Near-infrared Light Responsive Upconversion Nanoparticles for Imaging, Drug Delivery and Therapy of Cancers

Cancers have become serious threat to human health and life, and they are critical to develop safe and effective theranostic methods for diagnosis and therapy of tumors. In recent years, real time cancer theranostic visualization systems (RT-CTVS) based on light-responsive nanoparticles have been developed. Especially, upconversion nanoparticles (UCNPs) have excellent optical properties and unique near-infrared (NIR) responsive. The minimized photodamage, low autofluorescence and high penetration depth can be achieved with UCNPs. Therefore, UCNPs are widely used in real time NIR mediated visualization systems of cancer diagnosis and therapy. In this review, we focus on the latest developments of rare earth ions doped upconversion fluorescence nanoparticles. First, the synthesis methods of UCNPs were briefly introduced. Second, the strategies of UCNPs surface modifications, including the ligand exchange, ligand oxidation, ligand interaction, ligand free synthesis, layer by layer growth and surface silanization were summarized. Third, the recent research progress in applying UCNPs to construct NIR light stimuli-responsive RT-CTVS, including imaging, drug delivery and photodynamic therapy (PDT) were highlighted. Finally, some of the current problems and future effort directions in these fields were also proposed.