Surface plasmon resonance of Au/Ag metals for the photoluminescence enhancement of lanthanide ion Ln3+ doped upconversion nanoparticles in bioimaging

Deep tissue penetration, chemical inertness and biocompatibility give UCNPs a competitive edge over traditional fluorescent materials like organic dyes or quantum dots. However, the low quantum efficiency of UNCPs becomes an obstacle. Among extensive methods and strategies currently used to prominently solve this concerned issue, surface plasmon resonance (SPR) of noble metals is of great use due to the agreement between the SPR peak of metals and absorption band of UCNPs. A key challenge of this match is that the structures and sizes of noble metals have significant influences on the peak of SPR formants, where achieving an explicit elucidation of relationships between the physical properties of noble metals and their SPR formants is of great importance. This review aims to clarify the mechanism of the SPR effect of noble metals on the optical performance of UCNPs. Furthermore, novel research studies in which Au, Ag or Au/Ag composites in various structures and sizes are combined with UCNPs through different synthetic methods are summarized. We provide an overview of improved photoluminescence for bioimaging exhibited by different composite nanoparticles with respect to UCNPs acting as both cores and shells, taking Au@UCNPs, Ag@UCNPs and Au/Ag@UCNPs into account. Finally, there are remaining shortcomings and latent opportunities which deserve further research. This review will provide directions for the bioimaging applications of UCNPs through the introduction of the SPR effect of noble metals.