Synthesis of High Surface Area Hydroxyapatite Nanoparticles by

Mesoporous calcium phosphate using casein as a template: Application to bovine serum albumin sorption. Oberto Grangeiro da Silva , Marco Marciel Alves...
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Synthesis of High Surface Area Hydroxyapatite Nanoparticles by Mixed Surfactant-Mediated Approach Masafumi Uota,† Hiroshi Arakawa,‡ Nana Kitamura,‡ Takumi Yoshimura,†,‡ Junzo Tanaka,§ and Tsuyoshi Kijima*,†,‡ CREST, Japan Science and Technology Cooperation, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan, Department of Applied Chemistry, Faculty of Engineering, Miyazaki University, Miyazaki 889-2193, Japan, and Biomaterials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Received January 5, 2005 A new surfactant-mediated approach was developed to synthesize hydroxyapatite (HAp) nanoparticles with high surface areas by calcination of their precursors encapsulated with calcium stearate using mixed surfactant-containing reaction mixtures. Acidic aqueous solution of calcium phosphate was mixed with both or either nonaoxyethylene dodecyl ether (C12EO9) and polyoxyethylene(20) sorbitan monostearate (Tween 60) and then was treated with aqueous ammonium at 25 °C. The C12EO9-based single surfactant system yielded an aggregate of platy HAp nanoparticles 20-40 nm in size, whereas the Tween 60-based single and mixed systems led to lath-shaped HAp nanoparticles 2-8 nm wide and encapsulated with calcium stearate. On calcination at 500 °C, the stearate-encapsulated HAp nanoparticles in the latter two systems were deorganized into high surface area HAp nanoparticles. Particularly, the HAp nanoparticles in the mixed system exhibited a specific surface area as high as 364 m2 g-1 that is roughly 3 times larger than 160 m2 g-1 for those in the single system. The significantly high surface area for the former is attributed to much less adhesion of decapsulated HAp nanoparticles, which originated from the particleseparating effect of the C12EO9 molecules adsorbed on the outer surface of the stearate-encapsulated HAp nanoparticles to inhibit their agglomeration or interfacial coordination. The present results demonstrate that the mixed use of two different surfactants as a source of encapsulating and templating agent and a particle-separating agent is specifically effective for the synthesis of high surface area HAp nanoparticles.

Introduction Hydroxyapatite (HAp,Ca5(PO4)3(OH)) has attracted a lot of attention because of its extensive applications as bone and tooth implants, bone fillings, and adsorbents, among others.1 HAp is used mostly as powders and its usefulness depends on the powder properties such as particle size, surface area, and morphology. Nanostructured HAp particles with a higher surface area would be more desirable for their use in many fields including separation processes. Conventionally, HAp powders are synthesized by various methods based on dry processes such as solid-state reaction2and wet processes such as hydrolysis and precipitation.33 However, even the wet processes inevitably undergo particle agglomeration to yield HAp fine particles whose specific surface area is less than 100 m2 g-1 or normally around 20-60 m2 g-1.1c,3c Since the initial approach to the fabrication of reticulated calcium phosphate frameworks,4 the microemulsion technique was employed for the synthesis of HAp fine * Author to whom correspondence should be addressed. Tel: +81-985-58-7311; fax: +81-985-58-2876; e-mail: t0g102u@ cc.miyazaki-u.ac.jp. † Japan Science and Technology Cooperation. ‡ Miyazaki University. § National Institute for Materials Science. (1) (a) McConnell, D. Apatite: Its Crystal Chemistry, Mineralogy, Uilization, and Biologic Occurrences; Springer-Verlag: New York, 1973. (b) Mann, S. J. Chem. Soc., Dalton Trans. 1993, 1. Mann, S. Nature 1993, 365, 499. (c) Monma, H. Inorg. Mater. 1995, 2, 401. (d) LeGeros, R. Z. Clin. Orthop. Relat. Res. 2002, 395, 81. (e) Dorozkin, S. V.; Epple, M. Angew. Chem., Int. Ed. 2002, 41, 3130. (2) (a) Young, R. A.; Holcomb, D. W. Calcif. Tissue Int. 1982, 34, S17. (3) (a) Rhee, S. H.; Tanaka, J. J. Am. Cer. Soc. 1998, 81, 3029. (b) Tas, A. C.; Korksuz, F.; Timicin, M.; Akkas, N. J. Mater. Sci.: Mater. Med. 1997, 8, 91. (c) Rodriguez-Lorenzo, L. M.; Vallet-Regi, M. Chem. Mater. 2000, 12, 2460. (4) Walsh, D.; Hopwood, J. D.; Mann, S. Science 1994, 264, 1576.

