The Effect of Full-Scale Exchange of Ca²⁺ with Co²⁺ Ions on the Crystal Structure and Phase Composition of CaHPO₄·2H₂O

Abstract

The influence of ionic substitution in the Ca_1−xCo_xHPO₄·nH₂O compound was studied systematically for the first time. Among the fascinating features of these biomaterials is that they can be easily tailored for specific applications, for example, as biocements and bioceramics. Different molar concentrations of Co(NO₃)₂·6H₂O, Ca(NO₃)₂·4H₂O, and NaH₂PO₄·2H₂O compounds were employed in determining the starting solutions utilized in the present study. The experimental findings reveal that, when the Co/Ca molar ratio is below 0.67 (BCo4), Co doping (the partial substitution of Ca by Co) takes place in brushite as a monophase. However, in the Co/Ca 0.67–1.5 molar ratio range (BCo4–BCo6), biphasic Co₃(PO₄)₂·8H₂O/CaHPO₄·2H₂O crystals start to precipitate. Full Ca replacement by Co results in the precipitation of nanostructured monoclinic cobalt phosphate and orthorhombic ammonium cobalt phosphate hydrate. Subsequent X-ray photoelectron spectroscopy (XPS), powdered X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) analyses, along with thermogravimetric analysis (TGA), confirmed that the starting solution ratio of Co/Ca had a significant influence on the material’s microstructure, while tuning this ratio ultimately tailored the desired properties of the material for the intended applications.

1. Introduction

The enhanced biochemical and mineralogical features of calcium phosphates (CaPs) have facilitated wider applications of the material, especially in engineering and environmental domains. The end products feature excellent biocompatibility, high bioactivity, low toxicity, and a close resemblance with the phases of minerals found in osseous tissues [1,2,3,4]. Precursor applications of CaPs for bioceramics or biocements in dentistry and medicine are envisaged, especially when advanced materials are to be synthesized [5,6,7]. Further applications of CaPs can be found in the construction industry, the agricultural domain as fertilizers [8], and drug delivery/bone tissue engineering [6,9].

In addition to hydroxyapatite, dicalcium phosphate dihydrate (DCPD, CaHPO₄·2H₂O) is the most commonly employed CaP because of its high solubility [7,10] as well as high stability in environments in the 4.0–6.5 pH range and at temperatures below 60 °C [11,12]. It can also be quickly resorbed in the human body to form bone material since it becomes metastable at pH ~7.4 [13,14,15]. Therefore, DCPD is considered a suitable substitute either as a bone graft or a promoter of bone healing [16].

Cobalt (Co) is a crucial element in the production of red blood cells. This process is called erythropoiesis [17]. In several applications, Co has a similar function to Mn and Zn, and it replaces these elements in biochemical reactions [18]. Co also plays a major role in the biotin-dependent Krebs cycle, in which the body converts sugars into energy [19]. Co-based nanoparticles are very promising compounds, offering advantages in biomedical-related fields such as magnetic resonance imaging and drug delivery [20]. The toxicological effect of Co nanoparticles is mildly anti-proliferative against cancer cells, and yet they are safe towards normal cells [20]. Thus, these nanoparticles may fulfill the emerging need for drug delivery applications with no side effects for the patients, proving themselves to be biocompatible with human red blood cells.

Recent research in this domain has expanded to examine the inclusion of cobalt (Co) in different CaP systems [16,17,18,19,20,21,22] as Co is present in bone, dentine, and enamel (at concentrations below 1 ppm). It is also a constituent of vitamin B12 (also known as cobalamin) and could thus play a role in DNA synthesis and neurological function, given that vitamin B12 deficiency is associated with hematologic and neuropsychiatric disorders [16]. Extant evidence points to the potential of Co²⁺ for stabilizing hypoxia (low oxygen levels), inducible factor-1α (HIF-1α), and angiogenesis [16]. Cobalt phosphate is of great interest in additional applications and technological fields, being a catalyst for organic synthesis reactions such as styrene oxidation [20]. Previous studies have investigated the effect of doping cobalt in brushite cement [16]. However, as far as we are aware, no study has been conducted on the effect of full-scale calcium substitution by cobalt on brushite as a mineral, which is an important precursor of bone cements and bioceramics.

