ARTICLE pubs.acs.org/Langmuir
Bioinspired Crystallization of CaCO3 Coatings on Electrospun Cellulose Acetate Fiber Scaffolds and Corresponding CaCO3 Microtube Networks Lei Liu, Dian He, Guang-Sheng Wang, and Shu-Hong Yu* Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, School of Chemistry & Materials, University of Science and Technology of China, Hefei 230026, PR China
bS Supporting Information ABSTRACT: This article describes the mineralization behavior of CaCO3 crystals on electrospun cellulose acetate (CA) fibers by using poly(acrylic acid) (PAA) as a crystal growth modifier and further templating synthesis of CaCO3 microtubes. Calcite film coatings composed of nanoneedles can form on the surfaces of CA fibers while maintaining the fibrous and macroporous structures if the concentration of PAA is in a suitable range. In the presence of a suitable concentration of PAA, the acidic PAA molecules will first adsorb onto the surface of CA fibers by the interaction between the OH moieties of CA and the carboxylic groups of PAA, and then the redundant carboxylic groups of PAA can ionically bind Ca2þ ions on the surfaces of CA fibers, resulting in the local supersaturation of Ca2þ ions on and near the fiber surface, which can induce the nucleation of CaCO3 on the CA fibers instead of in bulk solution. Calcite microtube networks on the macroscale can be prepared by the removal of CA fibers after the CA@CaCO3 composite is treated with acetone. When the CA fiber scaffold is immersed in CaCl2 solution with an extended incubation time, the first deposited calcite coatings can act as secondary substrate, leading to the formation of smaller calcite mesocrystal fibers. The present work proves that inorganic crystal growth can occur even at an organic interface without the need for commensurability between the lattices of the organic and inorganic counterparts.
1. INTRODUCTION Biominerals occurring within living organisms are inorganic organic hybrid materials characterized by hierarchically ordered structures from the nanoscale to the macroscale.1 The organic components, biomacromolecules and organic matrices, direct the nucleation, growth habit, and orientation of inorganic fractions.2 For example, the nacreous layer of sea shells has multilayered structures in which polygonal aragonite platelets cover the insoluble organic matrix and stack along the vertical vector on the organic matrix layers.3 Calcium carbonate (CaCO3) is one of the most intensively studied minerals because of its abundance as a biomineral in nature and also its important applications in paints, plastics, rubbers, and papers.4 Many approaches have been widely chosen to mediate the morphology and polymorph of CaCO3 as extensively reviewed recently.5 Hence, a variety of insoluble matrices, including Kevlar,6 nylon,7 silkworm silk,8 eggshell membranes,9 polymer brushs,10 self-assemblied monolayers (SAMs),11 and liquid crystals12 as well as biomacromolecules13 and double hydrophilic block copolymers (DHBCs),14 as crystal modifiers have been exerted to control the size, shape, phase, and oriented aggregation of CaCO3 crystals. A great number of novel CaCO3 architectures have been achieved, such as pancakes,14a monodisperse spheres,14d concave mesocrystals,15 fibers,16 tubes,17 layered and multilayered structures,18 and complex single crystals.19 Besides, because of its r 2011 American Chemical Society
biocompatibility and nontoxic nature, synthetic CaCO3 structures targeted with other functional nanoparticles can fulfill some biological applications such as drug delivery,20 biosensing,21 and bone regeneration.22 However, CaCO3 macroscopic materials with complicated substructures from the nanoscale to the microscale have been rarely reported. Recently, many research groups have demonstrated that the nucleation, growth, and organization of biogenic CaCO3 minerals were subtly directed by an appropriate microenvironment created by insoluble organic networks fabricated from nano- or microfibers.23 Meanwhile, electrospinning, an interesting and versatile route to produce fibers with homogeneous diameters from the nanoscale to the microscale, has been proposed.24 The electrospun polymeric fibers have a wide variety of applications, including filters, drug-delivery vehicles, and scaffolds for tissue engineering. Calcium phosphate coatings have been achieved on different kinds of electrospun nanofibers, such as poly(L-lactic acid),25 poly(ε-caprolactone),26 and poly(lactic-co-glycolic acid).27 However, these coatings were not well developed either with many impurity nanoparticles or as a compact film covering the whole surface of the scaffold instead of single fibers. Cellulose acetate (CA) Received: February 25, 2011 Revised: April 7, 2011 Published: May 02, 2011 7199
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is the acetate ester of cellulose, one of the most abundant natural polysaccharides on earth. It is hydrophilic, environmentally friendly, biodegradable, and renewable with good processability.28 The most common form of CA has an acetate group on approximately two of every three hydroxyls in the main backbone chains. Therefore, the electrospun CA nanofibers may be an ideal matrix for directing the mineralization of CaCO3, and the templating effect of CA fibers is expected for the large-scale synthesis of CaCO3 tubular structures under mild conditions. In this article, we investigate the mineralization behavior of CaCO3 on an electrospun CA fiber network in the presence of poly(acrylic acid) (PAA) as a crystal modifier. Calcite thin film coatings composed of nanoneedles can be successfully prepared on the surfaces of CA fibers. After removal of the CA fibers from the CaCO3/CA composites by acetone, calcite networks composed of microtubes can be prepared. In addition, it has been found that the calcite thin film coating can serve as a secondary substrate, inducing the formation of thinner calcite fibers when the crystallization time is extended.
