Fabrication of Hydroxyapatite Hierarchical Hollow Microspheres and

Jan 26, 2012 - Hydroxyapatite nanorod-assembled hierarchical microflowers: rapid synthesis via microwave hydrothermal transformation of CaHPO 4 and th...
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Fabrication of Hydroxyapatite Hierarchical Hollow Microspheres and Potential Application in Water Treatment Shu-Dong Jiang,† Qi-Zhi Yao,‡ Gen-Tao Zhou,*,† and Sheng-Quan Fu§ †

CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, ‡School of Chemistry and Materials, and §Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, P. R. China. ABSTRACT: Many efforts have been made in fabricating three-dimensional (3D) ordered hydroxyapatite (Ca10(PO4)6(OH)2, HAp) nanostructures due to their growing applications as a bone cement, drug deliverer, tooth paste additive, dental implant, gas sensor, ion exchange, catalyst, etc. Here, we developed a new synthetic route to 3D HAp-based hollow microspheres through a water-soluble biopolymer (polyaspartic acid) assisted assembly from HAp nanorods. The as-obtained products were characterized by Xray diffraction (XRD), field emission scanning electron microscopy (FESEM), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), highresolution TEM (HRTEM), and Brunauer−Emmett−Teller (BET) gas sorptometry. SEM and TEM results showed that 3D HAp hollow microspheres are constructed by a number of one-dimensional (1D) nanorods as primary building units. The influences of the additive polyaspartic acid and reaction time on final morphology and assembled structure of the products were systematically investigated. On the basis of our experimental results, a phenomenological elucidation of the mechanism for growth of the hollow HAp architectures has been proposed. The time-dependent experiments unveil that the HAp hollow microspheres are fabricated following initial formation and subsequent transformation of amorphous calcium phosphate (ACP) spheres. In-depth investigations, based on control experiments and FT-IR, EDX, and XPS analyses, reveal that polyaspartic acid acts as both a chelating and a surface capping agent in the synthesis process. First, polyaspartic acid molecules via calcium ion accumulation induce formation of ACP. At the subsequent stage Ostwald ripening contributes to formation of the hollow microspheres, and polyaspartic acid molecules capping to the surface of HAp crystallites control growth of the short nanorod subunits. Moreover, the adsorption experiments of the hierarchical hollow HAp for different heavy metal ions were conducted, and the results exhibit that the hierarchical hollow HAp have unique selective adsorption activity for heavy metal Pb2+. In-depth investigation is still in progress.

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including hard templates such as polystyrene latex spheres,16 spherical silica,17 and carbon spheres18 and soft templates such as emulsion droplets,19 micelles,20 vesicles,21 and gas bubbles22 were extensively employed to prepare hollow structures. However, template-directed methods usually require tedious procedures, including surface modification, precursor attachment, and core removal. Thus, the search for template-free, simple, mild, high-yield, and environmentally benign methods to synthesize microspheres or nanospheres with a hollow interior is still an ongoing process and represents a major challenge. Recently, utilization of some physical phenomena, such as the Kirkendall effect,23 Ostwald ripening,24 and oriented attachment process,25 to fabricate hollow structures provides new alternative opportunities for template-free fabrication of hollow structures. Calcium phosphates are the main mineral constituents of bones and teeth. Most calcium phosphates are only frugally

INTRODUCTION Recently, the design and synthesis of materials with specific morphologies have attracted considerable attention because the dimensional and structural characteristics of these materials endow them with a wide range of remarkable properties and potential applications.1−3 Specifically, fabrication of hierarchical and complex nano/microstructures that assemble from nanoparticles, nanorods, nanoribbons, or nanobelts as building blocks at different levels have been proposed and partially realized in recent years.4−8 Among the different morphological nanostructures, hollow structures of nanometer to micrometer dimensions have received much attention because of their widespread potential applications in catalysis, drug delivery, chromatography separation, chemical reactors, controlled release of various substances, protection of environmentally sensitive biological molecules, and lightweight filler materials.9−15 To date, numerous efforts have been devoted to generate inorganic hollow structures including conventional template and newly emerging template-free methods. Template-directed synthesis has been demonstrated as a versatile approach to fabricate hollow structures. A wealth of templates © 2012 American Chemical Society

Received: December 4, 2011 Revised: January 19, 2012 Published: January 26, 2012 4484

