Kinetics and stoichiometry of reactions between the 2,2-diphenyl-1-picrylhydrazyl (DPPH) stable radical and 25 antioxidant compounds with different structure, molecular weight, number of -OH groups, and redox potential were investigated by recording the loss of DPPH(•) absorbance at 515 nm continuously for 10 min. A series of antioxidant concentrations was tested to determine linear response ranges and reaction saturation points. The primary feature distinguishing antioxidant activity--rate of initial reaction (<30 s)--was controlled by whether the dominant antioxidant mechanism was electron (very fast) or hydrogen atom (slow) transfer and by impairment of steric accessibility to the DPPH radical site by bulky ring adducts and multiple phenolic rings. Results raise serious questions regarding application of the DPPH assay for ranking antioxidants and natural extracts and suggest possible redirection of this assay to distinguish active reaction mechanisms by comparing reactions rates and patterns in different solvents and in 50% water/methanol mixtures at different pH values.

Reactive oxygen species (ROS) are crucial molecules in cancer therapy. Unfortunately, the therapeutic efficiency of ROS is unsatisfactory in clinic, primarily due to their rigorous production conditions. By taking advantage of the intrinsic acidity and overproduction of H 2 O 2 in the tumor environment, we have reported an ROS nanoreactor based on core−shell-structured iron carbide (Fe 5 C 2 @Fe 3 O 4 ) nanoparticles (NPs) through the catalysis of the Fenton reaction. These NPs are able to release ferrous ions in acidic environments to disproportionate H 2 O 2 into • OH radicals, which effectively inhibits the proliferation of tumor cells both in vitro and in vivo. The high magnetization of Fe 5 C 2 @Fe 3 O 4 NPs is favorable for both magnetic targeting and T 2 -weighted magnetic resonance imaging (MRI). Ionization of these NPs simultaneously decreases the T 2 signal and enhances the T 1 signal in MRI, and this T 2 /T 1 switching process provides the visualization of ferrous ions release and ROS generation for the supervision of tumor curing. These Fe 5 C 2 @Fe 3 O 4 NPs show great potential in endogenous environment-excited cancer therapy with high efficiency and tumor specificity and can be guided further by MRI.

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