2. EXPERIMENTAL SECTION Chemicals and Materials. All chemical reagents were of analytical grade and used as received without further purification. Calcium chloride (CaCl2, Mw = 110.99) and cellulose acetate (CA) were purchased from Shanghai Chemical Reagent Company. Dichloromethane (DCM) and methanol (MeOH) were purchased from Sinopharm Chemical Reagent Co. Ltd. Poly(acrylic acid) (PAA, Mw = 1800) was purchased from Sigma-Aldrich. Distilled water (18.2 Ω cm1) was obtained from simplicity 185 type Millipore apparatus. All glass bottles were soaked in a H2SO4 (98%)/H2O2 (7:3 v/v) solution at 90 °C for 4 h, rinsed with distilled water, and finally dried with acetone. Electrospinning of Cellulose Acetate. CA fibers were prepared by the electrospinning method employing equipment described elsewhere.24 In a typical electrospinning process, CA was dissolved in a DCM/MeOH (7:3 v/v) mixed solvent at a fixed concentration of 7 wt %. The freshly prepared solution was loaded into a 10 mL syringe with a needle at the tip that was fixed to a syringe pump later to squeeze out the CA solution. Then, the solution was electrospun at a 18 kV positive voltage, a 15 cm working distance, and a 0.5 mL/h squeeze rate at room temperature. The electrospun CA fibers were collected on a copper mesh at a collection time of 1 h. Mineralization of CaCO3. The crystallization of calcium carbonate was carried out using a slow gasliquid diffusion technique as described elsewhere.29 In a typical synthesis, 15 mL of an aqueous solution of CaCl2 (20 mM) with different PAA concentrations was poured into a glass bottle. Then a piece of electrospun CA fiber scaffold (1 cm 1 cm) was immersed in the freshly prepared solution for incubation. The bottle was covered with Parafilm, which was punched with four needle holes and placed in a closed desiccator. Another glass bottle filled with NH4HCO3 was also covered with Parafilm, punched with four needle holes, and placed at the bottom of the desiccator. After different periods of incubation at room temperature, the electrospun CA fiber scaffold was taken from the bottle, rinsed with distilled water, and dried at room temperature for further characterization. CaCO3 crystals were also formed at the airwater interface, and the samples floating on water were collected with a piece of glass. Characterization. The final crystals were examined by nitrogen sorption, X-ray powder diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), and selected-area electron diffraction (SAED). Nitrogen sorption results were obtained from a Micromeritics Tristar 3000 analyzer. XRD analyses were carried out on a Philips X’Pert
Figure 1. SEM images of the as-prepared electrospun CA fibers from a 7 wt % CA solution in DCM/MeOH (7:3 v/v) mixed solvent. (a) Lowand (b) high-magnification images of the sample. The inset in image a is a photograph of the electrospun CA fiber scaffold. Pro Super X-ray diffractometer equipped with graphite-monochromatized Cu KR radiation (λ = 1.54056 Å), and the operating voltage and current were maintained at 40 kV and 40 mA, respectively. SEM was performed on a Zeiss Supra 40 field-emission microscope. FT-IR spectra were recorded on a Magna-IR 750. TEM and SAED were performed on a JEOL JSM-2010F transmission electron microscope at 200 kV.
3. RESULTS AND DISCUSSION Figure 1 shows the images of the CA fibers prepared by electrospinning that exhibited a nonwoven fibrous structure. The diameter of the CA fibers ranged from 300 nm to 1.5 μm (Figure 1a). The surfaces of the fibers were very smooth, as shown in Figure 1b. The electrospun CA fiber network had a relatively more uniform porous structure with a high specific surface area of 4.39 m2 3 g1 (Supporting Information, Figure S1), leading to the exposure of more functional groups, such as hydroxyl in the main backbone of CA, to the bulk solution. Thus, it can substantially facilitate the interaction between acidic PAA molecules and CA fibers.18a The characteristic FT-IR absorption peaks of CA were 3467, 1751, 1371, and 1051 cm1, relative to the stretching vibrations of hydroxyl, carbonyl, methyl, and cyclic ester bonds respectively (Supporting Information, Figure S2a).30 The representative SEM images of CaCO3 crystals mineralized on the electrospun CA fibers without and with PAA in CaCl2 solutions are shown in Figure 2. It was found that PAA can dramatically affect the balance between the nucleation and growth of CaCO3 crystals, leading to significant changes in the 7200
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Figure 2. SEM images of the electrospun CA fibers after CaCO3 mineralization for 10 days (a, b) without and (c, d) with the PAA present in CaCl2 solutions. Insets in panel c: Cross-sectional view of a single CaCO3-coated CA fiber (top right) and a photograph of the CA fiber scaffold with uniform CaCO3 coatings (bottom left). The white arrow in the inset of panel c shows the electrospun CA fiber. (d) High-magnification image of the surface of a single fiber in panel c. [CaCl2] = 20 mM, [PAA] = 1 g 3 L1.