The above findings have prompted our study of the thermal properties, crystal morphology, mineralogy, and chemical composition of the materials produced when the calcium ions in brushite—as a precursor of different biomaterials—are gradually substituted (up to 100%) by Co ions. The results yielded fill an important research gap and may also be valuable for biphasic Co₃(PO₄)₂·8H₂O/CaHPO₄·2H₂O powder synthesis and for obtaining biomaterials tailored to diverse applications in medical and pharmaceutical industries.

2. Materials and Methods

2.1. Materials

The materials employed for the synthesis of Ca_xCo_1−xHPO₄·nH₂O compounds were used as received. The materials include cobalt (II) nitrate hexahydrate (Co(NO₃)₂·6H₂O), India; calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O) purchased from LOBA Chemie; and sodium dihydrogen orthophosphate dihydrate (NaH₂PO₄·2H₂O) procured by Techno Pharmchem, India. A water purification system (PURELAB option-Q, ELGA, Oxford, UK) was employed to prepare distilled water (0.055 µS/cm). The other materials used included a magnetic stirrer (ISOTEMP, Fisher Scientific, Singapore) and a digital analytical balance (EX324N, OHAUS, Parsippany, NJ, USA).

2.2. Preparation of Ca_1−xCo_xHPO₄·nH₂O Compounds

At room temperature, six Ca_1−xCo_xHPO₄·nH₂O compounds were synthesized using the wet dissolution–precipitation method [3]. The required molar ratios of Na₂HPO₄·2H₂O, Ca(NO₃)₂·4H₂O, and Co(NO₃)₂·6H₂O 0.5 mol/L solutions are shown in

Table 1. Molar proportions of NaH₂PO₄·2H₂O, Ca(NO₃)₂·4H₂O, and Co(NO₃)₂·6H₂O as well as Co/Ca molar ratios used for the synthesis of Ca_1−xCo_xHPO₄·nH₂O compounds.

ID	x	NaH2PO4·2H2O	Ca(NO3)2·4H2O	Co(NO3)2·6H2O	Co/Ca Molar Ratio
BCo0	0	1	1	0	0
BCo2	0.2	1	0.8	0.2	0.25
BCo4	0.4	1	0.6	0.4	0.67
BCo5	0.5	1	0.5	0.5	1.0
BCo6	0.6	1	0.4	0.6	1.5
BCo10	1	1	0	1	-

[3]. The synthesis of these compounds was based on the following reaction (1):

$$\alpha \mathrm{C}\mathrm{o}({\mathrm{N}\mathrm{O}}_3{)}_2·6{\mathrm{H}}_2\mathrm{O}+\left(1-\alpha \right)\mathrm{C}\mathrm{a}({\mathrm{N}\mathrm{O}}_3{)}_2+{\mathrm{N}\mathrm{a}}_2{\mathrm{H}\mathrm{P}\mathrm{O}}_4+{\mathrm{N}\mathrm{H}}_3+{\mathrm{H}}_2\mathrm{O}\to \left({\mathrm{C}\mathrm{a}}_{1-\alpha }+{\mathrm{C}\mathrm{o}}_{\alpha }\right)\mathrm{H}\mathrm{P}{\mathrm{O}}_4·2{\mathrm{H}}_2\mathrm{O}+{2\mathrm{N}\mathrm{a}\mathrm{N}\mathrm{O}}_3$$

(1)

The synthesis of brushite (represented as BCo0 in Table 1) involves the mixture of Na₂HPO₄·2H₂O solution with 100 mL of Ca(NO₃)₂·4H₂O solution, the flow rate of the latter being maintained at ~2 mL/min using a stopcock-based glass funnel. The mixture was continuously stirred at 450 rpm until a 1.0 molar ratio of Ca/P was attained (this took about 1 h). Stirring was conducted at room temperature for 60 min to ensure homogeneity. An ammonia solution (~15 moL/L, Labochemie, India) was added to adjust the pH to a range of 6–6.5. The obtained white precipitate was vacuum filtered using qualitative filter paper (45 µm, Ø 12 cm, Double Rings, China) coupled with a Buchner funnel. Deionized water was employed for the initial washing of the filter cake, followed by ethanol washing for agglomeration prevention. Each washing was conducted three times [3]. The sample was finally placed on a watch glass and was oven dried at 40 °C overnight (ED53/E2, Binder, Germany). The synthesis of BCo6, BCo5, BCo4, and BCo2 compounds involves a mixture of Ca(NO₃)₂·4H₂O and Co(NO₃)₂·6H₂O solutions with the molar ratio outlined in Table 1. An amount of 100 mL of the obtained solution was mixed with 100 mL of Na₂HPO₄·2H₂O solution at a flow rate of ~2 mL/min. A similar process was conducted for the synthesis of BCo10 using NaH₂PO₄·2H₂O and Co(NO₃)₂·6H₂O.

