In chemico methodology for engineered nanomaterials categorization according to number, nature and oxidative potential of reactive surface sites

Abstract

Methanol probe chemisorption quantifies the number of reactive sites at the surface of engineered nanomaterials, enabling normalization per reactive site in reactivity and toxicity tests, rather than per mass or physical surface area. Subsequent temperature-programmed surface reaction (TPSR) of chemisorbed methanol identifies the reactive nature of surface sites (acidic, basic, redox or combination thereof) and their reactivity. Complementary to methanol assay, dithiothreitol (DTT) probe oxidation reaction is used to evaluate oxidation capacity. These acellular approaches to quantify the number, nature, and reactivity of surface sites constitute a New Approach Methodology (NAM) for site-specific classification of nanomaterials. As a proof of concept, CuO, CeO2, ZnO, Fe3O4, CuFe2O4, Co3O4 and two TiO2 nanomaterials were probed, and a harmonized reactive descriptor was obtained: DTT oxidation rate per reactive site, or Oxidative Turnover Frequency (OxTOF). CuO and CuFe2O4 nanoparticles exhibit the largest reactive sites surface density and are the most oxidative in the series, as estimated by DTT probe reaction, followed by CeO2 NM-211 and, then, by titania nanomaterials (DT-51 and NM-101) and Fe3O4. DTT depletion in ZnO NM-110 was associated with dissolved zinc ions rather than the ZnO particles themselves, but the basic character of the ZnO NM-110 particles surface was evidenced by methanol TPSR. These acellular assays allow ranking the 8 nanomaterials into three categories with statistically different oxidative potential: CuO, CuFe2O4 and Co3O4 are the most reactive, ceria exhibits a moderate reactivity, and iron oxide and the titanias possess a low oxidative potential.