Figure 3. XRD patterns of the CaCO3/CA composites after CaCO3 mineralization for 10 days (a) without and (b) with the PAA present in CaCl2 solutions. [CaCl2] = 20 mM, [PAA] = 1 g 3 L1 (JCPDS card no. 72-1650).
final morphology of CaCO3 particles mineralized on CA fibers. When there was no PAA in the CaCl2 solution, large rhombohedral calcite crystals bound by the {104} faces were the dominant product (Figure 2a). The calcite rhombohedras were not well developed but have relief structures on (104) faces and obtuse corners. Some CA fibers were occluded in calcite rhombohedras as shown in Figure 2b. It can be observed that the relative intensity of the (104) faces of calcite in the XRD pattern was extraordinarily strong (Figure 3a). The sharp, strong diffraction peak can be attributed to the high crystallinity and large size of the calcite rhombohedras. While under the control of PAA, CaCO3 coatings composed of a large number of particles formed on each CA fiber after crystallization for 10 days at room temperature (Figure 2c). The
detailed surface structure of a single CaCO3-coated CA fiber is shown in Figure 2d, from which it can be seen that the surfaces of the fibers became very coarse. These particles, which can be considered to be nanoneedle aggregates, were compact and stacked tightly against each other (Figure 2d). The diameter of CaCO3-coated CA fibers was in the range of 1 to 2 μm, and a CaCO3 layer has a thickness of about 400 nm (top right inset in Figure 2c). All four characteristic absorption peaks of CA can also be observed in the FT-IR spectrum (Supporting Information, Figure S2b). The absorption peaks at 872 and 712 cm1 belong to calcite.31 The peak at 1385 cm1 was caused by the overlapping of the 1371 cm1 peak of CA and the 1407 cm1 peak of calcite. Compared with the rhombohedrals formed without PAA, some other diffraction peaks of calcite except for the (104) plane were obvious and can also be indexed as calcite in the XRD pattern shown in Figure 3b. On the basis of the above results, it can be concluded that the addition of PAA induced a transformation from heterogeneous nucleation to homogeneous nucleation in the early crystallization stage of CaCO3. In the case free of PAA, Ca2þ ions cannot adsorb on CA fibers and heterogeneous nucleation would occur in bulk solution with CO2 diffusion into the system. Once the nuclei formed, they suppressed the precursor diffusion in the nearby environment and facilitated the further homogeneous growth of existing crystals. Thus, rhombohedral calcite was formed via heterogeneous nucleation and subsequent homogeneous growth. When the growing CaCO3 crystals encountered the CA fibers, some fibers would be imbedded in these crystals, leading to some visible defects on the crystals (Figure 2b). On the contrary, in the presence of PAA, acidic macromolecules can be adsorbed onto the fiber surface through the interaction between the carboxylic groups of PAA and the OH moieties of CA.18a However, the redundant 7201
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Figure 4. (Top) Low- and (bottom) high-magnification SEM images of CaCO3 crystals deposited on electrospun CA fibers with different PAA contents in CaCl2 solutions. (a, e) 0.01, (b, f) 0.05, (c, g) 0.1, and (d, h) 0.5 g 3 L1. [CaCl2] = 20 mM. The crystallization time is 10 days.
