High monodisperse nanospheres Gd2O3: Yb3+, Er3+ with strong upconversion emission fabricated by synergistic chemical method

Gd 2 O 3 : Yb 3+ , Er 3+ monodisperse nanospheres with strong red color upconversion luminescence (UCL) and high re-dispersibility fabricated by synergistic chemical method, in presence of urea, adding polyol and followed by a subsequently step by step calcination are demonstrated. While the urea/RE ratio from 10/1 to 50/1 and the content change of polyethylene glycol varied, the size of GdOHCO 3 ·H 2 O: Yb 3+ , Er 3+ precursors increased from 200 nm to above 400 nm and remained the low standard deviation. The thermal convertion of GdOHCO 3 : Yb 3+ , Er 3+ into Gd 2 O 3 : Yb 3+ , Er 3+ by the multistep calcination between 650 and 1200 °C has accomplished to achieve the perfect spherical shape. The size of the resultant Gd 2 O 3 : Yb 3+ , Er 3+ nanospheres has changed and controlled from 150 to 350 nm. It was found that the addition of polyethylene glycol (PEG) uprised consideringly the size, which further homogenized the shape as well as enhanced the re-dispersability and suspension stability of Gd 2 O 3 : Yb 3+ , Er 3+ nanospheres. Gd 2 O 3 : Yb 3+ , Er 3+ monospheres with high activator concentration show the strong UCL with extremely sharp strong peaks and possess intense red color emission. The UCL intensity was enhanced drastically when the calcination temperature increased from 650 to 1200 °C. The UCL properties of Gd 2 O 3 : 18%Yb 3+ , 2%Er 3+ and Gd 2 O 3 : 7.5%Yb 3+ , 1.5%Er 3+ as the typical nanophosphors have been studied to understand more the UCL process. Based on these advanced features, Gd 2 O 3 : Yb 3+ , Er 3+ UCL nanospheres will open promising platform for application in biomedicine as well as toward to developing a single spherical particle nanosensor. Graphical abstract