Synthesis and dispersion of hydroxyapatite nanopowders

Materials Science and Engineering C 32 (2012) 1237–1240

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Short communication

Synthesis and dispersion of hydroxyapatite nanopowders S.K. Swain a, S.V. Dorozhkin b, D. Sarkar a,⁎ a b

Department of Ceramic Engineering, NIT - Rourkela, Orissa, India Kudrinskaja Square 1 - 155, 123242, Moscow, Russia

a r t i c l e

i n f o

Article history: Received 12 July 2011 Received in revised form 19 January 2012 Accepted 21 March 2012 Available online 30 March 2012 Keywords: Hydroxyapatite Nanostructures X-ray diffraction Infrared spectroscopy Microstructure Dispersability

a b s t r a c t Spherical, rod and fibroid hydroxyapatite [HAp, Ca10(PO4)6(OH)2] nanoparticles were prepared and dispersed in aqueous media. Temperature and solution pH were the key factors to synthesis of different morphology and crystallinity. Processing conditions were selected from ternary diagram of pH, temperature and Ca:P ratio. High hydroxyl ion concentration (12.25 ≥ pH ≥ 10.5) and low temperature (298 K) favored isotropic non-confined spherical particles, intermediate concentration (9.5 ≥ pH ≥ 7.75) and low temperature (303 K) initiated the anisotropic growth of rod shaped particles but low concentration (7 ≥ pH ≥ 5.25) and high temperature (353 K) accelerated one-dimensional fibroid morphology. The dispersed HAp–citrate complex exhibited a constant zeta potential and size distribution for six months. © 2012 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental

Nano scale hydroxyapatite (HAp) is a classic bioactive and biocompatible material, which simulates both of the dental mineral compartments and of bone [1]. However, the specific use of HAp depends on Ca:P ratio, crystallite size and their morphology [2]. The dispersed nano HAp particles have potential demand for gene carrier, protein delivery media, freeze casted porous scaffold and artificial bone regrowth because of high absorbability and binding affinity with versatile molecules [3–5]. Spherical morphology prefers as delivery media, whereas high aspect ratio enhances the mechanical properties of scaffolds [6,7]. Therefore, control over the size, morphology and composition of HAp phase has great importance for different bio-applications [8]. Usually, biocompatible HAp nanoparticle has tendency to coagulate without any external mechanical stirring or ultrasonic energy [9,10]. However, a common technique such as selective pH of solution and zeta potential favors the formation of homogenous suspension of nanopowders for a long duration [11,12]. In this backdrop, a common strategy has been adapted to synthesis of different morphology (spherical, rod and fibroid) of HAp nanoparticles and dispersed in aqueous media to prepare stable nanocolloids.

Hydroxyapatite nanoparticles were prepared from two common precursors; calcium acetate ((CH3COO)2Ca) and potassium dihydrogen phosphate (KH2PO4). Double distilled water was boiled and cooled down to prepare solutions. Both of the solutions [(CH3COO)2 Ca=0.1 M and KH2PO4 =0.06 M] were slowly added together and pH was controlled through NH4OH and tris-buffer (Tri-methylhydroxy aminomethane) for spherical and rod morphology, respectively. A specific tailor-made Ptsensor attached glass vessel was used to synthesis of these nano particles. Different 35 sets of experiments were conducted in account of Ca/P ratio, pH and temperature, wherein 22 numbers of particles were crystalline with desired morphology. However, only specific five number of powders exhibited Ca/P near to 1.67. The entire set of experimental conditions and their outcome was represented in Fig. 1. Three optimized conditions were considered to achieve the desired morphology and Ca/P ratio of pure HAp nanoparticles. Spherical HAp nano particles (NHS), rod HAp nanoparticles (NHR) and fibroid HAp nanoparticles (NHF) were developed at pH=12.25, temperature=298 K; pH=9.5, temperature=303 K and pH=5.25, temperature=353 K, respectively. The precipitates were washed, centrifuged at 14,000 rpm and finally freeze dried at 220 K and 120 Torr pressure. Three morphologies were prepared separately to characterize and prepare nanocolloids; each batch size was 60 g. Calcium (Ca) and phosphorus (P) was determined by complexometric titration with EDTA and gravimetric method through ammonium molybdate, respectively [13]. In addition, Ca/P ratio was also crosschecked through ICP technique. Phase analysis and specific surface area of HAp nano

