The radiative energy transfer from rare earth fluoride upconverting (UC) NaxLiyYF4:Yb3+,Er3+ nanoparticles to rhodamine dyes has been systematically studied in colloidal solutions at room temperature. The UC emission bands at 520 and 550 nm have been shifted to the longer-wavelength (ca. 600 nm) region suitable for biomedical applications. To decrease the optical length between the upconverting emitter and the fluorophore, the UC nanoparticles were decorated with titanate nanotubes coated with a dense layer of dye molecules providing possible resonance-energy transfer between them. The fabricated nanostructured composite shows efficient harvesting of UC emission within the proximity of the nanoparticles, allowing the local generation of light suitable for photodynamic therapy applications.