Emitter-Active Shell in NaYF₄:Yb,Er/NaYF₄:Er Upconversion Nanoparticles for Enhanced Energy Transfer in Photodynamic Therapy

To realize the potential of near-infrared (NIR) upconversion nanosensitizers for photodynamic therapy of cancer, upconversion luminescence and energy transfer (ET) efficiency from emitter donors to photosensitizer acceptors need to be improved. In the current work, upconversion nanoparticles (UCNPs) with a core/emitter-active shell structure were constructed to enhance not only the upconversion emission but also the ET from the nanoparticles to surface-anchored photosensitizers. The emitter was doped into the shell to bridge the migration of upconverted energy to the surface. NaYF₄:Yb,Er/NaYF₄:Er UCNPs and rose bengal (RB) photosensitizer were employed as an example. The upconversion emission was lifted by up to ∼81 times of the core counterpart. The bridge effect of the emitter-doped shell was obvious for the constructed nanophotosensitizer. The emission of the RB photosensitizer was up to ∼36 times that of the core counterpart. The NaYF₄:Yb,Er/NaYF₄:Er UCNPs also endow the RB photosensitizers with the most efficient reactive oxygen species production capability under NIR irradiation. In vitro photodynamic tests on glioma cells were conducted to validate the efficacy of the NaYF₄:Yb,Er/NaYF₄:Er/RB agent. Therefore, this work can facilitate the development of ET-based upconversion nanosystems.