1 m calcium nitrate

A well-established wet chemistry and hydrothermal treatment method was employed to synthesize nHA with nanometer size and crystallinity. In brief, 37.5 mL of a 0.6 M ammonium phosphate (Sigma Aldrich, St. Louis, MO) solution was added to 375 mL of water and adjusted to a pH of 10 with ammonium hydroxide (Fisher Scientific, Pittsburgh, PA). Subsequently, 1 M calcium nitrate (Sigma Aldrich, St. Louis, MO) was titrated into the above mixture at a rate of 5 mL/min while stirring. Precipitation of amorphous HA proceeded for 10 min at room temperature. The mixture containing HA precipitate was hydrothermally treated at 200 °C for 20 h in a 125 ml Teflon liner (Parr Instrument Company, Moline, IL). After 20 h, the nHA precipitate was centrifuged and rinsed thoroughly with water three times, dried at 80 °C for 12 h and ground into a fine powder and thoroughly characterized as described in our previous work.^{26 (link)-31 (link)}

A wet chemistry plus hydrothermal treatment method was used to synthesize nHA with good crystallinity and nanometer particle size as described in our previous papers^{32 (link)–36 (link)}. Briefly, 37.5 mL of a 0.6 M ammonium phosphate (Sigma Aldrich, St. Louis, MO) solution was added to 375 mL of water and adjusted to pH 10 with ammonium hydroxide (Fisher Scientific, Pittsburgh, PA). A 1 M calcium nitrate (Sigma Aldrich, St. Louis, MO) solution added dropwise into the above mixture while stirring. Precipitation of HA continued for 10 min at room temperature. Upon complete addition of calcium nitrate, the HA precipitate containing solution was hydrothermally treated at 200 °C for 20 h in a 125 mL Teflon liner (Parr Instrument Company, Moline, IL). After 20 h, the nHA precipitate was centrifuged and rinsed thoroughly with water three times, dried at 80 °C for 12 h and ground into a fine powder.