particles with smaller size and higher surface area,5 as effectively applied to the preparation of some other nanosized materials with controlled particle size and shape.6 Particularly, Bose and Saha synthesized HAp nanopowders with a specific surface area as high as 130 m2 g-1 and particle size between 30 and 50 nm by using nonionic surfactant emulsion.5d Another surfactant-templating approach has been extensively used in the preparation of various nanoporous or nanotubular framework materials including mesoporous silica7 and many other metal oxides.8 The templating synthesis using hydrophilic polymers yielded HAp nanoparticles with unique morphologies such as neuronlike mesostructures.9 Similar approaches were developed not only for the morphological control of HAp nanoparticles10 but also for their functionalization11 and coating12 with organic molecules. Despite many efforts, however, no evidence has (5) (a) Sonoda, K.; Furuzono, T.; Walsh, D.; Sato, K.; Tanaka, J. Solid State Ionics 2002, 151, 321. (b) Lim, G.; Wang, J.; Ng, S. C.; Gan, L. Mater. Lett. 1996, 28, 431. (c) Lim, G.; Wang, J.; Ng, S. C.; Gan, L. Langmuir 1999, 15, 7472. (d) Bose, S.; Saha, S. K. Chem. Mater. 2003, 15, 4464. (6) (a) Pileni, M. P. Nature Mater. 2003, 2, 145. (b) Pileni, M. P. Langmuir 1997, 13, 3266. (7) (a) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (b) Yanagisawa, T.; Shimizu, T.; Kuroda, K.; Kato, C. Bull. Chem. Soc. Jpn. 1990, 63, 988. (8) (a) Tremel, W. Angew. Chem. 1999, 38, 2175. (b) Ozin, G. A. Chem. Commun. 2000, 419. (9) (a) Antonietti, M.; Breulmann, M.; Goltner, C. G.; Colfen, H.; Wong, K. K. W.; Walsh, D.; Mann, S. Chem. Eur. J. 1998, 4, 2493. (b) Peytcheva, A.; Colfen, H.; Schnablegger, H.; Antonietti, M. Colloid Polym. Sci. 2002, 280, 218. (10) (a) Walsh, D.; Kingston, J. L.; Heywood, R.; Mann, S. J. Cryst. Growth 1993, 133, 1. (b) Kijima, T.; Yamaguchi, K.; Miyata, A.; Yada, M.; Machida, M.; Tanaka, J. Chem. Lett. 2000, 1324. (11) (a) Gonzalez-McQuire, M.; Chane-Ching, J.-Y.; Vignaud, E.; Lebugle, A.; Mann, S. J. Mater. Chem. 2004, 14, 2277. (b) Welzel, T.; Meyer-Zaika, W.; Epple, M. Chem. Commun. 2004, 1204.

