Capillary Electrophoresis-Assisted Identification of Peroxyl Radical Generated by Single-Walled Carbon Nanotubes in a Cell-Free System

The mechanism of how single-walled carbon nanotubes (SWCNTs) induce oxidative stress in cells has always been under debate. In the present paper, we have clearly identified the generation of peroxyl radical (ROO center dot) by the unmodified SWCNT and the poly(ethylene glycol)-functionalized SWCNT (SWCNT-PEG) in aqueous solutions using a simple analytical technique: capillary electrophoresis (CE). CE provides a sensitive detection platform for the oxidation product of the employed reactive oxygen species (ROS) indicator, 2,7-dichlorodihydrofluorescein (H2DCF). In addition, its high separation power eliminates the fluorescence quenching exerted by SWCNTs, which was a big problem that has been encountered in measurements with micro-plate readers, and facilitates the screening of a large variety of ROS scavengers, including small molecules, surfactant, and proteins, in our study. Furthermore, CE enables the simultaneous incubation of multiple ROS indicators for comparison of their oxidation efficiency, improving the detection specificity. The adsorption of oxygen on the SWCNT surface could be the source of the identified ROO center dot, because surface coverage by surfactant or proteins could inhibit the oxidation and the anaerobic experiment led to data agreeing with the Langmuir isotherm. The peroxyl radical can serve as the seed for the production of more-active ROS after the SWCNT enters the cell and, therefore, SWCNTs hold potential toxicity to the biological hosts.