Ligand-Promoted Surface Solubilization of TiO₂ Nanoparticles by the Enterobactin Siderophore in Biological Medium

Titanium dioxide nanoparticles (TiO₂-NPs) are increasingly used in consumer products for their particular properties. Even though TiO₂ is considered chemically stable and insoluble, studying their behavior in biological environments is of great importance to figure their potential dissolution and transformation. The interaction between TiO₂-NPs with different sizes and crystallographic forms (anatase and rutile) and the strong chelating enterobactin (ent) siderophore was investigated to look at a possible dissolution. For the first time, direct evidence of anatase TiO₂-NP surface dissolution or solubilization (i.e., the removal of Ti atoms located at the surface) in a biological medium by this siderophore was shown and the progressive formation of a hexacoordinated titanium-enterobactin (Ti-ent) complex observed. This complex was characterized by UV-visible and Fourier transform infrared (FTIR) spectroscopy (both supported by Density Functional Theory calculations) as well as electrospray ionization mass spectrometry (ESI-MS) and X-ray photoelectron spectroscopy (XPS). A maximum of ca. 6.3% of Ti surface atoms were found to be solubilized after 24 h of incubation, releasing Ti-ent complexes in the micromolar range that could then be taken up by bacteria in an iron-depleted medium. From a health and environmental point of view, the effects associated to the solubilization of the E171 TiO₂ food additive in the presence of enterobactin and the entrance of the Ti-enterobactin complex in bacteria were questioned.