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been presented on the formation of not only mesoporous HAp but also HAp crystals with high surface areas far beyond 100 m2 g-1. In contrast to the conventional templating method using a single surfactant, our recent study demonstrated the first synthesis of noble metal nanotubes by the templating approach using a mixed surfactant liquid crystal (LC).13 This motivated us to apply such a mixed surfactant template to the fabrication of nanostructured HAp. Here, we report the synthesis of HAp nanoparticles encapsulated with calcium stearate using polyethylene(20) sorbitan monostearate (Tween 60) and nonaethyleneglycol monododecyl ether (C12EO9) and their calcination into high surface area HAp nanoparticles. Experimental Section Materials. The reagent grade calcium nitrate Ca(NO3)2‚4H2O, potassium dihydrogen phosphate KH2PO4, nonaethyleneglycol monododecyl ether (C12EO9), and polyoxyethylene(20) sorbitan monostearate (Tween 60) were purchased from Wako Co. Ltd. and used without further purification. Synthesis of Calcium Phosphate in Single Surfactant Systems. Calcium nitrate, potassium dihydrogen phosphate, nitric acid, and water were mixed to obtain an acidic solution. Nitric acid was added to avoid the precipitation of calcium phosphate. The acidic solution was mixed with C12EO9 and then heated to 60 °C with stirring to obtain a clear mixed solution. The total molar ratio of Ca(NO3)2, KH2PO4, C12EO9, HNO3, and H2O was adjusted to 1.67:1:1:8:60. The mixed solution was cooled to 25 °C, and the resulting liquid crystalline mixture was treated with aqueous ammonia twice as much as nitric acid in molar amount, followed by standing at that temperature for 48 h. The resulting solid was centrifuged, fully washed with distilled water and then with ethanol, and finally dried in air. The reaction using Tween 60 in place of C12EO9 was done in the same manner as above. Synthesis of Calcium Phosphate in Mixed Surfactant System. The mixed solution of Ca(NO3)2, KH2PO4, C12EO9, HNO3, and H2O prepared as in the single surfactant systems was mixed with Tween 60 with shaking at 60 °C and then cooled to 25 °C to obtain a liquid crystalline mixture. The total molar ratio of Ca(NO3)2, KH2PO4, C12EO9, Tween 60, HNO3, and H2O was adjusted to 1.67:1:1:1:8:60. The subsequent procedure was the same as in the single surfactant system. Some portion of the as-grown samples obtained in the single or mixed surfactant system was calcined at 500 and 1000 °C for 5 h. Characterization. Powder X-ray diffraction (XRD) measurements were conducted with Cu KR radiation at a scanning rate of 2°/min typically by the reflecting method on a Shimadzu XDD1 diffractometer. Transmission electron microscopic (TEM) images were taken by a Hitachi H-800MU and a JEOL JEM2010. Scanning electron microscopy (SEM) was carried out by a Hitachi H-4100M. Energy-dispersive X-ray (EDX) microanalysis was performed with a HORIBA EMAX-5770. Thermogravimetric and differential thermal analyses (TG/DTA) were conducted by a Seiko TG/DTA320U with a heating rate of 10 °C /min in air. Fourier transform infrared (FT-IR) spectra were measured by the KBr pellet method using a Nippon Bunko FT/IR-300. Specific surface area was measured by the BET method using N2.

Results and Discussion In the typical fabrication process based on the mixed surfactant system, a mixed LC of Ca(NO3)2, KH2PO4, Tween 60, C12EO9, HNO3, and H2O at a molar ratio of 1.67:1:1:1:8:60 was treated with aqueous ammonia as a precipitant. The transmission electron microscope (TEM) image of the resulting solid, 1, showed an aggregate of (12) (a) Welzel, T.; Radtke, I.; Meyer-Zaika, W.; Epple, M. J. Mater. Chem. 2004, 14, 2213. (b) Bertoni, E.; Bigi, A.; Falini, G.; Panzavolta, S.; Roveri, N. J. Mater. Chem. 1999, 9, 779. (13) Kijima, T.; Yoshimura, T.; Uota, M.; Ikeda, T.; Fujikawa, D.; Mouri, S.; Uoyama, S. Angew. Chem., Int. Ed. 2004, 43, 228.

Figure 1. TEM or HRTEM images of (a, b) 1, the as-grown sample in the C12EO9/Tween 60 mixed surfactant system; (c, d) the 500 °C-calcined form of 1; (e) 2, the as-grown sample in the Tween 60-based single surfactant system; and (f) 3, the as-grown sample in the C12EO9-based single surfactant system.

lath-shaped particles 4-8 nm wide and above 100 nm in length, consisting of 2-3 nm wide core particles encapsulated with certain additional films (Figure 1a). The core and encapsulating materials were identified as HAp and calcium stearate (CaSt), respectively, from the lattice image characteristic of the former crystals (Figure 1b) as well as the XRD pattern of 1 (Figure 2a): the XRD pattern gave a series of diffraction peaks assigned to the reflections for CaSt, together with several broad peaks for a low crystalline HAp.14 The coexistence of HAp with a large amount of stearate anions was also confirmed by the FTIR spectrum of 1 giving rise to strong absorption bands attributable to CH2 and COO- groups, along with the band due to phosphate group (Figure 3a). The stearate anions resulted from the hydrolysis of Tween 60 succeeded by the neutralization process. The TG curve for CaSt alone showed two-step decomposition of the organic moiety into CaCO3 over the wide temperature range 200-420 °C, followed by the carbonate decomposition at around 600 °C (Figure 4d). In marked contrast, the solid 1 underwent one-step decomposition of the CaSt moiety at around 200300 °C catalytically enhanced by the nanoscale mixed HAp nanoparticles, preceded by the desorption of water at below 100 °C (Figure 4a). Furthermore, the subsequent carbonate decomposition occurred at much higher temperatures around 700 °C. This is probably because pyrolytically produced fine CaCO3 crystals are strongly adhered to the surface of HAp nanoparticles, as suggested by comparison with appreciably weak XRD peaks observed for the CaCO3 pyrolytically derived from CaSt (Figure S1). Actually, a very weak XRD peak (d ) 0.303 nm) assigned to the strongest reflection of CaCO3 was detected for the deorganized form, 1′, obtained by calcination of 1 at 500 °C in air (Figure 2b). The total conversion of 1 into HAp and (14) See Supporting Information A.

