Preparation of green emission and red emission ligand-free upconverting nanoparticles for investigation of the generation of reactive oxygen species applied to photodynamic therapy

Abstract

Two kinds of ligand-free upconversion nanoparticles (UCNPs) that exhibit green emission and red emission respectively were successfully synthesized for the study of reactive oxygen species (ROS) generation in photodynamic therapy (PDT). Fourier-transform infrared spectroscopy (FT-IR) confirmed the removal of oleate ligand from the surface of UCNPs after acid treatment. X-ray diffraction (XRD) and transmission electron microscope (TEM) analysis revealed that both oleate-capped and ligand-free UCNPs had the tetragonal phase and the single-crystal structure. The particle sizes analyzed by dynamic light scattering (DLS) were in line with the data from TEM. The photoluminescence (PL) spectra showed that the removal of the oleate ligand had no notable effect on the fluorescence intensities of both UCNPs. Two photosensitizers (PSs), merocyanine 540 (MC 540) and chlorin e6 (Ce6), were used as the energy acceptors and coupled with the energy donors (UCNPs) for luminescence resonance energy transfer (LRET) and ROS generation studies. By comparing the quenching efficiencies of PL spectra of the oleate-capped and ligand-free Ho3+-doped UCNPs/MC 540 pairs with those of Er3+-doped UCNPs/Ce6 pairs, it was found that the quenching efficiencies caused by the adsorption of MC 540 on the surface of both types of Ho3+-doped UCNPs were greater than those of Er3+-doped UCNPs/Ce6 pairs. Subsequently, the investigation of the generation of ROS was performed by using 9,10-anthracenediyl-bis(methylene) dimalonic acid (ABDA) to detect the production of reactive oxygen species. The results showed that among the four combinations of LRET pairs, the ligand-free Ho3+-doped UCNPs/MC 540 pair exhibited more pronounced ROS generation due to the occurrence of more effective LRET between the UCNP donor and the MC 540 acceptor.