Abstract
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.
Highlights
Hydroxyapatite (Ca5(PO4)3(OH); HAP, Table 1) has a wide range of occurrences and uses
Transformation pathways were found to be more complex, because Amorphous Calcium Phosphate (ACP) rapidly hydrolyses to the meta-stable octacalciumphosphate (Ca8(PO4)6x5H2O; OCP), which subsequently transforms to HAP [23]
Previous experimental studies on HAP formation suggested that in aqueous alkaline media, OCP occurs as an intermediate phase during the transformation of ACP to HAP [41]
Summary
Hydroxyapatite (Ca5(PO4)3(OH); HAP, Table 1) has a wide range of occurrences and uses Those span from products of biomineralization and post-sedimentation in natural surroundings [1,2,3], addons in composites for tailored properties e.g., of cements and water treatment agents [4,5,6] to innovative medical-related products e.g., for remineralization of teeth or for bone grafts [7,8,9,10]. ACP has already been identified as a transient phase during bone mineralization in the 1960ies by mere optical measures [11]. This observation was later confirmed in several studies by X-ray diffraction Transformation pathways were found to be more complex, because ACP rapidly hydrolyses to the meta-stable octacalciumphosphate (Ca8(PO4)6x5H2O; OCP), which subsequently transforms to HAP [23]
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