Two-color upconversion in rare-earth-ion-doped ZrO2 nanocrystals

To develop fluorescent labels for multicolor imaging, rare-earth-ion-doped ZrO2 nanocrystals were prepared by a complex precursor method. Laser excitation of 976nm induced single fluorescent bands of green and red upconversion (UC) in ZrO2:Er3+ and ZrO2:Er3++Yb3+ nanocrystals, respectively. A suppression ratio (SR) parameter was introduced, defined as the UC intensity ratio of the main band to all the other detected impurity bands, and SR values in the order of 10–100 were experimentally obtained, demonstrating the excellent monochromaticity of the UC labels. Thus, the two-color UC labels obtained are potentially ideal to be used for biological multicolor imaging.