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

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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.