Unveiling the composition of hydroxyapatite minerals and their (bio)-organic adlayer using X-ray photoelectron spectroscopy

Abstract

Although X-ray photoelectron spectroscopy (XPS) is a surface science technique, it is widely used for chemical analysis of hydroxyapatite (HAP) minerals. The use of biomimetic mineralization strategies further complicates the chemical context with possible adsorption of biomolecules. This work addresses methodological developments dealing with the reliability of XPS data and their interpretation on series of HAPs yielding variable amounts of carbonate and fluoride and different (bio)-organic adlayers. A specific focus is given here to: selectivity (adsorbed bio-organic compounds and inorganic constituents), precision (generation of statistically meaningful data) and accuracy (differential screening effects and factors influencing the quantification) of XPS data. Results show that exploring the information acquired for the (bio)-organic adlayer allows (i) to make a distinction between oxygen of organic and inorganic nature and (ii) to consider a differential screening effect. When exploring correlations between spectra guided by stoichiometry and charge balance, significant discrepancies are observed when quantifying phosphorus through the P 2p or the P 2s peak. The O/P ratio is shown to usefully evaluate the consistency of XPS data, while the interpretation of the Ca/P ratio with respect to surface chemistry should be approached cautiously, as quantification may require corrections that are rarely considered.