA, from room temperature to 800 °C at a heating rate of 20 °C/min in air atmosphere. All samples were dried in a vacuum oven at 35 °C for three days before testing.

Scheme 1. Structure of CMCNa/PDMC PEC Nanoparticle

3. RESULTS AND DISCUSSION 3.1. Mineralization of CMCNa/PDMC PEC NPs. The threshold task of this study was to confirm that water-soluble CMCNa/PDMC PEC NPs could be used as a template for mineralization. Barium sulfate was synthesized as targeted crystals in various concentrations of aqueous CMCNa/PDMC PEC solutions. Its morphology and structure were characterized with FESEM, EDX, XRD, and FT-IR, and were compared with BaSO4 synthesized in pure water and single component CMCNa or PDMC solutions. Morphologies of BaSO4 synthesized in water-soluble CMCNa/PDMC PEC NPs solutions were observed with FESEM (Figure 1a−f, PEC concentration is 2.0 g/L, 1.0 g/L, 0.50 g/L, 0.20 g/L, 0.10 g/L, and 0.05 g/L, respectively). The chemical composition and crystal structure of BaSO4/PEC-0.50 (Figure 1c) CMCNa/PDMC PEC NPs were analyzed with EDX and XRD, confirming that the major mass of these particles was BaSO4 crystals (Figures S1 and S2, Supporting Information). In Figure 1d,e, where CMCNa/PDMC PEC concentrations are relatively low, BaSO4 crystal presents flat

Structure of water-soluble CMCNa/PDMC PEC is shown in Scheme 1 and has been well studied. It was proved to be negatively charged with excessive carboxyl groups, existing as needle-like nanoparticles with a length of 300−550 nm and width of about 50 nm.13a,16 CMCNa and PDMC inside the PEC particles were combined together by electrostatic force between carboxyl groups and quaternary ammonium groups located on PDMC polymer chains, while excessive free carboxyl groups contribute to the water solubility of PEC. These free carboxyl groups were expected to be active functional groups to control the crystallization of BaSO4. 2.3. Mineralization of PEC NPs. Barium sulfate was synthesized as follows. 50 mL of 0.02 M Na2SO4 and 50 mL of 0.02 M Ba(OAc)2 solution were simultaneously dropped into 100 mL of PEC solution 2383

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Figure 1. BaSO4 crystal synthesized in CMCNa/PDMC complex, CMCNa, and PDMC solutions. (a−f) BaSO4 synthesized in CMCNa/PDMC PEC solutions with concentrations of 2.0 g/L, 1.0 g/L, 0.50 g/L, 0.20 g/L, 0.10 g/L and 0.05 g/L, respectively; (g) in 0.50 g/L CMCNa; (h) in 0.50 g/L PDMC; (i) pure water free of additives.

cm−1 is that of BaSO4. The ν(CO) absorption peaks of the ionized carboxyl group (1591 cm−1, Figure 2a−d) in BaSO4/ PEC show a red shift of about 14 cm−1, compared with that in CMCNa/PDMC (1604 cm−1). This red shift indicates that −CO bonds in carboxyl groups would be weakened, for Ba2+ would reduce electron density of carboxyl groups through electrostatic attraction. TGA (Figure 3) shows that these particles are composed of about 90−95 wt % BaSO4 and about 5−10 wt % CMCNa/PDMC PEC.

disk morphologies. When the CMCNa/PDMC concentration is as low as 0.05 g/L, BaSO4 particles show lamellar edges and rigid corners. When concentration of CMCNa/PDMC increases, BaSO4 particles turn to be spherical (Figure 1a−c). The surfaces of BaSO4 spheres are rough and have many pores with diameters of several hundred nanometers. Figure 1d (0.20 g/L CMCNa/PDMC) is the transition point between disk and spherical morphology. This set of images shows that the morphology of BaSO4 crystal is changed with the addition of CMCNa/PDMC PEC NPs. Chemical composition of BaSO4 crystals were analyzed with FT-IR and TGA, and these particles were confirmed to be a hybrid of BaSO4 crystals and CMCNa/PDMC. FT-IR spectra lines a−d in Figure 2 show coexistence of absorption peaks of CMCNa/PDMC and BaSO4, confirming that it is a hybrid of BaSO4 and CMCNa/PDMC. Peaks in the range of 1300 cm−1 to 1800 cm−1 are absorptions of CMCNa/PDMC and 605

Figure 3. Thermal gravimetric analysis of BaSO4 synthesized in CMCNa/PDMC PEC NPs solutions. (a−d) Concentrations of PEC are 2.0 g/L, 1.0 g/L, 0.50 g/L, 0.20 g/L, respectively.