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Moreover, PASP is used as a capping agent, through interacting with the surface of the HAp crystallite, leading to formation of the short nanorod subunits. This PASP-assisted method for synthesis of HAp hollow microspheres can be successfully carried out in aqueous medium. Therefore, such aqueous synthetic strategy may potentially be applicable to fabrication of other metal phosphates with assembled or hierarchical structures. Furthermore, we also conducted adsorption experiments of the hierarchical hollow HAp for different heavy metal ions. The results revealed excellent adsorption ability and selective adsorption activity of the hierarchical hollow HAp for Pb2+.

soluble in water, and some can be considered to be insoluble, but all dissolve in acids. The ratios of calcium and phosphate ions (R) in these pure compounds are between 0.5 and 2.0 and are factors to control the solubility of these minerals.26 Among these minerals, HAp with R = 1.67 is the most stable and least soluble in water. Because of the good properties of HAp in osteoconductivity, bioactivity, and biocompatibility and these properties depend on the morphology, crystallite size, composition, and structure,27,28 several methods, including precipitation hydrolysis, microwave irradiation, sol−gel, hydrothermal, or solvothermal process, etc., have been used to synthesize HAp with different morphologies and microstructures.29−42 For example, Kandori et al. synthesized fibrous HAp particles via a precipitation hydrolysis approach.29 Using the microwave irradiation method, Liu et al. fabricated the bowknot, flower, and rod-like structured HAp.30 Bigi et al. prepared hydroxyapatite gels and nanocrystals through a sol− gel process.31 Zhang et al. reported the synthesis of nanorods, microsheets, bur-like microspheres, and microflowers of HAp via the hydrothermal method at different pHs.32 In water/N,Ndimethylformamide (DMF) mixed solvent, Ma et al. prepared HAp hollow spheres by a solvothermal method at 200 °C for 24 h.33 He et al. reported that in the hexane−water−bis(2ethylhexyl) sulfosuccinate (AOT) system hollow apatite spheres were presented.34 Ma et al. synthesized agglomerated nanorods of HAp using monetite as a precursor in a NaOH solution and HAp microtubes using CaCl2 and NaH2PO4 in mixed solvents of water/N,N-dimethylformamide (DMF) by a solvothermal method at 160 °C for 24 h.35,36 Utilizing poly-Laspartic acid, Bigi et al. obtained hydroxyapatite with a greater length/width ratio by direct synthesis in aqueous solution.37 Diegmueller et al. synthetized hydroxyapatite with a small size in the presence of poly-L-aspartic acid.38 Recently, Wang et al. prepared hydroxyapatite hollow microspheres constructed by self-assembly of nanosheets using Ca(CH3COO)2, Na2HPO4, NaH2PO4, and sodium citrate in aqueous solution through a microwave-assisted hydrothermal method.39 Hagmeyer et al. reported nanoparticles of calcium phosphate assemble spontaneously within a few seconds into hollow spheres with a diameter around 200−300 nm in the presence of dissolved amino acids and dipeptides. 40 Wang et al. obtained hydroxyapatite with a three-dimensional architecture through a facile transformation process of hedgehog-like aragonite precursors.41 Xia et al. presented a mineralization method to prepare ion-doped hydroxyapatite spheres with a hierarchical structure that is free of organic surfactants and biological additives.42 In spite of these achievements, it appears that it is still a challenge to explore simple, mild, effective aqueous, and template-free strategies to synthesize HAp with hierarchical hollow structures. In this paper, we present a facile template-free route to prepare HAp hollow microspheres assembled with nanorods by use of deionized water as a solvent and polyaspartic acid (PASP) as both a chelating and a capping agent. Use of PASP to control the crystal structure and amorphous to crystalline transition is well known.43−45 PASP is a promising polycarboxylic sequestrant that is nontoxic, highly biodegradable, and water soluble.46 In addition, sequestrant consists of polymerized α- and β-aspartyl residues, each containing a carboxylic functional group that can combine with metal ions to form metal−PASP species. 47 Thus, in our route via coordination bonding with Ca2+ ions PASP is designed as the nucleation site for amorphous calcium phosphate particles.

