Synthesis and characterization of hydroxyapatite using polymerized complex method by chelation of calcium ions with organic phosphonic acid

Abstract

Hydroxyapatite (HAp) precursors were prepared by chelation of calcium ions with organic phosphonic acid by using calcium nitrate hydrate (Ca(NO 3 ) 2 ·4H 2 O) and 2-phosphonobutane-1.2.4-tricarboxylic acid (C 7 H 11 PO 9 ; PBTA) in distilled water in the temperature range of 60-130°C. HAp powder was then synthesized by heating the precursors at various temperatures. HAp precursors were pectized in a three-step reaction. First, calcium ions were chelated with carboxyl groups in PBTA in an early reaction. Second, chelation of calcium ions with phosphonic acid and/or the dehydrated condensation of carboxyl groups with phosphonic acid occurred at and above 80°C. Finally, dehydrated condensation of carboxyl groups proceeded in the range of 100-130°C. The homogeneity of the HAp precursors improved with an increase in their preparation temperature, which accelerated gelation. Powder obtained after heating the HAp precursor prepared at 130°C (preHAp-130) had a single crystalline HAp phase at 1000°C. In addition, the crystallization temperature of preHAp-130 was 200°C, which was lower than that of HAp precursors prepared using other polymerized complex methods. Powder obtained by heating HAp precursors at and below 800°C contained carbonate apatite, in which carbonate ions replaced hydroxyl and phosphate sites in the HAp crystal lattice. The Ca/P molar ratio of the HAp powder was higher than that in the stoichiometric composition of HAp (Ca/P = 1.67), because the organic components of the HAp precursors were not pyrolyzed; however, the powder obtained by heating preHAp-130 at 1000°C had stoichiometric HAp composition due to the thermal decomposition of the precursors. The particle size of the HAp powder prepared by heating preHAp-130 decreased with an increase in the heating temperature, and when the HAp particles were heated at and above 800°C, a three-dimensional network with micro and nanopores was formed.