Hierarchical hollow hydroxyapatite microspheres: microwave-assisted rapid synthesis by using pyridoxal-5'-phosphate as a phosphorus source and application in drug delivery.

Three-dimensional (3D) hydroxyapatite (HAP) hierarchical nanostructures, in particular hollow nanostructures, have attracted much attention owing to their potential applications in many biomedical fields. Herein, we report a rapid microwave-assisted hydrothermal synthesis of a variety of hydroxyapatite hierarchical nanostructures that are constructed by the self-assembly of nanorods or nanosheets as the building blocks, including HAP nanorod-assembled hierarchical hollow microspheres (HA-NRHMs), HAP nanorod-assembled hierarchical microspheres (HA-NRMs), and HAP nanosheet-assembled hierarchical microspheres (HA-NSMs) by using biocompatible biomolecule pyridoxal-5'-phosphate (PLP) as a new organic phosphorus source. The PLP molecules hydrolyze to produce phosphate ions under microwave-hydrothermal conditions, and the phosphate ions react with calcium ions to form HAP nanorods or nanosheets; then, these nanorods or nanosheets self-assemble to form 3D HAP hierarchical nanostructures. The preparation method reported herein is time-saving, with microwave heating times as short as 5 min. The HA-NRHMs consist of HAP nanorods as the building units, with an average diameter of about 50 nm. The effects of the experimental conditions on the morphology and crystal phase of the products are investigated. The hydrolysis of PLP under microwave-hydrothermal conditions and the important role of PLP in the formation of 3D HAP hierarchical nanostructures are investigated and a possible formation mechanism is proposed. The products are explored for potential applications in protein adsorption and drug delivery. Our experimental results indicate that the HA-NRHMs have high drug/protein-loading capacity and sustained drug-release behavior. Thus, the as-prepared HA-NRHMs are promising for applications in drug delivery and protein adsorption.