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EXPERIMENTAL SECTION All chemical reagents were of analytical grade and used as received without any further purification. In a typical synthesis procedure, 0.1472 g (1 mmol) of CaCl2·2H2O and 0.5 g (0.05 mmol) of PASP were dissolved in 10 mL of deionized water, and the mixture was stirred for 30 min to form a homogeneous solution A. Then, 0.0792 g (0.6 mmol) of (NH4)2HPO4 was dissolved in 7.5 mL of deionized water to form solution B with vigorous stirring. Solution B was introduced into solution A under continuous stirring, and some precipitate was formed. In order to keep the mixed solution clear, the pH of the solution was adjusted to 5.0 by addition of 0.1 M HCl or diluted ammonia solution. This solution was transferred into a Teflonlined stainless-steel autoclave of 20 mL capacity; the autoclave was maintained at 180 °C for 24 h and finally cooled to room temperature naturally. After washing with deionized water and absolute alcohol several times, the white precipitates were obtained and dried in vacuum at 60 °C for 3 h. Several analytical techniques were used to characterize the synthesized products. The powder X-ray diffraction (XRD) patterns of the as-synthesized samples were recorded with a Japan MapAHF X-ray diffractometer equipped with graphitemonochromatized Cu Kα irradiation (λ = 0.154056 nm), employing a scanning rate of 0.02° s−1 in the 2θ range of 3− 60°. Infared (IR) spectrum analyses were operated on samples pelletized with KBr powder in the range of 4000−400 cm−1 using an infrared Fourier transform spectrophotometer (Nicolet, ZOSX). Microstructures of the products were observed by JEOL JSM-2010 field-emission scanning electron microscopy (FESEM). X-ray photoelectron spectra (XPS) were taken on a Thermo ESCALAB 250 X-ray photoelectron spectrometer with Al Kα radiation. Selected area electron diffraction (SAED) patterns, high-resolution transmission electron microscopy (HRTEM) images, and transmission electron microscopy (TEM) images were obtained on a Hitachi model H-800 transmission electron microscope with an accelerating voltage of 200 kV. Energy-dispersive X-ray spectroscopy (EDX) analyses were obtained with an EDAX detector installed on the same TEM, and a copper grid was used. Nitrogen adsorption−desorption isotherms at the temperature of liquid nitrogen were measured with a Micromeritics Coulter (USA) instrument. Pb(NO3)2, CuCl2·2H2O, and Cd(NO3)2·4H2O were used as sources of Pb2+, Cu2+, and Cd2+ ions, respectively. In order to study the selective adsorption activity of the hollow HAp for different heavy metal ions, the as-prepared hollow HAp was mixed with the aqueous solution containing different heavy metal ions, including Pb2+, Cu2+, and Cd2+ (50 μg/mL, respectively). The effect of pH on the adsorption activity of the hollow HAp was carried out, and the pH varied from 1 to 6. 4485

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The pH of the solution was adjusted with HCl and/or NaOH and recorded with a pH meter. The above experiments were all performed at 25 °C in continuous stirring for 2 h. In addition, the effect of contact time on the adsorption activity of the hollow HAp was carried out for different time intervals. The supernatant of the suspension was collected by centrifugation at 4000 rpm for 10 min, and then the concentration of heavy metal ions in the supernatant was analyzed using inductively coupled plasma atomic emission spectroscopy (ICP-AES, optima 7300 DV).

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RESULTS AND DISCUSSION The composition and phase purity of the products were first examined by XRD, and the results reveal that pure HAp [Ca10(PO4)6(OH)2] was always obtained under the 180 °C hydrothermal conditions for 24 h. The representative XRD pattern of the products is shown in Figure 1a, and all diffraction

Figure 1. XRD patterns of the products synthesized for 24 h (a), 3 h (b), 30 min (c), and 20 min (d). Figure 2. FESEM (a, b) (scale bar =10 μm), FESEM (c) (scale bar =300 nm), and TEM (d, e) (scale bar =500 nm) images of the hollow HAp microspheres.

peaks can be indexed as hexagonal HAp (JCPDS file, No. 896440, a = 9.423 Å, c = 6.883 Å, and space group P63/m) except that our sample’s (002) reflection is significantly strong and narrow. Therefore, the synthesized products can be assigned to a pure phase hydroxylapatite. The stronger (002) reflection is most likely related to the preferential orientation of the HAp along the [001] direction. The other broadened diffraction peaks from our product may imply that the prepared product consists of nanocrystals and/or slight distortions of the crystal lattice occur in the crystal. The morphology and size of the as-synthesized products were characterized by FESEM and TEM. The panoramic FESEM image in Figure 2a provides a representative overview of the sample after 180 °C hydrothermal treatment for 24 h. It can be seen from Figure 2a that the product is composed of a large quantity of well-dispersed microspheres with an average diameter of 2.5 μm, which indicates that the PASP-assisted synthetic route can result in a good yield of HAp microspheres. In the higher magnification FESEM image (Figure 2b) some broken microspheres could be observed, revealing that the spheres have a hollow interior. More detailed information about the HAp hollow microspheres is shown in Figure 2c, from which one can see that the hollow HAp is a hierarchical nanostructure and the entire architecture is constructed by a single layer of radially oriented nanorods, self-wrapping to form