carboxylic groups of PAA can ionically bind Ca2þ ions, so Ca2þ ions would nonselectively gather on the surfaces of CA fibers, which led to the local supersaturation of Ca2þ ions, inducing the homogeneous nucleation and formation of uniform calcite coatings on the fiber surface (Figure 2c). As described above, polyelectrolyte PAA can have a great influence on the growth habit of CaCO3 crystals formed on electrospun CA fibers. Hence, we further investigated the influence of the concentration of PAA on the morphologies of the nanoscale building blocks of the CaCO3 coatings (Figure 4). When the concentration of PAA was very low, for example, 0.01 g 3 L1, calcite film coatings could not be achieved and aggregated particles with a size range of 38 μm were dispersed on the surface of the fiber scaffold (Figure 4a,e). By increasing the concentration of PAA to 0.05 g 3 L1, calcite coatings made up of carpolite-like building blocks were produced on the surface of each CA fiber and the conglomeration phenomena of carpolites on the fiber surfaces became obvious (Figure 4b,f). By further increasing the concentration of PAA up to 0.1 g 3 L1, CaCO3 film coatings can also be obtained (Figure 4c). However, the film coating was not well developed and part of the CA fibers can be observed (Figure 4g). Besides, CaCO3 nanoparticles with irregular morphology instead of an angular shape were observed as shown in Figure 4g. With a relatively higher initial concentration of PAA (0.5 g 3 L1) in CaCl2 solution, it can be seen that CaCO3 film coatings were uniform and the surface was relatively smooth (Figure 4d). The size of spherical CaCO3 nanoparticles decreased to 100270 nm as evaluated from Figure 4h. These results implied that the concentration of PAA played a crucial role in controlling the balance between the nucleation and growth of CaCO3 crystals and mediated the interaction of organic (CA fiber) and inorganic (CaCO3) components, which led to significant variations in the shapes and sizes of the developing crystals. What is more, it is worth noting that the concentration of PAA determined whether the nucleation event occurs on the CA fibers or in bulk solution. The heterogeneous nucleation in bulk solution instead of on the CA fibers becomes dominant if the concentration of PAA is too low (less than 0.05 g 3 L1) (Figure 4a), and homogeneous nucleation on the CA fibers will be enhanced when the concentration of PAA is more than 0.05 g 3 L1 (Figure 4bd). When the concentration of PAA increases, the initial pH value of the bulk solution will decrease. The initial pH can also play a role in the growth habits of CaCO3 crystals, leading to variations in final
shapes and sizes as previously reported.14c,32 To identify the dominant effect of the pH value versus the concentration of PAA, the pH value was adjusted by using HCl from pH 4.67 ([PAA] = 0.01 g 3 L1) to 3.81 ([PAA] = 0.1 g 3 L1). However, there were no significant morphological variations between the two samples obtained before and after adjusting the pH value with the same concentration of PAA ([PAA] = 0.01 g 3 L1), except that the latter sample had a larger size and its building blocks exhibited a 1D growth trend (Supporting Information, Figure S3). These results indicated that the initial pH indeed exerted some influence on the growth habits of CaCO3 crystals but the initial concentration of PAA still played a crucial role in mediating the nucleation event and growth of deposited CaCO3 crystals on CA fibers. In the present case, CaCO3 crystals can also be obtained at the airwater interface, which was different from that of the deposits formed in bulk solution as reported previously in the presence of block copolymers.14c These crystals were in the form of a calcite phase according to their XRD patterns (Supporting Information, Figure S4). Typical SEM images of the product with different concentrations of PAA in CaCl2 solution after crystallization for 10 days are shown in Figure 5. With a PAA concentration of 0.1 g 3 L1, calcite aggregates consisted of small microparticles (Figure 5a). Each microparticle displayed a pyramid-like shape with 3-fold symmetry and truncated structures (Figure 5d). However, when the concentration of PAA increased to 0.5 g 3 L1, no CaCO3 pyramids were found and aggregated calcite assemblies elongated along the crystallographic c-axis direction of calcite were observed (inset in Figure 5e).33 The stair edges of the elongated aggregates were parallel or nearly parallel to each other. Porous architectures composed of spherical nanoparticles can be observed with a PAA concentration of 1 g 3 L1 (Figure 5c,f). Increasing the concentration of PAA will result in a decrease in CaCO3 particle size but an increase in the porosity of whole architectures formed at the airwater interface. The growth process of CaCO3 coatings mineralized on the surfaces of electrospun CA fibers with the same PAA concentration in CaCl2 solutions was followed by time-dependent experiments. The transformation process of CaCO3 crystals can be clearly observed (Figure 6). Initially, when the CA fibers were incubated in CaCl2 solution for 1 day, there were only a few nanoparticles adhering to the surface of each fiber (Figure 6a). After the CA fibers incubated for 5 days, some of the initial nanoparticles aggregated with each other, indicating that a thin CaCO3 coating was developed on the surfaces of CA fibers. 7202
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Figure 5. (Top) Low- and (bottom) high-magnification SEM images of the CaCO3 crystals formed at the airwater interface with different concentrations of PAA in CaCl2 solutions: (a, d) 0.1, (b, e) 0.5, and (c, f) 1 g 3 L1. [CaCl2] = 20 mM. The crystallization time is 10 days.
Figure 6. SEM images of the electrospun CA fibers after mineralization for different reaction times: (a) 1, (b) 5, and (c, d) 15 days. (d) Magnified image of the square area in panel c. [CaCl2] = 20 mM, [PAA] = 1 g 3 L1.
