Hélène Terrisse

Natural opal is a widespread mineral formed by the aqueous alteration of silicate rocks. It occurs as a mixture of silica nano-to-micro-structures (e.g., nanograins, spheres) and silica hydrogel cement, with variations in the proportions of these components leading to significant differences in the physico-chemical properties of opals. However, the detailed process of their formation in nature and the influence of the mixing ratio are not fully understood, as opal has not been yet synthesized under geologically relevant conditions. This study aims to develop a method of opal synthesis in conditions close to continental weathering conditions (2O and OH). Moreover, the synthetic opal obtained with a 1:1 ratio shows the closest similarity to natural opal-AG. Finally, our results support the hypothesized multistage process for opal formation in nature.

This study presents an innovative approach in dye-sensitized photocatalytic systems for CO₂-to-CO conversion, using electrostatically assembled, noble metal-free components, achieving ∼100% CO selectivity and 1658 turnovers with an apparent quantum yield of 16.9%.

Tricalcium phosphate (TCP) is a food additive, labeled E341(iii), used in powdered food preparation, such as baby formula. In the United States, calcium phosphate nano-objects were identified in baby formula extractions. Our goal is to determine whether the TCP food additive, as is used in Europe, can be classified as a nanomaterial. The physicochemical properties of TCP were characterized. Three different samples (from a chemical company and two manufacturers) were thoroughly characterized according to the recommendations of the European Food Safety Authority. A commercial TCP food additive was identified as actually being hydroxyapatite (HA). It presents itself in the form of particles of different shapes (either needle-like, rod, or pseudo-spherical), which were demonstrated in this paper to be of a nanometric dimension: E341(iii) is thus a nanomaterial. In water, HA particles sediment rapidly as agglomerates or aggregates over a pH of 6 and are progressively dissolved in acidic media (pH < 5) until the complete dissolution at a pH of 2. Consequently, since TCP may be considered as a nanomaterial on the European market, it raises the question of its potential persistency in the gastrointestinal tract.

Silver biodistribution and gut toxicity of two different colloidal silver products were evaluated in mice after oral exposure. Biophysics-based methodologies provided novel insights into (nano)silver uptake, fate and toxicological effects.

Titanium dioxide is a food additive that has raised some concerns for humans due to the presence of nanoparticles. We were interested in knowing the fate of TiO₂ particles in the gastro-intestinal tract and their potential effect on digestive enzymes. For this purpose, we analysed the behaviour of two different food grade TiO₂ samples (E171) and one nano-sized TiO₂ sample (P25) through a standardized static in vitro digestion protocol simulating the oral, gastric and intestinal phases with appropriate juices including enzymes. Both E171 and P25 TiO₂ particles remained intact in the digestive fluids but formed large agglomerates, and especially in the intestinal fluid where up to 500 μm sized particles have been identified. The formation of these agglomerates is mediated by the adsorption of mainly α-amylase and divalent cations. Pepsin was also identified to adsorb onto TiO₂ particles but only in the case of silica-covered E171. In the salivary conditions, TiO₂ exerted an inhibitory action on the enzymatic activity of α-amylase. The activity was reduced by a factor dependent on enzyme concentrations (up to 34% at 1 mg mL⁻¹) but this inhibitory effect was reduced to hardly 10% in the intestinal fluid. In the gastric phase, pepsin was not affected by any form of TiO₂. Our results hint that food grade TiO₂ has a limited impact on the global digestion of carbohydrates and proteins. However, the reduced activity specifically observed in the oral phase deserves deeper investigation to prevent any adverse health effects related to the slowdown of carbohydrate metabolism.

Titanium dioxide is a metal oxide used as a white pigment in many food categories, including confectionery. Due to differences in the mass fraction of nanoparticles contained in TiO₂, the estimated intakes of TiO₂ nanoparticles differ by a factor of 10 in the literature. To resolve this problem, a better estimation of the mass of nanoparticles present in food products is needed. In this study, we focused our efforts on chewing gum, which is one of the food products contributing most to the intake of TiO₂. The coatings of four kinds of chewing gum, where the presence of TiO₂ was confirmed by Raman spectroscopy, were extracted in aqueous phases. The extracts were analysed by transmission electron microscopy (TEM), X-ray diffraction, Fourier Transform Raman spectroscopy, and inductively coupled plasma atomic emission spectroscopy (ICP-AES) to establish their chemical composition, crystallinity and size distribution. The coatings of the four chewing gums differ chemically from each other, and more specifically the amount of TiO₂ varies from one coating to another. TiO₂ particles constitute the entire coating of some chewing gums, whereas for others, TiO₂ particles are embedded in an organic matrix and/or mixed with minerals like calcium carbonate, talc, or magnesium silicate. We found 1.1 ± 0.3 to 17.3 ± 0.9 mg TiO₂ particles per piece of chewing gum, with a mean diameter of 135 ± 42 nm. TiO₂ nanoparticles account for 19 ± 4% of all particles, which represents a mass fraction of 4.2 ± 0.1% on average. The intake of nanoparticles is thus highly dependent on the kind of chewing gum, with an estimated range extending from 0.04 ± 0.01 to 0.81 ± 0.04 mg of nano-TiO₂ per piece of chewing gum. These data should serve to refine the exposure scenario.