hollow interiors with ca. 1 μm diameter. Note that the shells of the hemispheres are porous and that the constituent nanorods are aligned and radially oriented with their growth axes perpendicular to the surface of the hemisphere. Close examination of these images indicates that the lengths and diameters of the constituent nanorods are ca. 0.5−1 μm and 50 nm, respectively, giving a typical aspect ratio of ca. 15. As shown in Figure 2d, a typical TEM micrograph of the asobtained sample reveals that the hierarchical nanostructured HAp microspheres are with an average size similar to the FESEM observation. Figure 2e shows a typical transmission electron microscopy (TEM) image of a random HAp microsphere. The contrast between the light center portion in the superstructure and the black edge further confirms the hollow interiors of the unique self-wrapped nanorod arrays, which is consistent with FESEM observation. The TEM image of a constituent nanorod peeled from the wall of the hollow microspheres by a 10 min ultrasonication is shown in Figure 3a. The representative HRTEM image and corresponding fast Fourier transform (FFT) dots reveal that the constituent nanorods on the wall are a single-crystal entity and preferential [001] growth, as depicted in Figure 3b and 3c. The 4486

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Figure 3. TEM (a) and HRTEM (b) (scale bar = 20 nm) images and corresponding FFT pattern (c) and SAED pattern (d) of the single nanorod stripped from the hollow HAp microsphere.

Figure 4. FESEM images of products synthesized for 20 min (a, scale bar = 10 μm), 30 min (c, scale bar = 10 μm; d, scale bar = 1 μm), and 3 h (e and f, scale bar = 10 μm). TEM image of the product synthesized for 20 min (b, scale bar = 560 nm). Inset in a, c, and f, scale bar = 1 μm. Inset in b is SAED pattern.

representative SAED pattern collected from the marked area in Figure 3a is shown in Figure 3d. These spots can be indexed as hexagonal HAp with the zone axis [1̅10]. Moreover, the indexed spots also exhibit that the [001] direction is parallel to the nanorods, demonstrating that the [001] direction is the favorable growth direction for the HAp nanorods around the hierarchical structures, which is consistent with HRTEM observations (e.g., Figure 3b). To understand the formation details of the hollow HAp microspheres we conducted time-dependent experiments. Figure 4 displays FESEM and TEM images of the products synthesized for different reaction durations. It can be seen from panels a and b of Figure 4 that a 20 min hydrothermal reaction at 180 °C only yields solid particles with well-defined spherical morphology and with an average diameter of 1.5 μm. The TEM image of the spheres (Figure 4b) reveals that the smooth sphere should be solid, and the appearance of the diffuse rings in the SAED pattern (inset in Figure 4b) confirms the amorphous nature of the spherical particles. EDX analyses (Figure 5a) reveal that the solid amorphous spheres contain Ca, P, and O elements, indicating that in the presence of PASP the first generated product should be a poorly ordered amorphous calcium phosphate. The XRD result (Figure 1d) further identifies the amorphous nature of the initial mineralized product, corresponding to the SAED analysis (inset in Figure 4b). By comparison with a 20 min hydrothermal reaction, extending the hydrothermal time to 30 min resulted in a better crystallinity (Figure 1c) and the distinct surface texture of the spherical particles (Figure 4c and 4d) which may be due to secondary nucleation and growth of HAp nanocrystals on the surface of the microspheres. Moreover, some minor holes on the microspheres (indicated by arrows) can also be seen (Figure 4c and its inset). When the hydrothermal time is prolonged to 3 h, a large quantity of well-dispersed microspheres have formed (Figure 4e) and more obvious holes on

