The role of crystallinity of palladium nanocrystals in ROS generation and cytotoxicity induction

Palladium (Pd) nanocrystals with different crystalline forms exhibit distinct enzyme-like activities in generating reactive oxygen species (ROS). How such crystallinity-dependent catalytic activity regulates potential cytotoxicity remains to be elucidated. In the present work, Pd nanocrystals with four different crystalline forms were synthesized, and the underlying mechanisms involved in ROS-mediated cytotoxicity were systematically revealed. Pd nanocrystals with the {100} (nanocubes) and {111} (nanooctahedrons and nanotetrahedrons) facets triggered cytotoxicity by generating singlet oxygen (1O2) and hydroxyl radicals (OH˙), respectively. Meanwhile, Pd nanoconcave-tetrahedrons, which had both the {110} and {111} facets, induced ROS-mediated cytotoxicity via activating the superoxide (O2˙-) pathway. Consumption of protons and generation of hydroxide during intracellular ROS conversion resulted in pH alkalization, eventually leading to cell death. Our findings emphasize the importance of facet-dependent ROS generation promoted by Pd nanocrystals. Furthermore, alkalization is identified as a new biomarker for analyzing ROS-mediated cytotoxicity.