Optical multiplexing of upconversion in nanoparticles towards emerging applications

Lanthanide-doped upconversion nanoparticles (UCNPs) have received substantial attention in diversities of frontier applications because of their excellent and flexible photon upconversion performance. Advanced nano-synthetic techniques and core–shell nanostructure designs offer a versatile and powerful tool to smartly manipulate the dopant spatial distributions and energy transport channels, allowing for the fine tuning of interionic interactions and upconversion dynamics at a single nanoparticle level. In this review, the recent manipulation strategies of upconversion through internal chemical approaches and adopting external stimulus were systematically summarized and discussed, including tuning particle size, matrix composition, dopant concentration, spatial distribution, nanostructure design, energy flux regulation and heterostructure integration. Benefitting from the tunable multicolor and lifetime characteristics, the optical multiplexed UCNPs exhibit superior potential for emerging applications from nanophotonics to biomedicine. The challenges and perspective of optical multiplexed UCNPs in the near future are also depicted. This review should present a deep understanding of the flexible optical regulation of UCNPs and contribute greatly to their diversities of emerging applications.