Template-Free Growth of Novel Hydroxyapatite Nanorings: Formation Mechanism and Their Enhanced Functional Properties A. Joseph Nathanael, †,‡ Sun Ig Hong,* ,‡ D. Mangalaraj,* ,§ N. Ponpandian, § and Pao Chi Chen ⊥ † Department of Physics, Bharathiar University, Coimbatore 641 046, India ‡ Department of Nanomaterials Engineering, Chungnam National University, Daejeon 305-764, South Korea § Department of Nanoscience and Technology, Bharathiar University, Coimbatore 641 046, India ⊥ Department of Chemical and Materials Engineering, Lunghwa University of Science and Technology, Taoyuan, Taiwan * S Supporting Information ABSTRACT: Template-free, single crystalline novel hydroxyapatite (HAp) nanorings with an inner diameter of 70 nm were grown by a combined high gravity and hydrothermal approach. Nanodisks were suggested to be formed by oriented aggregation and Ostwald ripening of mostly calcium pyrophosphate nanospheres prepared initially by the high gravity method with a stepwise increase of flow rate of phosphate solution. The prolonged hydrothermal treatment of nanodisks appeared to induce the nanoring formation via acid penetration along the dislocations in HAp nanodisks. The presence of edge dislocations in the central region of nanodisks was confirmed by high resolution transmission electron microscopy. The mechanical evaluation of high molecular weight polyethylene (HMWPE) composite with various shaped HAp nanocrystals and in vitro cellular analysis of HAp nanocrystals revealed that mechanical and bioactive performances improved with an increase of the specific surface area of HAp nanocrystals. The enhanced mechanical performance of HMWPE/ HAp nanoring composite and the excellent cell viability for HAp nanorings are attributed to the superior interface bonding and cell activity, respectively, both of which are enhanced by the high specific surface area. 1. INTRODUCTION One of the most challenging processes in materials engineering is the controlled fabrication of materials with user-defined shapes to provide an increasingly precise control over the structures and hence their properties. Significant progress has been made in the fabrication of one- and two-dimensional (1D and 2D) nanomaterials such as nanoparticles, rods (wires, cables, tubes, ribbons, and helixes), and sheets. 1-7 Among those, hollow nanostructures such as nanocages, nanospheres, and nanorings have attracted much interest because these hollow cavities improve the functionality of these nanomaterials to be used in various applications. 8-12 However, it is still a challenge to develop such functional hollow ring-like shapes in exact circular, oval, or polygonal morphologies with large yield. It should be noted that in various studies, the ringlike nanostructures were either grown directly onto templates followed by template removal or rearranged from a particular part of raw material. 7,13-17 Development of free-standing ringlike nanostructures using self-organized tiny building blocks has been a challenging task until now. Recently, a few successes were made in the synthesis of free- standing rings (e.g., CdS, ZnO, Au, Cd(OH) 2 , and Ag 2 V 4 O 11 ) by solution-based processes based on different synthesis mechanisms, such as self-assembly of primary nanoparticles, central-etching of disks, and self-coiling of nanobelts. 7,18-22 Since nanorings have a unique structural texture, they exhibit novel properties caused by the presence of cavities. This cavities may greatly enhance the functionality of nanomaterials. 14-17,23 For example, 24 the cavities of gold nanorings influence an extremely uniform field enhancement effect. It has been suggested that in sensing and spectroscopy applications they could act as a resonant nanocavities for probing or carrying tiny nanostructures such as quantum dots, biomolecules etc. 24 Similarly α-Fe 2 O 3 nanorings exhibit excellent characteristics as a sensor. It has shown long-term stability, good reproducibility, and function as a highly sensitive electro-catalyst mainly due to the high surface-to-volume ratio and the unique network of interconnected pores in the nanorings (Fe 2 O 3 ). 12 Therefore, research for new synthesis methods to achieve such hollow ring-like morphologies is imperative. The mineralized tissues of vertebrates include bone, dentine, and calcified cartilage. All these tissues have hydroxyapatite (HAp; Ca 10 (PO 4 ) 6 (OH) 2 ) as their mineral component but with some modification of their organic matrix composition. The HAp has been studied extensively for cell cultures and has been found to possess good osteoconductive properties. 25,26 Received: March 26, 2012 Revised: May 27, 2012 Published: May 29, 2012 Article pubs.acs.org/crystal © 2012 American Chemical Society 3565 dx.doi.org/10.1021/cg3003959 | Cryst. Growth Des. 2012, 12, 3565-3574