*Review* **Recent Advances of Upconversion Nanomaterials in the Biological Field**

**Cunjin Gao 1, Pengrui Zheng 1, Quanxiao Liu 1, Shuang Han 1, Dongli Li 1, Shiyong Luo 1, Hunter Temple 2, Christina Xing 2, Jigang Wang 1,\*, Yanling Wei 3,\*, Tao Jiang 4,\* and Wei Chen 2,5,\***


**Abstract:** Rare Earth Upconversion nanoparticles (UCNPs) are a type of material that emits highenergy photons by absorbing two or more low-energy photons caused by the anti-stokes process. It can emit ultraviolet (UV) visible light or near-infrared (NIR) luminescence upon NIR light excitation. Due to its excellent physical and chemical properties, including exceptional optical stability, narrow emission band, enormous Anti-Stokes spectral shift, high light penetration in biological tissues, long luminescent lifetime, and a high signal-to-noise ratio, it shows a prodigious application potential for bio-imaging and photodynamic therapy. This paper will briefly introduce the physical mechanism of upconversion luminescence (UCL) and focus on their research progress and achievements in bio-imaging, bio-detection, and photodynamic therapy.

**Keywords:** UCNPs; UCL; RE-doped; bio-imaging; PDT; bio-detection