However, the coating was not well developed and not continuous, and the surfaces of CA fibers were still partially exposed to air, maintaining its smooth surface (Figure 6b). As the incubation time extended to 10 days, well-developed, continuous CaCO3 coatings consisting of nanoneedles with coarse surfaces were produced on each CA fiber (Figure 2c). By extending the incubation time to 15 days, the CaCO3 films acted as a second-tier substrate, leading to the formation of thinner CaCO3 fibers (Figure 6c). It can be observed that these smaller CaCO3 fibers protruding from calcite coatings were nanoparticle assemblies with a diameter of 140410 nm (Figure 6c,d). A polymer-induced liquid-precursor (PILP) process was found in a very early mineralization stage of carbonates under the control of acidic polymers such as PAA and polyaspartic acid, which usually led to very smooth polycrystalline coatings.34 Compared with
those studies, the PAA content in the present case was in the typical concentration range for the formation of PILPs. However, liquid precursor droplets of CaCO3 were not observed, and the surface of CaCO3 coatings was rough in the current system. These results indicated that the formation of CaCO3 coatings was not through the PILP process but under the synergetic control of the CA fiber templating effect and the mediation effect of PAA. More detailed information about a single calcite fiber can be further provided by TEM, HR-TEM, and SAED analyses (Figure 7). The diameter of the calcite fiber shown in Figure 7a was consistent with that estimated from their SEM image (Figure 6d). It can be concluded from the HR-TEM image (Figure 7b) that the growth direction of calcite fibers was along the [104] direction. The SAED pattern (Figure 7c) of a single calcite fiber showed the 7203
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Figure 7. TEM image of (a) a single calcite mesocrystal fiber, corresponding to HR-TEM image b, and (c) a SAED pattern.
Figure 8. (a) SEM image of calcite microtubes after the CaCO3/CA composite was treated with acetone for 30 min. (b) Magnified SEM image of the calcite microtubes shown in panel a. (Insets in a): Crossectional view of a single CaCO3 microtube (top right) and a photograph of a CaCO3 network after the removal of CA fibers (bottom left).
monocrystalline nature of calcite fibers. Combining the results of the SAED pattern and SEM and TEM images of a single calcite fiber, it can be confirmed that these calcite fibers were actually mesocrysals. There also existed an amorphous phase in the fiber as shown in the area enclosed in a white circle in Figure 7b. At a lower concentration of PAA, calcite fibers composed of nanoscale building blocks were obtained with an extended incubation time (Supporting Information, Figure S5). The fibers grown at a PAA concentration of 0.1 g 3 L1 exhibited stairlike structures, and nanoscale units with smooth facets of calcite were observed (Supporting Information, Figure S5a). When the PAA
concentration increased to 0.5 g 3 L1, the units became porous and maintained the characteristic rhombohedral shape of calcite (Supporting Information, Figure S5b). The formation mechanism of these fibers can also be attributed to the directed aggregation of a PAA-stabilized precursor composed of nanoscale units. The selective formation of calcite fibers on CaCO3 coatings is a combination of a diffusion-controlled process and an oriented assembly process directed by the microenvironment around them. It can be found that calcite fibers occasionally protruded from single nanoneedle aggregates and had similar diameters (Figure 6d), so the nanoneedle aggregates can be considered to be “seeds” for developing fibers. As fibers initially form, they will suppress the diffusion of the precursor into the nearby environment and facilitate the further growth of fibers. However, the precursor building blocks (i.e., PAA-stabilized crystalline nanoparticles) will continuously form around the fibers on exposure of the solution to CO2 gas. Subsequently, the crystalline precursors assembled to form a fibrous mesocrystal driven by the unit morphology, which was similar to a previous report.35 Figure 8 shows the SEM images of CaCO3/CA composite incubated for 10 days and subsequently treated with acetone for 30 min at room temperature. The treatment resulted in the formation of calcite microtubes by the dissolution of CA fiber cores (top right inset in Figure 8a), which obviously demonstrated the templating effect of CA fibers. The 1D structure was maintained well, and there were no obvious breaks and cracks on the calcite microtubes. Interestingly, there are some spherical nanoparticles adhered to the calcite tube surface (white dots in Figure 8a). From the magnified SEM image of the calcite microtube surface, it can be observed that these microtubes are composed of CaCO3 nanoneedles (Figure 8b) similar to those of calcite coatings (Figure 2d). Because of the large size of these tubes and the layer thickness of the CaCO3 coatings, the hollow nature of the calcite microtubes cannot be observed by TEM (Supporting Information, Figure S6). Hence, the acetone treatment procedure did not destroy the structure of final CaCO3 particles but simply removed electrospun CA fibers. Because of its inhibiting effect on the growth of CaCO3 crystals,36 PAA can induce the formation of CaCO3 films on various 2D solid matrices.18 By combining the influence of PAA with the templating effect of CA fibers, a possible growth mechanism of the calcite film coatings has been proposed as illustrated in Figure 9. Initially, acidic PAA molecules were adsorbed on the surfaces of CA fibers through the interaction between the OH moieties of CA and the carboxylic groups of PAA.18a The PAA macromolecules with redundant carboxylic groups can ionically bind Ca2þ ions on the surfaces of CA fibers, 7204
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Figure 9. Schematic illustration of the growth process of calcite film coatings deposited on electrospun CA fibers based on time-dependent experiments. (a) Adsorption of PAA molecules on the surfaces of CA fibers. (b) CaCO3 nanoparticles stabilized by PAA formed on and close to the surface of CA fibers. (c) CaCO3 nanoparticles aggregate to form isolated intermediates. (d) Intermediates fuse together to form calcite film coatings. (e) Microtubes obtained after calcite-coated CA fibers were treated with acetone. (f) Calcite mesocrystal fibers protruding from the surface of the CaCO3 coating with an extended incubation time. (df) White lines on the CaCO3 coatings denote the boundaries of nanoneedle aggregates.
