One of the most serious problems in fluorescence bio-imaging is the limitation of observation time due to color-fading of the fluorescence probes and damages on the bio system, both of which are caused by the excitation ultraviolet irradiation. This problem can be solved by using an upconversion emission of rare-earth doped inorganic nanophosphors under infrared excitation. For the application of nanophosphors as bio-imaging probes, surface modification by biocompatible polymers is needed to obtain biological functionality in organic tissues. In this paper, authors demonstrate the formation of poly(acrylic acid) (PAAc) and &alfa;-acetal-poly (ethylene glycol)-block-[poly (2-(N,N-dimethylamino) ethyl methacrylate)] multilayers on the Er-doped Y₂O₃ nanoparticles as an upconversion nanophosphor. The existence of the layers was confirmed by using zeta electrometry and infrared vibrational spectroscopy measurements.