2.3. Characterization Techniques

A Shimadzu XRD diffractometer-6000 (Japan) cobalt tube was employed to carry out qualitative mineralogical analysis on the synthesized BCo0–BCo10 samples with the 10–60° scanning range maintained at a 2°/min scan rate. Using the Inspect F50 (the Netherlands) apparatus via scanning electron microscopy, the establishment of product morphology was attained. The surface chemistry of the samples with elemental compositions was obtained by means of X-ray photoelectron spectroscopy using the XPS system (Thermo K Alpha spectrometer, USA). The mass loss (~100 mg) due to product heating from 40 °C to 750 °C, at 5 °C min⁻¹ increments under atmospheric helium conditions, was determined using a thermogravimetric (TG) analyzer (Netzsch, Germany, TG 209 F1 Libra). Inductively coupled plasma optical emission spectrometry (ICP-OES) was used to determine cobalt in the synthesized powders. The equipment used was a THERMOSCIENTIFIC (iCAP 7000 series).

3. Results and Discussion

3.1. Analysis of Microstructure and Mineralogy Content

Figure 1 shows the XRD patterns of all the samples with the inclusion of the standard brushite (CaHPO₄·2H₂O) and cobalt (II) orthophosphate octahydrate (Co₃(PO₄)₂·8H₂O). The findings from the quantitative mineralogy analysis confirm the precipitation of pure brushite (BCo0) after mixing solutions of Ca(NO₃)₂·4H₂O and NaH₂PO₄·2H₂O at a molar ratio of 1:1 Ca/P. The proportionate growth of three main planes was observed, including (020), (121-), and (141), with crystallization after nucleation. The monoclinic structure [23,24,25,26] of BCo0 is confirmed from every peak identified in the XRD pattern, while the progression of crystal growth along the (020) plane [3] is confirmed by the peak observed at 11.7° 2-theta.

The pattern obtained for BCo2 is similar to that produced by BCo0 as only the partial replacement of Ca with Co (Co doping) takes place at such a low Co/Ca molar ratio (BCo2). The intensity attributed to brushite peaks is more pronounced but lower than that attributed to the Bco0 spectrum as seen in the acquired pattern, especially the peaks corresponding to the (020), (121-), (040), and (141-) planes. This indicates that doping Co hinders the crystallization of brushite. BCo0 and BCo2 are characterized by the precipitation of monophasic Brushite minerals. This is an indication that all the Co ions substitute the Ca ions in the brushite lattice.

As reported in Figure 1, BCo4–BCo10 are not similar to BCo0 due to the occurrence of Co phosphate. As the figure depicts, the patterns are associated with BCo4, BCo5, and BCo6 (with Co/Ca molar ratios ranging from 0.67 to 1.5), biphasic compounds of brushite, and cobalt (II) orthophosphate octahydrate (Co₃(PO₄)₂·8H₂O) started to precipitate. The planes corresponding to Co₃(PO₄)₂·8H₂O (020), (200), (–131), and (200) are also depicted. A linear relationship exists between the Co/Ca molar ratio and the peak intensity of Co₃(PO₄)₂·8H₂O, as reflected by the XRD patterns. Finally, as shown in Figure 1 and Figure 2C, the pattern exhibited by BCo10 (which has a 1.0 Co/P molar ratio) affirms that there exists low-intensity peaks corresponding to nanostructured cobalt phosphate compounds—cobalt phosphate (Co₃(PO₄)₂) and ammonium cobalt phosphate (NH₄CoPO₄·H₂O).

As observed in Figure 2A (pertaining to BCo0), the precipitation of pure brushite results in plate-like crystals, which, similarly, has been reported elsewhere [10,11]. The dimensions of the observed plate-like crystals are ~500 nm × 5 µm × 10 µm. These dimensions are comparable to those described previously [20]. Typically, the precipitated brushite shows a flat-plate morphology in its crystallographic structure [15]. Confirmed from Figure 2B, which corresponds to BCo5, is that 50% of Ca replacement with Co leads to the formation of biphasic powder with monoclinic crystals of brushite, point (a), and monoclinic crystals of cobalt (II) orthophosphate octahydrate, point (b). Thus, the findings obtained from SEM corroborate the results of the XRD analysis presented in Figure 1.

