Dark Reactive Oxygen Species Generation in ReVO4:Eu3+ (Re = Gd, Y) Nanoparticles in Aqueous Solutions

Nanomaterials, which promote the generation of reactive oxygen species (ROS) and precipitate the onset of oxidative stress responsible for the development of a variety of adverse conditions, have a great prospect for various nanomedicine applications. In this paper, for the first time, we report the pro-oxidant activity of GdYVO4:Eu3+ and GdVO4:Eu3+ nanoparticles (NPs) in water solutions without any external stimuli (in the darkness). This dark pro-oxidant activity is shown to be associated with superoxide (O-2(center dot-)) and hydroxyl radical ((OH)-O-center dot) generation and is dependent on the size of NPs and, consequently, the concentration of surface defects, such as oxygen vacancies V-O. For small 2 nm GdYVO4:Eu3+ NPs, strong O-2(center dot-) and (OH)-O-center dot radical production has been revealed, whereas for larger GdVO4:Eu3+ NPs, low level of O-2(center dot-) and negligible level of (OH)-O-center dot radical production have been detected during the sample storage in the darkness. The possible mechanisms of the dark ROS generation in ReVO4:Eu3+ NPs are considered.