Does nanoparticle activity depend upon size and crystal phase?

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Abstract
A method to investigate the dependence of the physicochemical properties of nanoparticles (e.g., size, surface area and crystal phase) on their oxidant generating capacity is proposed and demonstrated for TiO2 nanoparticles. Gas phase synthesis methods that allow for strict control of size and crystal phase were used to prepare TiO2 nanoparticles. The reactive oxygen species (ROS) generating capacity of these particles was then measured. The size dependent ROS activity was established using TiO2 nanoparticles of nine different sizes (4–195 nm) but the same crystal phase. For a fixed total surface area, an S-shaped curve for ROS generation per unit surface area was observed as a function of particle size. The highest ROS activity per unit area was observed for 30 nm particles, and observed to be constant above 30 nm. There was a decrease in activity per unit area as size decreased from 30–10 nm; and again constant for particles smaller than 10 nm. The correlation between crystal phase and oxidant capacity was established using TiO2 nanoparticles of 11 different crystal phase combinations but similar size. The ability of different crystal phases of TiO2 nanoparticles to generate ROS was highest for amorphous, followed by anatase, and then anatase/rutile mixtures, and lowest for rutile samples. Based on evaluation of the entire dataset, important dose metrics for ROS generation are established. The implications of these ROS studies on biological and toxicological studies using nanomaterials are discussed.