Hydrothermal synthesis of poly(acrylic acid)-functionalized α-(β-)NaYF4:Yb, Er up-conversion nano-/micro-phosphors

Carboxylic acid-functionalized Yb3 +/Er3 + co-doped α- and β-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 α-(β-)NaYF4 upconversion 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 α-NaYF4:Yb3 +, Er3 + nanospheres could be obtained via PAA assisted hydrothermal process at the pH value of 4–5 and 160 °C for 1–4 h. The obtained product is a mixture of α and β phases NaYF4:Yb3 +, Er3 + when the reaction time increased from 5 to 24 h. When the solution was treated at 200 °C for 8–24 h at the pH value of 8–9, pure β-NaYF4:Yb3 +, Er3 + microspheres were obtained. The FE-SEM results showed that the morphology of pure α-NaYF4:Yb3 +, Er3 + was spherical nanoparticles with an average diameter of about 70 nm and the pure β-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 α-NaYF4:Yb3 +, Er3 + upconversion phosphors. The luminescence properties of α-(β-) NaYF4:Yb3 +, Er3 + phosphors were also studied. Under 980 nm excitation, the upconversion luminescence (UCL) intensity of β-NaYF4:Yb3 +, Er3 + microspheres was much stronger than that of α- NaYF4:Yb3 +, Er3 + nanoparticles. Moreover, both red and green upconversion were ascribed to the two-photon process.