Spatial Organization of Hydroxyapatite Nanorods on a Substrate via a Biomimetic Approach

The growth of uniform single-crystal HAp (hydroxyapatite) nanorods on specific sites on a substrate is achieved. HAp nanorods were synthesized under hydrothermal conditions in a solution containing calcium and phosphate ions, urea as a source of base, and with gelatin as a structure directing agent. The nanorods are hundreds of micrometers in length and 50-200 nm in width. Without gelatin, platelike HAp crystals were formed. To grow these uniform HAp nanorods on specific sites on a substrate, the surface of a gold-coated glass slide was coated by a self-assembled monolayer of 11-mercaptoundecanoic acid by stamping with a patterned PDMS (polydimethylsiloxane) stamp. The HAp nanorods were grown exclusively in the carboxylate functionalized regions. Selective area electron diffraction analyses show that the single-crystal HAp nanorods formed on the substrate grow along the crystallographic c-axis. In comparison to the nonpatterned sample, an increase of the X-ray diffraction intensity ratio of (00l)/(h00) peaks in the patterned sample indicates that nanorods are preferentially oriented with their c-axes perpendicular to the substrate, which was confirmed by SEM cross-sectional images. In this strategy, both the morphology and the deposition sites of HAp nanorods are controlled.