This is a copy of the slides presented at the meeting but not formally written up for the volume.

Upconverting nanoparticles (UCNs) are modified nanometer-sized composites which generate higher energy visible light from lower energy radiation (usually near-infrared (NIR)) by non-radiative transfer of photons between transition metal, lanthanide, or actinide ions doped into a solid-state host. These nanoparticles offer several advantages as imaging probes for live cells and tissues: high sensitivity of detection due to absence of autofluorescence from tissues, sharp emission peaks, less toxic components (than quantum dots (QDs)) and high depth of penetration and low phototoxicity of NIR light. Although the use of upconverting phosphors in nucleic acid assays, immunohistochemistry and immuno-assays have been demonstrated, no reports exploiting the advantages of these labels in live mammalian cell and tissue imaging have been demonstrated. Moreover, these assays usually utilize large sized reporters (>400nm). In this report, we present the synthesis and characterization of UCN and explore their effectiveness as live cellular and tissue labels. Nanoparticles with a nanocrystalline NaYF4 core doped with Yb3+ and Er3+ and coated with high molecular weight (25 kDa) PEI as surfactant was synthesized using a simple ‘one pot’ hydrothermal method. After characterization and biocompatibility tests, the UCN were conjugated to folic acid and targeted to mammalian breast carcinoma cells. To demonstrate tissue imaging, UCN were injected into live mouse and rat tissues and excited using a simplified NIR laser set-up. The nanoparticles obtained were spherical, about 50nm in diameter and with a narrow size distribution. They demonstrated sharp emission peaks at 653nm and 540nm when excited with a 980nm laser, and excellent stability when stored in phosphate buffered saline or incubated with complete serum at 37 deg C. The particles were found to be biocompatible with different cell types at different concentrations when incubated over varying time periods. Upon incubation, mammalian cancer cells took up the UCN and were imaged with high signal-to-background ratios. Continuous imaging of live cells could be performed without cell damage or death. UCN was injected into mouse skin and leg muscles and excited with NIR laser set at low power to prevent tissue damage. Visible phosphorescence was recorded from both sites. Phosphorescence could also be seen when UCN was injected in the skin and to a small depth of penetration in some muscles of rats.We conclude that these upconverting nanoparticles are promising labels for use in live cell and tissue imaging.