the microspheres can be found (Figure 4f and its inset). The corresponding XRD pattern is shown in Figure 1b. In comparison with Figure 1c and 1d, pointing to the long hydrothermal duration (3 h) leads to a much higher crystallinity. This should be associated with a progressive redistribution of matter from the interior to the exterior of the microspheres and leads to hollow particles. As a result, the solid particles are reconstructed into ordered nanorod-based mesoporous shells (Figure 2). To identify the effect of PASP on formation of the hollow HAp hierarchical structures, some control experiments were also carried out without PASP while other conditions remained unchanged. In this case, addition of the (NH4)2HPO4 solution led to white turbidness in the CaCl2 solution and even when the pH of the suspension was adjusted to 5 the white turbidness still remained undissolved and could be separated from the suspension by simple centrifugation. Figure 6a and 6b provides direct information about the typical shape and size of the white precipitate obtained without PASP. As can be seen, the product is an irregular sheet-like particle, and no hierarchical hollow structures occur. The corresponding XRD pattern (Figure 7a) indicates that the sheet-like particles are phase pure CaHPO4·2H2O (Monoclinic, JCPDS File No. 72-1240, and space group I2/a). However, while the suspension was hydrothermally treated for 24 h at 180 °C, the SEM images of the product show that only belt-like structures with an average length and width of about 50 and 1 μm can be obtained (Figure 6c and 6d). The XRD pattern (Figure 7b) of the beltlike structures is characteristic of hydroxyapatite, indicating that 180 °C hydrothermal treatment lead to conversion of sheet-like CaHPO4·2H2O to belt-like rather than hollow-like HAp. 4487

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Figure 7. XRD patterns of the products synthesized without PASP for original precipitate (a) and 24 h hydrothermal reaction (b).

exists in the current reaction system it would react with Ca2+ to form relatively stable Ca−PASP species, precluding generation of CaHPO4·2H2O crystals in solution. It appears that PASP acts as a strong chelating agent for Ca2+ ions in our experiment system. FT-IR and XPS analyses provide further supporting evidence that PASP played a key role in formation of the hollow HAp hierarchical structures. Figure 8a shows the typical FT-IR

Figure 5. EDX spectra (a) of the product synthesized for 20 min, and XPS spectra (b) of the hollow HAp microspheres.

Figure 8. FT-IR spectra of products synthesized for 20 min (a) and 24 h (b).

spectrum of the product obtained for a 20 min hydrothermal reaction at 180 °C in the presence of PASP. The single broad peak at 547 cm−1 is the most obvious characterization of the ACP spectrum and further confirms that the product is ACP rather than crystalline HAp.48 In addition, the FT-IR spectrum also shows the bands corresponding to organic functional groups in PASP molecule, such as C−H of methylene (2930 and 2848 cm−1) and the characteristic asymmetric and symmetric bands of COO− groups (1580−1650 and 1415 cm−1) in the Ca−PASP complex,49,50 suggesting that PASP molecules remain strongly associated with the ACP even after extensive washing. Therefore, we reasonably believe that PASP molecules are intimately associated with formation of ACP. It seems reasonable to assume that the high carboxyl-group density along the PASP backbone would provide the nucleation

Figure 6. FESEM images of the products synthesized without PASP for original precipitate (a, scale bar = 100 μm; b, scale bar = 2 μm) and 24 h hydrothermal reaction (c, scale bar = 30 μm; d, scale bar = 5 μm).

Furthermore, the abnormal strong (300) peak in Figure 7b further supports the belt-like feature of the hexagonal HAp, i.e., the much higher intensity of this peak can be attributed to the shape effects. However, in the presence of PASP, when the pH value of the suspension was adjusted to 5, no precipitation can be formed. Moreover, it has been reported that the metal ions complex with PASP to form metal−PASP species since PASP contains a mass of the carboxyl group.47 Thus, when PASP 4488