Unit cell parameters (Rietveld refined) for brushite and cobalt (II) orthophosphate octahydrate materials are presented in

Table 2. Results of XRD analysis showing monoclinic brushite unit cell (Rietveld refinement).

ID	wt%	a (Å)	b (Å)	c (Å)	βo	V (Å3)
Standard Brushite	-	5.8120	15.1800	6.2390	116.430	492.9101
BCo0	100	5.8151	15.2179	6.2664	116.413	496.3547
BCo2	100	5.8119	15.1867	6.2416	116.405	493.4316
BCo4	40.6	5.8145	15.2086	6.2580	116.408	495.6503
BCo5	31.2	5.8145	15.2086	6.2580	116.408	495.6503
BCo6	28.0	5.8151	15.2179	6.2664	116.413	496.3547
BCo10	0.0	-	--	-	-	-

and

Table 3. Results of XRD analysis (Rietveld refinement) showing the unit cell parameters for monoclinic cobalt (II) orthophosphate octahydrate.

ID	wt%	a (Å)	b (Å)	c (Å)	βo	V (Å3)
Standard cobalt (II) orthophosphate octahydrate [22]		10.021	l3.331	4.673	104.9	603.3
BCo0	0	-	-	-	-	-
BCo2	0	-	-	-	-	-
BCo4	56.5	10.021	13.331	4.6730	104.9	603.3
BCo5	68.8	10.021	13.331	4.6730	104.9	603.3
BCo6	72.0	10.021	13.331	4.6730	104.9	603.3
BCo10	0.0	-	--	-		-

. It is evident from the tabulated data that, at Co/Ca molar ratios below 0.67 (BCo4), only the brushite phase is formed. However, in the 0.67–1.5 range, the brushite phase gradually decreases to 28%, whereas the complete replacement of Ca with Co in the starting solutions (as is the case for the BCo10 sample) leads to the disappearance of the brushite structure. In turn, it is observed that an increase in the Co/Ca molar ratio from 0.67 to 1.5 (BCo4–BCo6) leads to an increase in cobalt (II) orthophosphate octahydrate from 56.5% to 72%, as reported in Table 3.

The findings further indicate that brushite unit cell parameters are dependent on the Co/Ca molar ratio (Figure 3). Although increasing the BCo0 and BCo2 Co ions in the starting solutions up to 20% results in the precipitation of one phase (brushite), as shown in Figure 3A, the unit cell volume of brushite decreased significantly, as reported in Figure 3B. This is an indication that the Ca ions are substituted by the smaller Co ions, and, thus, smaller unit cells of brushite are formed. On the other hand, the increase in Co ions in the starting solutions, up to 60% (BCo4, BCo5, and BCo6), led to the precipitation of two biphasic compounds, CaHPO₄·2H₂O and Co₃(PO₄)₂·8H₂O (Figure 3A). The Co ions partially contribute to the precipitation of Co₃(PO₄)₂·8H₂O, so the brushite unit cell gradually restores its original volume (Figure 3B). Meanwhile, the parameters pertaining to the cobalt (II) orthophosphate octahydrate unit cell remain constant when the molar ratio of Co/Ca increases from 0.67 to 1.5 (Table 3).

Finally, with the full replacement of Ca by Co, both brushite and cobalt (II) orthophosphate octahydrate are eliminated from the structure, while two crystalline phases—monoclinic cobalt phosphate crystals and orthorhombic ammonium cobalt phosphate hydrate crystals—are formed, as reported in

Table 4. Results of XRD analysis (Rietveld refinement) showing the crystalline phase composition and unit cell parameters for BCo10.

Product	wt%	Crystal Type	a (Å)	b (Å)	c (Å)	βo	V (Å3)
Co3(PO4)2	93.9	Monoclinic	5.92	10.334	4.750	91.1	290.5
NH4CoPO4·H2O	6.1	Orthorhombic	5.55	8.850	4.805	-	236.0

.

The elemental analysis shows that the cobalt content presented a linear trendline, as reported in Figure 4. In general, there is a clear agreement between the gradual increase in cobalt in the resulting compounds, and this corresponds to the proportions of cobalt in the initial solutions. These results confirm that cobalt was an essential part of the resulting compounds and had a role to play in the reactions at various stages.

