Fig. 1

Characterizations of Fe₃O₄ NPs. a TEM images of the Fe₃O₄ NPs (2.3 nm (left), 4.2 nm (middle) and 9.3 nm (right)). The inserts are the high magnification images. b Particle size distribution of Fe₃O₄ NPs. c XRD patterns of Fe₃O₄ NPs (2.3 nm (bottom), 4.2 nm (middle) and 9.3 nm (up)). d The release of Fe²⁺ from Fe₃O₄ NPs (2.3 nm, 4.2 nm, and 9.3 nm) in PBS with different pH values (pH 4.0, 5.0, 6.0, and 7.4). e Detection of generated ·OH from different-sized Fe₃O₄ NPs in pH 5.0 PBS using 3-CCA as fluorescence indicator. f The generation of ·OH from 2.3 nm Fe₃O₄ NPs in PBS with different pH values (pH 4.0, 5.0, 6.0, and 7.4). g The cytotoxicity of Fe₃O₄ NPs (1000 μg/mL) to MCF-7 cells in the presence and absence of H₂O₂