Ultra-small luminescent nanoparticles (NPs) are quite desirable for optoelectronic and biomedical applications.

Demands for multiplexed biological detection have driven the development of fluorescence encoding nanoprobes. We have synthesized water soluble dual mode emission core-shell rare earth nanoprobes (~30 nm) for fluorescence encoding. The nanoprobes were composed of ytterbium (Yb), erbium (Er) and/or thulium (Tm) ions codoped heterogeneous NaYF 4 /NaLuF 4 nanocrystals as cores and amorphous SiO 2 embedded with europium (Eu) or terbium (Tb) complexes as shells. Exited by both infrared light and ultraviolet light, the nanoprobes exhibited dual characteristic emissions which enable a novel spectral encoding strategy. The core nanocrystals exhibited tunable up-conversion emissions through various lanthanide ions doping. Combining these multiple up-conversion emissions of core nanocrystals with downconversion emissions of shell containing rare earth complexes, a large amount of distinct fluorescence codes can be generated.We have synthesized water soluble dual mode emission (upconversion and downconversion) core-shell rare earth nanoprobes (~30 nm) for fluorescence encoding.