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

Carboxylic acid-functionalized Yb 3   + /Er 3   + co-doped α - and β -NaYF 4 nano-/micro-spheres were prepared through a simple poly(acrylic acid) (PAA)-assisted hydrothermal process. Bovine serum albumin (BSA) protein was conjugated with Yb 3   + /Er 3   + co-doped α -( β -)NaYF 4 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 -NaYF 4 :Yb 3   + , Er 3   + 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 NaYF 4 :Yb 3   + , Er 3   + 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 -NaYF 4 :Yb 3   + , Er 3   + microspheres were obtained. The FE-SEM results showed that the morphology of pure α -NaYF 4 :Yb 3   + , Er 3   + was spherical nanoparticles with an average diameter of about 70   nm and the pure β -NaYF 4 :Yb 3   + , Er 3   + 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 -NaYF 4 :Yb 3   + , Er 3   + upconversion phosphors. The luminescence properties of α -( β -) NaYF 4 :Yb 3   + , Er 3   + phosphors were also studied. Under 980   nm excitation, the upconversion luminescence (UCL) intensity of β -NaYF 4 :Yb 3   + , Er 3   + microspheres was much stronger than that of α - NaYF 4 :Yb 3   + , Er 3   + nanoparticles. Moreover, both red and green upconversion were ascribed to the two-photon process.