Beam-induced photoreduction of copper in functionalized calcium phosphate nanoparticles during X-ray photoelectron spectroscopy analysis

• Calcium phosphate nanoparticles were successfully doped with copper ions using different metal-complexing agents. • Systematic XPS analysis revealed significant X-ray–induced photoreduction of Cu(II) to Cu(I). • More pronounced photoreduction at low copper loadings, reaching complete conversion at ∼ 1 wt% Cu • The extent of beam-induced reduction also depends on the complexing agent, with carbonate offering higher protection than EDTA or citrate. • Results highlight the need of complementary analysis and control of experimental parameters in XPS to prevent misinterpretation of copper oxidation states. The development of copper-based nanomaterials is being explored in diverse fields of high applicability, including medicine (antimicrobial) or, more recently, agriculture (nutrient supply and fungicide). Copper oxidation state can heavily influence the performances of the resulting nanomaterials, even determining their activity. While X-ray photoelectron spectroscopy (XPS) is the most widely used technique for distinguishing between Cu(I) and Cu(II), it is often hardly reliable due to beam-induced emerging features. We report here a complete study to evaluate the X-ray beam effect during the surface analysis of amorphous calcium phosphate (ACP) nanoparticles doped with increasing amounts of Cu. We found out that X-Ray induced the photoreduction of Cu(II) to Cu(I), being this effect more pronounced at lower copper loadings. In fact, total photoreduction of cupric to cuprous is found at the lowest doping ( ca. 1% wt of Cu). Photoreduction is also highly dependent on the metal-complexing agent used during nanomaterial preparation, exhibiting carbonate ions higher protective effect against copper reduction than EDTA or citrate. Our findings underscore the critical importance to consider experimental parameters (exposure time, beam voltage, temperature, pressure) during XPS of copper-based nanomaterials to avoid misinterpreting beam-induced effects.