Antimicrobial properties of co-doped tricalcium phosphates Ca3-2x(MˊMˊˊ)x(PO4)2 (M = Zn2+, Cu2+, Mn2+ and Sr2+)

The substituted (Ca2+/Cu2+), and co-substituted (Cu2+/Zn2+), (Cu2+/Sr2+), and (Sr2+/Mn2+) β-tricalcium phosphate (β-TCP)-based Ca3-2x(MˊMˊˊ)x(PO4)2 (M = Zn2+, Cu2+, Mn2+ and Sr2+) solid solutions have been synthesized using solid-state route. The powder X-ray diffraction study shows the formation of β-TCP-type structure as the main phase in all solid solutions. The crystal structures and chemical compositions were approved using Fourier-transform infrared (FT-IR) absorption spectra and energy-dispersive X-ray spectrometry (EDX) data, respectively. The unit cell parameters and volume of as-synthesized samples directly depend on the radius of the incorporated ions. The limits of the single-phase solid solutions were found based on the possible occupation of the crystal sites in β-TCP structure. For the divalent ions with small radii, such as Cu2+ or Zn2+, the limit composition was found as Ca2.571Mˊ0.429–xMˊˊx(PO4)2 for Mˊ and Mˊˊ – Cu2+ and Zn2+. The enlargement of the unit cell by incorporation of Sr2+ allows to extend the limit of solid solutions up to Ca2.5Sr0.5–xMˊx(PO4)2 for Mˊ – Cu2+ or Mn2+. The antibacterial properties were studied on 4 bacteria (S. aureus, P. aeruginosa, E. coli and E. faecalis) and 1 fungus (C. albicans). It has been showed that co-doped Ca2.5Sr0.25Cu0.25(PO4)2 sample exhibits the highest antimicrobial activity resulting in 92%, 96% and 96% inhibition growth rate for S. aureus, P. aeruginosa and E. faecalis, respectively. The antimicrobial properties are strongly related to the occupation of the crystal sites in the β-TCP structure by doping ions.