Ammonium-Induced Synthesis of Highly Fluorescent Hydroxyapatite Nanoparticles with Excellent Aqueous Colloidal Stability for Secure Information Storage

Abstract

In this paper, uniform hydroxyapatite (HA) nanoparticles, with excellent aqueous colloidal stability and high fluorescence, have been successfully synthesized via a citrate-assisted hydrothermal method. The effect of the molar ratio of ammonium phosphate in phosphate (RAMP) and hydrothermal time on the resultant products was characterized in terms of crystalline structure, morphology, colloidal stability, and fluorescence behavior. When the RAMP is 50% and the hydrothermal time is 4 h, the product consists of a pure hexagonal HA phase and a uniform rod-like morphology, with 120- to 150-nm length and approximately 20-nm diameter. The corresponding dispersion is colloidally stable, and transparent for at least one week, and has an intense bright blue emission (centered at 440 nm, 11.6-ns lifetime, and 73.80% quantum efficiency) when excited by 340-nm UV light. Although prolonging the hydrothermal time and increasing the RAMP had no appreciable effect on the aqueous colloidal stability of HA nanoparticles, the fluorescence intensity was enhanced. The cause of HA fluorescence are more biased towards carbon dots (which are mainly polymer clusters and/or molecular fluorophores constituents) trapped in the hydroxyapatite crystal structure. Owing to these properties, a highly fluorescent HA colloidal dispersion could find applications in secure information storage.