Sol–gel synthesis and characterization of hydroxyapatite nanorods

Abstract

In the present study hydroxyapatite (HA) nano-hexagonal rods with 70–90 nm diameter and 400–500 nm length are synthesized using a simple sol–gel route with calcium nitrate and potassium dihydrogenphosphate as calcium and phosphorus precursors respectively. Deionized water was used as a diluting media for HA sol preparation and ammonia was used to adjust the pH = 9. After aging, the HA gel was dried at 60 °C and calcined at different temperatures ranging from 300 to 700 °C. The dried and calcined powders were characterized for phase composition using X-ray diffractrometry, elemental dispersive X-ray and Fourier transform infrared spectroscopy. Rietveld analysis showed the calcined HA powders of high purity with a hexagonal unit cell structure. Calcination yielded HA nanopowders of increased particle size and crystallinity with increase in temperature. The particle size and morphology was studied using transmission electron microscopy. The aspect ratio (length to diameter ratio) of HA nanorods was measured to be between 6 and 7.

Introduction

Hydroxyapatite Ca₁₀(PO₄)₆(OH)₂ (HA) has great importance in materials chemistry because of its excellent biocompatibility (LeGeros, 2002) and good chromatographic properties for proteins purification (Kawasaki, 1991). In biomaterials, nano-HA is used as a drug delivery agent for anti-tumor and antibodies in the treatment of osteomyelitis; a bone infection that is often treated by excision of necrotic tissue and irrigation of the wound (Yamashita et al., 1998). HA has also been used as an acidic catalyst for different chemical reactions (Sebti, Tahir, Nazih, Saber, & Boulaajaj, 2002). All these HA specific applications are dependent on properties such as particle size, dimensional anisotropy, morphology, real microstructure, etc. which are of critical importance for optimization and applications. Nanorod morphology of HA has gain a lot of attention in recent years. For instance, in column chromatography application, rod shaped HA shows enhanced protein adsorption because of their charging surface efficiency (Kawachi et al., 2006, Vasiliev et al., 2008). HA crystals with nanorod features have shown to possess desirable biocompatibility and bioactivity because of better adsorbability, since the underlying van der Waal's interactions are proportional to the large (surface) area of the rods (Zandi et al., in press). Even HA present in human tooth and bone exhibits the form of nano-polycrystalline hexagonal nanorods (Cuisinier & Robinson, 2007).

HA powders can be synthesized using a number of methods including sol–gel processing (Bigi, Boanini, & Rubini, 2004), co-precipitation (Pang and Bao, 2003, Phillips et al., 2003), emulsion techniques (Lim, Wang, Ng, Chew, & Gan, 1997), batch hydrothermal processes (Kothapalli et al., 2005, Liu et al., 2003, Riman et al., 2002, Wang et al., 2006, Zhang et al., 2005), mechano-chemical methods (Rhee, 2002) and chemical vapour deposition (Darr, Guo, Raman, Bououdina, & Rehman, 2004). HA rods have been successfully synthesized using hydrothermal (Zhang and Vecchio, 2007, Zhang et al., 2005), wet chemical (Liu, Hou, & Wang, 2004), protein precursor method (Han, Li, Wang, Jia, & He, 2007), bio-mimetic techniques (Chen, Clarkson, Sun, & Mansfield, 2005), ultrasonic spray pyrolysis (An, Wang, Kim, Jeong, & Choa, 2007), etc. However not much has been reported on the sol–gel synthesis of HA nanorods. Sol–gel method has been popular because of well-known inherent advantages such as homogeneous molecular mixing (Kim & Kumta, 2004), low processing temperature and ability to generate nanocrystalline powders, bulk amorphous monolithic solids and thin films (Afshar et al., 2003, Liu et al., 2001). In the present work, a simple sol–gel based method is reported for the synthesis of HA nanorods having a hexagonal crystal structure, using calcium nitrate, potassium dihydrogenphosphate and ammonia as chemical precursors. Rods of 70–90 nm diameter and 400–500 nm length, i.e. with an aspect ratio >5 are reported in this study.