Rare earth doped upconversion nanoparticles (UCNPs) are a class of NIR excited anti-Stokes emitting luminescent materials. Huangxian Ju’s group has established many detection and therapeutic approaches using the unique optical properties of UCNPs, including simultaneous detection of multiple glycosyl groups of specific proteins from cell membranes (Angew. Chem. Int. Ed., 2016, 55, 5220), intracellular microRNA imaging (Anal. Chem., 2019, 91, 3374) and targeted delivery and controlled release of antitumor drugs in cancer cells (Biomaterials, 2018, 163, 55). However, due to the limited light absorption capacity of rare earth elements and the energy loss during long-range energy transfer process, the improvement of luminance intensity and energy transfer efficiency of UCNPs has become an urgent need.
To solve this bottleneck problem, Ju’s group designed an “Energy-Concentrating Zone (ECZ)” strategy for UCNPs, which composed of energy absorption layer, energy emission layer and inert core to shorten effectively the energy transfer distance, and used NIR dye 800CW bound on the surface of UCNPs via a double-phosphate small molecule (ADA) to enhance energy absorption (Figure 1). The strong light absorption capacity of 800CW enabled abundant energy transfer to Nd3+ in the energy absorption layer through short-range resonance energy transfer, and further transferred to Er3+ in the energy emission layer.Due to the introduction of an inert core that blocked inward transfer, energy was effectively confined in ECZ, which led to 3600 times stronger emission intensity than UCNPs with traditional structure. The tight assembly of acceptor molecule via ADA greatly improved the energy transfer efficiency of ECZ structured UCNPs to 60%.
With fluorescent dye as the acceptor molecule, the detection sensitivity of ECZ structured UCNPs was significantly improved, which reduced the detection limit of cellular Hg2+ by 3 orders of magnitude (Figure 2A). With photosensitizer as the acceptor molecule, the production efficiency of reactive oxygen species was also highly improved for efficient photodynamic therapy of cancer (Figure 2B). The development of ECZ structured UCNPs provides an important tool for the development of highly efficient bioanalysis and therapy.
This work was published in Angew. Chem. Int. Ed.58 (35), 12117-12122 (2019) (DOI: 10.1002/ anie.201906380) with the title of Boosting Luminance Energy Transfer Efficiency in Upconversion Nanoparticles with an Energy-Concentrating Zone. Dr. Xiaobo Zhang, Dr. Weiwei Chen and Dr. Xiaoyu Xie were the co-first authors of the work, Prof. Huangxian Ju and Prof. Ying Liu were the corresponding authors. Prof. Haibo Ma of the institute of theoretical and computational chemistry department of Nanjing university and Prof. Chenghui Liu of Shaanxi Normal University provided assistance in this work.
Figure 1. Schematic diagrams of UCNPs with energy concentrating- zone structures and simplified energy transfer paths and energy level diagrams.
Figure 2. Hg2+ detection method using N719 as energy receptor (A) and the principle and detection results of high-efficient active oxygen generation using photosensitizer RB as energy receptor (B)