Controlling Hydrothermal Synthesis Of Hydroxyapatite Nanoparticles And Their Liquid Crystal Phase Transition

Ordering a specimen in a continuous phase on the nanometer scale is of particular interest in materials science because of the outstanding optical and mechanical properties of such hybrids or nanocomposites. Bone tissues, which are peculiar natural examples of ordered nanocomposites, are highly organized composites with hierarchical structures in which the basic building blocks(apatite crystals) are generally in the nanometer size range; this organization ensure optimal physical and biological function. To simulate the natural properties of bone for biomedical applications, such as bone repair and bone fixation, much effort has been expended on the preparation of polymer-based hydroxyapatite nanocomposites. However, several issues limit the ability of currently available nanocomposites to reach ideal mechanical performance, including how to disperse nanoparticles homogeneously in the polymer matrix, how to macroscopically align the nanoparticles in the polymer matrix, and how to enhance interfacial bonding between the particles and the polymer matrix. Solving these challenges simultaneously will be a key to obtaining ideal mechanical performance from these materials, particularly for achieving macroscopically ordered materials through self-organization or self-assembly of hydroxyapatite nanorods in polymer matrixes. The self-assembled three-dimensional of nanoparticles in inorganic colloidal liquid crystal is similar to the structure of natural bone tissue. To our best knowledge, there have been no reports of hydroxyapatite liquid crystals.Firstly, we systematically evaluated the hydrothermal synthesis of hydroxyapatite nanocrystals with aid of sodium citrate. By controlling experimental parameters including the molar ratio of sodium citrate and calcium salt, hydrothermal temperature, hydrothermal time, pH values, hydroxyapatite nanorods with high aspect ratio, excellent colloidal stability and good crystallinity were obtained. When the dispersion of hydroxyapatite nanorods was concentrated beyond the critical particles concentration, the phase transition was firstly observed. Because it is well known that the strong interactions enable citrate ions to form complexes with many different metal ions, this case is more than an exception. The procedure should allow the design of many new types of LCs. As examples, LCs were successfully synthesized from Mg(OH)2 and Mg3(PO4)2 nanoplates.Secondly, we improved the previous reported LSS strategy. Large-scale of 5g hydrophobic hydroxyapatite particles were first synthesized using a simple hydrothermal route under low temperature and atmospheric pressure. The Liquid crystal phase transition of hydroxyapatite nanoparitcles in nonpoliared solvents was first observed. Besides that, it was recognized that acid environment of precursor is significant to the synthesis of needle-shaped nanocrystals, which was attributed to formation of amorphous calcium phosphate.Inspired by the our successful work on the hydrothermal synthesis of hydroxyapatite nanorods with well dispersion performance, we report a simple and versatile hydrothermal method that incorporates the use of sodium citrate to prepare water-dispersible Eu3+-doped hydroxyapatite nanorods with an average length of 50~80 nm and an average diameter of 10~30 nm. Dispersions of these hydroxyapatite nanorods, which are transparent with a slightly milky color under natural light and a bright red color when excited with 254 nm UV light, display zeta potentials of-35 mV and hydrodynamic diameters of 124 nm. These dispersions remain colloidally stable for a few months. Dispersions with these properties could be easily applied to security printing for confidential information storage and anti-counterfeiting technologies.In short, liquid crystal phase transition both in water and non-polarized solvents was first observed by precise controlling growth of hydroxyapatite nanoparticles. In addition, fluorescence of hydroxyapatite nanoparticles could be easily introduced by Eu3+ doping without loss of colloidal stability. These findings will bring great impact on the related fields of biomaterial fabrications, drug delivery and cell images, but also promoted the methodology of synthesis of inorganic liquid crystals.