In-situ Raman spectroscopy of amorphous calcium phosphate to crystalline hydroxyapatite transformation

Amorphous calcium phosphate (Ca₃(PO₄)₂xnH₂O; n = 3-4.5; ACP) is a precursor phase of the mineral hydroxyapatite (Ca₅(PO₄)₃(OH); HAP) that in natural settings occurs during both authigenic and biogenic mineral formation. In aqueous solutions ACP transforms rapidly to the crystalline phase. The transformation rate is highly dependent on the prevailing physico-chemical conditions, most likely on: Ca & PO₄ concentration, pH and temperature. In this study, we conducted a calcium phosphate precipitation experiment at 20 °C and pH 9.2, in order to study the temporal evolution of the phosphate mineralogy. We monitored and assessed the transformation process of ACP to crystalline HAP using highly time-resolved in-situ Raman spectroscopy at 100 spectra per hour, in combination with solution chemistry and XRD data. Transformation of ACP to crystalline HAP occurred within 18 h, as it is illustrated in a clear peak shift in Raman spectra from 950 cm^-1 to 960 cm^-1 as well as in a sharpening of the 960 cm^-1 peak. The advantages of this method are: •In-situ Raman spectroscopy facilitates quasi - continuous monitoring of phase transitions.•It is an easy to handle and non-invasive method.