3.2. FTIR Spectrum of Ca_xCo_1−xHPO₄·nH₂O Compounds

The infrared spectra (FTIR) of Ca_xCo_1−xHPO₄·nH₂O are reported in Figure 5. The broad absorption peak, BCo0 and BCo2, between 3500 cm⁻¹ and 2400 cm⁻¹ is due to the O-H stretching vibration in brushite [15]. However, these two monophasic samples, BCo0 and BCo2, are composed from brushite, as reported in Figure 1, so they exhibit similar FTIR spectra. The P-O-P asymmetric stretching vibration band was observed at 984 cm⁻¹ as a result of P=O stretching vibrations and other bands at 648 and 563 cm⁻¹, which may be attributed to (H-O-)P=O for acid phosphates [27,28]. Another two peaks were observed at 3480 and 1640 cm⁻¹, which indicated the existence of water, and the intensity of these peaks decreased due to the decreasing trend in brushite contents in the samples (BCo2, BCo4, BCo5, and BCo6).

As a result of increasing the Co contents in the starting solutions, from BCo2 up to BCo10, the samples showed the appearance of new peaks at 1079 and 560 cm⁻¹, which are indexed to Co–O bending vibrations. The presence of ammonium cations in the BCo10 compound was confirmed by infrared spectroscopy, showing bands at 2840 and 1440 cm⁻¹ [29,30].

3.3. Elemental and Chemical Composition of Ca_1−xCo_xHPO₄·nH₂O Compounds

An XPS analytical method was employed to characterize the prepared samples, and we then evaluated the effect of the Co/Ca ratio contained in the starting solution on the chemical state as well as the surface chemistry of the samples. Figure 6 shows the chemical states of Ca, P, and Co, the integral constituents of the synthesized Ca_1−xCo_xHPO₄·nH₂O compounds. Both the extent and the degree of substituting Ca with Co in the prepared brushite are influenced by the ratio of Co to Ca. A similar ratio affects cobalt (II) orthophosphate octahydrate precipitation, especially when utilizing the higher ratio of Co/Ca. The XPS spectra for a range of BCo2 to BCo10 samples clearly show the peaks corresponding to the Co 2p orbital, and the intensities of these peaks increase progressively. In contrast, the intensities corresponding to Ca 2s and Ca 2p peaks decrease proportionately with an increase in the Co/Ca ratio. It is noteworthy that the intensity of the P 2s peak remains nearly constant. This shows that the intensity of Co, P, and Ca peaks is closely related to the precipitation of cobalt (II) orthophosphate octahydrate (when the Co/Ca ratio is higher) as well as the degree of substitution of Ca with Co.

The influence of the Co/Ca ratio on the binding energies of Ca 2s, Co 2p, and P 2s peaks is presented in Figure 7A–C. Accordingly, increasing the Co/Ca molar ratio from 0 (BCo0) to 1.5 (BCo6) results in greater binding energies of the Ca 2s and P 2s peaks, respectively, from 438 eV to 444 eV and from 190 eV to 196 eV. Findings pertaining to the BCo10 sample indicate that the binding energy of P 2s is also around 196 eV once Ca is completely substituted with Co.

Moreover, our results show that an increase in the Co/Ca ratio to 0.67 (BCo4), and further to 1.5 (BCo6), causes an increment in the Co 2p peak intensity, as expected, whereas the precipitation of cobalt (II) orthophosphate octahydrate accompanies the substitution of Ca with Co. The acquired XRD patterns displayed in Figure 1 show BCo4 to be the first compound in the BCo0–BCo10 sequence that contains cobalt (II) orthophosphate octahydrate crystals. The fact that BCo10 comprises solely cobalt (II) phosphate would indicate that the Auger peaks corresponding to Co and Co 2p are characterized by the highest intensity.

From the aforementioned XPS observations, it can be said that the crystal structure of Ca_xCo_1−x·HPO₄·nH₂O compounds may be conveniently altered by increasing the starting solution ratio of Co/Ca, while Ca ions are partially substituted with Co, thereby increasing the binding energy peaks of P 2s and Ca 2s.

3.4. Thermogravimetric Analysis (TGA)

Figure 8 presents the results of TGA for BCo0–BCo10 compounds. The structural crystallography of brushite consists of compact sheets formed by parallel chains of ions, where the coordination of Ca ions is maintained by six ions of phosphate coupled with two oxygen atoms emanating from water molecules [31]. Therefore, as expected, the greatest mass loss occurs due to heating—the sample was heated from 80 °C to 220 °C, which facilitated the evaporation of the weakly bonded water present at the sample surface [32,33].

