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The dithiothreitol (DTT) assay is the most commonly used method to quantify the oxidative potential of fine particles.However,the reported DTT decay rates of carbon black (CB)materials vary greatly among different researchers.This might have resulted from either the intrinsic toxicity of CB or the unsuitability of the DTT assay protocol for CB particles.In the current study,the protocol of the DTT assay for CB materials has been carefully evaluated.It was found that the dispersion degree of CB particles in water has a great influence on the DTT decay rate of CB materials.For CB particles (special black 4A (SB4A)and Printex U) and single-walled carbon nanotube tube (SWCNT),the DTT decay rate after sonication for 10 min became 4.2,4.6 and 1.7 times higher than that without sonication.The rate continued to grow as a function of ultrasound time up to 30 min of sonication.Although the concentration of soluble transition metals and surface oxygen-containing species such as carbonyls increased slightly with sonication,they had no significant effects on the measured DTT activity,while the increase in the dispersion degree of aggregates was found to play a vital role in the observed enhancement of the DTT decay rates for different CB materials.Based on our results,30 min of sonication is recommended for sample dispersion when measuring the DTT decay rate of CB materials.