Surface State Control of Apatite Nanoparticles by pH Adjusters for Highly Biocompatible Coatings

Apatite nanoparticles are biocompatible nanomaterials, so their film formation on biodevices is expected to provide effective bonding with living organisms. However, the biodevice–apatite interfaces have not yet been elucidated because there is little experimental evaluation and discussion on the nanoscale interactions, as well as the apatite surface reactivities. Our group has demonstrated the biomolecular adsorption properties on a quartz crystal microbalance with dissipation (QCM-D) sensor coated with apatite nanoparticles, demonstrating the applicability of apatite nanoparticle films on devices. Therefore, it is important to clarify the biodevice–apatite nanointerfaces by characterizing their physicochemical properties. This research aims to control the apatite nanoparticle surfaces using different types of pH adjusters as well as to investigate biodevice–apatite interfaces. In this study, tetramethylammonium hydroxide, sodium hydroxide, and potassium hydroxide were used to adjust the pH during the synthesis of apatite nanoparticles. As a result, it was found that the ratio of Ca-deficient hydroxyapatite phase to B-type carbonate ion-substituted hydroxyapatite phase could be controlled by adjusting the OH– concentration and that the formation of B-type carbonate ion substituted hydroxyapatite phase was demonstrated in terms of the charge compensation because hydrogen phosphate ion in the non-apatitic layer would be diffused and substituted inside the apatite core structure by the replacement of carbonate ion. By contrast, the phosphate ions in the core structure were moved and contained in the non-apatitic layer, and the proportion of phosphate ions in the non-apatitic layer increased. The surface changes of the nanoparticles provide an effective biodevice surface coating. It was observed that the thickness of the electrophoretically deposited nanoparticles clearly increased with the proportion of phosphate ions in the non-apatitic layer. Furthermore, the formation of the hydration layer with immersion in biological fluid was evaluated. It was inferred that the water molecules in the hydration layer interacted with the substituted ions and remained as nonfreezing water layer on the top surface of the nanoparticles, while the abundant phosphate ions newly interacted with the water molecules in the non-apatitic layer, thus increasing the proportion of intermediate water. These results indicated that the hydrogen phosphate and phosphate ions were retained in the non-apatitic layer on the top surfaces of apatite nanoparticles, so that the thickness of the electrophoretically deposited film and the weight fraction of the hydrated layer can be controlled by the component ratio of phosphate ions in the non-apatitic layer. It is expected that surface coating technology using apatite nanoparticles will be applied for biodevices.