Luminescent biolabels are being eagerly investigated as a means of detecting cancer cells by bioimaging. Upconversion nanoparticles are a promising option to be used as biolabels for cancer cell detection. This process uses a near infrared beam (NIR λ = 980 nm) as the excitation source to upconvert the energy into light in the visible region. The present study, used Y2O3:Yb3+, Er3+ (1%, 10% mol) and Gd2O3:Yb3+, Er3+ (1%, 10% mol) capable of emitting red photons of λ = 660 nm. The nanoparticles were previously functionalized with aminosilanes and folic acid (UCNP-NH2-FA). Folic acid binds to the folate receptor on the surface of MCF-7 breast cancer cells, and this binding promotes internalization of the UCNPs via endocytosis. The UCNPs were characterized by TEM, EDS, and Fourier transform infrared. Cytotoxicity was also analyzed using the MTT (methy-134 thiazolyltetrazolium) colorimetric assay. The UCNPs-NH2-FA was noncytotoxic to the studied cancer cells and they were clearly localizable within the cell cytoplasm via confocal microscope.