Calcium Hydroxyapatite Coatings: Low-Temperature Synthesis and Investigation of Antibacterial Properties

In the present work, the low-temperature synthesis of substituted calcium hydroxyapatite (Ca10(PO4)6(OH)2, HAP) with copper and zinc ions on titanium substrates was performed. Initially, CaCO3 coatings were synthesised on titanium substrate using the sol-gel method at 550 °C in a CO2 atmosphere. Crystalline calcium hydroxyapatite was then synthesised from these CaCO3 coatings through the dissolution-precipitation method at low temperature (80 °C). X-ray diffraction (XRD) analysis, FTIR and Raman spectroscopies, and scanning electron microscopy (SEM) were employed to evaluate the phase composition, surface functional groups, crystallinity, and morphology of the coatings. The results showed the formation of hexagonal HAP particles with a size of 20 nm at low temperature, exhibiting high homogeneity in particle size distribution. In the calcium hydroxyapatite, some of the Ca2+ ions were replaced by Cu2+ ions. Heating the mixture of Ca(NO3)2 and Cu(NO3)2 solutions at 550 °C in a CO2 atmosphere led to the formation of copper hydroxide carbonate (malachite, Cu2(OH)2CO3) along with CaCO3. The reaction between the sol-gel precursor obtained and Na2HPO4 resulted in the formation of copper-substituted hydroxyapatite (Cu-HAP). Different synthesis methods were tested with Zn2+ ions, and on the surface of the coating, Zn(OH)(NO3)(H2O), Zn3(OH)4(NO3)2, and unreacted CaCO3 were formed. Antibacterial properties of the coatings were tested using the inhibition zone method. No inhibition zones were observed for HAP. However, in the Cu and Zn containing coatings, inhibition zones were observed in the presence of a colony of B. subtilis bacteria. However, no inhibition zones were detected in the presence of E. coli bacteria.