which led to the local supersaturation of Ca2þ on and close to the fiber surface (Figure 9a). Then, CaCO3 nanoparticles formed via homogeneous nucleation in supersaturated locations and were associated with PAA molecules (Figure 9b). These routes are similar to those occurring in the biomineralization process in vivo.37 Subsequently, CaCO3 nanoparticles tended to aggregate into larger isolated intermediates on the surfaces of CA fibers (Figure 9c). Finally, the intermediates grew larger and coalesced into an integrated calcite film coating (Figure 9d). In a word, the inhibiting effects on the growth of CaCO3 (in the direction vertical to the CA fiber surface) imposed by PAA molecules and the templating effects of CA fibers will result in the formation of calcite film coatings on CA fibers. In addition, calcite-coated CA fibers treated with acetone could eliminate CA fibers, leading to the formation calcite microtubes (Figure 9e). However, with the extended incubation time of CA fibers in CaCl2 solution, smaller calcite fibers would protrude from CaCO3 coatings through the aggregation process of nanoscale building blocks by using CaCO3 coatings as a secondary substrate (Figure 9f).
4. CONCLUSIONS We present the crystallization behavior of CaCO3 crystals on an electrospun cellulose acetate (CA) fiber scaffold using a facile gasliquid diffusion technique and further templating the synthesis of CaCO3 microtubes after the removal of CA fibers. In the absence of PAA, large rhombohedral calcite crystals were bound by {104} faces that studded the CA fiber framework. Calcite film coatings composed of nanoscale building blocks on the surface of each CA fiber can be produced under the control of PAA. The concentration of PAA played a crucial role in controlling the final sizes and shapes of the nanoscale building blocks of CaCO3. Meanwhile, calcite aggregates also formed at the airwater interface. When the incubation time of CA fibers in CaCl2 solutions was extended, thinner calcite fibrous mesocrystals grew along the [104] direction and protruded from the surfaces of the first synthesized CaCO3 film coatings, which acted as a secondary substrate. The formation process of calcite film coatings was attributed to the synergetic effects of inhibiting the effect of PAA and templating the effect of CA fibers. The present work proves that inorganic crystal growth can even occur at an organic interface without the need for commensurability between the lattices of the organic and inorganic counterparts. The asprepared calcite-coated CA fiber frameworks and corresponding
microtube networks may act as ideal candidates in the fields of tissue engineering and drug delivery.
’ ASSOCIATED CONTENT
bS
Supporting Information. Nitrogen sorption isotherm, FT-IR spectra, SEM and TEM images, and XRD patterns. This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Correspondence should be addressed to S. H. Yu, Fax: þ 86 551 3603040, E-mail:
[email protected].
