In this report we studied the properties of the dye IR806 and possible mechanisms of the dye sensitization effect on ytterbium-erbium co-doped upconversion nanoparticles. We found that the dye IR806 has two primary emission peaks in the NIR spectral range at around 850 nm and at around 950 nm. The intensity of these peaks were observed to be affected by the concentration of the dye and the addition of Gadolinium(III) chloride and Yttrium(III) chloride. Specifically increases in the intensity of the 950 nm peak relative to the 850 nm were of interest since ytterbium readily absorbs 950 nm and transfers this energy in the upconversion process. Our hypothesis is that the change in the intensity of the 850 nm and the 950 nm peak is associated with aggregation of the dye IR806 and the amount of monomers and dimers. Results from adding ytterbium-erbium co-doped upconversion nanoparticles in IR806-ethanol solution points to the picture of dimers being formed on the surface on the nanoparticles. This analysis is however based on the assumption that the 850 nm emission peak of IR806 is associated with monomers and that the 950 peak is associated with dimers, which is yet to be confirmed and further studies are therefore needed.