Calcium phosphates (CaPs) are the main inorganic constituents of human hard tissues. There are a lot of CaP phases, which have different physical properties and application. Human hard tissues mostly consist of calcium hydroxyapatite (HAp) which is the most stable CaP phase around neutral pH [1]. The second most abundant CaP phase is whitlockite. Magnesium whitlockite is a member of CaPs family, where some calcium ions are substituted by smaller magnesium ions. Chemical formula of magnesium whitlockite is Ca18Mg2(HPO4)2(PO4)12 [2]. Unfortunately, the preparation of phase-pure whitlockite remains very challenging and the obtained synthesis products often contain impurities. Nevertheless, this material is a promising candidate in orthopedic surgery because it is biocompatible and possesses excellent osteogenic properties. Whitlockite phase is more stable than HAp in acidic solution so it is beneficial in bone mineralization [1].
Doping of CaPs with other ions may result in the appearance of new physical and biological properties, which can be used in wider application area. Copper is an essential trace element, necessary for mammalian life and it plays a crucial role in the cross-linking of collagen and bone elastin [3]. It is important for metabolism processes because it acts like cofactor for many enzymes [4]. Copper exhibits antibacterial properties and limited cytotoxicity, due to these reasons it is a promising candidate for doping into CaPs matrices. Cu-substituted CaPs are well known for their antibacterial, angiogenic and osteogenic properties [5].
The main goal of this work was to synthesize single phase whitlockite powders. Copper whitlockite powders were synthesized by dissolution-precipitation method under hydrothermal conditions using calcium hydrogen phosphate dihydrate (CaHPO4·2H2O) and copper nitrate trihydrate (Cu(NO3)2·3H2O) as starting materials. The crystallinity, crystal structure and structural changes were evaluated by powder X-ray diffraction (XRD), Fourier-transform infrared (FTIR) and Raman spectroscopies. Scanning electron microscopy (SEM) was used for the characterization of morphological features of products. Chemical composition of synthesized powders was analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES).
The results of XRD, FTIR and Raman spectroscopies confirmed that synthesized CaP has whitlockite structure. It was demonstrated that optimal reaction temperature is 200°C, at this temperature was synthesized single phase whitlockite powders. Optimal pH range was determined to be from 6.4 to 6.6. Neighboring crystal phases were obtained at lower temperatures and higher/lower pH of the reaction medium. Also, it was found, that optimal reaction time is about 3 hours. Thermal stability studies revealed that copper-containing whitlockite is thermally unstable and decomposes after a heat treatment.