There has been a burgeoning trend in research and development for nanoparticles for potential bio-imaging application. Several classes of nanomaterials have been investigated. They include organic, metallorganic and polymer based dye, fluorescent proteins and semiconductor nanocrystals. Organic fluorophores suffer from a number of shortcomings including photobleaching and instability. Inorganic nanophosphors such as quantum dots seem ideal due to their bright fluorescence, size-dependent tunable emission and photostability. However, they usually contain toxic elements such as cadmium. Rare-earth (RE) nanophosphors, in comparison, offer an alternative for biological labeling and medical diagnostics due to their large Stokes shift, sharp emission spectra, long lifetime, multiphoton and up-conversion excitation, low toxicity and reduced photobleaching over quantum dots and organic phosphors molecule. These materials have the ability to emit intensely at various wavelengths by an appropriate choice of color-center elements instead of varying particle size.

In the current work, synthesis and characterization of various rare-earth (RE) doped and co-doped Y₂O₃ nanomaterials were undertaken. RE dopants such as terbium (Tb), europium (Eu) and erbium (Er) were successful doped, and ytterbium (Yb)-erbium (Er) was co-doped into yttrium oxide (Y₂O₃) nanomaterials. The as-synthesized nanomaterials are highly-dispersed in organic phase. X-ray diffraction (XRD) peaks confirms the crystallinity of the as-prepared Y₂O₃ samples and peaks can be assigned to cubic Y₂O₃. Room temperature photoluminescence (PL) spectra showed characteristic emission peaks of Tb, Eu and Er ions doped, and Er-Yb co-doped Y₂O₃ samples. Up-conversion spectra of green and red emissions were also shown for Er-Yb co-doped Y₂O₃ samples and the intensities of the green and red emissions could be altered by varying the dopant concentration. The nanocrystals were surface-functionalized with amine (NH₂) group via a reverse microemulsion method to render them water soluble and also afford them with the possibility of further functionalization by other bio-molecules. in vitro cytotoxicity studies showed that the synthesized nanocrystals have no appreciable toxicity on human hepatocellular carcinoma (Hep-G2) cells. Because of the Y₂O₃:RE nanomaterials' well-dispersity in water, low toxicity and good photoluminescence, they can potentially be used as fluorophores in bioimaging.