Carboxylic acid-functionalized Yb3+/Er3+ co-doped alpha- and beta-NaYF4 nano-/micro-spheres were prepared through a simple poly(acrylic acid) (PAA)-assisted hydrothermal process. Bovine serum albumin (BSA) protein was conjugated with Yb3+/Er3+ co-doped a-(A-)NaYE4 up conversion phosphors via free carboxylic acid groups on the surface of phosphors. The final products were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), infrared (IR) spectrophotometer, ultraviolet (UV) spectrophotometer and photoluminescence spectroscopy (PL). XRD results showed that pure a-NaYF4:Yb3+, Ei(3+) nanospheres could be obtained via PM assisted hydrothermal process at the pH value of 4-5 and 160 C for 1-4 h. The obtained product is a mixture of a and)3 phases NaYN:Yb3+, Er3+ when the reaction time increased from 5 to 24 h. When the solution was treated at 200 degrees C for 8-24 h at the pH value of 8-9, pure p-NaYF4:Yb3+, Er3+ microspheres were obtained. The FE-SEM results showed that the morphology of pure a-NaYF4:Yb3, Er3+ was spherical nanoparticles with an average diameter of about 70 nm and the pure beta-NaYF4:Yb3+, Er3+ was microsphere with an average diameter of about 700 nm, which is composed of spherical nanoparticles (30 nm). The FT-IR and UV spectra showed that BSA had been conjugated with a-NaYF4:Yb3+, Er3+ upconversion phosphors. The luminescence properties of alpha-(beta-) NaYF4:Yb3+, Er3+ phosphors were also studied. Under 980 nm excitation, the upconversion luminescence (UCL) intensity of 13-NaYF4:Yb3+, Er3+ microspheres was much stronger than that of a- NaYF4:Yb3+, Er (3+) nanoparticles. Moreover, both red and green upconversion were ascribed to the two-photon process. (C) 2012 Elsevier B.V. All rights reserved.