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Figure 4. (A) TG and (B) DTA curves for (a) 1, (b) 2, (c) 3, and (d) CaSt. Figure 2. XRD patterns of (a) 1, the (b) 500 °C- and (c) 1000 °C-calcined forms of 1, (d) 2, and (e) 3 (CuKR).14 For the asgrown samples 1, 2, and 3, most of the diffraction peaks indicated by open circles and squares are broad but assignable to the reflections for HAp and those for CaSt, respectively. The peak indicated by solid circle refers to the 104 reflection for CaCO3 and those by open triangles refers to the 200 and 220 reflections for CaO.

Figure 3. FT-IR spectra of (a) 1, the (b) 500 °C- and (c) 1000 °C-calcined forms of 1, (d) 2, (e) 3, and (f) calcium carbonate obtained by pyrolysis of CaSt at 500 °C for 2 h.

CaO at 1000 °C was further confirmed by the XRD pattern of the calcined product (Figure 2c). The TG data thus revealed that the as-grown solid 1 is a 1:0.93 mixture of HAp and CaSt (Table 1). The Ca/P molar ratio of 1.98 evaluated from the TG data was also in close agreement with 2.1 obtained by EDX analysis. For the 500 °C calcined product 1′, the IR spectrum also showed weak or broad bands at 873 and around 1440 cm-1 attributable to CO32-

group in CaCO3 (Figure 3b, f). More interestingly, the BET surface area of 1′ was as high as 364 m2 g-1. To our knowledge, this value is about 3 times as much as the highest surface area of 130 m2 g-1 so far reported.5d Such a high surface area HAp would result from much less sintering of precursory nanocrystals as the major phase with an accompanying increase in the average width of HAp particles from 4 to 8 to 8 to 20 nm, as suggested from the HRTEM image of 1′ (Figure 1c). On calcination at 1000 °C, the solid 1 was converted into nodular highly crystalline HAp particles of ∼300 nm in size (Figures 2c, 3c, S2). The Tween 60 based single surfactant system produced an aggregate of lath-shaped nanoparticles, 2, with nearly the same composition as 1 (Figure 1e, 2d, Table 1). The former product, however, was morpholosically characterized by HAp nanoparticles more cohered and dispersed in a continuous matrix of CaSt. The specific surface area of 2 calcined at 500 °C was 160 m2 g-1, being much less than half the value for 1 (Table 1). The lath-shaped HAp nanoparticles produced in both the Tween 60 based single and the mixed surfactant systems are similar in shape and size to HAp nanorods typically formed for the intercation with anionic templates10a and also those functionalized with amino acid11a or complexed with poly(acrylic acid).12b Nevertheless, no surface area data have been reported for all the latter nanocrystals, as grown or calcined. On the other hand, the C12EO9 based single surfactant system yielded almost surfactant-free platy particles of low crystalline HAp, 3, (Figure 1f), as previously observed for the HAp formed in the presence of nonionic surfactant.9a The platy HAp particles exhibited a specific surface area of as low as ∼8 m2 g-1(Table 1). It is thus marked that the combined effect of Tween 60 and C12EO9 would be responsible for the markedly high surface area of HAp observed in the mixed surfactant system. On the basis of the CaSt to HAp volume ratios for 1 and 2,15 the HAp core particles in these solids were covered with CaSt 3.2-3.5 times larger in volume, being consistent with the TEM observations. In these two reaction systems, on addition of Tween 60 the transparent mixture of Ca(NO3)2, KH2PO4, HNO3, and H2O with or without C12EO9 became a slightly translucent solution, because of the hydrolysis of Tween 60 into water-insoluble stearic acid and water-soluble poly(ethylene glycol) sorbitan, and (15) See Supporting Information B.

