Self-Assembly of Dandelion-Like Hydroxyapatite Nanostructures Via Hydrothermal Method Aidin Lak, z Mahyar Mazloumi, z Matin Mohajerani, z Amir Kajbafvala, z Saeid Zanganeh, z Hamed Arami, z and S. K. Sadrnezhaad w,z,y z Materials and Energy Research Center, PO Box 14155-4777, Tehran, Iran y Department of Materials Science and Engineering, Center of Excellence for Production of Advanced Materials, Sharif University of Technology, PO Box 11365-9466, Tehran, Iran Self-assembled dandelion-like hydroxyapatite (HAp) nanostruc- tures were successfully synthesized via a mild template-free hydrothermal process, using ethylenediaminetetraacetic acid (EDTA) as the surfactant. The obtained dandelion-like HAp nanostructures were between 5 and 8 lm in diameter and were composed of radially oriented nanorods with an average diameter of about 200 nm. The X-ray diffraction analysis and Fourier transform infrared spectroscopy were used to characterize the crystalline phase and purity of the synthesized nanostructures. The Brunauer–Emmett–Teller surface area of the dandelion-like nanostructures was measured to be about 39 m 2 /g. The results of thermal analysis revealed that dandelion-like HAp nanostruc- tures have appropriate thermal stability up to 12001C. Scanning electron microscopy and transmission electron microscopy ana- lyses showed that EDTA plays an important role in obtaining the dandelion-like morphology, because without it, only mono- dispersed HAp nanoparticles with an average diameter of about 125 nm were formed. The mechanism for the formation of dandelion-like HAp nanostructures was suggested based on the radial self-assembly of Ca-EDTA molecular complexes. I. Introduction S ELF-ASSEMBLY is considered as a spontaneous organization of the initial constitutes into patterned and super-molecular nanostructures. 1,2 During the past decade, chemical self-assem- bly of nanostructures and formation of complex architectures have attracted increasing attention in nanoscience and nano- technology, because of the versatile physical and chemical advantages of these structures. 2–4 Therefore research on the self-assembly of nanoparticles into various two-dimensional (2D) and three-dimensional (3D) nanostructures has expanded greatly. 5 One-dimensional (1D) nanostructures (e.g., nanorods, nanowires, and nanotubes) have various applications, especially in biomedical fields because of their unique and fascinating properties, 6 and therefore previous research has been focused on developing different synthesis methods for their high yield and cost-effective production. 7–9 There are two distinct self- assembly strategies concerning the formation of nanostructure materials. 10 One strategy uses template-assisted techniques, in which hard templates (e.g., carbon nanotubes or porous anodic alumina) 11,12 or soft templates (e.g., surfactants) 13 are used to control the shape and morphology of nanocrystals. Another strategy is attributed to template-free techniques, which provide nanostructure materials through various physical phenomena such as Ostwald ripening, 14 Kirkendall effect, 15 and oriented attachment mechanism. 16 However, template-assisted ap- proaches have been restricted rather than template-free routes because of their high production cost. Therefore, more concerns have been devoted to the latter. For many years, hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 , HAp) has attracted much interest, because of its close chemical and physical resemblance to the mineral constituent of human hard tissues. 17 Because of its excellent biocompatibility, bioactivity, and osteoconductivity, artificially synthesized HAp has been used for many biomedical applications such as in orthopedics, dental implants, matrices for controlled drug delivery, and bone cements. 18 It is also an attractive material for nonmedical ap- plications including use in gas sensors, catalysts, and host ma- terials for lasers, and in ion-exchange applications. 18,19 Therefore various methods such as chemical precipitation, 20,21 sol–gel, 22 microwave assisted techniques, 23 sonochemistry, 24 and microemulsion 25 have been developed for preparation of HAp. In addition, the hydrothermal method has been widely used for preparing well-crystallized, well-dispersed, homogeneous, and nonagglomerated HAp powders with Ca/P molar ratios close to the stoichiometric value (i.e., Ca/P 5 1.67 for pure HAp). 25,26 However, difficulty in controlling the morphology of HAp has been reported as a drawback for its hydrothermal synthesis. 27 Recently, chelating agent 28 and urea 29 were used to control and change the morphology of the HAp through the hydrothermal decomposition of chemical complexes. Carboxylic acids such as acetic, citric, lactic, and ethylenediaminetetraacetic acid (EDTA) were also used repeatedly as chelating agents. 30 There are few investigations that have reported fabrication of HAp nanostructures with 3D morphology. Liu et al. 31 have re- ported synthesis of HAp bowknot-like and flower-like nano- structures via microwave irradiation. Teng et al. 32 have reported fabrication of flower-like HAp nanostructures during 20 h of conventional diffusion process. Fabrication of flower-like po- rous carbonated HAp was reported by He et al. 33 However, hydrothermal preparation of dandelion-like HAp nanostruc- tures with the help of EDTA has never been reported elsewhere. In this paper, we report a facile and template-free hydrother- mal method for synthesis of self-assembled dandelion-like HAp nanostructures with high surface area. Such 3D nanostructures would be beneficial in making novel catalysts, molecular sieves, biosensors, and nanocomposites used in tissue engineering. 34,35 The effect of EDTA chelating agent on the morphology of HAp was also investigated. II. Experimental Procedures (1) Sample Preparation Nanostructure HAp was prepared via a hydrothermal synthesis technique. Analytical-grade CaCl 2 (Merck, Darmstadt, Ger- many), K 2 HPO 4 (Merck), EDTA, C 10 H 16 N 2 O 8 (Merck), KOH (Merck), and distilled water were used as starting mate- rials. These reagents were used without any further purification. R. Riman—contributing editor w Author to whom correspondence should be addressed. e-mail: sadrnezh@sharif.edu Manuscript No. 23779. Received September 25, 2007; approved June 12, 2008. J ournal J. Am. Ceram. Soc., 91 [10] 3292–3297 (2008) DOI: 10.1111/j.1551-2916.2008.02600.x r 2008 The American Ceramic Society 3292