Fig. 7
![Fig. 7](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2Fs12989-022-00465-y/MediaObjects/12989_2022_465_Fig7_HTML.png)
Possible causes of toxicity of USPIONs (< 5 nm): (1) high biodistribution of the ultrasmall NPs in heart; (2) ultrasmall NPs (< 5 nm) trigger the generation of ROS; (3) quick release of Fe2+ from ultrasmall NPs induces the ferroptosis based on hydroxyl radicals (·OH)