It can be inferred from the discussions above that CMCNa/ PDMC PEC NPs can act as effective templates for biomimetic mineralization of BaSO4. BaSO4 crystals turn out to be spherical with rough surface in high concentrations of CMCNa/PDMC PEC solutions, while disk-like in low concentrations. The mole ratio of PDMC to CMCNa can be tuned by adjusting the concentration of HCl used for PEC preparation (Section 2.2). Change of the PDMC to CMCNa

Figure 2. FT-IR spectrum for BaSO4 synthesized in CMCNa/PDMC PEC nanoparticle solutions. (a−d) Concentrations of PEC are 2.0 g/ L, 1.0 g/L, 0.50 g/L, 0.20 g/L, respectively. 2384

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Figure 4. Stepwise growth and cross-sectional morphology of BaSO4 synthesized in 0.50 g/L CMCNa/PDMC solution; Volumes of 0.02 M Na2SO4 and Ba(OAc)2 solutions added. (a) 0 mL, (b) 2.5 mL, (c) 5 mL, (d) 10 mL, (e) 50 mL, (f) cross-sectional morphology of e.

Figure 5. Barium sulfate mineralization process of CMCNa/PDMC PEC nanoparticles.

positively charged polyelectrolyte PDMC is not. CMCNa has been reported to regulate the growth of other crystals like CaCO3,17 BaCO3,18 and Cd(OH)2.19 It can be seen that CMCNa is an effective template for biomimetic mineralization synthesis of BaSO4, while PDMC is not. So in the case of CMCNa/PDMC PEC NPs, it is only the CMCNa segment that is responsible for mineralization of BaSO4. CMCNa/ PDMC PEC NPs are composed of CMCNa and PDMC chains, which are entangled together by interactions between opposite charges. Like single-component CMCNa, the effective sites for biomimetic mineralization are free carboxyl groups. In fact, most of the postive quaternary ammonium groups in PDMC chains are combined with excessive carbonyl groups in CMCNa chains and are not available for inorganic crystals. 3.2. Formation Mechanisms of Spherical BaSO4/PEC. The above discussion revealed that CMCNa/PDMC PEC NPs can be used as templates to synthesize spherical BaSO4 with rough surface. Stepwise growth of BaSO4 experiments revealed its formation mechanism of BaSO4/PEC. Figure 4a is the image of pristine CMCNa/PDMC PEC NPs, which shows clear needle-like structures. With the addition of Ba(OAc)2 and Na2SO4 (both of which are 2.5 mL), BaSO4 first nucleates on PEC NPs (Figure 4b), and rod-like particles about 1−2 μm in length are formed. Large particles about 2 μm in diameter were

ratio did not change the morphology of BaSO4 particles (Figure S3 Supporting Information). Water-soluble PEC NPs can also be prepared with CMCNa and other kinds of polycations like poly(diallyldimethylammonium chloride) (PDDA) and chitosan (CS). CMCNa/PDDA13b and CMCNa/CS13c PEC NPs could also be templates for biomimetic mineralization of BaSO4, which is also similar in shape with those reported here (Figure S4 Supporting Information). We also tried to synthesize CaCO3 with CMCNa/PDMC template, and also obtained spherical CaCO3 particles with rough morphologies (Figure S5 Supporting Information). In order to understand the different contributions of CMCNa and PDMC segments of CMCNa/PDMC PEC NPs to the formation of BaSO4 spheres, biomimetic behaviors of single component CMCNa and PDMC were studied as controlled groups. Figure 1g−i shows different morphologies of BaSO4 synthesized in 0.50 g/L CMCNa, in 0.50 g/L PDMC and free of polyelectrolyte additives. BaSO4 with no additives (Figure 1i) is lamellar with distinct edges and corners, which indicates a single crystal. Morphology of BaSO4 grown in 0.50 g/L PDMC (Figure 1h) is similar to that with no additives (Figure 1i), while BaSO4 grown in 0.50 g/L CMCNa (Figure 1g) is peanut-like with a smooth surface. CMCNa turns out to be able to change the morphology of BaSO4, while the 2385

