## 1 Introduction  
Nowadays, bone diseases, which are mainly caused by infections, defects, tumors, and injuries, are among the most common pathologies in clinical practices. It is certain that these defects require an appropriate bone treatment, without which irreversible consequences up to invalidity can occur. One of the most difficult but effective methods of treatment is surgery, with the subsequent replacement of the damaged area. One of the main aspects of bone treatment is the type and quality of materials used as a base for implants, which will determine the future successful healing of patients. Therefore, the creation and investigation of novel synthesized materials is gaining increasing interest in physics, biology, chemistry, and medicine [1; 2; 3; 4]. Currently, there are a number of suitable materials for bone substitution such as inert metals (titanium- and tantalum-based alloys), polymers (polycaprolactone, cellulose, collagen), and ceramics (bioglass, gypsum, calcium carbonate) [2; 5; 6]. However, it is considered that calcium phosphate (CP)-based compounds are the most favorable choice for the reliable restoration of human hard-tissue defects due to their chemical stability and compositional similarities to the inorganic, mineral phase of the bone. At the same time, processes of biomineralization have attracted scientific and technological attention for a long time in view of creating personalized, non-toxic materials for bone and dental tissue engineering [7; 8; 9], myocardial tissue regeneration [10], bioimaging and drug delivery [11; 12], tumor hypothermia treatment [13], tracking of the course of pathological calcification [14], etc. Several inorganic phases are involved in biomineralization, generally presented as non-stoichiometric carbonate-substituted calcium phosphates, among which the following are of the most importance: hydroxyapatite (HA, chemical formula Ca\({}_{10}\)(PO\({}_{4}\))\({}_{6}\)(OH)\({}_{2}\)), tricalcium phosphate (TCP, chemical formula Ca\({}_{3}\)PO\({}_{4}\)), dicalcium phosphate dihydrate (DCPD, CaHPO\({}_{4}\)\(\times\)2H\({}_{2}\)O, brushite), and octac calcium phosphate (OCP, Ca\({}_{8}\)H\({}_{2}\)(PO\({}_{4}\))\({}_{6}\)\(\times\)5H\({}_{2}\)O). At present, CP-based materials are commonly used in medicine and dentistry since they have shown excellent biocompatibility, osteoconductivity, and suitable mechanical properties. All of these CP materials provide the conditions for inducing osteoblast differentiation and improving the viability and proliferation of the bone cells [15; 16]. Such suitable properties have contributed to the use of these materials in a wide range of other biological applications, e.g., drug and gene delivery, as a filler for biocomposites, transfection processes, as fluorescing probes and biomarkers in nanomedical applications [17; 18]. At the same time, there are a lot of emerging applications in addition to the biomedical ones. For example, OCP has demonstrated a good capacity for the defluorization of water [19], which has previously been demonstrated for HA [20]. HA has also been applied as a catalyst and catalyst support [21]. HA and cation-doped HA in the form of nanoparticles were used for the environmentally friendly oxidative desulfurization of oil [22] and light gas oil [23]. Likewise, HA was presented as an efficient and reusable catalyst for the epoxidation of olefins and \(\alpha\), \(\beta\)-unsaturated ketones using hydrogen peroxide under relatively mild conditions [24]. Recently, the nitrate doping of CP nanoparticles was proposed for potential use as (P,N) nanofertilizers in agricultural applications [25], and EPR was recognized as one of the fastest methods of evaluating the presence and amount of nitrates. It is also known that HA is applied as an adsorbent for water purification [26], including the removal of nitrate due to its adsorption onto CP surfaces [27].