The gradual transformation of brushite to monetite (CaHPO₄) at the temperature near ~220 °C leads to the release of the chemically bound water [34], and the transformation extends to calcium pyrophosphate (Ca₂P₂O₇) when the temperature increases to ~400 °C [7]. These observations are in line with the findings reported here, as heating brushite (BCo0) to 750 °C resulted in approximately a 21.5 wt% mass loss [19], which is closely comparable with a 21 wt% theoretical mass loss [35]. On the other hand, the BCo2–BCo10 sample with a higher ratio of Co/Ca translates as greater mass loss (23–24%).

The brushite dehydration reaction is provided in Equation (2), whereas the formation of calcium pyrophosphate is described by Equation (3):

$${\mathrm{C}\mathrm{a}\mathrm{H}\mathrm{P}\mathrm{O}}_4·2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}\mathrm{a}\mathrm{H}\mathrm{P}\mathrm{O}}_4+2{\mathrm{H}}_2\mathrm{O}$$

(2)

$${2\mathrm{C}\mathrm{a}\mathrm{H}\mathrm{P}\mathrm{O}}_4\to {\mathrm{C}\mathrm{a}}_2{\mathrm{P}}_2{\mathrm{O}}_7+{\mathrm{H}}_2\mathrm{O}$$

(3)

The mass loss rate for Ca_x·Co_1−xHPO₄·nH₂O compounds with temperature is presented in Figure 9. In Figure 9A–D, the peaks related to the dehydration of two molecules of water found in pure brushite (BCo0) appear in the spectrum and are accompanied by other peaks resulting from an increase in the Co/Ca ratio from 0 (BCo0) to 1.5 (BCo6) [36]. However, as derived from the starting solution (BCo4–BCo6), presented in Figure 9C–E, when Ca is substituted with Co, the powders exhibit similar mass loss behavior with two peaks between 140 °C and 190 °C. Finally, a major peak around 160 °C and a hump in mass loss under 550 °C are seen (Figure 9F) with the full substitution of Ca by Co in brushite.

3.5. Phase Evolution When Ca_1−xCo_xHPO₄·nH₂O Compounds Were Precipitating

This study establishes the preservation of the original shape of brushite (plate-like crystals) when the starting solution ratio of Co/Ca does not go beyond 0.67. Above this value, cobalt (II) orthophosphate octahydrate with monoclinic crystals starts to precipitate alongside brushite. A nanostructured powder is formed as a result of fully substituted Ca in brushite by Co, as summarized in

Table 5. Phase evolution, crystal size, and structure as a function of Co/Ca molar ratio (x fraction of Co = 0 to 1) in solution.

X Fraction (Co)	Crystal Structure	Compounds Formed
0.4	Monoclinic	Brushite
0.4–0.6	Monoclinic + monoclinic	Brushite + cobalt (II) orthophosphate octahydrate
1	Monoclinic + orthorhombic	Cobalt phosphate + ammonium cobalt phosphate hydrate

.

4. Conclusions

In the present study, the substitution of Ca by Co in brushite’s crystal structure was investigated for varieties of Ca_1−xCo_xHPO₄·nH₂O biomaterial-based compounds that were subsequently characterized using XRD, TG, SEM, FTIR, and XPS techniques. The results obtained indicate that any starting solution molar ratio of Co/Ca below 0.67 (BCo4) leads to the partial substitution of Ca with Co; however, while the precipitated brushite structure remains unaffected, smaller unit cells are formed. The precipitation of cobalt (II) orthophosphate octahydrate with a monoclinic structure was observed as the molar ratio of Co/Ca increased from 0.67 to 1.5 (BCo4–BCo6). The full replacement of Ca ions by Co ions in brushite leads to the precipitation of nanostructured cobalt phosphate and ammonium cobalt phosphate hydrate. The biphasic compounds of CaHPO₄·2H₂O and CoH(PO₄)₂·8H₂O were successfully synthesized using the wet precipitation method.

This is insightful information, especially for synthesizing future biomaterials with desired characteristics and species composition tailored using a starting Co/Ca solution ratio that alters the material morphologies and composition. Still, further studies are needed to assess the biomaterial performance and specifically examine the physiochemical, biological, antibacterial, and mechanical properties. In addition, a comprehensive investigation on the effect of doping (using additional ions) should be carried out to ascertain the detrimental and beneficial effects of Co on biomaterials before making recommendations for medical applications.