⁎ Corresponding author. Tel.: + 91 661 2462207. E-mail address: [email protected] (D. Sarkar). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2012.03.014

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Fig. 2. Room temperature XRD pattern of a) NHS, b) NHR and c) NHF after freeze drying at 220 K and 120 Torr pressure.

Fig. 1. Hatched area of ternary phase diagram indicates the projected region for the formation of spherical (NHS), rod (NHR) and fibroid (NHF) morphology. Colored circular point represents the optimum conditions to achieve Ca:P ~ 1.67 for diversified morphology.

powders were studied from room temperature powder X-ray diffraction (XRD) analyzer and BET method, respectively. Furthermore, powder morphology and crystallinity were estimated through HRTEM and SAED (Selected Area Electron Diffraction) analysis. The dispersion of 50 mg NHR powder was executed in 400 ml double distilled water by the addition of (2 ml) 0.1 M citric acid and (2.5 ml) 0.1 M NaOH solutions and processed at 348 K and 350 rpm. The synthesized HAp particles and colloidal NHR–citrate complex was analyzed through FTIR technique. Particle size and zeta potential at different steps were measured during preparation of nanocolloids. 3. Results and Discussion A ternary phase diagram has been sketched out based on processing parameters; pH, temperature and Ca:P ratio (Fig. 1). Hatched region represents crystalline HAp particles with different morphologies. However, most of the particle appears as calcium phosphate beyond these hatched regions. High concentration of hydroxide (12.25 ≥ pH ≥ 10.5) ion and low temperature (298 K) favors isotropic growth of spherical shape, medium concentration (9.5 ≥ pH ≥ 7.75) and low temperature (303 K) initiates the anisotropic growth of rod, whereas low concentration (7 ≥ pH ≥ 5.25) and high temperature (353 K) accelerates the confined growth to fibroid morphology as represented in transmission electron microscopic microstructure. High solution pH adsorbs more OH − ions on the entire surface of HAp nuclei and favors non-confined threedimensional growth to spherical morphology [14]. However, at intermediate solution pH, a small amount of hydroxide ions are expected to release from tris-buffer and adsorbed on selective site of HAp nuclei, which results in a weak isotropic growth to nano rods. Hence, minimization of OH − concentration is advantageous to confined and directional growth of HAp nanoparticles. Low solution pH restricts the OH − ion adsorption on HAp nuclei, which dominates anisotropic growth and crystallizes at 353 K; i.e. growth of HAp nuclei to fibroid morphology enhances as similar mechanism proposed by Bu, et Al. [15]. Phase content, purity and crystallinity of these HAp nanoparticles are represented by XRD pattern in Fig. 2. The diffraction peaks are indexed as HAp hexagonal phase with a = b = 9.4180, c = 6.8840 and space group P63/m (JCPDS No. 09-0432). NHS and NHR are appeared as low crystalline, while NHF is well crystalline.