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Figure 6. Effect of NaCl on morphology of BaSO4 crystals. BaSO4 crystals from 0.50 g/L PEC solution with (a) 0 g/L NaCl, (b) 0.25 g/L NaCl, (c) 0.50 g/L NaCl, (d) 1.0 g/L NaCl, (e) BaSO4 crystals from 0.50 g/L CMCNa solution with 1.0 g/L NaCl, (f) BaSO4 crystals from 1.0 g/L NaCl solution, free of polymer.

According to the proposed mechanism, the needle-like structure is crucial in the formation of spheroid and rough crystals, for it guides the formation of rod-like primary mineralized BaSO4 particles. No surface pores would be expected if PEC templates were disintegrated, and this assumption was verified by ionic strength effect on the morphologies of BaSO4 crystals. A certain amount of NaCl was added into CMCNa/PDMC PEC NPs solution before mineralization of BaSO4.. BaSO4 crystals synthesized in CMCNa/PDMC solution with no more than 0.5 M NaCl appeared quite rough in surface (Figure 6a,b,c), while it turned to smooth when NaCl concentration was as high as 1.0 M (Figure 6d). The change of surface roughness coincides with the change of hydrodynamic radius monitored by dynamic light scattering, as shown in Figure 7.

observed at the wafer when the added volume was above 5 mL (Figure 4c,d) and they might be the cores for further crystal growth. These large particles could not suspend in water stably and would precipitate. With further addition of Ba(OAc)2 and Na2SO4, diameters of precipitates would expand to be 5−7 μm, as shown in Figure 4e. Cross section of a BaSO4 particle presents an annual ring structure. In addition, it is quite interesting that the periodic intervals between rings are about 400 nm, which coincides with the sizes of rod-like particles, as shown in Figure 4d, indicating that these ring structures might consist of rod-like particles. The growth of crystals might be involved with step-by-step aggregations of rod-like particles. From the above analysis of BaSO4/PEC, a formation process of BaSO4 crystals is proposed as depicted in Figure 5. Ba2+ first aggregates in the vicinity of CMCNa/PDMC PEC NPs because of the static electrostatic interactions between Ba2+ and carboxylic groups, followed by primary BaSO4 crystals formation on the surface of PEC NPs. The primary crystals of BaSO4 are about 19.5 nm in size, evaluated with the Sherrer equation (Table S1 Supporting Information). The interaction between PEC NPs and Ba2+ has been verified by red shift of carboxyl peak in FT-IR. With increasing BaSO4 mineralized on CMCNa/PDMC PEC NPs, they will aggregate due to a decrease in solubility and shielding of free charges, and the building blocks fused into a single particle. BaSO4 mineralized PEC NPs grow and aggregate outwardly in all directions, and thus spherical and annual ring structures are formed. Colfen1a,18 has also reported formation of large crystal particles via a “stacking and fusing” process, so-called nonclassic crystallization.1a,20 The irregular shapes of rod-like particles determine that they could not be assembled together tightly, and thus large pores are left both inside and outside the surface of crystals. Pores formation during assembly of microparticles was also reported elsewhere.21 BaSO4 synthesized in 0.50 g/L CMCNa/PDMC solution has a surface area of 48.9 m2/g measured with BET absorption method, which is much larger than BaSO4 synthesized in pure water (0.15 m2/g). The surface area 48.9 m2/g is comparable to some mesostructured BaSO4 reported in the literature.22 The surface area increase may be caused by surface roughness and loosened stacking of building particles. To support the mechanism suggested above, the effect of ionic strength on the morphology of BaSO4 crystal was studied.

