Enzyme-like Activity And Cytotoxicity Of Nano-palladium With Different Crystal Faces

Palladium nano-crystals are not only commonly used catalysts in synthesis,pharmaceutical manufacturing,and exhaust gas treatment,but also have broad prospects in the environmental engineering,usually as components of sensor arrays to detect heavy metal ions and organic molecules pollutants.In previous reports,palladium nano-crystals exhibit very diverse catalytic activities due to different crystal facets.Different from traditional catalytic hydrogenation reduction materials,palladium nano-crystals with the size in nanometer also exhibit oxidation properties.However,the mechanism of the conversion from reduction to oxidation properties is unclear yet.Meanwhile,with the properties of oxidase-,peroxidase-,catalase-,and superoxide dismutase-like activity,how palladium nano-crystals cause the excitation of molecular oxygen and how the oxygen species are converted in presence of palladium nano-crystals,have not been thoroughly studied yet.Therefore,for the purpose of studying the transformation of oxygen species and biological hazards of palladium nano-crystals with various facets,we firstly synthesized palladium nano-cubes with single{100}facets,nano-octahedrons with single{111}facets,nano-tetrahedrons with single{111}facets and nano-concave tetrahedrons with{110}and{111}mixed facets,respectively.Different reducing agents and shape-directing agent were used to dynamically control the attachment and growth of palladium atoms.These four kinds of palladium nano-crystals had different facet indexes,coordination densities,and atomic exposure rates.Subsequently,chemical tests were carried out to explore the balance between reactive oxygen species and redox capacity.Human gastric mucosal cells were then used as a cell model to explore the cytotoxicity and the relationship between cytotoxicity and palladium nano-crystals'chemical catalytic properties.The existence of oxygen and different reactive oxygen species in the cell and their transformation process after palladium nano-crystals'exposure were also discussed.The results show that different facets produce different reactive oxygen species.Palladium nano-cubes with{100}facets have higher surface energy,which excites triplet oxygen to a singlet state,and then dissociates molecular oxygen on the facets to exhibit its oxidative enzyme activity with the generation of hydrogen peroxide as byproduct.Peroxidase activities decompose hydrogen peroxide to hydroxyl radical.Palladium nano-octahedrons with{111}facets exhibit catalase-like activity and low cytotoxicity.Palladium nano-tetrahedrons with{111}facets have a larger proportion of corner atoms and a greater atomic exposure rates than that of palladium nano-octahedrons{111}.The oxygen dissociated on the surface of nanomaterials and hydrogen peroxide is generated simultaneously.Hydrogen peroxide is subsequently degraded and atomic oxygen is generated to cause oxidative damage.Palladium nano-concave tetrahedrons with{110}and{111}mixed facets can excite triplet oxygen to a singlet state,and then dissociates molecular oxygen on the facets to exhibit its oxidative enzyme activity and cause cytotoxicity.Peroxide hydrogens were generated as byproducts.Peroxidase activities decompose hydrogen peroxide to hydroxyl radical.Concave tetrahedrons also generate mass superoxide radical resulting in cytotoxicity.Palladium nano-crystals exhibit peroxidase-like properties in both chemical reactions and in vitro experiments,and the consumption of protons when decomposing hydrogen peroxide may further aggravate cytotoxicity.The above results show that the way nanomaterials cause oxidative stress is not limited to ROS.Both oxidase-and peroxidase-like activity have significant effects on cytotoxicity of palladium nano-crystals.The causes of cytotoxicity include both the production of ROS and oxidative properties such as oxidase-like activity.Furthermore,the balance of redox properties also contributes to the cytotoxicity of nanomaterials.In the process of the balance of redox properties of palladium nano-crystals with different facets,hydrogen peroxide plays a key role in the transformation of O₂ and ROS.The generation,degradation and conversion of hydrogen peroxide may cause cytotoxicity due to cascade mimic enzyme reactions of nanomaterials.Based on the above conclusions,we suggest that,in the preparation and application of nanomaterials,we should not only pay attention to the oxidation/reduction properties of new nanomaterials,but also discuss the possible health effects and related mechanisms caused by the oxidation/reduction properties of nanomaterials.