X-ray absorption spectroscopy shining (synchrotron) light onto the insertion of Zn2+ in calcium phosphate ceramics and its influence on their behaviour under biological conditions

Abstract

The present study gives a fine description of the Zn²⁺ location in Zn-doped Biphasic Calcium Phosphate (BCP) samples heat treated between 500 °C and 1100 °C. Structural considerations were used to explain the sample interactions with biological fluid (DMEM). X-ray Absorption Spectroscopy (XAS) experiments were used to characterize the powdered samples. The main results (1) indicate the presence of Zn²⁺ complexes physisorbed at the HAp surface for a sintering temperature of 500 °C, (2) confirm the insertion of Zn²⁺ into the β-TCP phase using a substitution mechanism for a sintering temperature around 700 °C, and (3) fully describe the insertion of Zn²⁺ into the HAp phase by an interstitial mechanism for heat treatment above 900 °C (composition Ca₁₀Zn_x(PO₄)₆(OH)_2−2xO_2x; x_max ∼ 0.25). The formation of the linear O–Zn–O entity with d_Zn–O = 1.72(2) Å has been clearly evidenced by Fourier transform amplitude fitting in the R-space (Zn two-fold coordination unambiguously described for the first time). The mineralisation stimulatory effect of Zn²⁺ incorporated into BCP has been explained by two different mechanisms. For samples heat treated between 500 °C and 800 °C, the stimulatory effect is attributed to the presence of soluble Zn²⁺ species: Zn²⁺ physisorbed at the HAp surface for sintering treatment around 500 °C and Zn²⁺ incorporated into about 20 wt% (weight percent) of the soluble Zn-doped β-TCP phase for sintering treatment around 800 °C. A sintering temperature above 900 °C led to the formation of an extremely weakly soluble and well-crystallized Zn-doped HAp phase which acts by facilitating the nucleation of a calcium phosphate phase at its surface.