Figure 7. Effect of NaCl on particles sizes of CMCNa/PDMC complex.

The hydrodynamic radius of CMCNa/PDMC PEC complex decreases with increasing NaCl concentration. This decrease happens because PEC NPs disintegrate into smaller subunits due to the presence of NaCl.23 CMCNa/PDMC still exists as associated PEC NPs below 0.50 M NaCl, whose particle sizes are above 200 nm and results in irregular crystals (Figure 6a− c), while its particle size decreases to ca. 35 nm at 1.0 M NaCl, close to that of single CMCNa chains, which indicates complete disintegration of PEC NPs, resulting in smoother crystals (Figure 6d). Compared with that in Figure 6a−c, crystals in Figure 6d are more similar to that in Figure 6e, which were 2386

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Though CMCNa/PDMC PEC NPs consisted of CMCNa and PDMC, it exerted a total different effect on the crystallization of BaSO4 compared with that of singlecomponent CMCNa or PDMC. BaSO4 crystals synthesized in CMCNa/PDMC PEC NPs were rough in surface, while those synthesized in CMCNa had a smooth surface. Surface pores of BaSO4 crystals would disappear if CMCNa/PDMC complexes were disintegrated at high concentrations of NaCl. In comparison with mineralization of CMCNa, conformation rigidity of CMCNa/PDMC PEC NPs may be one reason for the roughness of BaSO4 crystals. We can conclude that, in biomimetic mineralization of multi-component systems, crystal morphology is determined by both conformation structure and chemical composition of the templates system.

synthesized in 0.50 g/L CMCNa solution with 1.0 M NaCl. Both crystals morphology should be dominated by CMCNa chains instead of NaCl salt, considering pure NaCl solution would lead to polyhedral BaSO4 crystals instead of spherical ones (Figure 6f). From the discussion above, it can be seen that, if disintegrated at high ionic strength, CMCNa/PDMC PEC NPs would no longer guide the formation of spherical crystals with surface pores. In biomimetic mineralization of multicomponent polyelectrolyte, both chemical compositions and aggregation structures of templates determine the morphologies of crystals. CMCNa/PDMC PEC NPs would also disintegrate at low pH, and a study about the pH effect on the morphology of BaSO4 also reached the same conclusion (Figures S6 and S7 Supporting Information). An interesting question is why CMCNa/PDMC PEC works differently from single-component CMCNa in the biomimetic mineralization synthesis of BaSO4 crystals, although both active sites for mineralization are CMCNa segments. CMCNa/ PDMC PEC NPs templated BaSO4 crystal (Figure 1a−c) is rough, while CMCNa templated crystal (Figure 1g) is smooth. One difference between CMCNa and CMCNa/PDMC PEC NPs is the charge density, that is, the fraction of free −COO− groups. In single-component CMCNa, nearly all −COO− groups were ionized, while they were partial combined with PDMC in CMCNa/PDMC PEC NPs. The fraction of free −COO− groups in PEC depends on the mole ratio of PDMC to CMCNa. Since the morphology of BaSO4 did not change obviously with PEC templates of different PDMC to CMCNa ratios, charge density may not be the main factor that determined crystal morphology (Figure S3, Supporting Information). Another difference between CMCNa and CMCNa/PDMC is conformation change during mineralization. As reported in the literature,24 the CMCNa chains are in extended conformation and will collapse to a spherical state in the presence of divalent metallic ions. Pai24b found that conformation change of polyeletrolyte upon addition of divalent metallic ions plays an important role in the biomimetic mineralization of CaCO3, promoting the formation of the smooth texture of CaCO3. In mineralization of BaSO4, CMCNa should work in the same way, collapsing to spherical conformation with the addition of Ba2+ ions, leading to a smooth surface. On the other hand, CMCNa chains in CMCNa/PDMC PEC NPs were confined in the rigid needlelike PEC structure and would not collapse upon addition of Ba2+, leading to a rough surface.

