Plasmon-Enhanced Upconversion Fluorescence and Its Application

Lanthanide-doped upconversion nanoparticles (UCNPs) are capable of emitting visible light under near-infrared light excitation through a two-photon or multi-photon mechanism. Compared to other fluorescent materials such as organic dyes and quantum dots, UCNPs own superior physicochemical features such as chemical stability, high photostability, long-lived luminescence, large anti-Stokes shifts, narrow emission bands and deep penetration, which show potential applications in bioimaging, sensors, lasers, photodynamic therapy, solar cells and so on. However, the quantum yield of UCNPs is relatively low due to the small absorption cross-section of activator in UCNPs, limiting their further application. Therefore, how to improve the luminescence intensity of UCNPs has become a hotspot. A variety of methods such as core-shell nanostructure, phase transition and plasmon-enhanced upconversion have been developed in order to improve the fluorescence intensity of UCNPs. Among these methods, plasmon-enhanced upconversion as an efficient strategy has attracted extensive interests. In this review, three kinds of mechanisms about plasmon-enhanced upconversion luminescence are introduced firstly. Then construction methods of metal-UCNPs systems including chemical methods and physical methods, application of plasmon-enhanced upconversion luminescence in solar cells, bioimaging, bioassay, photothermal therapy and photocatalysis are discussed in detail. Finally, the limitations and directions for future research of plasmon-enhanced upconversion luminescence are also proposed.