’ ACKNOWLEDGMENT S.-H.Y. acknowledges funding support from the National Basic Research Program of China (2010CB934700), the National Natural Science Foundation of China (NSFC, nos. 91022032 and 50732006), and the International Science & Technology Cooperation Program of China (2010DFA41170). ’ REFERENCES (1) (a) Heuer, A. H.; Fink, D. J.; Laraia, V. J.; Arias, J. L.; Calvert, P. D.; Kendall, K.; Messing, G. L.; Blackwell, J.; Rieke, P. C.; Thompson, D. H.; Wheeler, A. P.; Veis, A.; Caplan, A. I. Science 1992, 255, 1098– 1105. (b) Mayer, G. Science 2005, 310, 1144–1147. (2) (a) Gotliv, B. A.; Addadi, L.; Weiner, S. ChemBioChem 2003, 4, 522–529. (b) Bezares, J.; Asaro, R. J.; Hawley, M. J. Struct. Biol. 2008, 163, 61–75. (3) Addadi, L.; Weiner, S Nature 1997, 389, 912–915. (4) Dalas, E.; Klepetsanis, P.; Koutsoukos, P. G. Langmuir 1999, 15, 8322–8327. (5) (a) Meldrum, F. C.; C€ olfen, H. Chem. Rev. 2008, 108, 4332– 4432. (b) Sommerdijk, N.; de With, G. Chem. Rev. 2008, 108, 4499– 4550. (c) Gower, L. B. Chem. Rev. 2008, 108, 4551–4627. (d) Chen, S. F.; Zhu, J. H.; Jiang, J.; Cai, G. B.; Yu, S. H. Adv. Mater. 2010, 22, 540–545. (6) Fu, G.; Valiyaveettil, S.; Wopenka, B.; Morse, D. E. Biomacromolecules 2005, 6, 1289–1298. (7) Ajikumar, P. K.; Lakshminarayanan, R.; Valiyaveettil, S. Cryst. Growth Des. 2004, 4, 331–335. (8) Cheng, C.; Yang, Y. H.; Chen, X.; Shao, Z. Z. Chem. Commun. 2008, 5511–5513. 7205
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Langmuir (9) Takiguchi, M.; Igarashi, K.; Azuma, M; Ooshima, H. Cryst. Growth Des. 2006, 6, 2754–2757. (10) Tugulu, S.; Harms, M.; Fricke, M.; Volkmer, D.; Klok, H. A. Angew. Chem., Int. Ed. 2006, 45, 7458–7461. (11) (a) K€uther, J.; Seshadri, R.; Knoll, W.; Tremel, W. J. Mater. Chem. 1998, 8, 641–650. (b) An, X. Q.; Cao, C. B. J. Phys. Chem. C 2008, 112, 6526–6530. (12) Nishimura, T.; Ito, T.; Yamamoto, Y.; Yoshio, M.; Kato, T. Angew. Chem., Int. Ed. 2008, 47, 2800–2803. (13) (a) Belcher, A. M.; Wu, X. H.; Christensen, R. J.; Hansma, P. K.; Stucky, G. D.; Morse, D. E. Nature 1996, 381, 56–58. (b) Metzler, R. A.; Evans, J. S.; Killian, C. E.; Zhou, D.; Churchill, T. H.; Appathurai, N. P.; Coppersmith, S. N.; Gilbert, P. U. P. A. J. Am. Chem. Soc. 2010, 132, 6329–6334. (14) (a) Chen, S. F.; Yu, S. H.; Wang, T. X.; Jiang, J.; C€olfen, H.; Hu, B.; Yu, B. Adv. Mater. 2005, 17, 1461–1465. (b) Gao, Y. X.; Yu, S. H.; Cong, H. P.; Jiang, J.; Xu, A. W.; Dong, W. F.; C€olfen, H. J. Phys. Chem. B 2006, 110, 6432–6436. (c) Gao, Y. X.; Yu, S. H.; Guo, X. H. Langmuir 2006, 22, 6125–6129. (d) Guo, X. H.; Yu, S. H.; Cai, G. B. Angew. Chem., Int. Ed. 2006, 45, 3977–3981. (15) (a) Wang, T. X.; Antonietti, M.; C€olfen, H. Chem.—Eur. J. 2006, 12, 5722–5730. (b) Geng, X.; Liu, L.; Jiang, J.; Yu, S. H. Cryst. Growth Des. 2010, 10, 3448–3453. (16) (a) Olszta, M. J.; Gajjeraman, S.; Kaufman, M.; Gower, L. B. Chem. Mater. 2004, 16, 2355–2362. (b) Zhu, J. H.; Song, J. M.; Yu, S. H.; Zhang, W. Q.; Shi, J. X. CrystEngComm 2009, 11, 539–541. (17) Fan, Y. W.; Wang, R. Z. Adv. Mater. 2005, 17, 2384–2388. (18) (a) Kato, T. Adv. Mater. 2000, 12, 1543–1546. (b) Kotachi, A.; Miura, T.; Imai, H. Cryst. Growth Des. 2006, 6, 1636–1641. (c) Wei, H.; Ma, N.; Shi, F.; Wang, Z. Q.; Zhang, X. Chem. Mater. 