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ASSOCIATED CONTENT

S Supporting Information *

EDX and powder XRD patterns of BaSO4/PEC-0.5. Estimation of primary crystal size with the Scherrer equation. Morphology of BaSO4 crystals synthesized with CMCNa/PDMC of different PDMC to CMCNa ratios. Morphology of BaSO4 crystals synthesized in CMCNa/PDDA and CMCNa/CS solution. Morphology of CaCO3 synthesized in CMCNa/ PDMC solution. The pH effect on the morphology of BaSO4 crystals. These material are available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (+86)-571-87953780; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by NNSFC (No. 51173160, 50633030), the National Basic Research Program of China (No. 2009CB623402), Zhejiang Provincial Natural Science Foundation of China (No. Y4100250), and Key Innovation the Fundamental Research Funds for the Central Universities (MOE Engineering Research Center of Membrane and Water Treatment Technology, Zhejiang University).

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REFERENCES

(1) (a) Colfen, H.; Yu, S. H. MRS Bull. 2005, 30 (10), 727−735. (b) Patel, P. A.; Eckart, J.; Advincula, M. C.; Goldberg, A. J.; Mather, P. T. Polymer 2009, 50 (5), 1214−1222. (2) (a) Zhu, J. H.; Yu, S. H.; Xu, A. W.; Colfen, H. Chem. Commun. 2009, 9, 1106−1108. (b) Sommerdijk, N.; de With, G. Chem. Rev. 2008, 108 (11), 4499−4550. (c) Palmer, L. C.; Newcomb, C. J.; Kaltz, S. R.; Spoerke, E. D.; Stupp, S. I. Chem. Rev. 2008, 108 (11), 4754− 4783. (d) Wang, T. X.; Colfen, H. Langmuir 2006, 22 (21), 8975− 8985. (e) Li, S.; Zheng, L.; Yu, L. J. Dispersion Sci. Technol. 2011, 32 (4), 601−603. (f) Gries, K.; Heinemann, F.; Gummich, M.; Ziegler, A.; Rosenauer, A.; Fritz, M. Cryst. Growth Des. 2011, 11 (3), 729−734. (g) Geng, X.; Liu, L.; Jiang, J.; Yu, S.-H. Cryst. Growth Des. 2010, 10 (8), 3448−3453. (3) (a) Neira-Carrillo, A.; Acevedo, D. F.; Miras, M. C.; Barbero, C. A.; Gebauer, D.; Colfen, H.; Arias, J. L. Langmuir 2008, 24 (21), 12496−12507. (b) Yu, S. H.; Colfen, H.; Xu, A. W.; Dong, W. F. Cryst. Growth Des. 2004, 4 (1), 33−37. (c) Shkilnyy, A.; Schone, S.; Rumplasch, C.; Uhlmann, A.; Hedderich, A.; Gunter, C.; Taubert, A. Colloid Polym. Sci. 2011, 289 (8), 881−888. (d) Xu, A. W.; Ma, Y. R.; Colfen, H. Biomimetic mineralization. J. Mater. Chem. 2007, 17 (5), 415−449.

4. CONCLUSION CMCNa-based water-soluble PEC NPs CMCNa/PDMC could be used as novel templates for biomimetic mineralization. BaSO4 synthesized in CMCNa/PDMC solutions has rough surfaces and an annual ring-like cross section. FT-IR spectrum confirmed the coexistence of BaSO4 and CMCNa/PDMC PEC in those spherical crystals. TGA showed that BaSO4/PEC-0.50 contained ca. 90−95 wt % BaSO4 and ca. 5−10 wt % CMCNa/ PDMC PEC NPs. A controlled group experiment shows that CMCNa segments are the active site for mineralization of BaSO4. In addition, a stepwise growth mechanism of CMCNa/ PDMC PEC NPs mineralization was proposed. BaSO4 primary crystals first nucleated on PEC NPs and mineralized PEC particles tended to aggregate ring by ring, and finally became large spherical BaSO4. The annual ring-like BaSO4 structure and formation mechanism was first reported here. 2387

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dx.doi.org/10.1021/cg3000476 | Cryst. Growth Des. 2012, 12, 2382−2388