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.