2007, 19, 1974–1978. (19) (a) Park, R. J.; Meldrum, F. C. Adv. Mater. 2002, 14, 1167–1169. (b) Li, C.; Qi, L. M. Angew. Chem., Int. Ed. 2008, 47, 2388–2393. (c) Finnemore, A. S.; Scherer, M. R. J.; Langford, R.; Mahajan, S.; Ludwigs, S.; Meldrum, F. C.; Steiner, U. Adv. Mater. 2009, 21, 3928–3932. (20) (a) Wei, W.; Ma, G. H.; Hu, G.; Yu, D.; Mcleish, T.; Su, Z. G.; Shen, Z. Y. J. Am. Chem. Soc. 2008, 130, 15808–15810. (b) Cheang, T. Y.; Wang, S. M.; Hu, Z. J.; Xing, Z. H.; Chang, G. Q.; Yao, C.; Liu, Y.; Zhang, H.; Xu, A. W. J. Mater. Chem. 2010, 20, 8050–8055. (21) Won, Y. H.; Jang, H. S.; Chung, D. W.; Stanciu, L. A. J. Mater. Chem. 2010, 20, 7728–7733. (22) (a) Popescu, D. C.; van Leeuwen, E. N. M.; Rossi, N. A. A.; Holder, S. J.; Jansen, J. A.; Sommerdijk, N. A. J. M. Angew. Chem., Int. Ed. 2006, 45, 1762–1767. (b) Yu, H. D.; Zhang, Z. Y.; Win, K. Y.; Chan, J.; Teoh, S. H.; Han, M. Y. Chem. Commun. 2010, 46, 6578–6580. (23) (a) Fabritius, H.; Walther, P.; Ziegler, A. J. Struct. Biol. 2005, 150, 190–199. (b) Nudelman, F.; Shimoni, E.; Klein, E.; Rousseau, M.; Bourrat, X.; Lopez, E.; Addadi, L.; Weiner, S. J. Struct. Biol. 2008, 162, 290–300. (c) Chien, Y. C.; Hincke, M. T.; Vali, H.; McKee, M. D. J. Struct. Biol. 2008, 163, 84–99. (24) (a) Greiner, A.; Wendorff, J. H. Angew. Chem., Int. Ed. 2007, 46, 5670–5703. (b) Li, D.; Xia, Y. N. Adv. Mater. 2004, 16, 1151–1170. (25) Chen, J. L.; Chu, B.; Hsiao, B. S. J. Biomed. Mater. Res. A 2006, 79A, 307–317. (26) Yang, F.; Wolke, J. G. C.; Jansen, J. A. Chem. Eng. J. 2008, 137, 154–161. (27) (a) Li, X. R.; Xie, J. W.; Lipner, J.; Yuan, X. Y.; Thomopoulos, S.; Xia, Y. N. Nano Lett. 2009, 9, 2763–2768. (b) Madurantakam, P. A.; Rodriguez, I. A.; Cost, C. P.; Viswanathan, R.; Simpson, D. G.; Beckman, M. J.; Moon, P. C.; Bowlin, G. L. Biomaterials 2009, 30, 5456–5464. (28) (a) Ding, B.; Kimura, E.; Sato, T.; Fujita, S.; Shiratori, S. Polymer 2004, 45, 1895–1902. (b) Luong, N. D.; Lee, Y. K.; Nam, J. D. Eur. Polym. J. 2008, 44, 3116–3121. (29) (a) Albeck, S.; Aizenberg, J.; Addadi, L.; Weiner, S. J. Am. Chem. Soc. 1993, 115, 11691–11697. (b) Albeck, S.; Weiner, S.; Addadi, L. Chem.—Eur. J. 1996, 2, 278–284. (30) Chen, W. J.; Su, Y. L.; Zheng, L. L.; Wang, L. J.; Jiang, Z. Y. J. Membr. Sci. 2009, 337, 98–105.
ARTICLE
(31) Shen, Q.; Wang, L. C.; Li, X. P.; Liu, F. L. Cryst. Growth Des. 2008, 8, 3560–3565. (32) (a) Li, M.; Mann, S. Adv. Funct. Mater. 2002, 12, 773–779. (b) Liu, L.; Hu, B.; Chen, S. F.; Liu, S. J.; Jiang, J.; Cai, G. B.; Yu, S. H. CrystEngComm 2010, 12, 3593–3598. (33) Neira-Carrillo, A.; Pai, R. K.; Fernandez, M. S.; Carreno, E.; Quitral, P. V.; Arias, J. L. Colloid Polym. Sci. 2009, 287, 385–393. (34) (a) Olszta, M. J.; Gajjeraman, S.; Kaufman, M.; Gower, L. B. Chem. Mater. 2004, 16, 2355–2362. (b) Cheng, X. G.; Gower, L. B. Biotechnol. Prog. 2006, 22, 141–149. (c) Zhu, J. H.; Song, J. M.; Yu, S. H.; Zhang, W. Q.; Shi, J. X. CrystEngComm 2009, 11, 539–541. (35) Kim, Y. Y.; Kulak, A. N.; Li, Y. T.; Batten, T.; Kuball, M.; Armes, S. P.; Meldrum, F. C. J. Mater. Chem. 2009, 19, 387–398. (36) Sugawara, A.; Ishii, T.; Kato, T. Angew. Chem., Int. Ed. 2003, 42, 5299–5303. (37) Addadi, L.; Weiner, S. Angew. Chem., Int. Ed. 1992, 31, 153–169.
7206
dx.doi.org/10.1021/la200738n |Langmuir 2011, 27, 7199–7206