The crystallinity of pure phase has been increased with increasing aspect ratio; highest for fibroid morphology. The calculated crystallite size as preferred (211) plane from NHS, NHR and NHF samples are 7 nm, 9 nm and 61 nm, respectively [16]. HRTEM images of these HAp powders and their SAED pattern has shown in Fig. 3. A close look reveals that the formation of spherical particles near to 10 nm diameter with strong concentric ring pattern (002), (211), (112) and (300) plane of polycrystalline HAp [17]. Rod shaped morphology exhibits near to 8 nm diameter with aspect ratio of ~5. The dotted SAED patterns are well matched with ring, which confirms more crystallinity of rod morphology compare to spherical powder. High crystallinity and purity of fibrous morphology as defined by XRD pattern supports the SAED pattern. Average diameter is gradually increased to 30–40 nm with very high aspect ratio for fibroid morphology. BET surface area of these freeze-dried NHS, NHR and NHF particles are estimated as 256 m 2/g, 217 m 2/g and 47 m 2/g and calculated particle size were 7, 9 and 70 nm, respectively. Elemental analysis in ICP method reveals that the Ca:P ratio is 1.671, 1.669 and 1.664 for spherical, rod and fibroid morphology, respectively. FTIR spectrum of NHS, NHR and NHF powders are shown in Fig. 4(A). The absorption bands are detected at wave numbers 3435, 1040, 605, and 570 cm − 1 for the synthesized HAp nanoparticles. The absorption band at 1430 cm − 1 has been detected for NHS due to the absorption of atmospheric CO2 on the surface, since HAp has more affinity towards the absorption of CO2 in high solution pH media compare to acidic media. The bands at wave number 1040 and 570 cm − 1 are associated with the characteristics of PO43− group, but bands at 3435 and 605 cm − 1 represents the characteristic stretching and bending modes for OH – group, respectively [18]. The stretching mode of OH – in NHR and NHF are broader than NHS due to the partial attachment of water molecules through van der Waals bonds during synthesis at low solution pH [19]. Thus, the synthesized HAp nano particles are composed of PO43− and OH − anions and free from other impurities. However, the hydroxyapatite–citrate complex exhibits different spectroscopic behavior due to the formation of hydrogen bond and carboxyl-calcium-carboxyl ([COO-]-Ca-[COO-]) complex as represented in Fig. 4(B) [20]. FTIR spectrum of HAp–citrate complex exhibits C–O spectra bands of ν as(COO –) at 1619 cm − 1 and ν s(COO –) at 1416 cm − 1, respectively [21]. Most intense and broad peak at 3458 cm − 1 is because of OH − ions of citrate group adsorbs on the HAp surface. The spectra band difference indicates the formation of citrate complex. Zeta potential (ξ; mV) has been measured to monitor the dispersion of nanoparticles. Dispersed nanoparticles are stable for the extent of more than six months and exhibits a constant zeta potential, −19.2 mV, −17.6 mV and −15.2 mV for

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Fig. 3. HRTEM and SAED represent the morphology and crystal pattern of NHS, NHR and NHF.

NHS, NHR and NHF, respectively. The estimated particle sizes in dispersed conditions are 21 nm, 32 nm and 150 nm for NHS, NHR and NHF, respectively. Relatively larger particle sizes are expected in dispersed condition because of the formation of HAp–citrate complex instead of only HAp nanoparticles. The negative zeta potential are favorable to make homogenous dispersion because of the superior chelating ability of citrate ions to complex Ca 2+ ions, which enhances (a) (b) 3435cm

A 1629cm

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570cm 1040cm

4. Conclusions The particle size of spherical, rod and fibroid morphology of HAp nanopowders were 7, 9 and 70 nm, respectively, which varies in stable colloids due to formation of hydroxyapatite–citrate complex. Aspect ratio and crystalline was enhanced with low solution pH and increase temperature. Dispersed HAp nanoparticles could be used as delivery media and preparation of porous scaffold with suitable solid loading. References

B 3430cm

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1416cm 1619cm

3458cm

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[1] [2] [3] [4] [5] [6] [7]

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the more negative electrophoretic mobility of HAp in citrate solution [22]. HAp–citrate complex increases the interfacial surface area due to the repulsion between particles and increases the dispersibility of these nano particles.

1500

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Wavenumber (cm-1) Fig. 4. FTIR spectrum of (a) NHS, (b) NHR and (c) NHF nanoparticles (A) and before and after dispersion of NHR Nanoparticles (B).

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