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sites for ACP through Ca2+ ion accumulation. Combining with our control experiments, it is not difficult to find that PASP should have a crucial role in inducing and transiently stabilizing the ACP in solution. Figure 8b reveals the representative FT-IR spectrum of the product obtained after a 24 h hydrothermal reaction at 180 °C in the presence of PASP. The set of bands and spectral features agree fairly well with the reported IR data for hydroxyapatite,32 further confirming that the product is HAp phase. Moreover, the peaks at 1601 and 1417 cm−1 related to COO− in the Ca−PASP complex also imply the strong association between PASP and HAp crystals,49,50 even though the intensities of vibration bands related to COO− significantly decline with respect to Figure 8a due to thermolysis of PASP during the hydrothermal process. Furthermore, we conducted XPS analysis to investigate the surface composition of the product. In Figure 5b it can be seen that the product contains O, P, Ca, C, and N elements. Thus, it can be further shown that PASP chains or the organic chelating ligands also exist on the surface of the product. In the synthesis of inorganic nanostructures, many organic additives have been employed for modifications of certain crystallographic surfaces. Absorption of the organic additives on those surfaces can change the order of free energies of different facets through their adsorption/capping interactions. This alteration may significantly affect the relative growth rates of different facets and thus leads to different morphologies for the final products.51 Chen et al. reported that −COOH- and −NH2terminated polyamidoamine dendrimers can be bound to naturally occurring hydroxyapatite enamel crystals, and the relatively strong binding of the crystals to the partially ionized carboxylic-acid-capped and amine-group dendrimer may suggest net positive and negative charge domain structures on the crystal surface.52,53 In addition, previous studies showed that the HAp crystal has two types of crystal planes with different charges: the basal plane of HAp is rich in OH− species and negatively charged, whereas the prism plane of HAp is rich in Ca2+ and positively charged.54,55 Moreover, the experiment of Diegmueller et al. suggested that PASP decreased the hydroxyapatite crystal sizes due to absorption of PASP onto the crystal surface, which inhibited or decelerated subsequent growth of the crystal planes.38 PASP is a polyelectrolyte with −COOH and −NH2 terminals. It was reasonable to deduce that the end of PASP would have a similar binding ability to that of −COOH- and −NH2-terminated polyamidoamine dendrimers. Combined with our FT-IR and XPS analyses results, we reasonably believe that at the formation stage of HAp a capping layer of PASP molecules has formed by the binding affinities of carboxyl groups and amino groups in PASP with the positive and negative charge crystal planes, which can inhibit or decelerate further growth of the crystal planes, together with the effect of the intrinsic crystal habit of HAp due to the lattice energy difference among its different crystal planes,56 leading to formation of the short nanorod subunits rather than the control belt-like structures with large size. Hence, in our experiments PASP might play three main roles in the process (one is to preclude generation of CaHPO4·2H2O crystals in solution, another is inducing and transiently stabilizing the amorphous calcium phosphate, and another is adsorption on the facets of HAp crystals) and inhibits or decelerates further growth of the crystal planes. Subsequent development from the amorphous calcium phosphate particles to HAp hollow microspheres is probably due to localized Ostwald ripening. According to the Gibbs−

Thompson equation and Fick’s first law the chemical potential of the particle increases with the decrease in particle size, meaning that the equilibrium solute concentration near a small particle is higher than that near a larger one. The resulting concentration gradients would lead to diffusion of molecularscale species from smaller particles to larger particles through solution.57 Usually the inner component in the solid spheres can be visualized as smaller spheres because they have higher curvature (i.e., higher surface energies and thus easily dissolved).58 Therefore, they are capable of diffusing to the outer shell by the dissolution−recrystallization process. Typically, this process is characterized by the initial deposition of amorphous microspheres that, although kinetically favored during the nucleation stage, become metastable with respect to more thermodynamically stable HAp as the supersaturation falls with time in the surrounding solution. Thus, the amorphous solid particles become coated with an ultrathin shell of a less-soluble crystalline phase until the system reaches equilibrium with respect to the surface layer. Unlike the external crystalline layer, however, the amorphous core remains out of equilibrium with the surrounding solution due to its higher solubility; thus, provided there is a diffusion pathway through the outer crystalline shell, the core will dissolve. As a consequence, supersaturation increases in the solution above the solubility product of the crystalline exterior layer and secondary nucleation of the crystalline HAp on the external surface occurs. Thus, the crystalline shell increases in thickness as the amorphous core becomes progressively depleted to produce intact hollow microspheres. This mechanism was also suggested for formation of calcium carbonate hollow spheres.59 Formation of the short nanorod subunits may be due to the contribution of both inhibiting or decelerating further growth of the crystal planes due to absorption of PASP onto the HAp crystal surface and the effect of the intrinsic crystal habit of HAp, as discussed above. The schematic illustration for formation of hollow HAp microspheres is shown in Figure 9.

Figure 9. Schematic illustration for formation of hollow HAp microspheres: (a) PASP-assisted formation of ACP microspheres; (b) HAp crystallization of the particle surface and reduced core density; (c) formation of hierarchical nanostructured HAp hollow microspheres.

