Lab on upconversion nanoparticles: optical properties and applications engineering via designed nanostructure

Over the past decade, high-quality lanthanide doped upconverting nanoparticles (UCNPs) have been successfully synthesized with the rapid development of nanotechnology. Due to the unique electron configuration of lanthanide ions, there are rich energy level structures in the near-infrared, visible and ultraviolet spectral range. However, for UCNPs, only a limited number of efficient upconversion excitation and emission have been generated due to the limited number of sensitizer (Yb(3+)) and activator (Tm(3+), Er(3+), and Ho(3+)) ions, and the application is mainly focused on the bio-imaging by using the upconversion luminescence of UCNPs. Recently, more and more researchers have started to focus on tuning of upconversion optical properties and developing of multi-functional UCNPs by using the combination of sub-lattice mediated energy migration, core@shell structural engineering and UCNPs based nanocomposites which greatly expands the range of applications for lanthanide-doped UCNPs. Therefore, a "nanolab" can be created on UCNPs, where the property modulation can be realized via the designed host-dopants combinations, core@shell nanostructure, energy exchange with "alien species" (organic dyes, quantum dots, etc.), and so on. In this paper, we provide a comprehensive survey of the latest advances made in developing lanthanide-doped UCNPs, which include excitation and emission energy levels guided designing of the UCNP nanostructure, the synthesis techniques to fabricate the nanostructure with optimum energy level structure and optical properties, the fabrication of UCNPs-based nanocomposites to extend the applications by introducing the additional functional components, or integrating the functional moiety into one nanocomposite.