EPR studies of upconversion effect on prooxidant activity of Gd2O3 nanoparticles in photodegradation of methylene blue under red light irradiation

Paramagnetic gadolinium oxide (Gd 2 O 3 ) nanoparticles (NPs) with an average size of 24 ± 2 nm and pro-/antioxidant properties were obtained by hydrothermal synthesis. Energy-dispersive X-ray spectroscopy analysis confirmed the presence of substitutional dopant defects within the Gd 2 O 3 crystal lattice, specifically holmium (Ho 3+ , 0.15 at%) and europium (Eu 3+ , 0.05 at%). The Ho 3+ and Eu 3+ impurity defects confer luminescent and pro-oxidant properties to the Gd 2 O 3 NPs through an upconversion mechanism. Using spin-trapping electron paramagnetic resonance spectroscopy with 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) to detect hydroxyl radicals (•OH), the influence of Gd 2 O 3 NPs on the photo-oxidative degradation process of methylene blue under red light irradiation (λ ex = 660 ± 5 nm) was determined to be concentration-dependent and bidirectional. The combination of paramagnetic and pro-oxidant properties renders Gd 2 O 3 NPs highly promising for application in controlled combination theranostics, specifically as a contrast agent for magnetic resonance imaging and an efficient upconversion photosensitizer for photodynamic therapy.