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Table 1. Preparation Conditions of Hydroxyapatite Samples and Their Characterization composition of starting surface area of mixture (mole ratio) composition content/wt % CaSt/ 500 °C-calcined structurea Ca(NO3)2/KH2PO4/C12EO9/ form/m2 g-1 Tween 60/HNO3/H2O HApb (X-ra y) HAp CaSt H2 O Ca/Pc morphology 1.67 1.67 1.67

1 1 1

1 0 1

1 1 0

8 8 8

60 lath-shaped Hap + CaSt 60 lath-shaped Hap + CaSt 60 platy HAp

45.9 44.1 94.4

51.8 54.3 2.6d

2.3 1.6 3.0

0.93 1.02 0

2.1 (1.98) 2.2 (2.01) 1.66

364 160 ∼8e

a CaSt: calcium stearate, (C H COO) Ca. b Mole ratio evaluated relative to the HAp composition of Ca (PO ) OH. c Mole ratio obtained 17 35 2 5 4 3 by EDX. The values calculated from TG data are given in parentheses. d Content of C12EO9. e As-grown form.

Scheme 1. Schematic Models Proposed for the Formation and Calcination of HAp Nanoparticles in the Single and Mixed Surfactant Ca(NO3)2/KH2PO4/H2O/Tween 60 or C12EO9 Systems: (A) Tween 60/C12EO9, (B) Tween 60, and (C) C12EO9 Systemsa

a The illustrations for A and B show a side view of as-grown or sintered lath-shaped HAp particles aggregated with their top surfaces directed upward.

the solution was further converted into a translucent pasty material by cooling to 25 °C. The pasty precursors for 1 and 2 thus formed showed no long spacing structure except for XRD peaks characteristic of stearic acid as well as a halo band around 2θ ) 22° (Figure S3a, b). This indicates that the precursory materials consist of stearic acid nanoparticles suspended in aqueous pasty phase of poly(ethylene glycol) sorbitan or C12EO9. On the subsequent addition of aqueous ammonium solution, the aqueous pasty phase changed from acidic to basic, leading to the precipitation of calcium phosphate as HAp as well as the deposition of CaSt, preceded by the dissolution of stearate ions through the neutralization of stearic acid. We can thus propose models for the formation and calcination of HAp nanoparticles in the Tween 60 based mixed and the single surfactant systems (Scheme 1). In both systems, the HAp nanoparticles would grow with bilayered stearate ions adsorbed on their surface because stearate ion is much more strongly attracted by the surface Ca2+ ion than all the other ions or molecules in the reaction mixture. Such surface adsorption would lead to the formation of thin and long nanoparticles, as reported previously for some other systems.10a,11a,12b The binding of stearate ions to the surface Ca2+ ions would also induce the epitaxial growth of CaSt on the HAp nanoparticle, resulting in the encapsulation of HAp nanoparticle with CaSt layer. Particularly in the mixed system, the C12EO9 molecules would be located between the CaSt-encapsulated nanoparticles to inhibit the agglomeration or interfacial coordination of CaSt-encapsulated HAp nanoparticles to a greater extent. This would cause eventually much less agglomeration of lath-shaped HAp nanopar-

ticles during calcination, leading to the formation of markedly high surface area HAp nanoparticles. Conclusion We demonstrated a new surfactant-mediated approach to synthesize HAp nanoparticles with a specific surface area of as extremely high as 364 m2 g-1, as well as their CaSt-encapsulated precursors using mixed nonionic surfactant-based reaction mixtures. Because stearic acid and its metal salts are insoluble or poorly soluble in water, it is impossible to employ these compounds as a starting material in place of Tween 60. In brief, the present approach successfully used Tween 60 as a combined source of both encapsulating and templating agents and C12EO9 as a particle-separating agent. This approach is based on the precipitation reaction due to an increase in pH, accompanied by the adsorption of stearate ions on the precipitate surface and the subsequent growth into encapsulating material. Thus, we can expect that our approach will be further applicable to the synthesis of a variety of metal oxide or hydroxide nanoparticles. This strategy could be also extended to the nanoparticle synthesis using some other hydrolyzable surfactants such as amide group containing ones. The HAp nanoparticles obtained by the present approach are also promising as high-performance adsorbents and precursory powders for implant materials. Acknowledgment. This study was supported by Grant-in-Aids for the CREST of Japan Science and Technology Coorporation (JST) and Promotion of Evolu-

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tional Science and Technology in Miyazaki Prefecture. We thank Ms. Chiharu Hoshiyama for her assistance in TEM investigation. Supporting Information Available: Experimental procedure; assignment of XRD pattens cited in Figure 2;

Uota et al. determination of the volume ratio of CaSt to HAp; XRD and TEM data for the characterization of the resulting and precursory phases. This material is available free of charge via the Internet at http://pubs.acs.org.

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