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

Gd₂O₃: Yb³⁺, Er³⁺ 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 GdOHCO₃·H₂O: Yb³⁺, Er³⁺ precursors increased from 200 nm to above 400 nm and remained the low standard deviation. The thermal convertion of GdOHCO₃: Yb³⁺, Er³⁺ into Gd₂O₃: Yb³⁺, Er³⁺ by the multistep calcination between 650 and 1200 °C has accomplished to achieve the perfect spherical shape. The size of the resultant Gd₂O₃: Yb³⁺, Er³⁺ 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 Gd₂O₃: Yb³⁺, Er³⁺ nanospheres. Gd₂O₃: Yb³⁺, Er³⁺ 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 Gd₂O₃: 18%Yb³⁺, 2%Er³⁺ and Gd₂O₃: 7.5%Yb³⁺, 1.5%Er³⁺ as the typical nanophosphors have been studied to understand more the UCL process. Based on these advanced features, Gd₂O₃: Yb³⁺, Er³⁺ UCL nanospheres will open promising platform for application in biomedicine as well as toward to developing a single spherical particle nanosensor.