The above mechanism is likely to be highly sensitive to the relative rates of dissolution of the amorphous solid particles and nucleation of the crystalline phase. When the former is comparatively slow the amorphous particles will transform in situ to solid crystalline microspheres, whereas when the latter is relatively slow the amorphous phase will completely dissolve prior to crystallization, which then takes place in free solution. Only when the rates are similar will the phase transformation process initiate specifically on the surface of the amorphous particles and remain localized as the particle core is depleted. Nitrogen adsorption−desorption isotherms are measured to determine the specific surface area and pore volume of the HAp hollow microspheres, and the corresponding results are 4489

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presented in Figure 10a. The isotherm of HAp samples is a combination of types I and IV (BDDT classification).60 In a

Figure 10. Nitrogen adsorption−desorption isotherms (a) and BJH pore size distribution (b) of the hollow HAp microspheres obtained after 24 h.

low relative pressure range (below 0.2) the isotherms exhibit high adsorption, indicating the presence of micropores (type I).61 At a high relative pressure range between 0.4 and 1.0 the sample exhibits a type H3 hysteresis loop according to BDDT classification;62 this type of hysteresis loop is traditionally associated with textural slit-like pores but recently has also been observed in structures consisting of voids surrounded by a mesoporous matrix63 or hollow particles with mesoporous walls.64 Accordingly, the hysteresis loops in our case could be assigned to the hollow chambers and mesoporous walls assembled by the nanorods. The plot of the pore size distribution (Figure 10b) was determined using the Barrett− Joyner−Halenda (BJH) method from the desorption branch of the isotherm. The average pore size of the HAp hollow spheres was ca. 15.9 nm, where the pore size represents the nanopores in the shell, not the hollow core. The BET specific surface area was around 164.73 m2/g, which is much higher than those of other spherical HAp particles.65 The high BET surface area, mesoporous structure, and nanoscale size are beneficial for ion adsorption, exchange, and diffusion. Figure 11a shows the selective adsorption activity of the hollow HAp for Pb2+, Cd2+, and Cu2+ coexisting in the adsorption system. The removal capacities of the hollow HAp for Pb2+, Cu2+, and Cd2+ were about 99.794, 66.52, and 38.78 mg/g, respectively. The results indicate that the hollow HAp

Figure 11. Selective adsorption activity of the hollow HAp microspheres for heavy metal ions coexisted in the adsorption system (a), effect of pH on heavy metal ion adsorption of the hollow HAp microspheres (b), and effect of contact time of the hollow HAp microspheres on different heavy metal ions (c).

has a much stronger adsorption activity for Pb2+ and Cu2+ than Cd2+. Gupta et al. found that the pH value of the solution is one of the important factors affecting the biosorption of metal ions.66 Removal of the heavy metal ions was investigated as a function of solution pH, and the effect of pH on the adsorption activity of the hollow HAP is presented in Figure 11b. It can be seen that the adsorption contents of the heavy metal ions gradually increase with the increase of pH. The maximal adsorption contents appear at pH 5 and then decrease with the further increase of pH. The effect of treatment time on removal of the different heavy metal ions by the hollow HAP was also studied, and the results are shown in Figure 11c. It shows a very 4490

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rapid adsorption of the heavy metal ions in the first 10 min. Subsequently, the adsorption rate decreases gradually and the adsorption contents reach equilibrium in about 45 min. Indepth investigation is still in progress.

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CONCLUSIONS In summary, hollow HAp microspheres assembled by 1D HAp nanorods can be manipulatively synthesized by a water-soluble biopolymer PASP-assisted hydrothermal route. Our results show that during formation of hollow microspheres PASP molecules play a crucial role in inducing and transiently stabilizing amorphous calcium phosphate (ACP), and Ostwald ripening is responsible for subsequent transformation of ACP to the hollow microspheres. Moreover, the adsorption experiments of the hierarchical hollow HAp for different heavy metal ions indicate that the hierarchical hollow microspheres have unique selective adsorption activity for heavy metal Pb2+. Therefore, such hollow microspheres can be potentially applied to heavy metal-contaminated water treatment.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86 551 3600533. Fax: 86 551 3600533. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Natural Science Foundation of China (NSFC) under Grant No. 41072029 and the Knowledge Innovation Program of the Chinese Academy of Sciences, Grant No. KZCX2-YW-QN501.

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