Large-scale synthesis of water-soluble luminescent hydroxyapatite nanorods for security printing

Luminescent hydroxyapatite nanoparticles , which have excellent biocompatibility, excellent photostability, and strong fluorescence, have received increasing attention as bioprobes in cell imaging . However, they are also excellent candidates for use in ink-jet security printing. Successful products for related applications usually require highly crystalline, mono-dispersible hydroxyapatite nanorods with good colloidal stability and high fluorescence in aqueous media. These requirements are hard to simultaneously satisfy using most synthetic methods. In this paper, we report a simple and versatile hydrothermal method that incorporates the use of sodium citrate to prepare water-dispersible Eu 3+ -doped hydroxyapatite nanorods. The hydroxyapatite nanorods obtained using this method are highly crystalline rod-shaped particles with an average length of 50–80   nm and an average diameter of 15–30   nm. Dispersions of these hydroxyapatite nanorods, which are transparent with a slightly milky color under natural light and a bright red color when excited with 241   nm UV light , display zeta potentials of −35   mV and hydrodynamic diameters of 120   nm. These dispersions remain colloidally stable for a few months. Dispersions with these properties could be easily applied to security printing for confidential information storage and anti-counterfeiting technologies.