Beam-Generated Sample Evolution to Emphasize Spatial Inhomogeneities: The Example of HPO42−-Containing Amorphous Calcium Phosphate

Abstract

Amorphous calcium phosphates (ACPs) represent a family of bioactive compounds particularly relevant to bone regeneration. However, due to their intrinsic metastability, their processing into 3D-shaped materials cannot be undergone by conventional sintering methods and requires cold sintering approaches. Also, their microstructure and local compositional changes still have to be explored in detail. To this aim, spectroscopy techniques are particularly appealing to probe local chemical environments at the microscale. Concerning ACPs, one question regards the distribution of (hydrogenated) phosphate species, as they may lead to various evolutionary trends. In this contribution, we purposely exploited the laser–beam interaction through Raman mapping to trigger the in situ HPO₄²⁻-to-P₂O₇⁴⁻ transformation. Analysis by a multivariate approach allowed us to spot P₂O₇⁴⁻ clusters resulting from HPO₄²⁻-rich initial domains. Moreover, a blue shift of the ν₁PO₄³⁻ band was noticed in the close vicinity of these clusters, thus evidencing a local evolution of the chemical composition of the ACP. These results, corroborated by differential thermal analysis, demonstrate the relevance of using the laser–sample interaction through local heating to probe the spatial evolution induced, in our case, by an ultrafast compaction process. Comparison of outcomes obtained using two different laser/power strategies finally evidenced the need to adapt the Raman analytical conditions to the behavior of the metastable material to analyze.