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.

Water-soluble α-(β-) NaYF4 nano-/micro-spheres were prepared via one-pot hydrothermal process by taking PAA as surfactant. The effect of reaction conditions including the pH value, temperature and reaction time on phase and morphology of products was studied. The upconversion luminescence of products was discussed. Finally, the BSA protein was successfully conjugated with α-NaYF4 upconversion nanophosphors.Figure optionsDownload as PowerPoint slideHighlights
► Water-soluble α-(β-) NaYF4 was prepared via one-pot hydrothermal process.
► Effect of reaction conditions on phase and morphology of products was studied.
► BSA protein was conjugated with α-NaYF4 upconversion nanophosphors successfully.