The article "Upconversion Nanoparticles: Design, Nanochemistry, and Applications in Theranostics" by Guanying Chen, Hailong Qiu, Paras N. Prasad, and Xiaoyuan Chen provides a comprehensive review of upconversion nanoparticles (UCNPs) and their applications in theranostics. UCNPs are a promising class of materials that can convert low-energy NIR photons into higher-energy emissions, making them suitable for various biomedical applications such as imaging, drug delivery, and therapy. The introduction covers the basic concepts of UCNPs, including their structure, upconversion mechanisms (excited-state absorption, energy transfer upconversion, cooperative sensitization upconversion, cross relaxation, and photon avalanche), and the advantages of using UCNPs over conventional contrast agents in bioimaging. The subsequent sections detail the design and optimization of UCNPs to achieve high efficiency and tunable emission colors. Key strategies include selecting novel host materials, tailoring local crystal fields, plasmonic enhancement, engineering energy transfers within lanthanide dopants, and suppressing surface-related deactivations. The article also discusses the nanochemistry involved in the controlled synthesis and surface engineering of UCNPs, emphasizing the importance of monodispersed size, uniform shape, and stoichiometric composition. It highlights the need for biocompatibility and the ability to target specific biomarkers through surface engineering. The theranostic applications of UCNPs are explored in detail, including biosensing, bioassays, high-contrast bioimaging, and drug delivery and therapy. The authors discuss the advantages of UCNPs in these applications, such as high efficiency, multicolor emissions, and deep tissue penetration. Finally, the article concludes with a discussion on the current status, challenges, and future perspectives in the field of UCNPs, emphasizing the potential of UCNPs in personalized medicine and advanced biomedical diagnostics and therapy.The article "Upconversion Nanoparticles: Design, Nanochemistry, and Applications in Theranostics" by Guanying Chen, Hailong Qiu, Paras N. Prasad, and Xiaoyuan Chen provides a comprehensive review of upconversion nanoparticles (UCNPs) and their applications in theranostics. UCNPs are a promising class of materials that can convert low-energy NIR photons into higher-energy emissions, making them suitable for various biomedical applications such as imaging, drug delivery, and therapy. The introduction covers the basic concepts of UCNPs, including their structure, upconversion mechanisms (excited-state absorption, energy transfer upconversion, cooperative sensitization upconversion, cross relaxation, and photon avalanche), and the advantages of using UCNPs over conventional contrast agents in bioimaging. The subsequent sections detail the design and optimization of UCNPs to achieve high efficiency and tunable emission colors. Key strategies include selecting novel host materials, tailoring local crystal fields, plasmonic enhancement, engineering energy transfers within lanthanide dopants, and suppressing surface-related deactivations. The article also discusses the nanochemistry involved in the controlled synthesis and surface engineering of UCNPs, emphasizing the importance of monodispersed size, uniform shape, and stoichiometric composition. It highlights the need for biocompatibility and the ability to target specific biomarkers through surface engineering. The theranostic applications of UCNPs are explored in detail, including biosensing, bioassays, high-contrast bioimaging, and drug delivery and therapy. The authors discuss the advantages of UCNPs in these applications, such as high efficiency, multicolor emissions, and deep tissue penetration. Finally, the article concludes with a discussion on the current status, challenges, and future perspectives in the field of UCNPs, emphasizing the potential of UCNPs in personalized medicine and advanced biomedical diagnostics and therapy.