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Solubilization of TiO2 nanoparticles by a bacterial siderophore

​​​​​​​​​Titanium dioxide nanoparticles are widely used, for their white pigment and photocatalytic properties, in consumer products (food, drugs, paint, etc.). Until now, they were considered chemically stable and insoluble. But for the first time, researchers at IRIG have demonstrated the solubilization of these nanoparticles in a biological medium by enterobactin, a bacterial siderophore. From a health and environmental point of view, the effects associated with solubilization should be questioned.​

Published on 27 June 2023

​With the emergence of nanotechnology, the nanoparticulate forms of TiO2 are found in an increasing number of daily-life products such as adhesives, paints, sunscreens, toothpastes, cosmetics as well as in the food industry. For example, the TiO2 food additive E171, contains a variable portion of TiO2 nanoparticles depending on the source, is now forbidden by the European community since August 7th 2022 but still used in many countries to make foods whiter and brighter,. The average of ingested amount of titanium has been estimated at about 10-50 mg per person and per day. TiO2 is considered as robust, chemically stable, and by common belief, insoluble. 

Commonly, robust and chemically stable TiO2 nanparticle was considered insoluble, while its dissolution could have an impact on antimicrobial properties, toxicity, health, and the environment. Size is considered the main physicochemical property affecting nanoparticle solubility; but various other parameters such as surface area, morphology and crystallinity must also be considered. The presence of organic ligands can also affect suspension stability, leading to nanoparticle dissolution.

Researchers at IRIG [collaboration] have taken an interest in organic siderophore ligands interaction with TiO2. Bacteria synthesize and secrete siderophores to capture the iron essential for their development. They have an extremely high affinity for iron (III) and exhibit a wide structural diversity: in particular, enterobactin (ent) forms the most stable complex with iron. The structure of enterobactin is composed of 3 catechol groups linked to a central lactone macrocycle (Figure). Its affinity for Fe(III) is so high that enterobactin is able to solubilize iron present in minerals such as olivine.
For the first time, studies have revealed that enterobactin binds covalently and forms complexes with Ti(IV), whose ionic radius is almost identical to that of Fe(III). In addition, Ti(IV) has a particular affinity for oxygenated ligands and could compete with hexacoordinated oxygenated metalloproteins or biomolecules.
The researchers were therefore interested in the binding of enterobactin to TiO2 nanoparticles, which could then dissolve. They showed that enterobactin, by binding to the surface of TiO2 nanoparticles, promotes Ti(IV) solubilization through the formation of Ti-ent complexes. This dissolution depends on intrinsic properties such as the size, surface defects and crystallographic shape of the nanoparticles.

In addition, the dissolution of the food additive TiO2 (E171) and the entry of the Ti-ent complex into Escherichia coli bacteria were also demonstrated. All these results raise questions about the possible impact, in terms of health or ecosystems, of the interaction between a powerful iron chelator such as enterobactin secreted by bacteria and TiO2 nanoparticles.

Figure: titanium atoms are represented by pink spheres, oxygens in red and nitrogens in blue, carbons in grey and hydrogens in white.

IRIG/LCBM, IRIG/SyMMES, CEA Saclay, National Institute of Materials Physics in Romania

Financial support
LabEx SERENADE (acronyme of Laboratory of Excellence for Safe(r) Ecodesign Research and Education applied to NAnomaterial Development).

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