Rare-earth doped yttrium oxide nano-particles modified with bio functional polymers are one of the good candidate for upconversion bio imager. The requirement to the particles for bio imaging application is a small particle size less than 50 nm, dispersion and chemical durability in body fluid. In this study, polyacrylic acid (PAAc) was used for both precursor precipitation and surface modification of the calcinated particles. The use of the PAAc resulted in small particle size at 33 nm. Also, the modification by adding 104 PAAc/particle resulted in chemical durability and dispersion of the particles under heavily acidic and basic conditions.